PLIF and chemiluminescence in a small laminar coflow methane-air diffusion flame at elevated pressures

Abstract

Fluorescence and chemiluminescence are significant diagnostic tools in combustion research. This experimental work studies OH PLIF and the OH electronically excited state OH* that provide beneficial insights regarding the physical extent and progress of elevated pressure combustion, particularly as regards these heat release zone and reaction zone markers. CH* chemiluminescence is also imaged since this optical diagnostic often serves as the first visual evaluation of a flame. A high-pressure combustion facility is built to study the fluorescence and chemiluminescence in a laminar coflow methane/air flame within its steady limits up to 11.4 atm. This research demonstrates how diagnostics in high-pressure combustion environments pose particular challenges because of the difficulty in accounting for quenching and line broadening in the measurements. The overall imaging results show that as the pressure rises there is a consistent thinning of the reaction zone while maintaining the same flame height, and there is an increasing concentration of soot near the flame tip. This work presents detailed high-pressure experimental design and procedures, including a discussion of the intrinsic physical challenges for soot interference associated with the optical interrogation of hydrocarbon fuel flames at these pressures. In addition, the main contributors to the fluorescence signal are identified and quantified versus pressure. Finally, we compare the experimental PLIF images with simulated PLIF obtained from CFD.