Numerical analysis of heat transfer between a wall and diesel spray: Effect on ignition characteristics

The heat transfer between the wall and spray has an important effect on the in-cylinder ignition characteristics of the diesel engine, especially during cold start conditions. To better understand the effect of heat transfer between the wall and spray at different wall temperatures on the ignition characteristics, a numerical analysis was conducted with a large-eddy simulation (LES) based on optical experiments. The results show that the spray/wall interaction led to a smaller Sauter mean diameter (SMD) and more small and large droplets compared to the free spray. Furthermore, the total surface area of droplets increases, and heat transfer between droplets and the surrounding hot gas is promoted, which is conducive to the ignition process. On the other hand, when the wall temperature is low, the spray is cooled by the wall during impingement, thus inhibiting ignition. As a result of this competing effect, the air entrainment rate increases with the increase of wall temperature due to the cooling effect weakening and gradually changing to a heating effect. However, when the wall surface temperature increases to a high temperature, the heating effect is suppressed and the local heat transfer coefficient decreases due to the Leidenfrost effect. It is worth emphasizing that the Leidenfrost effect only occurs near the stagnation point in this study, and the heat transfer between the wall and the vapor-phase spray still impacts the ignition process, resulting in a shorter ignition delay time (IDT) as the wall temperature increases. Interestingly, the low-temperature reactions of both the free spray and spray/wall interaction cases started at the same moment, and the transition period from the low-temperature ignition delay time (τ₁) to IDT shows a significant difference, indicating that the interaction between the wall and spray primarily affects the heat accumulation of the low-temperature reaction (LTR).