Numerical Simulation On Diesel Engine Flame Lift-off Length Based On Kiva-3v Software

The high particulate matter emission of direct injection diesel is an important factor to restrict the development and is also the focus of many researchers. A recent study indicates that the flame lift-off length of direct injection diesel has an important impact on the information progress of Soot through affecting the mix of fuel and air. The related parameters of diesel engine will also affect the flame lift-off length.By using KIVA-3V three-dimensional simulation software, the effects of diesel parameters on the flame lift-off length are studied in steady-state and unsteady-state conditions. Firstly, the three-dimensional model is built based on the parameters of the constant volume combustion bomb and the optical engine in Sandia national laboratory, and its validation is also validated by contrast with the experimental data. Then the changes of lift-off length are simulated in different conditions such as the initial temperature, the initial pressure, the diameter of injector nozzle hole, the injection pressure and the fuel temperature. Furthermore , the flame lift-off asymmetry in the optical engine is defined and studied; an evaluating coordinate system is built which contains two planes and one angle, the corresponding relationships between the flame lift-off asymmetry and angle of nozzle, turbulence intensity and oxygen mass fraction are obtained.Some conclusions can be made from the simulation results. The flame lift-off is ax-symmetric in steady-state condition that is in the constant volume combustion bomb or the optical engine without swirl. Initial environment temperature and density increases would reduce the flame lift-off length, and then reduce the entrained air rate. The temperature dependence of the flame lift-off length is greater than the density dependence, and the changes of the flame lift-off length are nonlinear. The flame lift-off length increases with the increases of diameter of injection hole and the injection pressure, and reduces with the increase of fuel temperature, and an estimated formula about flame lift-off length in steady-state condition is obtained by simply fitting using origin software. At last, the discord of variation trend between entrained air rate and lift-off length with these parameters was found.The flame lift-off asymmetry is evident in the simulation of optical engine. As the swirl rate increases, both the flame lift-off length and its asymmetry increase at first and then decrease, which indicate that a proper swirl rate would increase the entrained air rate. Increasing the angle of nozzle will increase the flame lift-off asymmetry and impede the flame propagation in the side near the wall which is harmful to the combustion. The standard of using the isotherm of 2100K as the edge of flame lift-off length is not suitable when the initial oxygen mass fraction is very low.