Energy & Fuels 1998, 12, 83-89
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Comparison of Hydrogen, Natural Gas, Liquified Petroleum Gas, and Gasoline Leakage in a Residential Garage Michael R. Swain* and Jeremy Shriber Department of Mechanical Engineering, University of Miami, P.O. Box 248294, Coral Gables, Florida 33124
Matthew N. Swain Analytical Technologies, Inc., 14057 SW 140th St., Miami, Florida 33186 Received July 7, 1997. Revised Manuscript Received September 17, 1997X
This paper compares the relative safety risks of four types of vehicle fuels. The fuels considered are hydrogen, natural gas, liquified petroleum gas (LPG), and gasoline. The accident scenario considered is a vehicle parked in a single-car garage for 2 h with a fuel-line leak. The comparison is made on the basis of the volume of combustible gas produced by each fuel. Only LPG and gasoline produced appreciable volumes of combustible gas.
Introduction The purpose of the following work was to predict fuel gas cloud motion, in a single-car garage subject to vehicle fuel-line leakage, using four different fuels. The results depicted the possible accident scenario of a vehicle parked in a closed single-car garage for 2 h while leaking fuel from a fuel line. Two hours was enough time for the leakage of hydrogen to reach a relatively steady-state condition. The four fuels of interest were hydrogen, natural gas, liquified petroleum gas (LPG), and gasoline. The work was accomplished in two stages. The first stage was computer model verification using experimental data gathered by GEOMET Technologies Inc., Germantown, Maryland.1 The second stage was prediction of fuel gas cloud motion inside a single-car garage for the four different fuels, hydrogen, natural gas, LPG, and gasoline, using the computer model. The computer model prediction was used to determine the volume of combustible gas formed after 2 h of leakage. The volume of combustible gas was used as a measure of risk. Description of GEOMET Experiments The primary objective of the GEOMET experiments was to gather data that could be used to determine the necessary vent size to keep hydrogen concentrations below 2% in a residential garage during the charging cycle of an electric vehicle. To that end, GEOMET designed and constructed a test garage. The GEOMET garage was designed to be used to determine the air exchange rate produced by a single vent in one wall. The GEOMET garage was more tightly sealed than a typical Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Hydrogen Emissions from EV Batteries Undergoing Charging in Residential Garages. GEOMET Final Report IE-2647; GEOMET Technologies, Inc.: Germantown, MD, October 22, 1993 (prepared for Electric Power Research Institute, Palo Alto, CA). X
residential garage. The GEOMET garage was sealed well enough to produce only 0.02 air changes per hour (ACH) air exchange rate with the single vent closed. The GEOMET research effort considered garage volume, hydrogen emission rate of the batteries during the charging cycle, and minimum expected air exchange rate (2.92 ACH). This was less than the expected air exchange rate specified by American Society of Heating Refrigerating and Air conditioning Engineers (ASHRAE) for a typical residential garage. The ASHRAE standard ANSI/ASHRAE 62-1989 including ANSI/ASHRAE addendum 62a-1990 “Ventilation for Acceptable Indoor Air Quality” 2 under the heading “Outdoor Air Requirement for Ventilation of Residential Facilities” for residential garages requires 100 cfm per car. The specification table includes the statement “normally satisfied by infiltration or natural ventilation”. Entries for other structures specify “installed mechanical exhaust capacity”, indicating that forced ventilation is necessary in those cases. ASHRAE expects that the 100 cfm per car (3.73 ACH for the GEOMET garage) can normally be supplied by infiltration or natural ventilation in an average garage. GEOMET measured and reported the airborne concentrations of hydrogen in the test garage as a function of hydrogen leakage rate, garage volume, area of ventilation between the garage and the outdoors, and the overall air exchange rate with the outdoors. The data are recorded for 2 h of leakage and is available in the GEOMET final report.1 The GEOMET data used for model verification were garage size (2.52 m × 6.59 m × 2.74 m), hydrogen leakage rate (1000 L/h), and air exchange rate (either 0.18 or 2.92 ACH). The hydrogen leakage rate was chosen to match the data recorded by GEOMET. The experimental setup for the GEOMET tests included a 4 × 8 sheet of plywood used to simulate the bottom of a vehicle. The plywood was parallel to and 40 cm above the floor. The hydrogen was released under the plywood, simulating a hydrogen leak trapping gas under a vehicle. The principal path of ventilation was through a single vent (2) Ventilation for Acceptable Indoor Air Quality. ANSI/ASHRAE 62-1989 (including ANSI/ASHRAE addendum 62a-1990).
