Mechnaism Study Of Ion Transportion In Porous Media

Study on Alkali Metal Thermoelectric Conversion is an important topic in the emergingenergy field. AMTEC（Alkali Metal Thermal to Electric Converter）is a generating set usingbeta"-Al2O3solid electrolyte characteristics that it serves as an insulator for electronics and agood conductor for ion. The AMTEC device has the advantages of high conversion efficiency,maintenance free and so on. Small change of BASE（Beta"Alumina Solid Electrolyte）performance can significantly affect its energy output. Therefore it is of great importance tostudy the structure and properties of BASE ceramic solid electrolyte and the migrationmechanism of working fluids in porous media.This dissertation is mainly to study on the migration mechanism of AMTEC workingfluid in solid electrolytic ceramic BASE, including BASE microstructure and its conductivemechanism, the affecting factor of sodium ion transfer in BASE ceramic porous media, theinfluence of microscopic shape, grain size, and current density of BASE surface electrode onelectrode performance, flow conditions of sodium ion within the electrode and the situation ofion in anode adsorption and cathode escape.Firstly, studied on BASE model and electrode model for AMTEC using Simulinksimulation method is carried out in this dissertation, including the influence of BASEefficiency on AMTEC and the effect of BASE ions impedance growth on the power output ofconverter. Secondly, in this dissertation a two-dimensional model is selected for numericalsimulation calculation. The micro-Nernst-Planck ion transport model coupled partialdifferential equations are used to describe ion migration in microscopic porous media,physical field coupling analysis software COMSOL Multiphysics is employed to solve themicroscopic model and variations of velocity field, concentration field, potential field,temperature field are analyzed. Finally, the influence of operating temperature, porosity andpressure on BASE performance is analyzed.The simulation results of the mathematical model of BASE and electrodes show that:BASE plays a decisive role on AMTEC performance while electrodes have relatively lesseffect on AMTEC performance. The simulation results of ion migration in BASE show that:under the drive of the electrochemical degree, it is necessary to take into account the influence of electric field coupling between ion and ion, ion and solid particles surface concerning themigration model of ion migration in porous media. Pressure gradient together with electricfield act as the driving force for the flow of sodium ions, where the pressure gradient is thedominating force. The integrated results of ion migration rate diagram in the BASE, potentialdiagram, temperature diagram and power diagram show that: when temperature conditionremains the same, with the increase of polarization voltage the current density increases, cellvoltage decreases and power output increases. As the current density increases to a certaindegree the output power begins to decrease gradually. The analysis results of the influencingelements of BASE show that: under the different hot side temperature conditions, with theincrease of temperature the electrode voltage, the current density and the output power allincrease; under different porosity conditions, as the porosity increases the pressure gradientdriven velocity increases and the concentration increases more rapidly; under different lowpressure side conditions, as the pressure increases the transportion speed increases so that theion accumulate continually at cathode but the power density of the electrode decrease.