Thermal conductivity of composite phase change materials to optimize fin structure of casing heat storage unit

This study aims to optimize the phase change material (PCM) and fin structure, in order to improve the heat storage efficiency of the casing unit. Paraffin, lauric acid, and stearic acid were mixed with three organic PCMs. Once the total volume was constant, the volume fraction of the three PCMs was adjusted to prepare the mixed organic PCMs. An optimal combination was selected with the highest thermal conductivity. Then, Al₂O₃, Fe₂O₃, Fe, and Cu nanoparticles with different mass fractions were doped into the organic PCMs, in order to prepare a variety of composite phase change heat storage materials. The results show that four types of nano-Cu with different weight fractions (0.5%, 1%, 1.5%, and 2%) were selected as the additives to improve the thermal conductivity of the original PCM samples of 0.2% SDBS. The thermal conductivity values were 0.371, 0.373, 0.374, and 0.370 W/(m·K), respectively, which were 13.46%, 14.07%, 14.37%, and 13.15% higher than that of the sample without Cu nanoparticles, respectively. The composite PCM with 1.5% Cu nanoparticles shared the highest thermal conductivity of 0.374 W/(m·K), which was 14.37% higher than that of the sample without Cu nanoparticles. The typical fin structure of casing was summarized into four shapes: triangular, parallelogram, rectangular, and fan-shaped. The heat transfer performance of the four shapes was numerically simulated, according to the same working conditions: the volume ratio of the fin to the heat storage material, and the heat exchange edge length. It was found that the arc-shaped and straight fin were combined to achieve the high melting rate of the fan-shaped structure. The three-dimensional numerical simulation was carried out on the heat storage device of the fin structure using FLUENT software. Once the Ste number changed from 0.3 to 0.7, the melting time was reduced from 7 207 to 904 s, where the whole melting time was shortened by 6 303 s. Once the Ste number was 0.7, there was the maximum temperature difference between HTF and PCM. The natural convection was particularly intense to further accelerate the melting process of the heat storage unit. The relationship between melting time and Ste number was fitted by dimensionless quantity. The heat flux corresponded to the different Ste numbers. Once the Ste number was 0.3, the peak value of heat flux reached 155 W at the beginning of melting and then decreased gradually. The reason was that the HTF with the temperature difference between the initial and PCM entered the inner tube for heat exchange, and then the heat flux rate increased rapidly to the peak, due to the superior thermal conductivity of the steel pipe, while the HTF was continued to transfer the heat into the steel pipe, finally the heat loss occurred in the flow process, resulting in the heat flow rate gradually decreasing until the end of melting. The peak heat flow reached 436 W at a Ste number of 0.7, due to the large temperature difference between the HTF inlet temperature and the PCM. The findings can also provide a strong reference to improve the heat storage efficiency of phase change heat storage units.