Experimental And Numerical Investigations Of In-cylinder Turbulence Characteristics With Multi-spatial Scales In Gasoline Engines

The turbulence in internal combustion engines?ICEs?plays an important role in the various kinds of physical and chemical processes,and significantly affects the performance of ICEs.The in-cylinder spatially multiple-scale turbulent flow,consisting of large-scale flow,medium-scale coherent structure and small-scale flow with high frequency,makes a complicated contribution to fuel-air mixing,pollutant formation and heat transfer processes.In order to study the spatially multiple-scale turbulence under the highly unsteady,compressible,rotational and anisotropic conditions,the three-dimensional particle image velocimetry?3D PIV?and large eddy simulation?LES?were used in the present study.This work consists of three parts:the steady measurement and evaluation of the tumble flow in a gasoline direct injection engine,the instantaneous three-dimensional flow features at multiple-spatial scales,and the turbulent characteristics and cycle to cycle variation?CCV?under various tumble intensities.The main contents of these three parts are described as follows:For the steady measurement and evaluation of the large-scale tumble flow,a steady measurement method combining the 3D PIV was used to study the distribution of the two-dimensional three-component?2D3C?velocities under different intake valve lifts.The effects of viewing area,pressure drop and measuring plane location on the calculated tumble ratio were investigated,and the reliability of this methodology was confirmed.It is found that the distribution of the out-of-plane velocity is more symmetric,but the in-plane velocity is nearly independent of the increase of valve lift.When the measurement plane moves downwards,the distribution of the out-of-plane velocity near exhaust valves is changed gradually from double peaks to a single peak.Furthermore,with the increasing valve lifts,the tumble ratio varies non-monotonously.Finally,the dependence of tumble ratio on calculation radius is decreased with the measurement plane moving downwards.The sensitivity of the tumble ratio on the pressure drop is the lowest at the measurement plane with distance of 0.5 times of the cylinder bore?0.5D?from the bottom of the cylinder.The mean tumble number obtained from the direct measurement method using 3D PIV is close to that obtained from the indirect measurement method using swirl torque meter at 0.5D.From the aspect of the in-cylinder transient turbulence features at multi-spatial scales,the three-dimensional velocity was obtained by a high-speed 3D PIV,and then the three-dimensional features of the ensemble averaged velocity fields,the fluctuating velocity fields and the transient turbulence features at middle and small scales were studied in detail in the intake and compression stroke.The results show that,for the ensemble averaged velocity fields,an obvious out-of-plane moving tendency is observed in the intake stroke,and the tendency decays later due to the strong tumble flow.In the later compression stroke,the out-of-plane moving tendency is intensified because of the break-up of the tumble flow.The mean kinetic energy and the turbulent kinetic energy increase significantly at a higher engine speed.The mean kinetic energy in different spatial directions changes similarly,while the evolution of the turbulent kinetic energy in the three spatial directions is different as a function of crank angle.The tumble center moves clockwise in the later stage of the compression stroke,and the instantaneous tumble scale is much smaller than the averaged tumble scale.The averaged vortex scale is decreased,but the mean vortex intensity is increased as the engine speed increases further.From the aspects of spatially multi-scale turbulence and its CCV under different tumble intensities,the multi-cycle LES of the cold flow in a non-fire gasoline direct injection engine equipped with variable intake valve system was performed.The LES results were validated against PIV results.A new method to identify coherent structures in in-cylinder flows was proposed,and the turbulence,therefore,could be decomposed reasonably.The effects of the variable tumble flow on the turbulence at multi-spatial scales and its CCV were investigated systematically.The results show that,at low and middle engine speeds?<3000 r/min?closing the tumble flap can enhance the tumble flow,and the maximum tumble ratio is proportional to the engine speed.The stronger tumble flows between 160²80^oCA ATDC results in a larger integral length scale in the vertical direction with its maximum value of 1.93×10^-2 m and the smaller Kolmogorov scale in the later stage of compression stroke.The minimum Kolmogorov scale is2.0×10^-5 m at 30^o CA BTDC.The scale and energy of each subfield decreases successively.At the stronger tumble flow,the energy ratio of the averaged flow increases,the energy dissipation decreases,and the kinetic energy maintains unchanged until the later stage of compression stroke.In the end of compression stroke,an obvious energy cascade phenomenon is observed at different vortex scales,and the medium-scale coherent structure plays a key role in the CCV of the whole flow field.Therefore,the strong tumble flow generated by the variable intake air system can reduce the CCV of the in-cylinder flow at low and middle engine speeds.