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Multi-Mechanisms Of Gas Transport In Nanoscale Pores Of Coal

Posted on:2020-06-23Degree:MasterType:Thesis
Country:ChinaCandidate:W R LiFull Text:PDF
GTID:2381330620465014Subject:Safety science and engineering
Abstract/Summary:PDF Full Text Request
The pore size distribution of pore network in coal is extensive,with a range of pore sizes varying from nanoscale to microscale.Different from Darcy flow,there are many non-linear gas transport mechanisms in nanopore due to the molecular diffusion effect of adsorbed or free gas with the pore size changing.This results in the coexistence of various transport mechanisms,such as slip flow,molecular diffusion of free gas and migration of adsorbed gas in nanopore.However,due to the unclear understanding of the mechanism of some gas transport mechanisms and the ambiguity of key parameters,there are great divergences in the application of theory model in engineering simulation,even a huge deviation between theoretical calculation and engineering practice.Therefore,it is particularly urgent need for an in-depth study the non-linear transport mechanism in nanopore.The main research work of this paper is as follows:The homogeneous nano-activated carbon was innovatively used to replace the raw coal to carry out the adsorption-desorption experiments under the supercritical condition of methane.Based on the theory of micropore volume filling,an isothermal adsorption model for nano-scale pore was established.Based on the adsorption-diffusion characteristics of methane and the theory of interface scientific adsorption potential energy under supercritical condition,the controversy of the key parameters in the surface diffusion model was solved,and the transport model of surface diffusion of adsorbed gas was deduced.A pore-slit slip flow transport model of gas was established based on the ratio of pore and slit porosity obtained by three-dimensional reconstruction technology of CT scanning.A unified model for gas transfer in nanopores of coupling molecular diffusion model of free gas and surface diffusion model of adsorbed gas was established,the equivalent apparent permeability model of mass flux was deduced accordingly.The pressure differential penetration experiments of nano-scale pore films under different rarefied gases were carried out by using a customed permeability experimental system,and the accuracy of the equivalent apparent permeability model proposed in this paper was verified.Finally,the lattice Boltzmann LBM-D2Q9 model is used to simulate mass transfer in the transition region of nano-scale circular pipe.It not only verifies the rationality of the transfer model,reveals more the law characteristics of gas microscopic non-linear flow.Results show that the supercritical DA isothermal adsorption model is more scientific and accurate in solving the nano-scale pore gas adsorption.The migration and diffusion of adsorbed gas on the pore wall is essentially different from the collision-diffusion of free gas,and its driving force comes from the potential energy gradient.Therefore,the mas transfer model of surface diffusion derived from interface science potential energy theory is more scientific and reliable.Compared with the deduced pore-slit slip flow model of gas in this paper,the traditional circular tube slip flow model of gas will significantly overestimate the mass transport flux.Meanwhile,the diffusion of free molecules driven by concentration gradient dominates the nonlinear fluid transport,and there is no negligible situation.The diffusion transport of free gas dominates in discontinous fluids,there is no negligible case.In theory,the surface diffusion coefficient ranges from 10-14 m2/s to 10-10 m2/s.In a pore with radius of 2nm,the contribution of surface diffusion in total flux can reach to 60%.Compared with the Darcy apparent permeability model,the modified unified apparent permeability model coupled with supercritical environment,adsorption potential energy and material mechanical response is more accurate,can reflects more thoroughly and comprehensively in gas migration and contribution changes of multiple transport mechanisms in nanopore.
Keywords/Search Tags:nano-scale pore, multi-mechanism transport mechanism of gas, supercritical adsorption model, 3D reconstruction technology, apparent permeability model
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