Hugues Prétrel

Pascal Zavaleta

Sylvain Suard

Experimental study on the effect of mechanical ventilation on cable tray fires in compartments

Keywords: cable fire, compartment, ventilation, vitiation, air intake, oscillation

This study deals with the effect of ventilation flow rate and the ventilation configuration on the behavior of a cable tray fire in a confined and mechanically ventilated enclosure. Results are based on large-scale fire tests performed within the framework of the OECD Nuclear Energy Agency (NEA) PRISME projects. The fire source, identical for all tests, consisted in five horizontal cable trays measuring 3 m in length, stacked vertically and filled with halogen-free flame retardant cable-type, non-powered cables. The installation consists of two rooms connected by an open doorway. Two ventilation configurations were considered with two different arrangements of the inlet and outlet branches. The analysis demonstrates the effects of ventilation flow rate and air intake position on the fire heat release rate (HRR). Decreasing the ventilation flow rate reduces the maximum HRR and delays combustion on the trays, which burn one after the other rather than simultaneously. This result is the consequence of oxygen vitiation near the cable trays, which reduces the horizontal flame spread on each cable tray. The analysis also discusses the phenomenon of low frequency oscillatory behavior of fire in a ventilated enclosure. The results highlight that the conditions of occurrence of oscillations combine two criteria: a low ventilation flow rate to favor the release of unburnt gases and an appropriate configuration of the ventilation air intake to generate ignition of premixed gases.

Introduction

Electrical cable fire remains a scenario of major interest in many fire risk assessment studies. Indeed, electrical cables are often encountered in very large quantities in industry and in the transportation sector. In nuclear power plants (NPP) in particular, approximately 50% of fires between 1970 and 2000 were caused by electrical equipment [START_REF] Suard | IAEA-TECDOC-1421 Experience gained from fires in nuclear power plants. Lessons learned[END_REF]. The review of fire safety assessment in many NPPs has shown that fire is one of the most important risk contributors [START_REF] Suard | IAEA-TECDOC-1421 Experience gained from fires in nuclear power plants. Lessons learned[END_REF] and fires involving electrical cables are considered to be one of the major scenarios of interest [START_REF]Collection and Analysis of Fire Events (2010-2013) -Extensions in the Database and Applications[END_REF]. Indeed, electrical cable is a complex material, and its ignition, thermal behavior, pyrolysis and flame propagation processes remain difficult to assess. In addition, actual arrangements of cables set on trays in an enclosure add complexity because of the specific geometrical cable arrangement (horizontal or vertical) and the influent environment consisting in a hot and sooty atmosphere. Understanding fire scenarios involving electrical cables and the prediction capabilities of numerical tools are therefore important issues [3].

Since the large cable fire at the Browns Ferry NPP in the USA on March 22, 1975 [4], one of the most severe fires to occur in such a facility, numerous research studies have been conducted to improve our knowledge of electrical cable fires [START_REF] Nowlen | Nuclear power plants: A unique challenge to fire safety[END_REF]. The first study focused on multi-scale tests and protocols enabling the characterization of the performance of cable exposed to fire [START_REF] Grayson | Assessing the fire performance of electric cables (FIPEC)[END_REF]. Large-scale test campaigns were then conducted to better understand the effect of cable tray features (vertical or horizontal orientation, number of trays, dimensions, loading, ceiling effect, etc.) [START_REF] Mc Grattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE) Phase 1 Horizontal Trays[END_REF] [START_REF] Mc Grattan | Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE), Phase 2: Vertical Shafts and Corridors[END_REF]. These tests led to proposing a simple predictive model, named FLASHCAT, designed to predict time variations in the fire heat release rate (HRR) of a cable tray fire. The effects of the environment in the case of fire in a confined enclosure were then studied within the framework of the OECD PRISME project addressing NPP compartment configurations [START_REF] Zavaleta | Improved Assessment of Fire Spread over Horizontal Cable Trays Supported by Video Fire Analysis[END_REF], [START_REF] Suard | Fire development in multi-compartment facilities: PRISME 2 project[END_REF] [START_REF] Suard | Overview of the OECD PRISME 3 Project[END_REF], [START_REF] Plumecocq | Horizontal cable tray fire in a well-confined and mechanically ventilated enclosure using a two-zone model[END_REF]. For this project, a fire test campaign named Cable Fire Spreading (CFS) focused on the behavior of a horizontal cable tray fire in a two-room mechanically ventilated set-up [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. The ventilation configuration comprised an air intake in the upper part of the fire room and an exhaust line in the adjacent room. The results highlighted the effect of the ventilation flow rate on the fire HRR, as well as the phenomenon of HRR oscillatory behavior. The results demonstrated the possible effect of the air intake position on oscillation occurrence. This topic has already been investigated mostly with pool fire, showing a significant influence of the air intake position on smoke distribution within the rooms and, consequently, on fire intensity [START_REF] Backovsky | Temperature Profiles In Forced-Ventilation Enclosure Fires[END_REF], [START_REF] Acherar | Impact of air intake position on fire dynamics in mechanically ventilated compartment[END_REF], [START_REF] Wang | Effects of air inlet or outlet position of a fan coil unit ventilation system on smoke movement and fire severity[END_REF], [START_REF] Matsuyama | Experimental Study on Fire Behavior in a Compartment Under Mechanical Ventilated Conditions: The Effects of Air Inlet Position[END_REF]. Changing the position of the air intake modifies the oxygen concentration field in the vicinity of the fire source and influences the severity of the fire.

The objective in this study is to investigate the effect of the ventilation features on the fire HRR, focusing specifically on two parameters: the ventilation flow rate and the ventilation layout. A new ventilation configuration, compared to that investigated in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], has been considered, with the air intake positioned in the adjacent room and the exhaust line in the fire room. A new set of fire tests have been conducted within the framework of the Cable Fire Propagation (CFP) campaign of the OECD PRISME 3 project [START_REF] Suard | Overview of the OECD PRISME 3 Project[END_REF], with the ultimate aim of demonstrating the effect of ventilation layout based on comparative analysis with fire tests presented in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. The first section introduces the new fire tests. Then a comparative analysis discusses the effects of the ventilation flow rate and of the air intake position. This analysis is supplemented by a discussion on the influence of the ventilation layout on the occurrence of combustion oscillations, as well as an analysis of the relationship between oxygen depletion and the fire heat release rate. Finally, all the conclusions are presented together in the last section.

