Study Of Gallium Melting Characteristics And Solid-liquid Phase Change Electroconvection Enhancement Via Lattice Boltzmann Method

Solid-liquid phase change coupled with natural convection widely exists in many engineering applications,such as industrial waste heat recovery,electronics cooling and solar energy storage.The study of fundamental characteristics of solid-liquid phase change is of great guiding significance for industrial production practice.In addition,because of low heat conductivity of phase change materials(PCM)solid-liquid phase change process tends to proceed very slowly,which limits the application of solid-liquid phase change in the efficient storage of solar energy.Thus,more and more studies focused on the solid-liquid phase change enhancement techniques.Previous numerical studies often cared about the melting without heat transfer in solid zone,i.e.,the isothermal melting.It is obviously different from the real process and some important characteristics will be ignored.In this study both isothermal and nonisothermal phase change is considered.Firstly,melting via heating from bottom and left side walls are simulated,and basic characteristics are analyzed.Then non-isothermal melting/solidification of gallium is studied,and significant difference in flow,heat transfer and melting rates are founded compared with isothermal melting.Phase change enhancement technique can be decided into passive and active enhancement techniques.For passive one,different heating modes without extra energy waste are conducted to improve the melting efficient by perfecting the synergy of velocity field and temperature gradient field.For active technique,electrohydrodynamics(EHD)enhancement technique,which just causes few extra energy waste,is applied to enhance the melting of pure dielectric PCM.The reminder of this article is organized as follows:Two kinds of lattice Boltzmann models are used,one for isothermal melting and another for non-isothermal phase change,respectively.As for the electrohydrodynamics phase change,Maxwell equations and relative Navier-Stokes equations are given and then simplified,then the coupled lattice Boltzmann models are established.Applying Chapman-Enskog analysis,the governing equations are obtained from relative lattice Boltzmann equations.Simulating melting via natural convection and Rayleigh-Bernard convection to study the flow,heat transfer and melting performance.Results show that melting with bottom heater is faster than melting with left heater in high Rayleigh number.Then non-isothermal phase change of pure gallium is simulated by the new lattice Boltzmann model.Two kind of boundary conditions are compared and adiabatic condition is adopted finally.Some characteristics in non-isothermal process show great difference from the isothermal one.By simulating the coupled melting-solidification cycle it is found that PCM has good stability in the cycle phase change process.Besides previous uniform wall-temperature(UWT),increase wall-temperature(IWT),decrease wall-temperature(DWT),convex wall temperature(CWT)and sink wall-temperature(SWT)are considered to compare melting efficiency.Results show that CWT via bottom heater has highest melting efficient.Filed synergy principle is introduced to estimate the convective heat transfer in melting performance,which indicates that new heating modes change the synergy of velocity field and temperature gradient field to influence melting.Electroconvection solid-liquid phase change is simulated to study the basic flow,heat transfer,melting and the charge density distribution characteristics.The four electric dimensionless parameters,i.e.electric Rayleigh number,nondimensional mobility parameter,injection strength and nondimensional charge-diffusion number are analyzed.Melting upon different directions of heating and electric field are considered.Results show that melting is faster when heating and injecting from left wall than bottom wall,and in this case ono-isothermal melting can be enhanced greatly by electroconvection.