“We found recently in an FAA test that if you enrich an atmosphere with a little bit of hydrogen, like 1-2% ... the flammability and burning characteristics of materials that are typical for cabins increase,” Catalin Fotache, FAA chief scientist and technical advisor for propulsion systems, said March 3 at the H2-Aero conference in Long Beach, California.
“You have significantly higher heat release rates and see significantly higher combustion rates,” Fotache said, describing the test results as “another worrisome picture” among the safety challenges created by the flammability characteristics of hydrogen compared with kerosene.
Hydrogen has a much wider flammability limit range when mixed with air, from 4-75% by volume compared with 0.5-5% for jet fuel. The FAA tests showed that hydrogen could change the flammability characteristics of cabin materials at much lower mixtures.
“While more work needs to be done on this aspect, it is something worse than [I expected]. If you had asked me, before seeing this data, I would have said you need to worry about 5-6%, but 1-2% is really low,” Fotache told the conference, organized by the Vertical Flight Society.
Such small volumes of hydrogen could result from microleaks, another major safety concern with emerging hydrogen propulsion systems. “These are extremely small leaks—5 micrograms per second. This kind of flow rate is so low ... they are very hard to measure, and you can barely see them,” he said.
“They're very low heat release, about 500 watts or a couple of light bulbs. If they're burning—and if you have a leak, you have to assume the hydrogen would be burning—you will not detect them with a flammability detector or volume detector like some of our [certification] applicants have proposed.”
Highlighting the concerns with ensuring the safety of hydrogen-powered aircraft, Fotache said, “These are areas that I think are ripe for further research to understand what is the behavior, and when are these leaks truly dangerous?”