Lattice Boltzmann Simulation Of Complex Flow And Heterogeneous Chemical Reaction In Porous Media

C/C and C/SiC composites are widely used in the aerospace field,such as the hot-end components?like nozzles?of the high-performance engine and the aircraft thermal protection system.In these applications,the composites are usually exposed to the high temperature oxidizing environment,which will suffer from the severe ablation problems.The kind of composites showed obvious characteristics of the porous media before and after the ablation.Therefore,the study on ablation from mesoscopic scale aspect?porous media scale?is essential to the understanding of the ablation mechanism.In this paper,we researched the flow?including multiphase flow?,phase transition and chemical reaction in porous media at pore scale by adopting the lattice Boltzmann method?LBM?.Firstly,we contrasted three LB collision models,namely the lattice Bhatnagar-Gross-Krook?LBGK?model,the multiple relaxation time?MRT?model and the entropic lattice Boltzmann model?ELBM?.The capability and accuracy of these models for simulating flow through porous media were evaluated.Next,we coupled the lattice Boltzmann method and the enthalpy-based method to research the natural convection melting in a square cavity,and analyzed the differences between 2D and 3D simulations by combining Gau&Viskanta's melting experiment.On this basis,we studied the melting process of porous media in a square cavity at pore scale.Then,we researched melting and multiphase flow problem by combining the enthalpy-based method and the multicomponent pseudo-potential model.We improved multicomponent pseudo-potential model with large density ratio,and developed a thermal multiphase flow model on this basis.Finally,we established a thermochemical ablation model of C/C composite based on the lattice Boltzmann method,in which the flow,diffusion,chemical reaction and heat transfer were comprehensive considered.We analyzed the ablation process of C fibers and C/C composite.The studies showed that:The LBGK and ELBM models produced a phenomenon that the permeability is increased by the viscosity,which is not suitable for flow simulation in porous media.However,the MRT model can overcome this defect,and we recommend an optimum combination of the relaxation parameters.For the simulation of the natural convection melting of gallium,the multicellular flow structure is got for 2D cases,yet 3D simulated results show the monocellular structure.The solid-liquid interface shapes and positions obtained from 3D cases are consistent with the experimental data,but 2D results have larger discrepancies with the experimental data.We believe that the correct flow is the monocellular structure for Gau&Viskanta's experiment.The interface appears as irregular shapes in convection-dominated melting of porous media.We find that when the porous medium skeleton melted,which forms the liquid film?or drop?coverage on the non-molten particles in the simulation of the melting of the porous medium and multiphase flow.We improve the multicomponent pseudo-potential model that performs accurately even for density ratio of O?1000?.The spurious current is much smaller than that of literature,and the interfacial tension can be adjusted independently from density ratio.By the simulation of chemical ablation of C/C composite,we find that H₂O leads to more ablation than CO₂.Take no account of the temperature source term,increasing the inlet temperature,inlet velocity,inlet concentration and chemical reaction rate will accelerate ablation.In the process of ablation of C/C composite,C matrix is faster consumed,resulting in surface roughness.Increasing the density ratio of C fibers to C matrix,the roughness of composite surface becomes larger.Through this research,it shows that the lattice Boltzmann method can be used for studying ablation of C-based composites at mesoscopic scale,which provides the idea and lays the foundation for the further study of ablation mechanism.