Flashback in diluted hydrogen flames

Abstract

Flashback has been studied experimentally in premixed hydrogen/air with a dilution gas. The experiments, finding a critical velocity that occurs boundary laser flashback, were conducted in a quartz glass tube with an inner diameter of 3, 4, 5, and 6 mm at the equivalence ratio between 0.6 and 3.1. N₂, Ar, He, and CO₂ were used as a dilution gas. The concentration of the dilution gas in the fuel mixture was 15 and 30 %. An infrared camera monitored flame development in the tube. The critical velocity gradient at the boundary of the gas stream, where the flashback is observed, has been systematically investigated and determined through experimental methods for each specific set of experimental conditions. The critical velocity gradient is independent of the tube diameter and dependent on the equivalence ratio, the dilution fraction, and the dilution gas. Generally, the magnitude of the flashback velocity gradient follows an order: undiluted hydrogen, Ar dilution, He dilution, N₂ dilution, and CO₂ dilution flames. The flashback velocity gradient reduces with increasing the dilution gas concentration. An experimental equation for the flashback velocity gradient indicates that the flashback at the boundary in the hydrogen flames is influenced by both the laminar flame speed and the Lewis number.