S0887-0624(97)00111-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
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installed in the center of the garage door. The GEOMET data described hydrogen concentration versus time at six locations in the garage. The GEOMET garage geometry was reproduced in the computer model, and the hydrogen concentration versus time calculated by the model was compared to the measured values from the GEOMET experiments to verify the model.
Description of Model FLUENT,3 a computational fluid dynamics software package that models fluid and heat-transfer problems with a finite difference scheme, was used to solve the continuity, momentum, and concentration equations with the appropriate boundary conditions for this work. A transient analysis for mass diffusion and transport with buoyancy was performed on hydrogen, methane, propane, and gasoline cloud formation. The governing equations employed by FLUENT are as follows:
continuity equation: ∂F + ∇FV ) 0 ∂t
(1)
momentum equations:
( ( (
) ) )
D(Fu) ∂p ∂2u ∂2u ∂2u ) + µ 2 + 2 + 2 + Fx Dt ∂x ∂x ∂y ∂z
(2)
D(Fv) ∂p ∂2v ∂2v ∂2v ) + µ 2 + 2 + 2 + Fy Dt ∂y ∂x ∂y ∂z
(3)
D(Fw) ∂p ∂2w ∂2w ∂2w ) + µ 2 + 2 + 2 + Fz Dt ∂z ∂x ∂y ∂z
(4)
diffusion equation: Dc ) ∇‚D∇c + c′′′ Dt
(5)
where F is the density of the fluid, u, v, and w are velocities in the x, y, and z directions, respectively, p is the pressure, Fx, Fy, and Fz are body forces in the x, y, and z directions, respectively, D is the diffusion coefficient of the species, c is the concentration of the species, t is time, and c′′′ is the local rate of production of species c per unit volume. Since there is no production of species in the domain, c′′′ is zero. Fx and Fz are zero. Body force due to gravity is acting in the y direction only. The density distribution in the flow field is provided by the equation of state:
p
F)
mj
∑ RT j M
(6)
j
where Mj is the molecular weight of species j, mj is its mass fraction, and R is the gas constant. Model Verification The GEOMET experiment included a single vent location. Forced ventilation was not used; however, (3) FLUENT is a trade name for Computational Fluid Dynamics Software Creare.X Inc., Hanover, New Hampshire.
some natural ventilation (air exchange between the garage and the outdoor air) existed owing to external air movement (wind). GEOMET used a tracer gas to quantify the natural ventilation in terms of air changes per hour (ACH). CO2 was used as the tracer gas. The tracer gas concentration was measured with a nondispersive infrared CO2 analyzer. The ACH values were converted to cfm, using the volume of the test garage, to generate input variables for the model. To simulate the natural ventilation in the first-order model, an air inlet was included on the wall opposite the wall with the outlet vent. The flow in the air inlet was set to match the cfm computed from the GEOMET ACH value. This produced good results for the low (0.18 ACH) ventilation rate experiment (within 0.3% for the maximum value of concentration reached) but did not work well for the high (2.92 ACH) ventilation rate (maximum concentration level was computed 33% low). This indicated that the flow patterns produced by the inlet vent (and therefore controlled by placement of the inlet vent) were not a significant effect in the low ACH rate case but were important at high ACH ventilation rates. The GEOMET data recorded the total air exchange rate but did not record the flow rate in or out of the vent as a function of time. A percentage 99.3% of the gases that entered or exited the garage (not including the gas added at the leak under the plywood) passed through the single vent in the garage door. To address the fundamental problem of how to model air exchange between the garage and the outside due to a single vent at the higher ACH rates, the model was run with air inlets at various locations closer to the outlet vent to reduce the efficiency of the ventilation in the model. This was consistent with earlier work done at the University of Miami indicating that the location of ventilation orifices affects the efficiency of hydrogen removal from a structure. Two runs with symmetric vents on the walls adjoining the inlet wall were modeled, together with two runs with symmetric vents surrounding the inlet. Those runs produced results that deviated from the experimental data enough to be cut short before they finished. A run was made with the outlet area matching the