Left: Turbulent
Right: Lifted

The United States and several other countries are considering adopting hydrogen as a fuel carrier. Nonetheless, many challenges must be addressed before hydrogen can be used as a a transportation fuel, particularly the fire hazards that hydrogen presents if it leaks.

Hydrogen has a number of unique properties that make it particularly prone to fire risks associated with leaks. Hydrogen has exceptionally wide flammability limits — with a nominal low limit of 4 percent and an upper limit of 75 percent in air — that make it more susceptible to ignition than other fuels. Also, a hydrogen leak can support combustion at a much lower leak rate than other fuels, and small hydrogen flames are virtually invisible to the naked eye. That means a small hydrogen leak could ignite and remain undetected indefinitely. Fire hazards and material degradation are distinct possibilities with such a flame. This could also potentially result in catastrophic consequences, such as detonation or a boiling liquid expanding vapor explosion (BLEVE). Though this risk is not unique to hydrogen, a potential explosion is significantly more probable than with other liquefied flammable gases because of hydrogen’s wide flammability limits.

LACER is investigating risks associated with fires caused by small hydrogen leaks. Our results are being used to improve codes and standards for hydrogen vehicles and other hydrogen systems. To do this, we are testing and analyzing the flammability limits of hydrogen under a variety of conditions. This work involves delivering hydrogen through microburners or controlled leaks of different sizes and shapes. The results of this research have enabled the identification of limits for stable hydrogen flames, and become the Society of Automotive Engineers (SAE) standard for ensuring the safe use of hydrogen. They can be found in SAE J2579, “Standard for Fuel Systems in Fuel Cell and Other Hydrogen Vehicles.”

Our research has also led to the production of the smallest flame ever reported — at just one-quarter of a watt.

The image on the left shows a near-extinction hydrogen flame.
Barely visible to the naked eye (look very carefully just above the top of the burner), we were able to photograph this flame in dim lighting with no flash. This picture was taken at the widest aperture setting and a 30-second exposure time.