NONLINEAR DYNAMICS OF LOW REYNOLDS NUMBER ROÜND JETS: PERIODIC ATTRACTORS AND TRANSITION TO CHAOS

IONUT DANAILA 1 -3 , JAN DUSEK 2 AND FABIEN ANSELMET 3 1 I.N.E.R.I.S., 60550 Verneuil en Halatte, France 2 I.M.F., 2, rue Boussingault, 67000 Strasbourg, France 3 I.R.P.H.E., 12, Av. General Ledere, 13003 Marseiile, France Direct numerical simulations have been shown [l, 2] to provide detailed information on the dynamics and coherent structures of the near field of a spatially developing axisymmetric jet. In [l] we demonstrated the shift from helical to axisymmetric structures with increasing diametral Reynolds number in the ränge [200; 500]. At the upper bound ofthis ränge, the varicose m = 0 mode is the most amplified (m is the azimuthai wave-number). The development of the unsteady flow is accompanied by the well known phenomena: 2D Kelvin-HelmhoItz instability, roll-up and pairing, streamwise filaments and side-jets. The onset of the asymptotic chaotic state is preceded by vortex rings reconnection and breakdown of the large structures due to strong stream-wise filaments. A similar transition process was observed in temporal simulations by Melander et al. [START_REF] Melander | Breakdown of a circular jet into turbulence[END_REF].

In this paper, we investigate the mechanisms leading to instationarity and transition tö chaos for Reynolds numbers dose to the lower bound of the mentioned ränge. The numerical implementation was the same äs that described in [l]. The 3-D Navier-Stokes (NS) solver Nekton based on the spectral element space discretization has been used to solve the incompressibie NS equations in a cylindrical domain of stream-wise length equal to 15 nozzle diameters D and diameter roughiy equal to 10 D. Numerical shear in a very small domain at the nozzle is responsible for the spontaneous onset of the instationarity.

For these low Reynolds numbers, the amplification of the helical mode is responsible for the jet syrametry breakingat the primary"instability. T)w to the axisymmetry of the base flow there exist two linearly independent, equally amplified, unstable modes, identified äs the counter-rotating helical modes m == ±1. Their mutual interaction leads, dose to the instationarity threshold, to a succession of 3 different regimes:

Seventh European Turbulence Conference, Saint-Jean Cap Ferrat, 30juin-3 juillet 1998, p. 105-108 Regime I, appearing for 0 < e = (Re -Recr)/Recr < 1.2%, is characterized by a limit cycle dynamics with a single helical mode present in the flow; Regime II (1.2 < e < 3%) having a limit torus dynamics generated by the presence of both m = ±1 modes with unequal amplitudes; and finally, Regime III (3_< e < 4.5%)_wlth, again, a limit cycleL dynamics, resulting from the interaction of the two m == ±1 modes with equal amplitudes. All these stages could easily be predicted by a 5-th order weakly nonlinear theory describing the interaction ofthe helical modes m == ±1 (see [START_REF] Danaila | Non-linear dynamics at a Hopf bifurcation with axisymmetry breaking m a jet[END_REF] for more details). The predictions of the theoretical model are in very good agreement with the results ofdirect numerical Simulation, äs illustrated in figure l.

The chaotic state sets in about at e == 5% above the instationarity threshold. Figure 2a shows the very slow decay ofthe limit cycle resulting äs an equilibrium ofthe two counter-rotating helical modes (Regime III). After very long transients, intermittent oscillations set in. The power spectra on the right side of the figure show that a new, about 50% higher, frequency appears. To detect the spatial structures responsibIeJor this new .frequency, we analyzed the flow-field by the Fourier analysis proposed in [4J, and applied with success in [START_REF] Danaila | Non-linear dynamics at a Hopf bifurcation with axisymmetry breaking m a jet[END_REF], to characterize the unstable symmetry brealdng modes. It consists in computing temporal Fourier modes through-out the flow-field in a sufficiently large time interval. In this case, we used a time intervai corresponding to 17 periods of rapid oscillations visible in Fig. 2a.

It appeared that only temporal Fourier modes Cn with n = l, 17 and 25 were really significant. The mode n = 17, corresponding to the basic frequency (denoted /i), being the strengest and the mode n == 25 (corresponding to /a) approaching progressively the level of mode n = 17, The obtained temporal modes (c") can be further decomposed into azimuthai Fourier modes with index-m. The equilibrium ofthewi = dbl helicat modes, characterizing the decaying Regime III, is thus expressed by comparing the Fourier coefficients ci7,±i. The iso-pressure surfaces of dominating modes {2Real[cr",me~i mö ], with n = 17,25 and m = l) are shown in Fig. 2b. It clearly appears that the second frequency is associated with another helical mode. It is interesting to note that the wavelengths of both modes have the same ratio äs their periods, showing that these two modes have the same phase velocity of about 0.5, value characteristic for a jet. The observed onset of chaos can be characterized äs type II intermittency. A similar behavior was observed in the simulations for 5% < e < 10% above the instationarity threshold, when the chaos is completely developed (Fig. 3). The same type II of intermittency was observed at high Reynolds numbers, in the forced jet experiments of Broze & Hussain [START_REF] Broze | Transition to chaos in a forced jet: intermittency. tangent bifurcations and hysteresis[END_REF]. It is also interesting to note that the transition to chaos for low Reynolds numbers involves only the interaction of helical modes, while for high Reynolds numbers, the breakdown into turbulence is due to the interaction between axisymmetric and helical modes [START_REF] Melander | Breakdown of a circular jet into turbulence[END_REF].

Figure l .

 l Figure l. Azimuthai velodty at a point located in the jet mixing layer, at 2 Bozzle diametere downstream. The period of oscillations is about 6 time units. Comparison between the simulated and theoretically predicted dynamics: limit cycle (regimes I and III) and limit torus (regime II).

Figure 2 .

 2 Figure 2. Direct numerical Simulation at e ?s 5%. a) Azimuthai velocity signal and corresponding spectra for the direct Simulation (same point äs in Fig. l), b) Iso-surfaces of low pressure characterizing the amplified modes; 0174 (up) corresponds to the helical mode of frequency /i and 025,1 (down) to the helical mode of frequency fs.

Figure 3 .

 3 Figure 3. Direct numerical Simulation at e w 9%. Azimuthai velodty at the same point äs in Fig. l. Intermittent part of the signal.


Joseph-Jean Pâques

Philippe Villeneuve De Janti

Jean-Luc Durka

Safety requirements for an automatic mining truck without onboard operator

A prototype of an automatic truck has been developed for ore haulage in underground gold mines in Quebec (Canada). The control system is a computerized guidance system that is designed to give standard underground mining haulage vehicles the ability to operate fully automatically without an onboard operator. It combines scanning lasers with the most successful elements of radio remote controls on the leading edge of mine automation. It uses also a collision avoidance system to detect specific transponders equipping all people and vehicles in the mine by using microwave signals. The presentation will deal with the development of the safety program and with what has been done in the real life-cycle of the project.

INTRODUCTION

A mining company in Quebec is planning to install a system of automatic trucks in its mine at Abitibi. This will be the first time such a system has been used in Canada, and the trucks will transport ore between the cutting face and the treatment plant with no human intervention. These automatic trucks are expected to operate in certain areas where there are also operator-driven vehicles and workers on foot.

As part of a safety assurance programme, the IRSST in cooperation with INERIS has been asked to develop and apply a safety programme to ensure that the automatically guided vehicle system is safe enough with respect to workers in the mine.

System operation is based upon three major components:

• A system to guide and operate the automatic trucks employing fixed markers (overhead reflecting strips and bar codes of the same type).

• An onboard device to detect people carrying passive transducers.

• A traffic control system employing command and communication facilities operating throughout the site.

To ensure the safety of such a system, special efforts must be made in its design and operation.

A particularly serious problem in obtaining an adequate level of safety is that workers and automatic machines are present in the same areas.

warn ure

Locations of the guidance system and the onboard computer

Principle of the guidance system

PARTICULARS OF THE SYSTEM

The system is made up of four parts:

• Control of the automatic truck.

• Control of traffic.

• The truck protection systems (against fire, overspeed, and so on).

• The collision avoidance system consisting of a transceiver and passive transducers attached to the helmets of the personnel (3 transducers) and vehicles (6 transducers) which receive the signal from the truck and transmit a signal at twice the frequency.

When the collision avoidance system receives this double frequency, it issues the order to stop the truck.

At the unloading point at each cycle, the system carries out a self-test to verify the integrity of its safety function (checking the performance of the detection system at the front and rear of the truck).

SAFETY ASSURANCE PROGRAMME

A programme of safety assurance was developed and applied in order to ensure that the self-guided vehicle system showed an adequate level of safety with regard to workers.

Evaluation of the system provided data about the level of safety resulting from the principles applied and, secondly, provided users with further information for optimising its use. It is based upon European standards, notably standard Etf 954 "Safety of machinery and safety-related system components", and on the draft international standard IEC 61508 "Functional safety of electrical/electronic/programmable electronic systems relative to safety".

The safety file was prepared in four main phases:

1. Analysis of system risks. 2. Definition of objectives and technical safety requirements for the system.

3. Validation of the design and construction of the system. 4. Evaluation of the operating safety of the system.

The safety file sets out all the analysis done together with the tests carried out on an experimental site, together with a summary of the necessary improvements to meet the safety objectives.

There are also certain recommendations concerning maintenance and the operating requirements for the machine: self-tests, periodic tests, and so on.

SAFETY FILE

Analysis of system risks

The aim of this phase is to identify any hazards of the system in each operating mode: loading the truck, transfer from the loading point to the unloading point, truck unloading and maintenance. The analysis was in four phases:

• Identification of hazards.

• Preliminary analysis of hazards to determine safety-related events.

