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Experimental study of air entrainment mode with natural ventilation using shafts in road tunnel fires
International Journal of Heat and Mass Transfer 56 (2013) 750–757
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Experimental study of air entrainment mode with natural ventilation using shafts in road tunnel fires C.G. Fan, J. Ji ⇑, Z.H. Gao, J.Y. Han, J.H. Sun State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 18 June 2012 Received in revised form 13 September 2012 Accepted 21 September 2012 Available online 3 November 2012 Keywords: Road tunnel Fire Natural ventilation Shaft Air entrainment
a b s t r a c t A set of burning experiments with n-heptane pool fire were conducted to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence in our previous work whereas this study quantificationally analyzes the effect of plug-holing on the smoke exhaust efficiency. The disturbance of smoke exhausting on the smoke–air interface, which causes different amounts of fresh air exhausted directly and indirectly, is investigated. Results show that using vertical shafts to discharge smoke leads to a strong mixing process between the smoke layer and the fresh air. Consequently, about 2/3 of the smoke exhausting rate of the shaft is air. Moreover, some of the entrained air mixes into the smoke layer downstream the shaft, resulting, in a small reduction amount of the spilling smoke, which also proves an inefficient smoke exhausting process. Hence, for the sake of improving effectiveness of natural ventilation, some significant improvements are needed in practice. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Road tunnel is an effective way to solve urban traffic problems. Nowadays, more and more urban road tunnels are under construction in cities like Tokyo, Moscow, Beijing and so on all over the world. However, owing to the special structure of tunnels, smoke and toxic gases induced by fires, such as carbon monoxide, which are the most fatal hazard to the people, will not easily be discharged. The smoke will hamper safe evacuation of occupants and affect firefighters from extinguishing the fire. In fact, 85% of the deaths in building fires are attributed to the toxic smoke according to statistics [1]. Compared with mechanical ventilation that requires a large space of excavation for installation of ventilation equipments along with a costly maintenance and electrical energy consumption, natural ventilation using vertical shafts can avoid air fans that will reduce the tunnel section height and does not consume power in the operation process. Furthermore, natural ventilation is particularly applicable to short tunnels with a light traffic flow [2]. Shafts not only contribute to discharge of high temperature smoke and weaken its impact on lining structures and equipments, but also facilitate airflow exchange in tunnels so as to improve interior air quality at ordinary times. In tunnel fires research, the critical velocity and backlayering length due to mechanical ventilation systems [3–8], the ⇑ Corresponding author. Tel.: +86 551 3606431(O); mobile: +86 13721101322. E-mail address: [email protected] (J. Ji). 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.09.047
maximum temperature beneath the ceiling [9–11] and the heat release rate [12,13] have been widely studied. However, less studies on the natural ventilation system with vertical shaft have been conducted. Wang et al. [14] conducted a set of burning experiments in a full-scale tunnel with roof openings, tested the effect of natural smoke exhaust and investigated the ceiling jet temperature and the backflow distance. Yoon et al. [15] investigated the natural ventilation pressure of vertical shaft in two road tunnels with results indicating that the ratio of natural ventilation pressure induced by the shaft to the mechanical ventilation pressure came to 29.26%, which had greatly improved the smoke exhausting efficiency compared to traditional natural ventilation without vertical shaft, thus verifying the feasibility of natural exhaust system with vertical shaft. Huang et al. [16] numerically studied the effect of the ventilation shaft arrangement and geometry on natural ventilation performance in a subway tunnel with FLUENT. However, in these former studies, the motion state of smoke within shaft has rarely been addressed. Ji et al. [17] conducted a set of burning experiments to investigate the effect of vertical shaft height on natural ventilation in urban road tunnel fires. Two special phenomena, plug-holing and turbulent boundary layer separation were observed, both of which will influence the effect of smoke exhaust. When the shaft height is relatively small, the boundary layer separation is significant. With the increasing of shaft height, the boundary layer separation becomes inconspicuous and the plug-holing occurs, leading to some ambient fresh air beneath the smoke layer being exhausted
C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
751
Nomenclature A coshaft cotunnel g hspill L _ a;shaft m _ a;spill m _ s;shaft m _ shaft m _ 0spill m _ 00spill m _ Dm
cross-sectional area of the shaft (m2) CO concentration in the smoke inside the exhaust shaft (ppm) CO concentration in the smoke under the tunnel ceiling (ppm) gravity acceleration (m/s2) thickness of spilling smoke (m) length size (m) air proportion of the exhausting rate (kg/s) mass flow rate of air entrained into the smoke layer and not exhausted by the shaft (kg/s) smoke proportion of the exhausting rate (kg/s) smoke exhausting rate of the shaft (kg/s) mass flow rate of spilling smoke without natural ventilation (kg/s) mass flow rate of spilling smoke with natural ventilation (kg/s) reduction amount of spilling smoke (kg/s)
directly, which will strongly decrease the smoke exhaust efficiency. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence. However, this previous wok of our group did not quantificationally analyze the effect of plugholing on the smoke exhaust efficiency. Although the investigation of mechanical smoke exhaust system has been receiving most attention and meanwhile providing reliable information in some former studies [18–20], delicate quantitative analyses on how natural ventilation using vertical shafts influences the air entrainment mode has rarely been addressed. The disturbance of smoke exhausting on the smoke–air interface, which causes different amount of fresh air exhausted directly and indirectly, has rarely been discussed. In order to investigate the air entrainment mode under natural ventilation in road tunnel fires, a set of burning experiments were conducted in the present work. The CO concentration in the vertical shaft and tunnel, smoke temperature and velocity were experimentally studied to quantify the effect of plug-holing on the smoke exhausting efficiency. This study may benefit the current design of natural ventilation system with vertical shaft in road tunnels. 2. Theoretically analysis The primary driving force of natural ventilation is the stack effect formed by temperature difference between the hot smoke in the shaft and the ambient air. Its effect mainly depends on the shaft height and the density difference between the smoke in the shaft and the ambient air. The smoke with relatively high temperature will induce a strong stack effect inside the shaft, which is in favor of smoke exhausting. However, when the exhausting velocity reaches a relative high level, some fresh air will be drawn directly into the smoke exhausting shaft from the lower layer in the tunnel due to the plug-holing effect as shown in Fig. 1. Simultaneously, the mixture process between the fresh air and smoke layer becomes stronger. A part of the mixed air is exhausted by the shaft and the rest stays in the smoke layer and keeps spreading along the tunnel ceiling. From the above analysis, the smoke exhausting rate of the shaft, _ shaft , can be classified into two parts: m
_ s;shaft þ m _ a;shaft _ shaft ¼ m m
Q Ta Tint Tmax Tshaft ushaft uspill W
heat release rate (kW) air temperature (K) smoke layer interface temperature (K) maximum smoke temperature vertically (K) smoke temperature of the shaft (K) smoke velocity of the shaft (m/s) velocity of spilling smoke (m/s) width of spilling smoke (m)
Greek symbols qa air density (kg/m3) qshaft smoke density of the shaft (kg/m3) qspill density of spilling smoke (kg/m3) Subscripts f full-scale tunnel m model tunnel
_ a;shaft should be kept as a smaller value to make smoke as much m as possible exhausted. The CO concentrations measured in the smoke under the tunnel ceiling and inside the exhaust shaft can reflect the air entrainment degree. In fact, the gas exhausted by the shaft can be regard as dilute smoke compared with that under the tunnel ceiling. The higher CO concentration at the top opening of shaft indicates better smoke exhausting efficiency. Therefore,
_ shaft m
coshaft _ s;shaft ¼m cotunnel
co _ shaft 1 shaft ¼ m _ a;shaft m cotunnel
ð3Þ
where coshaft denotes the CO concentration measured in the smoke inside shaft with natural ventilation, cotunnel denotes the CO concentration measured in the smoke under the tunnel ceiling without co natural ventilation and thus co shaft can reflect the air entrainment detunnel gree. Then the smoke proportion of the exhausting rate of the shaft, _ s;shaft , can be calculated as m _ shaft cocoshaft . On the other hand, the mass m tunnel _ 0spill without employing the flow rate of spilling smoke is defined as m _ 00spill with natural ventilation shaft for exhausting smoke and m respectively. The differentials of the two parameters, namely the _ On the whole, reduction amount of spilling smoke, is defined as Dm. with regard to the smoke layer, a few dilute smoke is exhausted from the shaft with some air mixing into the smoke layer, then Eq. (1) can be rewritten as:
ð1Þ
_ s;shaft is the smoke proportion of the exhausting rate and where m _ a;shaft is the air proportion. To achieve a better exhausting effect, m
ð2Þ
Fig. 1. Phenomenon of plug-holing.
