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Heat release rate and thermal fume behavior estimation of fuel cell vehicles in tunnel fires
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Heat release rate and thermal fume behavior estimation of fuel cell vehicles in tunnel fires Miho Seike a,*, Nobuyoshi Kawabata b, Masato Hasegawa c, Hirato Tanaka d a
Faculty of Engineering, Department of Mechanical Systems Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 9390398, Japan b Faculty of Production Systems Engineering and Sciences, Komatsu University, Nu, 1-3, Shicho-machi, Komatsu, Ishikawa, 9230971, Japan c Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 9201164, Japan d Ex-Deputy Director, Japan Institute of Country-ology and Engineering (JICE), Japan
highlights HRR of a carrier loaded with FCVs in a tunnel was estimated. Thermal fume of FCV and gasoline vehicle fires in inclined tunnel was investigated. Inclination caused faster thermal fume propagation of FVC fires was clarified.
article info
abstract
Article history:
This study proposes a heat release rate (HRR) estimation method for a carrier loaded with
Received 19 April 2019
fuel cell vehicles (FCVs) trapped in a tunnel fire. The carrier is divided into several parts,
Received in revised form
and the HRR is estimated by adding the HRRs of all system parts (carrier and FCVs). The
7 August 2019
HRR of one FCV was compared with that of a gasoline vehicle. The thermal fume behavior
Accepted 12 August 2019
in longitudinally inclined tunnel fires was also investigated. Even a modest inclination
Available online 6 September 2019
hastened the thermal fume propagation of the FCV fires. Of relevance to the prevention of tunnel fire disasters, the thermal fume behavior differed between FCV and gasoline fires.
Keywords:
For safety assessment of tunnel fires, the thermal fume behaviors of an FCV fire and
HRR
gasoline vehicle fire in a tunnel were investigated by the proposed method. In the case of
FCVs
no longitudinal inclination, the thermal fume of the FCV fire arrived earlier than that of the
Road tunnel
gasoline vehicle fire (by 1 min at x ¼ 200 m and over 4 min at x ¼ 250 m) because of the
Fire
emitted hydrogen gas. At 2% longitudinal inclination, the thermal fume of the FCV fire
Evacuation environment
propagated to the downstream side 4 min before that of the gasoline vehicle fire. At 4% longitudinal inclination, the thermal fume propagated 50 m downstream of the initial fire after 10 min. Therefore, after the hydrogen emission, the thermal fume of the FCV fire traveled faster than that of the gasoline vehicle fire. The proposed HRR estimation method can contribute to the risk analysis of various types of tunnel fires. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Seike), [email protected] (N. Kawabata), [email protected] (M. Hasegawa), [email protected] (H. Tanaka). https://doi.org/10.1016/j.ijhydene.2019.08.099 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction As part of the effort to replace traditional vehicle consumers of fossil fuels with cleaner alternatives, the commercial distribution of fuel cell vehicles (FCVs) recently began in Japan. Disaster prevention equipment that is compatible with fires caused by FCVs is imperative. Commercially available FCVs are fueled by hydrogen gas, which generates less smoke than gasoline during a fire. However, the hydrogen gas emissions of FCV fires immediately increase the ambient temperature, especially in enclosed spaces such as tunnels. Therefore, if the fire spreads to the hydrogen gas cylinder, the tunnel may sustain considerable damages. FCV risk in tunnels has been investigated in both experiments and simulations [39,41,46,49,54,55,59,60]. Middha and Hansen [46] discussed the temperature nearby (around ±10 m) a fire vehicle with hydrogen emissions. However, a tunnel is a long and large (albeit single and simple) space that becomes dotted with evacuees in the event of fire. Especially, evacuation decisions must be self-determined with respect to the tunnels. In Japan, tunnels are 500e1000 m long, numerous (totaling more than 1500 [64]), and lack emergency facilities. Against this background, we focused on the thermal fume behavior during an FCV fire in tunnels of several hundred meters in length. We considered that the hydrogen vessels are not broken by significant crushing, and that hydrogen gas unintentionally leaks from the vehicle, as assumed elsewhere [36]. Additionally, an experimental study by Ref. [2] assumed that the leaked hydrogen gas immediately ignited and combusted. We thus considered that the unburned hydrogen gas does not fill the tunnel and there is no risk of hydrogen deflagration [37e40,41e46,56,57]. Enozono and Tanaka's [2] study was inspired by the increasing use of fuel cells and new regulations issued by the Japanese government. They conducted fire experiments using a carrier loaded with FCVs in a full-scale tunnel at the Japan Construction Method and Machinery Research Institute. As hydrogen fueling stations are sparsely spaced, FCVs may be transported by trucks bearing full hydrogen fuel tanks. Additionally, restrictions on the transportation of hazardous materials through undersea tunnels and long tunnels should be reconsidered [1]. Although the number of hydrogen fueling stations in Japan has increased since 2005, it cannot meet the current demand. Tunnel spaces are typically long and enclosed, posing challenges to evacuation, rescue, and firefighting activities. Even small incidents in tunnels incur a high possibility of significant losses. Tunnel fires have caused significant mortality and economic loss. Examples are the 1999 Mont Blanc tunnel fire in France and Italy with 39 fatalities [4], the 1999 Tauern tunnel fire in Austria with 12 fatalities [5,6], and the 2001 St. Gotthard tunnel fire in Switzerland with 11 fatalities [7]. Since those incidents, disaster prevention methods have been widely examined by risk analysis techniques to mitigate the risk of tunnel fires. These techniques numerically simulate the heat release rate (HRR) during a fire. Previously, the HRR has been estimated by applying the oxygen consumption principle [8,9] measuring the maximum temperature with thermocouples [17], determining the mass loss rate on an
electronic balance [17], and measuring the radiant flux with a radiometer. However, useful data from these measurement methods must be acquired in full-scale experiments, which are expensive and depend on the environment and neighborhood of the tunnel, rendering HRR measurements difficult. Moreover, the HRR of FCVs in tunnels has not yet been investigated. Given the increasing popularity of FCVs as transportation options, investigating the HRR of FCVs in tunnel fires and the impact of a fire caused by several FCVs is necessitated from a safety perspective. With the aim of improving tunnel fire safety, the present work investigates the HRR of several carrier-loaded FCVs. First, it proposes and validates an HRR estimation method that sums the rated HRRs of the individual parts of the system (carrier and FCVs). Next, the proposed HRR calculation method was applied to the safety assessment of tunnel fires, confirming its suitability to risk analysis of various fire scenarios. The proposed method is then applied to two fire scenarios with different inclinations from the normal (0%, 2%, and 4%, which are usually seen in the tunnel longitudinal inclinations) in a tunnel, and the simulated thermal fume behavior is compared with the experimentally observed behavior. The first fire scenario involves a normal vehicle filled with 30 L gasoline; the second involves an FCV filled with 86 m3 hydrogen gas in a 70 MPa vessel (current FCVs are approximately 80 m3 and 70 MPa, respectively [22]).
Heat release rate (HRR) The HRR of a carrier loaded with four FCVs was estimated by summing the rated HRRs of the individual parts of the carrier and FCVs. Fire experiments using a carrier loaded with FCVs were then conducted in a full-scale tunnel at the Japan Construction Method and Machinery Research Institute [2], where conditions used were obtained from literature. In this experiment [2], jet fans providing air streams at 2 m/s were installed in each tunnel portal. The temperature, extinction coefficient, and air velocity were measured at 10 m from each portal (x ¼ 10 and 70 m), at x ¼ 60 m, and at four different heights (z ¼ 1.5, 3.0, 4.5, and 6.0 m). The HRR was not investigated. When the temperature (measured by a pressure relief device on the gas cylinder unit) reached 110 C, the hydrogen gas was discharged by manually opening the gas cylinder valves from a monitoring room (at that time, the safety valves for this type of cylinder had not been developed). The experimental tunnel was horseshoe-shaped with a length, width and height of 80 m, 12.4 m and 7.46 m, respectively (see Fig. 1). The origin of the Cartesian coordinate system was located at the center of the road surface at the tunnel entrance (left side of Fig. 1). The x-, y-, and z-axes corresponded to the tunnel length, width, and height, respectively. The following experimental scenarios were considered: 1. A small passenger car colliding with a carrier loaded with four FCVs. 2. Burning of a small passenger car. 3. The fire from a small passenger car spreading to an FCV loaded on a carrier.