• Preliminary analysis of risks.

• Deductive analysis using causal trees.

The analysis was applied to the entire installation, i.e., the automatic truck in its mine environment and its control system (excluding the protective devices). The analysis leads either to certain functions of the control system being identified as requiring handling with a high level of safety (guidance, braking) or, usually, to a protection system (such as collision avoidance, overspeed) being introduced.

Identification of hazards

The following figure shows how hazardous situations leading to a potential accident can be broken down:

Hazardous entity

Dangerous Phenomenon

Event or condition (mainly of a technical nature, sometimes human) (or failure)

Hazardous situation

Additional condition (human or technical in nature) (sometimes a failure)

Breakdown of potential accidents Rear view of a mining truck fitted with the receiving antenna of the collision avoidance system

Dangerous entities, i.e., the parts of the installation that can be associated with an intrinsic or potential hazard, or an external attack. The hazardous entity has to be exposed to an event or condition (failure of the command system) to lead to a hazardous situation.

Hazardous situations that result from the interaction of a hazardous entity or an external attack with the context of or the entire installation. This hazardous situation has to be associated with an additional condition if it is to be converted into a potential accident.

Potential accidents related to the interactions between the hazardous entities and the events causing a hazardous situation and their consequences.

The following table gives an example of an analysis using this model for three hazardous entities:

Analysis of hazardous entities

Preliminary analysis of hazards

The hazards were identified and risks analysed for the five operating modes of the system. Three of these modes are related directly to production (loading -transitunloading). One mode is associated with the maintenance of the vehicle and the last mode corresponds to a test run carried out at the beginning of each shift:

» Loading the truck -low speed (collision avoidance system not in service).

» Transit of truck from loading point to unloading point -high speed (collision avoidance system in service).

• Unloading truck -low speed (collision avoidance system not in service). This situation is similar to the loading station (layout of the unloading station to be taken into account).

• Maintenance (collision avoidance system not in service). This situation is regarded as similar to the loading station except as concerns the commands by guide rail and bar codes.

• First test circuit in automatic mode with driver on board (at the beginning of each shift).

Preliminary risk analysis according to EN 954

The risk analysis must also specify the level of safety with which the control and protection functions must be executed. According to a model proposed in standard EN 954-1, estimation of the risk takes into account the following three parameters:

• => Collision with a person on foot.

Existing preventive measures

Braking test carried at the mine entrance on flat ground at each loading cycle.

Shut down engine and apply brakes in the event of malfunction.

Preventive maintenance procedure.

Test of system integrity during each unloading cycle.

Shut down engine and apply brakes in the event of malfunction.

Several transducers per person or vehicle.

Cat.

2

The risk analysis may be qualitative and/or quantitative. Each of these approaches has advantages and drawbacks; for this reason a combination of the two is recommended for critical installations.

Fault trees

General fault trees were plotted for the main undesirable events.

They determine the various possible combinations of events that result in an undesirable top event and allow any common causes to be determined.

Objective and technical safety requirements

This phase involves defining the safety objectives at system level based upon the analysis of hazards and risks. As an example, the technical requirements in category 2 applicable to the equipment are:

1. For the collision avoidance function:

• The system should be designed in such a way that its functions self-test at appropriate intervals (at least every second during operation).

• The system should be capable of stopping the truck (shutting down the engine and applying the brakes) if a fault occurs (weak on-line coverage of faults: 60%).

• Malfunctions affecting safety (truck accidentally starting, bar codes wrongly read) and not tested on-line should be detected by off-line tests at appropriate intervals to maintain a level of safety better than 10" /year. 2. For the overspeed and braking functions:

• Malfunctions affecting the safety (failure of the truck transmission system, failure of the braking system) should be detected by off-line tests at appropriate intervals to maintain a level of safety better than lO'Vyear.

Validation of the phases of design and construction

Based upon the technical specifications laid down, this phase covers:

• Validation of the system with regard to the environmental constraints it is likely to encounter.

• Validation of the methodology used for developing the software to highlight the methods and facilities employed to ensure safety.

Qualification tests

• Climatic testing: Ability to withstand cold, dry heat, and temperature changes.

• Mechanical tests: Vibration and impacts.

Electrical tests

• Electrostatic discharges.

• Radiated EM fields.

• Interruptions to power supplies.

• Fluctuation in supply voltage.

Analysis of the methodology used for developing software

As a rule, the process of software development is broken down into activities that constitute the life cycle of a program. This entire breakdown is translated into "quality requirements" and is used to define the basic way of looking at the tasks of developing, testing and evaluating the software. Thus the aim of this analysis is to provide a qualitative view of the functional safety level of the program by means of an evaluation questionnaire adapted to the application for the following phases:

• Specifications of the software.

• Preliminary design of the software.

• Detailed design.

• Writing code.

• Unit tests on the software.

• Integration tests on the software.

• Utilisation procedures.

• Development and modification of the software.

Evaluation of the functional safety of the system

Functional safety is evaluated in 3 stages:

1. Structural analysis of the system as a whole (functional structure, common modes).

2. Evaluation of the hardware structure.

3. Evaluation of the integrity of the software.

Overall analysis of the system

The system must be capable of stopping the track (shutting down the engine and applying the brakes) when a fault occurs. Moreover it must be designed in such a way that its functions are self-tested every second during operation.

The safety architecture concept adopted by the manufacturer is of the lool type:

Senior «nit input (nitefa** «OnilHFiMlRt

Latin syxtem tompantat

' I F 1 pilotage 1 1 (; k 1rdjciMoini 1 Ontptft fot*rft« >n4 final
Architecture structure adopted

Overall analysis of the system shows that, in its present state, it does not present an adequate level of confidence for utilisation in a mine.

However, in view of the test procedures devised for detecting faults affecting:

• the collision avoidance system,

• the brakes,

• mistakes in reading or interpreting critical data from bar codes, consideration may be given to its use on an experimental site on condition that appropriate safety instructions and training arrangements are drawn up. The experimental site investigated is located at the Val d'Or (Quebec) mining research centre.

Diagram showing a mining vehicle integrated in the experimental environment investigated

Evaluation of the hardware structure

The hardware and its structure were evaluated on the basis of the data -occasionally preliminary -available.

Functional analysis

The functional analysis was done with the system broken down to level -4. It contains structural diagrams that explain the technical composition of systems providing the identified functions.

The following table gives an example of how the controlcommand function for the truck's path is broken down:

Level -1 Control-command of the truck's path and of the driving functions.

Level -2

Monitoring the track's path.

Level -3

Measuring the position of the truck centre line with respect to the guide rail.

Calculate the deviation of the truck centre line from the guide rail. The main thread running through this analysis is the level of detail of the functional analysis, and links the disclosure of malfunctions to the safety objectives.

Level

Function

To detect the presence of a vehicle or of a person in the path of the truck.

Failure mode

Spurious detection.

Cause of failure

Faulty transtnissio n relay.

Effect on the function

Truck stops even though no vehicle or person is present

Detection

None.

Disposition

Maintenance.

Evaluation of the software structure

The software structure was also analysed. The program operates in multi-task mode. It is loaded into the "RAM" memory. A CRC16 is performed only during loading. The system has no independent hardware watchdog.

CONCLUSION

The results of this study were used to prepare a safety file that will serve as a basis for the design and evaluation of these safety-related electronic systems.

The conclusions reached in the safety file resulted in recommendations being made to the designer and will serve as a basis for future experiments.

  Seriousness of damage: C • Time spent in the hazardous zone: F • Possibility of containing the hazard: P Measures excessive for the risk in question • -: Possible category requiring additional measuresThe synthesis of the risk analysis was presented in the following form

  valve right Diagram showing the principle of the path control system 4.5.4 Analysis of failure modes and their effects

Congrès Advances in Vehicles Control and Safety, AVCS'98, Amiens, 1-3 juillet 1998


Patrick Bonnet

Didier Jamois

A Rachid

Characterisation of fuel volatile compounds using a new ionic microsensor

Keywords: Catalytic microsensor, ionic emission, multilevel interrogation, volatile phase of diesel füel, diesel pollutiön l

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

The "Intemal combustion engines" project ofthe DIVA (Diagnostique Industriel et Vehicules Avances) [Industrial Diagnostics and Advanced Vehicles] research centre, fünded by Region Picardie, aims to develop a real-time system capable of modifying the operating parameters of a diesel engine so äs to optimise its pollutant emissions and füel consumption. In view of the varied compositions of diesel füels and the effects that composition has on the engine running, there is undoubted technical interest in being able to characterise the füel in the tank. Existing measurement facilities are too large to be carried on board the vehicie or can't discem variations in diesel füel composition. This A. Rachid Automatic Systems Laboratory LSA Jules Veme University ofPicardy UPJV Amiens, Rue du MoulinNeuf 80000 articie describes an ionic microsensor able to provide the engine Computer with useable data about the füel quality. The thermionic emission due to the catalytic combustion of a combustible gas on the surfäce of a heated platinum filament has been demonstrated in a number of research projects including those M. E. UMSTEAD et al. [l] and W. M. SEARS et al. [START_REF] Sears | Positive Ion Emission from a Platinum Hot Wire Gas Sensor[END_REF] for example. These authors showed that the total quantity of ions collected during this type of combustion depends on the filament temperature and on the nature of the oxidised molecule [3] [4]. The filament in these tests was heated continuously to a given temperature. The filaments used were typically a few centimetres long with a diameter of 0.125 mm.

m.

