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Smoke spreading analysis from an experimental subway scale model
Fire Safety Journal 86 (2016) 75–82
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Smoke spreading analysis from an experimental subway scale model ⁎
B. Giachetti, D. Couton, F. Plourde
crossmark
Institut Pprime, Dept. FTC, Axe COST, ENSMA, 1 Avenue Clement Ader BP40109, 86961 Chasseneuil du Poitou, France
A R T I C L E I N F O
A BS T RAC T
Keywords: Jet in cross flow Subway station Ventilation Stack effect
The aim of this study is to experimentally investigate buoyancy-induced upward flow interaction with forced convection by mechanical ventilation. We consequently tried to determine optimal ventilation strategy in case of fire arising in a confined subway station. To do so, a subscale approach was developed and measurements from Picture Imaging Velocimetry and thermal field investigations were carried out to provide comprehensive flow field analysis. The flow patterns involved have been described for several geometric configurations i.e. first without and then with passenger staircases. In staircases, limits of the momentum ratio determining presence of the stack phenomenon were identified for two staircase configurations. A need for compromise between smokefree staircases and a globally stratified platform environment was underscored. Extraction tends to decrease by 22% the safety height parameter in the station, but a concomitant 50% ventilation mass flow rate increase ensures that the two passenger accesses will be smoke-free. The results of this study correspond to the existing guidelines for prevention and ventilation in case of underground station fire.
1. Introduction Fire threat is a major concern in subway stations with severe consequences due to the presence of intensive crowds. Not only does smoke drastically reduce the visibility of evacuation routes but also, due to platform confinement, stack effect through upward staircases results in smoke propagation in areas from which people may consequently be unlikely to escape. Numerous investigations on subway fires have been carried out. Cheng et al. [1] used a numerical tool to investigate emergency ventilation procedures in case of fire in underground facilities. The results highlighted the efficacy of a push-pull ventilation model as a way to control exhaust-air temperature and smoke once fire breaks out. However, Gao et al. [2,3] showed that while mechanical ventilation obviously helps to control the horizontal spread of smoke, it has little effect on vertical dispersion. To reach their conclusions, Gao et al. [2] studied characteristic station geometries with roof heights as input parameters using the Large Eddy Simulation numerical technique. Kang [4] investigated smoke visibility based on line of sight from non-stationary numerical predictions, and showed how quickly visibility is blocked. Ventilation strategy is consequently crucial for safety purposes. Rie et al. [5] studied the optimal emergency mode of operating a comprehensive smoke ventilation system both numerically and experimentally. In the push-pull mode, turbulence may favor smoke dispersion and evacuation is not ensured. Yang et al. [6] found the extraction mode to be the most favorable one but they considered station design highly significant. Yuan et al. [7] underlined ⁎
the need to evaluate and optimize the ventilation modes of station platforms using high-fidelity CFD techniques without referring to measured boundary conditions as had Rie et al. [5], Yang et al. [6] and Park et al. [8]. All the works quoted above take into consideration the main features of a station platform, tunnels and openings such as staircases, ventilation systems and confined areas. The arrangement of the station itself may have a major impact on smoke propagation. Smoke extraction is also an issue in case of fire in a tunnel and Vauquelin and Mégret [9] experimentally investigated the influence of relative position between mechanical exhaust and fire. They concluded that in a tunnel the position of ceiling extraction with regard to fire position had only a negligible effect on smoke evacuation procedures. More recently, Li et al. [10] used FDS to determine a critical Froude number to predict the start of plug-holing in a tunnel under fire. Flow pattern in the ceiling was numerically investigated by Chen et al. [11] who demonstrated the importance of stack effect in a subway station with several openings; according to their findings, smoke spreads through the nearest vertical exit and moves toward the shaft of the smaller cross-sectional area. Chen et al. [12] extended their work by proposing an optimized ventilation strategy suited to a complex underground station with four exits. Their numerical results showed that stack effect is a key factor in smoke control near staircases; in their ventilation scenarios, at least one staircase remained smoke-free, thereby providing passengers with a way to escape. Stack effect (also known as “chimney effect”) is of decisive importance in smoke control when a fire occurs near staircases; in such a case, mechanical smoke
Corresponding author. E-mail address: [email protected] (F. Plourde).
http://dx.doi.org/10.1016/j.firesaf.2016.10.001 Received 27 April 2016; Received in revised form 1 September 2016; Accepted 10 October 2016 0379-7112/ © 2016 Elsevier Ltd. All rights reserved.
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Δρ = ρt −ρj [kg m−3]
Nomenclature
−1
T − Tt T j − Tt
[dimensionless]
Kinematic viscosity [m s ] Density [kg m−3]
( ) j t tun ext
V′2 ,
Average over time Jet Transversal flow (in upstream tunnel) Tunnel Extraction
Dimensionless number
Fr
Greek symbols
Δρ
2
Subscripts and overlines
W ′ 2 Longitudinal, lateral and vertical fluctuation velocity [m s−1] Wj Vertical velocity of jet at Z*=0 [m s−1] X , Y , Z Longitudinal, lateral and vertical Cartesian coordinates [m] X *, Y *, Z * Longitudinal, lateral and vertical Cartesian coordinates normalized by jet characteristic diameter: D hj U′ 2 ,
Dimensionless temperature: θ =
θ ν ρ
Hydraulic diameter [m] Mass flow rate [kg s−1] ṁ Dimensionless mass flow rate: ṁ *= ṁ [dimensionless] ext Local temperature [K] Longitudinal, lateral and vertical velocity component [m s−1] U *, V *, W * Longitudinal, lateral and vertical component normalized by jet characteristic velocity Wj [dimensionless]
Dh ṁ ṁ * T U, V , W
U2
ρ
Froude number: Fr= Δρt gD t
htun
ρj Wj 2
r
Momentum ratio: r=
Re
Reynolds number: Re=
ρt Ut 2 UDh ν
Difference of density between jet and transversal flow: on the roof of the station; they could be removed so as to provide the reference configuration i.e. with flat roof. To provide accurate measurement within the entire subscale station, fire was modeled by a hot jet, which was scaled from a similitude approach based on momentum ratio conservation. The dynamic and temperature of the jet were defined corresponding to a 5 MW heat release rate. Located at the station center, a hot gas jet was injected with a stationary velocity of 2.9 m s−1 and at a temperature of 410 K – these values were measured and controlled by PIV and thermocouples at the jet outflow and by a thermocouple located within the jet. It corresponded to a Reynolds number based on the jet hydraulic diameter of 0.05 m equal to Rej=5484 – i.e. corresponding to an axisymmetric turbulent jet. Smoke propagation was then studied from temperature propagation for several r ratio (Table 1) levels, which were modulated by controlling the extracted mass flow rate. The r parameter [13–19] is based on momentum ratio conservation between
control cannot prevent its spread throughout the staircases and possibly beyond. Up until now, the influence of ventilation management in subway stations has been studied mainly from a numerical approach. The present work is dedicated to experimental investigation of smoke propagation within a subscale station, and its objectives are: 1. to study the flow pattern in case of fire, 2. to understand the relationships between staircases and forced ventilation and, 3. to define momentum ratio range in conjunction with safety rules. A dedicated experimental test bench was developed to depict smoke spreading in a confined environment with free opening and transverse flow. To reduce the scale from real size to laboratory scale, a 1/24 downsize scaling coefficient on all the main characteristic features of a characteristic Parisian subway station was applied.