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Fig. 1 e Schematic of the tunnel fire experiment. The proposed method sums the rated HRRs of the individual burning parts of the carrier and FCVs, including (i) the individual vehicles without fuel (burning duration: 1200 s), (ii) hydrogen gas (burning duration: 180 s), (iii) the rear wheels of the carrier (burning duration: 1800 s), and (iv) the fuel in a 1 m2 pan (burning duration: 360 s). Here the burning duration of each part was determined by the method of [2]; and only the convective components of the HRR were examined. The experimental duration was approximately 30 min, although the final fire (rear wheels) continued to burn thereafter. The results were recorded from t ¼ 0e30 min (1800 s; see Fig. 2). The time of fire ignition to the end was determined by a video camera.
Estimation of the HRRs of the individual parts FCVs (Nos. 2 to 5 in Fig. 3) The gross calorific value of complete combustion of a gasoline vehicle is 7 GJ for a full tank [3], and approximately 1 GJ for a tank filled with 30 L of gasoline (30 L 34.6 MJ/L ¼ 1038 MJ). Hence, the gross calorific value of a vehicle without gasoline is 6 GJ. According to the [24]; the convective thermal component of complete vehicle combustion is between 40% and 80% of the gross calorific value of complete combustion. Additionally, the convective HRR is 67% of the total HRR [17,30]. In this study, the convective thermal component of a vehicle body was assumed as 70% of the complete combustion (i.e., 4.2 GJ). For comparison with the experimental results of [2]; the hydrogen volume (43.6 m3) and gas cylinder pressure (32.5 MPa) in this study were approximately half those of current FCVs (approximately 80 m3 and 70 MPa, respectively [22]). The assumed capacities were the available capacities of
the hydrogen tanks installed at the time of the 2004 experiment. The complete combustion of this volume of highpressure hydrogen yields 0.47 GJ has been reported by Ref. [2]. We used this value and decided the convective component. According to the [24]; the convective thermal component of a high-pressure hydrogen jet flame should reach 80% of the gross calorific value of complete combustion. Therefore, the convective thermal component of hydrogen gas was calculated as 0.4 GJ, and the convective calorific value of one FCV was estimated as 4.6 GJ. The blue plot in Fig. 2 shows the calculated HRR of a car fire from ignition to the end of fire. The HRR was proportional to the squared time (t2), and the fully developed state with maximum HRR (4.2 MW) began at t ¼ 150 s and continued for 900 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 240 s after the vehicle body ignition. The growth rate was adopted as the t2 curve, reported for fast fire growth in the Handbook of Tunnel Fire Safety (2nd edition;[11]). As shown in Fig. 2 (orange plot), the HRR of hydrogen was proportional to the time elapsed from the start of the ignition to the fully developed state (the maximum HRR of 3 MW was reached at t ¼ 20 s), and the fully developed state continued for 80 s. The total calorific values of the car and fuel over the entire experiment were approximately 4.6 and 0.5 GJ, respectively.
Rear wheels of the carrier (No. 6 in Fig. 3) The experimental carrier had 10 wheels in total: eight in the rear and two in the front. During the experiment, the fire occurred at the fuel pan located behind the carrier and gradually spread to the front. Therefore, the rear wheels began burning easily, whereas the front wheels and the driver's seat did not burn. Assuming that a single wheel weighs 28 kg, its theoretical calorific value is 32.9 MJ/kg, yielding approximately 0.92 GJ/wheel [3]; thus, the approximated total theoretical calorific value of the eight rear wheels was 7.36 GJ. Ref. [17,30] reported that HRR is 67% convective; accordingly, the convective thermal component was assumed as 70% and the convective calorific value was calculated as 5.16 GJ. As no differences in the wheel-ignition timings were distinguishable in the recorded video, all rear wheels were assumed to ignite at the same time. Therefore, the HRR of the wheels was estimated as the summation by summing those of the eight rear wheels. The HRR increased linearly to its maximum of 3.36 MW at t ¼ 300 s (calculated from the start of wheel ignition) and then remained steady for 1200 s (Fig. 2, gray plot).
Pool fire (No. 1 in Fig. 3) Fig. 2 e HRR of each component in the experimental system.
The pool fire consumed 35 L of gasoline, corresponding to an approximate calorific value of 1.2 GJ. Assuming a convective
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Fig. 3 e Numbers assigned to the different combustion areas. thermal component of 70%, the convective calorific value was approximately 854.7 MJ. In addition, the combustion time of the pool fire was assumed as 6 min. As shown in the HRR curve of pool fire combustion (green plot in Fig. 2), the HRR was proportional to the time elapsed from ignition till 60 s. After reaching its maximum of 2.8 MW at 60 s, the HRR plateaued until 300 s, and burned out at 350s. The HRR of the test case (i.e., the fire of the carrier with four FCVs ignited by the fuel pan) was re-estimated by determining and then summing the HRRs of the following components: 1) a vehicle without fuel, 2) a flaming hydrogen jet from a cylinder, 3) the rear wheels of the carrier, and 4) a 1 m2 pool fire. Fig. 4 shows the combustion parts and the corresponding HRRs of the carried FCVs along with their reference numbers. The curves were obtained by adding the blue and orange plots in Fig. 2. The hydrogen gas was released linearly at 32.5 MPa, and maximum HRR was in 20 s from releasing of the hydrogen gas. The release volume then remained constant before the hydrogen release rate linearly decreased to 0. In Ref. [2] experiment, where the temperature around the gas cylinder safety valve reached 110 C, the FCV was ignited from a pool fire before the hydrogen gas cylinder began ejecting hydrogen gases at t ¼ 250 s. The first FCV fire a (No. 2 in Fig. 3, blue plot in Fig. 4) spread to a second FCV at t ¼ 300 s (No. 3 in Fig. 3, orange plot in Fig. 4) located above the first FCV. Moreover, the fire from the first FCV (No. 2 in Fig. 3) spread to the carrier wheels (No. 6 in Fig. 3) at t ¼ 450 s. The third FCV (No. 5 in Fig. 3, gray plot in Fig. 4) caught fire from the driver seat of the first FCV at t ¼ 450 s. Finally, the fourth FCV (No. 4 in Fig. 3, green plot in Fig. 4) ignited at t ¼ 1200 s. The HRR was proportional to the squared time (t2), and fully
Fig. 4 e HRRs of FCV bodies with a fuel component added at 240 s after ignition of each body.
developed state with maximum HRR (4.2 MW, No.2 in Figs. 3 and 4) began at t ¼ 150 s and continued for 900 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 240 s after the vehicle body ignition. Second, at 300 s, the fully developed state with maximum HRR (No. 3 in Figs. 3 and 4) began at t ¼ 450 s and continued for 1350 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 540 s after the vehicle body ignition. Third, at 450 s, the fully developed state with maximum HRR (No. 5 in Figs. 3 and 4) began at t ¼ 600 s and continued for 1500 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 690 s after the vehicle body ignition. Finally, at 1200 s, the fully developed state with maximum HRR (No. 4 in Figs. 3 and 4) began at t ¼ 1350 s and continued for 2250 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 1440 s after the vehicle body ignition. As previously described, the overall HRR of the test case was determined by summing the HRRs of the different components of the carrier and vehicles. Fig. 5 shows HRR of the combined components, excluding the front wheels and the driver's seat of the carrier, estimated by the present method. The ignition times of these parts (0, 250, 300, 450, 480, and 1200 s for components No. 1, 2, 3, 6, 5, and 4, respectively) were determined in experiments [2]. The HRR simulated by the proposed method, which increased rapidly after ignition (Fig. 5), was input to the simulation and the calculated temperature was compared with the experimental results of [2].
Fig. 5 e HRR of the combined components (excluding the front wheels and the driver's seat of the carrier), estimated by the present method.
