Construction And Application Of Subcritical And Trans/Supercritical Homogeneous Jet/Spray Models

Accurate prediction of the fuel atomization process and spray structure in the internal combustion engine is an important prerequisite to the prediction of the combustion process and emission performance characteristics.Therefore,research on spray models has always been a hot topic;In addition,with the application of new combustion concepts and supercharging technology in recent years,the pressure in the cylinder is increasing,and the research on the trans/supercritical spray and mixing has become a new hotspot.In this paper,an Eulerian homogeneous model is used to study and explore the subcritical and trans/supercritical jet spray models.Main work completed in this thesis is as follows.(1)Construction of a subcritical homogeneous Euler-Lagrangian spray model and simulation of the spray in the cold state.Based on the Eulerian homogeneous model,a Euler-Lagrangian spray model was added to the open source CFD program OpenFOAM,thus,the spray calculation models in the program were expanded.The Eulerian near-field characteristics of the cold ECN spray experiment were simulated with the constructed new model.The differences in the calculation of spray flow and diffusion between different RANS turbulence models were analyzed,and the validity of the model was demonstrated.The calculation results show that for the high Reynolds number and high density difference sprays,the HDR(High density ratio)turbulence models which considered the density difference correction in the source term,can correctly simulate turbulence effects and diffusion between gas and liquid.In addition,compared with the experimental results,it is found that the prediction accuracy of the model is higher when the C?1 is taken as 1.6.In order to solve the discrete phase droplets,the gas-liquid interface area density transport equation was added in the Euler model program to establish a complete Euler-Lagrangian model.The results of a large eddy simulation show that,both the flow characteristics in near-field and the droplets distribution in the downstream of spray can be effectively described.(2)Construction of a trans/supercritical jet spray model algorithm.For the trans/supercritical jets,a new real fluid CFD solver was developed using the homogeneous model.The focus was on the coupling and interaction between thermodynamics and fluid mechanics.In the aspect of thermodynamics,the cubic equations of state were used to solve the internal energy,enthalpy,specific heat and transport coefficients of the fluid,and the density was obtained by directly solving the compression factor equation.In the aspect of fluid mechanics,the variable-density compressible Navier-Stokes equations were solved.The PISO algorithm was used to solve the coupling of velocity and pressure,and the pressure Poisson equation in the PISO algorithm was modified.Also the energy equation and the pressure equation were solved together.Finally,thermodynamic properties and a case of one-dimensional flow were validated respectively.(3)Simulation studies of transcritical and supercritical jet sprays.Numerical simulation of transcritical and supercritical jets were performed using the newly constructed solver.Compared with the experimental and Tim's simulation results,the current model is more accurate.The comparison between different transcritical jets shows that the difference in the initial temperature of the jet does not result in differences in the nature of the jet.The existence of pseudo-boiling under transcritical jet conditions results in the formation of a large density gradient,which produces an effect similar to a "solid wall" that inhibits turbulent diffusion and the development of mixing layer.However,under the supercritical jet,as the initial temperature of the jet has exceeded the pseudo boiling point temperature,the density change in the mixing layer was mainly caused by the temperature rise,so that the density gradient was smaller.These speeds up the mixing rate of the fluids,making the jet reach a self-similar state earlier.In addition,a comparison between the PR and SRK equations of state shows that the PR equation has higher accuracy for transcritical jet under the conditions when the chamber pressure is slightly above the critical pressure.For the supercritical jet or the transcritical jet with a higher chamber pressure,the accuracy of the PR and SRK equations tends to be consistent.(4)Thermodynamic analysis of pseudo-boiling phenomenon and the determination of pseudo-boiling range and "upper critical point" of pseudo-boiling behavior.Studies have shown that the transcritical pseudo-boiling phenomenon is a finite continuous process that varies with temperature and pressure.The strength of pseudo-boiling is directly related to the supercritical environmental pressure;the higher the pressure,the weaker the pseudo-boiling strength.That is,the Widom line is not infinitely extended,but a line segment of finite length;its starting point and ending point can be called "lower critical point" and "upper critical point" respectively.The "lower critical point" is the usual thermodynamic critical point(Pr=1);the "upper critical point",for conventional gases,occurs approximately at 3.5 times relative pressure(Pr=3.5),and for hydrocarbon fuels,approximately at 4 times relative pressure(Pr=4).At this "upper critical point",the density difference between the two sides of the Widom line tends to zero,the peak phenomenon of the constant pressure specific heat disappears,and the structure latent heat for the volume expansion in the pseudo-boiling also tends to zero.Once the "upper critical point" is exceeded,the constant pressure heating process makes the fluid directly change from a liquid-like state to a uniform supercritical state,and the transition state no longer exists.For fluids in a transcritical state,the theoretical significance and practical value of pseudo-boiling point are greater than the critical point.For the mixing layer,as the pseudo-boiling strength decreases,the damping effect of the"solid wall" is lower,and the thickness of the mixing layer is increased.According to the second law of thermodynamics,the decrease in the pseudo-boiling strength promotes the increase of entropy generation and accelerates the mixing of the injected fluid with the surrounding warm gas,making the mixing layer gradually retracted toward the nozzle.