Elementary structure ofthe platinum microfilament (dimensions in yia)

Figure 2: Diagram of platinum filament

The small size of this microfilament offers various advantages. To begin with, filament heating electrical power is low (typically 200 mW). Next, the very low thermal inertia ofthe filament ensures short response times. Thus it takes only about 10 milliseconds for the filament temperature to stabilise at a given value (between 400 and 1100°C). The electronic System used with the filament is designed to heat it at constant resistance in pulse mode [START_REF] De | Procede pseudocontinu d'interrogation d'un detecteur de gaz oxydable[END_REF] to different temperature plateaux the number, duration and value of which can be controlled by extemal digital signals. The heating temperature is controlled extemally by a digitally controlled potentiometer. The Cursor of this potentiometer can be set to 100 different values. Having regard to the practical heating ränge (from 400°C to 1100°C), the temperature resolution ofthe electronic System is 7 deg.C. The advantage of using heating cycles (or "interrogations") is twofold. First, the filament wears out more slowly than it does when heated continuously. Secondly the short response time of the filament means that several temperatures can be scanned in a single interrogation. An electronic control card containing the setpoint values corresponding to a given heating cycle (number of temperature plateaux, duration and temperature value of each plateau, and the hold time between two interrogations) is linked to the heating electronics. These values are caiculated first from the calibration curve of the platinum filament which corresponds to the value of its resistance äs a fünction of temperature (Rfil= A*T+R.o) and, secondly, from the sensor electronics calibration curve which represents the effective heating resistance for a given position of the digitally controlled potentiometer Cursor (Rheat = a *Pos 2 + ß*Pos + y). To detect ions, a current/voltage Converter was adapted to the ion collector electrode. Converter amplification is one volt per nanoampere. The output voltage from the converter is called the "sensor ionic response" (Vionic)-Thö filament heating signals (voltage Vheat äüd current Iheat) consist of the different plateaux corresponding to the setpoint values transmitted by the control card. The ionic response of the sensor consists of the different values obtained for each ofthese plateaux (figure 3).

0<Vi,e,t<3V 0<Iheat<400mA

,.------,

255m

0<V"",c<10V 30ms<T<300ms

Figure 3:Pattern ofthe raw responses ofthe sensor

In fact the actual signals show, at the beginning of each plateau, an unstable portion which represents the time necessary to reach the temperature to be controlled. For the operating temperatures selected, this period of instability never exceeds 10 ms.

Test bench

Characterising the sensor responses and investigating the different parameters that might affect these responses involve a very large number of operations. A test bench has been therefore developed to automate äs many operations äs possible. This test bench can record the simultaneous responses of four sensors to gases or vapours mix-tures. The vapours are generated using a motorised syringe (figure 4). This type of system can generate all the vapour concentrations needed for the purposesofthetest.

Figure 4: Generating combustible vapours

A data acquisition system simultaneously records the signals from the four sensors: these consist of the ionic responses together with the filament heating voltage and current. Automatic data processing gives the effective temperature of each plateau together with the corresponding ionic response value.

Tests carried out

The first task was to study the influence of the operating parameters (electrode-filament distance and bias voltage) and of the ambient measurement conditions (relative humidity and the proportion of oxygen present). The second phase was to investigate the response of the sensor in the presence of combustible molecules. Most of the molecules tested are listed in the following Some of these molecules were tested at different concentrations using different interrogation cycles (l, 6, 12, 24 or 100 temperature plateaux according to the molecules).

Reference tests

In order to take into account any Variation in the sensor ionic response with time, or response dispersion between sensors, reference tests were conducted periodically. Accordingly the sensors characteristics were recorded in N50 synthetic air, 0.69% butane and 1000 ppm acetone for an electrode-filament distance of 2 mm and a bias voltage of -80 V. There is no sensor ionic response for temperatures below 1100°C in air. The response in butane (6900 ppm) shows a maximum (100 mV or 100 pA) around 900 0 C. The response to acetone (1000 ppm) shows a maximum (220 mV) around 950°C. Making alloWance for the differences between the filaments reactive surface areas, all these results agree with the data from the literature. The ions created in combustion reactions desorb from the platinum filament at a velocity VQ and are then accelerated by the electric field produced by the bias voltage applied to the collecting electrode. The intensity of this acceleration is proportional to the bias voltage applied between the electrode and the filament, and inversely proportional to the electrode-filament distance. It can be seen from the curve in figure 5 that the ionic response is inversely proportional to the electrode-filament distance. The shape of the curves on figure 6 shows that the efficiency of ion collection is dependent of the applied accelerating field. The moving ions are slowed down and their direction changed essentially by impacts with neutral molecules in the surrounding atmosphere. Accordingly the number of ions collected on the electrode is a fünction of the accelerating electric field: this is certainly related to the detecting head geometry which was not optimised for maximum collection efficiency. The accelerating voltage (-80 V) and the reference electrode-filament distance (2 mm) were determined from these results for the four sensors.

Influence of the operating parameters of the sensor

Sensor response at 730-C äs a funcUon of electrode-fflament distance

Influence of measurement conditions

Quite apart from the nature and quantity of the combustible gas to be detected, other parameters can influence measurements. It was confirmed that variations in flow rates between 50 cnrVmin and 2 l/min do not change the ionic response. Figure 7 shows the influence of the oxygen percentage presence (from 20% to 6% by volume) on the sensor ionic responses at different temperatures under the reference conditions. As the oxygen concentration falls, the practically linear ionic response is also seen to fall. Also, it is possible to evaluate the power dissipated by the catalytic combustion reaction by measuring the heating parameters of the microsensor. However no fall in catalytic activity, obtained from microsensor heating parameters measures, is observed äs the oxygen concentration declines. It must therefore be concluded that although catalytic activity is necessary for ionic emission [START_REF] Sears | Design of Platinum Hot Wire Gas Sen sors[END_REF] during the different combustion stages, at least one specific stage in ion creation process is limited by the oxygen concentration. It can be seen however that for a given oxygen concentration the responses ratios for different temperatures remain the same. Consequently the discrimination potential of the sensor does not appear to be adversely affected by a fall in the oxygen concentration.

Responses to combustible molecules

Owing to the varied nature and proportions of the füel components, it was not possible at this stage to conduct systematic tests for each diesel fuel ingredient. This investigation attempts to determine the main sensor characteristics in the presence of representative molecules. To begin with, it was confirmed that the responses to simple gas mixtures were additive. On that basis it seems reasonable to assume that the separate investigation of the combustible molecules representative of those found in the diesel fuel volatile phase will give a good approach of the sensor ionic response in the presence of a real fuel. Moreover, the ionic response varies in a linear manner versus concentration at all the temperature plateaux. This significant property had already been demonstrated by SEARS [START_REF] Sears | Production of Positive Ions During the Catalytic Oxydation of Organic Vapors on Hot Platinum[END_REF] for a given heating temperature. The first molecules to be studied were the linear alkanes (frorn €3 to CK)). Figure 8 shows the sensor responses to four linear alkanes (propane, butane, pentane and heptane) between 580°C and 950°C. The increased ionic response for longer carbon chains had already been demonstrated in the literature for a given heating temperature. Here we observe the same phenomenon at all the temperatures even though there are relative variations in ionic response from one temperature to another. Indeed these variations suggest the possibility of discrimination with suitable data processing. Generally speaking, the ionic response shape varies with the molecular size of the alkane tested but, for high heating temperatures, the ionic response is proportional to the number of carbon atoms in the molecule. In fact this ruie applies only to linear alkanes. From the example shown in figure 9 it is possible to compare the molecular geometry influence on the ionic response obtained using alkanes containing 6 carbon atoms. A quatemary carbon atom presence, in other words a carbon atom linked to four other carbon atoms and not to a hydrogen atom, substantially increases the sensor ionic response (in the example, the response is 420 mV at 700°C for 2,2 dimethyl butane compared with 50 mV at 930°C for 2,3 dimethyl butane). Eise, the comparison of 2,3 dimethyl butane and 3-methyl pentane tends to show that the more branched a molecule, the higher its response (+ 40% at 930°C). Once again, the shape ofthe ionic response is seen to vary with the nature of the molecule. These results are in agreement with the literature [l]. Figure 10 illustrates the effect of different fünctions (acid, alcohol and aldehyde) on the ionic response for non-branched molecules containing four carbon atoms. The presence of the fünction and its nature can be seen to affect the level and shape of the sensor response. Just äs for the alkane series, the fünction location in a molecule is important. This is very well illustrated by figure 11 which shows that the alcohol fünction location in propanol can modify the response by a factor of 6 and completely change its shape (maximum around 700°C for propanol-1 and above 960°C for propanol-2). Figure 12 shows the influence of molecular size on the ionic response in the presence ofketones. Unlike the alkanes, the ionic response falls äs the length of the carbon chains associated with the ketone fünction increases. Indeed äs the size ofthe molecule increases, the ionic response tends towards that of the alkanes of equivalent molecular weight, äs if the fünction specificity is attenuated by the length of the carbon chains. Accordingly the fünction has most effect on molecules of small size. Figure 13 shows the responses obtained for a number of saturated and aromatic cycles. In fact these responses are extremely low (frequently below l mV). Only the cyclanes and light aromatic compounds are liable to give usable responses.

Influence of fünctions

Influence of fünction locatlon In the molecule

Composition ofa diesel fuel

Although the compositions of diesel fuels are extremely varied, essentially owing to additives, it is possible to discem major molecular families.

Figure 15 shows the average composition of a commercial diesel füel. On the basis of this typical diesel füel composition, an attempt can be made to determine what the main composition of its volatile phase will be. Since most of the molecules in diesel füel have a high molecular weight, the volatile phase consists essentially ofthe lightest ethers and alcohols, a few cyclanes and aromatics, and other light molecules resulting frorn imperfections in refining, the nature and quantities of which are not well known. However, it can be seen that the ionic responses to cyclanes and aromatics are low compared with those to ethers and alcohols. On the other hand, the presence of ethers and alcohols gives ionic responses at high levels and with fairly characteristic shapes.