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Simulation The simulations adopted the 3D computational fluid dynamics (CFD) code developed by Ref. [15]; which employs large eddy simulations with the standard Smagorinsky model. The governing equations of the heat and air flows in the tunnel were the continuity equation, momentum equation, equation of energy conservation, and equation of state. The Smagorinsky constant was taken as 0.15. The subgrid thermal diffusivity was estimated using the subgrid viscosity and the subgrid Prandtl number (set to 0.7). The pressure differences were approximately 0.1% of the barometric pressure. Therefore, the changes in density corresponded only to the changes in temperature. The combustion reactions were ignored, and the heat release source was represented in the energy and concentration equations; thus, the fire was treated as a heat source. The time progression was estimated using the explicit CrankeNicolson method; therefore, the continuity and momentum equations were solved simultaneously using the SMAC(Simplified Maker And Cell) method. The advection terms in momentum equation and energy equation were discretized with the fourth-order central-difference scheme and the third-order upwind-difference scheme, respectively. In refs. [62,63] it has been reported that the temperature reaction delay time was almost the same during the calculation by fourth-order central-difference scheme and third-order upwind-difference scheme in comparison with experimental results in a 1/10 model-scale tunnel. Upon comparing fourthorder central-difference scheme and third-order upwind-difference scheme, the temperature fluctuation behavior in fourth-order central-difference scheme could be confirmed, but almost the same behavior could be shown. Additionally, owing to diverge easily in the case of fourth-order centraldifference scheme, we adopted third-order upwind-difference scheme in energy equation. Other spatial differentials were subjected to second-order central-difference schemes. The smoke spreading, backlayering, and descent behaviors had been previously confirmed by comparing the numerical and experimental results of the developed model with those in full-scale tunnels[16,17,25e33,35]. Fig. 6 is a schematic of the tunnel used in the simulation. The Cartesian coordinate system was set up as described above. Jet fans installed outside the left portal in the experiments generated a longitudinal wind, so the directional and strength variations in the external (meteorological) wind could be considered negligible. Accordingly, a uniform velocity of 2 m/s and standard atmospheric pressure were applied
at the left and right portals of the external domain (blue and orange oblique lines in Fig. 6), respectively. The tunnel space was divided into uniform cells, while the exterior domain was divided into non-uniform cells to reduce the number of cells and the computational load. The HRR used in the calculation was estimated in Section Estimation of the HRRs of the individual parts. The result of this calculation can be compared with the experimental temperatures obtained by Ref. [2]. The total number of cells was 3.2 105 (181 57 31). In the tunnel space, the cell sizes were dx ¼ 0.5, dy ¼ 0.334, and dz ¼ 0.355 m (giving 160 (x) 37 (the maximum number of cells, y) 21 (the maximum number of cells, z) ¼ 1.0 105 cells in the tunnel). The fire sources of the HRR (Section Heat release rate (HRR)) were located from x ¼ 30e40.5 m, consistent with the experimental locations (see Table 1). The tunnel ceiling and walls were assumed to be constructed from concrete with a density of 2100 kg/m3, specific heat of 879 J/(kg$K), and thermal conductivity of 1.10 W/(m$K) [23]. In the experimental tunnel, in the outside exposed atmosphere, by setting the outside boundary condition as 100 W/(m$K) of the heat transfer efficiency, we modeled that the heat radiated based on the differences between outside atmosphere temperature and tunnel outside wall temperature. Since the wall thickness was assumed as 500 mm and was divided into nine layers to facilitate the heat conduction calculations inside the walls. In the present case, the tunnel wall was sufficiently thick to prevent heat transfer to the external wall. The nozzle of the hydrogen vessel was not modeled, because the automatic nozzle had not been developed at the time of [2] experiments. Instead [2], opened the nozzle when the nearby temperature reached 110 C and measured the opening time. Their reported time was adopted in the present simulation.
Table 1 e Locations of HRR estimations during 1 the simulation (center points only, unit: m).
Pool fire (No. 1) FCV (No. 2) FCV (No. 3) FCV (No. 4) FCV (No. 5) Rear Wheel (No. 6)
X
Y
z
30e30.5 30.5e35 30.5e35 35e39.5 35e39.5 33.5e34.5 39.5e40.5
1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2
0e2.1 1.8e3.6 3.6e5.0 1.8e3.6 3.6e5.0 0e1.0
Fig. 6 e Schematic of the tunnel used in the simulation.
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The simulated HRR estimated by summing the rated HRRs of the individual parts of the carrier and FCVs (orange data in Fig. 7) were compared with the experimental data (black data in Fig. 7) [2]. Panels (a) and (b) of Fig. 7 plot the temperature changes (DT) over time (5-sec intervals) at z ¼ 6 m and z ¼ 3 m, respectively. In both cases, the (x, y) coordinates were (60 m, 0 m). As shown in Fig. 7(a), the post-ignition temperature increased much more rapidly in the simulated cases than during the experimental trial. Moreover, the simulated temperature was maximized at t ¼ 720 s, in qualitative agreement with the experimental results. In Fig. 7(b), the experimental temperature rapidly increases after ignition, whereas the simulated temperature increase is delayed as same as Fig. (a). This rapid increase in the temperature may have been caused by the relatively close location of the measuring points to the fire source (the edge of the fire source was 25 m from the fire point), and the existence of various thermal fumes that hindered the formation of a uniform thermal component similar to that of pool fires. Referring to Ref. [2] reports, we considered only four types of fires; however, we expect that the driver seat of the carrier had also ignited but the flame was not visible, so was not reported. Accordingly, the present study might underestimate the actual HRR.
At z ¼ 3 m, the simulated temperature was lower than the experimental temperature. However, similar to the measurements at z ¼ 6 m, the simulated temperature distribution approximated the experimental distribution. Moreover, the maximum temperature at t ¼ 720 s qualitatively agreed with the experimental results. Panels (a) and (b) of Fig. 8 show the vertical temperature profiles of the two HRR estimation methods [2] at t ¼ 660 s and t ¼ 1000 s, respectively. In both cases, y ¼ 0 m and x ¼ 60 and 70 m. Fig. 8(i) illustrates the vertical temperature profile at x ¼ 60 m in both time instances (a) 660 s and (b) 1000 s. Although the simulated temperature distribution was lower than the experimental distribution, the three methods present similar distribution curves. Fig. 8(ii), which illustrates the vertical temperature profile at x ¼ 70 m in both time instances (a) 660 s and (b) 1000 s, reveals similar temperature distributions from the simulations and experiment between z ¼ 4.5 and 6 m. However, under z ¼ 4.5 m, the experimental results were 10 C higher than the temperatures reached in Fig. 8(a) and (b).
Fig. 7 e Experimental (black) and simulated (orange) temperature rise profiles at x ¼ 60 m and y ¼ 0 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8 e Experimental (black) and simulated (orange) vertical temperature rise profiles at y ¼ 0 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Therefore, the present estimating method accords with the experimental results.
FCV vs. gasoline vehicle fire This section compares the thermal fume behaviors of a FCV and gasoline vehicle fire in a tunnel. As the hydrogen gas tank of a commercial FCV holds approximately 80 m3 of fuel [22], the FCVs were assumed to be filled with 86 m3 hydrogen gas (twice the volume assumed in the simulation discussed in Section Heat release rate (HRR)). Meanwhile, the gasoline vehicles were assumed to be filled with 30 L of gasoline. The model tunnel was 60 m2 in crosssection, over 1 km long, 10 m wide, and 7.5 m high. These dimensions typify tunnel constructions worldwide. However, only a 500-m section was considered in the calculation method. The fire point was located at the center of the tunnel, 250 m from the origin point at the left side. The x, y, and z-axes were the longitudinal, span-wide, and vertical axis, respectively. The grid was sized as described in Section Heat release rate (HRR). Fig. 9 is a schematic of the calculation domain. Consistent with the experiments, the fire sources were located from x ¼ 248.5e252.5 m. To acquire the necessary data, the longitudinal inclination was set to 0%, 2%, and 4%, which are usually seen in the tunnel longitudinal inclinations, because almost all tunnels throughout the world are longitudinally sloped for drainage or to accommodate the geometry limitations. And the wind velocity was 0 m/s. A single FCV or gasoline vehicle fire was assumed to have occurred in a tunnel with no congested vehicles. Only the differences in temperature distribution and location of the thermal fuel arrival between the FCV and gasoline vehicle are analyzed here. The simulated calorific values, ignition times per vehicle, and fire-spreading times were considered to be similar to those of the previous simulation. The HRR plots of the fires from one FCV and one gasoline vehicle are shown in Fig. 10.
FCV filled with 86 m3 hydrogen gas As was done for the FCV filled with 43 m3 hydrogen gas, the maximum HRR of the fuel loaded in the FCV was estimated using the theoretical calorific value of 0.94 GJ. The convective component was considered to be 80% of the theoretical value [24]. The HRR reached its maximum of 10.2 MW at t ¼ 300 s (Fig. 10). We assumed that the fuel tank had been located in the rear. In this scenario, the hydrogen would be released 240 s after the front-part ignition, the time at which the fire had spread to the rear in the experiments [2].
