Amina Meslem
email: amina.meslem@univ-rennes1.fr

Pierre Bragança

Kodjovi Sodjavi

EXPERIMENTAL ANALYSIS OF MIXING VENTILATION EFFICIENCY USING A VORTEX DIFFUSER -COMPARISON TO A LOBED MULTI-CONE DIFFUSER

Keywords: Mixing ventilation, lobed diffuser, vortex diffuser, cold jet, PIV measurements, thermal comfort PRACTICAL

In a recent work, an innovative multi-cone diffuser integrating lobed inserts as vortices promoters was proposed, and was compared to its counterpart without lobed inserts. The former offers better mixing of the jet with the ambient air, and significantly improves thermal comfort.

In this study, the innovative diffuser is compared to a vortex diffuser under the same experimental conditions. The vortex diffuser is described in the scientific and technical A c c e p t e d M a n u s c r i p t literature as the best diffuser in terms of mixing efficiency due to the swirling effect, being thereby a reference for the innovative lobed diffuser.

The results demonstrate that the vortex diffuser does not produce the expected swirling effect. The rotation of the jet is prevented by the immediate attachment of the flow to the ceiling due to the Coanda effect. The jet behaves as a radial ceiling-attached flow having a poor mixing performance. It is believed that such a diffuser would be efficient when mounted on a long duct far away from the ceiling. Hence, it will be better suited for high ceilings, such as in industrial or commercial spaces. In this case, the proposed lobed diffuser becomes an appropriate and efficient mixing ventilation solution for rooms having low ceiling heights.

INTRODUCTION

Mixing ventilation of residential spaces, particularly in the cooling mode, suffers from a bad reputation among users who often complain of draft problem, thermal discomfort and sometimes of high noise level generated by the fan and the air distribution circuit. Thermal discomfort and draft are mainly due to the fact that it is difficult to master the trajectory of a A c c e p t e d M a n u s c r i p t cold jet and to control its mixing with the ambient air. The way in which the air introduced into the room circulates and reaches the occupied zone of the room, depends strongly on the geometry of the diffuser and its position in the room. In mixing ventilation, there are usually dead pockets in the occupied zone difficult to reach by fresh and clean air. The user would try unsuccessfully to resolve this problem by increasing the jet volumetric flow rate, which in contrary would generate draught issues.

Hence, the ventilation efficiency should be homogenous in the occupied zone and high induction performance of the ventilation jet is a solution to achieve this goal. High induction allows the mitigation of stagnation regions given the optimal mix of the air jet with indoor air.

As a result, the occupants would be more satisfied in terms of thermal comfort and air quality.

The improvement of thermal comfort by using lobed inserts [START_REF] Vialle | Diffusion device with lobe inserts and fan coil comprising such a device[END_REF] was demonstrated in our previous work [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF][START_REF] Bragança | Passive control strategy for mixing ventilation in heating mode using lobed inserts[END_REF], in cooling mode (radial jet generation) and heating mode (vertical jet generation), respectively. No additional energy cost and no noise level increase were observed. The concept of lobed inserts [START_REF] Vialle | Diffusion device with lobe inserts and fan coil comprising such a device[END_REF] consists in introducing vortices promoters in any type of commercial air diffuser without changes in its manufacturing process.

Contrary to the "built-in lobed diffuser", which we have considered previously [START_REF] Meslem | Passive mixing control for innovative air diffusion terminal devices for buildings[END_REF][START_REF] Nastase | Lobed grilles for high mixing ventilation -An experimental analysis in a full scale model room[END_REF], this concept allows the manufacturer to adopt the innovation without complicated and high-priced changes in the factory production line of the diffuser. For the purpose of the present study, the inserts were made of resin material and were built using a 3D printer. In the context of the industrial integration of the innovation in the multi-cone diffuser, two couples of inserts, one for each mode (cooling and heating), should be series-produced in parallel to the production line, and marketed with the diffuser as accessories.

The assembly work needed to install the lobed inserts into the diffuser is not difficult and can be done by the user itself, when two times a year, in summer and in winter, respectively, he must hand in to set the diffuser in the suitable mode.

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The large-scale vortices generated by the lobed diffuser [START_REF] Nastase | Primary and secondary vortical structures contribution in the entrainement of low Reynolds number jet flows[END_REF][START_REF] Meslem | Analysis of jet entrainment mechanism in the transitional regime by time-resolved PIV[END_REF][START_REF] Nastase | Vortex dynamics and mass entrainment in turbulent lobed jets with and without lobe deflection angles[END_REF] improve mixing of the ventilation jet with the ambient air [START_REF] Meslem | A comparison of three turbulence models for the prediction of parallel lobed jets in perforated panel optimization[END_REF][START_REF] Cao | A review of the performance of different ventilation and airflow distribution systems in buildings[END_REF]. It results in better distribution of the heating or cooling loads, reducing local discomforts related to dead pockets and draftiness due to high air velocities and/or to large temperature gradients in the occupied zone.

The proposed concept of lobed inserts [START_REF] Vialle | Diffusion device with lobe inserts and fan coil comprising such a device[END_REF] could be considered as a competitor to the vortex diffuser widely marketed and recognized nowadays for its high mixing efficiency. In fact, the vortex diffuser is described in the scientific [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF][START_REF] Awbi | Ventilation of Buildings[END_REF] and technical literature [START_REF] Schako | Swirl diffuser DHV -Technical documentation[END_REF] related with mixing ventilation, as the best air diffuser in terms of mixing efficiency. The vortex diffuser produces a spiral rotating (or swirling) jet, enhancing mixing between the jet and the ambient air. The swirling effect is ensured by a special design of the fixed swirl impellers arranged at the diffuser outlet. The claimed high mixing efficiency of the vortex diffuser [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF][START_REF] Awbi | Ventilation of Buildings[END_REF][START_REF] Schako | Swirl diffuser DHV -Technical documentation[END_REF] makes it a reference for the proposed innovative lobed diffuser [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF][START_REF] Bragança | Passive control strategy for mixing ventilation in heating mode using lobed inserts[END_REF]. It is worth noting that the efficiency of the vortex diffuser in the context of mixing ventilation of buildings, is not supported by detailed experimental investigations. Previous experimental studies on the vortex diffuser [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF] are two decades old, and were based on pointwise, intrusive and omnidirectional velocity sensors. Many engineering applications involve swirling jets, such as in combustion chambers, jet engines, turbo machinery, mixing tanks, mixing ventilation. The majority of published studies dealing with swirling jets are dedicated to the mixing process in combustion chambers, where swirl helps to improve fuel/air mixture and stabilize the flame.

The jet region of interest is limited to the vicinity of the diffuser outlet, and the results are obviously not extensible to the mixing ventilation application, due to the very different jet inlet conditions.

To our knowledge, there is no literature published to date on the behavior of the swirling jet in its far regions, which are so important in mixing ventilation. In addition, the influence of the potential increase in air induction using the vortex diffuser, on the flow behavior and the A c c e p t e d M a n u s c r i p t thermal comfort in the occupied zone is not discussed. In building ventilation application, previous experimental studies [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF] attempted to show in isothermal conditions, the higher efficiency of the vortex diffuser, relative to a multi-cone diffuser. Pointwise, intrusive and omnidirectional velocity sensors have been used along the trajectory of the ventilating jet, generated by a vortex diffuser and a reference multi-cone diffuser, respectively. Results analysis were based on the decreasing rate of maximum jet velocity max U , given by the

velocity decay coefficient v K (   X / A K U / U n v n max



, where n U and n A are jet inlet velocity and diffuser outlet area, respectively, and X is the axial distance from the diffuser outlet, which is directly related to the entrainment rate. The authors have found higher v K values for the vortex diffuser (2.2) compared to the multi-cone diffuser (1.9) and concluded that the jet produced by the vortex diffuser had a greater throw and therefore, a greater amount of ambient air was mixed with the jet .It seems that the very fact of a larger v K coefficient for a vortex diffuser, compared to the multi-cone diffuser, is contrary to a well-accepted principle in the literature [START_REF]Space air diffusion[END_REF], which states that the higher the entrainment rate of jet, the smaller the coefficient v K .This amazing result of [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF] deserves a clarification.