Compounds liable to have the greatest influence on Ionic response to Dfesel vapours

Temperature (in °C)

Figure 14 shows the ionic response obtained in the presence oftwo molecules representative ofthe two most volatile compounds families (ethers and alcohols). It can therefore be expected that the ionic sensor will be fairly sensitive to ethers and alcohols concentrations for different diesel füels, or to change in their composition by the progressive depletion of the most volatile compounds gaseous phase. Figure 15 shows typical experimental ionic responses obtained in the presence of diesel füel vapours once the liquid-vapour equilibrium has been reached, and then in unbalanced conditions (test volume flushed with air and renewed every two minutes), after one hour. In spite of the low Saturation diesel fuel vapour pressure [7], a quite satisfactory signal level is obtained. The ionic response obtained for diesel füel in static air shows a number of inflection points that are characteristic of a mixture of different kind of molecule. The large signal at high heating temperatures recalls the curve shape obtained for ether, with in addition a local maximum around 650°C, which suggests an alcohol-1 response (Figure 11). When the air above the fuel is renewed at a fairly rapid rate, the vapour concentration falls. This explains the fall in the average level of the ionic response observed. Since the fuel quantity used was very low for an extensive liquid-gas exchange surface, it is likely that the relative proportion of highiy volatile compounds feil with respect to other components. This could explain a relative fall in the signal attributed to the presence of ethers for high heating temperatures. It is now necessary to make measurements on a number of fuels in rigorous test conditions in order to ensure reproductibility and usability of ionic responses.

Conclusion

The ionic sensor described here can detect most volatile organic molecules. The ionic response äs a fünction of temperature varies with the nature and concentration of molecules. The multiple temperature plateaux pulse mode has renewed the interest in such a sensor since the research already undertaken by other teams. This property shows the discrimination capability of such a sensor in circumstances where only a single füel molecule has to be identified. Since the responses to mixtures are additive, it could possible to combine such a sensor and a suitable data processing System in order to identify the constituents of a gas. The tests conducted on diesel füel show that it is possible to detect certain molecules constituting its volatile phase. At this stage of the research it seems reasonable to believe that this sensor will differentiate certain variations in diesel füel composition.

Figure 1 : Schematic diagram ofthe sensor 2

 12 Figure 1: Schematic diagram ofthe sensor 2 Sensor description Like PIDs and FIDs, this sensor is composed of two parts (figure l): the first is based on the catalytic microsensor with platinum filament developed by LETI and INERIS [5], which generates the ions; the second part collects and measures the ions. The

Figure 5 Influence

 5 Figure 5

  Figure 8

  Figure 10

  Figurell

  Figure15

Table 1 : Principal molecules tested.

 1 table.

	Name of gas or vapour	Family	NbofC
	Methane	Alkane	l
	Propane	Alkane	3
	Butane	Alkane	4
	Pentane	Alkane	5
	Cyclohexane	Alkane	6
	2.2 Dimethyl butane	Alkane	6
	2.3 Dimethyl butane	Alkane	6
	Methyl pentane	Alkane	6
	N-Hexane	Alkane	6
	Heptane	Alkane	7
	Methyl-cyclo-hexane	Alkane	7
	Octane	Alkane	8

Table 2 : Mass composition of Diesel fuel [7].

 2

	Type of molecule (size) Proportions by mass
	Alcanes (C\Q a C^Q)	30%
	Cyclanes (>Cy)	35%
	Aromatics (>C(,)	20%
	Ethers(C2äCn)	10%
	Aleohols	<5%

Congres Advances in Vehides Control and Safety, Amiens, 1-3 juillet 1998, p. 507-513


R Branka

C Michot

GAP TESTS; COMPARISON BETWEEN UN GAP TEST AND CARD GAP TEST

ABSTRACT:

UN gap test, type 1(a) or 2(a), is the recommended test in the acceptance procedure for transport of explosives in class 1. Up to the revision of the UN Manual of Tests ans Criteria, four tests were applicable, including the French Card Gap Test (CGT), type 1(a)(iv) or 2(a)(iv). Th French CGT is nevertheless always in f.irce in the explosive safety at work French regulation. This leads to the necessary performance cf both tests in some occasions. In order to allow the performance of only one test, a comparison between UN gap test and French CGT has been made. Five different explosives or reactive substances nave been selected for a large scaling of results. The output of the comparison is such that any result in one test allows now to derive the result in the second test.

l -INTRODUCTION

The UN test l (a) 11 (a) known äs the "UN gap test" is now, in revision 2 of the UN Manual of tests and criteria [l] in force since l January 1997, the only detonation test for inclusion in class l explosive materials in the meaning ofthe regulations governing the transport ofdangerous goods.

In revision l ofthe UN Manual, 4 detonation tests previously existed together: -UN l (a) (i) / 2 (a) (i):

BAM test in 50/60 steel tube -UN l (a) (ii) / 2 (a) (ii):

TNO test in 50/70 steel tube -UN l (a) (iii) / 2 (a) (iii):

"gap test": detonation Initiation test -UN l (a) (iv) / 2 (a) (iv):

"gap test": detonation Initiation test through a barrier,

The final test, of French origin, is also used for the risk classification of pyrotechnic establishments for the application of the regulations governing safety at work (protection of workers). In practice this means that in France, for the classification of transport and pyrotechnic establishments, two tests must be carried out (the "UN gap test" and the "detonation initiation test through a barrier"), instead ofonly one (the detonation initiation test through a barrier).

The fact that 2 tests coexist can lead to significant additional costs for users and a lack of coherence in the classifications.

Accordingly the study perförmed consisted in defining the procedures for UN test l (a) / 2 (a)the "UN gap test" -that render it suitable for meeting the needs of the classification of pyrotechnic establishments in France. Since the two tests are of the same nature, it was in fact a question of defining the thicknesses of the methyl polymethacrylate (PMMA) barriers in the "UN gap test" equivalent to the limiting numbers N of340, 320, 300 and 240 cellulose acetate cards in the test on initiating detonation through a barrier.

II -STUDY PROGRAMME

The study programme involved, first, the detonation initiation test through a barrier (known äs "P5" at INERIS), starting the test at a selected value ofthe number of cards of cellulose acetate in order to limit the number of tests and, secondly, tests using the conditions of UN tests l (a) and 2 (a) but with a search for the limiting block (thickness of the PMMA barrier) that gives, äs in test P5, three negative results in three tests, with at least one positive result using the next smaller block.

In order to approach the conditions of test P5, we decided to conduct the "UN gap test" with blocks ofPMMA of thickness varying in 5 millimetre Steps.

-EXPLOSIVE MATERIALS USED

To permit comparison of the two tests, we chose materials giving limiting number of N cards in the P5 test covering a wide ränge of barrier thicknesses. These were the following explosive materials:

-Crushed tolite T/D (TNT) ofparticie size 0-1400 um, -Hexogen B (RDX) with partide size 0-800 um, Ammonium perchlorate with mean partide size 200 [im, -Dried musk-xylene (moisture content below 0.1%) the partide size of which was measured between 0 and 100 um, -Humidified musk-xylene containing about 8% water.

-DESCRIPTION OF THE INERIS P5 TEST

The principle of this test is to position the test piece of material between two explosive relays. The initiating relay is separated from the test piece by an inert barrier, consisting of a Stack of cellulose acetate cards. The second relay is in contact with the test piece. The assembly consisting of the relays, the tube containing the material to be tested, and the cards is 40 mm in diameter (tube ÖD).

Detonation ofthe ignition relay induces a fairly intense explosive wave in the material being tested depending on the thickness of the inert barrier. The limiting thickness of the barrier for which detonation ofthe second relay or controlled relay is not initiated is determined.

The thickness of the barrier is varied in steps of 5 acetate cards each of which is 0.19 mm in thickness. The density ofthe material is always measured prior to the tests.

The result is expressed äs the minimum number of cards (equivalent to a given thickness) that gives three negative results out of three tests while giving at least one positive result for the thickness of cards immediately below. The positive or negative result of a test is given by the penetration ofa steel plate 10 mm in thickness placed in contact with the Standard relay.

-DESCRIPTION OF THE UN GAP TEST

The procedure is given in the UN Manual of tests and criteria [l].

-CHANGES MADE TO THE UN TEST PROCEDURE

We made the following adaptations to the test described in the UN Manual:

-the steel tube was closed at both ends (and not only at the bottom) by a food-grade plastic film (thickness 7 ± l um) held in place by adhesive tape; a wooden Support (ofsquare cross-section measuring 100 x 100 mm, and 150 mm in length, having a recess 90 mm deep and 50 mm in diameter to accommodate the initiating relay and the block of PMMA, an axial hole 10 mm in diameter for the detonator, and a groove at the bottom for the electric detonator leads to pass is placed at the bottom so äs to hold in place the assembly consisting ofthe detonator, relay, block ofPMMA, and steel tube; a cardboard tube ofinside diameter 50 mm (outside diameter 56 mm, 300 mm in length) was used to hold the steel tube and block of PMMA when the length of the block of PMMA was such that the steel tube could not be held in the wooden support (i.e., for PMMA block lengths of45 mm or greater).

The other items used were:

-seamless steel tubes type TU37b (NFA 49-210) of ÖD 40 ± 2 mm, wall thickness 4.0 ± 0.1 mm and length 400 ± 5 mm. Each tube was measured internally prior to the test so that the Charge densities ofthe materials being tested were known, -blocks ofPMMA 50 ± l mm in diameter and lengths varying from 5 to 90 mm (± 0. l mm) in

Steps of 5 mm, steel washers (shims) 1.6 ± 0.2 mm in thickness, ID 40 mm and ÖD 50 mm, -mild steel Standard plates measuring 150 x 150 mm and 3.2 ± 0.2 mm thick, UN Standard detonators charged with 0.6 g ofpenthrite (PETN), -hexogen wax relays 95/5/0.5 CH, diameter 50 ± l mm, length about 50 mm, mass 160 g and density 1600 ± 50 kg/cm 3 .

