Structure of Laminar Methane-Oxygen Diffusion Flames at High Pressures

Abstract

Characteristics of laminar methane-oxygen diffusion flames were studied at sub-critical and supercritical pressures up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10 to 90 atmospheres in a high pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and oxygen diffusion flame: an inner luminous zone similar to the methane-air diffusion flames and an outer blue flame zone. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10 to 100 atmospheres at the higher methane flow rate. At the lower methane flow rate, above 10 atmospheres, the luminous flame zone started collapsing and solid carbonaceous structures appeared on the fuel nozzle rim. Further increase in pressure resulted in shrinking of the luminous zone into mostly solid material and only the blue flame zone was visible. For the higher methane flow rate, between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations reached a plateau and then began decreasing with increasing pressure.