Wall Temperature Effects on Ignition Characteristics of Liquid-phase Spray Impingement for Heavy-duty Diesel Engine at Low Temperatures

The effect of wall temperature on the ignition of vapor-phase spray impingement has been studied extensively, but the mechanism of the liquid-phase case is not clear. Thus, the visualization experiment was conducted at wall temperatures of 363–673 K using Mie-scattering, shadowgraph, and direct photography methods in a constant volume combustion vessel. The results reveal that, compared with the vapor-phase wetting-wall, when the liquid fuel impinges on the cold wall, the threshold temperature of the autoignition increases greatly to 363 K. Furthermore, the wall temperature strongly affects the evaporation and fuel-air mixing under the liquid impingement case. As the wall temperature increases from 423 K to 573 K, the enhanced evaporation results in a significant reduction in the spray radius, height, and area. However, the evaporation is inhibited and the spray parameters increase when the wall temperature further rises to 623–673 K. This characteristic may be caused by the Leidenfrost effect, which is the main difference from the vapor-phase impingement. Too low wall temperature (below 520 K) easily leads to misfire and unstable ignition, and the threshold temperature of the autoignition is greatly increased, while a higher wall temperature exceeding the Leidenfrost temperature is also not conducive to rapid and stable ignition. With the increase of wall temperature, the ignition delay first shortens from 4.1 ms to 3.2 ms gradually and then prolongs to 3.6 ms again, while the flame area and intensity first increase and then decrease, and the transition temperature is 573 K. When liquid wetting-wall inevitably appears in a heavy-duty diesel engine operating at low temperatures, it is necessary to raise the wall temperature moderately to ensure stable ignition. However, it should not exceed the Leidenfrost temperature, otherwise, evaporation will slow down and ignition will worsen due to the Leidenfrost effect.