Study of Diesel Spray Development from Mixture Formation and Evaporation to Initial Flame Development

Abstract

This study investigates effects of evaporation and mixture formation of diesel spray on ignition and initial flame development. Spray structures injected from a single-hole injector were observed by a shadowgraph technique using a constant-volume spray chamber. The spray chamber was set under the condition of temperature at 900K and pressure at 4MPa similar to the atmosphere in a real diesel engine. Analysis of the observed spray images shows that combustible mixture tends to be formed early at the midstream region. Evaporation at the inside of the spray is promoted at the midstream as well as the spray boundary including spray tip. Direct photography measurement reveals that OH radical first appears at the midstream region. High injection pressure enlarges the fuel vapor area and moves the OH radical early to the downstream region. Initial luminous flame development that is thought to be luminescence of soot can be observed at the center region of the spray downstream under the condition of low injection pressure of 100MPa; on the other hand, the initial luminous flame tends to develop at the outer region of the spray for high injection pressure of 140MPa. The equivalence ratio distribution obtained by the tracer-LAS measurement can explain the relation between the mixture formation and the initial luminous flame development. Analysis of the micro-scale spray images provides that the difference of the initial flame development depending on the injection pressure is partly caused by the droplet distribution of the spray. Large-size droplet tends to be distributed at outer of the spray boundary for higher injection pressure. Fuel vapor area is easy to expand outward for higher injection pressure. Under the condition of lower injection pressure, mixture formation at the inside of the spray is delayed; accordingly initial luminous flame develops at the inside of the spray.