Modeling of pressure effects on flame structure and soot formation of n-heptane/air co-flow laminar flames by skeletal reaction mechanism

Abstract

The conversion from chemical energy to thermal energy by the high-pressure combustion of hydrocarbon fuel/air is often accompanied by pollution emissions of PAHs and soot in aircraft gas turbines and diesel engines, and the measurement and analysis of PAHs and soot formations in the practical turbulent flame of engines are difficult. In the study, based on the simulation of the simple laminar co-flow diffusion flame of n-heptane/air by the developed skeletal reaction mechanism, the effects of pressure on flame structure and soot formation are investigated. The results indicate that flame height keeps constant at 0.7~3.0 MPa; the flame radius decreases with pressure as p−1/2; the maximum carbon conversion to soot (ηs,max) is proportional to pressure at 0.1~2.0 MPa; the maximum soot volume concentration (fv,max) increases with pressure as p2; the locations of fv,max and ηs,max along flame centerline are inconsistent, and fv,max and ηs,max occur respectively at the middle and lower parts of flame height; fv,max occurs in the region where the mixture fraction and temperature are respectively 0.08~0.09 and about 1200 K. The diffusion flame consists of three zones: fuel heating zone, fuel-rich reaction zone and oxidizer-rich reaction zone. n-C7H16 is firstly decomposed into small molecule gas (e.g., H2, CH4, C2H2,C2H4, C2H6, C3H4, C3H6, etc.) in the heating zone, and PAHs (C6H6, C8H8 and C10H8) and soot precursors (C2H2, C6H5, C6H6 and C2H4) are formed in fuel-rich reaction zone.