Numerical Investigation Of Boundary Layer Flow In A Gasoline Direct-Injection Engine

Near-wall turbulent boundary-layer has a substantial impact on the wall heat transfer process in internal combustion engines,and ultimately affects the engine efficiency,pollutant formation and component thermal stresses.However,the heat transfer processes are not well understood because of a lack of information on gas side velocity and temperature distribution.In the present study,the transient velocity and thermal boundary layers in a motored spark-ignition direct-injection engine are predicted using a Delayed Detached Eddy Simulation model.The main contents and conclusions are as follows:In the first part,the temporal evolution of the near-wall ensemble-averaged velocity,fluctuation velocity and dimensionless velocity are analyzed,then the effects of tumble movement on the spatial distribution of near-wall velocity profiles are summarized,the paper focus on the effects of engine speed and tumble ratio on the flow characteristics.The temporal evolution of the ensemble-averaged velocity is significantly influenced by the inlet jet,tumble,moving piston and valves during the intake and compression strokes.The significant changes of velocity fluctuation and Reynolds stress in different directions proved that the turbulence in the near-wall region is anisotropic.The dimensionless velocity profiles agree well with the law of the wall in the viscous sub-layer but deviate from the log layer.The thickness of the viscous sub-layer gradually extends and the dimensionless velocity in log-layer increases as the piston moves upward.The tumble movement causes difference in the flow direction and magnitude between different locations in the combustion chamber,or even leads to partial backflow or vortex structures.Finally,it is validated that the near-wall velocity magnitude increases as the engine speed or tumble ratio increases.In the second part,the temporal evolution of the near-wall ensemble-averaged temperature,fluctuation temperature and dimensionless temperature are analyzed,then the evolution of local wall heat flux,heat transfer coefficient and turbulent heat transfer are summarized,the paper focus on the effects of engine speed and tumble ratio on the heat transfer characteristics.The thickness of thermal boundary layer shows a non-monotonic variation with time during the intake and compression strokes.The dimensionless temperature profiles agree well with the law of the wall in the viscous sub-layer but tend to be uniform in the log-layer.The in-cylinder gas firstly absorb heat from the cylinder head and release heat after 260 CAD,the local wall heat flux increase as the piston moves upward.The heat transfer coefficient decreases before 260 CAD and gradually increases to a peak at TDC.The spatial distribution of turbulent heat flux shows a strong link between the velocity boundary layer and thermal boundary layer,and the peaks located in the buffer layer occupy about 70~85% of the local wall heat flux,which indicates that the forced convection heat transfer is the main heat transfer mechanic in internal combustion engines.Finally,it is validated that the local wall heat flux and heat transfer coefficient would increase as the engine speed up,but both the heat transfer coefficients would decrease as the engine tumble ratio increase.