Gas Mass Transfer In Uniform Network Model Of Porous Media Based On DSMC Method

The study on gas mass transfer in porous media involves extensive applications, such as gas-solid catalytic reaction, molecular sieves, porous membrane gas separation, gas adsorption, micro-reactor, etc. The pore tructure has great influence on gas flow and mass transfer in porous media, it becomes an important research aspect that how to model the pore structure of porous media efficiently. In this thesis, the framework of modeling gas flow and mass transfer in porous media is established in the transition region. Based on uniform pore network model and direct simulation Monte Carlo(DSMC) method, the basic "Node-Bond" model is studied intensively.In the first part, DSMC method is applied to study the general characteristic of microscale gas flows. The MFRBDSMC program can be used for simulations of forward/backward step facing flows, micro-nozzle flows, gas mixing, gas flows and mass transfer in uniform pore network structure of porous media, etc. The perturbation analysis is employed to solve two-dimensional Navier-Stokes equation with slip boundary condition, and function expressions of velocity, pressure and mass flow rate are derived. By compared with these perturbation analysis solutions of different typical slip models, the ability of predicting micro-flow is analyzed contrastively, and these results manifest that the Cercignani second slip model agrees with DSMC results optimally. When Kn_o number exceeds 0.052, the general characteristic of microscale gas flows begin to appear, such as the velocity at wall is no longer zero and increases along the flow direction; the pressure profile along the flow direction is nonlinearized by compressible effect, and linearized by rarefied effect; the mass flow rate raises with the increase of Kn_o number due to the rarefied effect. In addition, adopted the velocity profile data of DSMC simulation results in the slip flow regime, a new slip model is presented, which has a good agreement with DSMC simulation results as Kn_o number within 0.254.The "Node-Bond" model is a basic unit, which can constitute the gas mass transfer model of porous media uniform pore network model. Based on the correction rarefaction coefficient method proposed by Bekok et al. and micro-channel DSMC simulation results with Kn_o number ranging from 0.24 to 47.60, the function expression of "Bond" is derived. Several cross-shape micro-channel flow cases are simulated by DSMC program, and their cross sections are treated as "Node". As a result, the "Node-Bond" model is established resorted to the treatment of "Node" and nondimensional equivalent length(L'e) which accounts for the effect of cross section. Under these different conditions of pressure difference, gas species, height-length proportion, etc., these results calculated by "Node-Bond" model are compared with DSMC simulation results, and it is shown that their errors are between 3.69% and 12.65%.In the last part, the binary component gas mass transfer process is investigated. The DSMC simulation results of He-Ar indicate that the binary component gas mass transfer process has some other mechanisms except Knudsen diffusion; the molar fraction of Ar is higher than He's along the flow direction except near outlet where the two components vary fiercely, and in consequence, the molar fraction of He is higher than Ar's; the partial pressure of Ar is always higher than He's. Although there are two components in micro-channel, the Ar component which is heavier than He affected hardly by He component, and almost keep the same flow state as pure Ar component. According to above analysis, the gas mixture viscosity coefficient is corrected by DSMC simulation results. The function expression of "Bond" is modified to estimate the total mass flow rate by substituting the corrected gas mixture viscosity coefficient for the pure viscosity coefficient.