Investigation On The Adhered Fuel Film And Velocity Characteristics Near Wall During Diesel Spray/wall Impingement Process

As the state-of-art technology of IC engines,high pressure common rail system has been widely used in diesel engines.To achieve advanced combustion modes such as HCCI?PCCI,early injection is the basic means among many engine technologies.Once early injection strategy was employed in engines,many problem will occur.Due to the relative low temperature and pressure in the cylinder,the spray droplets is easily impinging the piston head or the cylinder wall.But the condition of the flow filed near the wall during impingement process is still unkown.But actually,the flow filed near the wall during impingement affects the air/fuel mixture formation,heat transfer,combustion and emission significantly.So this paper investigate the diesel spray/wall impingement focused on two aspects:(1)In this paper,the spray/wall impingement experiment optic platform was used to investigate the behaviour of adhered fuel after impingement under different conditions(different lube oil film thickness,different wall roughness,different wall temperature).Also,the characteristics of spreading radius and entrainment height during spray/wall impingement was analyzed.The results show that with the increase of lube oil film thickness,the diffusion and merge interaction between spray droplets and oil film become obvious which intensify the phenomenon of the dilution of oil film.The distribution of oil film thickness after impingement also become nonuniform.The increase of wall roughness make the drag force larger gained by diesel spray droplets during the spreading process.As a result,the film thickness at the central region and adhered fuel ratio both increase.The rise of wall temperature accelerates the speed of fuel evaporation adhered on the wall.With the increase of wall temperature,the film thickness and adhered fuel ratio decrease simultaneously.(2)CONVERGE software was used to simulate the velocity field,turbulent kinetic energy distribution near wall and the central angular velocity,the turbulent kinetic energy in the cylinder wall was also investigated under different conditions.The results show that with the increase of the distance of the wall,the local area with larger velocity appears in the velocity cloud at the same time,and the distribution range of the velocity is small and the uniformity decreases.The impingement angle mainly affects the spatial symmetry of the flow field during the process of spray/wall impingement.The decrease of the wall inclination makes the larger value of the velocity field move towards the downstream of the spray.The ambient temperature mainly affects the droplet of the fuel spray physicochemical properties such as fuel viscosity,surface tension and density,the higher the temperature,the velocity at the center of the wall increases,and the distribution of the velocity cloud at high temperature increases;the increase of ambient back pressure increases the kinetic energy of diesel spray.With the increase of environmental pressure,the distribution range of the larger velocity at the wall decreases obviously.With the increase of the impingement distance,the average TKE value in cylinder become larger,mainly because the increase of the impingement distance prolongs the time of free development stage of diesel spray.The decrease of the impingement angle increases the interference and guidance of the wall to the diesel spray.With the decrease of the impact wall angle,the average TKE value in the cylinder increases gradually.With the increase of ambient temperature,the average TKE value in the process of impacting the wall also gradually increases.This is because the absorption energy of diesel spray droplets from ambient also rise;The increase of the environmental back pressure can also increase the average TKE value in the cylinder,mainly because the resistance of ambient air to diesel spray increases with increasing pressure of the environment,diesel spray droplet movement increased,so that the average tank TKE value increased gradually.The decrease of the wall inclination increases the angular velocity in the Z axis.The increase of the ambient temperature and the increase of the ambient backpressure all contribute to the increase of the average Z-axis angular velocity in the cylinder,thus enhancing the mixing of the spray droplets and the air.