A Visualization Study of Soot Production and Oxidation Characteristics under Diesel Engine like Conditions 2017-32-0126
To explore the production and oxidation characteristics of soot in the flame of diesel jet under the condition equivalent to the direct injection diesel engine condition, the effect of three different important parameters (including injection pressure, injection duration, and oxygen concentration) are experimentally examined. For these purposes, a small CVCC (constant volume combustion chamber) with the volume of 60cc equivalent to the volume of combustion chamber of automotive diesel engine is used.
To obtain the experimental data of soot production and oxidation, in experiments, the ambient condition of temperature, pressure and oxygen concentration before injection timing are prepared by the combustion of lean hydrogen mixture (with help of 8 spark plugs) at a high temperature and pressure condition around 1000K and 4.5MPa. The common rail type injector with 8 injection holes for modern diesel engine is attached to this vessel. Injection pressures are set up at 80, 100, and 120 MPa. The effects of three important factors are analyzed including: (1) different injection pressures with constant injection mass; (2) different injection pressures with constant injection duration; (3) oxygen concentrations of 21% and 17% in volume. The optical window is attached on one side of this vessel and the full image of the spray flames can be visualized and recorded through this window by a high-speed color video camera. From the visualized color image of diesel flame, the flame temperature and the amount of soot inside the flame can be quantified by the two color method. This method is based on the theoretical principle that the light emission from the soot cloud is a thermal radiation of gray body which is a function of soot temperature and the KL factor, an index of the amount of soot.
The obtained results show that the injection duration and the injection rate are the dominant factors for heat release rate (HRR). On the contrary, within the experimental condition, the heat release rate is affected by both of the injection pressure and the oxygen concentration of ambient mainly at late combustion phase. The injection pressure and the injection duration give small effect on the level of flame temperature; however, the ambient oxygen concentration gives a large effect on the flame temperature.
Concerning the soot production and oxidation, all of the considered parameters have large effect. The interesting result is that the effect of injection pressure and ambient oxygen concentration on the maximum value of total KL factor is not so large. However, the soot production rate becomes lower as the decrease of injection pressure and as the decrease of ambient oxygen concentration, and results in the delayed arrival of the maximum total KL factor. Comparing with the HRR curve, it is revealed that the soot oxidation reaction is occurred under the very low HRR combustion in the case of low injection pressure and oxygen concentration condition.