2. Experimental set-up and measurement procedures
upward flow and longitudinal flow as r=
Fig. 1 shows a scale model of a subway station with dimensions of 1.05 m×0.25 m×0.47 m including characteristic exit areas. The main part corresponds to a platform connected by two tunnels located downstream and upstream from the station. Both tunnels are 0.22 m high and 0.5 m long. The downstream part of the tunnel is connected to an extraction mass flow rate blower while the upstream tunnel inflow is passive and controlled by honeycomb and porous material. Two vertical staircases with square dimensions of 0.1 m×0.1 m were located
ρj Wj 2 ρt Ut 2
where ρj Wj 2 and ρt Ut 2
correspond to jet and transverse momentum respectively. The r ratio range is used to underline the dynamic involved between the flows and corresponds to Froude number ranging from 0 to 0.183. Note that higher Froude numbers were studied but are not reported hereinafter. The jet temperature was chosen in order to enable oil particle seeding
Table 1 Boundary conditions with regard to r ratio function.
ṁ ext 0.10−3 (kg s−1)
ṁj 0.10−3 (kg s−1)
Tj (K)
r
Fr
0 10.3 26.7
4.9 4.9 4.9
410.7 410.7 410.7
→∞ 30 7.4
0 0.011 0.183
Reext =
Uext Dh tun νext
0 2335 6019
Table 2 Influence of r ratio on tilted jet angle, mean downstream tunnel condition and high thermal safety.
r Ut (m s−1) Tt (K) X* impact Z θ*≤0.2 Fig. 1. Schematic diagram of the experimental setup.
76
→∞ 0.08 311.8 0 1.5
30 −0.08 303.2 −0.7 2.5
7.4 −0.33 298.2 −2.0 4.5
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Fig. 2. View of P.I.V. devices and temperature sensors.
while jet velocity arises from momentum ratio conservation. Hot jet source also ensures safe reproducible experiments (Table 2). In the subscale station two openings were controlled i.e. inlet hot jet and downstream tunnels while three openings could be either inlet or outlet: the two staircases and the upstream tunnel. One of our main goals was to depict the hydraulic conditions through which passenger paths could be cleared of smoke. An underground subway station belongs to a given network and was simulated by corresponding pressure loss at upstream tunnel. Our reference case was characterized by equal distribution of mass flow rate between upstream tunnel (50%) and individual passenger access (50%). To depict flow field behavior, a standard 2D PIV triggered at 4 Hz with a window size of 0.260 m x 0.325 m and high resolution pixel level (1024×1248) provided the velocity flow field. This window is presented in Fig. 2. Two micrometric moving systems were used to accurately locate laser plane and cameras. All windows were superimposed with an average overlap of 30% to ensure a full domain investigation. Measurements of flow field in mid-plane (Y =0) required 25 interconnected windows. From a temporal viewpoint, 500 double pictures were
Fig. 3. Contours of U* and W * components for reference case (without accesses) in the mid-plane (Y =0) – dashed line represented the jet trajectory.
* * Fig. 4. Contours of U′2 and W′2 fluctuation velocity components for reference case (without accesses) in the mid-plane (Y =0).
Fig. 5. Contour of normalized temperature and fluctuation field in the station volume for reference case (without accesses) in the mid-plane (Y =0).
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Fig. 6. Contour of temperature evolution in the station for 3 r ratio values and temperature profiles for X* =−3 and X* =3 – cases without accesses. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
recorded for each window and were found accurate enough to depict first and second-order average data. A preliminary study (not shown in the present paper for the sake of simplification) involving from 100 to 2000 double images was carried out to depict the minimum data allowing both the average and the fluctuating fields to be representative. Main flow and jet behavior were consequently analyzed by characterizing particle behavior. Seeding of the jet was carried out through a Larskin nozzle device producing fine oil droplets (smaller than 5 µm), while the main airflow had previously been mixed with particles present in the room, generated by a commercial smoke producer (smaller than 2 µm). FlowManager 4.71 software from Dantec Dynamics was used to control the PIV experiment, image and double-pulsed laser. With the “cross-correlation” algorithm and an interrogation area of 32×32 pixels, the two velocity components (U and W in XZ planes) were determined. Spatial resolution of the velocity vector field was equal to 2.54 mm in the X -direction and 2.26 mm in the Y -direction. Smoke propagation behavior was depicted from thermal behavior analyses; measurements were carried out from thin laboratory K-type thermocouples (12.5 µm – frequency response of 128 Hz), and 3 were mounted on a single 3D movable axis (Fig. 2). Thin thermocouples were spatially shifted to avoid flow perturbation at the measurement location. This axis was located in the station within a X slot, rubber seal ensuring airtightness. XY plane investigations were carried out and five
X slots located at Z =0.045 m, 0.135 m, 0.225 m, 0.315 m and 0.405 m provided up to 15 measurement points in a XZ plane (3 thermocouples *5 slots with 0.015 mm between 2 thermocouples) at a given Y (for more than 90 X positions). Accuracy of +/−0.5 °C in temperature measurement was reached with a frequency rate equal to 25 Hz. Taking into account the differences of air temperature near the jet, glass windows, transversal air flow and sensitive thermocouple diameter, the influence of radiation on local air temperature was estimated inferior at 0.5 °C. In order to investigate the whole mid-plane (Y =0) with spatial resolution of 0.010 m, experiments were performed for more than 8 h with a time delay between each measurement point. Thermocouples were localized along glasses, in pipes and tunnels to control their changes in time and to measure flow temperature at extraction and injection. Mass flow rate extracted downstream from tunnel was determined by a dedicated blower and controlled by pressure loss diaphragm with an integrated temperature measurement probe. Similar equipment along the hot system ensured measurement accuracy of +/−2%. The procedure to conduct experimental measurement was carried out in 3 steps; first, hot jet in the station was set up during 3 h. Extracting flow was launched and additional hours were required to reach steady state conditions. Finally, measurement (velocity and temperature field) was undertaken to depict the whole flow field.