Fire test

Description of the ventilated enclosure

Fire tests were performed in a closed and mechanically ventilated enclosure which was almost the same as the facility used a few years earlier when conducting the tests discussed in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. For the CFP campaign, the enclosure, part of the IRSN DIVA facility, consisted in two identical rooms of about 111 m 3 (Height H=3.84 m, length L=4.9 m, width W=5.9 m) connected together by an open doorway (Height H=2.17 m and width W=0.79 m) as illustrated in Fig. 1. Walls were made of concrete (ρ = 2700 kg/m 3 , k = 0.22 W/K/m, Cp = 920 J/kg/K) of different thicknesses (300 mm and 350 mm). The ceiling was made of four layers: concrete (300 mm), an air gap (130 mm), Board Plus LTI insulated panels (10 mm), and "Monalite M1A" calcium silicate panels (13 mm, ρ = 720 kg/m 3 , k = 0.19 W/K/m, Cp = 960 J/kg/K). It is worth noting that the facility differs slightly from the one used for the tests reported in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], in particular with regard to the thickness of the side walls and the insulated material used for thermal protection of the ceiling. The ventilation consisted in an admission line and an exhaust line both connected to an industrial ventilation network equipped with fans. This study aimed to investigate two ventilation layouts, named A and B, as illustrated in Fig. 1. The first layout consisted in an admission line in the adjacent room and an exhaust line in the fire room (which contains the cable trays) and the second (named B and identical to the one set for the tests discussed in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]) was the opposite, as illustrated in Fig. 1. The two configurations selected are two examples of configurations that may be encountered in nuclear installations. They were chosen to study two opposite scenarios regarding the position of the fire with respect to the air intake. For both layouts, the inlet and outlet sections of the two lines were rectangular, with dimensions of 0.6 m in height and 0.3 m in width and were in the upper part of the rooms. Flow directions are indicated in Fig. 1.

Fire source

The fire source was identical to that used in the tests presented in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. It was composed of five horizontal ladder-type trays with width 450 mm and length 3000 mm (see Fig. 2). The vertical distance between trays was 300 mm. The trays were set on metallic frames with vertical and horizontal round iron tubes thermally insulated with rock wool. The cable trays were set against a vertical sidewall (t = 40 mm, l = 2600 m, H = 2200 mm) made of insulated material (Superwool 607 HT Board, 350 kg/m 3 , thermal conductivity  = 0.11 W/m/C at 600°C). Each tray was filled with 32 samples of cable with a length of 2400 mm. The cable contains poly(ethylene/vinyl acetate) (EVA) and polyethylene (PE) as polymeric materials and aluminum tri-hydrate (ATH) as halogen free flame retardant (HFFR). The chemical formula of PE/EVA copolymer is [C2H4]n for PE and [[C2H4]-[C4H6O2]]n for EVA [START_REF] Bascou | Cable tray fire tests simulations in open atmosphere and in confined and mechanically ventilated compartments with the CALIF3S/ISIS CFD software[END_REF]. The cable had a diameter of 20 mm and a load of 0.70 kg/m (measured onsite before the tests) and comprised twelve conductors with a 1.5 mm 2 section. Total cable mass was approximately 267 kg and the fraction of combustible matter was estimated at 20%. The cable arrangement on the trays was loose and the cables were not powered. The supplier reference was NU-SHX (ST) HX 1kV, which is the cable mainly used for signal cabling in Nuclear Power Plants, and passes the reaction to fire standard tests NF C 32-070 (C1), IEC 60332-1 (C2), IEC 60332-3-22,-23 and 24, IEC 61034 and IEC 60754-2. This cable-type is considered to be a "well-classified cable type" since it meets the requirements of the many standard tests mentioned above. This cable is identical to one of the two cables characterized at small scale in [START_REF] Meinier | Fire behavior of halogen-free flame retardant electrical cables with the cone calorimeter[END_REF]. The fire source was located against the largest sidewall of the fire room, at its center, as indicated in Fig. 1. The metallic frame that supported the trays was equipped with a propane burner located below the first tray and at its center. The whole system was set on two scales in order to measure fuel mass loss. The elevation of the first tray was at about 940 mm from the ground. The analysis here combines two test campaigns, the CFS campaign as already described in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF] and a new campaign, named CFP, performed within the framework of the OECD PRISME 3 project. The test parameters were the ventilation layout with two different configurations named A and B and the ventilation flow rate as indicated in Table 1. Three ventilation flow rates were considered for use with ventilation layout B (targeted ventilation of 3660, 1410 and 1010 m 3 /h, which corresponds to renewal rates rounded to 15, 6 and 4 h -1 based on the volume of the two rooms) and two ventilation flow rates for ventilation layout A (targeted ventilation flow rates of 3600 and 960 m 3 /h). Although the ventilation flow rates differ slightly between the two campaigns (CFS and CFP), these differences will be considered small enough (estimated not larger than 5%) to allow for comparison between the tests. In addition, a fire test named PR2_CFSS_2, conducted to characterize the same fire source in an open atmosphere, was also considered in this study, as it was in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF].

For all tests, the combustible matter was ignited using an 80 kW propane burner located at the center of the tray and 0.2 m below it. The burner was shut off as soon as the total fire heat release rate reached about 400 kW (about 3 to 4 minutes after ignition for all tests). For all tests, the procedure was the same. First the ventilation flow rate was set to a targeted value. Fire was then remotely ignited using the burner. Last, combustion progressed until extinction. Fig. 3 gives examples of images recorded during the experiments.