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C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
_ a;shaft þ m _ a;spill þ Dm _ shaft ¼ m _ m
ð4Þ
_ a;spill is the mass flow rate of air entrained into the smoke where m layer and not exhausted by the shaft. It can be concluded that a larger smoke exhausting rate is not presenting a better effect of natural ventilation because the entrained fresh air due to the mixture with smoke layer and the fresh air drawn directly from the lower layer due to the plug-holing effect may decrease the reduction amount of spilling smoke, leading to an unsatisfactory exhausting effect. The smoke exhausting rate of the shaft can be obtained by:
_ shaft ¼ qshaft ushaft A m
ð5Þ
where qshaft is smoke density, ushaft is smoke velocity and A is crosssectional area of the shaft. Applying the ideal gas law to the hot smoke and neglecting the pressure variation as it is small in comparing with the ambient pressure [21], qshaft can be gained from:
qshaft ¼
qa T a T shaft
ð6Þ
where Tshaft is smoke temperature in the shaft, qa is air density and _ spill , can Ta is air temperature. The mass flow rate of spilling smoke, m be gained by:
_ spill ¼ qspill uspill hspill W m
where qspill, uspill, hspill, W are density, velocity, thickness and width of spilling smoke respectively. Actually, W is equal to tunnel width and qspill can be calculated referring to Eq. (6). 3. Experiments The experiments were conducted in a small-scale model tunnel with a scale ratio of 1:6 [17,22]. The tunnel is 6 m long, 2 m wide and 0.88 m high. Its aspect ratio is determined based on a survey of many tunnels and is considered to be a general representation. The Froude modeling was applied to build up the physical scale model. The dimensional relationships between the fluid dynamics variable were derived from first principles by Morgan et al. [23] and also mentioned in NFPA 92B [24]. By holding the Froude number constant, the relationships can be simplified to obtain the required scaling laws. As the scaling laws of Froude modeling do not apply to conductive and radiative heat transfer processes, it is actually assumed that the heat transfer mechanisms in this research work were predominantly convective [25]. The heat release rates (HRR), Q, the temperature, T and the velocity, V, are scaled using Eqs. (8)–(10). L denotes the length size, and the subscript ‘m’ represents the model tunnel and the subscript ‘f’ represents the full-scale tunnel.
5=2 Qm Lm ¼ Qf Lf
ð8Þ
Tm ¼ Tf
ð9Þ
1=2 Vm Lm ¼ Vf Lf
Fig. 2. Schematic of experimental apparatus.
ð7Þ uniformly at the top opening of the shaft. Agilent 34980A measurement unit was used for the temperature measurements. Three velocity probes of hot-wire anemometer were installed vertically near the thermocouple tree at the right opening with distances of 3, 7 and 11 cm from the tunnel ceiling respectively. The average value of these measuring points was considered as the velocity of the smoke spilling from the tunnel. A velocity probe was set near each thermocouple at the top opening of the shaft. KANOMAX four channel measurement unit was used for velocity measurements. A CO concentration measure point was set at the top opening of the shaft or under the tunnel ceiling in cases with or without natural ventilation respectively. The CO concentration was measured by gas analyzer (Testo, Co. Ltd., Model 350XL). The measurement uncertainty was about 5% with resolution of 1 ppm, and its sampling interval was set to be 1 s. The fuel was n-heptane, which is volatiler and purer than gasoline. Four sizes of pool were used with the same sidewall height of 2 cm, as shown in Table 1. Fuel with thickness of 1 cm was applied in each experiment. A balance was positioned below the pool to record the mass loss history of the burning pool fire with sampling interval of 1 s. The fire source was located at 1.4 m away from the left end of the model tunnel to avoid the exit effect [5]. In order to simulate a vehicle fire situation more accurately, the fire pool was embraced by an iron frame covered by gypsum boards with thickness of 6 mm to symbolize one semi-enclosed space. All these experiments began with similar ambient temperature (15–20 °C). A laser sheet with an output power of 2 W and a sheet thickness of 1 mm was used as a visualization assistant tool to show the smoke flow patterns. A Digital Video was used to record the experimental phenomena.
ð10Þ
The distance between the shaft and the left opening of the tunnel is 4.2 m. The cross section of the shaft is 30 cm 30 cm. The shaft height is 80 cm, slightly smaller than the tunnel height, which is consistent with the situation of a full-scale tunnel with natural ventilation using vertical shafts in Nanjing, China [14]. The experimental set-up is shown in Fig. 2. A thermocouple tree with 32 thermocouples (K-type) at a 1 cm vertical interval was positioned at the shaft bottom and the other one with 16 thermocouples (K-type) at a 1.5 cm vertical interval under the ceiling at the right opening of the tunnel. Four thermocouples were fixed
4. Results and discussion 4.1. Mass loss rate and CO concentration The variations of mass loss rate in pools with time are presented in Fig. 3. As shown in Fig. 3, after the ignition in all cases, the mass loss rate firstly maintains a relatively low and stable value for a little long time. In a relatively short period before the burnout of fuel, the mass loss rates in all cases reach a high level. Between these two stages, there is one transition stage, corresponding to a sudden
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C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757 Table 1 Summary of test scenarios. Test No.