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inward-oriented with an average velocity of −0.23 m s−1. Fluctuating activities of the two components are given in Fig. 4 to underline the interaction between flows such as ascending jet and cross-flow along the ceiling and recirculation flow areas. The highest fluctuating rate is reached in the ascending jet flow with maximum fluctuating level of 12% and even 18% for the longitudinal and vertical components respectively. Fig. 5 displays the normalized average temperature distribution (θ = (T − Tt )/(Tj −Tt )) and thermal fluctuation activity in the XZ mid-plane (Y =0), thereby demonstrating the stratification arising within the station. From the mean temperature, the hottest region is clearly the jet area and the upper region of the station with temperatures reaching 320 K i.e. θ =0.23. On the contrary, colder flow is found in the lower part of the station. Temperature fluctuations mainly exhibit areas of strong interactions between jet and longitudinal flow as did fluctuating velocity. Fig. 6 displays the contour of average temperature in mid-plane (Y =0) for different ventilation conditions i.e. for r → ∞ and r = 7.4 while the preceding reference case was obtained for r = 30 (the latter was displayed again for the sake of comparison). In case of a lower r , crossflow is reinforced and helps to decrease temperature gradients by the increase of dilution. In the lower part (Z θ*≤0.2 < 4.5), flow field convection ensures constant low temperature and a sharp gradient is identified at around Z * = 3.50 . The reduction of extracted mass flow rate i.e. increasing of the r ratio favors mixing in the lower part with stratification similar to the reference case, i.e. Δθ / ΔZ * ≈0.085 for Z * < 3.75 while in the upper part of the station, temperature reaches a plateau respectively equal to 0.35 and 0.38 for r =30 and r → ∞. The boundary between the cold air at the bottom of the station and hot air is also visible for Z *=1.5 and Z * =4.5 for r → ∞ and r =7.4 respectively. To sum up, overall flow field behavior is subjected to pronounced changes with regard to r . To enhance stratification, temperature profiles at X *=−3 and X *=3 (in red triangles for r =30) were plotted. Significant gradients (Δθ / ΔZ *≈0.085) were exhibited at Z * < 3.75. Stratification is a crucial issue since it corresponds to a safety parameter. Air temperature higher than 363 K (θ =0.60) is fatal to breathing, while a temperature around 318 K (θ =0.21) is endurable, but only for a short period of time [23]. θ =0.20 has consequently been chosen as a reference level and the ventilation level must be optimized in conjunction with stratification behavior. A safety zone is defined to correspond to an area where the temperature is lower than θ =0.20 and a Z θ*≤0.2 height is consequently estimated. In this case Z θ*≤0.2 is equal to 2.5. Two other ventilation conditions were studied with r → ∞ i.e. without any flow field extraction and r =7.4. Fig. 7 shows the U * average horizontal velocity component from r → ∞ to r =7.4. When no extraction occurs (r → ∞) two reverse flow areas develop downstream and upstream from the tunnels. The flow separates from ceiling impact, thereby underscoring development of the reversal area between the jet and the vertical walls above the tunnels. Such symmetry obviously no longer exists in case of forced convection and the smaller the r ratio, the more the dissymmetry is amplified. With flow extraction equal to r =7.4, reverse flow in the upstream tunnel is decreased, while in the downstream ceiling tunnel reversed flow is strengthened compared to r =30. Interaction between the main flow and the jet results in an ascendant motion deviation with modified tilted angle impact on the celling. The impact is modified from X * = 0 to 2 for r → ∞ and r = 7.4 respectively. In parallel, the safety area expanses from Z θ*≤0.2 = 1.5 to Z θ*≤0.2 = 4.5; the increase of extraction flow rate helps to improve passenger safety. Analysis of the reference configuration helps to elucidate flow field behavior with regard to r . More specifically, our experimental approach highlighted four main areas on both sides of the jet as well as reversal areas in tunnels. Tilted jet angle is obviously enhanced with a higher extracted mass flow rate, i.e. a lower r ratio, and tilted behavior has been correlated in several literature reviews [24–26] notwithstanding a lengthwise change in area. Jet inclination helps to alter recirculation flow within the upstream tunnel. Furthermore, the safety zone Z θ*≤0.2
Fig. 7. Contours of longitudinal velocity component U* for 3 r values (r → ∞, 30 and 7.4) – without accesses.
3. Results and discussion 3.1. Characterization of smoke flow in subway station without passenger path As underlined in the literature review above, smoke propagation arises from aerothermal mechanisms between forced convection and buoyant flows; in addition, presence of free openings such as passenger paths render smoke propagation difficult to depict and ventilation strategy complex to predict. In the subscale setup, free openings upstream from the station tunnel and two passenger paths have been studied. However, to better illustrate the respective roles of these two passenger paths, we initially studied a reference case corresponding to a geometrical configuration with no passenger path, which is more likely to correspond to a hot jet in cross-flow section. This type of flow has been widely studied in the literature [20–22] and interaction between the cross-flow and the upward flow is characterized by 3D flow behavior. Our experimental investigations were carried out both in 3 XZ planes (Y =−0.125 m, Y =0 m, Y =0.125 m) and in XY planes (Z =0.015 m, 0.076 m, 0.135 m, 0.195 m, 0.255 m, 0.315 m, 0.375 m and 0.435 m). The configuration studied provided a r momentum ratio of 30. For the sake of simplification, the results presented hereinafter arise from mid-plane (Y * =Y / D=0 ). Fig. 3 presents the average U * and W * velocity components in the mid-plane (Y =0) and thereby helps to identify the jet. Note that (*) refers to non-dimensional data and that velocity components are normalized by the reference velocity of jet Wj while length is divided by jet diameter. The dashed lines in Fig. 3 correspond to the ascending jet flow center, which is deviated from the right to the left due to the cross-flow. The jet impinges upon the station ceiling at X *=−0.70 and flows develop along the latter. A smaller recirculation flow develops in the downstream tunnel and reattachment occurs due to the extracting condition. Note that in the opposite position i.e. in the upstream tunnel, a contrary flow field develops for Z * > 3, which leans against the ceiling and exits from the outlet opening. However, bulk velocity is 79
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Fig. 8. Contours of longitudinal (left) and vertical (right) velocity component U* and W * for 3 r values with two accesses.
