Abstract
Numerical simulations and experiments have been carried out to investigate the effect of fuel injection nozzles on the combustion and NO_X formation processes in medium-speed marine diesel engines. Spray and combustion phenomena were examined numerically using FIRE code. Wave breakup and Zeldovich models were adopted to describe the atomization characteristics and NO_X formation processes. Spray visualization experiment was performed in the constant-volume high-pressure chamber to verify the numerical results on the spray characteristics such as spray angle and spray tip penetration. Time-resolved spray behaviors were captured by high-speed digital camera and analyzed to extract the information on the spray parameters. Numerical analysis of fuel injection system was also performed to get the profiles of fuel injection rate, which should be given as an input data for the combustion analysis. Numerical results were verified with experimental data such as cylinder pressure, ROHR (Rate of Heat Release) and NO_X emission. Finally, the effects of fuel injection nozzles on the engine performance were investigated numerically to find the optimum nozzle parameters such as fuel injection angle, nozzle hole diameter and number of nozzle holes. From this study, the optimum fuel injection nozzle (nozzle hole diameter, 0.32 mm, number of nozzle holes, 8 and fuel injection angle, 148°) was selected to reduce both the fuel consumption and NO_X emission. The reason for this selection could be explained from the highest fuel-air mixing in the early phase of injection due to the longest spray tip penetration and the highest heat release rate after 19° ATDC due to the increased injection duration.
