A numerical investigation of microgravity ethylene/oxygen/nitrogen spherical diffusion flames at their soot free limit

Abstract

Soot, which is a byproduct of incomplete combustion from hydrocarbon/air flames, is a major cause of premature deaths and lung cancer among pollutants generated from burning of carbon-based fuels. It also is responsible for the reduction of combustion efficiency due to the decrease of thermal energy and radiative heat losses. In this study, the sooting behavior of spherical diffusion flames using ethylene as the fuel was investigated to improve the understanding of soot formation processes in diffusion flames. Microgravity experiments using the 2.2-s drop tower operated by NASA Glenn Research Center were conducted to observe the sooting behavior and provide guidance to the theoretical study of such flames. Fifteen flames that were found to reach their soot-free conditions between 1 and 2 seconds from the experiments were selected in this study. A numerical code with detailed chemistry and transport properties as well as realistic radiation model was adopted to simulate these flames. Results show that for the formation of soot particles, both the local temperature and carbon to oxygen atom ratio need to exceed certain critical values, which were found to be 1425 K and 0.47, respectively, for the flames investigated in this study.