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CFD simulations in Residential Garages

Hydrogen leaks inside a residential garage compared against gasoline, natural gas and propane leaks in the same environment were simulated by Swain (SwainMR:1998) using the FLUENT code. Calculations were based on the GEOMET Invalid BibTex Entry! garage geometry (2.52x6.59x2.74 m) with a single vent placed in the center of one wall of an otherwise sealed garage. The leak rates for fuels other than hydrogen were adjusted for equal size holes and equal energy flow rates in the fuel lines considering both laminar and turbulent flow where applicable. The results of the simulations show that for the lower leakage rate (1000 l/hr) and typical garage air exchange (2.92 ACH representing natural ventilation), hydrogen and methane did not create dangerous conditions while propane and gasoline did produce dangerous conditions in similar accident scenario. For the larger fuel leakage rate (7200 l/hr) and minimal air exchange rate (0.2 ACH) all four fuels produced very large combustible clouds after 2 hours of leakage. However, the energy content of the combustible clouds was different, with hydrogen being at most % that of the other fuels. Both natural gas and hydrogen filled the entire garage with a flammable mixture after two hours, while propane and gasoline filled just over half of the garage volume with a flammable mixture. At the higher air exchange rate (2.92 ACH), the hydrogen still filled the garage with a flammable mixture, which reached about 6.6% hydrogen concentration after two hours.

Boil-off from the cryogenic hydrogen tank of a car in a private garage was simulated by Breitung et al. (BreitungW:2000) using the GASFLOW code to calculate the temporal and spatial distribution of hydrogen and criteria to evaluate the flame acceleration and detonation potential. Boil-off was assumed occurring at a rate of 170 g day-1 and the boil-off release to occur intermittently in five pulses per day of 100 or 10 s time period each, which gave 0.34 or 3.4 g s-1 respectively.

CFD modelling (FLUENT) was used in Invalid BibTex Entry! to analyze H2 leak scenarios inside residential two vehicle garages. The study was based on parking a 5-passenger sedan with compressed H2 gas reservoir carrying capacity of 6 kg at 10,000 psi (689.5 bar) pressure. The HFCV was designed to comply with SAE J2578 and J2579 standards for H2 and fuel cells, which include provisions for safety systems onboard the vehicle. Such assumed mechanisms include the implementation of a hydrogen detector in each wheel well. Each detector was designed to signal a shut down and isolation H2 procedure upon detecting 1% H2. Another assumed mechanism includes the use of an on-board computer that is capable of shutting down H2 flow upon detecting a larger than 20 CFM leak (9.4 lt s-1) when the vehicle is dormant. In addition the HFCV was equipped with a valve that isolates H2 in the tank upon engine (fuel cell) shut down. This assumed isolation mechanism was designed to monitor and test for leaks upon vehicle shutdown and prior to start up by the on-board computer. In the study most of the modelling scenarios were based on a 20 CFM leak from beneath the vehicle. This leak rate corresponded to a fuel cell power output of about 50 kW. It was found that for the considered garage layout and scenarios no modifications to the baseline structures would appear to be necessary for vehicles equipped with safety systems that detect hydrogen leaks according to the chosen scenarios. CFD modelling of one configuration showed that a 1% hydrogen concentration would reach the wheel well within 28 seconds where detectors could initiate a shut off of the fuel supply. Since not all vehicles will be equipped with hydrogen detectors, or be configured like the chosen vehicle design, additional modelling provided data on the time for a 5% hydrogen concentration to accumulate at the garage ceiling. This information may be used by carmakers to develop strategies to limit the amount of time that the vehicle operates at zero speed before shutting off the fuel supply.

(GallegoE:2005)

Helium dispersion experiments in a private garage including a mockup of a car and performed by Swain Invalid BibTex Entry! were simulated by Papanikolaou and Venetsanos (PapanikolaouEA:2005) using the ADREA-HF code and the standard k- model. Helium was released below the car at a flow rate of 7200 l/hr. The predicted results were found generally in acceptable agreement with the experiment. For the case with the lowest vent size the vertical concentration gradient was found underestimated compared to the experiment. This was attributed to the turbulence model overestimating mixing under the given low flow conditions.

References

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