Pool length (cm)
Pool width (cm)
Pool area (cm2)
Shaft height (m)
Test designation
1 2 3 4 5 6 7 8
10 15 15 20 10 15 15 20
10 10 15 15 10 10 15 15
100 150 225 300 100 150 225 300
– – – – 0.8 0.8 0.8 0.8
1010 1510 1515 2015 1010s 1510s 1515s 2015s
500
4.0 3.5
1010 1510 1515 2015 1010s 1510s 1515s 2015s
2.5 2.0
400
CO concentration (ppm)
mass loss rate (g/s)
3.0
1010 1510 1515 2015 1010s 1510s 1515s 2015s
1.5 1.0 0.5
300
200
100
0
0.0 0
50
100
150
200
250
300
time (s)
0
50
100
150
200
250
300
350
time (s)
Fig. 3. Mass loss rates in different pools with and without natural ventilation.
Fig. 4. CO concentrations for different pools with and without natural ventilation.
increase of the burning rate. At the initial steady burning stage, the burning only happens on the surface of the oil layer. As time passes by, the whole oil layer is heated to boil, resulting in the significant increase of the mass loss rate [26,27]. In our experiments, the heat feedback from the heated iron frame and gypsum boards to the unburned fuel plays a supplementary role in the process of heating the fuel to boil. It can also be seen from Fig. 3 that for the cases with the same size pool with and without natural ventilation, the histories of mass loss rate are similar at both the initial steady burning stage and the boiling burning stage. The average mass loss rate at the steady burning stage in all cases is relatively small, between 0.1 and 0.3 g/s. The average value at the boiling burning stage is larger, between 1 and 3.5 g/s. For the same size pool fire, natural ventilation does not cause the significant change of the average mass loss rate at both stages. Roh et al. [28] found that for tunnel fires, the increase in longitudinal ventilation velocity leads to the enhanced burning rate of fuels, due to the fact that the oxygen supply effect prevails rather than the cooling effects as the ventilation velocity increases. It can be concluded from Fig. 3 that, natural ventilation caused by vertical shafts does not lead to the enhanced burning rate of fuels largely. The reason is that the velocity of longitudinal air supplying caused by the natural smoke exhausting is limited and not large enough to result in a more drastic fire situation. Fig. 4 shows the CO concentrations measured at the top opening of the shaft with natural ventilation and under the tunnel ceiling without natural ventilation. At the beginning of fires, the CO concentrations increase slowly with time, and after one certain moment increase by a large margin suddenly until the fuels burn out. Based on Figs. 3 and 4, the times of sudden increase of the CO concentrations are found to be the same as those of the mass loss rates. In cases with pool size of 100, 150, 225 and 300 cm2, the sudden increases occur at about 60, 80, 110 and 250 s respec-
tively. It can be known that at the initial steady stage, the oxygen supplying can meet the requirement of complete combustion of fuel, resulting in very low CO concentration in the smoke. However, at the boiling burning stage, the mass loss rates increase approximate nine times and the oxygen supplying cannot meet the requirement, resulting in higher CO concentration in the smoke. Consequently, a fairly large number of smoke with a high CO concentration forms inside the narrow tunnel structure. As a matter of fact, the concentration of harmful gases like CO should be low enough for safe evacuation of stranded occupants (50 ppm) [29], also to the benefit of firefighters for extinguishing the fire. Moreover, for the cases with pools of the same size, the peak value of CO concentration measured at the top opening of the shaft in cases with natural ventilation is significant lower than that measured under the tunnel ceiling in cases without natural ventilation at the boiling burning stage. In the presence of vertical shaft, when the hot smoke spreads to the area beneath the shaft, it will be exhausted through the shaft because of the stack effect. Based on the analysis in the previous section, under the action of vertical inertia force, the disturbance on smoke layer interface will be strengthened greatly, leading to a lot more fresh air mixed into the smoke layer and lower CO concentration of smoke in the shaft. The peak values of the CO concentration measured in cases with and without natural ventilation and their ratio are shown in Fig. 5. The ratio is about 1/3, indicating that 2/3 of the smoke exhausting rate of the shaft is the cold air in the lower layer inside the tunnel. Vauquelin [30] defined Ventilation System Output (VSO), the ratio of the flow rate of extracted smoke to the total extraction flow rate, to characterize the transverse ventilation system and found VSO obviously strongly decreased in case of plug-holing. In this study, the ratio in Fig. 5 has similar physical meaning with VSO.
C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
ratio without natural ventilation natural ventilation
0.8
500
100
400
80
300
0.4
200
0.2
100
0.0
0
ratio
0.6
temperature rise (K)
1.0
CO concentration (ppm)
754
natural ventilation without natural ventilation
60
40
20
0 100
150
200
250
300
0.55
2
pool area (cm )
0.60
0.65
0.70
0.75
0.80
0.85
0.90
height (m)
Fig. 6 presents the typical smoke flow configuration under the tunnel ceiling taken by the laser sheet. It can be seen that in the case with natural ventilation, the plug-holing phenomenon occurs and the area right under the bottom opening of the shaft is occupied by fresh air, indicating that a large amount of fresh air is indirectly exhausted by the shaft and an inefficient smoke exhausting process is obtained. The vertical temperature rises measured under the center of the shaft bottom opening at the boiling stage in cases of 2015 and 2015s are shown in Fig. 7. It can be seen that in the case without natural ventilation, the upper part of thermocouple tree is inside the smoke layer and the lower part where the temperature rises are almost zero inside the air layer. Nevertheless, in the case with natural ventilation, the temperature rises of all measuring points are approaching zero, which illustrates the whole thermocouple tree is located in the air layer as plug-holing occurs. 4.2. Temperature rise, velocity and smoke layer interface height The temperature rise and velocity curves measured under the ceiling at the right opening of the tunnel with natural ventilation in test 5 are shown in Figs. 8 and 9. Both the temperature rise and velocity increase slowly with time in the beginning and increase sharply at the boiling burning stage. After the fuel burns out, the curves start to descend. These trends are the same with that of CO concentration. In the following part, the values of temperature rise and velocity used for further analysis are both averaged from the boiling burning stage, whose starting and final times are based on the variation of mass loss rate with time as represented in Fig. 3.
Fig. 7. Temperature rises measured under the shaft location (pool2015).
25
0.87 m 0.855 m 0.84 m 0.825 m 0.81 m 0.795 m 0.78 m 0.765 m 0.75 m 0.735 m 0.72 m 0.705 m 0.69 m 0.675 m 0.66 m 0.645 m
20
temperature rise (K)
Fig. 5. Maximum CO concentrations and the ratio with and without natural ventilation.
15
10
5
0
-5 -50
0
50
100
150
200
250
300
350
time (s) Fig. 8. Temperature rise curves measured under the ceiling at the right opening of the tunnel in test 5.
