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Micro heat exchangers have been widely used in both of academic and industrial fields for controlling heat for decades. Especially, the printed circuit heat exchanger (PCHE) is applicable to high pressure and high temperature cases since the manufacturing process, diffusion bonding, gives high durability. However, flow does not occur uniformly in multiple channels, which causes a heat transfer difference between the channels that reduces the effectiveness of the system. Also, it makes the total pressure drop larger than when it is uniformly distributed. Flow distribution has been studied intensively for decades, but the mechanism has not been clarified yet, since flow in the header is much too complicated. As a step to reveal the mechanism, we focus on maldistribution and quantitatively investigate the phenomena. We prepared experimental devices, including a set of parallel micro-channels, inlet and outlet. We used water as a working fluid, and water soluble red ink for visualization. We measured the interface location to derive the flow speed at each channel. We focused on the dependency of channel length and flow speed. In the theoretical part, we derived the flow speed profile of the entire channels that shows good agreement with the experimental results. This study provides a theoretical basis on resolving the maldistribution problem. [ABSTRACT FROM AUTHOR]

Local single-phase flow heat transfer downstream a single pin fin in a microchannel was experimentally and numerically studied. Three distinct flow regimes, depending on the Reynolds number, were characterized, namely: laminar flow with steady wake, laminar flow with unsteady wake, and turbulent flow. Local temperature measurements with high spatial resolution were obtained by incorporating an array of micro resistance temperature detectors (RTDs) (∼55 µm × 55 µm) on the internal microchannel surface. Local surface temperatures were related to the flow structures under different flow regimes. An enhanced local heat transfer coefficient at the trailing edge of the wake region downstream the pillar was observed. It is believed to be a result of vortex shedding and large-scale flow mixing triggered by flow instability at high Reynolds number. The numerical model enabled a full conduction/convection conjugate analysis of the entire system including heat conduction within the solid substrates and heat losses to the surrounding environment. Local heat transfer coefficient downstream the pin fin at each Reynolds number was obtained. [ABSTRACT FROM AUTHOR]

Local heat transfer downstream a single pin fin in a microchannel was experimentally studied by incorporating an array of micro resistance temperature detectors (RTD) (∼55 μm × 55 μm) on the internal microchannel surface. Local temperature distribution with spatial resolution as high as 150 μm was obtained and was superimposed onto the velocity field to reveal the interaction between the flow structure and the local heat transfer at different regions downstream the pin fin. Initial result in which the surface temperature inside the steady wake region was lower than in the regions outside the recirculation zone was explained and linked to an interplay of fluid convection and solid substrate conduction. Ignoring this local interplay and processing the data without careful consideration to the conduction process resulted in misinterpretation of the heat transfer processes. To address this issue numerical thermal and fluid model of the entire device was simulated to provide local heat flux distribution. This in turn allowed to resolve the local heat transfer coefficient in the vicinity, and outside the region, of the pin fin. [ABSTRACT FROM AUTHOR]