Numerical Analysis On Melting Process Of Paraffin In A Vertical Annular Cylinder

Nuclear energy, as a kind of clean energy, has great advantages in environmental protection and economy. Nuclear safety is of paramount significance to the development of nuclear energy, thus the research on reactor safety analysis has attracted much attention. Severe accidents of nuclear reactors are beyond design basis accidents, of which core damage is the important sign of the beginning. For the purpose of investigating the melting process of the fuel inside the reactor during severe accidents, a fundamental study on numerical simulation of melting process was conducted.Paraffin wax was chosen as the computational object and a method called enthalpy-porosity method based on fixed mesh was applied in this numerical simulation. The enthalpy-porosity method is quite popular because re-meshing is not required. In addition, there is only one energy equation during the calculation, thus there is no need to consider the boundary conditions at the solid-liquid interface. This method is quite suitable for the numerical simulation of melting process. ANSYS FLUENT was selected as the simulation tool, and the melting/solidification based on the enthalpy-porosity method was used in this simulation to calculate the melting process of paraffin wax in a central heating vertical annular cylinder. Axial symmetry of the physical model was assumed for the computational domain, and a two-dimensional model was built with structured grids. On account of the results of the grid sensitivity study, the 15500 grids with the first layer of 0.2mm were chosen for all computations. Providing that the boundary of paraffin wax was at a constant temperature (T0=50℃), and the bottom and top of the heating rod domain was adiabatic. The initial temperature of the entire computational domain is Tintial=50℃,which was below the melting point of paraffin wax in order to keep the paraffin wax at solid state at the initial time. During the process of the simulation, it was found that the thermal resistance of the enclosure (thickness=5cm) is of important significance to the heat transfer process during the simulation of melting, also the thermal storage in the heating rod cannot be neglected in the simulation, therefore the setting of volume heat flux was more suitable than surface heat flux.The simulation results were validated against the experiment results at three different levels of heating power. Good agreements between experimental data and numerical results were achieved. And it turns out that extending the computational domain can make the simulation results more close to the experiment data. Natural convection and melt front interface were well predicted by simulation. However, considerable differences appeared due to mushy zone constant in the model.A recommended value of mushy zone constant (C=106) was obtained by comparison to the experiments. The influence of thermal conductivity coefficient of the phase change material and the bath temperature were also analyzed numerically. It was predicted that large values of thermal conductivity coefficient accelerated the melting process and decreased the maximum temperature. The qualitative analysis showed that higher water bath temperature reduced more heat loss, and made the melting process faster. The VOF model could simulate the interface between the liquid paraffin wax and the air, which makes it possible for simulating the volume expansion of liquid paraffin wax during melting process. Introducing the VOF model to the numerical simulation made, the numerical results of the temperature variation closer to the experimental results. However, as for the interface evolution between solid and liquid paraffin wax, there are differences between experimental results and numerical results that obtained by either with or without the VOF model.