| Summary: | 碩士 === 建國科技大學 === 機械工程系暨製造科技研究所 === 98 === This work numerically investigates the straightening effect of jet nozzle on heat transfer of an impinging jet onto a porous block. The cooling fluid is the air. The porous blocks are made of aluminum foams with the porosities of 0.97 and 0.93, and have the fixed length (L) and width (W). Two calculating modes are employed to build the systems with/without the straightener, respectively. Mode A, meaning the system with straightener, excludes the region of jet nozzle, whose jet velocity profile is uniform. Mode B, displaying the system without the straightener, includes a sufficiently long region of jet nozzle, whose velocity profile is uniform at the inlet of jet nozzle and changed at the exit of jet nozzle via numerical results. Relevant parameters are the jet Reynolds number (Re=250~1000), the relative jet nozzle width (wj/L=0.125~1), the relative impinging distance (C/L=0~0.5), and the relative porous block height (H/L=0.125~1). The results indicate that the Nusselt numbers predicted by Mode A are obviously different from those by Mode B. Decreasing the wj/L of Mode B leads the velocity profile of fully developed flow at the exit of nozzle, enhancing the total heat transfer effectively. However, a velocity profile of the saddle shape, obstructing the heat transfer, will appear as increasing the wj/L of Mode B. The maximum deviation of Nusselt number (Nu) between the systems with and without straightener is 102% in the case of small H/L. Decreasing the values of wj/L, C/L and H/L generally benefits the heat transfer. But at some ranges of parameters, Nu firstly increases and then falls with increasment of H/L. This work also suggests the optimal height (H/L) of the aluminum foams to have the maximum Nu for the reference of thermal design.
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