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Performance of the smoke extraction system for fires in the Busan–Geoje immersed tunnel
Tunnelling and Underground Space Technology 25 (2010) 600–606
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Performance of the smoke extraction system for fires in the Busan–Geoje immersed tunnel Eui Ju Lee a,*, Chang Bo Oh a, Kwang Chul Oh b, Yong Ho Yoo c, Hyun Joon Shin c a
Division of Safety Engineering, Pukyong National University, San 100, Yongdang-Dong, Nam-Gu, Busan, Republic of Korea Environmental Components Research Center, Korea Automotive Technology Institute, 74 Yongjung, Pungse, Chonan, Chungnam, Republic of Korea c Department of Fire and Engineering Service Research, Korea Institute of Construction Technology, 2311 Daehwa, Ilsan, Goyang, Gyeonggi, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 2 April 2010 Accepted 6 April 2010 Available online 24 April 2010 Keywords: Tunnel fires Isothermal model Thermal model Smoke extraction efficiency Longitudinal ventilation Natural ventilation
a b s t r a c t A new partial smoke extraction system for the Busan–Geoje immersed tunnel was investigated experimentally using simulated tunnel fires. The tests were performed in a 1:20-scale model tunnel with a smoke extraction duct between two traffic tubes. The fire corresponded to a 5-MW full-scale fire, based on Froude modeling. Isothermal and thermal experimental models were considered. The performance of the partial smoke extraction system was quantified under natural and longitudinal ventilation conditions. The results showed that the smoke extraction efficiency of the natural ventilation was 30% better than with longitudinal ventilation, because of smoke stratification in the tunnel. Additionally, the efficiency obtained from the thermal model was comparable to that from the isothermal model under both ventilation conditions. The results suggested that the use of a partial smoke extraction system without longitudinal ventilation improved the initial visibility during tunnel fires. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fire is a major concern when there are traffic accidents in tunnels. Once a fire occurs in a tunnel, temperatures increase rapidly because of the semi-closed structure. Existing smoke extraction systems are limited, so that fire-induced toxic gases can result in insufficient oxygen for breathing and smoke can reduce visibility. Small-scale models have been widely used in experimental studies of tunnel fires because full-scale experiments are expensive and sometimes cause structural damage. Reduced-scale experiments for tunnel fires make use of thermal, isothermal, or hydraulic models, depending on the treatment of the fire sources (PIARC, 1999). Isothermal and thermal models are considered in this study. In an isothermal model, a fire source is represented as a mixture of a light gas (generally helium) and air or nitrogen. Because the model uses ambient-temperature gases, it does not reflect the effects of radiative and convective heat transfer between the smoke and a wall. Megret and Vauquelin (2000) developed a model based on an analysis of the combustion process in heptane pool fires. In their study, an overall one-step reaction was assumed for the combustion of major products, and the burning and air entrainment rates into the flame were provided by other empirical correlations. Their * Corresponding author. Tel.: +82 51 629 6471; fax: +82 51 629 6463. E-mail address: [email protected] (E.J. Lee). 0886-7798/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2010.04.005
predicted flow rate and temperature of the smoke plume were in good agreement with international recommendations (Lacroix, 1998), which implies that the quantitative measurements is possible with isothermal model because the flow rate and temperature of the smoke can be evaluated at a given heat release rate. When the densimetric model is applied with Froude number (Fr) scaling, the heat release rate and volume flow rate of the model are reduced by a5/2 for a given scale reduction ratio, a, whereas the smoke temperature must remain constant to preserve the buoyancy effect (Megret and Vauquelin, 2000). The helium fraction within the injected mixture to produce the equivalent buoyant effect can be estimated from the ideal gas law. On the other hand, in a thermal model, real flames from a gas burner or a pool fire are used as the fire source so that the heat transfer with the surrounding environment, such as a wall, can be considered directly. However, a thermal model based on the Froude number scaling cannot match other flame-induced properties, such as the total smoke flow rate, because the scaling law for smoke yield is not valid for Froude number modeling. Thus, thermal models have been primarily applied to determine the effect of fire size, and are sometimes used for visualization purposes (Drysdale, 1998; Zhou and Gore, 1995; Weckman and Strong, 1996; Wu and Baker, 2000). Modern tunnels are often long, and various facilities and shape designs are available. Thus, the smoke control and extraction systems for new tunnels should be studied carefully. This paper describes the performance evaluation of a partial smoke extraction
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system for the immersed fixed-link tunnel between Busan and Geoje Island in Korea that is currently under construction. The cross section of the immersed tunnel is 26.46 m wide and 9.97 m high, and contains two road traffic tubes with unidirectional traffic. The typical cross-sectional dimensions of the tubes are 9.7 m wide by 5.0 m high. A 2.5-m wide central gallery is isolated between the two road tubes. It contains a smoke extraction duct, a service gallery, and an escape gallery, as shown in Fig. 1 (Odgaard et al., 2006). Although jet fans on the ceiling of each tube are operated as the main smoke control system, the smoke extraction duct of the central gallery is designed to be used during the initial stages of a fire. Because the extraction flow rate is not sufficient to remove all the smoke inside the tunnel, the partial smoke extraction sys-
Damper Jet Fans
tem is only effective during an evacuation. Based on the basic design of the immersed tunnel, all dampers are normally closed. In case of a fire, the dampers downstream of the fire location are opened and then extract the smoke through the extraction duct with longitudinal ventilation by jet fans. In this study, experiments with thermal and isothermal models were performed with a reduced-scale tunnel model to evaluate the performance of the smoke extraction duct in the central gallery. Natural and longitudinal ventilation flow conditions were considered to determine the efficiency of the smoke extraction duct as a function of the flow induced by the jet fans. The characteristics of the smoke inside the main tube obtained using both experimental models were compared to each other. 2. Experiments
Smoke Extraction Duct
2.1. Model tunnel The experiments were performed using a 1:20-scale model of the Busan–Geoje immersed tunnel with a slightly simplified cross section to avoid manufacturing difficulties as shown in Fig. 2. The reduced scale main tunnel had a rectangular cross section with a width of 500 mm and a height of 250 mm. The total length was 12 m, made up of 12 separate sections of equal length. The first section of the main tunnel, which included the fire source, was constructed from stainless steel to prevent heat damage. The other sections were constructed from 5-mm-thick transparent acrylic. The smoke extraction duct placed near the ceiling of the main tunnel was 150 mm wide and 60 mm high and was connected to the
Service Gallery
Escape Gallery
Fire Fig. 1. Schematic cross section of Busan–Goeje immersed tunnel.
Flow straightener Wind tunnel
Main tube Damper
Smoke extraction duct
Fire source
0.8 m 12 m
Suction fan
Blower
(a) Schematic diagram 150 mm
Damper
250 mm
Main tube
60 mm
Smoke extraction duct
500 mm
(b) Cross section Fig. 2. Configuration of the reduced model tunnel.
