CMC Model Applied to Marine Diesel Spray Combustion: Influence of Fuel Evaporation Terms

Abstract

This study presents an application of the conditional moment closure (CMC) combustion model to marine diesel sprays. In particular, the influence of fuel evaporation terms has been investigated for the CMC modeling framework. This is motivated by the fact that substantial overlap between the dense fuel spray and flame area is encountered for sprays in typical large two-stroke marine diesel engines which employ fuel injectors with orifice diameters of the order of one millimeter. Simulation results are first validated by means of experimental data from the Wärtsilä optically accessible marine spray combustion chamber in terms of non-reactive macroscopic spray development. Subsequently, reactive calculations are carried out and validated in terms of ignition delay time, ignition location, flame lift-off length and temporal evolution of the flame region. Finally, the influence of droplet terms on spray combustion is analyzed in detail. The effect of evaporation into the mixture fraction variance transport equation was seen to play a prominent role concerning autoignition and flame stabilization: both ignition delay and flame lift-off length are considerably increased when evaporation effects are included. This was found to be attributed to the strong spatial overlap between evaporation and combustion - typical for marine sprays - leading to an increase in the local scalar dissipation rate in the evaporating region. Overall, inclusion of evaporation terms resulted in improved agreement with experimental data. These findings are contrary to previous investigations for typical automotive diesel sprays reporting only a minor influence of evaporation. Consequently, this study constitutes an extension of former analyses to large marine fuel injection configurations and emphasizes the importance of such effects for the simulation of marine diesel sprays.