Measurements and elaborated variables

Numerous measurements were performed during these tests. The analysis presented in this article only considers measurements performed in the fire room, the mass loss rate, the gas temperature above the trays, the oxygen and unburnt gas concentrations, the pressure in the room and the flow rates in the ventilation circuit. The measurement positions are illustrated in Fig. 1. The fuel mass loss was measured using a two SARTORIUS IS300IGG weighing system. The burning rate was deduced from the time derivative of the mass signal. In order to determine the burning condition and assess vertical and horizontal flame spread velocity along the cable tray, Ktype thermocouples were set along each cable tray (7 for the CFS test campaign and 14 for the CFP test campaign). As described in [START_REF] Pretrel | Experimental investigation of the effects of a sidewall and cable arrangement on a horizontal cable tray fire in an open atmosphere[END_REF] , horizontal flame spread velocity is computed from the time taken for the flame front to pass over two successive thermocouples. Oxygen concentrations were measured using a SERVOMEX XANTRA 4100 gas analyzer connected to sampling points fixed at three elevations (0.7 m, 2.1 m and 3.3m from the ground) on the SE mast as illustrated in Fig. 1. The gas analyzer measured species concentrations on a dry gas sample. One sampling point with a M&A Thermo FID gas analyzer was considered below the ceiling for the measurements of total hydrocarbon concentration (values reported in this analysis correspond to equivalent CH4 concentration). Flow rates and pressure in the ventilation lines were also measured using the averaging Pitot probe technique and pressure transducers. The fire heat release rate (HRR) was determined based on oxygen consumption (OC) and carbon dioxide consumption (CDG) calorimetric methods updated for use in closed and ventilated compartments [START_REF] Janssens | Calorimetry[END_REF], [START_REF] Prétrel | Determination of the heat release rate of large scale hydrocarbon pool fires in ventilated compartments[END_REF].

Uncertainties

The measurement uncertainties were evaluated based on calibration tests, carried out before or after the tests, and on the data provided by the sensor manufacturers. The calibration tests consisted in obtaining the differences between standard values (from specific calibrated sensors or samples) and the measured values, read on the acquisition system. Two levels of uncertainty were defined following the procedure given by Hamins [START_REF] Hamins | Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications Vol 2 -Experimental Uncertainty[END_REF]. The standard uncertainties were obtained at room temperature without fire. A second level, the expanded uncertainty, took into account the fire test conditions and test repeatability, and was introduced through a coverage (or correction) factor, applied to the standard uncertainty. The uncertainties of elaborated variables (the flow rate in the ventilation, the flame spread velocity, the MLR and the HRR), determined from measured quantities, are assessed from qualification tests. The measurement uncertainties are indicated in In addition, the raw data from fire tests PR2_CFSS_2, PR2_CFS_3 and PR2_CFS_4 have been reprocessed along with the new tests PR3_CFP_D1, PR3_CFP_D2 and PR3_CFP_D3 in order to use identical processing programs (for filtering, smoothing, and the computation of elaborated variables such as the heat of combustion, the mass loss rate and the heat release rate). This leads to slight differences (not greater than 8%) in comparison to the results reported in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF].

Results

The results focus on four main points: the influence of ventilation on the fire intensity, the oxygen concentration within the enclosure, the consequences on the fire propagation and the oscillatory regimes.

3.1 Characterization of the pressure and flow rate in the ventilation network.

Before studying the influence of the ventilation configuration on the fire, it is important to characterise the aeraulic behaviour of the two ventilation configurations A and B in terms of flow rate in the supply and exhaust lines and in terms of pressure in the room. These experimental data allow the reconstruction of an equivalent fan curve that can be used in numerical simulations. Fig. 4 shows the temporal evolution of ventilation rates and pressure for all tests and the correlation between pressure and flow rates for the three ventilation flow rates. The results show the typical consequences of a fire in a mechanically ventilated compartment, i.e. an increase in pressure leading to a decrease in the flow rate in the intake line and an increase in the exhaust line. The two configurations A and B have similar air resistance for a given flow rate. Only the air resistance of the exhaust branch for the tests at about 1000 m 3 /h is slightly different between the two configuration A and B.

Effect of the ventilation layout and ventilation flow rate on the fire intensity

In a first step, the influence of the ventilation configuration and the ventilation rate on the characteristics of the fire heat release rate (HRR) also named 𝑄 ̇ and of the heat of combustion, H are analysed. Whatever the ventilation configuration, A or B, a decrease in the ventilation volumetric flow rate (named 𝑞̇𝑣) reduces the fire HRR with respect to the reference HRR in an open atmosphere, as illustrated in Fig. 5. The amplitude of the maximum HRR (about 2.0 MW in an open atmosphere) is reduced and the time taken to reach this value is increased. For ventilation configuration A, the ventilation flow rate of 3660 m 3 /h gives nearly the same amplitude (about 1.85 MW) as for the open atmosphere configuration. The reduction in the flow rate to 1010 m 3 /h significantly reduces the amplitude to about 0.9 MW. For configuration B, similar observations are reported. However, it is worth noting that the amplitudes of maximum HRR are similar for the two flow rates 960 m 3 /h and 1410 m 3 /h (about 0.4-0.5 MW). The ventilation configuration also influences the fire HRR. For the same ventilation flow rate, ventilation configuration A leads to higher HRR than configuration B. Oxygenation of the fire source is more efficient with configuration A (maximum HRR of 1845 kW), with the air intake in the adjacent room than with configuration B, with the air intake in the upper part of the fire room (maximum HRR of 1252 kW). Although the air intake in configuration B is closer to the fire source, its position near the ceiling is not favorable and the entrance of fresh air into the fire room through the doorway seems to be more efficient at feeding the fire source homogeneously. This effect of the ventilation configuration with a flow rate of about 3600 m 3 /h (PR3_CFP_D1 and PR2_CFS_4 tests) is also reported for a lower amplitude of about 1000 m 3 /h (PR3_CFP_D2 and PR2_CFS_3 tests). These tests provide evidence of the combined effects of ventilation flow rate and positions of the ventilation inlet and outlet on oxygenation in the vicinity of the fire source. In order to investigate whether the change in the fire HRR also results from a change in the chemical reaction, the heat of combustion (or global combustion enthalpy) H is discussed. This is determined experimentally as the ratio between the experimental HRR determined based on chemical methods and the measured fuel mass loss rate (MLR). The heat of combustion versus the mass loss over time is presented in Fig. 6. For all tests, the behavior is similar: the heat of combustion increases linearly with the mass loss from about 15 MJ/kg up to 30 MJ/kg. This range is in accordance with typical values for such cable, as also reported in [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], [START_REF] Meinier | Fire behavior of halogen-free flame retardant electrical cables with the cone calorimeter[END_REF]. This increase over time is explained as the change in the combustible matter. The highly endothermic dehydration of the Aluminum Tri-Hydroxide (ATH) halogen-free flame retardant contained in the cable, delays the burning of the polymeric materials, EVA/PE, in particular during the first stage of the tests. The heat of combustion then increases continuously as the fire spreads over the entire area of the cable trays, which leads to the progressive consumption of ATH. [START_REF] Zavaleta | Effects of cable tray configuration on fire spread[END_REF], [START_REF] Delfosse | Combustion of ethylene-vinyl acetate copolymer filled with aluminium and magnesium hydroxides[END_REF], [START_REF] Zilberman | Flame retardancy of some ethylenevinyl acetate copolymer-based formulations[END_REF]. Fig. 6. Variation in the heat of combustion versus the mass loss over time (m is the mass of fuel over time and minit is its initial value prior ignition)

