Study On Optic-thermo Characteristic Andvolume Change Effect Of Semitransparent Medium With Phase Change

The phase-change process of semi-transparent medium exists widely in energy utilization,materials processing,aerospace and other fields,such as solar thermal chemical reaction,droplet fuel combustion,plasma spraying,exhaust plume particle radiation,thermal protection of aircraft,optical fiber manufacturing and so on.The phase-change process of semi-transparent medium is a strongly nonlinear and complex multi-field coupling problem involving the radiative and conductive coupling,the thermo-optic effect,the absorption and release of latent heat,and the development of the phase transition interface.It may also involve material volume expansion/contraction and large deformation phenomena.The analysis of these effects and behavioral mechanisms in the phase-change process is of great significance for engineering applications.This thesis focuses on the optic-thermo characteristics and volume change effect of the phase-change process of semi-transparent media,and researches on algorithm implementation,characteristic law,and experimental research.The main purpose is to establish a mathematical model of the optic-thermo characteristics and volume change effect of the phase transition process,develop an integrated coupled radiation and phase-change algorithm suitable for the simulation of solid-liquid phase-change problems,study the optic-thermo characteristics and volume change effect of the phase-change process through numerical simulation and experiments,and provide support for practical applications.In terms of algorithm implementation,the mathematical model of the optic-thermo characteristics of semi-transparent media with phase-change process is established based on the energy equation in the form of equivalent heat capacity method and the radiative transfer equation（RTE）.The correction scheme of equivalent heat capacity method for the latent heat is introduced.Based on the basic theory of meshless reproducing kernel particle method（RKPM）,the coefficients of the modified function of the kernel function are obtained according to the regeneration conditions.Interpolation RKPM discrete schemes for the energy equation and RTE are constructed.A multi-dimensional uniform refractive index medium radiative and conductive coupling RKPM algorithm is developed and the processes of applying radiation and conduction boundary conditions are described.In order to solve the difficulty of applying Dirichlet boundary conditions in solving the radiation field,based on the boundary energy balance relationship,virtual particles are arranged at the boundary to deal with the radiative energy transfer.The RKPM discrete schemes for multidimensional semi-translucent and opaque radiation boundary conditions are given.Further,considering that the refractive index of the medium may change with temperature,phase state,and spatial position,and then affects the heat transfer process,based on the RTE of gradient refractive index,the variable refractive index medium RKPM algorithm is established by using RKPM to directly disperse the refractive index gradient term and with the angle redistribution term is processed by first order difference.In order to investigate the heat transfer process of spherical particles,the radiative and conductive coupling algorithm in spherical coordinates is established.Based on the analytical solutions and numerical examples,the accuracy and effectiveness of the above algorithms are analyzed.Aiming at the problem of volume expansion/contraction during the solid-liquid phase-change process,according to the density change,the support area and spatial location of the RKPM particles are reassigned,the treatment scheme considering the medium volume change in the RKPM is given.The numerical implementation of coupled radiation and phase-change algorithm has been completed under the RKPM framework.In terms of the optic-thermo characteristic law of semi-transparent media with phase-change,the thesis focuses on the effect of the interaction between radiative and conductive and the thermo-optic effect on the phase-change process.Through the simulation of solidification problems in the angular domain and 3D computational domain,the error and convergence of the established multi-dimensional integrated algorithm are analyzed.The influences of various thermal physical parameters and boundary conditions on temperatures,phase-change interface,liquid fraction,radiation heat flux in the phase transition process are investigated.The effects of thermo-optic effect on the phase transition process of constant temperature and non fixed temperature are simulated;the focus is on discussion of influence of the medium refractive index as a function of spatial position on the solidification process.Aiming at the interaction between molten fuel and coolant in light water reactor,the solidification processes of millimeter-sized spherical Corium and Alumina particles in water vapor are simulated,the thermo-optic effect influences of the change of Corium particles absorption coefficient and the change of Alumina particles optical constants with temperatures and spectrums on the phase-change heat transfer are analyzed.In terms of the volume change characteristic law of semi-transparent media with phase-change,the theoretical and experimental studies are carried out on the factors affecting and characteristics of the volume change of low melting point alkane materials during the phase-change process.In this thesis,in order to analyze the effect of volume change on the phase-change process,the melting and solidification processes of n-C₁₈H₃₈ are simulated,and the thermal radiation induced n-C₁₈H₃₈ melting process is experimentally studied.Then the unidirectional solidification experiment device is designed,the RKPM algorithm,which considered the volume change,is verified and the volume contraction laws of the typical alkane solidification processes are preliminarily analyzed.In addition,the alkane unidirectional melting experiment is used to study the melting process of alkanes at different pressures and heating surface temperatures,the influence of the Stefan number and pressure on the heat flux are analyzed.The water bath measuring device for solid-liquid phase change volume change is set up,the volume change laws of low-melting alkanes during phase transition are measured experimentally,and the change rates and time characteristics are grasped by phase-change experiments of single alkanes and binary mixed alkanes.Alkane/expanded graphite composite phase-change materials are prepared and the enhancement of heat transfer performance and volume change are studied.Finally,the application of volume change effect in the process of energy storage and conversion is discussed.The heat transfer periodicity of the phase-change heat exchanger,the key component of the phase-change driven power generation system,is analyzed.The matching principle of the melting/solidification time of the working fluid in the phase-change heat exchanger under the continuous driving condition is obtained.The model machine of n-C₁₆H₃₄ phase-change driven energy storage is designed,the experimental research on the phase-change energy storage process is carried out,the influence of different heat transfer conditions and precharge pressures on the energy storage process of nitrogen accumulator is analyzed,and the design basis is provided for the phase-change driven or apply work.