Numerical simulation of flow and heat transfer in melting-solidification problems in different cavities

Abstract

The solid-liquid phase change processes are one of the important areas of energy transfer processes. These melting and solidification processes are classified as moving boundary problems. It involves changes in the thermo-physical properties of phase change material (PCM). The progress of phase change processes is very different from one another. In this thesis, the flow and heat transfer involved in melting-solidification processes are analyzed for various boundary conditions. Since the experimental study of transient solid-liquid phase processes is very difficult the numerical simulation is adopted for detailed phase change study. Commercial software Ansys-fluent is used for simulation. The phase change model used is enthalpy-porosity model. This model helps to identify the solid-liquid interface of PCM. A source term is used in the momentum balance equation to include the velocity suppression of solid phase. The numerical model is validated with the experimental result. The experimentation is done for melting of PCM (RT 27) in a spherical cavity. The image analysis tool is used to compare the experimental and numerical results. The deviation of numerical results from experimental results is within the permissible limit. The deviation is due to the difficulty involved in the identification of semi-solid and solid phase. This validated model is used for simulation of melting and solidification processes in different cavities for different boundary conditions. The simulation results include the effect of thermal diffusivity of cavity material on the melting process in spherical cavity. The different cavity materials are aluminium, copper, and glass. The thermal diffusivity of copper is highest and for glass, it is very low. It shows the shape of solid PCM is very different for glass cavity than that of aluminium and copper cavity. The melting time is lowest for highest thermal diffusivity cavity material. The effect of wall temperature of the cavity is also studied. Stefan number is defined to express the wall temperature in dimensionless form. For both spherical and rectangular shape, the melting time is inversely proportional to Stefan number. It is also found that though the size of spherical cavity is higher than the rectangular cavity for same Stefan number melting time is not much more different. The dependency of meting time on Stefan number and area of the cavity is derived. The effect of initial shape of PCM in the rectangular cavity is studied and found that the distance between PCM and cavity wall controls the phase change process. After melting, solidification processes are simulated in spherical and rectangular cavity for different Stefan number. The solidification process is much slower than melting process the simulations are done with copper cavity to minimize the computational time. For both the shapes a detailed discussion on flow and heat transfer involved is phase change processes are discussed in the thesis. The casting or solidification of pure metal like zinc and aluminium are also simulated. Since the thermal conductivity of metals and very high and surrounding temperature is very less compared to the solidus temperature of metals the solidification process is very fast. The process is even a little fast for high thermal conductivity material aluminium. On the other hand, density of metals is not affected by the solidification process. The melting is repeated with flow boundary conditions. The charging or melting of PCM in heat exchanger for different flow rates and temperatures of high-temperature fluid (HTF) are studied to suggest an effective range of flow rate and temperature of HTF.