Numerical Investigation Of Multiscale Multiple Physicochemical Coupled Reactive Transport Processes In Energy And Enviromental Discipline

Multiscale multiple physicochemical coupled reactive transport processes are often encountered in energy and environmental disciplines.These processes can include several sub-processes involving(single-phase or multi-phase)fluid flow,heat and/or mass transport,charged particle conduction,chemical reaction and solid phase dissolution(melting)-precipitation(solidification).These processes often cover several orders of the length scales and present the multiscale characteristics.Such non-linear non-equilibrium reactive transport processes as well as the complex coupled interactions pose great challenges for developing relevant effective,robust and accurate numerical methods.The multi-scale characteristics further require novel simulation methods that can comprehensively consider coupled reactive transport processes in different length scales as well as the information exchange mechanisms.In the present dissertation,based on four typical science-engineering problems,namely the proton exchange membrane fuel cell(PEMFC),micro reactor,CO2 storage under subsurface and disposal of nuclear waster in salt crystal,six multiple physicochemical coupled reactive transport processes are proposed and investigated.These six processes are widely encountered in natural,scientific and engineering systems and are of great theoretical and application importance.Numerical methods for such multiscale multiple photochemical processes are developed to investigate in depth the reactive transport mechanism at different length scales with particular emphasis being put on the meso-/micro-scales,and the underlying information exchange mechanisms between different scales are revealed.The major contributions of this dissertation are as follows.1.On the background of gas diffusion layer(GDL)in PEMFC,the multi-component multiphase fluid flow in micro porous media is investigated.First,three-dimensional reconstruction technique is developed to reconstruct the porous structures of GDL based on the statistic information of the GDL SEM image.A numerical framework based on the lattice Boltzmann method(LBM)is developed which can consider the multiphase fluid flow,multi-component mass transport and electrochemical reaction at pole-scale.Effects of the GDL anisotropic structures on the mass transport and the current density distribution are explored.Effects of the air flow rate,GDL mixed wettability and the complex structures on the liquid water dynamic behaviors,the residual saturation and the removal time are studied.For the first time the co-existence of capillary fingering and the viscous coupling mechanisms of the liquid water transport are revealed.The desirable wettability of GDL under two different liquid water distributions in the GDL,namely capillary continuum flow and separated droplets/film,are illustrated.2.On the background of the gas channel(GC)in PEMFC,the multiphase flow,multi-component mass transfer and electrochemical reaction processes in micro-channel are studied.Commercial software FLUENT is adopted,in conjunction with the VOF for tracking liquid-gas interface as well as user defined functions(UDF)for boundary conditions and anisotropic transport coefficient,to study the effects of operating condition,GC surface wettability and the surface micro structures on the GC pressure drop,concentration distribution and the current density distribution.Liquid water film spreading on the GC top wall is identified as the most desirable flow pattern in the GC based on overall evaluations of current density magnitude,uniformity of current density distribution and pressure drop in the GC.Improvement to GDL structure is proposed to promote the formation of the desirable flow pattern and alleviate the water-flooding phenomena.Effects of GDL surface microstructures on the dynamic behaviors of a water droplet are investigated and the desirable GDL surface structure is directional distributions of carbon fibers in the flow direction which can reduce flooding in GC.Furthermore,a 1-D force analytic model for droplet is developed,which takes into account the effects of microstructures of GDL surface as well as the surface tension force due to droplet deformation and can predict droplet detachment size under different GDL contact angles and air flow rates.3.On the background of the process in a micro-reactor,the coupled fluid flow,heat transfer,mass transport and surface reaction in micro channel are explored.A general numerical scheme based on the LBM is established to study the reactive transport in a posted micro reactor.Effects of several operating and geometrical conditions including NH3 flow rate,operating temperature,post size,post insert position,post orientation,post arrangement and post orientation on the coupled physicochemical thermal processes are assessed.The possibility of adopting such posted micro-reactor as the power source for the PEMFC is considered and the design principle for the micro-reactor is proposed.4.On the background of Liesegang precipitation patterns,coupled diffusion,surface chemical reaction and nucleation and crystal growth mechanisms are studied.A mesoscopic model based on the LBM is proposed to simulate the formation of Liesegang precipitation patterns.Nucleation model is established on the basis of practical nucleation process,and subsequent crystal growth model is constructed which can predict a variety of crystal growth processes.In conjunction with LBM mass transport model a pore-scale model is developed which can take following aspects into consideration: mass transport,isotropic and anti-isotropic chemical reaction,nucleation,crystal growth and variation of solid structure.A wide range of precipitation patterns is predicted under different gel concentrations,including regular bands,treelike patterns,and transition patterns.For the first time the effects of colloid on Liesegang precipitation patterns are studied with numerical models,and the width exponent in the width law is more accurately predicted.5.On the background of CO2 storage under subsurface,the fluid low,mass transport,chemical reaction and fluid-solid interface coupled dissolution and precipitation processes are investigated.A mesoscopic physicochemical model and numerical methods for dissolution and precipitation processes are developed which can consider the interface coupled dissolution-precipitation driven by the chemical reaction dynamics.The microscale nonlinear dynamic evolution processes for one-phase dissolution and two-phase precipitation are obtained through simulation.Effects of dissolution and precipitation reaction kinetics,molar volumes of primary and secondary minerals,initial powder size and surface roughness of the primary mineral as well as nucleation and crystal growth mechanisms on the dissolution and precipitation processes are investigated.Four types of dissolution and precipitation processes are identified.Particularly,the micro-mechanism of the pseudomorph phenomenon,which widely exists in nature,is investigated in detail.Validation and modification to the existing model for the pseudomorph phenomenon based on experimental results are performed.6.On the background of disposal of nuclear waster in salt crystal,coupled multiphase flow with phase change,heat transfer,mass transport and dissolution-precipitation are investigated.A pore-scale model combined with LB multiphase model,the mass transport LB model and dissolution-precipitation model is developed.Care is taken to handle information on computational nodes undergoing solid-liquid or liquid-vapor phase changes to guarantee mass and momentum conservation.A general LB concentration boundary condition is proposed that can handle various concentration boundaries including reactive and moving boundaries with complex geometries.The pore-scale model is finally developed to study the thermal migration of a brine inclusion in a salt crystal.Multiphase reactive transport phenomena with phase transitions between liquid-vapor phases and dissolution-precipitation processes of the salt in the closed inclusion are simulated and the effects of the initial inclusion size and temperature gradient on the thermal migration are investigated.7.On the background of PEMFC and micro-reactor,a multi-scale modeling framework combining finite volume method(FVM)and the LBM is developed.Concentration reconstruction operator and a general scalar variable(including velocity component,temperature,concentration,electric potential,etc.)reconstruction operator are derived for transforming macro variables to corresponding distribution functions in LBM.The multi-scale modeling framework is adopted to investigate multi-scale fluid flow,concentration transport,proton conduction and electrochemical transport reaction,in PEMFC.In addition,the coupled fluid flow,heat and mass transport,and surface catalyst reaction in a micro-reactor with walled catalyst are investigated.The feasibility and accuracy of coarse(FVM)and fine(LBM)grid system in the coupled region are examined,and the factors affecting the maximum coarse-fine grid ratio are studied.