Fig. 10 e Temporal HRR curve of one FCV fire and one gasoline vehicle fire.
Gasoline vehicle The HRR of the vehicle body was calculated similarly to the HRR of the FCV filled with 43 m3 hydrogen gas, but subtracting the gasoline fuel HRR. Hence, the gross calorific value for the complete combustion of a fuel-filled gasoline vehicle was 7 GJ [3]. The convective thermal component of the vehicle body was assumed as 70% of the complete combustion (i.e., 4.9 GJ). Fig. 10 plots the HRR of a gasoline car fire from ignition to the fully developed state. The HRR was initially proportional to the squared time (t2) and plateaued (4.9 MW) at t ¼ 180 s from the beginning of car ignition until t ¼ 1020 s. Tunnel fire evacuation is hindered by several factors: increased air temperature, thermal radiation, toxic gas (generally carbon monoxide, CO), and limited visibility (high extinction coefficient) [19]. The present paper assumes an air temperature of 60 C [19]. For evacuation safety purposes, we thus focused on the thermal fume behavior. Fig. 11 shows the longitudinal distributions of the first arrival times of the thermal fumes in all cases, including fumes from the FCV and gasoline vehicles, at longitudinal inclinations of 0%, 2%, and 4%. The temperature was assumed as 20 C in the normal atmosphere, and 60 C at the thermal fume arrival time (a 40 C rise when the thermal fume arrived at z ¼ 6 m); therefore, no thermal fume backlayering is described in Fig. 11. In the case of 0% longitudinal slope (blue plot in Fig. 11), only the right side of the fire source was considered. At x ¼ 150 m, the thermal fumes of the FCV and gasoline fires arrived at t ¼ 292 s and t ¼ 238 s, respectively. The thermal fumes of the gasoline vehicle fire arrived 60 s earlier than those of the FCV fire, until 180 m. Once the FCV began emitting hydrogen gas at 240 s, its thermal fume traveled faster than that of the gasoline vehicle fire. For example, the thermal
Fig. 9 e Schematic of the calculation domain.
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Fig. 11 e Longitudinal distribution of the thermal fume arrival times (First arrival point of 60 C (i.e., an increase of 40 C from the environmental temperature of 20 C at z ¼ 6 m)).
Fig. 12 e Longitudinal thermal fume behaviors of the FCV and gasoline vehicle fires.
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Fig. 12 e (continued).
fumes of the FVC and gasoline vehicle reached x ¼ 200 m at t ¼ 334 s and t ¼ 397 s, respectively, meaning that the thermal fume arrived 60 s earlier from the FCV fire than from the gasoline vehicle fire. Meanwhile, the thermal fume from the FCV fire reached x ¼ 250 m at t ¼ 374 s, whereas that of the gasoline fire had not reached x ¼ 250 m after 10 min. Hence, the thermal fume propagated more quickly in the FCV fire than in the gasoline vehicle fire. In the case of 2% longitudinal slope (orange plot in Fig. 11), and investigating the right side of the fire source, the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 150 m at t ¼ 282 s and t ¼ 228 s, respectively. The thermal fumes in gasoline and FCV cases in 2% longitudinal inclination arrived 10 s earlier than at 0% longitudinal inclination. However, after the hydrogen gas began emitting at t ¼ 240 s, the thermal fume velocity was faster in the FCV fire than in the gasoline vehicle fire (as observed for no longitudinal slope). The thermal fumes of the FCV and gasoline vehicle fires
reached x ¼ 200 m at t ¼ 305 s and t ¼ 365 s, respectively, meaning that the thermal fume from the FCV fire arrived 60 s earlier than that of the gasoline vehicle fire. Finally, the thermal fumes of the FCV and gasoline fires reached x ¼ 250 m at t ¼ 330 s and t ¼ 491 s, respectively. In the case of 4% longitudinal slope (gray plot in Fig. 11), and again considering the right side of the fire source, the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 150 m at t ¼ 298 s and t ¼ 213 s, respectively. These thermal fume arrival times were similar to those at 2% longitudinal inclination, but once the hydrogen gas began emitting at t ¼ 240 s, the thermal fume traveled faster from the FCV fire than from the gasoline vehicle fire (as observed at no longitudinal inclination). The thermal fumes of the FCV and gasoline vehicles fires reached x ¼ 250 m at t ¼ 318 s and t ¼ 480 s, respectively, meaning that the thermal fume arrived 160 s earlier in the FCV fire than in the gasoline vehicle fire. Investigating the left side of the fire source, the thermal fumes of
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Fig. 12 e (continued).
the FCV and gasoline vehicle fires reached x ¼ 50 m at t ¼ 160 s and t ¼ 146 s, respectively. However, due to the large inclination, the thermal fume did not propagate to the left in either type of fire. Fig. 12 shows the longitudinal and vertical temperature distributions from 1 to 10 min, with contour increments of 40 C. In the case of no longitudinal inclination (0%), the thermal fume propagated symmetrically to the left and right in both the FCV and gasoline vehicle fires. Examining the left side of the fire source at 2% longitudinal inclination (see Fig. 11), the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 100 m at t ¼ 248 and t ¼ 205 s, respectively; that is, the thermal fume arrived 40 s earlier in the gasoline vehicle fire than in the FCV fire. However, once the hydrogen gas began emitting at t ¼ 240 s, the thermal fume traveled faster in the FCV fire than in the gasoline vehicle fire, as observed for no longitudinal inclination.
The thermal fume of the FCV fire reached x ¼ 150 m at t ¼ 360 s, but that of the gasoline fire did not reach this distance. Again, the thermal fume propagated faster in the FCV fire than in the gasoline vehicle fire. The simulation clarified that the thermal fume traveled faster from the FCV than from the gasoline vehicle, even at a small normal slope. This difference should not be ignored. Because the evacuees disperse in the longitudinal direction, they will be found at several hundred meters from the fire source. Especially, emergency evacuation facilities are designed for tunnels over 500 m long in Europe and over 1500 m long in Japan. Therefore, the evacuees must move through several hundred meters, even along short tunnels. If exiting the tunnel is necessary, evacuees could be situated at these distances from the fire source would detect the thermal fume before evacuating, exposing them to injury or death risk. Moreover, the risks are higher for weaker
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evacuees such as elderly persons with slow walking speed. Considering this situation, the infrastructure must be generalized to cope with disasters involving new energy vehicles. In the case of no longitudinal inclination, the thermal fume of the FCV fire arrived 1 min earlier at x ¼ 200 m and over 4 min earlier at x ¼ 250 m than that of the gasoline vehicle fire. This behavior is attributable to the hydrogen gas emission. At 2% longitudinal inclination, the thermal fume propagated downstream over 4 min earlier in the FCV fire than in the gasoline vehicle fire. In the 4% longitudinal inclination case, the thermal fume propagated 50 m downstream at 10 min after the initial fire.
Conclusions The HRR of carriers loaded with four FCVs were estimated experimentally and in numerical simulations of a fullscale model tunnel. The HRRs of all inflammable parts of the system were estimated and plotted as functions of time. The total convective component of the HRR was also estimated by summing the rated HRRs of the individual parts. The main findings of this study are summarized below: The HRR of multiple vehicles fires can be estimated by summing the HRRs of all parts of the system. Therefore, the proposed method can be used to estimate tunnel fire phenomena in the future. By comparing the calculated vertical and longitudinal temperature distributions with those of previous experiments, it was proven that the HRR of a carrier loaded with four vehicles can be estimated by simulating the firespreading times. We clarified that the proposed estimating method is in agreement with the experimental results. The proposed HRR calculation method was applied to the safety assessment of tunnel fires by investigating the thermal fume behavior of an FCV fire and a gasoline vehicle fire in a tunnel. In the case of no longitudinal inclination, the thermal fume of the FVC fire arrived 1 min earlier at x ¼ 200 m and over 4 min earlier at x ¼ 250 m than that of the gasoline vehicle fire, because of the emitted hydrogen gas. At a longitudinal inclination of 2%, the thermal fume of the FCV fire propagated to the downstream side over 4 min faster than that of the gasoline vehicle fire. At a longitudinal inclination of 4%, the downstream thermal fume propagation was 50 m at 10 min after the initial fire. Thus, even a modest inclination enhanced the thermal fume propagation of FVC fires comparing with gasoline vehicle fires.
Acknowledgement We are grateful to Mr. Yasuhito Ejiri (Morita holdings corporation) for his assistance.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.099.