When there is no extraction, smoke spreads along the vertical passenger exits, which consequently require transversal extraction for security purposes. With r =30, passenger access behaves oppositely. While the air flow in the downstream passenger exit still presents a characteristic velocity of W *=0.154, upstream access is driven by the cold air entering the station. These opposed motions increase stress on the jet and amplify its deviation. In this case, the jet impinges upon the ceiling station at X * =−1.50 while without access (reference case), jet impact occurs at X *=0.70 (cf. Fig. 8). Changed airflow direction in the upstream access also modifies stratification within the station. Contrarily to the case not involving extraction, hot temperature in the upper part increases. In the downstream section of the station (Fig. 9) the threshold limit of θ =0.20 is shifted from Z θ*≤0.2 =4.5 to Z θ*≤0.2 =3.5. In addition, the dissymmetry of horizontal velocity components is increased. In the upstream tunnel, reverse flow develops less strongly than in the reference case; this is due to the decrease of outlet flow rate while mean mass flow rate decreases from ṁ * =0.52 to 0.22. Such behavior (only one vertical passenger access is driven by stack effect) corresponds to the second regime identified. Smoke propagation is also limited to one access, and smoke control is thereby rendered more effective. For r =7.4, no stack effect is detected and both accesses correspond
increases between 2.5 and 4.5 i.e. an increase of 40% for an increase of the extraction mass flow rate of 2.6 to a r ratio decrease from 30 to 7.4 respectively. 3.2. Characterization of smoke flow in subway station with 2 vertical passenger accesses The two vertical passenger accesses at the station ceiling provide additional free inlets or outlets. As openings, they directly affect overall mass flow rate balance as well as smoke propagation. Figs. 8 and 9 present the U * and W * contours and temperature ratio θ contours in mid-plane for the three r levels studied respectively. Without extracting flow field (r → ∞), a hot plume develops and separates into the two vertical accesses with a characteristic velocity of W * =0.188. The ascendant motions are symmetrical and occur in the vertical walls above the tunnels. As regards jet impact, two reverse flows develop along the ceiling of the station, escaping from vertical walls and bypassing the opening created by the accesses. Lastly, the flows impinging upon the wall pass through the accesses; such phenomena create over-speed within the latter. The accesses significantly modify stratification. In this case, the safety limit (θ =0.20) is detected at Z θ*≤0.2 =4.5, which is pronouncedly higher than in a case without access (Z θ*≤0.2 =1.5); the safety zone is consequently enlarged. 80
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Fig. 9. Contours of temperature and fluctuation temperature field for 3 r values with two accesses – white lines correspond to the position of accesses.
upstream tunnel limits the impact of mean flow on the jet. Mode of access likewise limits transverse flow from inlet, which as a result declines from X *=−2 (without access) to X *=−1.20 the impinging jet location. This corresponds to the third identified regime i.e. no stack effect is present. Evacuation of the subway station can then be carried out under optimal safety conditions. In the station, temperature is blended and hot air regions (θ =0.10–305 K) are visible near the ground at the base of the jet, but in this case Z θ*≤0.2 based on θ =0.20 increases to 3.5 for r =30 to 4.5. The change of extraction flow rate from 30 to 7.4 removes stack effect and protects the bottom of the station from dangerous hot air (θ > 0.20). The fluctuations of temperature, Fig. 9, illustrate the entrance of hot air from the jet and the cold air from the outlet in the station. These limits are underlined by strong fluctuations, especially for r =7.4 for the jet, and r =30 for the entrance of cold air around X *=7 and X *=9. In order to better illustrate the three regime thresholds (stack effect in the two accesses, in only one access and no stack effect at all), several additional aspect ratios were studied from r =4 to 64 corresponding to Froude number of 0.626-0.002 and presence or absence of stack is identified from the temperature recorded within the accesses. All of the results, presented in Fig. 10, show that for r ≤ r2 , no stack effect occurs and that the two accesses are fed by outside air. For r ranging from r1 =32 to r2 =16, stack phenomena are limited to downstream access. Above r1, stack phenomena occurs in both accesses. The first stack effect appears in downstream access and can be explained by the jet inclination. The inlet air flow decrease in the
Fig. 10. Evolution of mean temperature in staircases (correspond to stack effect) according to r ratio and Froude number.
to air inlet in the station with a vertical velocity of around 0.24. Air inlet from the upstream tunnel tends to decrease (0.82→0.26) and reverse flow development along the ceiling is favored. The inlet decrease at 81
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upstream tunnel tends to strengthen the rising jet, which is influenced in the station downstream. Hot air can escape from the station through this opening, i.e. the influence of extraction flow rate is reduced. Flow organization in the station is then modified, especially the tilted jet angle. To ensure evacuation of the subway station and provide at least one smoke-free access, ventilation rate must yield a r ratio lower than the r1 threshold limit. 4. Conclusion A dedicated test bench was developed to study the relationship between hot ascending flow and transverse flow and the resulting potential stack effect phenomenon. The main goal in the present work was to find a strategy to provide areas free of smoke for passengers to escape in case of fire in subway station. To do so, the crossflow rate increase (decrease of the r momentum ratio) highlighted dilution in the reference case i.e. with no staircase. Temperature profiles were self-similar with a constant slope for Z * < 3.75 ( Δθ / ΔZ * ≈0.085) reaching a smaller slope ( Δθ / ΔZ * ≈0.012) in the higher part of the station and overall levels were scaled down with the r decrease. In the geometry studied, the average Z θ*≤0.2 increased by a ratio of 1.8 from r =30 to 7.4. Inlet flow strongly interacted with the jet and, as expected, it directly controlled the tilted resulting angle as well as overall flow field behavior. The presence of two accesses modified global flow field organization and our main result concerns the limit setting of the r momentum ratio of the 3 possible regimes: the two accesses free of smoke, one out of two with stack effect and the two accesses with smoke. The latter regime was obtained for the smaller extraction rate (i.e. for r > 32) with stack effect occurring in the two passenger accesses simultaneously. This is obviously the worst scenario in terms of safety, but it occurs only when extraction mass flow is not strong enough to sweep off smoke arising from the source. The intermediate regime corresponded to stack phenomenon arising in only one passenger access, while fresh air entered from the passenger access located in the side of the air ventilating inlet, in the upstream access in the case studied. Finally, the strongest mass flow rate (r < 16) may be due to the presence the two passenger accesses of smoke, which allow safe passenger evacuation. However, the Z θ*≤0.2 was decreased by the decrease of the r momentum ratio. From r →∞ to r =30 the Z θ*≤0.2 was reduced by 22%. One should also underline the sensitivity of the overall mechanisms; a change of 50% in extracting mass flow rate leads from stack phenomena in two accesses to no stack phenomena, underlining the sensitivity of ventilation control. References [1] L.H. Cheng, T.H. Ueng, C.W. Liu, Simulation of ventilation and fire in the
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Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Smoke spreading analysis from an experimental subway scale model ⁎
B. Giachetti, D. Couton, F. Plourde
crossmark
Institut Pprime, Dept. FTC, Axe COST, ENSMA, 1 Avenue Clement Ader BP40109, 86961 Chasseneuil du Poitou, France
A R T I C L E I N F O
A BS T RAC T
Keywords: Jet in cross flow Subway station Ventilation Stack effect
The aim of this study is to experimentally investigate buoyancy-induced upward flow interaction with forced convection by mechanical ventilation. We consequently tried to determine optimal ventilation strategy in case of fire arising in a confined subway station. To do so, a subscale approach was developed and measurements from Picture Imaging Velocimetry and thermal field investigations were carried out to provide comprehensive flow field analysis. The flow patterns involved have been described for several geometric configurations i.e. first without and then with passenger staircases. In staircases, limits of the momentum ratio determining presence of the stack phenomenon were identified for two staircase configurations. A need for compromise between smokefree staircases and a globally stratified platform environment was underscored. Extraction tends to decrease by 22% the safety height parameter in the station, but a concomitant 50% ventilation mass flow rate increase ensures that the two passenger accesses will be smoke-free. The results of this study correspond to the existing guidelines for prevention and ventilation in case of underground station fire.
