# Numerical Investigation of Latent Thermal Storage in a Compact Heat Exchanger Using Mini-Channels
###### Abstract
[11] showed that the fluid heat transfer coefficients for circular passages are inversely proportional to the hydraulic diameter, given a fixed flow rate. However, the increase in heat transfer coefficient is achieved at a considerable cost of an increase in the pressure drop. Besides the hydraulic diameter, with rectangular channels, the aspect ratio of the channels as a fraction of the width to the height can influence the heat transfer coefficient. [PERSON] [15], in a study on laminar flow in heat transfer in non-circular ducts, showed that, given a constant Graetz number (Gz = \(\frac{d_{\text{h}}}{1}\)RePr), the Nusselt number either under constant heat flux or constant wall temperature conditions increases with the increase in the aspect ratios of the channels. Rectangular mini-channels with low hydraulic diameters and high area-to-volume ratios have the potential to enhance the heat transfer on both the air and PCM sides with having relatively high PCM packing factors (PF). This is particularly beneficial in LTES applications related to air as the HTF, where there are needs for enhancement due to low thermal conductivities of air and PCMs. Mini/micro-channels can be fabricated conventionally using extrusion or Additive Manufacturing (AM) processes [16; 17; 18; 19; 20]. However, an extrusion process is associated with certain limitations and restrictions in producing complex features. Additive Manufacturing (AM), known as 3D printing, is an alternative new fabrication method capable of manufacturing complex geometric features, including the embedding of internal/external thermal enhancers and bypassing many limitations associated with conventional methods. However, 3D printing is also associated with certain disadvantages such as higher surface roughness, limited options in printing materials, and higher cost per part in mass productions, as compared to the conventional methods [21]. This paper numerically investigates the novel idea of incorporating mini-channel profiles into LTES components with dry air as the HTF. Three-dimensional simulations of transient phase change were performed using ANSYS Fluent 19.5. An enthalpy-porosity method is used in this commercial software for solid-liquid phase change simulations. In this method, the mushy zone at which the phase change is happening is dealt with as a porous structure, and the liquid flow in this zone is treated as a liquid flow through a porous structure [22; 23]. Two types of mini-channels were studied. The first channel is an existing extruded aluminum profile and the second channel, with and without external fins, is a conceptual design capable of being manufactured via 3D printing. The mini-channels have relatively similar volumes but different internal hydraulic diameters (\(\mathrm{d_{hchannel-1}}\) = 1.6 and \(\mathrm{d_{hchannel-1}}\) = 2.3) and aspect ratios (\(\mathrm{AR_{channel-1}}\) = 0.86 and \(\mathrm{AR_{channel-2}}\) = 20). The compact LTESs are modeled three-dimensionally for a laminar air flow regime. The thermal performance of the LTES in charging/discharging with varying number of channels and ranging flow rates was analyzed and evaluated.