In the literature related to fluid mechanics aspects of swirling jets, the strength of the swirl is gauged by the swirl number S [START_REF] Gupta | Swirl flows[END_REF], defined as the ratio of the axial flux of angular momentum to the axial flux of the axial momentum. Chigier and Chervinsky [START_REF] Chigier | Experimental Investigation of Swirling Vortex Motion in Jets[END_REF] established correlations between the velocity decay rate of the swirling jet, its axial spreading and its entrainment rate, and the swirl number S. It was particularly shown that in the first interval of S ranging between 0 and 0.4, the increase in S lead to an increase in entrainment rate and a decrease in v K . Hence, the axial velocity component decreases faster, whereas the induced tangential and azimuthal velocity components increase. To put a link between swirling jets fundamental studies [START_REF] Gupta | Swirl flows[END_REF][START_REF] Chigier | Experimental Investigation of Swirling Vortex Motion in Jets[END_REF] and studies on their application in mixing ventilation [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF], the use of A c c e p t e d M a n u s c r i p t omnidirectional anemometer to establish the law of axial jet velocity decay, as in [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF],

obviously leads to an artefact: the measured velocity ( max

U

) is larger than in reality, which can lead to inconsistencies in flow analysis.

The experimental studies in references [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF] were cited or were used as a reference data in a number of numerical studies [START_REF] Aziz | Experimental and Numerical Study of A/C Outletsand Its Impact on Room Airflow Characteristics[END_REF][START_REF] Sajadi | Numerical investigation of the swirling air diffuser: Parametric study and optimization[END_REF][START_REF] Tavakoli | Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers[END_REF] dealing with CFD simulation of jets from ceiling vortex diffusers. The most recent numerical study [START_REF] Tavakoli | Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers[END_REF] focuses on the effect of the angle between the impellers and the vertical axis, on the jet behavior and on the resulting air quality in the room (qualified by the predicted total age of the air [START_REF] Li | Total air age: an extension of the air age concept[END_REF][START_REF] Gan | Effective depth of fresh air distribution in rooms with single-sided natural ventilation[END_REF]). The numerical results

show that when increasing the jet angle, the jet eventually attaches to the ceiling. The transition angle value, is between 50° (vertical jet) and 55° (radial jet). The radial jet configuration is better in terms of air quality in the room, but the increase in angle beyond 55° decreases jet induction and air quality.

The present study is dedicated for the first time to a direct comparison under a cooling mode, of a multi-cone ceiling diffuser equipped with lobed inserts to a ceiling vortex diffuser. An experimental investigation is made on the airflow pattern and the thermal comfort generated in a full scale model room equipped with a heated manikin simulating an occupant. The climatic chamber is used to simulate summer mode under steady state thermal conditions.

Having regard to the complexity of the swirling jet, velocity fields were acquired using 2D

Particle Image Velocimetry (PIV), which gives two velocity components in the observed plane, without influence of the out-of-plane third component. As for thermal dissatisfaction and draft effect, the analysis is based on traditional pointwise measuring probes and the standards [START_REF] Iso | Ergonomics of the thermal environment --Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria[END_REF] and ASHRAE 55 [START_REF]Thermal environmental conditions for human occupancy[END_REF].

EXPERIMENTAL APPARATUS

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Test chamber and diffusers

A cubic laboratory chamber of an edge length of 3470 mm has been used for the experiments (Figure 1 a). A dropped ceiling was installed at height 2500 mm to simulate a standard room with low ceiling height. The temperature of the ambient air, T a , measured at the extraction of the chamber, is controlled with a hydraulic circuit of capillary tubes inserted in the six walls, and connected to a heat pump. The air flow is conditioned with an air handling unit equipped with a fan, a heater and a chiller. The air flow is then brought to the chamber using an air diffusion circuit including at its extremity a plenum box followed by the diffuser. The diffuser centered on the ceiling is installed at the extremity of the plenum box. The plenum box includes a divergent, a short duct equipped with a perforated plate normal to the duct, a convergent, and again a short duct equipped with a honeycomb to ensure at the outlet of the terminal duct without diffuser, a top-hat mean velocity profile and a very weak turbulence intensity of 2% [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF].

A simplified seated manikin (HM) of 8 heated parts and a total power of 81W [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF][START_REF] Dogeanu | Conception of a simplified seated thermal manikin for CFD validation purposes[END_REF] was used to simulate an occupant located at the center of the room (Figure 1 b). The manikin was designed to have a mean skin temperature of 32 °C in a room with ambient temperature at 26°C and with still air.

The initial flowrate, Q 0 =200 ±6 m 3 /h and the initial jet temperature T 0 (Table 1) are controlled with sensors placed between the air handling unit and the plenum box and again measured at the jet exit using a balometer from ACIN (Flowfinder mk2). The accuracy in flow rate measurement is ±3% of the reading.

Figure 1 (c andd) shows the two air diffusers compared in this study under cooling conditions (Table 1). The first diffuser LD (Figure 1 c) is a multi-cone diffuser marketed by the ALDES company under the reference AF842 [START_REF] Aldes | Documentation technique diffuseur série AF/AT 842[END_REF], in which we have introduced lobed inserts, to enhance the jet mixing with the ambient air. The reader will find in reference [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF] the hence, according to the numerical prediction of [START_REF] Tavakoli | Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers[END_REF], the jet should be vertical.

The manufacturer's technical documentation specifies that the vortex diffuser DHV is designed especially for cooling and isothermal supply air pattern and is suitable to be flushmounted in ceiling up to 4 m high, without causing draft in the occupied zone. It is also specified that the supply air from the DHV diffuser enters the room horizontally and the increased induction allowed by the impellers causes the temperature difference and velocity to be rapidly reduced.

To verify the manufacturer's specifications of the ceiling swirl diffuser (Figure 1 d), and to compare its efficiency to the one of the lobed multi-cone diffuser (Figure 1 c), the jet flow field (using 2D PIV, see the subsection §2.1) and the resulting thermal comfort in the occupied zone (Figure 1 a, b) were characterized in cooling mode with a difference between the ambient air temperature (T a ) and the jet temperature (T 0 ) of about 10 °C (see Table 1).

Thermal comfort analysis is based on the method of Fanger [START_REF] Fanger | Thermal Comfort, Analysis and Applications in Environmental Engineering[END_REF] proposed specially for nonuniform environments. A set of thermocouples (type K, accuracy of ±0. floor. These positions are those of ankles, waist, head of a seated occupant and head of a standing occupant, respectively [START_REF]Thermal environmental conditions for human occupancy[END_REF][START_REF] Fanger | Thermal Comfort, Analysis and Applications in Environmental Engineering[END_REF]. To cover the whole occupied zone, the 16 canes were installed in the chamber following the mesh given in (Figure1 b). Mean values ( air T and air V ) of the 64 measurement points are given in Table 1.

Wall-surface temperature (T p ) measurements were also performed. Each wall including the floor and the ceiling is divided into 4 subzones of equal dimensions. Each one is equipped with a thermocouple located in its center. Each wall surface has almost uniform temperature with a maximum difference of 1.7 °C between the 4 corresponding subzones. For the entire test chamber (i.e. 24 measurement points), the maximum difference is 3.3 °C; the corresponding mean value p T is given in Table 1. For mean radiant temperature (T mrt ) calculation, the method described in [START_REF] Fanger | Thermal Comfort, Analysis and Applications in Environmental Engineering[END_REF] and in [START_REF] Iso | Ergonomics of the thermal environment --Instruments for measuring physical quantities[END_REF] based on wall-surface temperatures of the 6 surfaces and a seated manikin with unknown azimuthal angle is applied.

For each experiment, the steady-state conditions was achieved after a period of about 6 h.

After this period, environmental parameter measurements were carried out for a minimum duration of 8 h with a sampling time of 1 min. The corresponding 480 samples are used to calculate statistical mean values given in Table 1. Personal parameters such as metabolic rate, mechanical work and clothing insulation, were fixed at 1.2 met, 0 W/m 2 and 0.7 clo respectively, to evaluate the thermal comfort of a seated occupant, wearing a working clothing ensemble and having a sedentary activity. Calculations of thermal comfort indices were performed with a home-made software and the obtained PMV-PPD values were verified using the ASHRAE 55 Comfort tool [START_REF]Thermal environmental conditions for human occupancy[END_REF][START_REF]Thermal comfort tool[END_REF].

The pressure loss was measured according to standard [START_REF] Afnor | Ventilation des bâtiments -Bouches d'air -Essais aérodynamiques et caractérisation pour applications en diffusion à mélange[END_REF] using a KIMO MP110 micromanometer. One pressure tap is installed at 1.5 D (where D=160 mm is the diffuser neck-diameter) upstream the diffuser and the second in the occupied zone outside the jet flow. According to ISO 10052 [START_REF] Iso | Acoustique --Mesurages in situ de l'isolement aux bruits aériens et de la transmission des bruits de choc ainsi que du bruit des équipements --Méthode de contrôle[END_REF], the diffuser should be considered as a technical equipment.