In a second stage, the effects of the ventilation flow rate and of the ventilation configuration, on the global quantities are considered. These are the total mass loss, m, maximum HRR, 𝑄 ̇𝑚𝑎𝑥 , the total heat release (THR=∫ 𝑄 ̇𝑑𝑡), the mean combustion enthalpy (H = THR/m) and fire duration, t (see Table 3). Results are also illustrated in Fig. 7. The trend reported regarding HRR time history is confirmed based on total quantities. For both ventilation configurations, reducing the ventilation flow rate reduces total mass loss, total maximum HRR, total heat release and mean combustion enthalpy. The effect of the ventilation configuration is also reported. Configuration A compared to configuration B improves combustion efficiency and leads to similar trends as increasing the flow rate for a given configuration. It is worth noting that fire duration does not follow the same trend as the other quantities but shows more erratic behavior. The results are explained by the uncertainty in determining the extinction time. Small flaming regions may remain until a very late stage of the test, increasing total fire duration unrealistically with respect to the initial conditions. Fig. 7 also shows the comparison with behavior in the open atmosphere. As reported in the HRR analysis, ventilation configuration A with a flow rate of 3660 m 3 /h is the scenario which is the most similar to the open atmosphere scenario. To quantify the combined effects of the ventilation flow rate and the ventilation configuration, a modified reference global equivalence ratio, also called a ventilation factor, noted 𝜙 𝑜 , is introduced. This is similar to the parameter taken into account in the well-stirred reactor approach, used to characterize the behavior of pool fire in a ventilated enclosure [START_REF] Prétrel | Effect of environmental conditions on fire combustion regimes in mechanically-ventilated compartments[END_REF]. It is defined as the ratio between the amount of oxygen consumed by the fire in an open atmosphere (𝑚̇𝑂 2 𝑐,𝑜𝑝𝑒𝑛 ) and the amount of oxygen brought to the enclosure by the mechanical ventilation prior to ignition (𝑚̇𝑣 ,𝑂2 𝑖𝑛𝑖𝑡 ):

𝜙 𝑜 = 𝑘 𝑚̇𝑂 2 𝑐,𝑜𝑝𝑒𝑛 𝑚̇𝑣 ,𝑂2 𝑖𝑛𝑖𝑡 = ( ∫ 𝑄 ̇𝑜𝑝𝑒𝑛 𝑑𝑡 𝑌 𝑂2 𝑜 ∆𝑡 𝑜𝑝𝑒𝑛 𝐸 𝑂2 ) 𝑘 𝑚̇𝑣 𝑖𝑛𝑖𝑡 (1)
Where 𝑚̇𝑣 𝑖𝑛𝑖𝑡 is the ventilation mass flow rate prior to ignition, 𝑄 ̇𝑜𝑝𝑒𝑛 the fire HRR in an open atmosphere, ∆𝑡 𝑜𝑝𝑒𝑛 the fire duration in an open atmosphere, 𝐸 𝑂2 the combustion enthalpy per mass of oxygen consumed (assumed to be constant and equal to 13.1 MJ/kg), and k is a correction coefficient. The term in brackets in relation [START_REF] Suard | IAEA-TECDOC-1421 Experience gained from fires in nuclear power plants. Lessons learned[END_REF] depends on the fire source and, for the present study, is therefore a constant and is only set here to make the ventilation factor dimensionless. The ventilation factor varies from zero to infinity. When it tends to zero, ventilation supplies much more oxygen than is needed for combustion. When it is equal to unity, ventilation supplies exactly the amount of oxygen needed for the burning that would be needed in an open atmosphere. When the ventilation factor is greater than 1, ventilation supplies less oxygen than the amount needed. In practice, in the case of a fire in a ventilated compartment, the amount of oxygen consumed by the combustion is only a fraction of the oxygen supplied by the ventilation network, the remaining available oxygen fraction being extracted by the ventilation outlet. The position of the air intake significantly influences the fraction of oxygen consumed by the fire, as has been reported previously based on the fire tests. Different ventilation configurations lead to different results even with the same ventilation flow rate. In order to take this and, in particular, the air intake position (difference between configurations A and B) into account, a corrective coefficient k is introduced in the definition of the ventilation factor. This value depends on the ventilation layout. Therefore, two values for each configuration, A and B, are sought, such as the global quantities (total mass loss, maximum HRR, total heat release and the mean combustion enthalpy), that are best correlated (i. The three variables decrease monotonically with the ventilation factor, which appears to be an appropriate ranking parameter for correlating the combined effects of the ventilation flow rate and the ventilation configuration. The higher the o, the lower the mass loss, total heat release rate, maximum fire HRR and combustion enthalpy. In addition, this analysis confirms that the two tests (configuration A with ventilation flow rate of 1010 m 3 /h, PR3_CFP_D2 test) and (configuration B with ventilation flow rate of 3600 m 3 /h, PR2_CFS_4 test) which give nearly the same value for o also result in similar global quantities. The reduction in the ventilation flow rate is compensated by a more efficient (with respect to air supply) ventilation configuration; the ratio of ventilation flow rate of 3600/1010 is similar to the value of k = 0.35). This result confirms the counterintuitive observation made previously that configuration B with the air intake in the upper part of the fire room is less efficient than configuration A with the air intake in an adjacent room connected to the fire room through a doorway. The results also demonstrate that the entrance of air in a compartment through a doorway is an effective means of compartment oxygenation compared to a local ingress through an intake line. The advantages of the doorway include a significant portion of inlet flow near the lower area of the compartment, which allows air supply to the lower area of the fire, and a significant flow rate controlled by the temperature difference at the doorway.

