Internal Flow Analysis of a Diesel Spray Using Two Stage Injection System

Diesel spray had a lot of fluctuations in itself. They were fluctuations of density and velocity in the spray. They promoted the air entrainment and the dispersion processes of the fluctuations made the mixture for combustion. Then the characteristics of the fluctuations were worth to investigate for better understanding of the diesel spray. In order to investigate the effect of initial fluctuations in the spray on spray characteristics, the two-stage injection diesel spray was used in this research work. The initial fluctuation level in the spray was easily controlled by the change of the injection dwell time of the two-stage injection. Then the effect of the dwell time on the internal flow and breakup length was studied first. Second, the breakup length in the conventional diesel spray was estimated by a special case of the small dwell time in the two-stage injection. Further the auto-correlation and cross-correlation of the spray fluctuations made by the two-stage injection were analyzed by a FFT method. The characteristic length of the mixing process was obtained as the correlation length in the spray. As the results, the internal breakup length in the spray was 30% longer than the breakup length obtained by the spray tip penetration. The minimum correlation length was almost consisted with the breakup length in the spray. The maximum correlation length was about 80 mm. It meant that the largest fluctuation in a spray which was made by an injection control could only maintained within 80 mm from a nozzle tip. In other words, eddies in the spray, even if they had large scale, they could only remained within 3 millisecond.