Velocity fluctuation analysis near detonation propagation limits for stoichiometric methane–hydrogen–oxygen mixture

This paper reports the results of an experimental study of the velocity fluctuation near the detonation limits for CH₄[sbnd]2H₂[sbnd]3O₂ mixture in tubes with a length (L_t) of 2.5 m and diameter (d) of 4-mm, 14-mm and 36-mm. Fiber optics are used to measure the local velocity along the test section. Results suggest that the velocity behavior is complicated in 4-mm diameter tube (with L_t/d = 625) at the condition near the detonation limits. As the initial pressure reduced from 55 kPa to as low as 16 kPa, it can be observed six propagation modes with different features, these are steady detonation, fast fluctuation-stable detonation, stuttering detonation, stuttering detonation-fast deflagration, fast deflagration, and flame propagation. In 14-mm diameter tube (L_t/d = 178.6), it can be observed three propagation modes: steady detonation, galloping detonation and flame. Only steady detonation and flame propagation modes exist in the 36-mm diameter tube (L_t/d = 69.4). Results indicate that initial and boundary conditions have strong influences on the propagation of the detonation wave in small-diameter tube. The boundary layer displacement thickness is more significant in the smaller tube and lower initial pressure, which causes more losses through the flow divergence and therefore the increasing of velocity deficits. It is believed that the competition between the losses from the tube wall and the detonation instability causes the complicated velocity fluctuation in the smaller diameter tube as it is approaching the propagation limits. Results also confirm d/λ is an appropriate length scale for determining detonation limit.