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main tunnel through openings that represented open dampers. The cross-sectional area of each damper was 2160 mm2, and the distance between them was 2 m. A wind tunnel was installed at the entrance to the main tunnel so that longitudinal ventilation by jet fans could be simulated. The air velocity was measured upstream of the main tunnel to evaluate the performance of the wind tunnel and the deviation of the velocity distribution in the cross section was ±5%, except in the vicinity of the wall. An exhaust fan was installed at the downstream end of smoke extraction duct. A flow straightener was placed ahead of the exhaust fan to extract smoke with a uniform velocity. The air velocity of the main tunnel induced by the wind tunnel was one of the most important experimental conditions. The longitudinal ventilation velocity was set to 0.5 m/s, which was the measured critical velocity for an equivalent 5-MW fire in the model tunnel with isothermal model. The sampling probe for measuring oxygen concentration was placed at 20 cm upstream of fire source (mixtures of air and helium) on the ceiling. As the longitudinal velocity in the main tunnel was increased, the oxygen concentration was increased. When the measured oxygen concentration was same as that of ambient air (21%), the minimum velocity was determined to critical velocity. The magnitude of the fire was determined from the maximum heat release rate of a fire in a tunnel corresponding to one large passenger car (PIARC, 1999). For natural ventilation conditions, no longitudinal velocity was provided in the main tunnel.
gas sampling was 10 cm downstream of each damper opening in the smoke extraction duct. A hotwire anemometer (Kanomax, 6162) with a resolution of 0.01 m/s was used to measure the flow velocity and the oxygen concentration in the tunnel was measured with a gas analyzer (Horiba, 3100A) having a response time of 60 s. 2.3. Thermal model The fire source for the thermal model was simulated by a pool fire using a stainless steel burner with coaxial pans for the liquid fuel and water. A fuel reservoir was used for the fuel to preserve a constant level inside the pan (Lee and Shin, 2004). Heptane was used as the fuel for consistency, because the equivalent heat release rate of the isothermal model was calculated based on heptane combustion (Megret and Vauquelin, 2000). The pan diameter was 9 cm for the 5-MW fire experiment, as determined from the empirical relationship proposed by Burgess et al. (1961), as follows:
p _ 001 ð1 ekbD Þ D2 DHC Q_ pool ¼ m 4
_ 001 ¼ 0:1 kg=m2 s, kb = 1.1 m1, where D is the pan diameter; and m and DHC = 44, 600 kJ/kg for heptane. This equation is valid for large-diameter hydrocarbon fires in which the radiative heat transfer term predominates. Zabetakis and Burgess (1961) recommended that this relationship be used to predict the burning rate of liquid pools with diameters greater than 0.2 m. Therefore, it was necessary to verify the validity of Eq. (1) for the 9-cm small-scale diameter fire used in our experiments by measuring the time-averaged regression rate in the free-burning condition. The burning rate, based on the measured regression rate of 0.93 mm/min, is about 11% larger than the empirical burning rate obtained from Eq. (1). Although the tunnel geometry and longitudinal ventilation could affect the heat release rate of a fire in a tunnel, their effects are negligible for relatively small fires in a large tunnel (Carvel et al., 2001). The temperature of hot gases was measured using six 0.25mm K-type thermocouples; Fig. 3 shows the positions of the thermocouples. The first thermocouple (TC1) was placed on the ceiling of main tunnel above the fire source, while the other five thermocouples (TC2–TC6) in the smoke extraction duct were located adjacent to the measuring position for the velocity and gas sampling, which was the same location that was used during the isothermal test. To evaluate the smoke extraction efficiency, both the total flow rate and gas concentration are required at the dampers. Combustion product concentrations, such as those of carbon monoxide, carbon dioxide, and oxygen, were measured and the gas concentration above the fire was used for the representative value produced by pool fires.
2.2. Isothermal model To simulate the fire-induced smoke with an isothermal model, a light gas consisting of a mixture of helium and air was continuously released upward from a circular nozzle. Helium, 99.9% pure, from a cylinder manifold and air from a compressor entered a mixing chamber and the mixture was released at a uniform velocity across a mesh. The air and helium flow rates were controlled separately by digital mass flow meters. Based on the densimetric model for a 1:20 reduced scale 5-MW fire (Megret and Vauquelin, 2000; Vauquelin and Wu, 2006), the nozzle diameter, total flow rate, and air to helium ratio were set to 8.0 cm, 10.08 dm3/s, and 38.4/61.6 (%), respectively. The convective heat release rate of the 5-MW fire was 3.5 MW because a 30% radiative loss fraction was included in the calculations. For the smoke distribution of the main tunnel, gas concentration was measured along the centerline of the tunnel where sampling holes at four elevations were located in each section as shown in Fig. 3. In the smoke extraction duct, we measured velocity as well as the gas concentrations for the evaluation of smoke extraction efficiency. The location of velocity measurement and
1m
Stainless Steel
10 cm
TC2 TC1
A
ð1Þ
TC4
TC3
B
C
TC5
D
TC6
E
35 cm 60 cm 60 cm 60 cm 35 cm
Fire Source
Sampling Hole
Fig. 3. Schematic diagram of the reduced scale model tunnel with the measuring positions for temperature, velocity, and gas concentrations for the isothermal and thermal models. A–E indicate the damper locations between the smoke extraction duct and main tube. Thermocouples are indicated by ‘‘TC.”.
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3. Results and discussion
20
3.1. Isothermal model 19
4
1.0
V0 V/V0 with longitudinal ventilation V/V0 with natural ventilation
0.8
V / V0
0.6 2 0.4
V0 (m/s)
3
1 0.2
0
0.0
A
B
C
D
E
Damper Location Fig. 4. Exhaust velocity V0 and normalized smoke extraction velocity V at each damper with or without longitudinal forced flow – with isothermal model.
