The influence of initial temperature on flame acceleration and deflagration-to-detonation transition

Abstract

The influence of initial mixture temperature on deflagration-to-detonation transition (DDT) has been investigated experimentally. The experiments were carried out in a 27-cm-inner diameter, 2 1.3-meter-long heated detonation tube, which was equipped with periodic orifice plates t o promote flame acceleration.Hydrogen-air-steam mixtures were tested at a range of temperatures up t o 650K and at an initial pressure of 0.1 MPa. In most cases, the limiting hydrogen mole fraction which resulted in transition to detonation corresponded t o the mixture whose detonation cell size, A, was approximately equal to the inner diameter of the orifice plate, d (e.g., d/A=1). The only exception was in dry hydrogen-air mixtures at 650K where the DDT limit was observed t o be 11 percent hydrogen, corresponding t o a value of d/A equal to 5.5. For a 10.5 percent hydrogen mixture at 650K, the flame accelerated to a maximum velocity of about 1 2 0 m/s and then decelerated to below 2 m/s. This observation indicates that the d/A= 1 DDT limit criterion provides a necessary condition but not a sufficient one for the onset of DDT in obstacle-laden ducts. In this particular case, the mixture initial condition (i.e., temperature) resulted in the inability of the mixture t o sustain flame acceleration t o the point where DDT could occur. It was also observed that the distance required for the flame to accelerate to the onset of detonation was a function of both the hydrogen mole fraction and the mixture initial temperature. For example, decreasing the hydrogen mole fraction or increasing the initial mixture temperature resulted in longer transition distances.--: