# Numerical Investigation of Latent Thermal Storage in a Compact Heat Exchanger Using Mini-Channels
###### Abstract  
This paper aims to numerically investigate the thermal enhancement of a latent thermal energy storage component with mini-channels as air passages. The investigated channels in two sizes of internal air passages (channel-1 with \(\mathrm{d_{h}}=1.6\) mm and channel-2 with \(\mathrm{d_{h}}=2.3\) mm) are oriented vertically in a cuboid of \(0.15\times 0.15\times 0.1\) m\({}^{3}\) with RT22 as the PCM located in the shell. The phase change is simulated with a fixed inlet temperature of air, using ANSYS Fluent 19.5, with a varying number of channels and a ranging air flow rate entering the component. The results show that the phase change power of the LTES improves with by increasing the number of channels at the cost of a decrease in the storage capacity. Given a constant air flow rate, the increase in the heat transfer surface area of the increased number of channels dominates the heat transfer coefficient, thus increasing the mean heat transfer rate (UA). A comparison of the channels shows that the thermal performance depends largely on the area to volume ratio of the channels. The channel type two (channel-2) with a slightly higher area to volume ratio has a slightly higher charging/discharging power, as compared to channel type one (channel-1), at a similar PCM packing factor. Adding fins to channel-2, doubling the surface area, improves the mean UA values by 15-31% for the studied cases. The variation in the total air flow rate from 7 to 24 L/s is found to have a considerable influence, reducing the melting time by 41-53% and increasing the mean UA values within melting by 19-52% for a packing factor range of 77.4-86.8%. With the increase in the air flow rate, channel type two is found to have considerably lower pressure drops than channel type one, which can be attributed to its higher internal hydraulic diameter, making it superior in terms of achieving a relatively similar charging/discharging power in exchange for significantly lower fan power. Such designs can further be optimized in terms of pressure drop in future work, which should also include an experimental evaluation. P 2021 24 June 2  
[PERSON] and [PERSON] [7] analyzed a plate encapsulating PCM system working with air and reported that the thermal performance could be improved by decreasing the dimensions of the air channels and the PCM slabs. [PERSON] et al. [8] investigated an LTES system, enhanced with graphite, and concluded that in cases with the PCM-graphite composite, as the storage medium, the thermal performance of the LTES was controlled by the resistance on the air side, due to the reduced resistance of the storage medium. On the other hand, in cases where pure PCM was employed, the resistances on the PCM and the airside were competitive. [PERSON] et al. [10] performed a parametric study of an air-PCM heat exchanger for ventilation systems, considering square channels of 2 \(\times\) 2 cm\({}^{2}\), 4 \(\times\) 4 cm\({}^{2}\) and 6 \(\times\) 6 cm\({}^{2}\). They found that as the channel dimension decreased, the charging/discharging power increased, and the best thermal performance was achieved for the smallest channels. However, in the study, the influence of reducing the hydraulic diameter of the channels and subsequently the increase in the heat transfer coefficient of air and its effect on the thermal performance was lacking. The hydraulic diameter (d\({}_{\text{h}}\)) of the flow passages can be reduced significantly by using mini- and micro-channels. Mini/micro-channels proved to be significantly beneficial in various industries and applications such as the refrigeration/heat pump industry and electronics cooling [11; 12; 13; 14]. [PERSON] et al. [11] classified the passages based on the dimensions of the hydraulic diameter (d\({}_{\text{h}}\)) to conventional channels for d\({}_{\text{h}}\) > 3 mm, mini-channels for 0.2 mm \(<\) d\({}_{\text{h}}\) < 3 mm and micro-channels for 0.01 mm \(<\) d\({}_{\text{h}}\) < 0.2 mm. [PERSON] et al.