Study On Modelling Turbulent Wall Heat Flux And Wall Film Heating Inside Diesel Engine Cylinder

In-cylinder wall heat transfer of diesel engine can influence engine performance and reliability,due to the effects of wall heat transfer on near-wall combustion efficiency and on thermal load of combustion chamber structure.Therefore,considering the tendency of diesel engine strengthening nowadays,the study on diesel engine wall heat transfer modelling can contribute to the strengthened engine simulation and design,making new engines have more efficient and cleaner combustion and higher reliability than before.In order to address the key technologies related to milltary diesel engine,in this thesis,(1)anisotropy of engine in-cylinder near-wall turbulence was studied,and new prediction model was established,(2)heating model of engine in-cylinder evaporating wall film was established.Direct numerical simulation solver code was design and written for compressible reacting turbulent flow simulation.Based on OpenFOAM,this solver can calculate turbulence with direct numerical simulation method,predict combustion with reaction kinetics,eliminate numerical error waves with non-reflecting boundary conditions,impose inlet turbulence boundary condition with data mapping method,and initialize turbulence field with artificial vortex evolvement method.The compressible inert and reacting turbulent flow were calculated with our newly-written solver.The inert turbulent flow result was analyzed and compared with published data from the literature,in this way the code was verified and calculation case validated.Based on the inert and reacting turbulent data,the two-point correlation and correlation function in near wall turbulence were calculated,and then the correlation function and length scale in inert and reacting flow were compared.Results showed that a)more notable turbulence anisotropy was found in near wall region of reacting flow than in inert flow,b)turbulence anisotropy was surprisingly found in central region of reacting flow,and it should not have been there following the knowledge from the literature.The inert flow assumption used in original elliptic blending turbulence model was validated in reacting flow,based on inert and reacting turbulence data.The new features of two-point correlation and correlation function in reacting flow were found.Assumptions used in original elliptic blending model was modified with new findings,and new elliptic blending model for reacting flow was developed.The new model was applied in turbulent wall heat flux prediction,as well as the original model and other existing model,results showed that the new model can more accurately predict the turbulent wall heat flux in burnt zone than all others models can.The wall film heat transfer equation was solved analytically,considering evaporation at film surface as boundary conditions for the governing equation.Then a new approach to modelling evaporating wall film heating in engine cylinder was established.The new approach was applied in evaporating film heating calculation,compared with methods from the literature.Results showed that our new approach had great advantages in accuracy over methods from the literature.At last,the performance of methods from literature was analyzed,and limitations of assumptions used in these methods were exposed.