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Available online at www.sciencedirect.com
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Heat release rate and thermal fume behavior estimation of fuel cell vehicles in tunnel fires Miho Seike a,*, Nobuyoshi Kawabata b, Masato Hasegawa c, Hirato Tanaka d a
Faculty of Engineering, Department of Mechanical Systems Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 9390398, Japan b Faculty of Production Systems Engineering and Sciences, Komatsu University, Nu, 1-3, Shicho-machi, Komatsu, Ishikawa, 9230971, Japan c Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 9201164, Japan d Ex-Deputy Director, Japan Institute of Country-ology and Engineering (JICE), Japan
highlights HRR of a carrier loaded with FCVs in a tunnel was estimated. Thermal fume of FCV and gasoline vehicle fires in inclined tunnel was investigated. Inclination caused faster thermal fume propagation of FVC fires was clarified.
article info
abstract
Article history:
This study proposes a heat release rate (HRR) estimation method for a carrier loaded with
Received 19 April 2019
fuel cell vehicles (FCVs) trapped in a tunnel fire. The carrier is divided into several parts,
Received in revised form
and the HRR is estimated by adding the HRRs of all system parts (carrier and FCVs). The
7 August 2019
HRR of one FCV was compared with that of a gasoline vehicle. The thermal fume behavior
Accepted 12 August 2019
in longitudinally inclined tunnel fires was also investigated. Even a modest inclination
Available online 6 September 2019
hastened the thermal fume propagation of the FCV fires. Of relevance to the prevention of tunnel fire disasters, the thermal fume behavior differed between FCV and gasoline fires.
Keywords:
For safety assessment of tunnel fires, the thermal fume behaviors of an FCV fire and
HRR
gasoline vehicle fire in a tunnel were investigated by the proposed method. In the case of
FCVs
no longitudinal inclination, the thermal fume of the FCV fire arrived earlier than that of the
Road tunnel
gasoline vehicle fire (by 1 min at x ¼ 200 m and over 4 min at x ¼ 250 m) because of the
Fire
emitted hydrogen gas. At 2% longitudinal inclination, the thermal fume of the FCV fire
Evacuation environment
propagated to the downstream side 4 min before that of the gasoline vehicle fire. At 4% longitudinal inclination, the thermal fume propagated 50 m downstream of the initial fire after 10 min. Therefore, after the hydrogen emission, the thermal fume of the FCV fire traveled faster than that of the gasoline vehicle fire. The proposed HRR estimation method can contribute to the risk analysis of various types of tunnel fires. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Seike), [email protected] (N. Kawabata), [email protected] (M. Hasegawa), [email protected] (H. Tanaka). https://doi.org/10.1016/j.ijhydene.2019.08.099 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction As part of the effort to replace traditional vehicle consumers of fossil fuels with cleaner alternatives, the commercial distribution of fuel cell vehicles (FCVs) recently began in Japan. Disaster prevention equipment that is compatible with fires caused by FCVs is imperative. Commercially available FCVs are fueled by hydrogen gas, which generates less smoke than gasoline during a fire. However, the hydrogen gas emissions of FCV fires immediately increase the ambient temperature, especially in enclosed spaces such as tunnels. Therefore, if the fire spreads to the hydrogen gas cylinder, the tunnel may sustain considerable damages. FCV risk in tunnels has been investigated in both experiments and simulations [39,41,46,49,54,55,59,60]. Middha and Hansen [46] discussed the temperature nearby (around ±10 m) a fire vehicle with hydrogen emissions. However, a tunnel is a long and large (albeit single and simple) space that becomes dotted with evacuees in the event of fire. Especially, evacuation decisions must be self-determined with respect to the tunnels. In Japan, tunnels are 500e1000 m long, numerous (totaling more than 1500 [64]), and lack emergency facilities. Against this background, we focused on the thermal fume behavior during an FCV fire in tunnels of several hundred meters in length. We considered that the hydrogen vessels are not broken by significant crushing, and that hydrogen gas unintentionally leaks from the vehicle, as assumed elsewhere [36]. Additionally, an experimental study by Ref. [2] assumed that the leaked hydrogen gas immediately ignited and combusted. We thus considered that the unburned hydrogen gas does not fill the tunnel and there is no risk of hydrogen deflagration [37e40,41e46,56,57]. Enozono and Tanaka's [2] study was inspired by the increasing use of fuel cells and new regulations issued by the Japanese government. They conducted fire experiments using a carrier loaded with FCVs in a full-scale tunnel at the Japan Construction Method and Machinery Research Institute. As hydrogen fueling stations are sparsely spaced, FCVs may be transported by trucks bearing full hydrogen fuel tanks. Additionally, restrictions on the transportation of hazardous materials through undersea tunnels and long tunnels should be reconsidered [1]. Although the number of hydrogen fueling stations in Japan has increased since 2005, it cannot meet the current demand. Tunnel spaces are typically long and enclosed, posing challenges to evacuation, rescue, and firefighting activities. Even small incidents in tunnels incur a high possibility of significant losses. Tunnel fires have caused significant mortality and economic loss. Examples are the 1999 Mont Blanc tunnel fire in France and Italy with 39 fatalities [4], the 1999 Tauern tunnel fire in Austria with 12 fatalities [5,6], and the 2001 St. Gotthard tunnel fire in Switzerland with 11 fatalities [7]. Since those incidents, disaster prevention methods have been widely examined by risk analysis techniques to mitigate the risk of tunnel fires. These techniques numerically simulate the heat release rate (HRR) during a fire. Previously, the HRR has been estimated by applying the oxygen consumption principle [8,9] measuring the maximum temperature with thermocouples [17], determining the mass loss rate on an
electronic balance [17], and measuring the radiant flux with a radiometer. However, useful data from these measurement methods must be acquired in full-scale experiments, which are expensive and depend on the environment and neighborhood of the tunnel, rendering HRR measurements difficult. Moreover, the HRR of FCVs in tunnels has not yet been investigated. Given the increasing popularity of FCVs as transportation options, investigating the HRR of FCVs in tunnel fires and the impact of a fire caused by several FCVs is necessitated from a safety perspective. With the aim of improving tunnel fire safety, the present work investigates the HRR of several carrier-loaded FCVs. First, it proposes and validates an HRR estimation method that sums the rated HRRs of the individual parts of the system (carrier and FCVs). Next, the proposed HRR calculation method was applied to the safety assessment of tunnel fires, confirming its suitability to risk analysis of various fire scenarios. The proposed method is then applied to two fire scenarios with different inclinations from the normal (0%, 2%, and 4%, which are usually seen in the tunnel longitudinal inclinations) in a tunnel, and the simulated thermal fume behavior is compared with the experimentally observed behavior. The first fire scenario involves a normal vehicle filled with 30 L gasoline; the second involves an FCV filled with 86 m3 hydrogen gas in a 70 MPa vessel (current FCVs are approximately 80 m3 and 70 MPa, respectively [22]).
Heat release rate (HRR) The HRR of a carrier loaded with four FCVs was estimated by summing the rated HRRs of the individual parts of the carrier and FCVs. Fire experiments using a carrier loaded with FCVs were then conducted in a full-scale tunnel at the Japan Construction Method and Machinery Research Institute [2], where conditions used were obtained from literature. In this experiment [2], jet fans providing air streams at 2 m/s were installed in each tunnel portal. The temperature, extinction coefficient, and air velocity were measured at 10 m from each portal (x ¼ 10 and 70 m), at x ¼ 60 m, and at four different heights (z ¼ 1.5, 3.0, 4.5, and 6.0 m). The HRR was not investigated. When the temperature (measured by a pressure relief device on the gas cylinder unit) reached 110 C, the hydrogen gas was discharged by manually opening the gas cylinder valves from a monitoring room (at that time, the safety valves for this type of cylinder had not been developed). The experimental tunnel was horseshoe-shaped with a length, width and height of 80 m, 12.4 m and 7.46 m, respectively (see Fig. 1). The origin of the Cartesian coordinate system was located at the center of the road surface at the tunnel entrance (left side of Fig. 1). The x-, y-, and z-axes corresponded to the tunnel length, width, and height, respectively. The following experimental scenarios were considered: 1. A small passenger car colliding with a carrier loaded with four FCVs. 2. Burning of a small passenger car. 3. The fire from a small passenger car spreading to an FCV loaded on a carrier.
