Large Eddy Simulation And Molecular Dynamics Simulation On Fluid Injection Under Transcritical And Supercritical Conditions

In the increasing depletion of global fossil energy and stringent environmental regulations,it is of utmost importance to develop efficient and low-pollution engines.The proposal and implimentation of these new concept engines must be widely related to high injection pressure,high pressure boost,high back pressure and other technical measures.For most hydrocarbon fuels,the critical pressures are mainly distributed in the range of 1.5-3.0MPa,and the spray and atomization environment of the internal combustion engine has approached or exceeded the fuel critical point.Therefore,it is necessary to conduct a significantly research and explore the spray atomization and evaporation process of fuel in the cylinder of the internal combustion engine under transcritical and supercritical conditions.In this thesis,large-eddy simulation and molecular dynamics simulation are performed to deeply understand the transcritical/supercritical jets and interface phenomena,thus we can gain insight into mixing processes between the injected fluids and surrounding gas based on both macroscopically and microscopically.The main work and conclusions of this thesis are as follows:(1)The PISO algorithm was modified to include the real fluid state equations and anormaly transport properties,which are adapted to the transcritical and supercritical jets investigation.The one-dimensional shock tube and two-dimensional nozzles are used to verify the algorithm,and results show that the modified algorithm can be used in transcritical and supercritical injection simulation.Afterwards,Reynolds-averaged simulations of transcritical/supercritical liquid nitrogen jets are carried out,focusing on the effects of cuboidal equations of state,the PR and SRK equations and turbulence models on the jet characteristics.(2)The cryogenic liquid nitrogen injection is simulated by using large-eddy simulation,and main attentions are paid on the effects of transcritical/supercritical injections,pseudo-critical points,and ambient pressure on supercritical jets.Firstly,by comparing the mixing characteristics of transcritical and supercritical jets,it is found that supercritical jet surfaces tend to form unstable vortices,which promote the mixing of the jets with the surrounding fluids,forming a shorter "liquid core" region,so the jets can fasterly reach the self-similar state.Moreover,by studying the relationship between the isobaric specific heat and the density gradient,it is found that the large density gradients are relatively easy to form in the large specific heat capacity regions.(3)Based on the transcritical jet model,a "cold" fluid model without initial density stratification was constructed,and the effects of pseudo-boiling and pseudo-critical points on the supercritical jet were discussed.The results showed that the low temperature jet fluid tends to develop downstream and form larger deni sty areas due to the effects of the pseudo-critical point.At the same time,the pseudo-critical temperature basically corresponds to the region with largest density gradient,indicating that the distribution of temperature,density and density gradient are affected by the pseudo-critical point,but the velocity field and turbulent kinetic energy are little affected.When the fluid temperature transit across the pseudo-critical temperature i.e.pseudo-boiling point,the thermal energy absorbed from environment is mainly used for the expansion of its own volume,rather than increasing its temperature,which is similar to the subcritical boiling point.The traditional thermodynamic critical point is only significant under critical pressure,and should be replaced by pseudo-boiling point under high supercritical conditions.Therefore,for supercritical fluid,the pseudo-boiling point has more theoretical and practical significance than the critical point.(4)The diesel characterization fuel n-heptane injected into supercritical environment is simulated by large eddy simulation.The effects of critical properties,thermal properties and transport properties on the characteristics of transcritical and supercritical fuel jets are investigated.Similar to cryogenic liquid nitrogen jets,supercritical fuel jets tend to easier mixing with surrounding fluids and form smaller"liquid core" regions,as well as forming smaller density gradient regions.Due to the "dissolved" effect of the ambient fluid,the pseudo-boiling phenomenon of the mixed fluids is suppressed compared to a single fluid,and the sspecific heat of constant pressure at the pseudo-critical temperature is reduced.The study of the breakup characteristics of the jets shows that the transcritical jet has a "liquid" core zone,while the supercritical jet moves directly into the "gas" dense core zone and its interface thickness is relatively great.Through POD analysis of the transcritical/supercritical flow field,it is found that supercritical jets are more likely to generate vortex structures,and the distribution range of vortex structures is relatively wide.Under supercritical jet conditions,vortices are easier to form and develop radially.(5)Based on the OPLS-AA full-atomic potential function,the vapor-liquid-gas box model is used to study the evaporation characteristics of n-heptane under transcritical/supercritical and different ambient pressure conditions.The results show that with the increase of the nitrogen ambient pressure,the gas-liquid energy exchange increases and a larger mixed layer thickness is formed.Compared to the transcritical environment,the nitrogen molecules in the supercritical model can diffuse to the liquid phase earlier,so the temperature of the liquid phase rises faster,and a uniform supercritical fluid is quickly formed.At the same time,non-equilibrium molecular dynamics simulations of the liquid-phase n-heptane injection into the subcritical/supercritical environments are performed based on the united-atomic potential function.The results show that the gas-liquid interface gradually widens with the increase of the ambient pressure,and a continuous transition between liquid phase and gas phase appears.The interface thickness based on the density curve broadens with the increase of the ambient pressure,which is consistent with the conclusion obtained by using the interface thickness defined by the density gradient in the macro scale simulation.