Solid-liquid phase change heat transfer includes two processes: solidification of the substance (liquid becomes solid) and melting (solid becomes liquid), the substance is heated to the melting point and absorbs a large amount of heat during the melting process, and the latent heat is released during the solidification process when it is cooled to the freezing point.
Solid-liquid phase transition and heat transfer are common phenomena in nature, such as the formation of volcanic rocks, the evolution of ice and the thawing of the earth, etc., and are also important processes in the field of engineering technology, such as the refrigeration of food, polymer processing, solidification and crystallization of castings, preparation of amorphous alloy materials, refining of semiconductor materials, storage of heat or cold energy, etc.
Solid-liquid phase change heat transfer has the advantages of high heat flux density, high thermal efficiency and low pressure, which has important research significance and application value.
Mathematical models and governing equations for solid-liquid phase transition heat transfer are usually based on the concept of a continuum medium, while assuming the isotropy and uniformity of the solid-liquid phases. Since the solid-liquid interface is directly affected by the physical properties of the substance, the solid-liquid phase change heat transfer can be divided into two categories according to different materials: problems with a single phase transition temperature and a clear solid-liquid interface (pure substance).
The problem of a phase transition temperature in a certain range with the zone of coexistence of two phases (mixture). The heat transfer of solid-liquid phase transition can be divided into two categories according to the different characterization quantities: temperature model (temperature is the only dependent variable, and the energy equation is established in the solid phase and liquid phase regions respectively)
Enthalpy model (temperature and enthalpy are dependent variables, and enthalpy is used to distinguish between solid and liquid phases, without partitioning). The characteristics and difficulties of solid-liquid phase transition heat transfer lie in the moving solid-liquid interface, and are also affected by factors such as the relative flow of liquid, the volume change of solid-liquid phase transition, and boundary thermal resistance.
In the early stage, the solution of solid-liquid phase transition heat transfer mainly used analytical methods, including exact analysis and approximate analysis. Only a few idealized solid-liquid phase transition heat transfer with simple boundary conditions can be accurately solved for a few one-dimensional semi-infinite, infinitely large regions, mainly based on the Neumann problem and the generalized Neumann problem.
The approximation analysis mainly includes the integration method, the quasi-steady state method, the perturbation method, the thermal resistance method, the successive approximation method, etc., which mainly solves the one-dimensional monotonic interface phase transition problem and the very few two-dimensional problems. Numerical methods are the main solutions to the multi-dimensional solid-liquid phase transition heat transfer problem under complex conditions.
There are two main models for numerical methods to deal with solid-liquid phase transitions: the separated two-phase model (interface tracking method) and the mixed two-phase model (fixed grid method). The separated two-phase model treats the two phases as two regions, which can reflect the phase transition process in more detail, but the calculation process needs to trace the interface, so the computational effort is large.
The hybrid two-phase model believes that there is no strict interface in the phase transition process, and the two phases coexist, and the calculation is simple but cannot accurately display the interface characteristics. In addition, Monte Carlo and lattice Boltzmann methods are being used to calculate the heat transfer process of solid-liquid phase transition.
Due to the shortcomings of low thermal conductivity of phase change materials, especially organic phase change materials, the enhanced heat transfer of solid-liquid phase change is also an important problem that must be solved.
And there are two main types of strengthening methods: adding high thermal conductivity metal or non-metallic solid particles to improve the thermal conductivity of phase change materials; Strengthened structures such as metal foam, metal fins, and expanded graphite are used to strengthen heat transfer to phase change materials.