Hence, the time average equivalent sound pressure level with A-weighting, Leq (dBA), must

be measured with a recommended averaging time of 6 s [START_REF]Ministère de l'Écologie du Développement durable et de l'Énergie Ministère du Logement et de l'Égalité des territoires[END_REF]. Following standards specifications, Leq (dBA) was recorded at two locations in the chamber, P1 and P2 shown in Figure 1 a. These positions are corresponding [START_REF] Iso | Acoustique --Mesurages in situ de l'isolement aux bruits aériens et de la transmission des bruits de choc ainsi que du bruit des équipements --Méthode de contrôle[END_REF] respectively to the nearest position to the diffuser located in the occupied zone, and the limit of the occupied zone in the case where people are seated.

PIV measurements

The airflows in indoor spaces of buildings are usually complex and difficult to characterize. Nowadays with current advanced laser and camera technologies, PIV technique becomes a powerful method getting more and more used to study room airflow distributions [START_REF] Cao | A review of the performance of different ventilation and airflow distribution systems in buildings[END_REF][START_REF] Cao | Particle image velocimetry (PIV) application in the measurement of indoor air distribution by an active chilled beam[END_REF][START_REF] Cao | PIV measurement of the attached plane jet velocity field at a high turbulence intensity level in a room[END_REF]. In fact, this method offers a high spatial resolution compared to traditional pointwise measurements techniques, making easier the analysis of large scale flows.

In the present study, the jet flow from the diffuser is characterized using a PIV measurement campaign conducted using a 2D2C Dantec Dynamics PIV system of 2 Hz acquisition rate. It is composed of a DantecHiSense 11M CCD camera with a sensor of 4000×2672 pixels, and a dual-cavity 2×200 mJ laser. The airflow is seeded with Helium Filled Soap Bubbles (HFSB).

Unlike the traditional aerosol-based tracer, this enables to increase the field size without peak locking effect [START_REF] Raffel | Particle Image Velocimetry A Practical Guide[END_REF].

The HFSB generation uses a commercial generator of Sage Action (model 5 -modified, Beyond the global PIV fields obtained using HFSB (Figure 4 a, b), the analysis of the wall jet generated by each diffuser requires a high spatial resolution. Hence, an aerosol-based PIV measurement campaign has also been conducted. In this case, the same PIV system is used along with olive oil droplets generated by a laskin generator. A total of 4 PIV windows of 374×139 mm each with a vector spacing of 1.5× 1.5 mm were assembled to construct the ceiling jet field of 1274×139 mm in size. For each PIV window, 1000 pairs of images were acquired and processed through an adaptive multi-grid correlation algorithm handling the subpixel window displacement. The final size of the interrogation window was 64×64 pixels and 32×32 pixels, respectively, with an overlap of 50%.

The lobed multi-cone diffuser LD (Figure 1a) generates a conical ceiling attached jet. The flow is axisymmetric and therefore does not require any special precaution when vertical cross section is made. Conversely, the vortex diffuser equipped with impellers (Figure 1

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Hence, before choosing vertical cross section of the jet coming from VD, horizontal crosssections at different heights Z ranging from 25 mm to 80 mm have been performed (Figure 4 c) using olive oil droplets-based PIV system. The fields are 708×466 mm in size with a vector spacing of 5.7× 5.7 mm. A number of 1000 pairs of images were acquired and processed following the same procedure as previously for the wall jet PIV fields. The resulting fields (Figure 4 c) show on the one hand, that there is no swirling effect around Z, and indicate on the other hand, the persistence of a multi-jet behavior. Thus, a vertical section along one elementary jet (dashed line in Figure 3b) will be sufficient to describe the dynamics of the VD flow.

RESULTS AND DISCUSSION

Air flow analysis

Velocity distributions in the half-middle cross section of the room are given in Figure 4 PIV measurements [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF]. In this figure the reference length, n A , used for axis normalization is the square root of the neck-area (A n = πD 2 /4). The corresponding neck velocity W n =Q 0 /A n , is used as the reference velocity. The typical profile of the radial velocity component, U, of the wall jet, exhibits a maximum value, U max , and the axial position Z where U=0.5 ×U max is denoted Z 0.5 .

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As shown in our previous work [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF], the jet from the LD it ceiling-attached and spreads radially till ceiling/wall corner. At the corner, the jet separates from the ceiling, impinges on the opposite wall, attaches to it, and spreads downwards as a vertical attached wall jet. It has been shown in [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF] that the flow above the manikin has similar velocity profiles than those of a plume issued from a humanlike seated thermal manikin [START_REF] Borges | Analysis of the interaction of thermal plumes within office environment using a thermal manikin[END_REF]. The jet from the vortex diffuser VD exhibits a similar behavior, but with higher velocities (Figure 5 On the decrease of the maximum velocity (Figure 5 a), the change of the curve slope at the distance X √A n ⁄  5 is the trace of the jet attachment to the ceiling, and the transition from free jet velocity profiles to wall jet velocity profiles (Figure 6). In fact, upstream X = 900 mm (X √A n ⁄ = 6.3) the jet is free; downstream this distance the jet becomes ceiling-attached.

Beginning with X = 900 mm, normalized velocity profiles are in a good agreement in the inner region Z/Z 0.5 < 1 (Figure 7) with similarity laws of two-dimensional wall-jets given in references [START_REF] Rajaratnam | Turbulent jets. 1976[END_REF][START_REF] Verhoff | The two-dimensional turbulent wall jet with and without an external stream[END_REF]. The outer region (Z/Z 0.5 < 1) bears the trace of impellers leading to velocity values lower than reference laws. Nevertheless, the data is getting closer to the reference laws as X increases, indicating that the imprint of inlet conditions has a tendency to disappear in the far field.

Regarding the comparison of LD and VD, recall that the initial conditions (Table 1) and the volumetric flowrate (Q 0 =200 ±6 m 3 /h) were the same for the two diffusers. The higher velocities observed in VD case are due to the multi-jet configuration (Figure 4 c) resulting from impellers imprint, which seems persistent since the jets do not merge over the observation distance Z= 80 mm. The vortex jet is here radial contrary to the numerical predictions in [START_REF] Tavakoli | Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers[END_REF], where the simulated vortex jet would be vertical for impellers angle of 45°. Furthermore, the jet rotation seems to be stopped by the Coanda effect.

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To confirm our observations, an elementary jet is considered (Figure 8 a-b), from which tangential velocity component (V in Figure 8b) and radial velocity component (U in Figure 8b) are extracted. It could be observed in Figure 8 f, that the position of U max do not change anymore beginning with X=120 mm. Thus, we confirm here that jet attachment to the ceiling (Figure 4 b) clearly stopped the swirling effect claimed by the manufacturer [START_REF] Schako | Swirl diffuser DHV -Technical documentation[END_REF]. It is worth noting that jet attachment to the ceiling was also observed in isothermal conditions with Q 0 = 200 m 3 /h. This is not surprising since at high Reynolds number Re (here Re = 26 000, see Table 1) and low

Archimedes number Ar (here Ar = 0.006 and Ar = 0, respectively), the jet near field is dominated by the inertia force. It is believed that at low volumetric flow rate and large temperature difference (i.e. low Re and high Ar), the jet would no longer be attached to the ceiling. In this case, the buoyancy force dominate the jet behavior and opposes the Coanda effect. Additional tests should be conducted to find the corresponding initial critical conditions.

Anyway, the result of the present study is the need for a revision (by the manufacturer) of the method of mounting the vortex diffuser so that the swirling effect can actually occur. In our opinion, it will be more suitable in mounting the vortex diffuser on a long duct far away from the ceiling, so that the Coanda effect would not occur. Hence, VD will be better suited for high ceilings, such as in industrial or commercial spaces. In this case, the multi-cone diffuser flush-mounted in the ceiling and equipped with lobed inserts [START_REF] Vialle | Diffusion device with lobe inserts and fan coil comprising such a device[END_REF] becomes an appropriate and efficient mixing ventilation solution for rooms having low ceiling height. The lobed inserts A c c e p t e d M a n u s c r i p t will be useful in this case to improve the thermal comfort in the occupied zone with respect to the same diffuser without inserts [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF].