18
O2 (%)
The isothermal model experiments were conducted using either longitudinal or natural ventilation conditions. The total volumetric flow rate of the smoke extraction duct was set to 5:6 103 m3/s based on the basic design of the immersed tunnel. For the longitudinal ventilation, the velocity of the main tunnel model was determined to be 0.5 m/s experimentally as the critical velocity of simulated 5 MW fire. Fig. 4 shows the smoke velocity extracted from the main tunnel through each opened damper. The velocity was normalized by the exhaust velocity, V0, of each damper when no fire-induced flow existed. The normalized velocity indicates the increased exhaust flow rate of each damper due to the fire source and values above unity (V/V0 > 1) indicate that the buoyant plume generated in the fire passed through the smoke extraction dampers. As shown in Fig. 4, V0 increased as the flow moved downstream because of the accumulation of air through the previous dampers, while the non-dimensional extraction velocity decreased rapidly. In the case of natural ventilation, the normalized velocity at damper A was more than twice those at dampers C–E, suggesting that a damper near a fire source can extract more of the fire-induced plume. A similar trend was observed for longitudinal ventilation, but the normalized extraction velocity was smaller than that observed for natural ventilation. This might be due to the stratification of the smoke and the density difference. For natural ventilation, more low-density gas existed near the ceiling and a larger flow rate could be extracted at the same amount of fan power consumption. The ability to extract smoke flow can be estimated from the velocity after last damper E in the smoke extraction duct. If the total flow rate for longitudinal ventilation was determined to sum of the longitudinal flow and the fire source flow, the smoke flow rate extracted by the duct was about 15% of the total flow rate in the experimental condition. Fig. 5 shows the oxygen concentration of the mixture flowing through the dampers. Because the fire-induced smoke was simulated by a mixture of helium and air in the isothermal model, a lower oxygen concentration indicates a higher concentration of smoke. The measured oxygen concentration showed a different trend for the two ventilation conditions. Longitudinal ventilation provided a nearly uniform oxygen concentration of about 19% at all dampers because the air–helium mixture from the simulated fire source was diluted by the forced air flow from the wind tunnel. Hence the concentration was kept uniform throughout the entire
17
16
15
Longitudinal ventilation Natural ventilation
A
B
C
D
E
Damper Location Fig. 5. Oxygen concentration measured at each damper – with isothermal model.
tunnel. However, the oxygen concentration for natural ventilation varied from 16% to 18.5% with increasing axial distance from the fire source. To visualize the smoke distribution in a vertical plane of the main tunnel, the gas concentration was measured and quantified as a representative smoke concentration, which is referred to as the smoke dilution index (SDI):
SDIð%Þ ¼
X O2 ;m X O2 ;s 100 X O2 ;a X O2 ;s
ð2Þ
where X O2 ;m , X O2 ;s , and X O2 ;a are the volumetric mole fraction of oxygen measured at the sampling holes, at the exit of the fire source, and in the ambient air, respectively, and X O2 ;a is 0.21. The SDI can vary from 0 to 100; a zero SDI implies that no smoke is extracted or mixed with any other gas such as air, while 100% SDI means that all smoke inside tunnel is completely extracted. Fig. 6 depicts the smoke distribution along a vertical plane in the tunnel with and without longitudinal ventilation. In a real tunnel fire, fire-induced hot smoke rises to the ceiling, due to its buoyancy, and then moves downstream, eventually descending far downstream due to heat loss to the tunnel wall. In the isothermal model, this effect is ignored because there is no heat transfer between the wall and the smoke. The results show that the smoke layer was stratified only when the smoke extraction system was engaged and the damper near the fire extracted smoke exceptionally well as shown in Fig. 6a. Fig. 6b shows that the longitudinal ventilation resulted in a de-stratification of the smoke layer. The 90% SDI contour was reduced in size, and the 95% SDI contour disappeared entirely. The increased velocity of the flow passing over the fire generated turbulence to a degree where smoke stratification was not achieved. The dilution of the smoke due to convective mixing enhanced the phenomenon as well. This is also evident from the oxygen concentrations shown in Fig. 5. 3.2. Thermal model The thermal model was applied to the tunnel fire with and without longitudinal ventilation, similar to the isothermal model. Although the physical properties of smoke flow, such as temperature and gas concentrations, are not governed by the scaling law, data from reduced-scale experiments provide a good indication of steady-state combustion and smoke conditions. The measured data were collected over the 10 min period after ignition and averaged. For longitudinal ventilation, the concentrations of three major species, CO, CO2, and O2, were measured at each damper of the
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(a) Natural ventilation (Vcr=0 m/s)
(b) Longitudinal ventilation (Vcr =0.5 m/s) Fig. 6. Smoke distributions as indicated by the smoke dilution index (SDI) along a vertical plane in the tunnel – with isothermal model.
smoke extraction duct. These are listed in Table 1, along with the velocity and temperature. The concentrations of carbon monoxide and carbon dioxide decreased at the dampers located farther downstream and the small gradients of the concentrations indicated the rapid mixing between the plume and longitudinal flow at the front of the tunnel. On the other hand, the oxygen concentration was low at the damper close to the fire. These trends coincided with those of the isothermal model, but there was some quantitative discrepancy, compared with the values shown in Fig. 5, which stem from the fact that Froude number scaling is not valid for the yields of combustion products in reduced-scale experiments. Table 2 shows the velocity, gas concentrations, and temperature at each damper with natural ventilation. At all damper sections, the natural ventilation increased both the CO and CO2 concentrations and decreased the oxygen concentration, compared with values obtained with longitudinal ventilation. This was expected because the total air flow rate required to dilute the smoke was
Table 1 Flow properties measured at the smoke extraction dampers with longitudinal ventilation (with thermal model). Location
Velocity (m/s)
CO (ppm)
CO2 (vol.%)
O2 (vol.%)
Temperature (°C)
A B C D E
0.06 0.20 0.32 0.43 0.65
12 11 10 8 8
0.31 0.29 0.27 0.23 0.21
20.49 20.51 20.53 20.59 20.62
28.9 26.8 24.4 23.5 22.2
Table 2 Flow properties measured at the smoke extraction dampers for natural ventilation (with thermal model). Location
Velocity (m/s)
CO (ppm)
CO2 (vol.%)
O2 (vol.%)
Temperature (°C)
A B C D E
0.08 0.20 0.33 0.42 0.73
29 30 21 17 10
0.90 0.98 0.68 0.51 0.26
19.56 19.48 19.88 20.10 20.45
28.9 26.8 24.4 22.5 22.2
somewhat limited for natural ventilation. The velocity and temperature in the exhaust duct were nearly the same for the two ventilation modes, although the CO concentration at the first damper differed by a factor of three. The ratio of CO to CO2 has sometimes been used as an indicator for reaction completeness. The ratio of CO to CO2 measured at the last damper E, where the mixing was the best, was 3:8 103 for both ventilation modes. This result supports the fact that the forced ventilation effects on the heat release rate can be neglected for relatively small pool fires in a large tunnel (Carvel et al., 2001). 3.3. Smoke extraction efficiency To assess the performance of the smoke extraction duct, the smoke extraction efficiency was evaluated. The efficiency at each damper is defined by the ratio of the extracted volume flow rate to the volume flow rate of the smoke produced. Assuming that all fluid inside the tunnel is incompressible for the isothermal model, the smoke portion is directly related to the helium or oxygen concentrations and the efficiency can be deduced from Eq. (3), using the oxygen concentration:
gisothermal ¼
X O2 ;a X O2 ;d Q d X O2 ;a X O2 ;s Q s
ð3Þ
where Qd is the volume flow rate into the damper and Qs is the volume flow rate issued from the fire source. This relationship has a similar form to that of the traverse ventilation system (Vauquelin and Megret, 2002). The efficiency can therefore be calculated by measuring the volume flow rate, Qd, and oxygen concentration, X O2 ;d , at each damper, and by making use of the known values Qs and X O2 ;s for the fire source. There are two ways to evaluate the smoke extraction efficiency for the thermal model. One is to use the ratio of the energy increase of the ambient air to the heat generated by the fire source, but this must be applied only to a strictly adiabatic system so that the heat transfer to the wall can be neglected. The other is to use the volume flow rate of a combustion product extracted at the damper divided by the total flow rate generated by the fire source. Because no insulating material was applied to the tunnel model used for this study, it is very difficult to create adiabatic conditions experimentally. The efficiency calculated from the heat ratio differed by a few percent from the value
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gthermal ¼
Q d X CO;d Q s X CO;s
ð4Þ
40
30
Efficiency (%)
obtained using the flow rates, which indicates that the mixed hot gas lost a large quantity of heat to the wall. Thus, the latter option was used to calculate the smoke extraction efficiency. The total flow rate generated by the pool fire was required and a specific species was selected as a representative combustion product. Because it is difficult to measure the fire-induced flow, including entrainment directly, the total flow rate of the thermal model was assumed to be same as the value used for the isothermal model (Vauquelin and Megret, 2002). The presumed flow rate was 10.08 dm3/s for the 5MW fire and carbon monoxide was selected as a representative species, because it is usually formed with soot during fuel-rich incomplete combustion. The efficiency of the thermal model was expressed as
20
Thermal model Isothermal model
10
0 A
B
C
D
E
Damper Location
where XCO,d is the mole fraction of carbon monoxide measured at the damper sections and XCO,s is the mole fraction from the fire source, which was measured at the ceiling above the pool fire burning under quiescent ambient air; the value was 23 ppm. Fig. 7 shows the smoke extraction efficiency with longitudinal ventilation. The accumulated efficiency increased gradually at dampers located further from the fire. The last damper, E, had the best performance. The efficiency values from both the isothermal and thermal models were in good agreement, except for dampers B and C. The efficiency values at the last damper, E, were identical, at 24%. However, the smoke extraction performance differed with natural ventilation. As shown in Fig. 8, the first three damper sections, A–C, removed approximately 90% of the smoke from the tunnel, and hence the increasing rate of accumulated efficiency slowed farther downstream. The isothermal and thermal models showed similar trends, but the thermal model overestimated the efficiency at most dampers. The final smoke extraction efficiency was over 30% higher than that obtained with longitudinal ventilation. The smoke stratification that was observed with natural ventilation might have improved the performance of the smoke extraction duct, especially close to the fire source. The models used in this study were based on the Froude number conservation. Therefore, the heat release rate and the flow rate extracted through the duct could be extrapolated to real scale under two ventilation conditions. However, the flow effect or thermal effect in tunnel fires cannot be fully considered with the results because other dimensionless variables such as Re number were not conserved. For instance, the exhaust velocity at each damper in
Fig. 8. Smoke extraction efficiency at each damper with natural ventilation.
Fig. 4 might be different to the value in real scale tunnel because the pressure loss in the duct cannot be scaled with Fr modeling. These experimental results suggest an operating procedure for the smoke extraction duct in the event of a fire. When the jet fans are running with the smoke extraction system during the initial stages of a fire, the smoke and air will mix nearly uniformly across the entire cross section of the tunnel, thus diluting the smoke prior to removal even though the fans make it possible to control the level of smoke downstream without back layering. However, when the extraction dampers are open without longitudinal ventilation, the smoke stratification is preserved and results in a higher smoke extraction efficiency, which is better while evacuating the tunnel. 4. Conclusions The immersed Busan–Geoje fixed-link tunnel under construction will have a smoke extraction duct between the two road tubes for use in the event of a fire. To evaluate the performance of the partial smoke extraction system in the central gallery, two experimental models were tested in a 1:20 reduced-scale tunnel. A heptane pool fire was used to simulate the fire source in the thermal model and an isothermal jet with a mixture of lightweight gases was used to represent buoyant smoke in the isothermal model. Longitudinal ventilation was considered to characterize the effect of jet fans during smoke extraction and the results were compared with natural ventilation. The experimental findings were as follows:
40
Thermal model Isothermal model
Efficiency (%)
30
20
10
0
A
B
C
D
E
Damper Location Fig. 7. Smoke extraction efficiency at each damper with longitudinal ventilation.
1. With natural ventilation, smoke was removed through the dampers near the fire source and the smoke removal efficiency was relatively high. The amount of extracted smoke with longitudinal ventilation was similar at all dampers and the accumulated efficiency increased gradually along the tunnel. 2. When the partial smoke extraction system was operated during the early stages of a tunnel fire, smoke was extracted prior to mixing with the air inside the tunnel, which resulted in smoke stratification and improved smoke extraction performance. 3. The isothermal model was useful for predicting the smoke extraction efficiency and the smoke distribution inside the tunnel qualitatively and the results were in good agreement with those obtained from the thermal model after some assumptions were introduced. 4. Visibility in the event of a fire might be improved significantly by employing a smoke extraction duct, but careful operation of the jet fans is necessary to allow a safe evacuation.
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Acknowledgement This work was supported by the Pukyong National University Research Fund in 2006 (PKS-2006-010). References Burgess, D.S., Strasser, A., Grumer, J., 1961. Diffusive burning of liquid fuels on open trays. Fire Research and Abstract Reviews 3 (3), 177. Carvel, R.O., Beard, A.N., Jowitt, P.W., 2001. The influence of longitudinal ventilation systems on fires in tunnels. Tunnelling and Underground Space Technology 16 (1), 3–21. Drysdale, D., 1998. An Introduction to Fire Dynamics. John Wiley & Sons, New York. Lacroix, D., 1998. The new PIARC report on fire and smoke control in road tunnels. In: Proceedings of the Third International Conference on Safety in Road and Rail Tunnels, Nice, France, pp. 185–197. Lee E.J., Shin, H.J., 2004. Smoke characteristics of kerosene pool fires. In: Proceedings of Council on Tall Buildings and Urban Habitat (CTUBH) Conference, Seoul, Korea, pp. 950–956. Megret, O., Vauquelin, O., 2000. A model to evaluate tunnel fire characteristics. Fire Safety Journal 34 (4), 393–401.
