96/00712 Measuring the laminar burning velocity of methane/diluent/air mixtures within a constant-volume combustion bomb in a micro-gravity environment

Abstract

The laminar burning velocity of methane and air has been determined for a range of equivalence ratios, with various levels of carbon dioxide and nitrogen as diluents. Carbon dioxide and nitrogen are both present in natural gas, and in biogas there can be as much as 40% carbon dioxide. Furthermore, mixtures of carbon dioxide and nitrogen can be used to simulate the products of combustion, thus enabling measurements of the burning velocity to be made that correspond to conditions of exhaust-gas recirculation. The data presented here cover higher levels of diluents than previously reported, with up to 60% of either nitrogen or carbon dioxide as part of the fuel. Because of the effects of buoyancy during the combustion process within a constant-volume bomb, the centre of the flame kernel rises and the flame-front deviates from that of an ideal sphere at low values of burning velocity. This invalidates the analysis methods where the centre of the kernel is assumed to be stationary and the flame front is a perfect sphere. The problem may be overcome by use of a micro-gravity environment. Comparisons are reported between a detailed thermodynamic analysis and an approximate analysis, for computing the laminar burning velocity from the pressure/time record: it is concluded that the approximate analysis is satisfactory. A discussion is reported on the effects of flame stretch, from which it can be argued that, for the mixtures investigated here, the effects of stretch are negligible after the end of the initial pressure-rise period.