To avoid the subjectiveness of confirming smoke layer position employing the naked eye observation, the N-percent rule [31] which has been affirmed by other studies about the fire-induced smoke in buildings [32,33] is considered to determine the smoke layer interface height in this study. The interface temperature, Tint, is defined by the following formula [33]:
T int T a ¼ ðT max T a Þ N=100
ð11Þ
where T max is the measured maximum smoke temperature vertically. Previous study [27,32,33] recommended that the N should
Fig. 6. Typical configurations of smoke layer under the tunnel ceiling.
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some fresh air will be entrained into the smoke layer. A part of the mixed air is exhausted by the shaft and the rest stays in the smoke layer and keeps spreading along the tunnel ceiling. Meanwhile, in tests with natural ventilation, the smoke layer temperature rise at the right opening of the tunnel decreases compared with those without natural ventilation because of the entrainment of cold air.
velocity (m/s)
0.3
0.85 m 0.81 m 0.77 m
0.2
4.3. Exhaust effect of shaft
0.1
0.0
-0.1 -50
0
50
100
150
200
250
300
350
time (s) Fig. 9. Velocity curves measured at the right opening of the tunnel in test 5.
0.90
0.85
height (m)
0.80
0.75
0.70
smoke interface 0.65 0
10
20
30
40
50
60
temperature rise (K) Fig. 10. Vertical temperature profiles at the right opening of the tunnel in test 4.
be between 10 and 30 and the value N = 30 is chosen in this study. This method is an easily applicable and unambiguous way to determine the smoke layer interface height. An important advantage of this method is that it is not time-consuming [33]. Fig. 10 presents the vertical temperature profiles under the ceiling at the right opening of the tunnel in test 4, the smoke layer interface height determined by the N-percent rule is 0.715 m. The smoke interface height and temperature rises at the right opening of the tunnel in all tests are represented in Table 2. In tests with natural ventilation, although some smoke has been exhausted by the shaft, the smoke interface height at the right of the shaft does not increase much compared with those without natural ventilation. Furthermore, it has a little decrease for the test of pool of 100 cm2. As the mixture process between the fresh air and smoke layer becomes stronger due to the smoke exhausting of the shaft,
In terms of Eqs. (5)–(7) and the analysis in the previous sections, the mass flow rates of spilling smoke in cases with and with_ 00spill and m _ 0spill ) and the smoke exhausting out natural ventilation (m _ shaft ) at the boiling stage are calculated as shown rate of the shaft (m in Fig. 11. In cases without natural ventilation, the mass flow rates of spilling smoke increase with the pool area. After employing the shaft for exhausting smoke, the mass flow rates of spilling smoke have some degree of decrease. The smoke exhausting rate of the shaft is 0.064 kg/s in the case with 100 cm2 pool and between 0.075 and 0.08 kg/s for the other cases. It is thus clear that the smoke exhausting rate of the shaft is relatively large whereas the _ is not much, between reduction amount of spilling smoke (Dm) 0.006 and 0.022 kg/s for all the cases. This is due to the fact that the major part of the smoke exhausting rate of the shaft is air on account of the plug-holing effect firstly. Secondly, the disturbance of smoke exhausting on the smoke–air interface leads to much fresh air entrained into the smoke layer, some of which entrains into the smoke layer downstream the shaft and spills from the tunnel. Based on Eq. (4), the mass flow rate of air entrained into the _ a;spill ), the mass flow smoke layer and not exhausted by the shaft (m _ a;shaft ) and the reducrate of air portion exhausted by the shaft (m _ are presented in Fig. 12. tion amount of spilling flow rate (Dm) For all the cases, the air proportion of the smoke exhausted by the shaft is about 2/3 and the ratio of all the air including the part exhausted by the shaft and that entrained into the smoke layer and _ a;shaft and m _ a;spill ) to the smoke not exhausted by the shaft (m exhausting rate of the shaft is between 70% and 92%. Therefore, the decrease of spilling flow rate seems to be small compared with the air portion exhausted from the shaft and entrained into the downstream smoke as a whole. Several researchers have addressed this issue on the mechanical smoke exhausting. Chow et al. [18] conducted a set of experiments in one atrium and found that the mass flow rate of air across the smoke layer interface could be 30% of the mass exhaust rate of the mechanical smoke exhaust system. Shi [19] experimentally studied the mechanical exhaust efficiency in compartment fire and claimed that when the exhausting vents were located near the smoke–air interface, a large amount of fresh air were entrained into upper smoke layer. Ji et al. [20] conducted experiments to investigate the influence of smoke vent height and exhausting velocity on mechanical smoke exhausting efficiency and found that the fresh air entrained into the smoke layer was up to 48% of the mechanical exhausting rate. Therefore, in practice projects, the engineers must consider making some improvements on shaft design to avoid the
Table 2 Smoke interface height and temperature rise at the right opening of the tunnel. Pool area (cm2)
100 150 255 300
Smoke interface height (m)
Smoke layer temperature rise (K)
Without natural ventilation
Natural ventilation
Without natural ventilation
Natural ventilation
0.725 0.715 0.71 0.715
0.71 0.71 0.715 0.715
13.4 23.2 35.0 45.4
9.3 20.2 25.2 39.6
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some improvements on shaft design to avoid the plug-holing phenomenon as far as possible. Our findings could be beneficial to researchers in better understanding the behavior of air entrainment with natural ventilation using shafts in road tunnel fires. Future work will be focused on numerical modeling investigation on the influence of other related parameters including shaft geometry on natural smoke exhausting effectiveness in tunnel fires.
mass flow rate (kg/s)
0.08
0.06
mspill'
0.04
Acknowledgements
mspill'' This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 50904055 and National Key Technology Research and Development Program under Grant No. 2011BAK07B01-02.
mshaft
0.02
0.00 100
150
200
2
250
300
pool area (cm )
References
Fig. 11. Mass flow rates of smoke spilling from the right opening of the tunnel and exhausted by the shaft.
mass flow rate (kg/s)
0.08
0.06
mshaft
0.04
ma,shaft ma,spill Δm
0.02
0.00 100
150
200
250
300
2
pool area (cm ) Fig. 12. Mass flow rates of air portion exhausted by the shaft and entrained into the downstream smoke and reduction amount of spilling smoke.