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Fig. 1 e Schematic of the tunnel fire experiment. The proposed method sums the rated HRRs of the individual burning parts of the carrier and FCVs, including (i) the individual vehicles without fuel (burning duration: 1200 s), (ii) hydrogen gas (burning duration: 180 s), (iii) the rear wheels of the carrier (burning duration: 1800 s), and (iv) the fuel in a 1 m2 pan (burning duration: 360 s). Here the burning duration of each part was determined by the method of [2]; and only the convective components of the HRR were examined. The experimental duration was approximately 30 min, although the final fire (rear wheels) continued to burn thereafter. The results were recorded from t ¼ 0e30 min (1800 s; see Fig. 2). The time of fire ignition to the end was determined by a video camera.
Estimation of the HRRs of the individual parts FCVs (Nos. 2 to 5 in Fig. 3) The gross calorific value of complete combustion of a gasoline vehicle is 7 GJ for a full tank [3], and approximately 1 GJ for a tank filled with 30 L of gasoline (30 L 34.6 MJ/L ¼ 1038 MJ). Hence, the gross calorific value of a vehicle without gasoline is 6 GJ. According to the [24]; the convective thermal component of complete vehicle combustion is between 40% and 80% of the gross calorific value of complete combustion. Additionally, the convective HRR is 67% of the total HRR [17,30]. In this study, the convective thermal component of a vehicle body was assumed as 70% of the complete combustion (i.e., 4.2 GJ). For comparison with the experimental results of [2]; the hydrogen volume (43.6 m3) and gas cylinder pressure (32.5 MPa) in this study were approximately half those of current FCVs (approximately 80 m3 and 70 MPa, respectively [22]). The assumed capacities were the available capacities of
the hydrogen tanks installed at the time of the 2004 experiment. The complete combustion of this volume of highpressure hydrogen yields 0.47 GJ has been reported by Ref. [2]. We used this value and decided the convective component. According to the [24]; the convective thermal component of a high-pressure hydrogen jet flame should reach 80% of the gross calorific value of complete combustion. Therefore, the convective thermal component of hydrogen gas was calculated as 0.4 GJ, and the convective calorific value of one FCV was estimated as 4.6 GJ. The blue plot in Fig. 2 shows the calculated HRR of a car fire from ignition to the end of fire. The HRR was proportional to the squared time (t2), and the fully developed state with maximum HRR (4.2 MW) began at t ¼ 150 s and continued for 900 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 240 s after the vehicle body ignition. The growth rate was adopted as the t2 curve, reported for fast fire growth in the Handbook of Tunnel Fire Safety (2nd edition;[11]). As shown in Fig. 2 (orange plot), the HRR of hydrogen was proportional to the time elapsed from the start of the ignition to the fully developed state (the maximum HRR of 3 MW was reached at t ¼ 20 s), and the fully developed state continued for 80 s. The total calorific values of the car and fuel over the entire experiment were approximately 4.6 and 0.5 GJ, respectively.
Rear wheels of the carrier (No. 6 in Fig. 3) The experimental carrier had 10 wheels in total: eight in the rear and two in the front. During the experiment, the fire occurred at the fuel pan located behind the carrier and gradually spread to the front. Therefore, the rear wheels began burning easily, whereas the front wheels and the driver's seat did not burn. Assuming that a single wheel weighs 28 kg, its theoretical calorific value is 32.9 MJ/kg, yielding approximately 0.92 GJ/wheel [3]; thus, the approximated total theoretical calorific value of the eight rear wheels was 7.36 GJ. Ref. [17,30] reported that HRR is 67% convective; accordingly, the convective thermal component was assumed as 70% and the convective calorific value was calculated as 5.16 GJ. As no differences in the wheel-ignition timings were distinguishable in the recorded video, all rear wheels were assumed to ignite at the same time. Therefore, the HRR of the wheels was estimated as the summation by summing those of the eight rear wheels. The HRR increased linearly to its maximum of 3.36 MW at t ¼ 300 s (calculated from the start of wheel ignition) and then remained steady for 1200 s (Fig. 2, gray plot).
Pool fire (No. 1 in Fig. 3) Fig. 2 e HRR of each component in the experimental system.
The pool fire consumed 35 L of gasoline, corresponding to an approximate calorific value of 1.2 GJ. Assuming a convective
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Fig. 3 e Numbers assigned to the different combustion areas. thermal component of 70%, the convective calorific value was approximately 854.7 MJ. In addition, the combustion time of the pool fire was assumed as 6 min. As shown in the HRR curve of pool fire combustion (green plot in Fig. 2), the HRR was proportional to the time elapsed from ignition till 60 s. After reaching its maximum of 2.8 MW at 60 s, the HRR plateaued until 300 s, and burned out at 350s. The HRR of the test case (i.e., the fire of the carrier with four FCVs ignited by the fuel pan) was re-estimated by determining and then summing the HRRs of the following components: 1) a vehicle without fuel, 2) a flaming hydrogen jet from a cylinder, 3) the rear wheels of the carrier, and 4) a 1 m2 pool fire. Fig. 4 shows the combustion parts and the corresponding HRRs of the carried FCVs along with their reference numbers. The curves were obtained by adding the blue and orange plots in Fig. 2. The hydrogen gas was released linearly at 32.5 MPa, and maximum HRR was in 20 s from releasing of the hydrogen gas. The release volume then remained constant before the hydrogen release rate linearly decreased to 0. In Ref. [2] experiment, where the temperature around the gas cylinder safety valve reached 110 C, the FCV was ignited from a pool fire before the hydrogen gas cylinder began ejecting hydrogen gases at t ¼ 250 s. The first FCV fire a (No. 2 in Fig. 3, blue plot in Fig. 4) spread to a second FCV at t ¼ 300 s (No. 3 in Fig. 3, orange plot in Fig. 4) located above the first FCV. Moreover, the fire from the first FCV (No. 2 in Fig. 3) spread to the carrier wheels (No. 6 in Fig. 3) at t ¼ 450 s. The third FCV (No. 5 in Fig. 3, gray plot in Fig. 4) caught fire from the driver seat of the first FCV at t ¼ 450 s. Finally, the fourth FCV (No. 4 in Fig. 3, green plot in Fig. 4) ignited at t ¼ 1200 s. The HRR was proportional to the squared time (t2), and fully
Fig. 4 e HRRs of FCV bodies with a fuel component added at 240 s after ignition of each body.
developed state with maximum HRR (4.2 MW, No.2 in Figs. 3 and 4) began at t ¼ 150 s and continued for 900 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 240 s after the vehicle body ignition. Second, at 300 s, the fully developed state with maximum HRR (No. 3 in Figs. 3 and 4) began at t ¼ 450 s and continued for 1350 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 540 s after the vehicle body ignition. Third, at 450 s, the fully developed state with maximum HRR (No. 5 in Figs. 3 and 4) began at t ¼ 600 s and continued for 1500 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 690 s after the vehicle body ignition. Finally, at 1200 s, the fully developed state with maximum HRR (No. 4 in Figs. 3 and 4) began at t ¼ 1350 s and continued for 2250 s. Then, the hydrogen leaking from the tanks of the vehicles was assumed to ignite 1440 s after the vehicle body ignition. As previously described, the overall HRR of the test case was determined by summing the HRRs of the different components of the carrier and vehicles. Fig. 5 shows HRR of the combined components, excluding the front wheels and the driver's seat of the carrier, estimated by the present method. The ignition times of these parts (0, 250, 300, 450, 480, and 1200 s for components No. 1, 2, 3, 6, 5, and 4, respectively) were determined in experiments [2]. The HRR simulated by the proposed method, which increased rapidly after ignition (Fig. 5), was input to the simulation and the calculated temperature was compared with the experimental results of [2].
Fig. 5 e HRR of the combined components (excluding the front wheels and the driver's seat of the carrier), estimated by the present method.
