Cellular confinement of tunnel sections between two air curtains

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Building and Environment 42 (2007) 3352–3365 www.elsevier.com/locate/buildenv

Cellular confinement of tunnel sections between two air curtains Sanjeev Guptaa, Michel Pavageaua,, Juan-Carlos Elicer-Corte´sb a

De´partement Syste`mes Energe´tiques et Environnement, Ecole des Mines de Nantes, GEPEA, UMR CNRS 6144, 4, rue Alfred Kastler, BP 20722, 44307 Nantes Cedex 3, France b Departamento de Ingenierı´a Meca´nica, Universidad de Chile, Beauchef 850/50 Piso, Casilla 2777, Santiago, Chile Received 25 November 2005; received in revised form 23 January 2006; accepted 25 August 2006

Abstract This paper compares and discusses the efficiency of various air curtains designed to ultimately operate a bilateral aeraulic confinement of a short section of a tunnel or corridor-like geometry. The tested arrangements include single- and twin-jet air curtain systems with and without air return ducts. Jet opening ratios equal to 10 and 20 are considered, with air discharge velocities ranging from 1 to 10 m/s. The facility and the experimental approach used in this work are described and justified in many details. Results are presented in terms of leakage flow rates. Information is also provided about relevant air change rates and system time constants. The global efficiency of an air curtain is greatly improved by the adjunction of an air recirculation circuit due to enhanced system stability. A critical jet exit velocity corresponding to transition from the laminar to the turbulent regime of the channel flow inside the air curtain diffuser was found. For discharge velocities larger than this critical velocity an asymptotic behaviour is observed for every air curtain tested. At the lower nozzle exit velocities, the non-recirculated twin-jet system exhibits a weird behaviour. Unexpectedly, the results show that a single jet with a given opening ratio may perform better than a twin-jet of same height with a lower opening ratio. However, the results show that the performance of an air curtain is much dependant of the conditions under which this air curtain is operated demonstrating therefore that great care should be exercised in any attempt of comparison. r 2006 Elsevier Ltd. All rights reserved. Keywords: Bilateral cellular confining; Parallel twin jets; Gaseous barriers; Concentration decay method; Tunnel; Fire safety; Indoor air quality

1. Introduction Air curtains are devices useful in situations where it is desired to separate from each other two adjacent environments to preserve given climatic characteristics (in the broad sense of the term) in one at least of these environments while permitting traffic of people, vehicles, materials or objects between the two separated areas. The working principle of an air curtain is based on the discharge of a stream of air across an opening to create a singularity that hampers the free air movement through this opening. Diffusion and transport of heat, moisture, particulate matter and odours between the two areas separated aeraulically in this way can thus be kept to a minimum. Roberton and Shaw [1] add that air curtains can

Corresponding author. Tel.: +33 2 51 85 82 67; fax.: +33 2 51 85 82 99.

E-mail address: [email protected] (M. Pavageau). 0360-1323/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2006.08.026

also reduce transfers of bacteria and radioactive particles. Often defined in reference to an unprotected door, the efficiency of air curtains ranges between 60% and 85% [2–8] while that of standard solid material or plastic doors is usually about 90–95%. Air curtains are commonly used at the customer entrance of retail shops, malls, commercial and public buildings. They are also often mounted in the front of refrigerated food counters and open-shelves. Other applications of air curtains involving various device arrangements have been investigated or sometimes simply suggested by Grasmuck [9], Simper [10], Etkin and McKInney [11], Partyka [12], Szatmary [13], Rydock et al. [14], Pavageau et al. [15], Hu et al. [16], Bridenne et Coffinier [17], Lewis [18], Gupta et al. [19] among many others. The work presented in this paper comes within a larger (and longer) research programme aimed initially at designing air-curtain-based combined ventilation-safety systems for road tunnels.