Additional small scale aerosol-based PIV measurements [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF] were performed for the vortex diffuser in isothermal conditions at Q 0 =200 ±6 m 3 /h (Figure 9), with the objective to compare (Figure 10) the corresponding velocity decay of VD to the literature [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF][START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF]. The comparison reveals a very good agreement except near the jet exit for X √A n ⁄ <4. In this region, our data falls between those of Chuah et al. [START_REF] Chuah | Airflow Characteristics of Circular Ceiling Diffusers[END_REF] and Shakerin and Miller [START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF].

The v K -value of the VD is given in Table 2. In this Table, the v K -values previously obtained [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF] for the conventional multi-cone diffuser without lobed inserts (CD) and with lobed inserts (LD), respectively, are included for comparison.

For the VD, the obtained v K -value (1.9) is about 16% lower than the one (2.2) obtained twenty years ago by Shakerin and Miller [START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF] by using omnidirectional anemometer.

Taking into account that the v K -value is extremely sensitive to the fitting region, the gap could be considered low. Furthermore, in the author's study the vortex diffuser was as in our case flush-mounted in the ceiling, which leads to say that the flow was probably attached and the swirling effect inhibited. Consequently, tangential and normal components vanished following the jet attachment, leading to a relatively good approximation of U max by the omnidirectional sensor.

For the conventional multi-cone diffuser (CD), the obtained value (1.6) is about 19% lower than the one obtained by the authors [START_REF] Shakerin | Experimental study of Vortex Diffusers[END_REF]. The tendency for the two diffusers is the same, and differences are principally due to the different measurement techniques.

As for the lobed multi-cone diffuser (LD), its coefficient is 38% lower than that of our vortex diffuser (VD), due to higher mixing efficiency with the former.

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Despite the observations made above on the behavior of the jet issued from the vortex diffuser VD flush-mounted in the ceiling, which is not in line with expectations, we wanted to enrich the comparison of the two diffusers LD and VD, by addressing in the next section the issue of thermal comfort conditions generated in the occupied zone.

Thermal comfort analysis

The assessment of global thermal comfort is based on the PMV/PPD indices [START_REF] Fanger | Thermal Comfort, Analysis and Applications in Environmental Engineering[END_REF] considering a seated occupant, wearing a working clothing ensemble and having a sedentary activity. The corresponding personal parameters are 1.2 met for metabolic rate, 0 W/m 2 for mechanical work and 0.7 clo for clothing insulation. The environmental parameters (T air , V air , RH and T mrt ) have been measured in the 64 nodes of the occupied zone (Figure 1 b).

Following the procedure described in [START_REF] Fanger | Thermal Comfort, Analysis and Applications in Environmental Engineering[END_REF] for non-uniform thermal environment, spatial distributions of the 64 nodes in terms of thermal comfort levels (A, B, C) provided in ISO 7730 standard [START_REF] Iso | Ergonomics of the thermal environment --Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria[END_REF] were considered (Figure 11) for VD and LD, respectively. The calculations of thermal comfort indices were performed with a home-made software and the obtained PMV-PPD values were verified using the ASHRAE 55 Comfort tool [START_REF]Thermal comfort tool[END_REF]. The corresponding statistic distributions and mean values are given in Table 3 and Table 4, respectively.

While on average (Table 3) the results indicate that the two diffusers generate similar levels of thermal comfort, with a slight advantage for the LD, this is not the case when the 64 nodes are statistically analyzed (Table 4). One can observe that all the 64 nodes fall into the category A for LD, compared to 77% for VD (Table 4 and Figure 11). The VD case exhibits (Figure 11) higher air speeds (L) and lower air temperatures (T), leading to negative mean PMV-value (Table 3) corresponding to a slightly cold sensation.

A c c e p t e d M a n u s c r i p t

Acoustic and pressure loss data analysis

Figure 12 a1-a2, presents at positions P1 and P2 (Figure 1 b), global sound pressure levels as a function of the initial volumetric flow rate Q 0 . These global sound pressure levels reflect in the same time the possible noise generation of the diffuser and of the air handling unit and air distribution ducts. The air diffusion system is equipped with a plenum box, on which the diffuser is mounted. Two cases were considered: in the first one the measurements were performed without any diffuser mounted on the plenum, and the other case corresponds to the operation of the system with VD. The sound pressure levels of the VD itself were extracted (Figure 12b) and compared to the multi-cone diffusers (CD and LD) data from [START_REF] Bragança | Airflow characteristics and thermal comfort generated by a multicone ceiling diffuser with and without inserted lobes[END_REF]. As recommended by ISO 10052 standard [START_REF] Iso | Acoustique --Mesurages in situ de l'isolement aux bruits aériens et de la transmission des bruits de choc ainsi que du bruit des équipements --Méthode de contrôle[END_REF], mean values of pressure levels were calculated as representative values of global noise generated in the occupied zone. The mean value is an average weighted by the coefficients 2/3 and 1/3 of sound pressure levels in P1 and P2, respectively. Figure 12 c shows the total pressure losses for VD, CD and LD, measured following EN 12238 standard [START_REF] Afnor | Ventilation des bâtiments -Bouches d'air -Essais aérodynamiques et caractérisation pour applications en diffusion à mélange[END_REF], as a function of the inlet volumetric flow rate Q 0 .

The difference in pressure loss generation (Figure 12 c) between VD and LD increases with Q 0 . For Q 0 = 200 m 3 /h, the pressure loss of VD (47 Pa) is almost eight times higher than the one generated by LD (6 Pa). As for the sound pressure levels related to the diffuser noise generation (Figure 12 b), a very important difference, from 7 to 10 dBA, is recorded with an advantage for LD.

CONCLUSIONS

An innovative ceiling multi-cone diffuser equipped with lobed inserts (LD), which act as vortices promoters for the ventilation jet, has been confronted to a vortex diffuser (VD), which is specially manufactured for efficient mixing ventilation of buildings. The vortex The study shows that the swirling effect vanishes rapidly near the diffuser exit due to the Coanda effect and the subsequent jet attachment to the ceiling. The jet behaves as a radial ceiling attached flow having a poor mixing performance.

Thermal comfort is less satisfactory for the vortex diffuser VD than in the case of the lobed multi-cone diffuser LD. For pressure drop and sound pressure level, VD is significantly less efficient than LD. For Q 0 = 200 m 3 /h, the pressure loss of VD is almost eight times higher than the one generated by LD, with a 7 dBA increase in sound pressure level in the occupied zone.

Based on this study, it is believed that the VD will really generate a swirling jet if mounted on a long duct far away from the ceiling. Hence, it will be better suited for high ceilings, such as in industrial or commercial spaces. In this case, the proposed lobed diffuser LD becomes an appropriate and efficient mixing ventilation solution for rooms having low ceiling heights.

A c c e p t e d M a n u s c r i p t A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u

  s c r i p t geometrical details of this diffuser and of the introduced lobed inserts. The advantage of the multi-cone diffuser with lobed inserts has been demonstrated in cooling mode with respect to its counterpart without inserts. The multi-cone lobed diffuser LD is compared in the present study to the ceiling vortex diffuser VD (Figure1 d), presently distributed by SCHAKO under the reference DHV -NW 160[START_REF] Schako | Swirl diffuser DHV -Technical documentation[END_REF]. About that special diffuser, the SCHAKO Company advances the commercial argument of the high induction of ambient air in the jet ensured by the special design of the fixed swirl impellers. The impellers have an inclination angle of 45°,

  3 °C) and hot-sphere anemometers (TSI 8475, accuracy of ±3% of the reading) was used for environmental parameters (air temperature and air speed) measurements. The sensors were placed on 16 vertical canes (Figure1 b). Each cane bears 4 sensors at the vertical positions specified in ASHRAE 55-2013 standard [24], i.e., 0.1 m, 0.6 m,1.2 m and 1.8 m relatively to the room A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

  Sound pressure levels were measured with and without the diffusers using a handheld type 2250 Sound Level Meter of Class 1 Precision (tolerance of ±0.7 dB) from Bruel & Kjaer.

Figure 2a )

 2a Figure2a) equipped with two injectors. The soap is produced by Sage Action (reference SAI

  b) generates a discontinuous radial flow (Figure 3 b) with a potential rotating effect around the Z-axis. If not well chosen, a vertical 2D view of the flow can lead to misinterpretations.

Figure 5

 5 Figure 5 and are extracted from highly resolved fields issued from small scale aerosol-based

  a), a larger expansion (Figures 5 b) and a suction zone that interacts with the thermal plume of the manikin.