plug-holing phenomenon as far as possible and the criterion on the critical shaft height of the plug-holing occurrence in our previous work [17] can be referred to. It is also shown in Fig. 12 that the reduction amount of spilling flow rate has a trend to increase with the pool size. As a larger fire can induce a thicker smoke layer under the tunnel ceiling, the plug-holing effect will be weakened and thus more hot smoke will be exhausted and less fresh air will be entrained into the smoke layer. 5. Conclusions The principle purpose of the present work is to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires by means of small-scale experiments. It is suggested that the stack effect inside the shaft induce a strong mixing effect on the hot smoke layer and the cold air. For this reason, a portion of air will be inhaled into the shaft directly and indirectly, and some of the entrained air mixes into the smoke layer downstream the shaft and keeps spreading along the tunnel ceiling, which lead to an inefficient smoke exhausting process. The experimental results and analyses can be expected to be of great technical interest as basic data for the use of natural ventilation with shafts for smoke control. In practice projects, the engineers must consider making
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of air entrainment mode with natural ventilation using shafts in road tunnel fires C.G. Fan, J. Ji ⇑, Z.H. Gao, J.Y. Han, J.H. Sun State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 18 June 2012 Received in revised form 13 September 2012 Accepted 21 September 2012 Available online 3 November 2012 Keywords: Road tunnel Fire Natural ventilation Shaft Air entrainment
a b s t r a c t A set of burning experiments with n-heptane pool fire were conducted to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence in our previous work whereas this study quantificationally analyzes the effect of plug-holing on the smoke exhaust efficiency. The disturbance of smoke exhausting on the smoke–air interface, which causes different amounts of fresh air exhausted directly and indirectly, is investigated. Results show that using vertical shafts to discharge smoke leads to a strong mixing process between the smoke layer and the fresh air. Consequently, about 2/3 of the smoke exhausting rate of the shaft is air. Moreover, some of the entrained air mixes into the smoke layer downstream the shaft, resulting, in a small reduction amount of the spilling smoke, which also proves an inefficient smoke exhausting process. Hence, for the sake of improving effectiveness of natural ventilation, some significant improvements are needed in practice. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Road tunnel is an effective way to solve urban traffic problems. Nowadays, more and more urban road tunnels are under construction in cities like Tokyo, Moscow, Beijing and so on all over the world. However, owing to the special structure of tunnels, smoke and toxic gases induced by fires, such as carbon monoxide, which are the most fatal hazard to the people, will not easily be discharged. The smoke will hamper safe evacuation of occupants and affect firefighters from extinguishing the fire. In fact, 85% of the deaths in building fires are attributed to the toxic smoke according to statistics [1]. Compared with mechanical ventilation that requires a large space of excavation for installation of ventilation equipments along with a costly maintenance and electrical energy consumption, natural ventilation using vertical shafts can avoid air fans that will reduce the tunnel section height and does not consume power in the operation process. Furthermore, natural ventilation is particularly applicable to short tunnels with a light traffic flow [2]. Shafts not only contribute to discharge of high temperature smoke and weaken its impact on lining structures and equipments, but also facilitate airflow exchange in tunnels so as to improve interior air quality at ordinary times. In tunnel fires research, the critical velocity and backlayering length due to mechanical ventilation systems [3–8], the ⇑ Corresponding author. Tel.: +86 551 3606431(O); mobile: +86 13721101322. E-mail address: [email protected] (J. Ji). 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.09.047
maximum temperature beneath the ceiling [9–11] and the heat release rate [12,13] have been widely studied. However, less studies on the natural ventilation system with vertical shaft have been conducted. Wang et al. [14] conducted a set of burning experiments in a full-scale tunnel with roof openings, tested the effect of natural smoke exhaust and investigated the ceiling jet temperature and the backflow distance. Yoon et al. [15] investigated the natural ventilation pressure of vertical shaft in two road tunnels with results indicating that the ratio of natural ventilation pressure induced by the shaft to the mechanical ventilation pressure came to 29.26%, which had greatly improved the smoke exhausting efficiency compared to traditional natural ventilation without vertical shaft, thus verifying the feasibility of natural exhaust system with vertical shaft. Huang et al. [16] numerically studied the effect of the ventilation shaft arrangement and geometry on natural ventilation performance in a subway tunnel with FLUENT. However, in these former studies, the motion state of smoke within shaft has rarely been addressed. Ji et al. [17] conducted a set of burning experiments to investigate the effect of vertical shaft height on natural ventilation in urban road tunnel fires. Two special phenomena, plug-holing and turbulent boundary layer separation were observed, both of which will influence the effect of smoke exhaust. When the shaft height is relatively small, the boundary layer separation is significant. With the increasing of shaft height, the boundary layer separation becomes inconspicuous and the plug-holing occurs, leading to some ambient fresh air beneath the smoke layer being exhausted
C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
751
Nomenclature A coshaft cotunnel g hspill L _ a;shaft m _ a;spill m _ s;shaft m _ shaft m _ 0spill m _ 00spill m _ Dm
cross-sectional area of the shaft (m2) CO concentration in the smoke inside the exhaust shaft (ppm) CO concentration in the smoke under the tunnel ceiling (ppm) gravity acceleration (m/s2) thickness of spilling smoke (m) length size (m) air proportion of the exhausting rate (kg/s) mass flow rate of air entrained into the smoke layer and not exhausted by the shaft (kg/s) smoke proportion of the exhausting rate (kg/s) smoke exhausting rate of the shaft (kg/s) mass flow rate of spilling smoke without natural ventilation (kg/s) mass flow rate of spilling smoke with natural ventilation (kg/s) reduction amount of spilling smoke (kg/s)
directly, which will strongly decrease the smoke exhaust efficiency. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence. However, this previous wok of our group did not quantificationally analyze the effect of plugholing on the smoke exhaust efficiency. Although the investigation of mechanical smoke exhaust system has been receiving most attention and meanwhile providing reliable information in some former studies [18–20], delicate quantitative analyses on how natural ventilation using vertical shafts influences the air entrainment mode has rarely been addressed. The disturbance of smoke exhausting on the smoke–air interface, which causes different amount of fresh air exhausted directly and indirectly, has rarely been discussed. In order to investigate the air entrainment mode under natural ventilation in road tunnel fires, a set of burning experiments were conducted in the present work. The CO concentration in the vertical shaft and tunnel, smoke temperature and velocity were experimentally studied to quantify the effect of plug-holing on the smoke exhausting efficiency. This study may benefit the current design of natural ventilation system with vertical shaft in road tunnels. 2. Theoretically analysis The primary driving force of natural ventilation is the stack effect formed by temperature difference between the hot smoke in the shaft and the ambient air. Its effect mainly depends on the shaft height and the density difference between the smoke in the shaft and the ambient air. The smoke with relatively high temperature will induce a strong stack effect inside the shaft, which is in favor of smoke exhausting. However, when the exhausting velocity reaches a relative high level, some fresh air will be drawn directly into the smoke exhausting shaft from the lower layer in the tunnel due to the plug-holing effect as shown in Fig. 1. Simultaneously, the mixture process between the fresh air and smoke layer becomes stronger. A part of the mixed air is exhausted by the shaft and the rest stays in the smoke layer and keeps spreading along the tunnel ceiling. From the above analysis, the smoke exhausting rate of the shaft, _ shaft , can be classified into two parts: m
_ s;shaft þ m _ a;shaft _ shaft ¼ m m
Q Ta Tint Tmax Tshaft ushaft uspill W
heat release rate (kW) air temperature (K) smoke layer interface temperature (K) maximum smoke temperature vertically (K) smoke temperature of the shaft (K) smoke velocity of the shaft (m/s) velocity of spilling smoke (m/s) width of spilling smoke (m)
Greek symbols qa air density (kg/m3) qshaft smoke density of the shaft (kg/m3) qspill density of spilling smoke (kg/m3) Subscripts f full-scale tunnel m model tunnel
_ a;shaft should be kept as a smaller value to make smoke as much m as possible exhausted. The CO concentrations measured in the smoke under the tunnel ceiling and inside the exhaust shaft can reflect the air entrainment degree. In fact, the gas exhausted by the shaft can be regard as dilute smoke compared with that under the tunnel ceiling. The higher CO concentration at the top opening of shaft indicates better smoke exhausting efficiency. Therefore,
_ shaft m
coshaft _ s;shaft ¼m cotunnel
co _ shaft 1 shaft ¼ m _ a;shaft m cotunnel
ð3Þ
where coshaft denotes the CO concentration measured in the smoke inside shaft with natural ventilation, cotunnel denotes the CO concentration measured in the smoke under the tunnel ceiling without co natural ventilation and thus co shaft can reflect the air entrainment detunnel gree. Then the smoke proportion of the exhausting rate of the shaft, _ s;shaft , can be calculated as m _ shaft cocoshaft . On the other hand, the mass m tunnel _ 0spill without employing the flow rate of spilling smoke is defined as m _ 00spill with natural ventilation shaft for exhausting smoke and m respectively. The differentials of the two parameters, namely the _ On the whole, reduction amount of spilling smoke, is defined as Dm. with regard to the smoke layer, a few dilute smoke is exhausted from the shaft with some air mixing into the smoke layer, then Eq. (1) can be rewritten as:
ð1Þ
_ s;shaft is the smoke proportion of the exhausting rate and where m _ a;shaft is the air proportion. To achieve a better exhausting effect, m
ð2Þ
Fig. 1. Phenomenon of plug-holing.