A diagram ofthe set-up is attached.

//.5 -STUDY PROGRAMME

The test programme consisted in:

-performing the P5 test on the 5 materials, -performing the UN gap test on the same 5 materials, using the same procedure to make the results comparable.

The objective is to be able to define the thicknesses ofPMMA barriers in the UN test equivalent tothe limitine numbers N of340, 320, 300 and 240 cards in the P5 test.

TEST RESULTS

The results are summarised in the following table:

IV.1 -REMARKS CONCERNING THE UN GAP TEST PROCEDURE

It became clear during the tests that the procedure could be improved: the height of the wooden support block can be increased still fürther in order to cover all possible sizes ofthe PMMA barrier, which would make it possible to eliminate the cardboard guide tube and make the assembly more stable, -the Standard steel plates are very thin and are often ejected or shredded. This makes it difficult to compare the tests since the Perforation diameters are difficult to measure, -the interpretation ofthe test results should be better defined. In fact, it is not sufficient to say -äs in the UN gap test -that a test is positive when the Standard plate is perforated or when the steel tube containing the explosive material is completely fragmented. It is essential to define the state ofthe tube that determines whether a test is positive or negative. We propose to define a positive test in the following manner: reference plate perforated or steel tube: tube fragmented into at least 5 pieces. Ifthe tube is fülly opened up, flattened and/or in between l to 4 pieces, the test is negative.

IV.2 -CORRELATION BETWEEN RESULTS

To make it possible to correlate the results obtained in the two tests, we have plotted, on the following graph, the results ofthe UN gap test äs a fünction ofthe results ofthe INERIS/P5 test.

P5(numberofcards)

It was also possible to compare the two tests by inspecting two documents:

-document STANAG 4488 appendix B [START_REF]NATO standardization agreement (STANAG 4488, l ;t draft) Explosives, shock sensitivity test(s)[END_REF] which gives the shock pressure leaving a barrier äs a fünction ofthe number ofcellulose acetate cards for a test identical with the P5 test, -an artide by J. A. SANCHIDRIAN [START_REF] Sanchidriän | Analytical and numerical study of the shock pressures in the gap test[END_REF] which gives the shock pressure leaving a barrier consisting ofcellulose acetate cards or blocks ofPMMA for different types of explosive.

We give below the thickness ofPMMA (SANCHIDRIAN's artide) äs a fünction ofthe number of cards (STANAG 4488) in the same conditions äs the UN gap test and the INERIS/P5 test, and using the shock pressures given in each document (the explosives used were hexogen for SANCHIDRIAN's results and hexogen wax 95/5/0.5 CH in STANAG 4488. The two types of explosive have similar detonation velocities: 8750 m/s for hexogen of relative density 1.76 [START_REF] Meyer | Explosives[END_REF] and 8350 m/s for hexogen wax of relative density 1.60 [START_REF] Calzia | Les substances explosives et leurs nuisances[END_REF], which results in fairly similar detonation pressures. It can be seen that practically the same curve is obtained in both cases (see the coefficients ofthe equations ofthe lines given on the curves). It can be concluded from these correlations that there is equivalence between the two methods, and the following correspondence can be deduced:

V-CONCLUSIONS

It is possible to show equivalence between the two tests so äs ultimately to replace the test currently used for classifying risk in pyrotechnic establishments in France by the UN gap test.

The procedure of the UN gap test should be modified in order better to define the criteria governing the results ofeach test (see paragraph 4. l).

  Correlation of results between the INERIS P5 and UN tests correlation y = 0,2548x R' = 0.9889 200 250

  according to STANAG 4488 with hexogen wax 95/5/0,5CH of relative density 1

  It therefore seems possible to define a more straightforward UN gap test similar to the simplified INERIS/P5 test[START_REF] Bigourd | Card gap test -Epreuve alleges pour le classement des produits explosibles[END_REF]

	Limiting number of cards in the P5 test	Equivalent thickness ofPMMA in the UN gap test
		(mm)
	340	86.7roundedto 85
	320	81.6 roundedto 80
	300	76.5 rounded to 75
	240	61,2 rounded to 60

Twenty-Fourth International Pyrotechnics Seminar, Monterey, 27-31 juillet 1998, p. 85-92.


C Didier

F Wojtkowiak

R Salmon

J P Barriere

P Gaget

TREATMENT OF OLD DISUSED MINE OPENINGS

According to French regulations, the main objectives for the treatment of old abandoned mines consist of : preventing collapses and dangerous subsidence, controlling mine gas emissions and closing old abandoned mine openings.

In the Herault coal basin, underground exploitation started very early, in particular by the way of shallow adits dug in hillsides. Today more than 600 adits have been listed on old maps but a lot of them have not yet been precisely located.

Charbonnages de France and INERIS have analyzed all risks generated by these old openings and have established a methodology able to determine the closure procedure which can guarantee the safety of the opening. This methodology can be adapted to an adit (size, state of...) and to the physical characteristics of the site (gas, water, location...).

In order to limit the number of configurations, two of the above factors have been retained : the state and the location of the gallery. After analyzing all the possible configurations, closure principles have been determined for each of them.

The paper : discusses geographical and historical background of the Herault basin exploitation, list the specific parameters which influence the choice of process able to secure the adits, present the general methodology used.

Finally, the treatment of an adit located under inhabited areas is presented.

INTRODUCTION

According to French regulations, the main objectives for the treatment of old abandoned mines consist of : preventing collapses and dangerous subsidence; controlling mine gas emission and mine dewatering; closing old abandoned mine openings.

As part of the treatment of the Graissessac coal basin, Charbonnages de France and INERIS have analysed all the risks generated by numerous disused adits located in this area. A general method has been established to determine the closure procedure that can guarantee the safety of the opening. It is adaptable to various configurations (geometrical parameters, present state of galleries, location...) and site physical characteristics (geology, presence of gas or water). This paper firstly presents the geographical and historical backgrounds of the Graissessac coalfield. Then the specific parameters that influence the choice of a particular process able to secure the adits are calculated. Finally the applied general methodology and the treatment of a particular case are presented.

GEOGRAPHICAL AND HISTORICAL CONTEXTS OF THE GRAISSESSAC COALFIELD

The Graissessac coal field lies in the North-East of the Département de l'Herault, about 50 km West of Montpellier. It belongs to the stephanian coal fields located in the Massif Central of France. It is an E-W synclinal, about 20 km long and 3 km wide.

According to previous studies, the area may have been exploited for its mineral resources since Antiquity. The very first documents relating to coal extraction in Graissessac date back to XII th century but the extraction process was industrialised during the XIX th century.

For a long time, underground exploitation remained artisanal and generated numerous mine openings. Although the number of mine shafts is quite low (around 10), more than 600 disused adits have been listed from old maps, some of them still not being precisely located. These adits are present in various shapes and sizes and most of them have been dug into hillsides.

Underground exploitation ended in 1964 but coal continued to be extracted by way of large open pit mines which have been definitely closed in 1993. In accordance with French regulations, Charbonnages de France have recently analysed the kind of risks which could result from the presence of these adits and treat them in order to avoid accidents.

The hummocky topography of the site compounded with the low agronomic quality of soils limited the development of cultivated lands in the valley floors, in the vicinity of the few villages founded by miners. Moreover, the hillsides are not easily accessible on account of fairly dense vegetation. On the one hand this prevents to a large extent frequentation of the sites, but, on the other hand, it limits the spectrum of stabilising techniques requiring heavy vehicles.

Even though the Graissessac basin presents a low damage potential as a whole, it seemed necessary, however, taking into consideration the proximity of some galleries to urban areas and to main traffic roads to recommend a number of closure principles.

SITE SPECIFICATIONS

In order to ensure the ease of the process, a methodology adaptable to different kinds of orifices and various environmental contexts has been achieved. Different configurations have been defined to make the classification of galleries as systematic as possible.

For simplification and clarity, the number of basic parameters has been limited to reduce the quantity of crossed cases.

Basic parameters

As far as the Graissessac Basin is concerned, two basic parameters are taken into consideration. These are: the perceptible state of the underground gallery; the gallery environment (i.e. its proximity with sensitive installations).

State of gallery

This parameter depends essentially on the age of the gallery, its geological context as well as the kind of support adopted. As most galleries have either collapsed, degraded to a large extent or have been subject to some kind of treatment or other, it is now rare to have access to old underground works. Three « States of gallery » were considered to describe adits: those of the galleries that are not located. Although their presence is not in doubt (thanks to maps, statements,...), it has not yet been possible to locate them with precision on the ground ; those that have collapsed, caved in or have been backfilled. Galleries can collapse as a result of either the natural ageing of the structure or human activities. Some of these are easily percieved. Most of them are naturally or artificially remodeled and can be observed as subsidence phenomena on the surface of the hillsides. Galleries in such a state permit the circulation and emission of the gases, and threaten the safety of the environment ; those that are still opened or were only walled in. Open galleries are relatively rare in the basin. Galleries closed off by masonry are quite numerous. Holes in thes walls which are due to acts of vandalism or which have been designed to allow the circulation of water enable also the emission of mine gases. Hence this type of treatment is not considered to be satisfactory over a long period and it is often necessary to remove the wall.

Position of Gallery

The proximity of a gallery opening to sensitive installations such as habitations or traffic roads greatly influences the nature of risks susceptible to affect the site. Three possible « position of gallery » were defined : proximity to urban areas or regularly frequented areas such as stadiums, orchards, gardens.... Such a configuration requires optimal security for gallery openings in order to reach a tolerable level of residual risk. galleries that are not located in urban areas but extend under or next to roads. In such cases, levels of risk are lower than for the previous case but the regular circulation of people and the extra loads due to motorised engines have to be taken into consideration. galleries located in areas not easily accessible and far from any sensitive structure. The low damage potential in these cases does not imply treatments as severe as those required by the previous cases in order to reach a comparable residual level of risk. In such a case, one should focus on the limitation of gas circulation or accumulation in cavities close to the surface.