Figure 8 c

 8 gives a schematic of the vortex diffuser with expected flow trajectory in the horizontal plan due to impellers orientation. A zoom on the elementary jet field considered is provided in Figure8 d. In Figure8e, it is observed that near the diffuser outlet, the tangential component V reaches in absolute value 15% of the maximum velocity U max of the radial component U (Figure8 f), but falls down and approaches zero very quickly.

A c c e p t e d M a n u

  s c r i p t diffuser is equipped with fixed swirl impellers arranged at the diffuser outlet to ensure flow rotation around the diffuser axis.

A c c e

  p t e d M a n u s c r i p t

Figure 1 :Figure 2 :Figure 3 :Figure 4 :Figure 5 :Figure 6 :Figure 7 :Figure 8 :Figure 9 :Figure 10 :

 12345678910 Figure 1: (a) Sketch of the climatic chamber -1 perforated plate, 2 honeycomb, 3 diffuser, 4 dropped ceiling, P1 and P2 acoustic measurement points; (b) positions in the horizontal plane of the room of the heated manikin (HM), and of 16 verticals canes of hot-sphere anemometers and thermocouples; (c) multi-cone diffuser AF842 from ALDES [26] equipped with lobed inserts [2]; (d) swirl diffuser DHV from SCHAKO [14] Figure 2: (a) Sage Action HSFB generator with the corresponding two injectors; (b-c) Designed injector to increase HSFB flux: (b) photo of the injector, (c) Computer aided design drawing -sectional view

Figure 11 :Figure 12 :

 1112 Figure 11: Distributions of PPD, air velocity and temperature in the occupied zone: vortex diffuser VD in cooling mode (a) and lobed multi-cone diffuser LD in cooling mode (b)

Table 1 :

 1 Experimental conditions

			Diffuser					LD_CJ	VD_CJ
			T 0 (°C)					13.4±0.2	13.5±0.2
			T a (°C)					23.8±0.4	23.2±0.4
	Re			W	n		A	n	/			26 000	26 000
	Ar		g		A	n		T	/	T	a	W	2 n	0.0064	0.0059
			mrt T (°C)				24.6±0.7	24.0±0.7
			p T (°C)					25.0±0.7	24.3±0.7
			T op (°C)					24.1±0.5	23.0±0.5
			T	air	(°C)				22.4 ± 0.4	22.0± 0.2
			V	air	(m/s)				0.070± 0.017	0.091± 0.023
	V airmin /V airmax (m/s)	0.05/0.13	0.02/0.27
			RH (%)					42±2	42±2
			Q t (W/m²)				58.0	54.0

LD_CJ: Lobed Diffuser_Cold Jet; VD_CJ: Vortex Diffuser_Cold Jet

Table 2 :

 2 Comparison of v K -values obtained in isothermal conditions

	Diffuser	Case	K	v	Fitting region
	Multi-cone	CD IJ	1.62	⁄ 3 < X √A n 400 < X < 1300 < 9
	Multi-cone	LD IJ	1.19	⁄ 3 < X √A n 400 < X < 1300 < 9
	Vortex	VD IJ	1.93	6.3 < X √A n ⁄ 900 < X < 1450 < 10.2

IJ: isothermal jet -CD: conventional diffuser -LD: lobed diffuser

Table 3 :

 3 Mean values of PMV and PPD indices

	LD	VD

Table 4 :

 4 Statistic distribution of PPD index for the two diffusers

	Cat. ISO 7730	Conditions	LD	VD
	A	PPD < 6%	100	77
	B	6 < PPD < 10%	0	23
	C	10 < PPD < 15%	0	0
	Out	PPD >15%	0	0


P Le Maout

P S Laquintinie

C Lahuec
email: cyril.lahuec@imt-atlantique.fr

F Seguin

J-L Wojkiewicz
email: jean-luc.wojkiewicz@imt-lille-douai.fr

N Redon

L Dupont

A low cost, handheld E-nose for renal diseases early diagnosis

In the last decade, the use of electronic olfaction systems for the early diagnosis of several pathologies by breath analysis has been investigated. In this study, an electronic nose including seven polyaniline sensors has been developed. An impedance measurement circuit and a micro-computer to process the sensor responses were studied to give a prediagnosis conclusion. The measurement accuracy is 97% when it is exposed to a simulated human breath and different concentration of ammonia, from 500 ppb to 2.8 ppm. The described prototype weights about 300 g and can be used for 14 hours with a smartphone battery.

I. INTRODUCTION

With 10% of the global population affected by chronic kidney disease [START_REF] Dupont | facts about kidney disease[END_REF], the development of a non-invasive, easy to use and cost-effective solution to diagnose kidney pathologies is of utmost importance. In order to check patients for kidney diseases two tests are currently in use: a blood and urine tests. The first one assesses the glomerular filtration rate while the second one the abnormal presence of proteins. Both of these procedures are simple and cost-effective but require specific operator and laboratory. Moreover, blood tests are invasive. Studies have shown that patients with renal diseases have higher amount of ammonia in breath than healthy ones [START_REF] Davies | Quantitative analysis of ammonia on the breath of patients in end-stage renal failure[END_REF]. Analysing breath composition could lead to simple diagnosis tools to diagnose kidney diseases. As human breath is a complex mixture of gases and volatile compounds. It has been shown that combining cross sensitive sensors with classification algorithms is a very effective method to associate a pattern response of sensor to a disease [START_REF] Scott | Data analysis for electronic nose systems[END_REF]. Such systems are referred to as electronic noses (E-nose). They are composed of:

1) an array of sensors chosen for their cross-sensitivity to chemical compounds of interest, 2) a signal processing stage to extract relevant features from the sensors signals, 3) a classification algorithm to associate the sensor array responses to an healthy or ill breath. Different electronic noses already exist. However, they are usually not fully handheld systems as they require a personal computer type system to exploit the measurements [START_REF] Chiu | Towards a fully integrated electronic nose SoC[END_REF]. Another drawback is their cost, about few thousands of dollars. See for instance the Cyranose 320 R [START_REF]Sensigent company[END_REF]. This paper presents a low cost efficient handheld E-nose for early diagnosis of renal diseases. The simulated human breath which is used to test the system is described in section II. Section III explains the choices made to design the proposed E-nose. Section IV and V describe the experimentation and its results respectively. Section VI concludes the paper.

II. HUMAN BREATH

Human breath contains hundreds of compounds. Concentration of compounds ranges from few ppt up to thousands of ppb [START_REF] Phillips | Variation in volatile organic compounds in the breath of normal humans[END_REF]. The variability of the composition between individuals is important, and depend on many factors as age, gender or health condition. A standard composition of a simulated human breath has been made, based on mean concentration given in [START_REF] Phillips | Variation in volatile organic compounds in the breath of normal humans[END_REF] [START_REF] Turner | A longitudinal study of methanol in the exhaled breath of 30 healthy volunteers using selected ion flow tube mass spectrometry[END_REF]. Its composition is : 800 ppb of ammonia (±10%), 500 ppb of acetone (±0.5%), 500 ppb of methanol (±1%), 200 ppb of ethanol (±0.5%), 100 ppb of isoprene (±5%) and 20 ppb of acetaldehyde (±5%). Relative humidity during injection increases to 70 %. Carbon dioxide is not added because it does not react with the formulated sensors.

III. PROPOSED ELECTRONIC NOSE SYSTEM

The proposed electronic nose is based on a single-board computer, a Raspberry Pi. Its low cost, size, computational power and various of available interfaces (I2C, GPIO, SPI) made it a versatile and powerful control unit for the electronic nose. Both the measurement Front End and Raspberry Pi have the size of a credit card as shown in Figure 2. The developped sensor array is 2.5 × 0.5 × 0.1cm 3 . Packaging the complete system costs about 50$, and weights 300 g. Therefore, a handheld E-nose is then possible.

A. Custom Ammonia Sensors

A good candidate to detect the target compound, ammonia, in the ppb range, is the polyaniline (PANI). It is an intrinsically conductive polymer. Moreover, PANI is a low cost material. When in presence of ammonia, the equivalent electrical impedance varies in time. The electrical model of a polyaniline sensor is a resistance and a capacitive element in parallel, Figure 1a [START_REF] Vaghela | Biopolymer-Polyaniline Composite for a Wide Range Ammonia Gas Sensor[END_REF]. By changing the structure of the polymer matrix, and surface/volume ratio between formulations, different impedance variations, i.e. differentiated responses, are observed. The sensors were manufactured by drop coating a polyaniline solution on gold interdigitated electrodes deposited on an ceramic substrate, Figure 1b. The worst case sensors sensitivity is in the order of 5% impedance variation per ppm when equilibrium is reached. Seven PANI sensors with different formulation are used [START_REF] Merian | Ultra sensitive ammonia sensors based on microwave synthesized nanofibrillar polyanilines[END_REF][10], which is enough for correct olfactive pattern recognition [START_REF] Scott | Data analysis for electronic nose systems[END_REF].

