(244f) Enhancement of Thermal and Hydraulic Characteristics of Microchannel Heat Sinks By Employing sCO2 As Coolant

Micro-scale thermal management has become a challenge currently with the extraordinary shrinkage of microchips in computing and communication devices [1]. The operating temperatures associated with microchips must be kept within stipulated limits that required cooling systems that are extremely compact and exhibit excellent heat transfer characteristics. Besides, the heat flux rates from microelectronics have gone out to be considerably above the projected values for the year 2020 [2] with recent nanotechnological advancements. Henceforth, the compact heat removal system's design has become intricate, requiring novel coolants and heat sink geometries to enhance thermal and hydraulic associated with the cooling systems.

Liquid-cooling systems have already replaced the air-cooling systems due to the limited heat removal rates associated with the air, but further research is still needed for superior thermal cooling systems. The microchannel heat sink presents itself as a favorable solution, as demonstrated by its compact size and high heat dissipation capabilities [2]. However, extremely high-pressure drops associated with the microchannel heat sinks (MCHS) are a concern that needs to be addressed to reduce the pump size and the overall size of the heat-removing systems. The literature shows that an appropriate choice of coolant can play a crucial role in improving a microchannel heat sink's performance. Various coolants have been examined for MCHS by different researchers, such as methyl alcohol [1], R410a [2], and R134a [3]. Additionally, nanofluids initially industrialized by Choi and Eastman [4] have recently gained researchers' attention. A recent study [5] indicated that thermohydraulic characteristics associated with the mini channel heat sinks could be enhanced considerably using alumina nanofluid. Similar findings have been reported for the copper (Cu)/water [6], copper-oxide(CuO) /water [7], diamond/glycerin and diamond/water [8], TiO₂/water [9], and ZnO/ethylene glycol [10]. But at the same time, studies [6-10] suggest the improved thermal performance utilizing nanofluids comes at the cost of pressure drops that in turn will increase the pump size.

The literature discussed above indicated a dire need for a coolant that can enhance both thermal and hydraulic characteristics of MCHS for an efficient cooling system. In this reference, supercritical carbon dioxide (sCO₂) operating close to its critical point can be of great value because of its promising thermophysical properties. Hence, the thermohydraulic performance of the sCO₂-cooled microchannel heat sinks (MCHS) are investigated numerically and compared with the conventional water-cooled MCHS in the current work for the first time to the author’s best knowledge. Header geometry for both sCO₂-cooled and water-cooled MCHs are also optimized for uniform flow distribution; else, maldistribution can considerably influence the performance of the MCHS [11–15]. Finally, the effect of different operating conditions on the thermohydraulic performance of the proposed sCO₂-cooled MCHS is examined to locate its optimal operating conditions. To perform the conjugate heat transfer calculations mentioned above, a 3D Reynolds Averaged Navier–Stokes (RANS) model is established and validated for both coolants, and shear stress transport (SST) turbulence model is used to model the flow turbulence. The computational geometry of both fluid and solid domains and its hexahedral mesh generated using ANSYS ICEM-CFD are shown in Fig. 1a. The mesh's same topology was enforced in the interface between fluid and solid domain to minimize the interface losses. Thermophysical properties of the changes substantially near the critical point, and it is essential, therefore, to integrate these variations in ANSYS-CFX during the numerical calculation for accurate prediction of the thermohydraulic characteristic of sCO₂-cooled MCHS. Thermophysical properties of are incorporated via a high-resolution real gas property (RGP) file to capture sharp changes in its properties.

Results suggest both distributor and collect header geometry can impact the thermohydraulic performance of the MCHS severely. It is found that rectangular header geometry minimizes the flow maldistribution (Fig. 1b) and results in a uniform temperature of the MCHS's base. The comparison of the performance of the MCHS with two coolants reveals that sCO₂-cooled MCHS is up to 32% higher in comparison with water-cooled MCHS (Fig. 1d). Further base temperature with the former was found less in comparison with the latter. It is also found that the sCO₂-cooled MCHS can withstand the off-load heating loads without any change in the base temperature. Moreover, it is found that the sCO₂-cooled MCHS reduces the pressure drop up to 7 times (Fig. 1e) compared to water-cooled MCHS, while the friction factor for the proposed coolant is up to 10 times smaller than the water-cooled MCHS. Finally, based on overall performance evaluation criteria performance of sCO₂-cooled MCHS was found up to 2.2 times better (Fig. 1f) than the water-cooled MCHS.

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