TREATMENT TECHNIQUES SPECIFIED TO EACH CONFIGURATION

Each configuration requires specific treatment. Table 1 summarises the configurations in relation to the two parameters as defined above. A slip is associated with each of the 9 configurations. On such a slip, the risk incurred as well as the main objectives to be achieved are gathered. Various techniques of stabilisation are then proposed. They provide principles that have to be adapted to the site.

It should be noticed that in the case of galleries located in an urban area, the configuration should not be expected to be associated with one of these slips as the complexity of the situation as well as structure vulnerabilities, close to or above galleries, are too specific in each case. Hence a specific study has to be carried out for such galleries. Hence for a given configuration the operating company is simply requested to define both « position of the gallery » and « state of the gallery » parameters. This couple is associated with a slip as defined above. The operating company can then apply the recommended treatments explained on the slip to satisfy the French regulation.

State

You will find further the slip number 4, corresponding to a collapsed gallery located far from any sensitive structure.

EXAMPLE OF A SPECIFIC STUDY -THE SAINT-JOSEPH MINE CASE

Background

The Saint-Joseph mine was dug in 1859 and definitively closed in 1870. Unfortunately there are few documents describing the main entrance gallery.

Geological Context

Only pieces of information on surface geology were accessible and observations are limited to :

in the gardens located close to buildings called « Les Bâtisses », one can notice the presence of sandstone with soil. Because of dense vegetation making any visual observation very difficult, it has been assumed that the opening of the gallery was situated within this formation ; « Les Bâtisses » were founded on fill made of mining materials. If we assume the height of the fillslope being approximately equal to the thickness of the insitu layer, it is around 5 m thick (fig 2).

Current state of the gallery

Before the visit of the adit, the opening of the Saint-Joseph gallery was closed by masonry presenting a split in its lower part allowing water circulation.

The gallery, slightly inclined, is 2-meter wide and 3-meter high. A visit to the mine in March 1998 showed the gallery to be in a good state of as a whole along the first 600 meters. It was not possible to visit further as the gallery was obstructed by some collapsed material.

SLIP N°4

Configuration

State of the gallery: collapsed, caved in or backfilled Position of the gallery: far from any urban area and not susceptible to extend under a road

Encured risks

Destabilisation of materials obstructing the opening Gas emissions through collapsed or backfilled materials and gas accumulation in voids near surface Collapses above residual voids located back from the opening.

Objectives to be reached

To achieve the closure of the gallery entrance if necessary in order to avoid any penetration To limit air circulations and avoid gas accumulations.

Proposed techniques

As the adit is located in an area of low damage potential, it would be uneconomic and useless to proceed to a systematic treatment of residual voids back from the gallery opening. Indeed the probability of occurrence of a residual void, a brittle and rapid propagation of a sinkhole through the soil and to the surface, and the presence of a person on this place, at the right moment is definitively very low and the risk encured can be considered as acceptable.

Hence the treatment would simply consist of ensuring the non accessibility of the gallery and avoiding gas accumulation.

Even if the gallery is partly or fully collapsed, it is however necessary to ensure a sufficient obstruction of its entrance. If the volume of collapsed material at the entrance is not sufficient, one should proceed to either : its clearing and a higher quality backfilling of the entrance, the collapse of surface material at a sufficient distance back from the entrance, according to the topography, a hillock of surface materials may block up the gallery opening. This hillock should keep its obstruction characteristics despite being affected by some residual consolidation or some debris slipping from it.

The necessity of ensuring complete obstruction of the gallery entrance by one way or another is imposed by the second objective, that is avoiding gas circulations. Finally any hollows or dead ends should be treated, as these could enable some gas accumulation and hence, be of great danger.

Land use

The opening of the gallery is located next to a road. Hence vehicle may easily access the gallery and the spectrum of treatment techniques is not limited.

Along the first 100 meters, the gallery is located under private gardens. It then goes beneath some buildings called « Les Bâtisses ».

Hydrogeology

Mine dewatering is achieved through the gallery. The average flow rate is 19 m 3 /h and reached around 40 m 3 /h in September 1995. Water comes from a source located 10 meters back from the opening. Its average conductivity is low (less than 300 S/cm), no deposit has been noticed at the exit and the concentrations of typical mine chemical substances are low (these are HCO 3 , Fe, Mn, SO 3 , Na, Mg...). These arguments may lead us to assume that most of this water is meteoric and does not come from old mine workings. Water is diverted towards Le Clédou, a river, for irrigation purposes.

A water source with quite a high flow rate has been noticed 50 meters back from the opening.

It is assumed to come from some brook on the surface. Beyond this source, the gallery is dry.

Pit gas emanation

During the exploitation, a fire damp hurt someone in the Saint-Joseph mine area on 25 September 1869. In October 1997, atmospheric measures showed some outward air circulations. They revealed the following concentrations: O 2 8.7%, CO 2 9.3%, CH 4 0%.

Even though in March 1998 the atmospheric conditions were more satisfying and allowed a complete visit of the gallery, these data suggest the necessity to take special recommendations concerning pit gas emission treatment.

Risks encured

collapse and subsidence are expected to affect both mine and surface. Under this threat, no sensitive installations were build within an area 100 m back from the entrance ; the masonry is not sufficient to guarantee no gas circulation and no human penetration.

Methodology and recommended treatments

To avoid a sinkhole developing through the soil and affecting the surface, it was decided to backfill the mine along a sufficient distance to ensure an acceptable level of safety. In order to assess the height to which a sinkhole may extend, some volumetric calculations were carried out. These calculations are based on the principle of a circular chimney that develops until the volume of collapsed material equals the volume available in the gallery (that is a cone) and the volume of the chimney itself by virtue of soil natural bulking characteristics. Hence the main parameters taken into consideration are:

-gallery geometric parameters, -bulking factor, considered to be in the range 1.25 to 1.35, -the angle of the cone that is the angle of repose of caved rock within the gallery.

The models used were those of a rectangular and a semi-elliptic gallery to frame the actual result. Calculations showed that the maximum height of a chimney would be between 44 m and 49 m. These yielded quite accurate value and it has been possible to decide from which point no backfill would be required to ensure safety on surface. The same calculations were computed with backfill corresponding to 80% of void volume and to 50% void. A factor of safety has been chosen taking into consideration the lack of information on parameters as well as the damage potential of surface structures. As two sensitive structures are located above the mine and the thickness of covering materials has been found not to be sufficient, a backfill of the full gallery volume is recommended. As shown on figure 2, it has been possible to propose a « by step » backfilling procedure, each zone backfilled separated from the other by a wall to limit fill dispersion.

As far as the opening is concerned, an integral treatment is recommended to block up the gallery entrance by one way or another as explained before.

Figure

  Fig 1 showing the models used for the actual section of Saint-Joseph Gallery


S Brohez'

C Delvosalle'

G Marlair

A Tewarson

ACCURATE CALCULATIONS OF HEAT RELEASE IN FIRES

In the field of reaction-to-fire test, the application of the Oxygen Consumption (OC) and Carbon Dioxide Generation (CDG) calorimetry have been used extensively both at bench-scale and in large-scale to calculate heat release rates for well-ventilated fires. Both methods, however, provide only approximate heat release rates since average values of the heat of combustion per unit mass of oxygen consumed and carbon dioxide generated are used. For the calculations of accurate heat release rates, actual elemental compositions of the materials and net heat of complete combustion values are needed. In addition, effects of large amounts of soot formed in under-ventilated fires need to be accounted for in the calculations.

This paper deals with a modification of the CDG calorimetric relationships for the generation of smoke. The soot modified CDG calorimetric relationships show that for wellventilated fires, the soot correction is generally negligibly small, but it becomes important for under-ventilated fires.

Introduction

Fire is often considered as the most hazardous accidental event which may affect safety in the chemical industries. The fire damage may be thermal or non thermal. As examples, the fire plume may transport a variety of toxic effluents, which may injure the staff of the industrial premises and the fire fighters, as well as the inhabitants in the neighbourhood. Intense radiation produced by big fires may cause serious burn injuries and generate "domino effects" to previously non affected equipment in the vicinity and result in related phenomena such as jetfires, fireballs, BLEVEs 1 . Moreover, polluted extinction waters while unconfined may greatly affect the aquatic environment. Although several ambitious projects were recently carried out in the field, there is still much work to be performed to get validated techniques capable of predicting (and keeping under acceptable control) the consequences of indoor and outdoor large chemical fires.

Heat release rate evaluation is a common practise in any fire risk assessment procedure. Usual method to measure heat release rate is based on the application of the oxygen consumption (OC) [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF][START_REF] Parker | An Investigation of the Fire Environment in the ASTM E84 Tunnel Test[END_REF][START_REF] Huggett | Estimation of the Rate of Heat Release by Means of Oxygen Consumption[END_REF][START_REF] Hinkley | Rates of heat Output and Heat Transfer in the Fire Propagation Test[END_REF] and carbon dioxide generation (CDG) calorimetry [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF]. The OC calorimetry states that heat of complete combustion per unit mass of oxygen consumed is approximately constant for many carbon and hydrogen containing organic solids, liquids and gaseous compounds [START_REF] Huggett | Estimation of the Rate of Heat Release by Means of Oxygen Consumption[END_REF][START_REF] Thornton | The Relation of Oxygen to the Heat of Combustion of Organic Compounds[END_REF]. Hugget [START_REF] Huggett | Estimation of the Rate of Heat Release by Means of Oxygen Consumption[END_REF] found an average value of the heat of combustion to be 13.1 Id per gram of oxygen consumed with a ± 5% variation. For a large number of liquids, gases, and solids, Tewarson [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF] found the average value to be 12.8 kJ per gram of oxygen consumed with a + 7% variation.

