Level Set Method For Simulations Of Liquid Atomization,Evaporation And Combustion

Liquid atomization,evaporation and combustion are interrelated and interacted on each other,which presents a great challenge for studying the heat and mass transfer as well as chemical reaction over an irregular evolving interface.To this end,the present thesis has developed a series of advanced numerical methods in the framework of level set method,which lay a research foundation for the multi-physical coupling mechanisms of liquid atomization,evaporation and combustion.The conventional level set method suffers from mass non-conservation as time evolves.Based on the previous work,which implicitly coupled the VOF method to introduce a mass remedy procedure,the present thesis has further proposed a high-fidelity,low-cost and massconserving level set method.This method utilizes a sigmoid function to simplify the reconstruction of VOF scalar and therefore can easily obtain the liquid fraction in each cell;assesses the physical significance of the signed curvature and its effects on mass non-conservation to improve the core criterion for distributing the remedy proportion to each cell adjacent to the interface;and optimizes the update of level set scalar by avoiding the iterations and eliminating the error tolerance.Besides,the upwind scheme for curvature is implemented to deal with small and/or thin liquid structures,whose characteristic scale is less than 4 times the mesh size.The method,after numerical validations,has been applied to swirling atomization.The complex evolution of liquid structures from sheets,ligaments to droplets has been successfully captured while the unphysical mass loss has been effectively reduced,demonstrating the good conservation property of the method.Most importantly,the accuray of predicting the interface location has been inproved and the additional CPU cost of the remedy procedure has been cut by 90% so that the method is efficient and suitable for large-scale applications.To couple the effects of interfacial heat and mass transfer,the level set method for simulations of the non-uniform evaporation on irregular evolving interfaces has then been developed.For this aim many difficulties arise,such as the liquid structures are complex and changeable;the distribution of the temperature and concentration fields are heterogeneous;the intense heat release when combustion exists can cause the coexistence of evaporation and boiling.Due to the local Stefan flow generated by the evaporation,advancing the level set function becomes challenging as the velocity is discontinuous and should take into consideration of the Stefan flow.That is,the key lies in developing an interface-resolved model to accurately calculate the local evaporation rate.Therefore,the present thesis has developed a revised heat flux based model(Re HFM),in which the the evaporation rate is derived from the conservation law for the interface and no hypothesis is involved,and such an interface-resolved evaporation model is accurate and applicable.The devloped level set method,afeter numerical vvalidations,has been successfully applied to the non-uniform evaporation of moving droplets,demonstrating that the method has break through the limitations of the traditional point source assumption as the detailed information of heat and mass transfer on irregular evolving interfaces can be obtained.A feature of the Re HFM lies in enforcing a prescribed Robin boundary condition.To the authors' best knowledge,however,there is no finite difference discretization method that can effectively deal with the Robin boundary conditions on irregular evolving interfaces.The present thesis has proposed such a novel finite difference discretization method with the focus on the accuracy and efficiency.Finally,in order to consider the effect of chemical reaction,the present thesis has developed a coupling computational method for evaporation and combustion on irregular evolving interfaces.The combustion process severely consumes fuel vapor and violently releases heat,which will greatly increase the inhomogeneity of the surrounding fields and affect the processes of interface evolution and evaporation.The effects appear as the source terms in the governing equations of temperature and various species concentrations,and induce potential risks on the numerical performance.The present thesis utilizes the one-step global reaction mechanism to enclosure the source terms,the level set method to implicitly describe the gas–liquid interface,and the ghost fluid method to accurately address the jump conditions on the interface.To the authors' best knowledge,this is the first computational method with the capability to handle simultaneous interface evolution,heat and mass transfer and chemical reaction.The method,after numerical validatios,has been applied to simulate the evaporation as well as combustion during droplet collision.It is implied that the irregular interface and the non-uniform evaporation can affect the overall combustion characteristics,which reveals the necessity and significance for developing such a coupling method for simultaneous interface evolution,heat and mass transfer and chemical reaction in the present thesis.