inlet area and placed side by side at the GEOMET vent location. This run was also cut short. Observation of gas movement in the garage model indicated that large-scale eddies were generated even when the air inlet directly adjoined the outlet. The air entered the garage and dropped, owing to lack of buoyancy, then flowed along the floor and across the room. The fresh air entering the garage was of a higher density than the hydrogen-air mixture in the garage. The inlet air temperature was the same as the temperature of the gases in the garage. This generated large-scale circulation and reduced the concentration gradients in the room by mixing the hydrogen and air. Three runs were made with an inlet surrounding the outlet but with portions of the inlet skewed to flow in directions other than perpendicular to the wall to reduce large-scale circulations. The best of those runs yielded results 12% high at the leanest sensor location and 13% low at the richest sensor location. Discussions with GEOMET indicated that the experimental apparatus had some air leakage at the seams of the experimental garage model (0.7% of the total 2.92
Comparison of Vehicle Fuels
ACH). Unfortunately, the locations at which the 0.02 ACH was occurring were not identified by GEOMET. To estimate where the air leakage may have been occurring, air inlets, and outlets, at various locations along the seams were also run in the model. Two runs were made with an additional inlet around the edge of the garage door. The total inlet flow was held at 2.92 ACH. The best of those runs yielded results 1.6% low at the leanest sensor location and 30% low at the richest sensor location. Four runs were made with additional inlets along various edges of the garage. The best of those runs yielded results 39% high at the leanest sensor location and 3.4% high at the richest sensor location. No additional attempts were made to estimate the locations of the air leakage. The model was also run with a pulsing air inlet flow to determine if the large-scale circulations were partially due to the steady nature of the inlet air flow assumed in the model. The inlet air flow in the GEOMET experiments was driven by wind and therefore unsteady in nature. An air inlet with a harmonically varying flow rate was used to provide ventilation and minimize the generation of large-scale circulation. The intent was to see if this would more closely simulate the random nature of the wind-driven ventilation in the GEOMET experimental data. A run was made with no outlet and the inlet specified as a harmonically varying pressure boundary. This produced a periodic flow in and out of the inlet location. Fresh air entered during the in flow portion of the harmonic cycle, and the mixture exited during the out flow portion. The model would not converge probably because the inlet had no significant friction and changes in pressure too easily produced large flow rates. Four runs were made with periodic flow inlets placed at various locations including a single inlet surrounding the outlet. The best of those runs yielded results 16% low at the leanest sensor location and 28% low at the richest sensor location. Large-scale circulation still existed, and in fact, periodic fluctuations in the circulation flows were sometimes generated at a frequency (lower) different from the harmonic driving the air inlet flow rate. It was decided that the time required to reconstruct the experimental air flow rate fluctuation schedule, together with the proper values for the other variables in the model, was too large to warrant the small improvements that may have resulted. The best results were obtained when the inlet and outlet for air (needed by the model) were placed close together. The best fit was with an air inlet surrounded by the outlet and showed a hydrogen concentration 12% high at the leanest sensor location and 7% low at the richest sensor location. The inlet and outlet were in the same location as the vent in the GEOMET experiment. The model showed somewhat smaller concentration gradients between the various sensor locations than did the experimental data. However, the average concentration in the garage model matched the experimental data very well. Therefore, the model was used in this form to conduct the final modeling. Choice of Leakage Rates The hydrogen leakage rate chosen for this work was 1000 L/h. This value was chosen to be consistent with the GEOMET experimental data.
