The Similarity Study of the Transient Heat Transfer of Impinging Flames under CI Engine-Like Conditions

The optimization of engine combustion systems based on scaled model experiments can reduce the cost of the development of large-bore marine diesel engines. Illustrating the transient heat transfer similarity of impinging flames would be beneficial to scaled engine model experiments in the development and optimization of large-bore compression ignition engines. In this work, the investigation of the similarity of the transient heat transfer of wall-impinging flames was performed in a high-pressure high-temperature constant-volume vessel. Two different injectors featuring different hole sizes and different flame impingement distances were applied to simulate the diesel spray impinging flames under the large-bore and the small-bore compression ignition engine-like conditions with a geometry similarity ratio equal to 0.7. By varying the injection parameters such as injection pressure and injection duration, the scaling laws based on constant injection pressure, constant engine speed, and constant lift-off length were achieved and examined. Two-color pyrometry was used to record and compare the flame luminosity and the temperature and soot distributions between the large-type and small-type impinging flames. The fast-response thermocouple was installed on the impinging wall to calculate the local transient heat flux through the wall. The similarity ratio of the heat transfer coefficient and transient Nusselt number and Reynolds number using different scaling laws was theoretically verified. The results indicate that different scaling laws show good performance in predicting the spray-impinging flame tip penetration, flame height, and radius of the impinging flame between the large-type and small-type impinging flames. Theoretical analysis was conducted to derive the similarity ratio of heat transfer coefficient. Results show that the scaling rule based on engine speed exhibits great potential for predicting the transient heat transfer through the impinging wall, and heat transfer correlations of the impinging flame under engine-like conditions.