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The use of air curtains as devices able to slow down or stop smoke propagation in case of tunnel fire or, more generally, in corridor-like geometries has formed the subject of a very limited number of studies. Sakurai et al. [8] studied a system of air curtains for fire safety applications in corridors of multi-storey buildings or in subways. Their system was based on the use of horizontal air curtains operated in a push–pull principle. They concluded that their system was efficient as long as two important parameters, namely the discharged air velocity and the tunnel height to jet nozzle width ratio, were chosen appropriately. Also in buildings, Cumo et al. [3] investigated the efficiency of air curtains blown vertically upwards. Their system was also based on the push–pull principle. Unlike Sakurai et al. [8], the authors suggested to use high induction nozzles through which discharged air streams can reach higher values. The few other air curtain arrangements investigated so far in tunnels involve only one air curtain [20–23]. In the corresponding cases, the jet is fed by air taken from one of the two areas separated by the curtain. The air intake is located at the ceiling, usually at a few metres from the curtain. As per our current knowledge, the only air curtain actually installed in a road tunnel is in the underground interchange A13 of the A86 West Underground Link-up of Paris, France [20]. At a discharge air velocity equal to 30 m/s and for an inclination angle of the curtain equal to 351, the system has an almost zero leakage up to a pressure difference of 80 Pa. However, the use of a single air curtain can prevent smoke propagation in one direction only, namely in the direction opposite to the curtain inclination when this one is blown at a fixed angle as is usually the case. As the location of an incidental fire with respect to the position of the air curtain is impossible to predict, the usefulness of such systems is thus questionable. The designs considered in the present work consist in combined ventilation-safety systems that can be operated both in nominal and incidental situations (Fig. 1). Appropriate vane manipulations allow switching easily and rapidly from one of these situations to the other one.

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In our system, the tunnel is partitioned into adjacent immaterial cells within which it is possible to locally confine smoke in case of fire. Thus, with our system, smoke propagation is always prevented in two directions. In a selfrescue phase, people can easily escape the smoky part of the tunnel whatever the direction they choose, and reach cleaner and safer areas. Death risks by asphyxiation are reduced. Additionally, the system makes it possible for rescue teams to access the incident place more freely from either tunnel ends, and to take efficient action faster. Finally, smoke confining and recirculation reduce oxygen supply locally, and contribute to fire self-extinguishment. An experimental facility was built to physically model one cell of the system whose principle is depicted in Fig. 1. Experiments were performed to investigate the dynamical behaviour and the confining efficiency of various air curtain arrangements under isothermal conditions. Detailed results concerning the flow dynamics of the tested configurations can be found in Gupta [24] and will be soon published in another article. The work includes a detailed analysis of the topology of the impingement region where coherent structures have been educed and characterized statistically [25]. The present paper focuses on the results of tracer gas experiments performed to assess and intercompare the confining efficiency of single and twin-jet air curtains at various discharge velocities. This paper is organised as follows. Section 2 provides a description of the experimental facility designed for this work. The experimental protocol used in our tracer gas experiments is presented. The results of our experiments are given and discussed in Section 3. They are mainly presented in terms of leakage flow rates. Conclusions and perspectives are drawn eventually. 2. Experimental apparatus and procedure 2.1. Experimental set-up The experimental facility used in this study consists of a 6 m long horizontal open tunnel with a 0.3  1 m rectangular

Fig. 1. Principle of an air-curtain combined ventilation/safety system for tunnels (a) nominal situation (b) accidental scenario 1: fire starts between two air curtains (c) Accidental scenario 2: fire starts below an air curtain.

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Fig. 2. Schematic view of experimental set-up.

cross-sectional area. Only half of the whole set-up is schematically depicted in Fig. 2, the installation being symmetric geometrically. The dimensions of the facility correspond approximately to a 1:17th scaled down model of a short section of a standard road tunnel. The front wall and the floor of the tunnel are transparent (8 mm thick perpex plates) to allow Particle Image Velocimetry (PIV) measurements. The floor-to-ceiling clearance can be varied to cover different values of the opening ratio, H/e, where H is the nozzle-to-plate distance and e is the nozzle width. In the experiments described here, H was kept constant to 0.3 m. Each blowing unit (only one unit is shown in Fig. 2) consists of two fully independent feeding circuits. It can be operated in single-jet or twin-jet mode. The so-called exterior jet is always fed with air taken from the ambiance. The interior feeding circuit can be operated so that the interior jet is fed with air taken either from the ambiance or from inside the tunnel. Recirculated air curtain designs refer to the latter case. The discharge air streams are always blown vertically downwards, spanning the wind tunnel cross-section completely. The inlet section of the exterior feeding circuit is fitted with shutters so as to ensure a sufficient stability of the exit jet flow at low discharge velocities. Both feeding circuits comprise flow straighteners, filters and honeycombs in order to achieve uniform velocity profiles at the nozzle exit. The terminal diffuser has a 10:1 contraction ratio. The splitting plate in the diffuser is 1.5 mm thick. The exit section of the blowing unit is 0.03 m wide, the nozzle exit sections of each blowing circuit being equally wide.