Our contribution specifically focuses on CDG calorimetry that similarly states that heat of complete combustion per unit mass of carbon dioxide generated is approximately constant for most organic liquids and gaseous compounds and for many organic solids [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF]. The heat of combustion per unit mass of carbon dioxide generated for a large number of gases, liquids and solids is 13.3 kJ/g with a ± 11% variation. Since oxygen is v. reactant and carbon dioxide is a product, the heat release rates from OC and CDG calorimetry are found to be similar, except for low heat release rates, where measurement accuracy of oxygen consumption becomes an important factor. The heat release rates, calculated from OC and CDG calorimetry using average values of the heat of combustion per unit mass of oxygen consumed or carbon dioxide generated, are only approximate. For accurate heat release rate calculations, both OC and CDG calorimetry require actual elemental compositions of the materials and net heat of complete combustion needs to be measured or calculated from bond energies and heats of formation.

Heat release rate calculations for under-ventilated fires also need to be improved if large amounts of soot are released in addition to various gaseous effluents. This paper presents a modified relationship for the CDG calorimetry to account for conversion of fuel-carbon into soot.

Theoretical analysis

The OC and CDG calorimetry are implemented both at bench-scale and in large-scale reactionto-fire tests [START_REF] Janssens | Calorimetry[END_REF][START_REF] Peacock | Analysis of Large-scale Fire Test Data[END_REF][START_REF] Tewarson | Flammability Parameters of Materials: Ignition, Combustion and Fire Propagation[END_REF][START_REF] Marlair | Modélisation Incendie Simulant les Transferts et les Risques A L'environnement[END_REF][START_REF] Marlair | A Review of Large-Scale Fire Testing Focusing on the Fire Behaviour of Chemicals[END_REF][START_REF] Marlair | Experimental Evaluation of the Fire Hazard of Organophosphorous Insecticides by Use of Various Test Methods[END_REF]. Since most of these tests are cairied out in open systems, a hood must be placed above the experimental test rig to exhaust the products plus dilution air. Measurement of gas flow rate and temperature and composition of gases (molar fractions of oxygen, carbon dioxide and carbon monoxide) are used to calculate the heat release rate.

In the CDG calorimetry, the heat release rate is calculated from the measurement of the mass generation rate of CO 2 , corrected from the mass generation rate of CO [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF]: Where, the average value of E co = 13.3 MJ.kg A ± 11% and E co = 11.1 MJ.kg' 1 ± 18%.

q = E C o 2 (m C o 2 ~ Ko, ) + E'co
The 11 and 18 % variations suggest that energy produced respectively per unit mass of carbon dioxide or carbon monoxide generated is slightly more fuel dependent than the energy generated per unit mass of oxygen consumed [START_REF] Janssens | Oxygen Consumption Calorimetry[END_REF]. The measurement accuracy for low heat rele tse rates, on the otr sr hand, is higher for the generation rates of CO and CO2 than the consumption rate of O2 [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF].

The combustion reactions generating carbon dioxide and carbon monoxide may be written respectively as

C a H b O c N d + (fl + ^-|) O 2 -5&U a CO 2 + b -H 2 O + ^N2 (2) C a H b O c N d + <?Y-~) O 2 -^U aCO + b -H 2 O + ^N2 (3)
where, E fue i = energy release per unit mass of fuel consumed for complete combustion (kJ.kg 1 )

E'fiiei = energy release per unit mass of fuel consumed for incomplete combustion to CO (kJ.kg' 1 )

As the value of energy release E fael is known for complete combustion of the fuel, the energy release E CO] follows from Since the water vapour is removed from the sampling line before the measurement of O 2 , CO 2 and CO molar fractions, the mass flow rate of CO 2 in the incoming air follows from

c -2±-= _^L (1-Y£ O )YCO (7)
where rh a = mass flow rate of the incoming air (kg.s' 1 ) M a = molar weight of the incoming air (kg.s' 1 )

Y H o = molar fraction of water vapour in the incoming air

Yco 3 = measured 1

molar fraction of carbon dioxide in the incoming air

The mass flow rates of CO 2 and CO in the exhaust duct are given respectively by

(8) m co = 3^(]-y ) Y A (9) M , , v l H,O ) 1 co \7J CO M e
where fh e = mass flow rate in the exhaust stack (kg.s' 1 ) M e = molar weight of the gas in the exhaust stack (kg.s' 1 ) Y Hl o = molar fraction of water vapour in the exhaust flow Yco 2 Yco = measured molar fraction of CO 2 and CO respectively (during the combustion test)

Lastly, the resulting equation to calculate heat release rate by CDG calorimetry is given by

q = ^-(1-Y H2O ) (E CO2 Y c % M CO2 +E< CO Y C A O M co )--^-(l-Y^o) Y C A °2 (10)
The molar fraction of water vapour in the exhaust stack is rarely measured because water vapour is in most cases removed before the gas stream is analysed. However, using the water vapour molar balance [START_REF] Marlair | Experimental Evaluation of the Fire Hazard of Organophosphorous Insecticides by Use of Various Test Methods[END_REF], the conservation of the mass flow of nitrogen (2) and the algebraic expression of the molecular weight of the exhaust gas [START_REF] Janssens | Oxygen Consumption Calorimetry[END_REF], one may obtain a solvable system of three equations with three unknowns {Y H ^0 , M e and m a ):

fn CO2 -m CO + M COi + M C0 J2a ^ ' m " a_v° \(1-Y A ° Y A °\ M Me K 1 Y H,O) V r 0 2 ~rC0 2 ) 1V1 e M e = M O Y O2 +M CO Y CO2 +M CO Y CO+ M H2O Y H2O +(J-Y Oi -Y CÛ2 -Y CO -Y H2O )M N2 (13) with, Y t -Y A (J -Y Hfi ) i: O 2 , CO 2 and CO ( 14
)
Due to the large dilution factor of the combustion gases by the air entrained into the exhaust hood, it can generally be assumed that M e is nearly equal to M a .

Basic experimental validation at INERIS

Experiments were performed with gas burner at bench-scale (at INERIS, France) by use of a fire calorimeter derived from both the FMRC 50 kW flammability apparatus [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF] and the Rhône-Poulenc test-rig [START_REF] Menguy | Méthodes calorimétriques: une voie prometteuse[END_REF]. The gas burner was fed with a constant mass flow rate of methane as fuel. The mass flow rate of diluted fire gases in the exhaust duct was regulated to a value of 100 kg.h" 1 . The measurements were made for O 2 , CO 2 , CO and unburned hydrocarbons molar fractions in the exhaust duct, the ambient air moisture and the volumetric flow rate of gases in the exhaust duct. As expected for all the experiments, the carbon monoxide and unburned hydrocarbons molar fractions were negligible. Figure 1 shows the heat release rates calculated from the OC calorimetry q o% according to Janssens's formulae [START_REF] Janssens | Oxygen Consumption Calorimetry[END_REF] on a wet basis (with ambient air moisture correction)and the CDG calorimetry q co according to equation ( 10) also on a wet basis (with water vapour correction). As expected, the curves show that both calculation methods are in perfect agreement. Further experiments were carried out with different mass flow rates of methane, to get different heat release levels with the same overall exhaust flow. The ratio of the heat release rate calculated from the CDG calorimetry on a dry basis to that calculated on a wet basis was plotted versus the latter parameter (figure 2). Figure 2 referring to these experiments, where the heat release rate has been increased within a range of 4:1, shows a very weak influence of Y H ^O in terms of correcting factor of the heat release rate (3% at maximum). It comes to a conclusion that this factor is simply negligible or easily kept under control by the user, for instance by provision of adequate operating conditions in the experiments. In fact, it is a common practice to report rate on a dry basis (as it is also done when OC calorimetry is used), only by neglecting water vapour molar fractions and providing large dilution factor of the fire gases.

Effect of incomplete combustion of fuel carbon into soot on heat release rate calculation

The production of soot from flaming combustion may be affected by many factors [START_REF] Ostman | Smoke and Soot[END_REF]. The effect of ventilation has been studied for many materials [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF][START_REF] Dod | Smoke Emission Factors From Medium Scale Fires: Part2[END_REF]. It has been shown that the soot production increases with restricted ventilation. The soot production also depends on the chemical nature of the material being burned. Compounds such as methyl alcohol, carbon monoxide and formaldehyde do not produce soot in significant quantities. For other compounds, under identical conditions in diffusion flames, the soot production is higher for unsaturated hydrocarbons than the corresponding saturated hydrocarbons. The general trend observed in the sooting tendency follows the order [START_REF] Glassman | Soot Formation in Combustion Processes[END_REF] Aromatics > alkynes > alkenes > alkanes Another known fact is that halogen compounds accelerate the rate of soot formation.