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Simulation The simulations adopted the 3D computational fluid dynamics (CFD) code developed by Ref. [15]; which employs large eddy simulations with the standard Smagorinsky model. The governing equations of the heat and air flows in the tunnel were the continuity equation, momentum equation, equation of energy conservation, and equation of state. The Smagorinsky constant was taken as 0.15. The subgrid thermal diffusivity was estimated using the subgrid viscosity and the subgrid Prandtl number (set to 0.7). The pressure differences were approximately 0.1% of the barometric pressure. Therefore, the changes in density corresponded only to the changes in temperature. The combustion reactions were ignored, and the heat release source was represented in the energy and concentration equations; thus, the fire was treated as a heat source. The time progression was estimated using the explicit CrankeNicolson method; therefore, the continuity and momentum equations were solved simultaneously using the SMAC(Simplified Maker And Cell) method. The advection terms in momentum equation and energy equation were discretized with the fourth-order central-difference scheme and the third-order upwind-difference scheme, respectively. In refs. [62,63] it has been reported that the temperature reaction delay time was almost the same during the calculation by fourth-order central-difference scheme and third-order upwind-difference scheme in comparison with experimental results in a 1/10 model-scale tunnel. Upon comparing fourthorder central-difference scheme and third-order upwind-difference scheme, the temperature fluctuation behavior in fourth-order central-difference scheme could be confirmed, but almost the same behavior could be shown. Additionally, owing to diverge easily in the case of fourth-order centraldifference scheme, we adopted third-order upwind-difference scheme in energy equation. Other spatial differentials were subjected to second-order central-difference schemes. The smoke spreading, backlayering, and descent behaviors had been previously confirmed by comparing the numerical and experimental results of the developed model with those in full-scale tunnels[16,17,25e33,35]. Fig. 6 is a schematic of the tunnel used in the simulation. The Cartesian coordinate system was set up as described above. Jet fans installed outside the left portal in the experiments generated a longitudinal wind, so the directional and strength variations in the external (meteorological) wind could be considered negligible. Accordingly, a uniform velocity of 2 m/s and standard atmospheric pressure were applied
at the left and right portals of the external domain (blue and orange oblique lines in Fig. 6), respectively. The tunnel space was divided into uniform cells, while the exterior domain was divided into non-uniform cells to reduce the number of cells and the computational load. The HRR used in the calculation was estimated in Section Estimation of the HRRs of the individual parts. The result of this calculation can be compared with the experimental temperatures obtained by Ref. [2]. The total number of cells was 3.2 105 (181 57 31). In the tunnel space, the cell sizes were dx ¼ 0.5, dy ¼ 0.334, and dz ¼ 0.355 m (giving 160 (x) 37 (the maximum number of cells, y) 21 (the maximum number of cells, z) ¼ 1.0 105 cells in the tunnel). The fire sources of the HRR (Section Heat release rate (HRR)) were located from x ¼ 30e40.5 m, consistent with the experimental locations (see Table 1). The tunnel ceiling and walls were assumed to be constructed from concrete with a density of 2100 kg/m3, specific heat of 879 J/(kg$K), and thermal conductivity of 1.10 W/(m$K) [23]. In the experimental tunnel, in the outside exposed atmosphere, by setting the outside boundary condition as 100 W/(m$K) of the heat transfer efficiency, we modeled that the heat radiated based on the differences between outside atmosphere temperature and tunnel outside wall temperature. Since the wall thickness was assumed as 500 mm and was divided into nine layers to facilitate the heat conduction calculations inside the walls. In the present case, the tunnel wall was sufficiently thick to prevent heat transfer to the external wall. The nozzle of the hydrogen vessel was not modeled, because the automatic nozzle had not been developed at the time of [2] experiments. Instead [2], opened the nozzle when the nearby temperature reached 110 C and measured the opening time. Their reported time was adopted in the present simulation.
Table 1 e Locations of HRR estimations during 1 the simulation (center points only, unit: m).
Pool fire (No. 1) FCV (No. 2) FCV (No. 3) FCV (No. 4) FCV (No. 5) Rear Wheel (No. 6)
X
Y
z
30e30.5 30.5e35 30.5e35 35e39.5 35e39.5 33.5e34.5 39.5e40.5
1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2 1.8e3.2
0e2.1 1.8e3.6 3.6e5.0 1.8e3.6 3.6e5.0 0e1.0
Fig. 6 e Schematic of the tunnel used in the simulation.
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The simulated HRR estimated by summing the rated HRRs of the individual parts of the carrier and FCVs (orange data in Fig. 7) were compared with the experimental data (black data in Fig. 7) [2]. Panels (a) and (b) of Fig. 7 plot the temperature changes (DT) over time (5-sec intervals) at z ¼ 6 m and z ¼ 3 m, respectively. In both cases, the (x, y) coordinates were (60 m, 0 m). As shown in Fig. 7(a), the post-ignition temperature increased much more rapidly in the simulated cases than during the experimental trial. Moreover, the simulated temperature was maximized at t ¼ 720 s, in qualitative agreement with the experimental results. In Fig. 7(b), the experimental temperature rapidly increases after ignition, whereas the simulated temperature increase is delayed as same as Fig. (a). This rapid increase in the temperature may have been caused by the relatively close location of the measuring points to the fire source (the edge of the fire source was 25 m from the fire point), and the existence of various thermal fumes that hindered the formation of a uniform thermal component similar to that of pool fires. Referring to Ref. [2] reports, we considered only four types of fires; however, we expect that the driver seat of the carrier had also ignited but the flame was not visible, so was not reported. Accordingly, the present study might underestimate the actual HRR.
At z ¼ 3 m, the simulated temperature was lower than the experimental temperature. However, similar to the measurements at z ¼ 6 m, the simulated temperature distribution approximated the experimental distribution. Moreover, the maximum temperature at t ¼ 720 s qualitatively agreed with the experimental results. Panels (a) and (b) of Fig. 8 show the vertical temperature profiles of the two HRR estimation methods [2] at t ¼ 660 s and t ¼ 1000 s, respectively. In both cases, y ¼ 0 m and x ¼ 60 and 70 m. Fig. 8(i) illustrates the vertical temperature profile at x ¼ 60 m in both time instances (a) 660 s and (b) 1000 s. Although the simulated temperature distribution was lower than the experimental distribution, the three methods present similar distribution curves. Fig. 8(ii), which illustrates the vertical temperature profile at x ¼ 70 m in both time instances (a) 660 s and (b) 1000 s, reveals similar temperature distributions from the simulations and experiment between z ¼ 4.5 and 6 m. However, under z ¼ 4.5 m, the experimental results were 10 C higher than the temperatures reached in Fig. 8(a) and (b).
Fig. 7 e Experimental (black) and simulated (orange) temperature rise profiles at x ¼ 60 m and y ¼ 0 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8 e Experimental (black) and simulated (orange) vertical temperature rise profiles at y ¼ 0 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Therefore, the present estimating method accords with the experimental results.
FCV vs. gasoline vehicle fire This section compares the thermal fume behaviors of a FCV and gasoline vehicle fire in a tunnel. As the hydrogen gas tank of a commercial FCV holds approximately 80 m3 of fuel [22], the FCVs were assumed to be filled with 86 m3 hydrogen gas (twice the volume assumed in the simulation discussed in Section Heat release rate (HRR)). Meanwhile, the gasoline vehicles were assumed to be filled with 30 L of gasoline. The model tunnel was 60 m2 in crosssection, over 1 km long, 10 m wide, and 7.5 m high. These dimensions typify tunnel constructions worldwide. However, only a 500-m section was considered in the calculation method. The fire point was located at the center of the tunnel, 250 m from the origin point at the left side. The x, y, and z-axes were the longitudinal, span-wide, and vertical axis, respectively. The grid was sized as described in Section Heat release rate (HRR). Fig. 9 is a schematic of the calculation domain. Consistent with the experiments, the fire sources were located from x ¼ 248.5e252.5 m. To acquire the necessary data, the longitudinal inclination was set to 0%, 2%, and 4%, which are usually seen in the tunnel longitudinal inclinations, because almost all tunnels throughout the world are longitudinally sloped for drainage or to accommodate the geometry limitations. And the wind velocity was 0 m/s. A single FCV or gasoline vehicle fire was assumed to have occurred in a tunnel with no congested vehicles. Only the differences in temperature distribution and location of the thermal fuel arrival between the FCV and gasoline vehicle are analyzed here. The simulated calorific values, ignition times per vehicle, and fire-spreading times were considered to be similar to those of the previous simulation. The HRR plots of the fires from one FCV and one gasoline vehicle are shown in Fig. 10.
FCV filled with 86 m3 hydrogen gas As was done for the FCV filled with 43 m3 hydrogen gas, the maximum HRR of the fuel loaded in the FCV was estimated using the theoretical calorific value of 0.94 GJ. The convective component was considered to be 80% of the theoretical value [24]. The HRR reached its maximum of 10.2 MW at t ¼ 300 s (Fig. 10). We assumed that the fuel tank had been located in the rear. In this scenario, the hydrogen would be released 240 s after the front-part ignition, the time at which the fire had spread to the rear in the experiments [2].
Fig. 10 e Temporal HRR curve of one FCV fire and one gasoline vehicle fire.
