Similarity Analysis Of Diesel Spray Mixture Formation Process

In the development process of the large marine diesel engine,it can be much difficult to use the same size of the single-cylinder engine for bench test.Therefore,the ability to accurately predict the engine's performance by an existing model would be very meaningful.Based on similarity theory,the present paper deduces dominant dimensionless numbers in the diesel spray combustion process and proves the similarity of diffusion combustion,premixed combustion and spray process,respectively.Conditions for some parameters including engine speed,injection duration are also established to realize the similarity of different size engines.Three commonly-used scaling rules are then derived from those conditions.It is also deduced that the scaling rule that keeps the same engine speed is more preferable when premixed combustion is remarkable.Based on the long tube injection rate test system,two injectors with different diameters are tested to validate that all three scaling rules can realize the similarity of injection rate.Furthermore,a Z-type experimental setup for the high-speed Shadowgraph imaging technique is established based on the constant volume vessel.Similarities of non-evaporating free spray,wall-impinging spray and split-injection are compared under the above three scaling rules,respectively.The theoretical analysis and experimental results reveal that the scaling rule which keeps the same injection pressure is more preferable for the prediction of spray angle,penetration and excess air ratio of the large nozzle,while the other two scaling rules with reduced injection pressure result in narrower spray angle and longer spray tip penetration than the large nozzle.Although the spray tip penetrates slower and impinges upon the wall later,the large nozzle and the pressure rule have the higher rebound height and rebound radius than the other two rules.With respect to the split-injection,pressure rule is more favorable for the short pulse spray in the pilot period and post period,especially for spray characteristics after the end of fuel injection(EOI).