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C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
_ a;shaft þ m _ a;spill þ Dm _ shaft ¼ m _ m
ð4Þ
_ a;spill is the mass flow rate of air entrained into the smoke where m layer and not exhausted by the shaft. It can be concluded that a larger smoke exhausting rate is not presenting a better effect of natural ventilation because the entrained fresh air due to the mixture with smoke layer and the fresh air drawn directly from the lower layer due to the plug-holing effect may decrease the reduction amount of spilling smoke, leading to an unsatisfactory exhausting effect. The smoke exhausting rate of the shaft can be obtained by:
_ shaft ¼ qshaft ushaft A m
ð5Þ
where qshaft is smoke density, ushaft is smoke velocity and A is crosssectional area of the shaft. Applying the ideal gas law to the hot smoke and neglecting the pressure variation as it is small in comparing with the ambient pressure [21], qshaft can be gained from:
qshaft ¼
qa T a T shaft
ð6Þ
where Tshaft is smoke temperature in the shaft, qa is air density and _ spill , can Ta is air temperature. The mass flow rate of spilling smoke, m be gained by:
_ spill ¼ qspill uspill hspill W m
where qspill, uspill, hspill, W are density, velocity, thickness and width of spilling smoke respectively. Actually, W is equal to tunnel width and qspill can be calculated referring to Eq. (6). 3. Experiments The experiments were conducted in a small-scale model tunnel with a scale ratio of 1:6 [17,22]. The tunnel is 6 m long, 2 m wide and 0.88 m high. Its aspect ratio is determined based on a survey of many tunnels and is considered to be a general representation. The Froude modeling was applied to build up the physical scale model. The dimensional relationships between the fluid dynamics variable were derived from first principles by Morgan et al. [23] and also mentioned in NFPA 92B [24]. By holding the Froude number constant, the relationships can be simplified to obtain the required scaling laws. As the scaling laws of Froude modeling do not apply to conductive and radiative heat transfer processes, it is actually assumed that the heat transfer mechanisms in this research work were predominantly convective [25]. The heat release rates (HRR), Q, the temperature, T and the velocity, V, are scaled using Eqs. (8)–(10). L denotes the length size, and the subscript ‘m’ represents the model tunnel and the subscript ‘f’ represents the full-scale tunnel.
5=2 Qm Lm ¼ Qf Lf
ð8Þ
Tm ¼ Tf
ð9Þ
1=2 Vm Lm ¼ Vf Lf
Fig. 2. Schematic of experimental apparatus.
ð7Þ uniformly at the top opening of the shaft. Agilent 34980A measurement unit was used for the temperature measurements. Three velocity probes of hot-wire anemometer were installed vertically near the thermocouple tree at the right opening with distances of 3, 7 and 11 cm from the tunnel ceiling respectively. The average value of these measuring points was considered as the velocity of the smoke spilling from the tunnel. A velocity probe was set near each thermocouple at the top opening of the shaft. KANOMAX four channel measurement unit was used for velocity measurements. A CO concentration measure point was set at the top opening of the shaft or under the tunnel ceiling in cases with or without natural ventilation respectively. The CO concentration was measured by gas analyzer (Testo, Co. Ltd., Model 350XL). The measurement uncertainty was about 5% with resolution of 1 ppm, and its sampling interval was set to be 1 s. The fuel was n-heptane, which is volatiler and purer than gasoline. Four sizes of pool were used with the same sidewall height of 2 cm, as shown in Table 1. Fuel with thickness of 1 cm was applied in each experiment. A balance was positioned below the pool to record the mass loss history of the burning pool fire with sampling interval of 1 s. The fire source was located at 1.4 m away from the left end of the model tunnel to avoid the exit effect [5]. In order to simulate a vehicle fire situation more accurately, the fire pool was embraced by an iron frame covered by gypsum boards with thickness of 6 mm to symbolize one semi-enclosed space. All these experiments began with similar ambient temperature (15–20 °C). A laser sheet with an output power of 2 W and a sheet thickness of 1 mm was used as a visualization assistant tool to show the smoke flow patterns. A Digital Video was used to record the experimental phenomena.
ð10Þ
The distance between the shaft and the left opening of the tunnel is 4.2 m. The cross section of the shaft is 30 cm 30 cm. The shaft height is 80 cm, slightly smaller than the tunnel height, which is consistent with the situation of a full-scale tunnel with natural ventilation using vertical shafts in Nanjing, China [14]. The experimental set-up is shown in Fig. 2. A thermocouple tree with 32 thermocouples (K-type) at a 1 cm vertical interval was positioned at the shaft bottom and the other one with 16 thermocouples (K-type) at a 1.5 cm vertical interval under the ceiling at the right opening of the tunnel. Four thermocouples were fixed
4. Results and discussion 4.1. Mass loss rate and CO concentration The variations of mass loss rate in pools with time are presented in Fig. 3. As shown in Fig. 3, after the ignition in all cases, the mass loss rate firstly maintains a relatively low and stable value for a little long time. In a relatively short period before the burnout of fuel, the mass loss rates in all cases reach a high level. Between these two stages, there is one transition stage, corresponding to a sudden
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C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757 Table 1 Summary of test scenarios. Test No.
