The thermal conductivity of composite phase change materials and the fin structure of casing phase change heat storage unit are optimized

In order to solve the problem of low heat storage efficiency of casing phase change heat storage unit, this paper optimizes the phase change material and fin structure. Paraffin, lauric acid and stearic acid were mixed with three organic phase change materials, and when the total volume was constant, the volume fraction of the three phase change materials was changed to prepare the mixed organic phase change materials, and a combination with the highest thermal conductivity was selected. Then, Al₂O₃, Fe₂O₃, Fe and Cu nanoparticles with different mass fractions were doped with organic phase change materials to prepare a variety of composite phase change heat storage materials. The results show that under the premise of maintaining the composite phase change material samples of 0.2% SDBS, four kinds of nano-Cu with different weight fractions (0.5%, 1%, 1.5%, 2%) were selected as additives to improve the shortcomings of the low thermal conductivity of the original PCM. At 1%, 1.5% and 2%, the thermal conductivity was 0.371, 0.373, 0.374 and 0.370 W/(m \cdot K), respectively, which was 13.46%, 14.07%, 14.37% and 13.15% higher than that of the sample without Cu nano. The composite PCM with 1.5 wt% Cu nanoparticles had the highest thermal conductivity of 0.374 W/(m \cdot K), which was 14.37% higher than that of the sample without Cu nanoparticles. Based on the research and analysis of the typical fin structure of the casing by previous scholars, the fin is summarized into four basic shapes: triangular, parallelogram, rectangular and fan-shaped, based on the same working conditions, the volume ratio of the fin to the heat storage material is the same, and the heat exchange edge length is the same, the heat transfer performance of the above four shapes is numerically simulated, and it is found that the melting rate of the fan-shaped structure is high, so a structure combining arc-shaped fin and straight fin is proposed. The three-dimensional numerical simulation of the heat storage device of the fin structure was carried out using FLUENT software, and it was found that when the Ste number changed from 0.3 to 0.7, the melting time was reduced from 7207 s to 904 s, and the whole melting time was shortened by 6303 s. When the Ste number is 0.7, the temperature difference between HTF and PCM reaches the maximum, and the natural convection is particularly intense, which further accelerates the melting process of the heat storage unit. Based on the model, the relationship function between melting time and Ste number is fitted by dimensionless quantity. The change of heat flux corresponding to different Ste numbers was studied, and it was found that when the Ste number was 0.3, the peak value of heat flux reached 155 W at the beginning of melting, and then gradually decreased, the reason for this phenomenon was that the HTF with the temperature difference between the initial and PCM entered the inner tube for heat exchange, and the heat flux rate increased rapidly to the peak due to the superior thermal conductivity of the steel pipe, and with the progress of the heat exchange process, the HTF continued to transfer heat to the steel pipe, and heat loss occurred in the flow process, resulting in the heat flow rate gradually decreasing until the end of melting. At a Ste number of 0.7, the peak heat flow reaches 436 W, which is due to the large temperature difference between the HTF inlet temperature and the PCM. The research results can provide a reference for improving the heat storage efficiency of phase change heat storage units.