Significant NOx Formation at the Tip of Diesel Spray Flames and Its Reduction by Enhanced Mixing in the Tip Region

Similarities in the structure of spray flames suggest that higher fuel injection speeds would reduce NOx emission as the fuel residence time in the reaction zone would shorter. However, in diesel combustion it is commonly known that NOx emissions increase when the fuel injection velocity is increased. The authors have assumed that the mixing time scale is significantly large at the spray tip region where most of the NOx in the emissions is formed. The increase in NOx by the higher injection velocity in engines can be explained as the mixing time scale increases corresponding to the penetration length relative to the nozzle diameter. The purpose of this paper is to confirm this assumption and to show an effective method to reduce NOx emissions based on the analysis. Experiments were made to measure NOx from a jet flame injected in a closed vessel with different injection speeds and injection periods. The results of the experiment and numerical simulation show agreement with the assumptions above. Pre-injection of inert gas and air was attempted to reduce residence time in the spray tip region: the aim was to eliminate the weak mixing zone at the tip and to create a fuel flow similar to a well developed jet structure. The NOx emissions decreased with this method, and the results provide clues to ways of reducing NOx in diesel spray flames.