Melting behavior of the latent heat thermal energy storage unit with fins and graded metal foam

Metal foam can effectively improve the melting rate of latent heat thermal energy storage units (LHTESU). However, the existing metal foam structure can't simultaneously solve the problem of non-uniform melting caused by natural convection and slow melting rate in horizontal shell-and-tube LHTESU. To this end, the structure with fins and graded metal foam is proposed to further enhance thermal performance. The porosity of the metal foam increases gradually from the inner tube to the outer shell to reduce thermal resistance and improve thermal conduction globally. The fins (the porosity is 0) are employed to further enhance local heat transfer and balance the difference of melting rate between upper and lower parts. The correctness of the current model is verified by the one-phase Stefan problem with explicit solutions and research results in previous literature. With consideration of natural convection, the enthalpy-porosity method is adopted to compare the melting behavior of LHTESU strengthened by uniform metal foam, graded metal foam, fins and uniform metal foam, and fins and graded metal foam. The results show that the existence of the fins improves the uniformity of the temperature field and significantly shortens the melting time. Based on the same amount of metal materials, the proposed structure obtains the best strengthening effect in the four different structures. Furthermore, the effects of different structural parameters (the fins angle, fins thickness, and porosity gradient) on the melting process are discussed. It can be found that there are optimal values for the three structural parameters. Compared with the graded metal foam structure, the new structure (the fins angle is 25 degrees, the fins thickness is 3 mm and the porosity gradient is 2%) further reduces the total melting time by 27.23% and increases the thermal energy storage rate by 36.52%.