Gasoline vehicle The HRR of the vehicle body was calculated similarly to the HRR of the FCV filled with 43 m3 hydrogen gas, but subtracting the gasoline fuel HRR. Hence, the gross calorific value for the complete combustion of a fuel-filled gasoline vehicle was 7 GJ [3]. The convective thermal component of the vehicle body was assumed as 70% of the complete combustion (i.e., 4.9 GJ). Fig. 10 plots the HRR of a gasoline car fire from ignition to the fully developed state. The HRR was initially proportional to the squared time (t2) and plateaued (4.9 MW) at t ¼ 180 s from the beginning of car ignition until t ¼ 1020 s. Tunnel fire evacuation is hindered by several factors: increased air temperature, thermal radiation, toxic gas (generally carbon monoxide, CO), and limited visibility (high extinction coefficient) [19]. The present paper assumes an air temperature of 60 C [19]. For evacuation safety purposes, we thus focused on the thermal fume behavior. Fig. 11 shows the longitudinal distributions of the first arrival times of the thermal fumes in all cases, including fumes from the FCV and gasoline vehicles, at longitudinal inclinations of 0%, 2%, and 4%. The temperature was assumed as 20 C in the normal atmosphere, and 60 C at the thermal fume arrival time (a 40 C rise when the thermal fume arrived at z ¼ 6 m); therefore, no thermal fume backlayering is described in Fig. 11. In the case of 0% longitudinal slope (blue plot in Fig. 11), only the right side of the fire source was considered. At x ¼ 150 m, the thermal fumes of the FCV and gasoline fires arrived at t ¼ 292 s and t ¼ 238 s, respectively. The thermal fumes of the gasoline vehicle fire arrived 60 s earlier than those of the FCV fire, until 180 m. Once the FCV began emitting hydrogen gas at 240 s, its thermal fume traveled faster than that of the gasoline vehicle fire. For example, the thermal
Fig. 9 e Schematic of the calculation domain.
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Fig. 11 e Longitudinal distribution of the thermal fume arrival times (First arrival point of 60 C (i.e., an increase of 40 C from the environmental temperature of 20 C at z ¼ 6 m)).
Fig. 12 e Longitudinal thermal fume behaviors of the FCV and gasoline vehicle fires.
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Fig. 12 e (continued).
fumes of the FVC and gasoline vehicle reached x ¼ 200 m at t ¼ 334 s and t ¼ 397 s, respectively, meaning that the thermal fume arrived 60 s earlier from the FCV fire than from the gasoline vehicle fire. Meanwhile, the thermal fume from the FCV fire reached x ¼ 250 m at t ¼ 374 s, whereas that of the gasoline fire had not reached x ¼ 250 m after 10 min. Hence, the thermal fume propagated more quickly in the FCV fire than in the gasoline vehicle fire. In the case of 2% longitudinal slope (orange plot in Fig. 11), and investigating the right side of the fire source, the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 150 m at t ¼ 282 s and t ¼ 228 s, respectively. The thermal fumes in gasoline and FCV cases in 2% longitudinal inclination arrived 10 s earlier than at 0% longitudinal inclination. However, after the hydrogen gas began emitting at t ¼ 240 s, the thermal fume velocity was faster in the FCV fire than in the gasoline vehicle fire (as observed for no longitudinal slope). The thermal fumes of the FCV and gasoline vehicle fires
reached x ¼ 200 m at t ¼ 305 s and t ¼ 365 s, respectively, meaning that the thermal fume from the FCV fire arrived 60 s earlier than that of the gasoline vehicle fire. Finally, the thermal fumes of the FCV and gasoline fires reached x ¼ 250 m at t ¼ 330 s and t ¼ 491 s, respectively. In the case of 4% longitudinal slope (gray plot in Fig. 11), and again considering the right side of the fire source, the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 150 m at t ¼ 298 s and t ¼ 213 s, respectively. These thermal fume arrival times were similar to those at 2% longitudinal inclination, but once the hydrogen gas began emitting at t ¼ 240 s, the thermal fume traveled faster from the FCV fire than from the gasoline vehicle fire (as observed at no longitudinal inclination). The thermal fumes of the FCV and gasoline vehicles fires reached x ¼ 250 m at t ¼ 318 s and t ¼ 480 s, respectively, meaning that the thermal fume arrived 160 s earlier in the FCV fire than in the gasoline vehicle fire. Investigating the left side of the fire source, the thermal fumes of
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Fig. 12 e (continued).
the FCV and gasoline vehicle fires reached x ¼ 50 m at t ¼ 160 s and t ¼ 146 s, respectively. However, due to the large inclination, the thermal fume did not propagate to the left in either type of fire. Fig. 12 shows the longitudinal and vertical temperature distributions from 1 to 10 min, with contour increments of 40 C. In the case of no longitudinal inclination (0%), the thermal fume propagated symmetrically to the left and right in both the FCV and gasoline vehicle fires. Examining the left side of the fire source at 2% longitudinal inclination (see Fig. 11), the thermal fumes of the FCV and gasoline vehicle fires reached x ¼ 100 m at t ¼ 248 and t ¼ 205 s, respectively; that is, the thermal fume arrived 40 s earlier in the gasoline vehicle fire than in the FCV fire. However, once the hydrogen gas began emitting at t ¼ 240 s, the thermal fume traveled faster in the FCV fire than in the gasoline vehicle fire, as observed for no longitudinal inclination.
The thermal fume of the FCV fire reached x ¼ 150 m at t ¼ 360 s, but that of the gasoline fire did not reach this distance. Again, the thermal fume propagated faster in the FCV fire than in the gasoline vehicle fire. The simulation clarified that the thermal fume traveled faster from the FCV than from the gasoline vehicle, even at a small normal slope. This difference should not be ignored. Because the evacuees disperse in the longitudinal direction, they will be found at several hundred meters from the fire source. Especially, emergency evacuation facilities are designed for tunnels over 500 m long in Europe and over 1500 m long in Japan. Therefore, the evacuees must move through several hundred meters, even along short tunnels. If exiting the tunnel is necessary, evacuees could be situated at these distances from the fire source would detect the thermal fume before evacuating, exposing them to injury or death risk. Moreover, the risks are higher for weaker
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evacuees such as elderly persons with slow walking speed. Considering this situation, the infrastructure must be generalized to cope with disasters involving new energy vehicles. In the case of no longitudinal inclination, the thermal fume of the FCV fire arrived 1 min earlier at x ¼ 200 m and over 4 min earlier at x ¼ 250 m than that of the gasoline vehicle fire. This behavior is attributable to the hydrogen gas emission. At 2% longitudinal inclination, the thermal fume propagated downstream over 4 min earlier in the FCV fire than in the gasoline vehicle fire. In the 4% longitudinal inclination case, the thermal fume propagated 50 m downstream at 10 min after the initial fire.
Conclusions The HRR of carriers loaded with four FCVs were estimated experimentally and in numerical simulations of a fullscale model tunnel. The HRRs of all inflammable parts of the system were estimated and plotted as functions of time. The total convective component of the HRR was also estimated by summing the rated HRRs of the individual parts. The main findings of this study are summarized below: The HRR of multiple vehicles fires can be estimated by summing the HRRs of all parts of the system. Therefore, the proposed method can be used to estimate tunnel fire phenomena in the future. By comparing the calculated vertical and longitudinal temperature distributions with those of previous experiments, it was proven that the HRR of a carrier loaded with four vehicles can be estimated by simulating the firespreading times. We clarified that the proposed estimating method is in agreement with the experimental results. The proposed HRR calculation method was applied to the safety assessment of tunnel fires by investigating the thermal fume behavior of an FCV fire and a gasoline vehicle fire in a tunnel. In the case of no longitudinal inclination, the thermal fume of the FVC fire arrived 1 min earlier at x ¼ 200 m and over 4 min earlier at x ¼ 250 m than that of the gasoline vehicle fire, because of the emitted hydrogen gas. At a longitudinal inclination of 2%, the thermal fume of the FCV fire propagated to the downstream side over 4 min faster than that of the gasoline vehicle fire. At a longitudinal inclination of 4%, the downstream thermal fume propagation was 50 m at 10 min after the initial fire. Thus, even a modest inclination enhanced the thermal fume propagation of FVC fires comparing with gasoline vehicle fires.
Acknowledgement We are grateful to Mr. Yasuhito Ejiri (Morita holdings corporation) for his assistance.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.099.
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