Concentration Measurements of OH and CH Radicals in Laminar Partially Premixed and Prevaporized Jet A / Palm Methyl Ester Blend Flames

Abstract

Palm methyl ester (PME) is an attractive alternate fuel that is becoming popular in Asia and has the potential for a wider application. In this study, the near-burner flame characteristics in the laminar partially premixed flames of Jet A, PME and their blends (25%, 50% and 75% by volume) were investigated. Planar laser-induced fluorescence (PLIF) technique was used to determine the relative concentration of OH and CH radicals in the near-burner region of the flames. The fuel was prevaporized by injecting into a hot air stream and passed through a 9.5 mm stainless steel tube which served as the injector. The burner exit equivalence ratios of 2, 3 and 7, were selected to simulate a wide range of (high to low) partially premixed modes of combustion that exist in practical combustors. The PLIF measurements indicated that the OH and CH concentrations closely followed the temperature profiles at the equivalence ratio of 2. The blend-fuel flames had higher OH and CH concentrations than the neat fuel flames at equivalence ratios of 2 and 3. At the equivalence ratio of 7, a significant reduction in OH concentration was observed in the near-burner region of the flames having biofuel content, and the CH concentration was comparatively higher than OH concentration of corresponding flames. The presence of high CH concentration in the region of peak NO concentration indicated that the NO formation at this condition was likely through the Fenimore mechanism. The concentration measurements of OH and CH radicals revealed a non-monotonic variation with the proportion of biofuel in the fuel blend and suggested a complex competition between the growth and oxidation of poly aromatic hydrocarbons (PAH) and soot particles, and the formation of combustion products.