1. Introduction Fire threat is a major concern in subway stations with severe consequences due to the presence of intensive crowds. Not only does smoke drastically reduce the visibility of evacuation routes but also, due to platform confinement, stack effect through upward staircases results in smoke propagation in areas from which people may consequently be unlikely to escape. Numerous investigations on subway fires have been carried out. Cheng et al. [1] used a numerical tool to investigate emergency ventilation procedures in case of fire in underground facilities. The results highlighted the efficacy of a push-pull ventilation model as a way to control exhaust-air temperature and smoke once fire breaks out. However, Gao et al. [2,3] showed that while mechanical ventilation obviously helps to control the horizontal spread of smoke, it has little effect on vertical dispersion. To reach their conclusions, Gao et al. [2] studied characteristic station geometries with roof heights as input parameters using the Large Eddy Simulation numerical technique. Kang [4] investigated smoke visibility based on line of sight from non-stationary numerical predictions, and showed how quickly visibility is blocked. Ventilation strategy is consequently crucial for safety purposes. Rie et al. [5] studied the optimal emergency mode of operating a comprehensive smoke ventilation system both numerically and experimentally. In the push-pull mode, turbulence may favor smoke dispersion and evacuation is not ensured. Yang et al. [6] found the extraction mode to be the most favorable one but they considered station design highly significant. Yuan et al. [7] underlined ⁎
the need to evaluate and optimize the ventilation modes of station platforms using high-fidelity CFD techniques without referring to measured boundary conditions as had Rie et al. [5], Yang et al. [6] and Park et al. [8]. All the works quoted above take into consideration the main features of a station platform, tunnels and openings such as staircases, ventilation systems and confined areas. The arrangement of the station itself may have a major impact on smoke propagation. Smoke extraction is also an issue in case of fire in a tunnel and Vauquelin and Mégret [9] experimentally investigated the influence of relative position between mechanical exhaust and fire. They concluded that in a tunnel the position of ceiling extraction with regard to fire position had only a negligible effect on smoke evacuation procedures. More recently, Li et al. [10] used FDS to determine a critical Froude number to predict the start of plug-holing in a tunnel under fire. Flow pattern in the ceiling was numerically investigated by Chen et al. [11] who demonstrated the importance of stack effect in a subway station with several openings; according to their findings, smoke spreads through the nearest vertical exit and moves toward the shaft of the smaller cross-sectional area. Chen et al. [12] extended their work by proposing an optimized ventilation strategy suited to a complex underground station with four exits. Their numerical results showed that stack effect is a key factor in smoke control near staircases; in their ventilation scenarios, at least one staircase remained smoke-free, thereby providing passengers with a way to escape. Stack effect (also known as “chimney effect”) is of decisive importance in smoke control when a fire occurs near staircases; in such a case, mechanical smoke
Corresponding author. E-mail address: [email protected] (F. Plourde).
http://dx.doi.org/10.1016/j.firesaf.2016.10.001 Received 27 April 2016; Received in revised form 1 September 2016; Accepted 10 October 2016 0379-7112/ © 2016 Elsevier Ltd. All rights reserved.
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Δρ = ρt −ρj [kg m−3]
Nomenclature
−1
T − Tt T j − Tt
[dimensionless]
Kinematic viscosity [m s ] Density [kg m−3]
( ) j t tun ext
V′2 ,
Average over time Jet Transversal flow (in upstream tunnel) Tunnel Extraction
Dimensionless number
Fr
Greek symbols
Δρ
2
Subscripts and overlines
W ′ 2 Longitudinal, lateral and vertical fluctuation velocity [m s−1] Wj Vertical velocity of jet at Z*=0 [m s−1] X , Y , Z Longitudinal, lateral and vertical Cartesian coordinates [m] X *, Y *, Z * Longitudinal, lateral and vertical Cartesian coordinates normalized by jet characteristic diameter: D hj U′ 2 ,
Dimensionless temperature: θ =
θ ν ρ
Hydraulic diameter [m] Mass flow rate [kg s−1] ṁ Dimensionless mass flow rate: ṁ *= ṁ [dimensionless] ext Local temperature [K] Longitudinal, lateral and vertical velocity component [m s−1] U *, V *, W * Longitudinal, lateral and vertical component normalized by jet characteristic velocity Wj [dimensionless]
Dh ṁ ṁ * T U, V , W
U2
ρ
Froude number: Fr= Δρt gD t
htun
ρj Wj 2
r
Momentum ratio: r=
Re
Reynolds number: Re=
ρt Ut 2 UDh ν
Difference of density between jet and transversal flow: on the roof of the station; they could be removed so as to provide the reference configuration i.e. with flat roof. To provide accurate measurement within the entire subscale station, fire was modeled by a hot jet, which was scaled from a similitude approach based on momentum ratio conservation. The dynamic and temperature of the jet were defined corresponding to a 5 MW heat release rate. Located at the station center, a hot gas jet was injected with a stationary velocity of 2.9 m s−1 and at a temperature of 410 K – these values were measured and controlled by PIV and thermocouples at the jet outflow and by a thermocouple located within the jet. It corresponded to a Reynolds number based on the jet hydraulic diameter of 0.05 m equal to Rej=5484 – i.e. corresponding to an axisymmetric turbulent jet. Smoke propagation was then studied from temperature propagation for several r ratio (Table 1) levels, which were modulated by controlling the extracted mass flow rate. The r parameter [13–19] is based on momentum ratio conservation between
control cannot prevent its spread throughout the staircases and possibly beyond. Up until now, the influence of ventilation management in subway stations has been studied mainly from a numerical approach. The present work is dedicated to experimental investigation of smoke propagation within a subscale station, and its objectives are: 1. to study the flow pattern in case of fire, 2. to understand the relationships between staircases and forced ventilation and, 3. to define momentum ratio range in conjunction with safety rules. A dedicated experimental test bench was developed to depict smoke spreading in a confined environment with free opening and transverse flow. To reduce the scale from real size to laboratory scale, a 1/24 downsize scaling coefficient on all the main characteristic features of a characteristic Parisian subway station was applied.