Oxygen concentration within the enclosure

As suggested previously, the reduction in the fire HRR because of a change in the ventilation flow rate and the ventilation configuration is due to the change in the reduction in oxygen concentration in the vicinity of the burning areas. To demonstrate this link, oxygen measurements in the fire room are investigated. Three measurement points located at three elevations (bottom, z = 0.7 m, middle z = 2.2 m, top, z = 3.3 m) on the vertical mast SE (see Fig. 1) are considered. Fig. 9-a shows the time variations of these sensor responses. For all tests, the oxygen concentration follows the same trend as the fire HRR and decreases similarly in line with the HRR increase. In addition, as expected, vertical stratification is reported with lower concentration in the upper part because of smoke accumulation and, consequently, higher concentration in the lower part of the room.

This stratification is also highlighted in Fig. 9-b, which shows the vertical concentration profiles at the time of maximum fire HRR. The results clearly show the effect of the ventilation flow rate in configuration A (PR3_CFP_D1 test 3660 m 3 /h and PR3_CFP_D2 test with 1010 m 3 /h). For configuration B, the effect of the ventilation flow rate is less pronounced, possibly because for this configuration B, the position of the sampling points on mast SE was not representative of the average oxygen in the vicinity of the fire source. This is one limitation in considering a reduced number of points to characterize the oxygen fields. For this scenario, the large fire area and a specific ventilation configuration (in particular, configuration B) induce a complex, probably highly tri-dimensional, flow which makes it difficult to determine a representative oxygen concentration from three measurement points. Fig. 9-c shows the effect of the ventilation factor on oxygen concentration at the three locations (top, middle and bottom). The trends reported on the global quantities are also observed on the oxygen concentration. Increasing the ventilation factor leads to fire scenarios where the oxygen content in the enclosure is lower. The results also show the similarity between the two tests, CFS_4 and CFP_D2, according to the ventilation factor. For less complex fire sources, such as a pool fire, for instance, the effect of oxygen vitiation on the fire HRR is often taken into account by introducing a correction factor on the MLR ( [START_REF] Peatross | Ventilation effects on compartment fire characterization[END_REF], [START_REF] Quintiere | A theory for flame extinction based on flame temperature[END_REF]), aimed at reducing the burning rate proportionally to the oxygen concentration nearby. By analogy to this approach, the maximum HRR normalized by its value in open atmosphere is expressed in relation to an oxygen concentration at the same time, averaged over the bottom and middle locations (Fig. 10). For the configuration A, the results confirm the expected trend and also that the reduction in the maximum HRR is correlated with the reduction in the oxygen concentration in the vicinity of the fire source. However, for configuration B, the data diverge significantly around the linear relationship. Two main explanations are proposed. The first is the uncertainty entailed in determining experimentally a representative average oxygen concentration in the vicinity of the fire source. This effect is strengthened for the configuration B for which the flow around the fire is more complex. This result highlights the important issue in measuring a representative oxygen concentration. The second explanation is the fact that, in the case of a fire involving solid materials over a significant volume (such as with five cable trays), the relationship sought is not the relation HRR=f(O2) but more probably the relationship between the local flame spread velocities on each tray and the local HRR per unit of area (HRRPUA) in relation to the local oxygen concentrations.

A comparison between the correlation obtained from the present tests and that of Peatross & al. [28] is proposed by assuming that the MLR ratio and the HRR ratio are equal. Similar trends are reported which confirms the behaviour reported regarding the effect of oxygen. However, the HRR ratio obtained on these tests seems to decrease more slowly with decreasing oxygen than the Peatross & al. prediction for MLR. Further data is needed to go further in the physical explanation.

Consequences on the flame propagation

To identify the physical mechanisms that reduce fire HRR in the case of vitiation in the vicinity of the fire source, two key variables regarding flame propagation are analyzed : the ignition time of the fifth tray, tign, and the mean flame spread velocities for each tray, U, (indicated in Table 4). Fig. 11-left shows the increase in the ignition time (based on a threshold temperature of 500°C) of the fifth tray with the ventilation factor. The more poorly the enclosure is oxygenized, the longer the time needed to ignite the upper tray. This result also confirms the similitude, reported in the global quantities analysis, between the two tests (1010 m 3 /h and configuration A) and (1410 m 3 /h and configuration B). The changes to the ventilation configuration compensate for the decrease in the ventilation flow rate. This feature is then also reported for the fifth tray ignition time. Fig. 11-center discusses the evolution of the flame spread velocity with the tray index. For the open atmosphere fire test (black triangle), the velocity increases with the tray index because of the preheating mechanisms induced by the fire plume generated by combustion of the lower trays. The combustion on each tray contributes to heating up the cable tray positioned above. In this configuration, the fifth tray is therefore the one that benefits the most from this pre-heating effect and its horizontal flame spread velocity is the highest. For the other tests, the velocity remains similar for the lower trays, for which the velocity is about 1 mm/s and is weakly affected by the ventilation factor. Differences are mainly reported for the upper trays for which the effect of the vitiation reduces the flame spread. For some tests, some unusual variation of flame spread is reported. There is no explanation associated with a particular physical phenomenon. The explanation is rather to be found in the calculation method which is based on temperature measurements. Locally, and especially for intermediate cable trays, larger values of the flame spread velocity can be obtained due to the presence of cable trays above and below.