Odgaard, S.S., Jensen, O.P., Kasper, T., Yoon, Y.H., Chang Y., Park, R.Y., 2006. Design of long immersed tunnel for highway in offshore conditions Busan–Geoje fixed link. In: Proceedings of the ITA-AITES 2006 World Tunnel Congress and 32nd ITA General Assembly, Seoul, Korea. Permanent International Association of Road Congresses (PIARC) Committee on Road Tunnels, 1999. Fire and Smoke Control in Road Tunnels. Ref.05.05.B. Vauquelin, O., Megret, O., 2002. Smoke extraction experiments in case of fire in a tunnel. Fire Safety Journal 37 (5), 525–533. Vauquelin, O., Wu, Y., 2006. Influence of tunnel width on longitudinal smoke control. Fire Safety Journal 41 (6), 420–426. Weckman, E.J., Strong, A.B., 1996. Experimental investigation of the turbulence structure of medium scale methanol pool fires. Combustion and Flame 105 (3), 245–266. Wu, Y., Baker, M.Z.A., 2000. Control of smoke flow in tunnel fires using longitudinal ventilation system – a study of the critical velocity. Fire Safety Journal 35 (4), 363–390. Zabetakis, M.G., Burgess, D.S., 1961. Research on the Hazards Associated with the Production and Handling of Liquid Hydrogen. US Bureau of Mines, Pittsburgh, PA (RI5707). Zhou, X.C., Gore, J.P., 1995. Air entrainment flow field induced by a pool fire. Combustion and Flame 100 (1), 52–60.
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Performance of the smoke extraction system for fires in the Busan–Geoje immersed tunnel Eui Ju Lee a,*, Chang Bo Oh a, Kwang Chul Oh b, Yong Ho Yoo c, Hyun Joon Shin c a
Division of Safety Engineering, Pukyong National University, San 100, Yongdang-Dong, Nam-Gu, Busan, Republic of Korea Environmental Components Research Center, Korea Automotive Technology Institute, 74 Yongjung, Pungse, Chonan, Chungnam, Republic of Korea c Department of Fire and Engineering Service Research, Korea Institute of Construction Technology, 2311 Daehwa, Ilsan, Goyang, Gyeonggi, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 26 January 2010 Received in revised form 2 April 2010 Accepted 6 April 2010 Available online 24 April 2010 Keywords: Tunnel fires Isothermal model Thermal model Smoke extraction efficiency Longitudinal ventilation Natural ventilation
a b s t r a c t A new partial smoke extraction system for the Busan–Geoje immersed tunnel was investigated experimentally using simulated tunnel fires. The tests were performed in a 1:20-scale model tunnel with a smoke extraction duct between two traffic tubes. The fire corresponded to a 5-MW full-scale fire, based on Froude modeling. Isothermal and thermal experimental models were considered. The performance of the partial smoke extraction system was quantified under natural and longitudinal ventilation conditions. The results showed that the smoke extraction efficiency of the natural ventilation was 30% better than with longitudinal ventilation, because of smoke stratification in the tunnel. Additionally, the efficiency obtained from the thermal model was comparable to that from the isothermal model under both ventilation conditions. The results suggested that the use of a partial smoke extraction system without longitudinal ventilation improved the initial visibility during tunnel fires. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fire is a major concern when there are traffic accidents in tunnels. Once a fire occurs in a tunnel, temperatures increase rapidly because of the semi-closed structure. Existing smoke extraction systems are limited, so that fire-induced toxic gases can result in insufficient oxygen for breathing and smoke can reduce visibility. Small-scale models have been widely used in experimental studies of tunnel fires because full-scale experiments are expensive and sometimes cause structural damage. Reduced-scale experiments for tunnel fires make use of thermal, isothermal, or hydraulic models, depending on the treatment of the fire sources (PIARC, 1999). Isothermal and thermal models are considered in this study. In an isothermal model, a fire source is represented as a mixture of a light gas (generally helium) and air or nitrogen. Because the model uses ambient-temperature gases, it does not reflect the effects of radiative and convective heat transfer between the smoke and a wall. Megret and Vauquelin (2000) developed a model based on an analysis of the combustion process in heptane pool fires. In their study, an overall one-step reaction was assumed for the combustion of major products, and the burning and air entrainment rates into the flame were provided by other empirical correlations. Their * Corresponding author. Tel.: +82 51 629 6471; fax: +82 51 629 6463. E-mail address: [email protected] (E.J. Lee). 0886-7798/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2010.04.005
predicted flow rate and temperature of the smoke plume were in good agreement with international recommendations (Lacroix, 1998), which implies that the quantitative measurements is possible with isothermal model because the flow rate and temperature of the smoke can be evaluated at a given heat release rate. When the densimetric model is applied with Froude number (Fr) scaling, the heat release rate and volume flow rate of the model are reduced by a5/2 for a given scale reduction ratio, a, whereas the smoke temperature must remain constant to preserve the buoyancy effect (Megret and Vauquelin, 2000). The helium fraction within the injected mixture to produce the equivalent buoyant effect can be estimated from the ideal gas law. On the other hand, in a thermal model, real flames from a gas burner or a pool fire are used as the fire source so that the heat transfer with the surrounding environment, such as a wall, can be considered directly. However, a thermal model based on the Froude number scaling cannot match other flame-induced properties, such as the total smoke flow rate, because the scaling law for smoke yield is not valid for Froude number modeling. Thus, thermal models have been primarily applied to determine the effect of fire size, and are sometimes used for visualization purposes (Drysdale, 1998; Zhou and Gore, 1995; Weckman and Strong, 1996; Wu and Baker, 2000). Modern tunnels are often long, and various facilities and shape designs are available. Thus, the smoke control and extraction systems for new tunnels should be studied carefully. This paper describes the performance evaluation of a partial smoke extraction
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system for the immersed fixed-link tunnel between Busan and Geoje Island in Korea that is currently under construction. The cross section of the immersed tunnel is 26.46 m wide and 9.97 m high, and contains two road traffic tubes with unidirectional traffic. The typical cross-sectional dimensions of the tubes are 9.7 m wide by 5.0 m high. A 2.5-m wide central gallery is isolated between the two road tubes. It contains a smoke extraction duct, a service gallery, and an escape gallery, as shown in Fig. 1 (Odgaard et al., 2006). Although jet fans on the ceiling of each tube are operated as the main smoke control system, the smoke extraction duct of the central gallery is designed to be used during the initial stages of a fire. Because the extraction flow rate is not sufficient to remove all the smoke inside the tunnel, the partial smoke extraction sys-
Damper Jet Fans
tem is only effective during an evacuation. Based on the basic design of the immersed tunnel, all dampers are normally closed. In case of a fire, the dampers downstream of the fire location are opened and then extract the smoke through the extraction duct with longitudinal ventilation by jet fans. In this study, experiments with thermal and isothermal models were performed with a reduced-scale tunnel model to evaluate the performance of the smoke extraction duct in the central gallery. Natural and longitudinal ventilation flow conditions were considered to determine the efficiency of the smoke extraction duct as a function of the flow induced by the jet fans. The characteristics of the smoke inside the main tube obtained using both experimental models were compared to each other. 2. Experiments
Smoke Extraction Duct
2.1. Model tunnel The experiments were performed using a 1:20-scale model of the Busan–Geoje immersed tunnel with a slightly simplified cross section to avoid manufacturing difficulties as shown in Fig. 2. The reduced scale main tunnel had a rectangular cross section with a width of 500 mm and a height of 250 mm. The total length was 12 m, made up of 12 separate sections of equal length. The first section of the main tunnel, which included the fire source, was constructed from stainless steel to prevent heat damage. The other sections were constructed from 5-mm-thick transparent acrylic. The smoke extraction duct placed near the ceiling of the main tunnel was 150 mm wide and 60 mm high and was connected to the
Service Gallery
Escape Gallery
Fire Fig. 1. Schematic cross section of Busan–Goeje immersed tunnel.