B. Front End: Impedance measurement

Since the impedance of the sensors is complex, it is chosen to use the AD5933 impedance measurement integrated circuit from Analog Device. The circuit has built-in calibrations for impedance measurements and temperature. Moreover, the AD5933 has an I2C port for communication with Raspberry Pi. The impedance to measure is excited with a sinusoidal voltage signal at 1 kHz and 30 kHz. The sensor response signal is then sampled at a rate of 2 Hz (controlled by the Raspberry) for computing the real and imaginary parts of the impedance. These values are then communicated to the Raspeberry via the I2C port. The measurement accuracy of the AD5933 is 0.5%. It yields an error of concentration measurement of 100 ppb, in line with the sensors sensitivity. To measure the impedance of the seven sensors, a 8:1 analog multiplexer (ADG708) is inserted between the AD5933 and the sensor array. The 8 th multiplexer input is used for impedance calibration purposes.

C. Feature extraction

Sensors responses processing is done on the Raspberry Pi. The first step consists in filtering the curves with a moving average operation to reduce the noise and remove any incoherent measures. The resistance is then calculated from the real and imaginary values provided by the measurement front-end circuit:

R = Re 2 (Z) + Im 2 (Z) Re(Z) (1)
where Re(Z) and Im(Z) are the real and the imaginary parts of the impedance Z, respectively. Computed R values are stored in memory before features are extracted from the memorized points. Typically, only the maximum variation is extracted from the curve, Figure 3. However, it has been shown that transient features such as integrals and/or derivative could also help increase the classification performance [START_REF] Yan | Feature selection and analysis on correlated gas sensor data with recursive feature elimination[END_REF]. The set of features extrated is thus:

• the maximum derivative of the signal: δ R ,

• the integral value: R, • the maximum variation: ∆ R . For each sensor, features are extracted during both the adsorption and desorption phases, Figure 3, yielding a total of 42 features (six extracted features for the seven sensors).

D. Classification stage

Typically, the classification is performed in two steps. First, the set of extracted features is transformed into a reduced set of dimensions by means of a specific algorithm. Second, the actual classification algorithm works on the reduced set of dimensions. The classification process required Several combinations are possible. Since predicting which one yields the best results is impossible, testing is necessary [START_REF] Leopold | Comparison of classification methods in breath analysis by electronic nose[END_REF]. Validating a classifier requires to randomly separate the data into training and testing databases. Typically, the training database represents 60% of the data randomly chosen. This selection process is eased on the Rasberry Pi card thanks to the Python Scikit-Learn librairie having many algorithms implemented.

IV. EXPERIMENTAL SET-UP

Moisted air, simulated human breath and ammonia are mixed and injected for five minutes into the exposure chamber in which the sensor array is. The amount of ammonia added in the mixture varies from 500 ppb to 2 ppm. Three classes are defined: Healthy, Uncertain, and Unhealthy. They correspond to three concentration ranges of ammonia : 500 ppb -1.1 ppm, 1.1 ppm -1.7 ppm, 1.7 ppm -2.8 ppm, respectively. The exposure chamber is in a climatic room, with a controled temperature of 27 • C, Figure 4. The flow rate is at 8 L.min -1 . After an exposition, a purified and dry air is injected during five minutes. A total of 100 cycles are then realised with non specific order in the concentration of ammonia added in the mixture. An ammonia analyser and a humidity sensor is used to control the real concentration of ammonia and the relative humidity inside the exposure chamber.

V. RESULTS

A. Sensor responses

Figure 5 shows typical responses of 2 different sensors. Exposed to a mixture of simulated human breath and 1.5 ppm of ammonia, sensors have similar responses but different ∆ R , δ R and R (table I). For a same concentration of ammonia, sensor responses vary. For exemple, the resistance variation for sensor 2 varies from 6.1% to 9% for 2 ppm of ammonia. Thus, a predicted concentration error of 600 ppb is possible. This is enough to mistake a healthy sample as a unhealthy one.

Absorption Desorption

∆ R (%) δ R R ∆ R (%) δ R R (×10

B. Classication results

After testing various combinations of dimension reduction and classification algorithms, the best combination is LDA and SVM, respectively. LDA reduces the number of dimensions from 42 down to two by linearly combining the original features. Figure 6 shows the projection of the database with LDA algorithm and is already efficient to discriminate the different classes with a classification accuracy of 93%. Then SVM algorithm is applied to the 2-D dataset. SVM principle is to calculate an hyperplan that segregate at best the different classes. The SVM algorithm classifies the samples with an accuracy of prediction of 97%. These results are detailed on the confusion matrix in Figure 7: row gives the class of a tested sample, and column the class predicted by the classifier. Accuracy is calculated by dividing the sum of correctly classified samples by the total sample tested. In the present case, a healthy or uncertain sample is always correctly classified. The confusion matrix points out that the predicted errors are only with unhealthy samples: 92% are accurately classified but 8% are predicted as uncertain. This is in line with the LDA results in Figure 6, where unhealthy and uncertain groups are close from each other. Errors between healthy/unhealthy would be more problematic in a diagnosis help system. Crossing responses from several sensors based on PANI with a classification stage is thus appropriate to reduce the error.

When used without dimensional reduction with the 42-D vectors, the SVM accuracy drops to 94%. The use of LDA for dimensionnal reduction is therefore necessary. Moreover it allows an easier hardware implantation of the classification stage. Further works will focus on confronting the proposed system with a more diverse set of simulated breath samples and real human breath.

C. System power requirements

The power consumption of the system is 1.35 W when idle, 1.5 W during measurement and 2.1 W during classification stage. The measurement front-end power consumption is only 0.1 W so the next step is to optimize the controller and classification parts with a more dedicated circuit.

For instance, this electronic nose prototype could be powered for 14 hours with a 5000 mAh battery. VI. CONCLUSIONS The electronic nose system presented in this paper allows detecting different levels of ammonia concentration in a simulated human breath with an accuracy of 97%.

Moreover, it is a low cost, portable and easy to use diagnosis system for a general practitioner. Such a system could allow a broader access to point of care testing for populations living in areas lacking of medical care services. Further work will heavily focus on further reducing the size and power consumption of the system by proposing a hardware implantation of all its components.

Fig. 1 .

 1 Fig. 1. (a) Equivalent electrical model of the sensor and its Nyquist diagram are shown. (b) Polyaniline sensor and interdigitated electrode dimensions.

Fig. 2 .

 2 Fig. 2. (a) Architecture of the electronic nose system and (b) the picture of different parts of the electronic nose. The sensor array card is composed of 100 sensors even if only the seven outlined ones are used.

Fig. 3 .

 3 Fig. 3. Typical gas sensor response with the different features extracted.

Fig. 4 .

 4 Fig. 4. Illustration of the E-nose laboratory test bench.

Fig. 5 .

 5 Fig. 5. Resistance variation of sensors 1 and 2 to 1.5 ppm of ammonia and human breath composition.

Fig. 6 .

 6 Fig. 6. Application of the linear discriminant algorithm.

Fig. 7 .

 7 Fig. 7. Normalized confusion matrix of the SVM algorithm. Healthy and Uncertain samples are always well predicted. Unhealthy ones are correctly classified at 92%.

TABLE I RESPONSE

 I FEATURES OF SENSORS 1 AND 2 TO 1.5 PPM OF AMMONIA.