The effect of the vitiation of the flame spread velocity is illustrated in Fig. 11-right, which shows the effect of the ventilation factor on the velocities for the lower (Tray 1) and upper (Tray 5) trays. The velocity is about 8 mm/s in an open atmosphere and drops down to about 1 mm/s in the case of poorly ventilated fire tests. For Tray 1, the velocity remains constant and is not affected by the ventilation factor. For Tray 5, the velocity decreases significantly with the ventilation factor. The increase in the ventilation factor and therefore the under-ventilated conditions mainly affect the burning of the upper trays. To highlight the correlation between the change in flame spread velocity and the decrease in HRR, the burning combustion on each tray is analyzed based on the temperature measurements. For this purpose, the three fire tests performed in the CFP campaign are considered, for which a fine mesh of fourteen temperature probes was set up on each tray in order to identify the combustion area based on a threshold limit of 500°C. These measurements allow us to identify three typical periods of combustion in the course of the test: a flame propagation period, during which the flame propagated horizontally over the tray (the burning area increases with time); a full combustion period, during which the whole tray burns entirely (the burning area is constant and corresponds to the total tray area; and an extinction period, during which the extinction front progresses until complete extinction (the burning area decreases with time). Results are presented in Fig. 12 showing the three periods for each tray and the fire HRR.

The most-ventilated fire test, with 3660 m 3 /h and ventilation configuration A (PR3_CFP_D1 test), produces the most intense fire. The propagation period starts on the lower Tray 1 and lasts the longest time because of low flame spread (1.1 mm/s). For the other trays (2, 3, 4 and 5), this period starts with a delay due to the time the flame takes to spreads vertically. The propagation period is shorter because of a greater flame spread velocity. The shortest propagation period is reported for the highest tray (Tray 5), with a flame spread velocity of 3.1 mm/s. Once the propagation period over each tray has ended, all trays burned entirely and simultaneously during a period of about 400 s which coincided with the peak HRR. Finally, an extinguishing phase begins with the appearance of an extinguishing front and a progressive reduction of the burning area on each cable tray.

The effect of ventilation flow rate on the fire HRR is analyzed, taking account of the fire test with the same configuration A but with a lower ventilation flow rate 1010 m 3 /h (PR3_CFP_D2 test). A first observation is the disappearance of the full burning period over the two lower trays (Trays 1 and 2). The extinction period began before the flame propagated over the entire tray. A second point is a delay in the combustion of the uppermost tray (Tray 5); mainly because of low flame spread velocity (0.5 mm/s) and therefore, of a long propagation period. This delay in the combustion of the fifth tray combined with early extinction induces no period during which the five trays burn entirely and simultaneously. For this test, there is only one period (between 3000 and 4000 s) during which Trays 4 and 5 burn entirely and simultaneously. The consequences on the fire HRR are a reduction in the maximum HRR and a delay in the burning of the tray. Of the three tests considered, the test characterized as the "least" ventilated (flow rate of 1440 m 3 /h in ventilation configuration B) shows similar trends with more pronounced behaviors. The two first trays (1 and 2) and the fifth tray show no period of full burning over the entire tray area. Only Trays 3 and 4 show a period of full burning. The ignition of Tray 5 happens very late and, in addition, the propagation period is the longest because of weak flame propagation (0.8 mm/s). Finally, there is no period during which several trays burn entirely and simultaneously. The consequence of such behavior is a low fire HRR (not greater than 500 kW). These results clearly show that the reduction in the fire HRR because of the change in the ventilation features results from a modification of the tray ignition time and horizontal flame spread velocities.

Occurrence of oscillatory behavior

An important result found from these fire tests relates to the occurrence of oscillatory behavior of the fire. This phenomenon seems to be very similar to that identified at both large and reduced scales in the case of pool fire in a forced ventilated enclosure [30] [31]. This is explained as a coupling mechanism between, on the one hand, combustion near the extinction limit leading to high fluctuations in the production of combustion gases because of the release of unburnt gases and, on the other hand, the mechanical ventilation flow rate. The coupling variable is the room pressure, a consequence of the fire heat release rate and simultaneously controlled by the ventilation flow rate. In the case of a cable fire, these phenomena have also been reported and described for the test PR2_CFP_3 as a result of the combustion of unburnt gases (or total hydrocarbon species CNHM) produced to a highly sufficient level near the ceiling [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. One objective of the new CFP campaign was to further our understanding of this phenomenon in investigating the effect of ventilation flow rate and air intake position on this phenomenon. Pressure in the fire room and the concentration of unburnt gas species have been investigated to highlight the oscillatory behavior of the fire HRR. For the five tests discussed here, time variations relative to these two variables are shown in Fig. 13.

As reported with regard to fire HRR time variations (see Fig. 5), two tests show oscillatory behavior and correspond to the tests with low ventilation flow rates of about 1000 m 3 /h. About eight cycles were reported with a period of 210 s for configuration B [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF]. Only two complete cycles were reported with a slightly longer period of about 280 s for configuration A. For both tests, the oscillation occurrence is correlated with the significant production of unburnt gases with a similar amplitude of about 20,000 ppm.

Comparison with the other tests with the same configuration (A or B) also shows that the increased ventilation flow rate suppressed the occurrence of oscillations. The result demonstrates that the mechanism of oscillation seems to be related to a combination of two features: a low ventilation flow rate to induce the production of unburnt gases, and a specific configuration of the ventilation layout ensuring the arrival of fresh air near the zone of high concentration of the unburnt gases and therefore favoring generation of a premixed flammable mixture. Configuration B with the ventilation flow rate of 960 m 3 /h combines these two features. In changing the configuration (B to A), the conditions were nearly achieved and only an onset of oscillation was reported. The amount of unburnt gases is significant (about 25,000 ppm) but the arrival of fresh air is possibly not adequate to induce the combustion of unburnt gases. In changing the ventilation flow rate (960 m 3 /h to 1440 m 3 /h) in configuration B, the condition of high production of unburnt gases concentration is reached (about 25,000 ppm) but not the condition of premixed flammable mixture generation.