When the blowing unit is operated in the twin-jet mode, the initial velocity U0 of each jet can be adjusted independently in the range 1–15 m/s by varying the rotation speed of the fans. A monitoring system consisting of 1.5 mm outer diameter total head tubes connected to standard micromanometers provides on-line information about outlet velocities, the signals from the pressure transducers being systematically recorded during measurements. The turbulence level at the nozzle exit is about 1%. From qualification and validation experiments, it was checked that the flow at the nozzle exit was uniform in the spanwise direction. No potential periodic component in the main flow caused by the fan vane passage frequency could be noticed in any case. It should be observed that the present air curtains, when they are operated in the twin-jet (or double-jet) mode, contrast with traditional double-jet air-curtain systems based on the discharge of a laminar slow jet and a fast jet [7,26,27]. The latter are better suited for small size installations where the slower jet can be chosen wide enough to hit the impact surface at the tip of its potential core, i.e., at x=e  5. 2.2. Test configurations The primary objective of this study was to experimentally estimate the effectiveness of single or double-jet aircurtain systems. The four arrangements schematically depicted in Fig. 3 were investigated for various exit flow velocities. During experiments, the pressure difference

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Fig. 3. Tested configurations: (a) single-jet curtain without recirculation (SJ), (b) single-jet curtain with full recirculation (SJR), (c) double-jet curtain without recirculation (DJ) and (d) double-jet curtain with partial recirculation (DJR).

between the tunnel ends was approximately 0.02 Pa. The two curtains forming the confinement cell were always operated in the same mode (symmetrically). The single-jet curtain of Fig. 3a was achieved by closing the nozzle outlet of the interior feeding circuit. In this case, the jet was fed from the ambiance. In contrast, the single-jet curtain of Fig. 3b was fed by air drawn from the confined area through the recirculation circuit of the blowing unit. In this case, only the interior feeding circuit was used. The nozzle outlet of the exterior feeding circuit was closed. Thus, the jet total width e was equal to 15 mm for these two single-jet configurations (only half of the diffuser unit was used). It was 30 mm for both double-jet air curtains (Figs. 3c and d). Keeping H constant, the resulting opening ratio ðH=eÞ is 20 for the SJ & SJR arrangements, and 10 for the DJ & DJR systems. For the 4 configurations of Fig. 3, six jet exit velocities ranging from 1 to 10 m/s were tested. Table 1 gives the corresponding exit flow rates and Reynolds numbers. The latter is based on the nozzle exit velocity U0 and the jet nozzle effective width e. It is given by Re ¼ U 0 e=n where n is the kinematic viscosity of the air–ethane mixture. For the double-jet curtains of Figs. 3c and d, the same velocity was used at the exit of both feeding circuits. Measurements of the flow dynamics of double-jet curtains with exit velocity ratio of interior jet to exterior jet greater or smaller than 1

Table 1 Mean exit velocities, mean flow rates and Reynolds numbers for single and double jets U0 (m/s)

1

2

3

5

7

10

Single jet Re

1000

2000

3000

5000

7000

10,000

Double jet Re

2000

4000

6000

10,000

14,000

20,000

Single jet Q0 (m3/s)

0.015

0.03

0.045

0.075

0.105

0.15

Double jet Q0 (m3/s)

0.03

0.06

0.09

0.15

0.210

0.3

have shown that such configurations could only perform worse than systems with identical exit velocities [24]. The main reason is that mean shear and Reynolds stresses along the jet symmetry plane of a twin-jet air curtain made of two streams discharged at different velocities take much higher values than with equal discharge velocities. Moreover, a velocity defect at the nozzle exit induces a more or less pronounced bend of the jet with higher shear in the impingement region. Thus, mass transfers across the curtain occur faster. Consequently, configurations with a

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velocity differential at nozzle exit were not further investigated. 2.3. General methodology The tracer gas decay method (ASTM Standard Practice for Measuring Air Leakage Rates by the Tracer Dilution Method E741-83) was used to determine the leakage rates across the investigated air-curtain systems. The method relies on the observation of the concentration decay with time, CðtÞ, of a tracer gas with uniform initial concentration C0 within the confined area. The tracer gas must be a passive scalar, i.e., must not react. Ethane (C2H6), which meets this criterion and whose density is close to that of air, was used in the present case. Great care was exercised in the construction of our experimental facility to avoid undesirable leakage and to ensure that the tracer gas was not removed in any manner other than removal by the air curtains themselves. All the ‘‘air-curtain+enclosure’’ systems investigated in this work turned out to behave like first-order systems. Thus, an expression of the leakage flow rate QL of our systems can be derived analytically from a single-zone mass balance expressed in volumetric terms as dC ¼ QL ðC  C amb Þ þ E, (1) dt where C is the concentration of ethane within volume V, Camb the outdoor (ambient) concentration of ethane, QL the flow rate of fresh outside air into the enclosure ¼ flow rate of tracer-laden air from the confined area (leakage flow rate), V the control volume, E the source/sink term within the control volume. Assuming that QL, Camb and E are constant, and setting C* equal to the difference between the indoor and outdoor tracer gas concentrations, the solution of Eq. (1) is   E E C  ðtÞ ¼ þ C  ðt ¼ 0Þ  expðAtÞ, (2) QL QL V

where A ¼ QL =V denotes the outside air exchange rate (often referred to as air changes per hour—ACH) within the control volume. In the absence of source term within the control volume, and assuming ethane concentrations within the control volume much larger than the ambient concentration of ethane, Eq. (2) reduces to CðtÞ ¼ C 0 expðAtÞ,

