Characteristics of the transient heat transfer of impinging flames and correlation analysis using a new characteristic velocity under CI engine-like conditions

Abstract

Understanding the transient heat transfer mechanism of impinging flames is a crucial pathway for further improving the thermal efficiency of already efficient compression ignition (CI) engines. In this paper, the investigation of the transient heat transfer of wall-impinging flames was performed in a high-pressure constant-volume vessel. Fast-response thermocouples were installed in the impinging wall to record the transient heat flux. Two-color pyrometry was employed to estimate the mean temperature inside the flame region. Firstly, the transient heat transfer characteristics were investigated under varied ambient density, oxygen concentration, temperature, and injection pressure conditions. The optical flame velocity calculation method was applied to this extensive dataset to develop a heat transfer correlation between Nu and Re with which transient heat transfer coefficients were calculated and compared with the experimental results. From this analysis, cumulative fuel injection velocity was developed as the new characteristic parameter to characterize the transient heat transfer of the wall-impinging flames. Results indicate that the effect of fuel injection pressure on the heat flux is more significant than that of ambient gas conditions, with higher injection pressure causing higher heat flux through the impinging wall. No obvious linear tendency of transient Nu and Re was found when using the optical measurement results of the wall-impinging flame as the characteristic parameter. Instead, the cumulative fuel injection velocity shows a strong linear trend of transient Nu and Re during the transient heat transfer processes of the wall-impinging flame. Moreover, the heat transfer coefficients from the new cumulative fuel injection velocity well fit the experimental results.