2. Experimental set-up and measurement procedures
upward flow and longitudinal flow as r=
Fig. 1 shows a scale model of a subway station with dimensions of 1.05 m×0.25 m×0.47 m including characteristic exit areas. The main part corresponds to a platform connected by two tunnels located downstream and upstream from the station. Both tunnels are 0.22 m high and 0.5 m long. The downstream part of the tunnel is connected to an extraction mass flow rate blower while the upstream tunnel inflow is passive and controlled by honeycomb and porous material. Two vertical staircases with square dimensions of 0.1 m×0.1 m were located
ρj Wj 2 ρt Ut 2
where ρj Wj 2 and ρt Ut 2
correspond to jet and transverse momentum respectively. The r ratio range is used to underline the dynamic involved between the flows and corresponds to Froude number ranging from 0 to 0.183. Note that higher Froude numbers were studied but are not reported hereinafter. The jet temperature was chosen in order to enable oil particle seeding
Table 1 Boundary conditions with regard to r ratio function.
ṁ ext 0.10−3 (kg s−1)
ṁj 0.10−3 (kg s−1)
Tj (K)
r
Fr
0 10.3 26.7
4.9 4.9 4.9
410.7 410.7 410.7
→∞ 30 7.4
0 0.011 0.183
Reext =
Uext Dh tun νext
0 2335 6019
Table 2 Influence of r ratio on tilted jet angle, mean downstream tunnel condition and high thermal safety.
r Ut (m s−1) Tt (K) X* impact Z θ*≤0.2 Fig. 1. Schematic diagram of the experimental setup.
76
→∞ 0.08 311.8 0 1.5
30 −0.08 303.2 −0.7 2.5
7.4 −0.33 298.2 −2.0 4.5
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Fig. 2. View of P.I.V. devices and temperature sensors.
while jet velocity arises from momentum ratio conservation. Hot jet source also ensures safe reproducible experiments (Table 2). In the subscale station two openings were controlled i.e. inlet hot jet and downstream tunnels while three openings could be either inlet or outlet: the two staircases and the upstream tunnel. One of our main goals was to depict the hydraulic conditions through which passenger paths could be cleared of smoke. An underground subway station belongs to a given network and was simulated by corresponding pressure loss at upstream tunnel. Our reference case was characterized by equal distribution of mass flow rate between upstream tunnel (50%) and individual passenger access (50%). To depict flow field behavior, a standard 2D PIV triggered at 4 Hz with a window size of 0.260 m x 0.325 m and high resolution pixel level (1024×1248) provided the velocity flow field. This window is presented in Fig. 2. Two micrometric moving systems were used to accurately locate laser plane and cameras. All windows were superimposed with an average overlap of 30% to ensure a full domain investigation. Measurements of flow field in mid-plane (Y =0) required 25 interconnected windows. From a temporal viewpoint, 500 double pictures were
Fig. 3. Contours of U* and W * components for reference case (without accesses) in the mid-plane (Y =0) – dashed line represented the jet trajectory.
* * Fig. 4. Contours of U′2 and W′2 fluctuation velocity components for reference case (without accesses) in the mid-plane (Y =0).
Fig. 5. Contour of normalized temperature and fluctuation field in the station volume for reference case (without accesses) in the mid-plane (Y =0).
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Fig. 6. Contour of temperature evolution in the station for 3 r ratio values and temperature profiles for X* =−3 and X* =3 – cases without accesses. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
recorded for each window and were found accurate enough to depict first and second-order average data. A preliminary study (not shown in the present paper for the sake of simplification) involving from 100 to 2000 double images was carried out to depict the minimum data allowing both the average and the fluctuating fields to be representative. Main flow and jet behavior were consequently analyzed by characterizing particle behavior. Seeding of the jet was carried out through a Larskin nozzle device producing fine oil droplets (smaller than 5 µm), while the main airflow had previously been mixed with particles present in the room, generated by a commercial smoke producer (smaller than 2 µm). FlowManager 4.71 software from Dantec Dynamics was used to control the PIV experiment, image and double-pulsed laser. With the “cross-correlation” algorithm and an interrogation area of 32×32 pixels, the two velocity components (U and W in XZ planes) were determined. Spatial resolution of the velocity vector field was equal to 2.54 mm in the X -direction and 2.26 mm in the Y -direction. Smoke propagation behavior was depicted from thermal behavior analyses; measurements were carried out from thin laboratory K-type thermocouples (12.5 µm – frequency response of 128 Hz), and 3 were mounted on a single 3D movable axis (Fig. 2). Thin thermocouples were spatially shifted to avoid flow perturbation at the measurement location. This axis was located in the station within a X slot, rubber seal ensuring airtightness. XY plane investigations were carried out and five
X slots located at Z =0.045 m, 0.135 m, 0.225 m, 0.315 m and 0.405 m provided up to 15 measurement points in a XZ plane (3 thermocouples *5 slots with 0.015 mm between 2 thermocouples) at a given Y (for more than 90 X positions). Accuracy of +/−0.5 °C in temperature measurement was reached with a frequency rate equal to 25 Hz. Taking into account the differences of air temperature near the jet, glass windows, transversal air flow and sensitive thermocouple diameter, the influence of radiation on local air temperature was estimated inferior at 0.5 °C. In order to investigate the whole mid-plane (Y =0) with spatial resolution of 0.010 m, experiments were performed for more than 8 h with a time delay between each measurement point. Thermocouples were localized along glasses, in pipes and tunnels to control their changes in time and to measure flow temperature at extraction and injection. Mass flow rate extracted downstream from tunnel was determined by a dedicated blower and controlled by pressure loss diaphragm with an integrated temperature measurement probe. Similar equipment along the hot system ensured measurement accuracy of +/−2%. The procedure to conduct experimental measurement was carried out in 3 steps; first, hot jet in the station was set up during 3 h. Extracting flow was launched and additional hours were required to reach steady state conditions. Finally, measurement (velocity and temperature field) was undertaken to depict the whole flow field.