(3)

where C0 is the initial concentration of ethane within the control volume at time t ¼ 0. By measuring the decay rate of C within the control volume, the determination of the slope of the plot lnðC=C 0 Þ ¼ f ðtÞ ¼ At ¼ ðQL =V Þt gives an estimate of QL. 2.4. Experimental protocol In practice, the source of ethane was located in the middle of the measurement volume. The confined volume

was filled with ethane until an initial concentration of about 8000 ppm was reached. The fans were then switched on. During filling, the ends of the tunnel were closed to limit gas losses by diffusion to the ambience. Note that it is not necessary to know the initial concentration of ethane C0 with a high accuracy since we are interested primarily in the decay rate of CðtÞ within the measurement volume. No particular device was used to mix the tracer gas inside the confinement volume before measurements. The air circulation induced in the confinement cell by the air curtain was enough to ensure a rapid homogenisation of the initial tracer gas concentration inside the volume. During experiment, the decrease in CðtÞ within the confinement cavity was monitored on line using a fast ionisation detector (FID) for hydrocarbon concentration measurements. The time response of the detector was about 0.1 s in the present case. The output signal from the FID was recorded digitally at a sampling frequency of 1 Hz until the concentration of ethane inside the measurement volume had decreased to almost the ambient value. The slope of the plots C ¼ f ðtÞ was determined from linear least square fits of the data. The spatial uniformity of C has been checked for experimentally through measurements where two FIDs were used, i.e., where the concentration of ethane in the confined area was measured at two points simultaneously. As shown in Fig. 4, these preliminary experiments have shown also that the decay rate of ethane concentration within the test area was position independent. This comes from the fact that the fluid within the control volume is not at rest. In fact, recirculation zones of fluid may exist within the confined area. Recall that their number and size depend much on the air-curtain opening ratio H=e, the height to length ratio H=L of the confined area, boundary conditions (free length between the air curtain and the tunnel end for example), the jet initial velocity U0 and the presence or not of recirculation conduits (see [24,28,29], for example). Consequently, multipoint averaging measurements were not necessary in the present case. The 50 cm long sampling capillary tube was placed in the middle of the control volume.

2.5. Control volumes The determination of QL requires estimates for A and V. The quantity A is deduced from measurement. The control volume V is equal to the total volume of the confined area. Its value must be prescribed. For enclosures sealed with an air curtain, there seem to be no convention as to where the separation surface between the enclosure and the ambiance should be set. In fact, the box model used in the present approach does not account for such details. It only accounts for the net flux between the confined area and the exterior, regardless of the actual characteristics of the openings (windows, doors, ventilation systems, porous walls, etc.) through which air exchanges take place.

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Fig. 4. Tests of concentration decay homogeneity within the control volume.

In the present work, we set the separation surface between the confined area and the ambiance at the nozzle symmetry plane of the blowing unit. Case depending, the control volume delimited this way includes part of or the entire jet flow forming the air curtain. The approach may thus sound inconsistent with the approximation of a wellmixed control volume at rest that underlies the box model used for analysis. However, it is rather convenient. The relative error made this way in the estimation of V is negligible in comparison with the value of the volume of the confined areas considered here. It is less than 1%. Moreover, the resulting overestimation of V is almost the same for all the investigated air curtain arrangements. It thus forms a systematic error that can be easily corrected for if need be. The estimation of QL is not much affected by this choice for V. The control volumes used in the present work are depicted in Fig. 5a and b for air curtain arrangements without and with air recirculation, respectively (due to symmetry, only half of the arrangements is represented for simplification). For the experimental configurations involving a full or partial recirculation of air (Figs. 3b and d), the control volume includes the volume of the air recirculation ducts (twice 0.36 m3), which is of the same order of magnitude of the volume between two curtains (0.90 m3). This comes down to assume implicitly that arrangements with recirculated air behave like or at least

Fig. 5. Control volumes for (a) non-recirculated and (b) recirculated air curtains.