AGGLOMERATION OF A FINE AND HYDROPHOBIC POWDER BY SPRAY DRYING

INTRODUCTION

Agglomeration has wide applications in the food industry since the formation of larger and permanent aggregates of finely divided solids allows better flow characteristics and an improved control of solubility [START_REF] Dumoulin | Qualité des poudres obtenues par pluvérisation[END_REF][START_REF] Cordier | Etude de la granulation humide des poudres cosmétiques par assiette granulatrice et mélangeur à haut-cisaillement[END_REF]. In the case of cocoa, the main reason for agglomeration is to enhance its dispersion rate for instant beverages. 'Instant' means the agglomerated cocoa powder can be dispersed in water or in milk by stirring the product in the liquid for a short time [START_REF] Jensen | Methods of instantizing powders for the preparation of food drinks[END_REF][START_REF] Schubert | Instantization of powdered food products[END_REF][START_REF] Hla | Wetting behaviour of instantized cocoa beverage powders[END_REF]. The low dispersibility of cocoa powder in liquid is due to its physical properties (small particle size diameter, high cohesivity) and its composition (10-12% cocoa butter content). A previous study [START_REF] Vu | Improving the dispersion kinetics of a cocoa powder by size enlargement[END_REF] has shown that cocoa dispersion kinetics can be improved by increasing the slowest step of the dispersion, the powder wetting by the liquid. Among several possibilities for achieving this increasing the grain size and porosity by agglomeration appears to have many advantages. Several techniques have been used to produce instant cocoa powders which require additives such as binders and surfactants [START_REF] Wieland | Cocoa and chocolateprocessing[END_REF][START_REF] Jensen | Agglomeration, instantizing, and spray drying[END_REF] or more recently, a continuous thermal agglomerator [START_REF] Omobuwajo | Thermal agglomeration of chocolate drink powder[END_REF]. And alternative is spray drying, a technique widely used in the food and pharmaceutical fields to dry heat-sensitive materials. In this work, spray drying has been investigated as a potential technique to obtain agglomerated cocoa powder from finely dispersed cocoa powder.

MATERIALS AND METHODS

CHARACTERIZATION

OF AGGLOMERATED AND NON-AGGLOMERATED COCOA POWDERS

The cocoa powder used here was provided by Cargill Ltda., Brazil. It is a low fat cocoa powder (10-12% of cocoa butter) and it is insoluble in water.The non-agglomerated raw powder and the resultant spray dried granules are both characterized in the same way. The mean diameter was measured using a Malvern laser diffraction instrument with edible oil as a dispersant phase to separate the granules without breaking them. The true density was measured with a Micromeritics Accupyc helium pycnometer. The aerated (ρ a ) and packed (ρ p ) densities were measured with a Hosokawa powder tester. These two densities were used to calculate the following indexes : the Carr index (I C ) that is representative of the powder flowability and the Hausner index (H R ) that corresponds to a measure of the cohesivity between the grains or the granules :

I C = ρ p -ρ a ρ p H R = ρ p ρ a
The intra-particular porosity of the grains and the intra-granular porosity of agglomerated granules were measured with a Micromeritics mercury porosimeter using a maximum pressure of 413 MPa. The inter-particular or inter-granular porosities were calculated from these porosities and the measured values of true and aerated densities. The morphology of the grains and the granules were observed with a scanning electronic microscope (S.E.M.). Suspensions of cocoa powder in demineralized water were prepared with different solid concentrations and their dynamic viscosities were measured with a Brookfield DVII viscometer in a cell controlled at 20°C. The aim was to identify the suitable solid concentration in aqueous suspension for the agglomeration tests since, the initial solid concentration in the feed and the resultant suspension viscosity can strongly influence the physical characteristics of the final dried particles from a spray drying processes [START_REF] Oneda | The effect of formulation variables on the dissolution and physical properties of spray-dried microspheres containing organic calcium salts[END_REF]. From the data obtained (Figure 1), a mass fraction of 20% was chosen for the spray drying agglomeration tests because the viscosity of a 20% mass fraction suspension is still low enough not to hinder droplet formation. A rotary atomizer APV Nordic Anhydro PSD52 with a drying capacity of 7 kg water per hour was used in the experiments (Figure 2). The speed of rotation of the atomizer was 30 000 rpm. The outlet temperature was 90°C, and the inlet temperature and the suspension flow were chosen to keep the outlet temperature constant. The large granules of the agglomerated powder were collected at the bottom of the drying chamber and the fine granules were carried by the air flow and collected in a cyclone. The two size population of granules were obtained and called "large spray dried granules" and "fine spray dried granules" and they were characterized in the same way as for the non-agglomerated cocoa powder.

AGGLOMERATION BY SPRAY DRYING

RESULTS AND DISCUSSION

DIAMETERS AND SIZE DISTRIBUTIONS

The characteristic diameters and the span of the size distribution for the powder and the granules are given in Table 1. It can be seen that the spray drying is a good method for agglomerating cocoa powder as the mean diameter changes from 15.7 µm to 40.8 µm for the large granules and to 24.8 µm for the fine ones. We can also note that there is a wide size distribution for the non-agglomerated cocoa powder (span ~ 3), but narower for the agglomerated granules. Agglomeration by spray drying reduces the amount of fine particles..

Material

PARTICLE AND GRANULE MORPHOLOGY

Figures 3 and4 show the differences between non-agglomerated cocoa grain powder and the spray dried granules. It can be seen that the non-agglomerated cocoa grains are irregular, and the spray dried granules are predominantly spherical and porous (Table 3). Their rounded shape can be attributed to drying,by atomisation. The suspension is broken into small liquid droplets by the rotary atomiser, and these spherical droplets are instantly dried by the cocurrent flow of hot air . The solids contained in these droplets remains aggregated, the original particles being still identifiable (Figure 3c and3d), and the liquid volume initially between the powder grains is substituted by air volume, that induces a porosity in the formed agglomerates.

(a) (b)

FLOWABILITY AND COHESIVITY

The non-agglomerated cocoa powder is very cohesive probably because of its irregular slab shaped particles. These irregularities produce an entanglement of the grains, and then fragile agglomerates. The agglomerates produced by spray drying are spherical and solid interactions between granules are less important than for the grains leading to a lower cohesivity for granules than for grains and consequently improved flowability (Table 2). Due to its physical characteristics, the agglomerated powder is easier to handle.

Material

POROSITIES

The non-agglomerated cocoa powder is characterized by an inter-particular porosity, that can be calculated from its true and aerated densities. However, the spray dried cocoa granules present an intra-granular porosity, equivalent to the inter-particular porosity of nonagglomerated powder. The results are shown in Table 3.

Material

CONCLUSIONS

Spray drying appears to be an attractive method for the preparation of cocoa powder agglomerates which seems to come close the desired properties for rapid dispersing beverages. Briefly, the technique brings about modifications to the raw cocoa powder, by increasing the mean diameter of the original grains, inducing an intra-granular porosity and decreasing the cohesivity between the resulting cocoa granules. The spray drying process thus modifies some physical properties of a cocoa powder that are the important characteristics in the dispersion kinetics of a powder in a liquid. Quantification of the dispersion kinetics is presently under evaluation.

ACKNOWLEDGEMENT

We would like to thank Cargill Ltda. Brazil for providing the cocoa powder.

Figure 1 .Figure 2 .

 12 Figure 1. Dynamic viscosity vs. mass volume of cocoa dispersions in demineralized water at 20°C suspension feed hot air 1. atomizing

Figure 3 .Figure 4 .

 34 Figure 3. S.E.M. for raw cocoa powder, magnitude 150X and 750X

Table 1 .

 1 Mean diameters and span measured by a laser diffraction in edible oil

	d 10	d 50	d 90	Span
	(µm)	(µm)	(µm)	(d 90 -d 10 )/d 50

Table 2 .

 2 Densities and mechanical properties of cocoa powders

		ρ true	ρ aerated	ρ packed	I C	Flowabilit	H R	Cohesivity
		(g.cm -3 )	(g.cm -3 )	(g.cm -3 )	(%)	y	(-)
	Non-agglomerated	1.45	0.28	0.54	47.9 Very bad 2.1	Very
	powder							strong
	Large spray dried	1.45	0.31	0.54	41.5 Medium 1.4	Strong

Table 3 .

 3 Porosites for cocoa grains and granules

		Intra-	Intra-granular	Inter-	Sum of
		particular	porosity	granular	porosities
		porosity	(%)	porosity	(%)
		(%)		(%)
	Non-agglomerated powder	0	no	83.0	83.0
	Large spray dried granules	0	17.5	60.8	78.3
	Fine spray dried granules	0	17.2	62.2	79.4


H Lochard
email: hubert.lochard@enstimac.fr

M Sauceau

E Rodier

J J Letourneau

J Fages

USE OF RESS AND SAS PROCESSES FOR POWDER GENERATION OF AN ACTIVE SUBSTANCE

INTRODUCTION

Particle generation using supercritical processes gives active substances with high purity and with improved properties of dissolution. However, these techniques may require a costly initial investment. Therefore, it is high value products such as those of the pharmaceutical industry which remain the most concerned by their industrial development.

Many of the new chemical entities coming out from the research departments of the pharmaceutical industry are poorly water soluble. As water is obviously the basis of any biological fluid, one the largest challenges the scientists have to address nowadays is: how to make these molecules more soluble and thus bio-available and active for living organisms?

