Effect Of Fuel Temperature On The Internal Flow Characteristcs Of Diesel Injection Nozzle

Improvements in the fuel injection systems of diesel engines can substantially improve the combustion of the fuel and reduce the emission of harmful pollutants. So the scholars over the world have paid great attention to the fuel injection system and done a lot of research on the fuel spray atomization mechanism. After long-term unremitting efforts, people gradually realized that the breakup and atomization of liquid jets are the result of multiple factors, aerodynamic, cavitation and turbulence perturbations are three basic reasons. The current research on the aerodynamic instability is more mature, so further study with the internal nozzle flow is very necessary. At present,the experimental study on nozzle cavitation made remarkable progress:defining nozzle cavitation flow macroscopic structure and its influencing factors. However, nearly all previous experiments of cavitation in injections nozzles have been conducted at fixed temperature. While in the context of realistic engine operation conditions, fuel temperatures can vary from below0℃to over150℃at the injector tip under highload firing conditions. In fact, diesel oil has widely different transport properties such as vapour pressure, viscosity, surface tension and density.This paper analyzes the relationship of the diesel physical parameters such as vapor pressure,viscosity,surface tension and density with temperature changes. Diesel vapor pressure is increased the most with increasing temperature among the four physical parameters, followed by diesel viscosity and surface tension decreased with increasing temperature. The diesel density also decreases with the increase of temperature and the smallest changes in the four properties. Then,this paper discusses the variations of cavitation number, Reynolds number, Weber number, and Ohnesorge number with diesel fule temperature and injection pressure. The increase of the diesel fuel temperature causes a increase of the caviation number because of the increase of the vapor bubble number as the bubble growth rate was determined by the thermodynamic properties. Reynolds number and Weber number are increasing with increasing temperature. The Ohnesorge number curve which incorporates all the main fluid pproperties is simlar with the viscosity and shows that the OH number and injection pressure are independent of each other.Finally, with CFD simulaiton software this paper has selected three injection pressure values and four temperature points to simulate the internal nozzle flow and obtained the following conclusions:First, the increase of the diesel fuel temperature causes the higher levels of cavitation flow and the decrease of the mass flow rate;Second, the increase of the diesel fuel temperature greatly enhances the cavitation development which may be attributed to significantly improve the diesel vapor pressure by the increase of diesel fuel temperature, thereby resulting in more cavition inside the nozzle hole;Third, in non-cavitation flow, increased energy and reduced viscosity were the main reason of the increasing flow velocity in internal nozzle. However, in cavitation flow, the impact on the internal nozzle flow velocity is mainly reflected in the extent of cavitation development. The cavitation development will cause the increase of internal nozzle flow velocity,which may be due to the increase of the levels of in nozzle cavitation largely reducing the liquid phase flow area;Fourth, after the supercavitation flow, changing the diesel fuel temperature has little effect on the internal nozzle flow.