Vortex ring combustion in an atmosphere of the same mixture as the combustible

Abstract

Vortex ring combustion experiments have been conducted in an atmosphere of the same mixture as the combustible. The propagating flames have been recorded by a digital high-speed camera with an image intensifier, and the variations of flame diameter and flame speed with vortex strength have been determined. From this experiments, it is found that the Lewis number effect appears in the near-limit flame behavior. For the mixture whose Lewis number is less than unity, such as lean methane and rich propane mixtures, burning is intensified at the head, and a flame of small diameter can propagate even for large values of the maximum tangential velocity, although the flame speed is smaller than the maximum tangential velocity. For the mixture whose Lewis number is larger than unity, such as rich methane or lean propane mixtures, the burning is weakened at the head, and the flame is prone to extinction even at small values of the maximum tangential velocity, although the flame speed is not small, almost equal to the maximum tangential velocity. It is also found that with an increase in the maximum tangential velocity, the flame diameter is decreased, and correspondingly, the rate of increase of the flame speed is decreased, that is, the flame speed is saturated. In this experiment, there is no dilution by entrainment of the ambient mixture. Hence, the observed decrease in the flame diameter is caused by pure flame-flow interaction at the head region. The measured flame speeds are compared with theories, which have the finite flame diameter taken into consideration. As the result, it is found that the finite flame diameter approximation by Asato et al., the theory by Umemura and Tomita, and a steady-state momentum flux conservation model proposed in this paper can describe some parts of the measured flame speeds.