Local reaction zone thickness of a high intensity turbulent premixed flame

Abstract

The present study has been performed to explore the detailed aspects of the reaction zone of a high intensity turbulent premixed flame, classified as a distributed reaction zone flame based on the Klimov-Williams criterion. A co-axial burner, and a micro-electrostatic probe were used to examine the local reaction intensity profiles across the flame. The measured ion current records imply that unburned mixtures and burned gases exist separately in the flame zone and local reaction zones of thicknesses much thinner than the flame zone can be defined. Assuming that the half value period of an ion curent fluctuation is a measure of the local reaction zone thickness in the direction passing the probe, the probability density function of it was examined to explore the aspects of local reaction zones in the flame zone. In order to interpret the probability density function distrubution of half value period, the ion current fluctuations to be recorded were predicted on the basis of the knowledge of ion current characteristics at premixed flame measurements. Considering the fluctuations of the inclined angle and velocity of the local reaction zones passing the probe, the probability density functions of the half value period were predicted for various thicknesses of the local reaction zones. Comparing the predicted probability density functions with the experimentally obtained ones, it was concluded that even in the extremely intense turbulence which satisfies the distributed reaction zone condition based on the Klimov-Williams criterion, the local reaction zone thickness is close to that of the laminar flame. This implies necessity to discuss the structure of the distributed reaction or to establish a novel criterion for existence of the distributed reaction zone not based on the characteristics of unbruned mixture turbulence.