Ignition of lean hydrogen–air mixtures

A series of tests were conducted to investigate how difficult lean hydrogen–air mixtures (4–10% hydrogen concentration) are to ignite. The commonly reported lean limit of combustion for hydrogen is 4.0%. The tests that were used to determine this assumed a quiescent hydrogen–air mixture and no upper limit on ignition energy or gap size for ignition. In this work, ignition tests were conducted with: (1) quiescent hydrogen–air mixtures and ignition arc lengths up to 3.2 mm (0.126 in), (2) mixtures in motion and arc lengths up to 1.5 mm (0.060 in), and (3) low-velocity mixtures with common electrical appliances as ignition sources. This work showed that the probability of igniting lean hydrogen–air mixtures is strongly affected by the ignition source, where successful ignition was a strong function of arc length and a weaker function of gas velocity and ignition energy. The tests were conducted at 26– 27 ∘ C (78– 80 ∘ F ).