Pool length (cm)
Pool width (cm)
Pool area (cm2)
Shaft height (m)
Test designation
1 2 3 4 5 6 7 8
10 15 15 20 10 15 15 20
10 10 15 15 10 10 15 15
100 150 225 300 100 150 225 300
– – – – 0.8 0.8 0.8 0.8
1010 1510 1515 2015 1010s 1510s 1515s 2015s
500
4.0 3.5
1010 1510 1515 2015 1010s 1510s 1515s 2015s
2.5 2.0
400
CO concentration (ppm)
mass loss rate (g/s)
3.0
1010 1510 1515 2015 1010s 1510s 1515s 2015s
1.5 1.0 0.5
300
200
100
0
0.0 0
50
100
150
200
250
300
time (s)
0
50
100
150
200
250
300
350
time (s)
Fig. 3. Mass loss rates in different pools with and without natural ventilation.
Fig. 4. CO concentrations for different pools with and without natural ventilation.
increase of the burning rate. At the initial steady burning stage, the burning only happens on the surface of the oil layer. As time passes by, the whole oil layer is heated to boil, resulting in the significant increase of the mass loss rate [26,27]. In our experiments, the heat feedback from the heated iron frame and gypsum boards to the unburned fuel plays a supplementary role in the process of heating the fuel to boil. It can also be seen from Fig. 3 that for the cases with the same size pool with and without natural ventilation, the histories of mass loss rate are similar at both the initial steady burning stage and the boiling burning stage. The average mass loss rate at the steady burning stage in all cases is relatively small, between 0.1 and 0.3 g/s. The average value at the boiling burning stage is larger, between 1 and 3.5 g/s. For the same size pool fire, natural ventilation does not cause the significant change of the average mass loss rate at both stages. Roh et al. [28] found that for tunnel fires, the increase in longitudinal ventilation velocity leads to the enhanced burning rate of fuels, due to the fact that the oxygen supply effect prevails rather than the cooling effects as the ventilation velocity increases. It can be concluded from Fig. 3 that, natural ventilation caused by vertical shafts does not lead to the enhanced burning rate of fuels largely. The reason is that the velocity of longitudinal air supplying caused by the natural smoke exhausting is limited and not large enough to result in a more drastic fire situation. Fig. 4 shows the CO concentrations measured at the top opening of the shaft with natural ventilation and under the tunnel ceiling without natural ventilation. At the beginning of fires, the CO concentrations increase slowly with time, and after one certain moment increase by a large margin suddenly until the fuels burn out. Based on Figs. 3 and 4, the times of sudden increase of the CO concentrations are found to be the same as those of the mass loss rates. In cases with pool size of 100, 150, 225 and 300 cm2, the sudden increases occur at about 60, 80, 110 and 250 s respec-
tively. It can be known that at the initial steady stage, the oxygen supplying can meet the requirement of complete combustion of fuel, resulting in very low CO concentration in the smoke. However, at the boiling burning stage, the mass loss rates increase approximate nine times and the oxygen supplying cannot meet the requirement, resulting in higher CO concentration in the smoke. Consequently, a fairly large number of smoke with a high CO concentration forms inside the narrow tunnel structure. As a matter of fact, the concentration of harmful gases like CO should be low enough for safe evacuation of stranded occupants (50 ppm) [29], also to the benefit of firefighters for extinguishing the fire. Moreover, for the cases with pools of the same size, the peak value of CO concentration measured at the top opening of the shaft in cases with natural ventilation is significant lower than that measured under the tunnel ceiling in cases without natural ventilation at the boiling burning stage. In the presence of vertical shaft, when the hot smoke spreads to the area beneath the shaft, it will be exhausted through the shaft because of the stack effect. Based on the analysis in the previous section, under the action of vertical inertia force, the disturbance on smoke layer interface will be strengthened greatly, leading to a lot more fresh air mixed into the smoke layer and lower CO concentration of smoke in the shaft. The peak values of the CO concentration measured in cases with and without natural ventilation and their ratio are shown in Fig. 5. The ratio is about 1/3, indicating that 2/3 of the smoke exhausting rate of the shaft is the cold air in the lower layer inside the tunnel. Vauquelin [30] defined Ventilation System Output (VSO), the ratio of the flow rate of extracted smoke to the total extraction flow rate, to characterize the transverse ventilation system and found VSO obviously strongly decreased in case of plug-holing. In this study, the ratio in Fig. 5 has similar physical meaning with VSO.
C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757
ratio without natural ventilation natural ventilation
0.8
500
100
400
80
300
0.4
200
0.2
100
0.0
0
ratio
0.6
temperature rise (K)
1.0
CO concentration (ppm)
754
natural ventilation without natural ventilation
60
40
20
0 100
150
200
250
300
0.55
2
pool area (cm )
0.60
0.65
0.70
0.75
0.80
0.85
0.90
height (m)
Fig. 6 presents the typical smoke flow configuration under the tunnel ceiling taken by the laser sheet. It can be seen that in the case with natural ventilation, the plug-holing phenomenon occurs and the area right under the bottom opening of the shaft is occupied by fresh air, indicating that a large amount of fresh air is indirectly exhausted by the shaft and an inefficient smoke exhausting process is obtained. The vertical temperature rises measured under the center of the shaft bottom opening at the boiling stage in cases of 2015 and 2015s are shown in Fig. 7. It can be seen that in the case without natural ventilation, the upper part of thermocouple tree is inside the smoke layer and the lower part where the temperature rises are almost zero inside the air layer. Nevertheless, in the case with natural ventilation, the temperature rises of all measuring points are approaching zero, which illustrates the whole thermocouple tree is located in the air layer as plug-holing occurs. 4.2. Temperature rise, velocity and smoke layer interface height The temperature rise and velocity curves measured under the ceiling at the right opening of the tunnel with natural ventilation in test 5 are shown in Figs. 8 and 9. Both the temperature rise and velocity increase slowly with time in the beginning and increase sharply at the boiling burning stage. After the fuel burns out, the curves start to descend. These trends are the same with that of CO concentration. In the following part, the values of temperature rise and velocity used for further analysis are both averaged from the boiling burning stage, whose starting and final times are based on the variation of mass loss rate with time as represented in Fig. 3.
Fig. 7. Temperature rises measured under the shaft location (pool2015).
25
0.87 m 0.855 m 0.84 m 0.825 m 0.81 m 0.795 m 0.78 m 0.765 m 0.75 m 0.735 m 0.72 m 0.705 m 0.69 m 0.675 m 0.66 m 0.645 m
20
temperature rise (K)
Fig. 5. Maximum CO concentrations and the ratio with and without natural ventilation.
15
10
5
0
-5 -50
0
50
100
150
200
250
300
350
time (s) Fig. 8. Temperature rise curves measured under the ceiling at the right opening of the tunnel in test 5.
To avoid the subjectiveness of confirming smoke layer position employing the naked eye observation, the N-percent rule [31] which has been affirmed by other studies about the fire-induced smoke in buildings [32,33] is considered to determine the smoke layer interface height in this study. The interface temperature, Tint, is defined by the following formula [33]:
T int T a ¼ ðT max T a Þ N=100
ð11Þ
where T max is the measured maximum smoke temperature vertically. Previous study [27,32,33] recommended that the N should
Fig. 6. Typical configurations of smoke layer under the tunnel ceiling.