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can be modelled by standard air-recirculation-free arrangements, as shown in Figs. 6a and b. It is consistent with the use of an integral box model as this model is unable to make the distinction between the configurations (a) and (b) of Fig. 6. Reality may be different, as shown by Kristoffersen et al. [30]. However, according to these authors, with the present duct volume ratio (Vduct/Vroom) and the typical transport delays in the recirculation duct, our approach may only slightly overestimate the actual supply of fresh air through an air curtain fitted with a return duct. We shall nevertheless keep in mind that the leakage rates presented for air curtains with a ventilation return system are ‘‘apparent’’ leakage rates. They reflect the leakage rate of systems ‘‘recirculated air curtain+enclosure’’ rather than of the relevant ‘‘recirculated air curtain’’ alone. It is also interesting to note that the arrangement depicted in Fig. 6a can also be viewed as a single jet with

a discharge flow rate Q0/2 across the opening of an enclosure in which air would be in movement (Fig. 6c). The question then may arise of the way the results obtained for double-jet recirculated air curtains should be analysed and interpreted or at least presented. Are we dealing with a double jet (flow rate Q0; width e) or a single jet (flow rate Q0/2; width e/2)? Again, the box model used for analysis does not make any difference between these two possible perspectives. It merely allows us to state the existence of a leakage flow rate, and to estimate it, independently of the way in which the actual system is regarded. 3. Discussion of results Results are presented in terms of QL, which characterizes a given arrangement better than the air change rate A. The latter quantity indeed is rather representative of a pair ‘‘aircurtain+confined area’’ as it is referred to the confined volume V. This, however, implies that, for given far field boundary conditions on the ambience side, a characteristic leakage flow rate QL can be associated with a given aircurtain arrangement (nozzle design, jet initial velocity, presence or not of a return duct of given size) regardless of the dimensions of the confined volume. It is thus implicitly assumed that what ‘‘happens within the confined area’’, i.e., the air movements within it, has no influence on the air-curtain structure and, subsequently, on the amount of air exchanged through the air curtain. This hypothesis is not completely true as an air curtain always interacts with its environment (see [24]). This may limit the applicability of our results to arrangements geometrically homothetic to the configurations considered here. Results concerning QL are presented with an estimated uncertainty of 5%. This value takes into account the experimental variability observed during repeatability tests. It also accounts for the uncertainty in the determination of the slope of C ¼ f ðtÞ in the most unfavourable case. 3.1. Unilateral versus bilateral confining

Fig. 6. Different ways to look at a recirculating double-jet air curtain (a) actual configuration, (b) Box model and (c) alternative interpretation.

Prior to giving results about the leakage flow rates characteristic of the systems investigated in this study, we shall make a short comparison between unilateral and bilateral confining. The former terminology refers to the protection of an area exhibiting only one opening to the ambiance (Fig. 7a). The latter refers to the confining of an area with two open ends (Fig. 7b). In what follows, we shall simply use the terminology ‘‘cavity’’ and ‘‘tunnel’’ to refer to unilateral and bilateral confining, respectively. In the cavity configuration only half of the tunnel was used, i.e., only one curtain was operated for a given air-curtain configuration. A solid plate was placed at the tunnel symmetry plane to close it and form the cavity. For the cavity configuration, the control volume for single and double air curtains without recirculation (SJ and DJ, respectively) was 0.45 m3 while for single and double air curtains with recirculation (SJR and DJR, respectively)

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3.2. Leakage rates

Fig. 7. (a) Configuration ‘‘cavity’’: unilateral confining and (b) configuration ‘‘tunnel’’: bilateral confining.

The leakage rates measured for each tested configuration are plotted versus jet flow rate Q0 in Fig. 9. The estimated leakage rates vary between 3% and 25% of the jet flow rate. At first sight, QL increases almost linearly with Q0 (or with Reynolds number, which is also proportional to eU0) whatever air-curtain system is considered. Present results confirm that, as long as the jet keeps contact with the impingement wall, one should use as low discharge velocities as possible to achieve more both efficient and economical air curtains. For a given jet flow rate, i.e., for a given energy input, the two SJR configurations (H=e equal to 10 or 20) lead to leakage flow rates of the order of 5–6% of the jet exit flow rate. Note that the right-most point on the SJR-H/e ¼ 10 plot should be disregarded due to the very low accuracy of measurements at that point. These two configurations prove to be the most efficient systems. However, such a configuration does not correspond to a replicable pattern able to form similar adjacent cells. Clearly, the SJR configuration with H=e equal to 20 performs better than the DJR and DJ configurations with H=e equal to 10 although all these configurations correspond to the same value of H, namely 30 cm. This result seems to contrast with well-known earlier findings asserting that the efficiency of an air curtain decreases with increasing H=e (see among others [21]). However, this statement hardly holds here considering that we are opposing different air-curtain designs (a simple jet and double jets). Furthermore, earlier studies [31,32] clearly showed the existence of a critical opening ratio ðH=e ¼ 8Þ for which turbulence intensity in the impingement region reached a maximum. In his study of the kinematics of single jets with opening ratios ranging between 5 and 50, Maurel [33] found that H=e was critical when it was 10.

