Atomisation and Combustion Studies of Diesel Sprays
This thesis addresses two potential problems facing investigators involved in Diesel engine development: fuel atomisation and soot formation. A particular concern has been cavitation in nozzles and its influence on the spray and combustion, investigated through impingement studies, scaled-up transparent nozzle studies and evaluation of hydro grinding effects in a real-size nozzle. Besides cavitation, air dilution effects on soot formation were studied using Laser Induced Incandescence (LII) and two-colour pyrometry.
A method was developed to study the instantaneous fuel jet momentum and nozzle discharge coefficient for a non-stationary injection process. The time-resolved fuel jet momentum and nozzle discharge coefficient provides information about the transient phenomena taking place during the injection period due to cavitation. Scaled-up transparent nozzles were used to observe the flow structure within the nozzle hole and to evaluate its effect on the jet emerging from the nozzle flow. The asymmetric distribution of cavitation discovered within the hole had a strong influence on the spray pattern. Under realistic engine conditions, two nozzles each with a hole along the nozzle axis and the same momentum distribution in time, but different inlet geometries, were chosen to study the cavitation effects on combustion. Even though there were differences in the internal turbulence and nozzle discharge coefficient due to cavitation, no major differences were observed in spray angle, penetration, ignition delay, flame structure, temperature or soot concentration between these two equivalent nozzles.
LII measurements revealed that soot formation occurred on the inside boundary of the flame periphery where the temperatures are high and oxygen concentration is depleted. In addition, no soot formation was observed initially in the central core of the spray. During later stages, soot production in the interior of the flame caused the soot concentration to be higher in the central region close to the tip of the flame. In diluted environments, the effective ambient oxygen concentration available for reactions is lower, so the reaction rates and flame temperatures are reduced, which delays the first appearance of soot near the nozzle tip and decreases the overall soot formation. However, due to lack of oxygen, the soot that is formed is not effectively oxidised in the later stages, particularly close to the flame tip where very low temperatures were observed.