The authors have reported that a minichannel flow system had a high heat transfer coefficient. We investigated the heat transfer and flow structure of single and array minichannels combined with an impingement flow system experimentally and numerically. The diameter $D$ of the channel was $1.27mm$, and length to diameter ratio $L∕D$ was 5. The minichannel array was so-called shower head, which was constructed by 19 minichannels located at the apex of equilateral triangle, the side length $S$ of which was $4mm$ a single stage block was used to investigate the heat transfer without impinging flow system. Two stage blocks were combined in series to compose an impingement heat transfer system with an impingement distance of $H$. $H∕D$ ranged from 1.97 to 7.87. The dimensionless temperature increased as the impingement distance became short. A comparison of heat transfer performance was made between minichannel flow and impingement jet by comparing the single- and two-stage heat transfer experiments. It was found that dimensionless temperature of the minichannel exceeded that of the impingement jet. The mechanism of high heat transfer was studied numerically by the Reynolds-averaged Navier-Stokes equation and $k-ω$ turbulence model. The limiting streamline pattern was correlated well to the surface heat flux distribution. The high heat transfer was achieved by suppressing the development of boundary layer under strong pressure gradient near the channel inlet. This heat transfer mechanisms became dominant when the channel size fell into the region of the minichannel.

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