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    Numerical investigation of the effects of geometric structure of microchannel heat sink on flow characteristics and heat transfer performance
    (Elsevier France-Editions Scientifiques Medicales Elsevier, 2019) Bayrak, Ergin; Olcay, Ali Bahadır; Serincan, Mustafa Fazıl
    A numerical study was performed to investigate the thermal-hydraulic performance of different microchannel heat sink (MCHS) designs for cooling channels in a lithium-ion battery including various geometric modifications. Comparative analysis was performed to determine which design is the best in terms of the heat transfer, the pressure drop, the overall performance and the temperature distribution on the baseline wall. It was observed that local modifications in channels can ensure suitable fluid mixing between core flow and near wall regions; therefore this situation enhances heat transfer performance considerably compared to MCHS with no cavity and rib (MC-NCR). However, the vortices obviously occurred in cavities. Although this phenomenon was helpful for the symmetrical cavity and rib (MC-SCR) in terms of heat transfer enhancement, it was opposite for the asymmetrical cavity and rib (MC-ACR) due to intensive recirculation zones. In addition, the large vortex bubbles especially seen after the last cavity or rib cause pick temperatures because this trapped flow could not be surpassed and carried. Results indicate that as the MC-SCR shows the best thermal performance owing to dominant jetting and throttling effect and convenient longitudinal and transverse vortices, asymmetrical cavity (MC-AC) is the best uniform temperature distribution on baseline wall.
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    Numerical study on mechanisms underlying the heat transfer enhancement of upward supercritical CO2 flow at low Reynolds numbers near the pseudo-critical region through a microtube
    (Pergamon-Elsevier Science Ltd, 2025) Bayrak, Ergin; Ahn, Hojin
    The buoyancy and thermal acceleration effects of supercritical CO2 flow near the pseudo-critical region have been widely mentioned as the mechanism of heat transfer enhancement in the literature. However, most publications deal with turbulent flows and do not discuss the details of how these effects alter flow structure and enhance heat transfer. The present study numerically investigated mechanisms underlying the heat transfer enhancement of upward supercritical CO2 flow through a microtube, 0.5 mm in diameter, at low Reynolds numbers. The heat transfer enhancement was closely associated with the appearance and disappearance of the M-shaped velocity profile. When the M-shaped profile started forming by the buoyancy effect, the first local maximum of the heat transfer coefficient appeared as the thermal acceleration of the boundary layer entrained fluid from the wall region. The fluid entrainment carried thermal energy from the wall toward the core, thus enhancing the heat transfer. When the M-shaped profile started disappearing due to the thermal acceleration in the core region, the second maximum appeared in some cases due to abrupt turbulence developed by two forces in the opposite direction: one force dragging the local maximum velocity in the M-shaped profile and the other force accelerating the core region.