Lattice Boltzmann Simulation Of Flow And Mass Transfer Characteriatics Of CO₂ Absorption In Nanofluids

The greenhouse effect caused by carbon dioxide emission is one of the major problems facing the world today.As the main place for CO₂ generation,CO₂ capture of flue gas after combustion from thermal power plants can effectively alleviate global warming.In the process of CO₂ capture after combustion,the performance of absorbent plays a decisive role in the capture efficiency and energy consumption of the system.Therefore,it has great significance to study the low energy consumption absorbent and its flow transfer characteristics in the process of CO₂ absorptionNanofluid consists of solid particles and base liquid.As an absorbent,it can combine the selective adsorption characteristics of solid with the absorption fluidity and stable kinetic properties of base liquid,and has a good application prospect in the separation and purification of CO₂ gas.However,the transfer characteristics of nanofluids are very complex,and the theory of mass transfer is not mature.Therefore,in this paper,the flow transfer characteristics of nanofluids in the process of CO₂ absorption were studied,including lattice Boltzmann simulation study of Rayleigh convection characteristics in the process of CO₂ absorption by pure solution;A solid-liquid two-phase flow model of nanofluids was established to study the Rayleigh convection characteristics during CO₂absorption by nanofluids;The lattice Boltzmann method coupled with the cellular automata model was used to construct a prediction model for the effective diffusion coefficient of nanofluids in the process of CO₂ absorption,and the enhanced mass transfer mechanism of nanofluids in the process of CO₂ absorption was studied.The main conclusions are as follows:（1）Based on the modified LKS model and the LBGK model,the Rayleigh convection characteristics of CO₂ absorption in standing pure solution were studied.The results showed that the existence of disturbance at the interface is a necessary condition for the occurrence of Rayleigh convection.In the value range of disturbance intensity10^-6<C_D<10^-5,the maximum error between simulation and theoretical prediction of critical time is about 10%;The simulated and experimental results are in good agreement in terms of the structure shape,the number of structures and the development rate of the Rayleigh convection;The mass transfer fluxes of methanol and ethanol in the process of CO₂ absorption were predicted by the small eddy model and the permeation model,the surface divergence model and the hybrid model,the results show that the fitting degree of the hybrid model is more than 0.9,which means that the hybrid model can well predict the mass transfer process of the base liquid absorption of CO₂.（2）The lattice Boltzmann method coupled with the cellular automata probability model was used to establish the solid-liquid two-phase flow model of nanofluid.The Rayleigh convection characteristics in CO₂ absorption process of nanofluid were studied,and the influence of solid particles on the Rayleigh convection was analyzed.The results showed that Brownian motion of nanoparticles intensifies the uneven concentration distribution at the gas-liquid interface,which causes the Rayleigh convection to occur in advance;Rayleigh convection promoted the renewal of nanoparticles at the gas-liquid interface and the surface of the base liquid;The critical time of Rayleigh convection in nanofluids increases with the increase of particle size and decreases with the increase of particle concentration.（3）The prediction model of effective diffusion coefficient in the process of CO₂absorption by nanofluids was established,and the mechanism of enhanced mass transfer in the process of CO₂ absorption by nanofluids was studied.The results show that the simulation results are in good agreement with the experimental results if Brownian motion and grazing effect are both considered in the calculation of effective diffusion coefficient,and there is a large deviation between the simulation results and the experimental results if only a single enhancement factor is considered;The effective diffusion coefficient decreased with an increase in particle size and increased with an increase in fluid temperature;When the particle surface adsorption rate k₁>1,both the instantaneous equilibrium model and the dynamic model are suitable for predicting the effective diffusion coefficient of CO₂ absorption by nanofluids.When the surface adsorption rate of particles is very low（k₁<<1）,the dynamic model is applicable to the prediction of the effective diffusion coefficient of CO₂ absorption by nanofluids.