Hydrogen Flammability, Detection, and Fire Safety

Concerns about climate change are driving efforts to develop hydrogen-powered systems as replacements for many current applications utilizing hydrocarbon fuels. The physical properties of hydrogen differ from those for hydrocarbon fuels. As a result, the mixing and combustion behaviors of hydrogen differ in significant ways from hydrocarbons and must be taken into account when engineering systems for safe operation and fire prevention. The differences between hydrogen and typical hydrocarbon fuels are particularly important when hydrogen is released into enclosed spaces such as buildings, garages, and tunnels. Example applications include hydrogen-fueled automobiles parked in residential garages and stationary fuel cells located within a building.

As hydrogen-powered automobiles come on the market, it is likely that they will be parked in the existing stock of residential garages. For this reason, it is important to understand the implications of potential hydrogen leaks in typical residential garages. Two such parameters are hydrogen leak location and the size and spatial distribution of leaks. Investigations focused on losses of hydrogen from an enclosure during and following a release are limited.

In the report, "Experimental Characterization of Helium Dispersion in a ¼-Scale Two-Car Residential Garage,"  (March 2011) a series of experiments are described in which helium was released at constant rates into a 1.5 m × 1.5 m × 0.75 m enclosure designed as a ¼-scale model of a two car garage. The purpose was to provide reference data sets for testing and validating computational fluid dynamics (CFD) models and to experimentally characterize the effects of a number of variables on the mixing behavior within an enclosure and the exchange of helium with the surroundings. Helium was used as a surrogate for hydrogen, and the total volume released was scaled as the amount that could be released by a typical hydrogen-fueled automobile with a full fuel tank. Temporal profiles of helium were measured at seven vertical locations within the enclosure during and following 1 h and 4 h releases. Idealized vents in one wall sized to provide air exchange rates typical of actual garages were used. The effects of vent size, number, and location were investigated using three different vent combinations. The dependence on leak location was considered by releasing helium from three different points within the enclosure. A number of tabulated quantitative measures are used to characterize the experiments. The complete experimental measurement results for each condition are available on the internet as described in Appendix A:

Appendix A—Downloadable Data Files of Experimental Results for the Eighteen Tests

The measured helium volume fractions (reported as percentages) at eight locations and differential pressure in Pascals as a function of time in seconds are available on the internet for the eighteen experiments discussed in this report. The results for each experiment are stored as individual Microsoft Excel files that are collected in a single zip file located at http://www.nist.gov/el/fire_protection/buildings/upload/HeliumDispersionDataSets.zip (18 Microsoft Excel files, zipped, 53Mb.) 

Reduced-scale garage constructed of poly(methyl methacraylate) to study the release and disperson of helium (used as a hydrogen surrogate). Photo credit: NIST

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