Energy & Fuels, Vol. 12, No. 1, 1998 85 Table 1
hydrogen methane propane
category 1 leak (L/h)
category 2 leak (L/h)
1000 300 118
1000 680 602
The wide range of leak geometries in the real world was broken into two categories. Category 1 was simple puncture type leaks that tend to be roughly circular in cross section. Category 1 leaks have a relatively small area with high gas velocity due to the small surface area of the leak passage. Category 2 was labyrinth type leaks such as long, narrow cracks or those associated with exfoliating fuel-line corrosion. Category 2 leaks have a relatively large area with low gas velocity due to viscous friction over the large surface area of the leak passage. Category 1 leaks were turbulent in character, while category 2 leaks were laminar in character. If the leak was category 1, the energy flow rates in the hydrogen and methane leaks were equal because the fuel-line pressures were set to give equal energy flow rates in the normal fuel-line flow, which was turbulent in character. If the leak was category 2, the flow rate of the two gases was inversely proportional to their viscosities and directly proportional to the fuel-line pressure because the leak flow was laminar in character. In category 2 the larger laminar leak orifice area produced a relative increase in the methane flow (through the larger area) due to the lower viscosity of methane relative to hydrogen. Table 1 lists the fuel flow rates used for the modeling of gaseous fuel leaks in this work. Category 1 flow rates were obtained by assuming the fuel lines for each fuel were of equal internal diameter and the flow was turbulent in each fuel line at normal, maximum flow rate, operation. It was also assumed that fuel-line pressure was set to achieve equal energy flow rate in each fuel line. It then follows that a category 1 leak, with passageways large enough to produce turbulent flow, would yield an equal energy leak flow rate for each gas. Therefore, the category 1 flow rates for the three gases were calculated by calculating the flow rate of methane and propane, equivalent to a 1000 L/h flow rate of hydrogen, and by using the ratio of the lower heating value (LHV) of the two gases to the LHV of hydrogen (Table 2). The category 2 flow rates were calculated recognizing that the laminar leak flow rates for two gases are inversely proportional to their dynamic viscosities and directly proportional to their leak pressure drops. The equation for category 2 flow rate calculation is
µfuel A ∆Pfuel B Qfuel B ) Qfuel A µfuel B ∆Pfuel A
(7)
where Q is volumetric flow rate, µ is dynamic viscosity, ∆P is leak pressure drop, fuel A is hydrogen, and fuel B is either methane or propane. The volumetric flow rate of hydrogen Qfuel A ) 1000 L/h. The values used for the other variables are given in Table 2. A case could be made for not using a lower ∆P for methane and propane. For example, the cross-sectional area of the fuel lines could be different for the three
86 Energy & Fuels, Vol. 12, No. 1, 1998
Swain et al. Table 2
hydrogen methane propane gasoline
LHVa
dynamic viscosityb
∆Pc
lean limit of combustiond
103 000 (239 578) 343 000 (797 818) 872 000 (2 028 272)
18.2 × (87.1) 22.7 × 10-8 (108.7) 16.6 × 10-8 (79.5)
∆Phydrogen 0.85∆Phydrogen 0.55∆Phydrogen
4.1 5.3 2.1 1.3
10-8
a In units of Btu lb-1 mol-1. Values in parentheses are in J g-1 mol-1. b In units of lb ft s ft-2. Values in parentheses are in µP. c In units of kPa. d In units of %.
Figure 1. Surface of constant 1% hydrogen concentration at 10 s, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 2. Surface of constant 1% hydrogen concentration at 5 min, 1000 L/h, and 2.92 ACH air exchange rate.
gases to yield equal energy flow in the fuel lines at equal pressure. If this assumption were made, the category 2 flow rates of methane and propane would need to be increased by 18% and 82% respectively. Results The comparison of the four fuels (hydrogen, natural gas, propane, and gasoline) was made beginning with hydrogen leakage under the GEOMET test conditions. The hydrogen leakage flow rate was 1000 L/h, and the air exchange rate was 2.92 ACH. The comparison was made based on the “volume of combustible gas” produced by each of the four fuels when leaked into a single-car garage. The “volume of combustible gas” was defined as the volume of gases in the garage with a fuel gas concentration richer than the lean limit of combustion. Figures 1-8 show the position of a surface of constant 1% hydrogen concentration versus time. The figures represent the growth of the fuel gas cloud at 10 s, 5 min, 30 min, 45 min, 1 h, 11/4 h, 13/4 h, and 2 h. During the first minute of leakage, minor changes in cloud size occurred (Figure 1 depicts the basic shape during this period) as the cloud begins to form under the plywood sheet representing the vehicle in the GEOMET test. These changes are the result of eddies under the plywood in the developing flow of the initially quiescent garage. After 5 min of leakage (Figure 2) the cloud begins to show preferential movement toward the edge of the plywood closest to the leak source. As the hydrogen reaches the edge of the plywood and rises, air is entrained with the rising hydrogen, and this flow steers the leaking hydrogen toward the nearest edge. Also, the fresh air entering the inlet at the garage door is more dense than the air-hydrogen mixture in the room and flows down the inside of the garage door and across the floor toward the plywood. This flow also tends to move the cloud toward the back of the garage and the rear edge of the plywood. In the period between 5 and 45 min (Figures 2-4) the hydrogen cloud shows an increasing preference for the rear edge of the
Figure 3. Surface of constant 1% hydrogen concentration at 30 min, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 4. Surface of constant 1% hydrogen concentration at 45 min, 1000 L/h, and 2.92 ACH air exchange rate.
