Convergence Issue on Modelling Phase Change Material (PCM)

Hello! I'm Dier , last term undergraduate student of mechanical engineering and I am working on modeling a heat transfer phenomenon on phase change material (PCM). This is the model 2D annular pipe with PCM on the outer side of the annular and the heat transfer fluid (HTF) on the inner side of the pipe. The thickness of inner pipe is 1.5 mm with inside diameter 0.0127 m and the material si Cu. The inside diamter of the outer pipe is 0.0258 m. The HTF I used is water. The properties of PCM I used is temperature dependent and I defined that with C code language VS 2017. The boundary conditions are:Inlet velocity : 0.2499 m/s, Inlet temp   : 305.65 K, Tsolidus PCM : 300.15 K, Tliquidus PCM : 300.65 K. Because of the Re is 4051 so, I modeled the simulation using k-epsilon with Realizable and enhanced wall treatment. I used SIMPLE method for the solver and 2nd order for momentum and energy, PRESTO! for the pressure. URF are set to default. But when I ran the calculation, floating point exception occurred. I used time step size equal to 0,1 s and the number of time steps are 36000 with maximum iteration 20/iteration. But when I changed the density of the PCM to constant value, the model was converged but the result aren't valid with the experiment data. Does anyone knows what went wrong with my model? Any answer will really help me and will really appreciated. Thank you


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