The fact that the concentration is lower for the test with the longest period of oscillation (configuration B and 960 m 3 /h) may be explained by the burning of "unburnt gases" itself, which mechanically leads to a reduction in the concentration of unburnt gases. For the other tests, the lack of ignition of unburnt gases or the fact that the unburnt gas temperature may not be hot enough to combust as mentioned in [START_REF] Prétrel | Experimental and numerical study of low frequency oscillatory behaviour of a large-scale hydrocarbon pool fire in a mechanically ventilated compartment[END_REF] maintain a high concentration of unburnt gases. Based on these new fire tests, criteria for oscillatory behavior seem to be (i) a low ventilation flow rate (for this scenario of 1010 m 3 /h) leading to local extinction on the upper trays and then enhancing production of unburnt gases, and (ii) a specific air intake position that favors the formation of premixed flammable mixture.

It is worth noting that the test with configuration A and a low ventilation flow rate (1010 m 3 /h) leads to high production of unburnt gases but also to combustion enthalpy very near the value in an open atmosphere (combustion efficiency close to unity). This result is explained by the fact that the generation of unburnt gases is not necessarily linked to combustion with low combustion efficiency but more probably to local extinction of the burning region, which then releases unburnt pyrolysis gases.

It is also worth noting that the oscillatory period of several hundreds of seconds is similar to the period obtained for fire tests involving pool fire in a 120 m 3 mechanically-ventilated room (periods of about 150-200 s reported in [START_REF] Prétrel | Experimental and numerical study of low frequency oscillatory behaviour of a large-scale hydrocarbon pool fire in a mechanically ventilated compartment[END_REF]). These results seem to confirm the similitude between the two types of experiment and that the oscillatory behavior does not depend on the type of fuel (solid or liquid). Including the results obtained at reduced scale for pool fire [START_REF] Mense | Experimental and numerical study on low-frequency oscillating behaviour of liquid pool fires in a small-scale mechanically-ventilated compartment[END_REF], these new results seem to indicate that such periods of oscillation get affected by the volume of the compartment. Table 5.summarizes the present data with two other contributions involving pool fire at small and large scale showing that the large the enclosure's volume has led to larger period of oscillation. Additional results are of course needed to further our understanding of the low frequency oscillatory behavior of fire in a compartment. Nevertheless, these large-scale experiments demonstrate the influence of compartment volume in the theoretical description of fire oscillatory behavior.

Conclusions

The objective of this study was to investigate the influence of ventilation features and, in particular, the ventilation flow rate and the position of the air intake on the behavior of a cable tray fire in a ventilated enclosure. The study combines two series of large-scale fire tests performed within the framework of the OECD PRISME programs. The test parameters are ventilation flow rate and two ventilation configurations with air intake located in the vicinity of the fire source or in an adjacent room. For all tests, the enclosure was identical, with a two-room assembly connected via a doorway, together with the same fire source, consisting in five horizontal cable trays measuring 3 m in length filled with qualified non-powered cables. The results highlight the influence of ventilation features on the fire heat release rate (HRR). The main outcomes are the following.

For this configuration, reducing the ventilation flow rate significantly reduces the fire HRR in comparison to its amplitude in an open atmosphere. In addition, for a given ventilation flow rate, the position of the air intake in the adjacent room leads to a higher fire HRR than when positioned in the upper part of the fire room. This counterintuitive result demonstrates the efficiency of a doorway for blowing air into a fire room.

A new ventilation factor, 𝜙 𝑜 , with a corrective coefficient k is proposed as a parameter to enable quantifying the combined effect of the ventilation flow rate and the ventilation configuration on fire behavior. The decrease in the mass burnt, the total heat release, the combustion enthalpy and the maximum HRR due to a change in the ventilation flowrate and the ventilation configuration is satisfactorily correlated with this parameter. The corrective coefficient in the ventilation factor shows that ventilation of the fire room via an air intake in the upper part of the fire room is 35% less efficient, for this configuration, than a doorway facing the fire and blowing air into fire room.

The reduction in the ventilation flow rate, together with the change in the ventilation configuration, also modify the oxygen concentration field in the vicinity of the fire. The decrease in oxygen concentration is correlated with a change in flame spread velocity and, consequently, the fire HRR. The influence of the ventilation flow rate and ventilation configuration on the fire HRR is mostly the result of the influence of oxygen concentration on flame spread velocities.

The decrease in the fire HRR because of the reduction in the ventilation flow rate and the change in the ventilation configuration is a consequence of the reduction in the flame spread velocities on each tray. For this configuration, the trays mostly affected by a slowdown in flame propagation are the upper trays, which are rapidly engulfed by a smoke layer and a vitiated environment. The consequence is a delay in the burning of the upper trays and then a reduction in the maximum HRR and an increase in fire duration.

The new tests provide additional information to improve our understanding of low frequency oscillatory behavior of a fire in a ventilated compartment. The occurrence of oscillations seems likely to be a combination of two features: the production of unburnt gases induced by local extinction and therefore by low ventilation flow rate, and the specific position of the air intake with respect to the fire source, which promotes the formation of premixed flammable gases. In addition, comparative analysis with other literature results shows that the period of oscillation is probably linked to the enclosure dimensions.

Fig. 1 .

 1 Fig. 1. Illustration of the facility showing ventilation layout A (left), and the two ventilation layouts A and B (right)

Fig. 2 .

 2 Fig. 2. Pictures of the cable section (a), a side view of the fire source (b) and the fire room (c)

Fig. 4 .

 4 Fig. 4. Time variation of the pressure and the flow rate in the ventilation lines (top) and relation P=f(Q) for the three ventilation flow rates (bottom).

Fig. 5 .

 5 Fig. 5. Time variation of the fire HRR for the two ventilation configurations A (left) and B (right) and for different ventilation flow rates

Fig. 7 .