plywood. During this period the gases above the end of the plywood are increasing in hydrogen concentration but have not yet reached 1%. At 1 h (Figure 5) the concentration of the rising column of hydrogen over the plywood has increased sufficiently to become visible at the 1% concentration level. Note that a roughly triangular volume of hydrogen-air mixture at the ceiling has risen above 1% concentration. The large-scale circulations developing in the room account for the asymmetrical shape of the hydrogen cloud. Between 11/4 h (Figure 6) and 13/4 h (Figure 7) slightly more than the top half of the room reaches a concentration greater than 1%. By 2 h (Figure 8) the room was reaching a relatively steady-state condition with the plane of 1% hydrogen
Comparison of Vehicle Fuels
Figure 5. Surface of constant 1% hydrogen concentration at 1 h, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 6. Surface of constant 1% hydrogen concentration at 11/4 h, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 7. Surface of constant 1% hydrogen concentration at 13/4 h, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 8. Surface of constant 1% hydrogen concentration at 2 h, 1000 L/h, and 2.92 ACH air exchange rate.
concentration positioned just below the vent. The hydrogen-air mixtures above the plane are over 1% concentration, while those below the plane are less than 1% concentration. However, the concentration gradient from floor to ceiling was not large. This was not due to hydrogen’s high molecular diffusion but rather to mixing caused by hydrogen’s buoyancy. Figures 9 and 10 depict the concentration gradient after 2 h of leakage. Figure 9 shows the surface of constant 1.13% hydrogen concentration. Note that the surface lies essentially at the ceiling of the garage. A “stalactite” of lean mixture
Energy & Fuels, Vol. 12, No. 1, 1998 87
Figure 9. Surface of constant 1.13% hydrogen concentration at 2 h, 1000 L/h, and 2.92 ACH air exchange rate.
Figure 10. Surface of constant 0.6% hydrogen concentration at 2 h, 1000 L/h, and 2.92 ACH air exchange rate.
exists over the plywood because the hydrogen is flowing around the plywood and rising in a somewhat hollow column. Figure 10 shows the surface of constant 0.6% hydrogen concentration. Note that the surface lies essentially at the floor of the garage. Also, note the path of fresh air (lean mixture) flowing down from the vent, across the room, and under the plywood (Figure 10). The hydrogen did not produce appreciable volumes of combustible mixture (4.1% hydrogen concentration) under these conditions. The surface of constant 4.1% hydrogen concentration would have been positioned within inches of the leak location. The volume of potentially burnable gases was less than 6 × 10-6 of the garage volume. The simulation for natural gas leakage was run using methane. The flow rate of methane was chosen based on an assumption of equal energy flow rate in the fuel line of the vehicle under normal conditions. This was a conservative assumption from the standpoint of critically assessing the safety of hydrogen. The hydrogenpowered vehicle would have approximately 2.7 times higher fuel economy (LHV) than the natural gas-fueled vehicle. If this were taken into account when choosing the relative fuel-line pressures, the leakage rate of hydrogen would have to be reduced by a factor of 2.7 below those used for this modeling. Methane did not produce appreciable volumes of combustible mixture under either category 1 or category 2 leak conditions. The surface of constant 5.3% methane concentration would have been positioned within inches of the leak location. Figure 11 shows the surface of constant 0.8% methane concentration. Methane, being lighter than air, behaved in a manner similar to hydrogen. Although the hydrogen concentration varied from 1.13% (Figure 9) at the ceiling of the garage to 0.6% (Figure 10) at the floor, methane concentration varied from 0.83% at the ceiling to 0.4% at the floor. Once again, the upper portion of
88 Energy & Fuels, Vol. 12, No. 1, 1998
Figure 11. Surface of constant 0.8% methane concentration at 2 h, 680.0 L/h, and 2.92 ACH air exchange rate.
Figure 12. Surface of constant 2.1% propane concentration at 2 h, 117.8 L/h, and 2.92 ACH air exchange rate.
