Experimental study on the interactions of wall temperature and impingement distances and their effects on the impinged diesel spray ignition and combustion characteristics

Abstract

This study examines the interactions of wall temperature and impingement distances and their effects on the impinged diesel spray ignition and combustion characteristics experimentally in a modified constant-volume combustion chamber. Four wall temperatures of 570 K, 620 K, 700 K, and 800 K, and four impingement distances that correspond to severe wall-wetting, slight wall-wetting, critical wall-wetting, and non-wall-wetting separately are tested. The results indicate increasing wall temperature can promote auto-ignition and the following combustion process, denoted by shorter ignition delays and higher flame area (FA) and spatially integrated natural luminosity (SINL) values, but the promotional effect generally weakens with the increase of the impingement distance. When the wall temperature is high enough, the spray/wall impingement can accelerate fuel evaporating and mixing with the entrained air, in which case decreasing the impingement distance is positive to auto-ignition and combustion. This trend is basically the opposite in low-wall temperature cases where the cooling effect plays a leading role. Besides, the normal distances from the ignition points to the wall decrease with the increment of the wall temperature, and the changes are more remarkable at lower impingement distances, which means that increasing the wall temperature may enhance auto-ignition and combustion in the near-wall region, probably leading to an enlarged temperature gradient in the boundary layer and strengthened heat transfer.