NOx Reduction in Diesel Combustion by Enhanced Mixing of Spray Tip Region

In diesel combustion it is commonly known that NOx emissions increase when the fuel injection velocity increases. On the other hand, increased fuel velocity reduces NOx in steady jet flames due to a decreased residence time in the flame region. To answer this contradiction, the authors have made variety of experiment and numerical simulation. The results indicated that the large NOx formation in diesel engine is due to the weak mixing intensity in the spray tip region, where the flow and turbulence structure is quite different from the continuous jet flames. The fact indicates that there is a possibility of reducing NOx from diesel engines by enhancing mixing intensity at the spray tip region to the level of continuous jet flame. As one of the attempts to make the velocity profile of diesel spray similar to the steady jet, an inert gas was injected prior to the fuel injection in a model apparatus in atmospheric pressure condition. The result showed that the flame apparently became less luminous by the pre-injection of nitrogen, and the NOx emission index was two-thirds of the non pre-injection case. Numerical simulation also showed the effect of pre-injection for the reduction of NOx. The paper presents the experimental and numerical simulation results together with photographic analysis of enhanced mixing of spray tip region when water was injected as the pre-injection for increasing mixing.