Initiation of detonation wave in a non-uniform hydrogen-air premixed gas

Abstract

There is increasing interest in the use of hydrogen as an energy source in fuel cells, and such cells are expected to find practical applications in the near future. However, the reaction rate of a hydrogen-air mixture is so high that the deflagration wave generated during ignition can easily become a detonation wave, even though only a small amount of energy is supplied to the premixed gas. Such a detonation wave can cause serious damage because of the high-pressure and temperature at the wavefront. Despite such concerns, the onset conditions for producing a detonation wave in a non-uniform mixture of hydrogen and air have not yet been fully clarified. In the present study, these conditions were investigated by changing the concentration of hydrogen to understand the onset condition of detonation wave. A vertical detonation tube was divided into two chambers using a slide valve; the upper chamber was filled with air and the lower chamber with hydrogen. A hydrogen concentration gradient was produced by opening the valve for a specific period of time. A pair of electret sensors was used to determine the concentration of hydrogen and the equivalence ratio by measuring the speed of sound in the premixed gas. The onset conditions for detonation were investigated by changing the overall equivalence ratio, φ , and the elapsed time, t d , from the onset of diffusion. It was found that for φ = 1.67 and t d ≥ 540 s, a detonation wave was produced leading to a large increase in pressure. Furthermore, the results indicated that the local equivalence ratio in the vicinity of the spark plug had an important influence on the initiation of the detonation wave.