Investigation On The Mechanism Of Cavitation And Gas Ingestion In The Diesel Nozzle With A Mini-sac At The End Of Injection

As one of the most thermal efficient power sources,diesel engine is commonly used in different applications such as agriculture,industry machinery,and transportation and other fields.With the development of industry,a serious of strict emission regulations have been promulgated accordingly to reduces atmosphere pollution.Saving energy and reducing the emission of harmful substances is one of important problems that people have to face.As the upstream boundary of diesel engine fuel injection,the quality of fuel atomization not only affects the dynamic performance and economy performance of diesel engine,but also affects its emission performance.At present,a larger number of experiments and numerical simulations indicate that fuel atomization is affected by the external flow field factors such as aerodynamics in addition to the internal flow field of the nozzle.Therefore,it is very important to study the internal flow field of the nozzle.With the extensive application of multiple injection technology in diesel engines nozzle,the specific gravity of the injection fuel at the non-stable stage increases accordingly.Therefore,the dynamic process of fuel injection during the needle value opening and closing stage deserves further study.The injection conditions and nozzle structures are widely adopted to study the process in no-stable fuel injection.However,there are rarely reports on cavitation and gas ingestion in the diesel nozzle with a mini-sac,especially in the process of end of injection.Based on the existing conditions,a visual experimental platform for transparent nozzle is built by ourselves to collect the visual images of flow fields in the diesel nozzle with a mini-sac at different injection conditions through a method that the high-speed camera combined with backing-image technology.The experimental results indicate that with the increase in orifice diameters and mass flux,cavitation is more likely to occur in the diesel nozzle with a mini-sac.Meanwhile,cavitation and gas ingestion occurred in the orifice and in the sac in turn at the end of injection.And the maximum cavitation and gas ingestion in the nozzle are mainly determined by the cavitation and gas ingestion in the sac.Cavitation gradually extends to the orifice outlet along the wall surface of the orifice.The low pressure zone in the nozzle is the precondition of Cavitation,which leads to the low pressure environment in the nozzle mainly caused by throttling,fuel stretching,fuel inertia outflow and so on.In addition,with an increase in mass flow and orifice aperture decrease,the incipient velocity of gas ingestion in the nozzle is faster,the longer the gas ingestion bubble in the orifice is when it is standing still.However,it takes a long time for the gas ingestion bubble to fuse in the nozzle under the action of surface tension.Based on the calculation platform of OpenFOAM in the Linux system and volume of fluid(VOF)method,a two-phase three-component(liquid-vapor-gas)multiphase flow solver with cavitation model in the nozzle is constructed.In addition,the reliability of this model is verified by Such’s quasi-two-dimensional nozzle experiment.consequently,the effect of different Mesh resolution,mass flux at fuel inlet on cavitation and gas ingestion in the nozzle were simulated.Based on the fuel volume fraction,pressure distribution,streamline distribution,velocity distribution,Q isosurface and vapor isosurface nephogram in the nozzle,the variation of cavitation and gas ingestion in the nozzle was analyzed.It is found that vortex cavitation is dominant in the sac at the end of injection,and with an increase in mass flow,the vortex core in the sac is decomposed into several small vortex cores.The cavitation zone gradually extends to orifice outlet along the wall of the orifice.Finally,the variation of cavitation and gas ingestion in the nozzle at the end of injection is analyzed through changing mass flux rate of fuel inlet,aperture and grid resolution.The results indicated that the higher mass flow rate and aperture,resulting in the more cavitation and gas ingestion.The larger the grid resolution is,the greater the density of the Q isosurface and the vapor isosurface in the orifice is.Cavitation collapse is one of the important factors causing gas ingestion,and the maximum cavitation determines the maximum gas ingestion volume.