The soot production (in term of yield) may be as high as the carbon monoxide production both for well-ventilated fires and under-ventilated fires [START_REF] Tewarson | Generation of Heat and Chemical Compounds in Fires[END_REF][START_REF] Hietaniemi | Fires at Chemical Warehouses, A Cone Calorimeter Study on the Burning Characteristics and Fire Effluent Composition of Selected Chemical Compounds[END_REF][START_REF] Anderson | Simulated Fires in Substances of Pesticide Type[END_REF]. A modified equation is thus proposed in order to take account of the soot production for application of CDG calorimetry: Although the soot production depends on several parameters such as the type of flame and the nature of the burning fuel, the properties of the carbon produced are little affected by these parameters [START_REF] Palmer | The formation of carbon from gases[END_REF]. Chemical analysis shows that (mature) soot particles generated from hydrocarbons burning in diffusion flames contain about 1% by weight of hydrogen and oxygen [START_REF] Palmer | The formation of carbon from gases[END_REF][START_REF] Gaydon | Solid Carbon in Flames[END_REF]. It can thus be assumed that soot particles are principally graphitic carbon particles. Whilst chemical processes of soot formation are very complex, the overall combustion reaction which produced soot may be written as:

<i = Eco 2 (™ C o 2 -™co 2 ) + E '
C a H b O c N d + {-A -C -) O 2 _£*-> a C + b -H 2 O + ^ N 2 (16)
Considering the oxidation reaction of carbon to carbon dioxide

C + O 2
AHs >CO 2 [START_REF] Hietaniemi | Fires at Chemical Warehouses, A Cone Calorimeter Study on the Burning Characteristics and Fire Effluent Composition of Selected Chemical Compounds[END_REF] and according to the Hess law, the heat release E" s per unit mass of carbon generated follows from:

^ ( 18
)
where AH S = energy release per unit mass of carbon consumed (32,7 MJ. kg' 1

of C) M s = molecular weight of carbon (12 kg.kmol 1 )

Since condensable species and soot particles are removed from the sampling line before measurement of O 2 , CO 2 and CO molar fractions, the mass flow rates of carbon dioxide and carbon monoxide in the exhaust stack are given by:

Y A ( I -V 7 -Y } 1 CO? V J L, 1 cond.n * S I M CO2 M e m CO m e V A Yco V-XY^-Ys) (20) M co M e where

Yoond n molar fractions of condensable species such as H 2 O, acid gases (HCl,...) Y s molar fraction of graphitic carbon

The mass flow rate of soot can be calculated from optical smoke measurements. According to Beer-Lambert law, the attenuation of monochromatic light over a given optical path through the smoke generated by the burning fuel can be expressed as

*• <)
where It has been shown that mass concentration of soot particles is proportional to the light extinction coefficient k [START_REF] Foster | Attenuation of Light by Wood Smoke[END_REF][START_REF] Seader | Correlation of the Smoke Tendency of Materials[END_REF]:

I
C s = j~ ( 22
)
where, C s = mass concentration of smoke particles (kg. m' 3 ) o s = specific extinction area per unit mass of soot (m 2 .kg~'j

The value of the specific extinction area <J S is roughly constant for fuels burning in flaming mode and producing primarily graphitic carbon soot. As an example, a practical value of this specific extinction coefficient as has been quoted by Mulholland et al [START_REF] Mulholland | The Basis for a Smoke Concentration Measurement Using Light Extinction[END_REF].

In open systems, since the attenuation of light through smoke is measured in the exhaust stack, the mass flow rate of soot follows as:

, = CF-^ (23)
'"S ^S ' e \*-J )

where V e = volume flow rate in the exhaust chimney at the actual temperature where the photometer is located (m~'.s~')

The molar fraction of soot in the exhaust stack can be written as m M

Lastly, the heat release rate calculated from CDG calorimetry is given by

^â, n -Y s ){E co Y c A o M COi + E' co Y c A o M co )-^(l-Y°2 O )Y c i 2 + E" S ^ (25)
As in the previous case (where the production of soot is insignificant), to calculate the molar fraction of water vapour, the mass flow rate of the incoming air and the molar weight of the exhaust gas, the following system of three equations with three unknowns can be solved:

(26)

-">" M e -"*-M a \ M COi M co • M s ) 2a (•' ~ YQ 2 -Y C02 -Y c0 -£ Y condn -Y s ) jy. (1-Y° ) (1-Y A ° -Y A ° ) M e e W 1 H 2 O) \ L 1 0 2 1 CO 2 ) e M e = M O2 Y Û2 +M CO2 Y CO2 +M C0 Y C0 +ZA/ OTi /^B +M S Y S +Y N2 M N2 (28) with, Y, = Y A (1 -S Y cmd " -Y s ) i: 0 2 ,C0 2 and CO (29) n Y N2 =0-Y 02 -Y C02 -Y co -Z Y cond , n -Y s ) (30)

Discussion at the light of experimental results from Tewarson

A first assessment of the relevancy of such a modified equation for the calculation of heat release rate by CDG calorimetry has been studied using experimental data of Tewarson [START_REF] Tewarson | Ventilation-Controlled Combustion of Polymers[END_REF]. Generalised correlation between yields of combustion products and equivalence ratio have been established by

To assess the influence of soot production on the heat release rate calculation, the ratio (equation 34) of the heat release rate calculated by the carbon dioxide generation principle (without soot correction) to the heat release rate corrected for the soot generation has been plotted versus equivalence ratio (figure 3):

-Polyethylene . Polystyrene 1,2 1,6 2
Equivalence ratio As it can be seen from figure 3, in wellventilated fire conditions, the correction accounting for soot generation on the heat release rate calculation is negligible in both examples given (PE and PS). On the other hand, in under-ventilated fire conditions, the situation moves. The higher is the equivalence ratio (e.g. the lower is the fire ventilated), the higher is the underestimation of the heat release rate, if the contribution of soot keeps neglected. The real significance of this underestimation in practical application is influenced by the nature of the tested material.

Conclusion

2 AHco

 2 where, Mfuel = molecular -weight of fuel (kg.kmol 1 ) M co = molecular weight of CO2 (44 kg.kmol 1 ) a = number of carbon atom in the molecular structure of the burning fuel According to Hess law and considering the oxidation reaction of carbon monoxide to carbon dioxide CO + -O energy release per unit mass of CO consumed for the burning of CO (10.1 MJ.kg 1 CO) M co = molecular weight of CO (28 kg.kmol 1 )

Figure 1 :Figure 2 :

 12 Figure 1 : HRR calculated by OC and CDG calorimetry (burning product = CH4)

  o = intensity of incident light I = intensity of the transmitted light through smoke L = optical path length through smoke (m) k = light extinction coefficient (m 1 )

Figure 3 :

 3 Figure 3: Influence of soot production on heat release rate calculation by CDG Calorimetry.

Table 1 :

 1 The equivalence ratio is a parameter which characterises the degree of ventilation in reaction-to-fire test. For polymers such as polyethylene (PE) and polystyrene (PS), the values of f Uwv , a , p and £ are listed in table 1. Correlation coefficients to account for the effects of ventilation on the generation rate ofCO 2 , CO and soot, from Tewarson[START_REF] Tewarson | Ventilation-Controlled Combustion of Polymers[END_REF].

		Tewarson for polymers:
		(31)
	where,
	ft vc	= yield of i for ventilation-controlled fires (gram of i produced per gram of burned fuel)
	ftwv	= yield of i for well-ventilated fires (gram ofi produced per gram of burned fuel)
	O	= equivalence ratio
	a, p, Ç = correlation coefficients

  CDG calorimetry is a useful method to calculate heat release in fires. For accurate heat release rate calculations, molecular compositions of the materials are needed as in any other practical method. If the molecular composition of the mateial being burned is known, the water vapour molar fraction in the exhaust stack can be calculated to improve the measurement of the heat release rate by CDG calorimetry ii required. The sooting tendency of materials is another important consideration for accurate calculations of heat release. A modified equation applying for CDG calorimetry has been proposed and shortly discussed in terms of relevancy, especially for ventilation-controlled fires. La Communauté française de Belgique (Belgium) -Le Ministère de l'Aménagement du Territoire et de l'Environnement (France). Ejuei Energy release per unit mass of fuel burned for complete combustion (kJ.kg 1 )E coEnergy release per unit mass of CO 2 produced for complete combustion to CO2 (kJ.kg" 1 )E' coEnergy release per unit mass of CO produced for incomplete combustion to CO (kJ.kg 1 ) E" s Energy release per unit mass of carbon produced for incomplete combustion to graphitic carbon (kJ.kg 1 ) ft Yield of i (gram of i produced per gram of burned fuel) Energy release per unit mass of CO consumed in the burning of CO (kJ.kg 1 ) AH S Energy release per unit mass of carbon consumed in the burning of carbon (kJ.kg 1 ) a P % Correlation coefficients a s Specific extinction area per unit mass of soot (rn^kg" 1 ) <D Equivalence ratio Refers to molar fractions in the analyser prior to the test

	/	Refers to burning fuel
	vc	Ventilation-controlled ares
	wv	Well-ventilated fires
	Superscripts
	A A 0	Refers to molar fractions in the analyser
	Acknowledgement
	This work has been sponsored by the following public institutions:
	-Nomenclature
	a	Stoichiometric coefficient for carbon
	b	Stoichiometric coefficient for hydrogen
	c	Stoichiometric coefficient for oxygen
	d	Stoichiometric coefficient for nitrogen
	I o	Intensity of incident light
	/ k	Intensity of the transmitted light through smoke Light extinction coefficient (m" 1 )
	L m M	Optical path length through smoke (m) Mass flow rate (kg.s" 1 ) Molecular weight (kg.kmol" 1 )
	q	Heat Release Rate (kW)
	V	Volume flow rate in the exhaust duct (m 3 ^"1 )
	Y	Molar fraction
	AH CO Subscripts
	a	Refers to incoming air
	e	Refers to gases in the exhaust stack

0

Refers to incoming air

Boiling Liquid Expanding Vapour Explosions CfflSA'98, Prague,

23-28 août 1998

The nomenclature is the same as that used by Janssens[START_REF] Janssens | Oxygen Consumption Calorimetry[END_REF]. The superscript A refers to mole fraction of any species (except H 2 O and soot) measured in the analyser (which is different from the corresponding molar fraction in the exhaust flow).

It is assumed that N 2 does not participate in the oxidation reactions because temperatures in fires are usually too low to allow high production of nitrogen oxides.