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inward-oriented with an average velocity of −0.23 m s−1. Fluctuating activities of the two components are given in Fig. 4 to underline the interaction between flows such as ascending jet and cross-flow along the ceiling and recirculation flow areas. The highest fluctuating rate is reached in the ascending jet flow with maximum fluctuating level of 12% and even 18% for the longitudinal and vertical components respectively. Fig. 5 displays the normalized average temperature distribution (θ = (T − Tt )/(Tj −Tt )) and thermal fluctuation activity in the XZ mid-plane (Y =0), thereby demonstrating the stratification arising within the station. From the mean temperature, the hottest region is clearly the jet area and the upper region of the station with temperatures reaching 320 K i.e. θ =0.23. On the contrary, colder flow is found in the lower part of the station. Temperature fluctuations mainly exhibit areas of strong interactions between jet and longitudinal flow as did fluctuating velocity. Fig. 6 displays the contour of average temperature in mid-plane (Y =0) for different ventilation conditions i.e. for r → ∞ and r = 7.4 while the preceding reference case was obtained for r = 30 (the latter was displayed again for the sake of comparison). In case of a lower r , crossflow is reinforced and helps to decrease temperature gradients by the increase of dilution. In the lower part (Z θ*≤0.2 < 4.5), flow field convection ensures constant low temperature and a sharp gradient is identified at around Z * = 3.50 . The reduction of extracted mass flow rate i.e. increasing of the r ratio favors mixing in the lower part with stratification similar to the reference case, i.e. Δθ / ΔZ * ≈0.085 for Z * < 3.75 while in the upper part of the station, temperature reaches a plateau respectively equal to 0.35 and 0.38 for r =30 and r → ∞. The boundary between the cold air at the bottom of the station and hot air is also visible for Z *=1.5 and Z * =4.5 for r → ∞ and r =7.4 respectively. To sum up, overall flow field behavior is subjected to pronounced changes with regard to r . To enhance stratification, temperature profiles at X *=−3 and X *=3 (in red triangles for r =30) were plotted. Significant gradients (Δθ / ΔZ *≈0.085) were exhibited at Z * < 3.75. Stratification is a crucial issue since it corresponds to a safety parameter. Air temperature higher than 363 K (θ =0.60) is fatal to breathing, while a temperature around 318 K (θ =0.21) is endurable, but only for a short period of time [23]. θ =0.20 has consequently been chosen as a reference level and the ventilation level must be optimized in conjunction with stratification behavior. A safety zone is defined to correspond to an area where the temperature is lower than θ =0.20 and a Z θ*≤0.2 height is consequently estimated. In this case Z θ*≤0.2 is equal to 2.5. Two other ventilation conditions were studied with r → ∞ i.e. without any flow field extraction and r =7.4. Fig. 7 shows the U * average horizontal velocity component from r → ∞ to r =7.4. When no extraction occurs (r → ∞) two reverse flow areas develop downstream and upstream from the tunnels. The flow separates from ceiling impact, thereby underscoring development of the reversal area between the jet and the vertical walls above the tunnels. Such symmetry obviously no longer exists in case of forced convection and the smaller the r ratio, the more the dissymmetry is amplified. With flow extraction equal to r =7.4, reverse flow in the upstream tunnel is decreased, while in the downstream ceiling tunnel reversed flow is strengthened compared to r =30. Interaction between the main flow and the jet results in an ascendant motion deviation with modified tilted angle impact on the celling. The impact is modified from X * = 0 to 2 for r → ∞ and r = 7.4 respectively. In parallel, the safety area expanses from Z θ*≤0.2 = 1.5 to Z θ*≤0.2 = 4.5; the increase of extraction flow rate helps to improve passenger safety. Analysis of the reference configuration helps to elucidate flow field behavior with regard to r . More specifically, our experimental approach highlighted four main areas on both sides of the jet as well as reversal areas in tunnels. Tilted jet angle is obviously enhanced with a higher extracted mass flow rate, i.e. a lower r ratio, and tilted behavior has been correlated in several literature reviews [24–26] notwithstanding a lengthwise change in area. Jet inclination helps to alter recirculation flow within the upstream tunnel. Furthermore, the safety zone Z θ*≤0.2
Fig. 7. Contours of longitudinal velocity component U* for 3 r values (r → ∞, 30 and 7.4) – without accesses.
3. Results and discussion 3.1. Characterization of smoke flow in subway station without passenger path As underlined in the literature review above, smoke propagation arises from aerothermal mechanisms between forced convection and buoyant flows; in addition, presence of free openings such as passenger paths render smoke propagation difficult to depict and ventilation strategy complex to predict. In the subscale setup, free openings upstream from the station tunnel and two passenger paths have been studied. However, to better illustrate the respective roles of these two passenger paths, we initially studied a reference case corresponding to a geometrical configuration with no passenger path, which is more likely to correspond to a hot jet in cross-flow section. This type of flow has been widely studied in the literature [20–22] and interaction between the cross-flow and the upward flow is characterized by 3D flow behavior. Our experimental investigations were carried out both in 3 XZ planes (Y =−0.125 m, Y =0 m, Y =0.125 m) and in XY planes (Z =0.015 m, 0.076 m, 0.135 m, 0.195 m, 0.255 m, 0.315 m, 0.375 m and 0.435 m). The configuration studied provided a r momentum ratio of 30. For the sake of simplification, the results presented hereinafter arise from mid-plane (Y * =Y / D=0 ). Fig. 3 presents the average U * and W * velocity components in the mid-plane (Y =0) and thereby helps to identify the jet. Note that (*) refers to non-dimensional data and that velocity components are normalized by the reference velocity of jet Wj while length is divided by jet diameter. The dashed lines in Fig. 3 correspond to the ascending jet flow center, which is deviated from the right to the left due to the cross-flow. The jet impinges upon the station ceiling at X *=−0.70 and flows develop along the latter. A smaller recirculation flow develops in the downstream tunnel and reattachment occurs due to the extracting condition. Note that in the opposite position i.e. in the upstream tunnel, a contrary flow field develops for Z * > 3, which leans against the ceiling and exits from the outlet opening. However, bulk velocity is 79
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Fig. 8. Contours of longitudinal (left) and vertical (right) velocity component U* and W * for 3 r values with two accesses.
