An experimental and computational study on soot formation in a coflow jet flame under microgravity and normal gravity

摘要

Upon the completion of the Structure and Liftoff in Combustion Experiment (SLICE) in March 2012, a comprehensive and unique set of microgravity coflow diffusion flame data was obtained that covers weak flames near extinction to strong, highly sooting flames, which enabled us to study the gravitational effects on phenomena such as liftoff, blowout and soot formation. This paper focuses on the different sooting behaviors of CH_4 and C_2H_4 coflow jet flames in microgravity and normal gravity. The microgravity experiment was carried out in the Microgravity Science Glovebox on board the International Space Station, while the normal gravity experiment was performed at Yale utilizing a copy of the flight hardware. Computational simulations of microgravity and normal gravity flames were also carried out to facilitate understanding of the experimental observations. The unique set of data serves as an excellent test case for developing more accurate computational models. Experimentally, the flame shape and size, lift-off height, and soot temperature were determined from line-of-sight flame emission images taken with a color digital camera. Soot volume fraction was determined by performing an absolute light calibration using a thermocouple. The microgravity sooting flames are found to have lower soot temperatures and higher volume fraction compared to their normal gravity counterparts. The soot distribution tends to shift from the centerline of the flame to the wings from normal gravity to microgravity. Computationally, the vorticity-velocity formulation of the governing equations was employed to describe the chemically reacting flow, and the resulting system of fully coupled, highly nonlinear equations was solved by a damped, modified Newton's method. The two-dimensional axisymmetric model coupled finite-rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation included detailed treatment of the inception, surface growth, oxidation, coalescence and thermophoresis of soot particulates. The influence of soot formation on radiative heat loss and scavenging of gaseous species was also taken into consideration.