Flow straightener Wind tunnel
Main tube Damper
Smoke extraction duct
Fire source
0.8 m 12 m
Suction fan
Blower
(a) Schematic diagram 150 mm
Damper
250 mm
Main tube
60 mm
Smoke extraction duct
500 mm
(b) Cross section Fig. 2. Configuration of the reduced model tunnel.
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main tunnel through openings that represented open dampers. The cross-sectional area of each damper was 2160 mm2, and the distance between them was 2 m. A wind tunnel was installed at the entrance to the main tunnel so that longitudinal ventilation by jet fans could be simulated. The air velocity was measured upstream of the main tunnel to evaluate the performance of the wind tunnel and the deviation of the velocity distribution in the cross section was ±5%, except in the vicinity of the wall. An exhaust fan was installed at the downstream end of smoke extraction duct. A flow straightener was placed ahead of the exhaust fan to extract smoke with a uniform velocity. The air velocity of the main tunnel induced by the wind tunnel was one of the most important experimental conditions. The longitudinal ventilation velocity was set to 0.5 m/s, which was the measured critical velocity for an equivalent 5-MW fire in the model tunnel with isothermal model. The sampling probe for measuring oxygen concentration was placed at 20 cm upstream of fire source (mixtures of air and helium) on the ceiling. As the longitudinal velocity in the main tunnel was increased, the oxygen concentration was increased. When the measured oxygen concentration was same as that of ambient air (21%), the minimum velocity was determined to critical velocity. The magnitude of the fire was determined from the maximum heat release rate of a fire in a tunnel corresponding to one large passenger car (PIARC, 1999). For natural ventilation conditions, no longitudinal velocity was provided in the main tunnel.
gas sampling was 10 cm downstream of each damper opening in the smoke extraction duct. A hotwire anemometer (Kanomax, 6162) with a resolution of 0.01 m/s was used to measure the flow velocity and the oxygen concentration in the tunnel was measured with a gas analyzer (Horiba, 3100A) having a response time of 60 s. 2.3. Thermal model The fire source for the thermal model was simulated by a pool fire using a stainless steel burner with coaxial pans for the liquid fuel and water. A fuel reservoir was used for the fuel to preserve a constant level inside the pan (Lee and Shin, 2004). Heptane was used as the fuel for consistency, because the equivalent heat release rate of the isothermal model was calculated based on heptane combustion (Megret and Vauquelin, 2000). The pan diameter was 9 cm for the 5-MW fire experiment, as determined from the empirical relationship proposed by Burgess et al. (1961), as follows:
p _ 001 ð1 ekbD Þ D2 DHC Q_ pool ¼ m 4
_ 001 ¼ 0:1 kg=m2 s, kb = 1.1 m1, where D is the pan diameter; and m and DHC = 44, 600 kJ/kg for heptane. This equation is valid for large-diameter hydrocarbon fires in which the radiative heat transfer term predominates. Zabetakis and Burgess (1961) recommended that this relationship be used to predict the burning rate of liquid pools with diameters greater than 0.2 m. Therefore, it was necessary to verify the validity of Eq. (1) for the 9-cm small-scale diameter fire used in our experiments by measuring the time-averaged regression rate in the free-burning condition. The burning rate, based on the measured regression rate of 0.93 mm/min, is about 11% larger than the empirical burning rate obtained from Eq. (1). Although the tunnel geometry and longitudinal ventilation could affect the heat release rate of a fire in a tunnel, their effects are negligible for relatively small fires in a large tunnel (Carvel et al., 2001). The temperature of hot gases was measured using six 0.25mm K-type thermocouples; Fig. 3 shows the positions of the thermocouples. The first thermocouple (TC1) was placed on the ceiling of main tunnel above the fire source, while the other five thermocouples (TC2–TC6) in the smoke extraction duct were located adjacent to the measuring position for the velocity and gas sampling, which was the same location that was used during the isothermal test. To evaluate the smoke extraction efficiency, both the total flow rate and gas concentration are required at the dampers. Combustion product concentrations, such as those of carbon monoxide, carbon dioxide, and oxygen, were measured and the gas concentration above the fire was used for the representative value produced by pool fires.
2.2. Isothermal model To simulate the fire-induced smoke with an isothermal model, a light gas consisting of a mixture of helium and air was continuously released upward from a circular nozzle. Helium, 99.9% pure, from a cylinder manifold and air from a compressor entered a mixing chamber and the mixture was released at a uniform velocity across a mesh. The air and helium flow rates were controlled separately by digital mass flow meters. Based on the densimetric model for a 1:20 reduced scale 5-MW fire (Megret and Vauquelin, 2000; Vauquelin and Wu, 2006), the nozzle diameter, total flow rate, and air to helium ratio were set to 8.0 cm, 10.08 dm3/s, and 38.4/61.6 (%), respectively. The convective heat release rate of the 5-MW fire was 3.5 MW because a 30% radiative loss fraction was included in the calculations. For the smoke distribution of the main tunnel, gas concentration was measured along the centerline of the tunnel where sampling holes at four elevations were located in each section as shown in Fig. 3. In the smoke extraction duct, we measured velocity as well as the gas concentrations for the evaluation of smoke extraction efficiency. The location of velocity measurement and
1m
Stainless Steel
10 cm
TC2 TC1
A
ð1Þ
TC4
TC3
B
C
TC5
D
TC6
E
35 cm 60 cm 60 cm 60 cm 35 cm
Fire Source
Sampling Hole
Fig. 3. Schematic diagram of the reduced scale model tunnel with the measuring positions for temperature, velocity, and gas concentrations for the isothermal and thermal models. A–E indicate the damper locations between the smoke extraction duct and main tube. Thermocouples are indicated by ‘‘TC.”.
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3. Results and discussion
20
3.1. Isothermal model 19
4
1.0
V0 V/V0 with longitudinal ventilation V/V0 with natural ventilation
0.8
V / V0
0.6 2 0.4
V0 (m/s)
3
1 0.2
0
0.0
A
B
C
D
E
Damper Location Fig. 4. Exhaust velocity V0 and normalized smoke extraction velocity V at each damper with or without longitudinal forced flow – with isothermal model.