Fig. 8. Comparison of the leakage flow rates measured in tunnel and cavity configurations for various air curtain designs. Results are presented in terms of QLTunnel =2QLCavity .

it was 0.81 m3. For the tunnel configuration, the control volume was twice as much as the volume for the corresponding cavity configuration for all air-curtain systems. Fig. 8 compares the ratio of leakage rates estimated in the tunnel and cavity configurations. The results clearly show that the ratio QLTunnel =2QLCavity tends to reach a value equal to 0.9 with increasing jet flow rate. The ratio approaches unity for the double-jet curtain with recirculation. Thus, in spite of a possible weak mutual influence of the two air curtains present in bilateral confining (especially at the lower jet exit velocities), everything seems to happen in the tunnel configuration as if a solid wall was present at the tunnel symmetry plane. Considering this, we shall limit the presentation of our results to the observations made for the cavity configuration.

Fig. 9. Estimated leakage flow rates for various air-curtain designs (unilateral confining).

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Fig. 10. Air-curtain stability.

This result, which has been later explained by a study of the dynamics of the vortical structures in the impact region [34], has not been validated for double-jet air curtains as only one value of H/e was considered in the present study. It could perhaps explain what was observed in our experiments. The DJ configuration is in first approximation comparable to a single-jet of opening ratio equal to 10. Both configurations indeed correspond to an air curtain supplied with air taken only from the ambiance. The only difference lies in the presence of a splitting plate in the diffuser for the DJ configuration. This perspective naturally leads us to compare the DJ configuration ðH=e ¼ 10Þ with the SJ configuration ðH=e ¼ 20Þ. Fig. 9 shows that these two arrangements exhibit similar performances. Again, this may appear contradictory with the rule according to which the efficiency of an air curtain should decrease with increasing H=e. However, for the same reasons as above, information is missing to definitely conclude that the rule is not violated. The effect of varying H=e can be examined by comparing the two SJ systems or the two SJR systems,

which were both tested for H=e equal to 10 and 20. At a given air supply flow rate (i.e., at a given Reynolds number or at a given energy input) and for the same confined volume, the leakage flow rate increases with an increase in H=e for both configurations. However, QL doubles when H=e is doubled in the latter case whereas QL is multiplied by a factor 4 when H=e is doubled in the former case. Results tend thus to show that QL ¼ f ðH=eÞ depends strongly on the considered air-curtain design. For the same opening ratio H=e ¼ 20, Fig. 9 shows that the SJR configuration is more efficient than the SJ configuration at a given input flow rate. The ratio of the corresponding leakage flow rates is equal to 4. For H=e ¼ 10, Fig. 9 shows that the SJR configuration is still more efficient than the SJ configuration at a given input flow rate. In this case, however, the ratio of the corresponding leakage flow rates is only 2. This is just another way to present the results given in the paragraph just above. However, it helps to better understand that quantification of the relative efficiency of several aircurtain designs requires more than a simple description of the air-feeding system. Full information about the size of

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Fig. 11. Summary of observations (H ¼ constant) (a) SJ configuration, (b) DJ configuration, (c) SJR configuration and (d) DJR configuration.

the considered arrangements is necessary. Present results clearly show the importance of being vigilant in the comparison of air-curtain systems with similar features but with different designs. The effects of the presence of a recirculation circuit are worth being examined. Fig. 9 shows that, for a given input flow rate, the SJR configuration is more efficient than the SJ configuration. Similarly, the DJR configuration exhibits a lower leakage flow rate than the DJ configuration at a given discharge flow rate. Even though the ratio of leakage flow rates is of the order of 4 and is only about 1.5 in the former and the latter case, respectively, the benefit gained from the implementation of a recirculation feeding circuit is obvious. Within the range of H=e investigated here, it is worth noting that the result is H=e independent. A closer examination of the flow dynamics for the 4 configurations considered here for comparison revealed that when a recirculation loop was implemented much more stable curtains were achieved. The recirculation loop greatly attenuates the natural flapping motion of an air curtain. This can be seen also in Fig. 10 where lnðC=C 0 Þ ¼ f ðtÞ is given for various air curtain arrangements. The plots obtained for the SJR and the DJR configurations exhibit much less data fluctuations. We ought to add however that this is probably true as long as the intake of the recirculation loop is not located too far from the aircurtain section. If the intake funnel of the recirculation feeding circuit is too close from the air discharge section, an air curtain may not function properly. Fig. 11 schematically depicts the four arrangements investigated for a constant tunnel height H. The number of ‘‘+’’ within the cavity indicates the degree of impervious-