The bio-availability of these new molecules depends on their rate of absorption by the gastro-intestinal tract. This in turn is highly dependable on their dissolution rate. The dissolution depends further on the particle size, morphology, crystalline form and specific surface area. Particle generation processes like RESS and SAS can positively and strongly influence these parameters.

After a presentation of the experimental set-up, this paper presents the main results obtained for both SAS and RESS processes. The opportunity of using the former or the latter according to the particle production parameters is then discussed.

MATERIALS AND METHODS

The experiments were carried out in a polyvalent pilot plant (Separex, France) designed to perform extraction, RESS (with and without co-solvent) and SAS. A schematic diagram of the apparatus is shown in figure 1. Carbon dioxide is cooled and stored in a liquid CO 2 storage tank [START_REF] Sauceau | Proc 7 th Meeting on Supercritical Fluids[END_REF]. It is circulated by a membrane pump (Lewa, Germany) (3). Compressed CO 2 passes then through a heat exchanger (4) and becomes supercritical.

In RESS configuration, supercritical CO 2 flows through a stainless steel basket containing the organic compound to be extracted which has been placed in a two liters extraction vessel (5). Then, the mixture is precipitated through a nozzle (6) with a diameter of 340 µm (Spraying system, France) and collected in a porous bag (7). If necessary, a cosolvent injection (1) can be performed immediately upstream of the heat exchanger (13).

In SAS configuration, the solution (9) is injected into the CO 2 stream before reaching the nozzle (Spraying system, France) by an HPLC pump (8) (GILSON, 307 piston pump) and sprayed. The powder formed is collected in a porous bag. CO 2 -solvent mixture is depressurised (10) and separated in cyclonic separators (11). After purification through an active carbon bed (12) and cooling (14), the condensed CO 2 runs back to the liquid CO 2 storage tank.

Powder obtained was characterised by a Scanning Electron Microscope (XL30, Philips, Netherlands) and the specific surface area is measured using the BET method (ASAP, 2010 Micromeritics).

RESULTS

Equilibrium solubility data of the compound in supercritical CO 2 were measured on a continuous flow apparatus [1] at medium temperatures. In these conditions this compound has a very low solubility in pure supercritical CO 2 . For example, at 45°C and 202 bars the solubility of the solid was measured at 6.9 10 -3 g per kg of CO 2 [START_REF] Sauceau | Proc 7 th Meeting on Supercritical Fluids[END_REF]. Therefore, the SAS process appeared to be the most suitable for temperatures below 65°C.

However, for higher temperatures, by extrapolating experimental measurements with a Chrastil correlation [START_REF] Sauceau | Proc 7 th Meeting on Supercritical Fluids[END_REF] [3], it appeared that the solubility was no longer negligible: 0.15 g/kg at 300 bars, 80°C and 0.3 g/kg at 300 bars and 100°C. Therefore, the RESS process became feasible provided the working temperature was high enough.

SAS Process

The choice of a solvent is a compromise between several elements: its toxicity, its miscibility with supercritical CO 2 and the solubility of the drug in the solvent. Dimethylsulfoxide (DMSO) is adapted provided that its residue is low in the crystallised powder. Several studies have already been performed on DMSO behaviour in supercritical CO 2 [4] and its use for powder generation [5].

Three main parameters were studied (cf Table 1): the solute concentration in the DMSO solution , C= 30, 115 and 200 g/l of DMSO; the volumic flow rate ratio antisolvent/solvent = 50, 262, 500 and 750 and the density of CO 2 ( a function of pressure and temperature) =500 kg/m 3 (140 bars, 65°C), 650 kg/m 3 (165 bars, 60°C), 800 kg/m 3 (210 bars, 50°C) and 900 kg/m 3 (270 bars, 40°C). These process conditions are schematically represented in figure2. As shown in figure 2, crystallisation failed at the lowest CO 2 density and lowest volumetric ratio CO 2 /DMSO. That corresponds to the lowest mass ratio (QC0 2 /QDMSO = 27.5). As a result of insufficient anti-solvent, high supersaturation could not be achieved, crystallisation was too slow and most of the powder had crystallised after the particle formation vessel. When the trial was successful, all the powder obtained was collected in the porous bag. It looked very expanded, with a low apparent density of about 30 kg/m 3 .

Figure 2. Diagram of experimental conditions for SAS process

Because of the shape of the particles (fibres or rods), laser scattering granulometry was unsuitable, thus microparticle size and shape were characterised with a Scanning Electron Microscope (SEM). Three kinds of particles can be distinguished. The first one corresponds to the failed attempts performed at the lowest mass ratio (QC0 2 /QDMSO = 27.5), it corresponds to Figure 3 (experiment G). Rods observed are heterogeneous, short (1-3 µm) and thick (diameter=0.7-1.2µm). The second one corresponds to attempts performed at intermediate mass ratio (100 ≤ QC0 2 /QDMSO ≤ 450). Fibres are more homogeneous, thinner (d=0.35µm) and longer (5-20 µm), an example is presented in Figure 4 (experiment PC). Figure 5 (experiment F') represents fibres obtained for the highest mass ratio (QC0 2 /QDMSO > 450), they were about the same length but are much thinner (d=0.09µm). In some experiments, the fibres were interconnected and formed a network. In others, two kinds of particles can coexist in the same powder. Surface area analysis by the BET method showed a significant increase in the specific surface area (table 3) when crystallisation was successful. High CO 2 /DMSO ratio and high CO 2 density seem to further high specific surface area, but specific surface area cannot be correlated to particle shape. The influence of compound concentration in the injected solution is not clear.

RESS process

We carried out 3 RESS experiments. The experimental conditions and results are summarised in table 2. Post-expansion pressure and temperature in the crystallisation vessel were set at 80 bars and 60°C. Under these conditions, carbon dioxide is in a single phase and can be recycled. All tests were successful. The powder obtained was fine-grade, with a low apparent density (about 12 kg/m 3 ), pulverulent and electrostatically charged. The yield calculated in table 2 is the ratio of the mass of powder collected over the theoretical mass predicted by the Chrastil model [START_REF] Chrastil | [END_REF]. This low yield can be explained by an incomplete saturation of supercritical fluid (insufficient residence time, too high CO 2 flow rate, too low surface contact). This series of experiments shows that SAS and RESS processes can be successfully used on the same molecule: SAS with DMSO at medium temperatures and RESS at higher temperatures. The characteristics of the powders obtained are summarised in table 3. In both cases, we have observed a great effect on apparent density and specific surface area and a decrease in the size distribution. However, influence of experimental conditions on powder characteristics is not clear. Particle morphology is different and more work is needed to distinguish between the properties of the different samples obtained: do they have the same crystal morphology? Do they have the same in vivo rate? Further work is in progress in our laboratory to allow a complete characterization of the different powders obtained.

It should be added that the production rate is much lower with RESS, which may be a major drawback for a scaling-up of the process. Nevertheless, the final choice for a specific process to be industrialized must take into account all the scientific and economic aspects.

Figure 1 .

 1 Figure 1. Schematic diagram of the supercritical set-up

Figure 3 . experiment G Figure 4 . experiment PC Figure 5 .

 345 Figure 3. experiment G Figure 4. experiment PC Figure 5. experiment F'

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Figure 6 . experiment RESS 1 Figure 7 . experiment RESS 1 Figures 6

 61716 Figure 6. experiment RESS 1 Figure 7. experiment RESS 1

Table 1 .

 1 Experimental conditions for SAS process

		C		Volumic ratio	Mass ratio	Specific
	Expt name	(g/l)	ρ CO2 (kg/l)	QCO2/QDMSO	QCO2/QDMSO	surface Area (m 2 /g)
	A	30	0.5	500	275.7	16.7
	B	30	0.8	500	441.8	27.8
	C	30	0.5	50	27.5	11.3
	D	30	0.8	50	44.0	12.4
	E	200	0.5	500	275.7	16.5
	F	200	0.8	500	441.8	21.7
	G	200	0.5	50	27.5	6.7
	H	200	0.8	50	44.0	18.6
	M	115	0.8	262.5	230.6	15.7
	I	115	0.65	500	356.0	29
	F''	200	0.9	500	495.5	27
	F'	200	0.8	750	664.3	18.3
	I'	115	0.65	750	539.2	14.3
	E'	200	0.5	750	412.9	14.6
	PC	115	0.65	262.5	188.0	15.5

Table 2 .

 2 Experimental conditions and results for RESS process

	RESS 1	RESS 2	RESS 3

Table 3 .

 3 Results summary for SAS and RESS process