Numerical study on enhanced melting heat transfer of PCM by the combined fractal fins

The melting heat transfer characteristics of PCM were studied in different combined types of heat exchangers. The effect of combining two different bifurcation level fractal fins was considered for PCM melting heat transfer in a phase change heat exchanger. Based on fractal theory and enthalpy-porosity method, a PCM melting model of two-dimensional unsteady heat transfer was established in a combined fractal fin heat exchanger. The influences of some factors, such as combined types of fractal fins, bifurcation level, heat transfer area of the fins and the spacing of the fins, were discussed for the melting heat transfer process. The results show that the complete melting time of PCM in a combined fractal fin heat exchanger is reduced by 68% compared with the traditional fractal fin heat exchanger. The melting time shows decreasing first and then gradually stabilizing as the bifurcation level number and heat transfer area of the combined fractal fins increase. The melting rate of PCM is the fastest in the Case 4 heat exchanger, and the complete melting time of PCM is the shortest in the Case 3 heat exchanger. Fin spacing and heat transfer area have a significant impact on the complete melting time. On this basis, the thermal performance coefficients between the fins zone and PCM zone are defined. The complete melting time of PCM is found to be exponentially negatively correlated with the thermal performance coefficient by curve fitting. The temperature distribution uniformity coefficient is proposed to evaluate the temperature distribution uniformity during the melting process of PCM. It is found that the temperature distribution uniformity of Case 2 has the best at the end of melting. The above results provide a theoretical basis for structural design of the heat exchanger with combined fractal fins to enhance PCM melting heat transfer.