18
O2 (%)
The isothermal model experiments were conducted using either longitudinal or natural ventilation conditions. The total volumetric flow rate of the smoke extraction duct was set to 5:6 103 m3/s based on the basic design of the immersed tunnel. For the longitudinal ventilation, the velocity of the main tunnel model was determined to be 0.5 m/s experimentally as the critical velocity of simulated 5 MW fire. Fig. 4 shows the smoke velocity extracted from the main tunnel through each opened damper. The velocity was normalized by the exhaust velocity, V0, of each damper when no fire-induced flow existed. The normalized velocity indicates the increased exhaust flow rate of each damper due to the fire source and values above unity (V/V0 > 1) indicate that the buoyant plume generated in the fire passed through the smoke extraction dampers. As shown in Fig. 4, V0 increased as the flow moved downstream because of the accumulation of air through the previous dampers, while the non-dimensional extraction velocity decreased rapidly. In the case of natural ventilation, the normalized velocity at damper A was more than twice those at dampers C–E, suggesting that a damper near a fire source can extract more of the fire-induced plume. A similar trend was observed for longitudinal ventilation, but the normalized extraction velocity was smaller than that observed for natural ventilation. This might be due to the stratification of the smoke and the density difference. For natural ventilation, more low-density gas existed near the ceiling and a larger flow rate could be extracted at the same amount of fan power consumption. The ability to extract smoke flow can be estimated from the velocity after last damper E in the smoke extraction duct. If the total flow rate for longitudinal ventilation was determined to sum of the longitudinal flow and the fire source flow, the smoke flow rate extracted by the duct was about 15% of the total flow rate in the experimental condition. Fig. 5 shows the oxygen concentration of the mixture flowing through the dampers. Because the fire-induced smoke was simulated by a mixture of helium and air in the isothermal model, a lower oxygen concentration indicates a higher concentration of smoke. The measured oxygen concentration showed a different trend for the two ventilation conditions. Longitudinal ventilation provided a nearly uniform oxygen concentration of about 19% at all dampers because the air–helium mixture from the simulated fire source was diluted by the forced air flow from the wind tunnel. Hence the concentration was kept uniform throughout the entire
17
16
15
Longitudinal ventilation Natural ventilation
A
B
C
D
E
Damper Location Fig. 5. Oxygen concentration measured at each damper – with isothermal model.
tunnel. However, the oxygen concentration for natural ventilation varied from 16% to 18.5% with increasing axial distance from the fire source. To visualize the smoke distribution in a vertical plane of the main tunnel, the gas concentration was measured and quantified as a representative smoke concentration, which is referred to as the smoke dilution index (SDI):
SDIð%Þ ¼
X O2 ;m X O2 ;s 100 X O2 ;a X O2 ;s
ð2Þ
where X O2 ;m , X O2 ;s , and X O2 ;a are the volumetric mole fraction of oxygen measured at the sampling holes, at the exit of the fire source, and in the ambient air, respectively, and X O2 ;a is 0.21. The SDI can vary from 0 to 100; a zero SDI implies that no smoke is extracted or mixed with any other gas such as air, while 100% SDI means that all smoke inside tunnel is completely extracted. Fig. 6 depicts the smoke distribution along a vertical plane in the tunnel with and without longitudinal ventilation. In a real tunnel fire, fire-induced hot smoke rises to the ceiling, due to its buoyancy, and then moves downstream, eventually descending far downstream due to heat loss to the tunnel wall. In the isothermal model, this effect is ignored because there is no heat transfer between the wall and the smoke. The results show that the smoke layer was stratified only when the smoke extraction system was engaged and the damper near the fire extracted smoke exceptionally well as shown in Fig. 6a. Fig. 6b shows that the longitudinal ventilation resulted in a de-stratification of the smoke layer. The 90% SDI contour was reduced in size, and the 95% SDI contour disappeared entirely. The increased velocity of the flow passing over the fire generated turbulence to a degree where smoke stratification was not achieved. The dilution of the smoke due to convective mixing enhanced the phenomenon as well. This is also evident from the oxygen concentrations shown in Fig. 5. 3.2. Thermal model The thermal model was applied to the tunnel fire with and without longitudinal ventilation, similar to the isothermal model. Although the physical properties of smoke flow, such as temperature and gas concentrations, are not governed by the scaling law, data from reduced-scale experiments provide a good indication of steady-state combustion and smoke conditions. The measured data were collected over the 10 min period after ignition and averaged. For longitudinal ventilation, the concentrations of three major species, CO, CO2, and O2, were measured at each damper of the
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(a) Natural ventilation (Vcr=0 m/s)
(b) Longitudinal ventilation (Vcr =0.5 m/s) Fig. 6. Smoke distributions as indicated by the smoke dilution index (SDI) along a vertical plane in the tunnel – with isothermal model.
smoke extraction duct. These are listed in Table 1, along with the velocity and temperature. The concentrations of carbon monoxide and carbon dioxide decreased at the dampers located farther downstream and the small gradients of the concentrations indicated the rapid mixing between the plume and longitudinal flow at the front of the tunnel. On the other hand, the oxygen concentration was low at the damper close to the fire. These trends coincided with those of the isothermal model, but there was some quantitative discrepancy, compared with the values shown in Fig. 5, which stem from the fact that Froude number scaling is not valid for the yields of combustion products in reduced-scale experiments. Table 2 shows the velocity, gas concentrations, and temperature at each damper with natural ventilation. At all damper sections, the natural ventilation increased both the CO and CO2 concentrations and decreased the oxygen concentration, compared with values obtained with longitudinal ventilation. This was expected because the total air flow rate required to dilute the smoke was
Table 1 Flow properties measured at the smoke extraction dampers with longitudinal ventilation (with thermal model). Location
Velocity (m/s)
CO (ppm)
CO2 (vol.%)
O2 (vol.%)
Temperature (°C)
A B C D E
0.06 0.20 0.32 0.43 0.65
12 11 10 8 8
0.31 0.29 0.27 0.23 0.21
20.49 20.51 20.53 20.59 20.62
28.9 26.8 24.4 23.5 22.2
Table 2 Flow properties measured at the smoke extraction dampers for natural ventilation (with thermal model). Location
Velocity (m/s)
CO (ppm)
CO2 (vol.%)
O2 (vol.%)
Temperature (°C)
A B C D E
0.08 0.20 0.33 0.42 0.73
29 30 21 17 10
0.90 0.98 0.68 0.51 0.26
19.56 19.48 19.88 20.10 20.45
28.9 26.8 24.4 22.5 22.2
somewhat limited for natural ventilation. The velocity and temperature in the exhaust duct were nearly the same for the two ventilation modes, although the CO concentration at the first damper differed by a factor of three. The ratio of CO to CO2 has sometimes been used as an indicator for reaction completeness. The ratio of CO to CO2 measured at the last damper E, where the mixing was the best, was 3:8 103 for both ventilation modes. This result supports the fact that the forced ventilation effects on the heat release rate can be neglected for relatively small pool fires in a large tunnel (Carvel et al., 2001). 3.3. Smoke extraction efficiency To assess the performance of the smoke extraction duct, the smoke extraction efficiency was evaluated. The efficiency at each damper is defined by the ratio of the extracted volume flow rate to the volume flow rate of the smoke produced. Assuming that all fluid inside the tunnel is incompressible for the isothermal model, the smoke portion is directly related to the helium or oxygen concentrations and the efficiency can be deduced from Eq. (3), using the oxygen concentration:
gisothermal ¼
X O2 ;a X O2 ;d Q d X O2 ;a X O2 ;s Q s
ð3Þ
where Qd is the volume flow rate into the damper and Qs is the volume flow rate issued from the fire source. This relationship has a similar form to that of the traverse ventilation system (Vauquelin and Megret, 2002). The efficiency can therefore be calculated by measuring the volume flow rate, Qd, and oxygen concentration, X O2 ;d , at each damper, and by making use of the known values Qs and X O2 ;s for the fire source. There are two ways to evaluate the smoke extraction efficiency for the thermal model. One is to use the ratio of the energy increase of the ambient air to the heat generated by the fire source, but this must be applied only to a strictly adiabatic system so that the heat transfer to the wall can be neglected. The other is to use the volume flow rate of a combustion product extracted at the damper divided by the total flow rate generated by the fire source. Because no insulating material was applied to the tunnel model used for this study, it is very difficult to create adiabatic conditions experimentally. The efficiency calculated from the heat ratio differed by a few percent from the value
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gthermal ¼
Q d X CO;d Q s X CO;s
ð4Þ
40
30
Efficiency (%)
obtained using the flow rates, which indicates that the mixed hot gas lost a large quantity of heat to the wall. Thus, the latter option was used to calculate the smoke extraction efficiency. The total flow rate generated by the pool fire was required and a specific species was selected as a representative combustion product. Because it is difficult to measure the fire-induced flow, including entrainment directly, the total flow rate of the thermal model was assumed to be same as the value used for the isothermal model (Vauquelin and Megret, 2002). The presumed flow rate was 10.08 dm3/s for the 5MW fire and carbon monoxide was selected as a representative species, because it is usually formed with soot during fuel-rich incomplete combustion. The efficiency of the thermal model was expressed as
20
Thermal model Isothermal model
10
0 A
B
C
D
E
Damper Location
where XCO,d is the mole fraction of carbon monoxide measured at the damper sections and XCO,s is the mole fraction from the fire source, which was measured at the ceiling above the pool fire burning under quiescent ambient air; the value was 23 ppm. Fig. 7 shows the smoke extraction efficiency with longitudinal ventilation. The accumulated efficiency increased gradually at dampers located further from the fire. The last damper, E, had the best performance. The efficiency values from both the isothermal and thermal models were in good agreement, except for dampers B and C. The efficiency values at the last damper, E, were identical, at 24%. However, the smoke extraction performance differed with natural ventilation. As shown in Fig. 8, the first three damper sections, A–C, removed approximately 90% of the smoke from the tunnel, and hence the increasing rate of accumulated efficiency slowed farther downstream. The isothermal and thermal models showed similar trends, but the thermal model overestimated the efficiency at most dampers. The final smoke extraction efficiency was over 30% higher than that obtained with longitudinal ventilation. The smoke stratification that was observed with natural ventilation might have improved the performance of the smoke extraction duct, especially close to the fire source. The models used in this study were based on the Froude number conservation. Therefore, the heat release rate and the flow rate extracted through the duct could be extrapolated to real scale under two ventilation conditions. However, the flow effect or thermal effect in tunnel fires cannot be fully considered with the results because other dimensionless variables such as Re number were not conserved. For instance, the exhaust velocity at each damper in
Fig. 8. Smoke extraction efficiency at each damper with natural ventilation.
Fig. 4 might be different to the value in real scale tunnel because the pressure loss in the duct cannot be scaled with Fr modeling. These experimental results suggest an operating procedure for the smoke extraction duct in the event of a fire. When the jet fans are running with the smoke extraction system during the initial stages of a fire, the smoke and air will mix nearly uniformly across the entire cross section of the tunnel, thus diluting the smoke prior to removal even though the fans make it possible to control the level of smoke downstream without back layering. However, when the extraction dampers are open without longitudinal ventilation, the smoke stratification is preserved and results in a higher smoke extraction efficiency, which is better while evacuating the tunnel. 4. Conclusions The immersed Busan–Geoje fixed-link tunnel under construction will have a smoke extraction duct between the two road tubes for use in the event of a fire. To evaluate the performance of the partial smoke extraction system in the central gallery, two experimental models were tested in a 1:20 reduced-scale tunnel. A heptane pool fire was used to simulate the fire source in the thermal model and an isothermal jet with a mixture of lightweight gases was used to represent buoyant smoke in the isothermal model. Longitudinal ventilation was considered to characterize the effect of jet fans during smoke extraction and the results were compared with natural ventilation. The experimental findings were as follows:
40
Thermal model Isothermal model
Efficiency (%)
30
20
10
0
A
B
C
D
E
Damper Location Fig. 7. Smoke extraction efficiency at each damper with longitudinal ventilation.
1. With natural ventilation, smoke was removed through the dampers near the fire source and the smoke removal efficiency was relatively high. The amount of extracted smoke with longitudinal ventilation was similar at all dampers and the accumulated efficiency increased gradually along the tunnel. 2. When the partial smoke extraction system was operated during the early stages of a tunnel fire, smoke was extracted prior to mixing with the air inside the tunnel, which resulted in smoke stratification and improved smoke extraction performance. 3. The isothermal model was useful for predicting the smoke extraction efficiency and the smoke distribution inside the tunnel qualitatively and the results were in good agreement with those obtained from the thermal model after some assumptions were introduced. 4. Visibility in the event of a fire might be improved significantly by employing a smoke extraction duct, but careful operation of the jet fans is necessary to allow a safe evacuation.
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Acknowledgement This work was supported by the Pukyong National University Research Fund in 2006 (PKS-2006-010). References Burgess, D.S., Strasser, A., Grumer, J., 1961. Diffusive burning of liquid fuels on open trays. Fire Research and Abstract Reviews 3 (3), 177. Carvel, R.O., Beard, A.N., Jowitt, P.W., 2001. The influence of longitudinal ventilation systems on fires in tunnels. Tunnelling and Underground Space Technology 16 (1), 3–21. Drysdale, D., 1998. An Introduction to Fire Dynamics. John Wiley & Sons, New York. Lacroix, D., 1998. The new PIARC report on fire and smoke control in road tunnels. In: Proceedings of the Third International Conference on Safety in Road and Rail Tunnels, Nice, France, pp. 185–197. Lee E.J., Shin, H.J., 2004. Smoke characteristics of kerosene pool fires. In: Proceedings of Council on Tall Buildings and Urban Habitat (CTUBH) Conference, Seoul, Korea, pp. 950–956. Megret, O., Vauquelin, O., 2000. A model to evaluate tunnel fire characteristics. Fire Safety Journal 34 (4), 393–401.
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