 7 Fig. 7. Effect of ventilation flow rate and ventilation configuration on global quantities (mass burnt, maximum HRR, total heat released, fire duration and mean combustion enthalpy).

  e.: show a monotonous trend) with the ventilation factor. The results are presented in Fig. 8. The best results are obtained for k = 0.35 for configuration A and k = 1 for configuration B. The ratio 1/0.35 means that configuration B supplies to the fire source only 35% of the air supplied in configuration A. For a given ventilation flow rate, configuration B is 35% less efficient than configuration A for supplying air to the fire source. The values of k are therefore probably correlated to the studied configurations. At this stage, a generalization of the value of k for other scenarios requires further results from either experiments or numerical simulations.

Fig. 8 .

 8 Fig. 8. Variation in dimensionless mass loss, total heat released, maximum HRR and combustion enthalpy versus the ventilation factor (in the dotted circle, the two tests that give similar results but with two different ventilation flow rates and air intake position: k = 0.35 for configuration A and k = 1 for configuration B)

Fig. 9 .

 9 Fig. 9. Oxygen concentration within the fire room: (a) time variation of oxygen concentration at three elevation (bottom z = 0.7 m, middle z = 2 m and top z = 3.3 m) (b) vertical oxygen concentration profile at the time of maximum HRR, indicated by the dashed line in plots (a), (c) Variation in the mean oxygen concentration (considering bottom and center elevation) versus the ventilation factor

Fig. 10 .

 10 Fig. 10. Dimensionless maximum HRR versus the corresponding oxygen concentration at the same time in the vicinity of the fire source (error bars stand for the deviation between the sensors located at two elevations, 0.7 m, bottom, and 2 m, center, on the mast NE); the dotted line stands for the correlation of the present data, the dash-line for the Peatross et al. correlation.

Fig. 11 .

 11 Fig. 11. Evolution of ignition time of the fifth tray versus the ventilation factor -Horizontal mean flame spread velocity versus the tray index for the six tests (center) -Evolution of the horizontal mean flame spread velocity of the lower and upper tray versus the ventilation factor (right)

Fig. 12 .

 12 Fig. 12. Comparison between the fire HRR and the periods of flame spread, full burning and extinction on each tray for the three tests PR3_CFP_D1, PR3_CFP_D2 and PR3_CFP_D3.

Fig. 13 .

 13 Fig. 13. Time variations in the room pressure and the concentration of total hydrocarbon for the five tests in a ventilated enclosure (for all plots, pressure is in Pa and concentration in ppm.10 3 ) -The two lower plots show a detailed view during the period of oscillatory behavior for the two tests with ventilation flow rate of 1010 m 3 /h for configuration A and 960 m 3 /h for configuration B.

Table 1 :

 1

	Test grid (* test presented in [13], "PR3" stands for PRISME3 project and "PR2" stands
	for PRISME2 project )
	Name	Targeted ventilation flow rate	Ventilation configuration
		(m 3 /h)
	PR3_CFP_D1	3660	A
	PR3_CFP_D2	1010	A
	PR2_CFS_3*	960	B
	PR3_CFP_D3	1410	B
	PR2_CFS_4*	3600	B

Table 2 .Table 2 :

 22 Measurement uncertainties ((abs) stands for absolute in SI unit and (rel) stands for relative in %).

	Physical variable	Range	Standard	Coverage	Expanded
			Uncertainty	Factor	Uncertainty
	Mass	(0 -600) kg	0.004 kg	2	0.008 kg (abs)
	Flame temperature	(0-1300) °K	8°K	2	16°K (abs)
	O2 molar fraction	(0 -25) %	0.2 %	2	0.4 % (abs)
	HCT molar fraction	(0 -100000) ppm	1000 ppm	2	2000 ppm (abs)
	Pressure	(0 -1300) °K	2°K	2	4°K (abs)
	Flow rate	-	-	-	7% (rel)
	Flame spread velocity	-	-	-	20% (rel)
	MLR	-	-	-	5% (rel)
	HRR	-	-	-	10% (rel)

Table 3 :

 3 Main results related to combustion

		PR2_CFSS_2 PR3_CFP_D2 PR3_CFP_D1 PR2_CFS_3 PR3_CFP_D3 PR2_CFS_4
	𝑞̇𝑣 𝑖𝑛𝑖𝑡 (m 3 /h)	-	1010	3660	960	1410	3600
	Config.	open	A	A	B	B	B
	minit (kg)	227.4	267.2	268.8	252.1	263.0	234,0
	m (kg)	105.4	94.9	115.8	72.8	83.4	101.0
	t (s)	5580	6985	6960	6030	6960	4980
	𝑄 ̇𝑚𝑎𝑥 (kW)	2031	924	1845	565	444	1252
	THR (MJ)	2809	2329	2896	1412	1719	2416
	H (MJ/kg)	26.7	24.5	25.0	19.4	20.6	23.9

Table 4 :

 4 Main results related to flame spread velocities

		PR2_CFSS_2 PR3_CFP_D2 PR3_CFP_D1 PR2_CFS_3 PR3_CFP_D3	PR2_CFS_4
	𝑞̇𝑣 𝑖𝑛𝑖𝑡 (m 3 /h)	-	1010	3660	960	1410	3600
	Config.	open	A	A	B	B	B
	tign (s) (5th tray)	600	1124	1034	2117	2187	616
	U (mm/s) Tray 1	0.8	1.3	1.1	2.4	1.2	0.5
	U (mm/s) Tray 2	1.1	0.8	1.5	2.6	1.3	5.7
	U (mm/s) Tray 3	2.7	2.4	2.3	3.8	4.3	1.4
	U (mm/s) Tray 4	3.0	3.5	3.2	0.2	1.2	6.2
	U (mm/s) Tray 5	6.4	0.5	3.1	1.3	0.8	9.6

Table 5 :

 5 Period of oscillation for three types of experiments

	Fire source	Fuel	Volume	Period	Reference
	Pool fire/D = [12 , 22] mm	Dodecane	1.75 m 3 (one room)	50	[30]
	Pool fire/D = [618 , 714] mm	Heptane/Dodecane	110 m 3 (one room)	[150-120]s	[31]
	Five horizontal trays	Well qualified cable	230 m 3 (two connected	[210-280]s	Present
			rooms)		and [14]

Acknowledgements

The authors would like to thank all the partners involved in the OECD PRISME 3 project for their financial support and the fruitful discussions throughout the project.