Effect of the circumferential and radial graded metal foam on horizontal shell-and-tube latent heat thermal energy storage unit

The latent heat thermal energy storage unit (LHTESU) strengthened by metal foam can effectively store solar energy and realize the sustainable utilization of solar energy. In this paper, the metal foam with a twodimensional (radial and circumferential direction) porosity gradient is proposed for the problem of slow melting rate and non-uniform melting in the horizontal shell-and-tube LHTESU. The non-uniform index and thermal energy storage rate are introduced to evaluate different metal foam structures. Under the same amount of metal foam, the influence of one-dimensional and two-dimensional graded structures is numerically studied. The results show that circumferential graded structure can alleviate the non-uniform melting problem. Compared with the uniform structure, the non-uniformity index decreases by 36.63% and the thermal energy storage rate is increased by 38.60%. In addition, the optimum circumferential porosity gradient also increases with the increase of Ra*. The strengthening effect of the circumferential and radial graded structures is also compared. It can be found that with the increase of porosity gradient, the strengthening effect of the circumferential graded structure increases gradually and that of radial graded structure first increases and then decreases. When the circumferential graded structure is combined with the radial graded structure, the two-dimensional porosity gradient structure can further improve the thermal performance of LHTESU. Compared with the uniform structure, the thermal energy storage rate can be increased by 47.40%.