When there is no extraction, smoke spreads along the vertical passenger exits, which consequently require transversal extraction for security purposes. With r =30, passenger access behaves oppositely. While the air flow in the downstream passenger exit still presents a characteristic velocity of W *=0.154, upstream access is driven by the cold air entering the station. These opposed motions increase stress on the jet and amplify its deviation. In this case, the jet impinges upon the ceiling station at X * =−1.50 while without access (reference case), jet impact occurs at X *=0.70 (cf. Fig. 8). Changed airflow direction in the upstream access also modifies stratification within the station. Contrarily to the case not involving extraction, hot temperature in the upper part increases. In the downstream section of the station (Fig. 9) the threshold limit of θ =0.20 is shifted from Z θ*≤0.2 =4.5 to Z θ*≤0.2 =3.5. In addition, the dissymmetry of horizontal velocity components is increased. In the upstream tunnel, reverse flow develops less strongly than in the reference case; this is due to the decrease of outlet flow rate while mean mass flow rate decreases from ṁ * =0.52 to 0.22. Such behavior (only one vertical passenger access is driven by stack effect) corresponds to the second regime identified. Smoke propagation is also limited to one access, and smoke control is thereby rendered more effective. For r =7.4, no stack effect is detected and both accesses correspond
increases between 2.5 and 4.5 i.e. an increase of 40% for an increase of the extraction mass flow rate of 2.6 to a r ratio decrease from 30 to 7.4 respectively. 3.2. Characterization of smoke flow in subway station with 2 vertical passenger accesses The two vertical passenger accesses at the station ceiling provide additional free inlets or outlets. As openings, they directly affect overall mass flow rate balance as well as smoke propagation. Figs. 8 and 9 present the U * and W * contours and temperature ratio θ contours in mid-plane for the three r levels studied respectively. Without extracting flow field (r → ∞), a hot plume develops and separates into the two vertical accesses with a characteristic velocity of W * =0.188. The ascendant motions are symmetrical and occur in the vertical walls above the tunnels. As regards jet impact, two reverse flows develop along the ceiling of the station, escaping from vertical walls and bypassing the opening created by the accesses. Lastly, the flows impinging upon the wall pass through the accesses; such phenomena create over-speed within the latter. The accesses significantly modify stratification. In this case, the safety limit (θ =0.20) is detected at Z θ*≤0.2 =4.5, which is pronouncedly higher than in a case without access (Z θ*≤0.2 =1.5); the safety zone is consequently enlarged. 80
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Fig. 9. Contours of temperature and fluctuation temperature field for 3 r values with two accesses – white lines correspond to the position of accesses.
upstream tunnel limits the impact of mean flow on the jet. Mode of access likewise limits transverse flow from inlet, which as a result declines from X *=−2 (without access) to X *=−1.20 the impinging jet location. This corresponds to the third identified regime i.e. no stack effect is present. Evacuation of the subway station can then be carried out under optimal safety conditions. In the station, temperature is blended and hot air regions (θ =0.10–305 K) are visible near the ground at the base of the jet, but in this case Z θ*≤0.2 based on θ =0.20 increases to 3.5 for r =30 to 4.5. The change of extraction flow rate from 30 to 7.4 removes stack effect and protects the bottom of the station from dangerous hot air (θ > 0.20). The fluctuations of temperature, Fig. 9, illustrate the entrance of hot air from the jet and the cold air from the outlet in the station. These limits are underlined by strong fluctuations, especially for r =7.4 for the jet, and r =30 for the entrance of cold air around X *=7 and X *=9. In order to better illustrate the three regime thresholds (stack effect in the two accesses, in only one access and no stack effect at all), several additional aspect ratios were studied from r =4 to 64 corresponding to Froude number of 0.626-0.002 and presence or absence of stack is identified from the temperature recorded within the accesses. All of the results, presented in Fig. 10, show that for r ≤ r2 , no stack effect occurs and that the two accesses are fed by outside air. For r ranging from r1 =32 to r2 =16, stack phenomena are limited to downstream access. Above r1, stack phenomena occurs in both accesses. The first stack effect appears in downstream access and can be explained by the jet inclination. The inlet air flow decrease in the
Fig. 10. Evolution of mean temperature in staircases (correspond to stack effect) according to r ratio and Froude number.
to air inlet in the station with a vertical velocity of around 0.24. Air inlet from the upstream tunnel tends to decrease (0.82→0.26) and reverse flow development along the ceiling is favored. The inlet decrease at 81
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upstream tunnel tends to strengthen the rising jet, which is influenced in the station downstream. Hot air can escape from the station through this opening, i.e. the influence of extraction flow rate is reduced. Flow organization in the station is then modified, especially the tilted jet angle. To ensure evacuation of the subway station and provide at least one smoke-free access, ventilation rate must yield a r ratio lower than the r1 threshold limit. 4. Conclusion A dedicated test bench was developed to study the relationship between hot ascending flow and transverse flow and the resulting potential stack effect phenomenon. The main goal in the present work was to find a strategy to provide areas free of smoke for passengers to escape in case of fire in subway station. To do so, the crossflow rate increase (decrease of the r momentum ratio) highlighted dilution in the reference case i.e. with no staircase. Temperature profiles were self-similar with a constant slope for Z * < 3.75 ( Δθ / ΔZ * ≈0.085) reaching a smaller slope ( Δθ / ΔZ * ≈0.012) in the higher part of the station and overall levels were scaled down with the r decrease. In the geometry studied, the average Z θ*≤0.2 increased by a ratio of 1.8 from r =30 to 7.4. Inlet flow strongly interacted with the jet and, as expected, it directly controlled the tilted resulting angle as well as overall flow field behavior. The presence of two accesses modified global flow field organization and our main result concerns the limit setting of the r momentum ratio of the 3 possible regimes: the two accesses free of smoke, one out of two with stack effect and the two accesses with smoke. The latter regime was obtained for the smaller extraction rate (i.e. for r > 32) with stack effect occurring in the two passenger accesses simultaneously. This is obviously the worst scenario in terms of safety, but it occurs only when extraction mass flow is not strong enough to sweep off smoke arising from the source. The intermediate regime corresponded to stack phenomenon arising in only one passenger access, while fresh air entered from the passenger access located in the side of the air ventilating inlet, in the upstream access in the case studied. Finally, the strongest mass flow rate (r < 16) may be due to the presence the two passenger accesses of smoke, which allow safe passenger evacuation. However, the Z θ*≤0.2 was decreased by the decrease of the r momentum ratio. From r →∞ to r =30 the Z θ*≤0.2 was reduced by 22%. One should also underline the sensitivity of the overall mechanisms; a change of 50% in extracting mass flow rate leads from stack phenomena in two accesses to no stack phenomena, underlining the sensitivity of ventilation control. References [1] L.H. Cheng, T.H. Ueng, C.W. Liu, Simulation of ventilation and fire in the
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