The Numerical Simulation Study On Transient Cavitation Flow Characteristics In Diesel Engine Injector

For diesel engines, the quality of fuel spray directly affects cylinder combustion process. Good atomization quality effectively raises fuel efficiency, thus improves fuel economy and reduces emissions of pollutants. Many studies have shown that cavitation inside diesel injector nozzle plays a crucial role for primary atomization. However, this phenomenon will cause cavitation erosion and affect nozzle performance. In general, the study of cavitation has a great significance.The objective of present work is to analyze the influence of nozzle geometric parameters and injection conditions on nozzle internal transient cavitation flow characteristics and out flow characteristics of nozzle with numerical simulation method. Firstly, this study validated the accuracy of physical model by comparing with experimental data of Arcoumanis C. At the same time, the Mixture model, Standard - turbulent model and Schnerr and Sauer cavitation model were determined as more reasonable model for simulating nozzle internal cavitation flow. On this basic, transient simulation was conducted by using CFD platform with dynamic grid technique. The results show that nozzle internal cavitation is closely related to needle valve motion. Cavitation generates rapidly at the beginning of needle valve rising stage. With the increase of needle lift, cavitation weakens sharply, then keeps steady. Moreover, cavitation enhances again at the end of needle valve declining stage.Secondly, this paper studied the influence of nozzle geometric parameters on cavitation. The results show that cavitation is inhibited as the needle maximum lift increases, but the effect is not clear. With the increase of nozzle wall roughness height, nozzle length to nozzle diameter ratio or nozzle entrance circle radius, cavitation is inhibited obviously. At the same time, with the increase of needle valve maximum lift, the mass flow rate, COD(discharge coefficient), average velocity and average turbulent kinetic energy at the nozzle exit will be increased simultaneously. The increase of roughness height makes average velocity and mass flow rate at the nozzle exit declined, but COD doesn’t show a clear change. With the increase of nozzle length to nozzle diameter ratio, average velocity and average turbulent kinetic energy at the nozzle exit will be declined. At the beginning of needle rising stage, mass flow rate and COD are increased when L/D is less than 4, while decreased when L/D is greater than 4. Average velocity, mass flow rate and COD at the nozzle exit all will be increased with the increase of nozzle entrance circle radius, but average turbulent kinetic energy changes little.Finally, this paper studied the influence of injection conditions on cavitation. The results show that the increase of needle valve rising velocity, falling velocity or injection pressure will enhance the cavitation. At the same time, the mass flow rate, COD, average velocity and average turbulent kinetic energy at the nozzle exit all will be increased as the needle valve rising velocity increases, while decreased with the increase of needle valve falling velocity. With the increase of injection pressure, mass flow rate, average velocity and average turbulent kinetic energy at the nozzle exit are greatly increased, but the change of COD is not very noticeable.