Soot Formation in Freely-Propagating Laminar Premixed Flames: Introduction

摘要

An investigation of soot formation within freely-propagating laminar premixed flames is described. The findings of the research are relevant to several problems of society, as follows: the particulate soot pollutant emissions from flames, the radiant heat loads from combustion processes, the hazards of terrestrial and spacecraft fires, and the practical development of methods of computational combustion. The research was carried out in two phases, as follows: (1) study of soot formation in freely-propagating laminar premixed flames, and (2) study of the optical properties of soot needed to develop nonintrusive optical ways of finding soot structure within freely-propagating laminar premixed flames. It is widely recognized that soot processes in flames are a major unresolved problem of combustion science. This observation has motivated numerous studies of the physical, optical and reactive properties of soot during combustion processes. This research has also disclosed some problems concerning studies of soot formation in burner-stabilized laminar premixed flames that frequently have been used for measurements of soot phenomena, as follows: (1) flame structure is sensitive to minor burner construction details so that laboratory-to-laboratory reproducibility is not very good, (2) consistent burner behavior over the lengthy test programs needed to define soot formation properties are hard to achieve due to poorly understood effects of burner aging, (3) it is difficult to accurately define near burner flame properties due to the intrusion of the flame holder, (4) it is difficult to quantify conductive, convective and radiative heat losses to the burner that are needed to define flame temperature properties, (5) it is difficult to define flow velocities near the burner where the flow is multidimensional and velocities change rapidly with streamwise distance due to effects of buoyancy, and (6) it is difficult to carry out fundamental numerical simulations of premixed flame properties due to the multidimensional effects and the complex geometries of flame holders (e.g., porous plates). These problems can largely be avoided, however, by studying freely-propagating laminar premixed flames at conditions where the flames have stable and smooth spherical flame surfaces as they propagate. The feasibility of experiments along these lines to study processes of soot formation is considered in the first part of the present study.