Figure 13. Surface of constant 2.1% propane concentration at 2 h, 602.4 L/h, and 2.92 ACH air exchange rate.
the garage contained very small concentration gradients (hundredths of a percent) and the lower portion contained small concentration gradients (tenths of a percent). The results shown in Figure 11 are for category 2 leaks. The results for category 1 leaks show similar trends but at lower concentrations of methane. The combustible cloud was less that 6 × 10-6 of the garage volume. The simulation for LPG was run using propane. The flow rates for propane were chosen in the same manner as the methane flow rates. The flow rate of propane corresponding to a category 1 hydrogen leak of 1000 L/h was 118 L/h of propane. The flow rate of propane corresponding to a category 2 hydrogen leak of 1000 L/h was 602 L/h of propane. Propane (C3H8) has a molecular weight of 44, making it approximately 50% denser than air. Figures 12 and 13 show the surface of constant 2.1% propane concentration for the two leak flow character assumptions. A value of 2.1% concentration is the lean limit of combustion for propane. The volume of gas below the surface is burnable. Figure 12 shows the surface of constant 2.1% propane concentration assuming a category 1 leak. A layer of combustible gas mixture covers more than half the floor. Figure 13 shows the surface of constant 2.1% propane concentra-
Swain et al.
Figure 14. Surface of constant 1.3% gasoline concentration at 2 h, 1 drop/s, and 2.92 ACH air exchange rate.
tion for a category 2 leak. Note that in Figure 13 the lower 25% of the garage is filled with a burnable mixture. The simulation for gasoline was run using a blend of nine hydrocarbon constituent gases. These were condensed from a GC analysis conducted at 13 gasoline stations in Paulboro, New Jersey by Mobil Oil Company. Each of the nine hydrocarbons was modeled independently using the appropriate molecular weight and diffusion coefficient. The gasoline flow rate was 6 cm3/ min (1 drop/s) and was assumed to fully evaporate. This was a conservative estimate of the leakage of gasoline from a fuel line damaged in the same manner as that which produced the hydrogen leak rates used above. For example, if a hole is drilled in a fuel line with a no. 80 drill 0.343 mm (0.0135 in.) in diameter, a pressure head of 14.3 cm (55/8 in.) of gasoline produced the gasoline leakage rate used, 6 cm3/min (1 drop/s). A hydrogen pressure of 363 kPa (52.6 psi) is needed to produce the 1000 L/h hydrogen leakage rate from the same fuel line. A value of 363 kPa (52.6 psi) for the fuel-line pressure is considerably higher than the 207 kPa (30 psi) commonly used in gaseous fuel lines. Figure 14 shows the surface of constant 1.3% gasoline vapor concentration. The value 1.3% is the lean limit of combustion for gasoline vapor. A layer of combustible gas covered nearly 80% of the floor. The volume of potentially burnable gases was approximately 2% of the volume of the garage. Conclusions4 (1) For a hydrogen leakage rate of 1000 L/h the combustible cloud did not extend more than 10 cm (4 in.) from the leak. (2) For a methane leakage rate of 300 L/h (category 1 leak) the combustible cloud did not extend more than 10 cm (4 in.) from the leak. (3) For a methane leakage rate of 680 L/h (category 2 leak) the combustible cloud did not extend more than 10 cm (4 in.) from the leak. (4) For a propane leakage rate of 118 L/h (category 1 leak) the combustible cloud covered half the floor of the garage (Figure 12). (5) For a propane leakage rate of 602 L/h (category 2 leak) the combustible cloud filled over 25% of the garage with combustible mixture (Figure 13). (4) The results depicted in this study were produced with FLUENT, a computational fluid dynamics modeling program, and require experimental verification.
Comparison of Vehicle Fuels
(6) For a gasoline leakage rate of 1 drop/s the combustible cloud covered over 80% of the floor of the garage (Figure 14). Acknowledgment. This work was supported by the Ford Motor Company under their “Direct-HydrogenFueled Proton-Exchange-Membrane (PEM) Fuel Cell
Energy & Fuels, Vol. 12, No. 1, 1998 89
System for Transportation Applications” contract with the U.S. Department of Energy No. DE-AC02-94CE50389 through a subcontract from Directed Technologies, Inc. of Arlington, Virginia. EF970111S