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C.G. Fan et al. / International Journal of Heat and Mass Transfer 56 (2013) 750–757 0.4
some fresh air will be entrained into the smoke layer. A part of the mixed air is exhausted by the shaft and the rest stays in the smoke layer and keeps spreading along the tunnel ceiling. Meanwhile, in tests with natural ventilation, the smoke layer temperature rise at the right opening of the tunnel decreases compared with those without natural ventilation because of the entrainment of cold air.
velocity (m/s)
0.3
0.85 m 0.81 m 0.77 m
0.2
4.3. Exhaust effect of shaft
0.1
0.0
-0.1 -50
0
50
100
150
200
250
300
350
time (s) Fig. 9. Velocity curves measured at the right opening of the tunnel in test 5.
0.90
0.85
height (m)
0.80
0.75
0.70
smoke interface 0.65 0
10
20
30
40
50
60
temperature rise (K) Fig. 10. Vertical temperature profiles at the right opening of the tunnel in test 4.
be between 10 and 30 and the value N = 30 is chosen in this study. This method is an easily applicable and unambiguous way to determine the smoke layer interface height. An important advantage of this method is that it is not time-consuming [33]. Fig. 10 presents the vertical temperature profiles under the ceiling at the right opening of the tunnel in test 4, the smoke layer interface height determined by the N-percent rule is 0.715 m. The smoke interface height and temperature rises at the right opening of the tunnel in all tests are represented in Table 2. In tests with natural ventilation, although some smoke has been exhausted by the shaft, the smoke interface height at the right of the shaft does not increase much compared with those without natural ventilation. Furthermore, it has a little decrease for the test of pool of 100 cm2. As the mixture process between the fresh air and smoke layer becomes stronger due to the smoke exhausting of the shaft,
In terms of Eqs. (5)–(7) and the analysis in the previous sections, the mass flow rates of spilling smoke in cases with and with_ 00spill and m _ 0spill ) and the smoke exhausting out natural ventilation (m _ shaft ) at the boiling stage are calculated as shown rate of the shaft (m in Fig. 11. In cases without natural ventilation, the mass flow rates of spilling smoke increase with the pool area. After employing the shaft for exhausting smoke, the mass flow rates of spilling smoke have some degree of decrease. The smoke exhausting rate of the shaft is 0.064 kg/s in the case with 100 cm2 pool and between 0.075 and 0.08 kg/s for the other cases. It is thus clear that the smoke exhausting rate of the shaft is relatively large whereas the _ is not much, between reduction amount of spilling smoke (Dm) 0.006 and 0.022 kg/s for all the cases. This is due to the fact that the major part of the smoke exhausting rate of the shaft is air on account of the plug-holing effect firstly. Secondly, the disturbance of smoke exhausting on the smoke–air interface leads to much fresh air entrained into the smoke layer, some of which entrains into the smoke layer downstream the shaft and spills from the tunnel. Based on Eq. (4), the mass flow rate of air entrained into the _ a;spill ), the mass flow smoke layer and not exhausted by the shaft (m _ a;shaft ) and the reducrate of air portion exhausted by the shaft (m _ are presented in Fig. 12. tion amount of spilling flow rate (Dm) For all the cases, the air proportion of the smoke exhausted by the shaft is about 2/3 and the ratio of all the air including the part exhausted by the shaft and that entrained into the smoke layer and _ a;shaft and m _ a;spill ) to the smoke not exhausted by the shaft (m exhausting rate of the shaft is between 70% and 92%. Therefore, the decrease of spilling flow rate seems to be small compared with the air portion exhausted from the shaft and entrained into the downstream smoke as a whole. Several researchers have addressed this issue on the mechanical smoke exhausting. Chow et al. [18] conducted a set of experiments in one atrium and found that the mass flow rate of air across the smoke layer interface could be 30% of the mass exhaust rate of the mechanical smoke exhaust system. Shi [19] experimentally studied the mechanical exhaust efficiency in compartment fire and claimed that when the exhausting vents were located near the smoke–air interface, a large amount of fresh air were entrained into upper smoke layer. Ji et al. [20] conducted experiments to investigate the influence of smoke vent height and exhausting velocity on mechanical smoke exhausting efficiency and found that the fresh air entrained into the smoke layer was up to 48% of the mechanical exhausting rate. Therefore, in practice projects, the engineers must consider making some improvements on shaft design to avoid the
Table 2 Smoke interface height and temperature rise at the right opening of the tunnel. Pool area (cm2)
100 150 255 300
Smoke interface height (m)
Smoke layer temperature rise (K)
Without natural ventilation
Natural ventilation
Without natural ventilation
Natural ventilation
0.725 0.715 0.71 0.715
0.71 0.71 0.715 0.715
13.4 23.2 35.0 45.4
9.3 20.2 25.2 39.6
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some improvements on shaft design to avoid the plug-holing phenomenon as far as possible. Our findings could be beneficial to researchers in better understanding the behavior of air entrainment with natural ventilation using shafts in road tunnel fires. Future work will be focused on numerical modeling investigation on the influence of other related parameters including shaft geometry on natural smoke exhausting effectiveness in tunnel fires.
mass flow rate (kg/s)
0.08
0.06
mspill'
0.04
Acknowledgements
mspill'' This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 50904055 and National Key Technology Research and Development Program under Grant No. 2011BAK07B01-02.
mshaft
0.02
0.00 100
150
200
2
250
300
pool area (cm )
References
Fig. 11. Mass flow rates of smoke spilling from the right opening of the tunnel and exhausted by the shaft.
mass flow rate (kg/s)
0.08
0.06
mshaft
0.04
ma,shaft ma,spill Δm
0.02
0.00 100
150
200
250
300
2
pool area (cm ) Fig. 12. Mass flow rates of air portion exhausted by the shaft and entrained into the downstream smoke and reduction amount of spilling smoke.
plug-holing phenomenon as far as possible and the criterion on the critical shaft height of the plug-holing occurrence in our previous work [17] can be referred to. It is also shown in Fig. 12 that the reduction amount of spilling flow rate has a trend to increase with the pool size. As a larger fire can induce a thicker smoke layer under the tunnel ceiling, the plug-holing effect will be weakened and thus more hot smoke will be exhausted and less fresh air will be entrained into the smoke layer. 5. Conclusions The principle purpose of the present work is to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires by means of small-scale experiments. It is suggested that the stack effect inside the shaft induce a strong mixing effect on the hot smoke layer and the cold air. For this reason, a portion of air will be inhaled into the shaft directly and indirectly, and some of the entrained air mixes into the smoke layer downstream the shaft and keeps spreading along the tunnel ceiling, which lead to an inefficient smoke exhausting process. The experimental results and analyses can be expected to be of great technical interest as basic data for the use of natural ventilation with shafts for smoke control. In practice projects, the engineers must consider making
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