Investigation On Soot Formation Of Laminar Co-Flow Diffusion Flames

The diffusion flames in micro-gravity have different features from the counterparts in normal gravity. It is very important to study these features for the understanding of combustion fundamental processes and verifying the existing combustion theories and developing new ones and improving fire safety in spacecraft. Taking the candle flames and those laminar diffusion flames as examples, the paper investigated diffusion flame features experimentally.In the first chapter, the significance of micro-gravity combustion was introduced, and the existing studies on candle flame and in laminar diffusion flame, as well as the challenges and opportunities in those regions were reviewed. On the basis of the above work, the major aims of the paper were presented.In the second chapter, the experimental investigation analyzes the soot formation in co-flow diffusion flames using the laser light full-field extinction measurements. A standard co-flow configuration constructed with two concentric tubes is used in this study. The fuel stream ejects from an inner nozzle, and the oxidizer is supplied through an outer tube. The fuel used in this paper is the acetylene. The interaction between the flow and soot processes and the effects of oxygen contents in the oxidizer stream on the soot formation are studied with the flames operating at normal ambient pressure.To examine the soot formation effects of oxygen concentrations on the oxidizer, five flames with respective oxygen concentrations of 15%, 21%, 30%, 50%and 80% are used. In order to study the interaction between the flow and soot processes, the fuel inflow and oxidizer inflow are controlled and metered using mass flow meters. The flow rates of the fuel are varied from 3.8 cm/s to 16.4 cm/s. The results indicate that the oxygen concentration and the fuel flow rate have effects on the soot volume fraction value and the radial distribution. When oxygen concentration increase the soot volume fraction peak value and flame height decrease but the radial distance increase. While the fuel flow rates increase the soot volume fraction and the fuel height increase. In the third chapter, the candle flame in low ambient pressure was studied experimentally. The experimental results show that shape of candle flame in the low ambient pressure is different from that in normal ambient pressure. In the normal ambient pressure environment the shape of candle flame is upward feather shape, but in the low ambient pressure it is hemispherical. The study showed that the candle flame shape is controlled mainly by the aerodynamic characteristics of the flow field near the candle, so that the diminishing of naturalconvection in low air pressure is responsible for the hemispherical shape of candle flame. The result of experiment in the low ambient pressure and normal gravity environment show that the candle flame becomes dim blue and hemispherical, when the ambient pressure diminished to below 0.2atm. This indicates that the low pressure may not only affect the chemical reaction rate, but it also diminished the effect of buoyancy. So in low ambient pressure we can get some characteristics that exit in low gravity environment. The study of flame color and flame shape reveals the mechanism of soot formation. In the fourth chapter, results and conclusions are given and further work is planed.