ness reached. The larger the number of ‘‘+’’ is, the more impervious is the considered arrangement. The addition of a second jet to the SJ configuration (11a) to form the DJ configuration (11b) results in an only slight increase in efficiency of the system. By adding a jet to configuration (11a) we actually reduce the opening ratio by a factor 2. This should make the DJ configuration more efficient than the SJ configuration. However, as was demonstrated by a detailed examination of the flow dynamics of these two arrangements [24], this is counterbalanced by a noticeable increase in the turbulence level within the jet in configuration (11b). Corresponding effects are finally attenuated by suppression of the deviation of the simple-jet curtain towards the exterior of the cavity when a second jet is added. The importance of a jet deviation towards the interior of the cavity is further instanced by comparing cases (11c) and (11d). The addition of a second jet reduces again the opening ratio by a factor two, and the level of turbulence within the jet also increases. However, this time, the addition of a second jet at the exterior of the cavity straightens the initial jet, which is almost no more deviated towards the interior of the cavity. The resulting effect is a decrease in the efficiency of the system. We thus see that the addition of a second jet to a single-jet curtain design may result in adverse and positive effects in terms of efficiency depending on the initial single-jet configuration. Present findings also suggest that the imperviousness of an air curtain is the result of a subtle balance between various mechanisms and parameters whose relative weight depends on the considered situation. Although we ought to be very cautious in the comparison of two systems (see above), one may imagine with little

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Fig. 12. Theoretical view of certain equivalence between the SJ and SJR configurations.

effort that, to some extent, the SJ and the SJR arrangements both correspond to the confining of a given volume by means of a single-jet fitted with a recirculation circuit. Fig. 12 may help to better understand this theoretical view. The SJ configuration is here viewed as a system that would confine the ambient volume. Accepting this, the perspective comes down to consider one single air-curtain arrangement, namely a recirculated single-jet air curtain. Thus one would expect that the leakage flow rate characteristic of this air curtain be the same in both situations. It is obviously not the case. The point is here that these two systems actually distinguish themselves by different boundary conditions on the confined side (and on the opposite side of the confined volume). We just want here to point out again the importance of providing an accurate description of the operating conditions under which air curtains are tested and characterized. Information about the behaviour and efficiency of a given air-curtain design should thus always be accompanied by information regarding the conditions under which these characteristics were evaluated. Incidentally, this makes difficult the comparison of results from the literature since the probability of finding fully comparable experimental arrangements remains rather low. In Fig. 13, we have plotted QL =Q0 versus Q0. This figure highlights a few fundamental differences with respect to the behaviour of the air-curtain arrangements tested here, especially at the lower discharge velocities. The curves obtained for the SJR and the DJR configurations exhibit the same tendency. For Q0 below 0.05 m3/s with the SJR configuration and for Q0 smaller than 0.1 m3/s with the DJR configuration, QL =Q0 decreases sharply as Q0 increases. It then seems to tend toward a constant value.

Fig. 13. Ratio QL/Q0 versus Q0.

Thus, it seems than an asymptotic flow regime exists beyond which the ratio QL =Q0 remains minimum and cannot be improved further. In the present case, the value of Q0 from which the slope of the plot QL =Q0 ¼ f ðQ0 Þ changes correspond exactly to U0 ¼ 3 m/s. This value is associated with transition to turbulence within the plane channels forming the diffuser, as the Reynolds number of the corresponding channel flows is 3000. In contrast to what precedes for the SJR and DJR configurations, the plots obtained for the SJ and DJ configurations differ substantially in shape. This is quite surprising at first sight insofar as nothing fundamental distinguishes these two configurations in which air is

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supplied from the ambience. We again find the previously observed marked change of slope from Q0 equal to about 0.05 m3/s for the simple-jet air curtains whereas the plot for the double-jet air curtain exhibits a maximum for Q0 equal to about 0.1 m3/s. These two critical values of Q0 correspond again to U0 ¼ 3 m/s. The reasons for which the DJR air curtain behaves in this way are still unclear. Further experiments, in particular for other opening ratios, are necessary. 3.3. Air change rates and time constants For information only, we present in Fig. 14 the air change rate A in function of nozzle exit flow rate Q0. The air change rate is here expressed in min1. For the comparison to make sense, results are presented for identical confining volumes as A depends on V. In the present case, V was arbitrarily chosen equal to 0.45 m3, which corresponds to the volume of half the working section of the tunnel when H ¼ 30 cm. The air change rate ranges between about 0.5 and 8 volumes/min depending on the considered air-curtain configuration and nozzle exit velocity. The average value is here about 3 volumes/min for the investigated configurations. From experiments performed on SJR air curtains with an inclination of 301 and H=e ¼ 12 (e ¼ 20 mm and e ¼ 40 mm), Dufresne de Virel et al. [21] showed that the air change rate increased of about 80% when doubling the nozzle exit velocity, which is in rather good agreement with the present results. The same observation was made on recirculated single-jet air curtains with H=e ¼ 24. It should be noted that the velocities used in the experiments of Dufresne de Virel et al. [21] were much higher than those used in the present study. Our findings are nevertheless

