Effects of thermal and chemical flame-surface interaction on flame quenching

Abstract

Incomplete combustion due to quenching in a narrow confinement has been a major problem for realization of a reliable micro combustion device. In most micro combustors, effects of flows are absent in the quenching because the flow is laminar and no severe stretch is present. In such circumstance, quenching is caused either by excessive heat loss or by radical depletion to the surface of the microscale combustors. An experimental investigation was carried out to examine the relative significance of these two causes of quenching of a methane-air premixed flame. A premixed jet burner with a rectangular cross section at the exit was built. At the burner exit, the flame stands between two walls with adjustable distance. The gap between the two walls at which quenching occurs was measured at different wall surface conditions: stainless steel 304, alumina, and quartz. The results were analyzed to estimate the relative significance of heat loss and radical quenching. The measurements indicate that the quenching distance is not governed by physical and chemical properties of the surface at low temperature. At high temperature, however, the surface characteristics strongly impinge on the quenching distance, implying that radical removal at the wall plays a significant role in the quenching process. In the second part of this thesis, I described the OH LIF measurements to qualitatively estimate the amount of radicals quenched out by chemisorption in the vicinity of the surface. From these measurements it was found that there is no significant OH radical recombination at low temperature surface conditions; in the case of 500℃, however, we observed reduced OH fluorescence intensity indicating significant radical quenching on the surface. When we further raised the temperature of plates to 700℃, OH fluorescence intensity increased again inasmuch as transition of $CH_4$ reaction mechanism has been converted from low temperature reaction mechanism to high temperature one a...