Numerical and Experimental Analysis of the Wall Film Thickness for Diesel Fuel Sprays Impinging on a Temperature-Controlled Wall

Analysis of spray-wall interaction is a major issue in the study of the combustion process in DI diesel engines. Along with spray characteristics, the investigation of impinging sprays and of liquid wall film development is fundamental for predicting the mixture formation. Simulations of these phenomena for diesel sprays need to be validated and improved; nevertheless they can extend and complement experimental measurements. In this paper the wall film thickness for impinging sprays was investigated by evaluating the heat transfer across a temperature controlled wall. In fact, heat transfer is significantly affected by the wall film thickness, and both experiments and simulations were carried out to correlate the wall temperature variations and film height.

The numerical simulations were carried out using the STAR-CD and the KIVA-3V, rel. 2, codes. Different wall impingement models and liquid film approaches were analyzed and numerical results were compared with measurements available in the literature and with experiments carried out at Chalmers. The conditions for which the simulations were performed correspond to those found during the compression stroke in a diesel engine. The fuel used was a 2-component model fuel (Idea, 70 % n-decane and 30 % α methylnaphthalene).

The experiments were carried out in the Chalmers high-pressure/high-temperature spray rig for non-evaporating and evaporating conditions. The spray chamber was equipped with a temperature controlled wall, including coaxial thermocouples for recording the surface temperature. The time histories of the surface temperatures were used to calculate the local heat fluxes applying a 1-dimensional transient heat conduction model.