Fig. 15. Time constant (unilateral confining) for various arrangements and control volumes.

rather consistent with these results since at the larger velocities we observe that A ¼ QL =V tends to be proportional to Q0 =V . Eq. (3) describes a first order system. We denote by t ¼ 1=A the time constant of this system, i.e. the time at which only 37% of the initial concentration C0 is still present in the confined volume. Fig. 15 gives for information the time constants obtained in the present study. Naturally, t decreases as Q0 increases, and increases with V. The present estimates of t are consistent with earlier findings of Pavageau et al. [15] on a facility of size similar to the present one. In the relevant experiments, the confined volume was 2.3 m3. With a recirculated double-jet air curtain with an opening ratio of 10 (e ¼ 5 cm), a time constant equal to 150 s was found for Q0 ¼ 0.0675 m3/s (U0 ¼ 2.7 m/s). The ratio QL/Q0 was about 0.17 which in good agreement with the present results (Fig. 13). From Fig. 15, we would obtain t ¼ 175 s for an equivalent discharge flow rate and the same confined volume. 4. Conclusion

Fig. 14. Air change rate (unilateral confining)—referred to V ¼ 0:45 m3— unilateral confining.

The study presented in this paper was an effort to compare the behaviour and the efficiency of various types of air-curtain systems including single- and double-jet devices. It was initiated with the objective to find a solution to multi-cellular confining in tunnels or corridor-like geometries. Experiments based on the concentration decay method were performed under isothermal conditions in order to assess the confinement efficiency of several air curtain designs. The results are presented in terms of leakage rates. Information about relevant air change rates and time constants is also provided. A critical velocity corresponding to the transition from the laminar to the turbulent regime of the channel flow

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developing within the diffuser was found. For discharge velocities larger than this critical velocity an asymptotic behaviour is observed for every air curtain tested. The leakage flow rate then varies almost linearly with aircurtain supply flow rate. At the lower discharge velocities, the investigated arrangements behave similarly except the so-called DJ configuration, which exhibits a weird behaviour. Results thus suggest the existence of an optimal discharge velocity for all the investigated configurations except the DJ one, which should be discarded. A recirculated single jet with the recirculation nozzle intake located within the confined space proved to be the best solution among the arrangements tested in this study. This air-curtain configuration performs better than the other tested arrangements although they exhibit a lower opening ratio. This result is contrary to what can be usually found in the literature. It could be explained after consideration of the stability of the air curtains tested here. More generally, the presence of an air recirculation loop allows achieving much more stable devices, which results in decreased leakage flow rates. Present results tend thus to indicate that stability is a quite important parameter in terms of imperviousness. For a given opening height, at equivalent control volumes and jet discharge flow rates, the air exchange rate obtained with a recirculated double jet is between 30% and 40% lower than the air exchange rate obtained with a nonrecirculating single jet. The discrepancy would be slightly less for confined spaces with two openings fitted with symmetrical air-curtain devices. Consistently with the results of Kristoffersen et al. [30], higher system time constants are obtained in the presence of recirculation ducts. The efficiency of recirculated air curtains increases with the volume of the recirculation conduits. Finally, this paper demonstrates the difficulty to experimentally derive intrinsic characteristics of a given air curtain. The behaviour and efficiency of an air-curtain design depend much on the overall conditions under which this air curtain is operated. To some extent, this makes problematic the comparison of results from the literature since the probability of finding evaluation works based on fully comparable experimental arrangements is rather low. Acknowledgements The study reported in this paper was supported by the EMN and the French Embassy in New Delhi (India) in the form of a Ph.D. grant. It was additionally supported by CONICYT (Chile) under grants Fondecyt No 1040498 and No 7040164. It could not have been completed without the assistance of the technical staff of the laboratory of the EMN. We thank, in particular, Mr. Y Gouriou, Mr. F.X. Blanchet and Mr. E. Chevrel for their valuable and unstinting help in the construction and instrumentation of the experimental facility designed for this work.

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