| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 100 === The heat transfer performance in the front surface of a serpentine heat exchanger was investigated. The heat exchanger is composed of rectangular tubes. During operation, gas vertically contacts with the front surface of the tubes and then it passes through a narrow spacing in between two neighboring tubes. Before the gas flows into the narrow spacing, it would impinge on the lateral side of the tubes. This work focuses on the impingement heat transfer in this region. A staggered-grid finite-difference method was used to discretize the continuity, Navier-Stokes and energy equations into a set of finite difference equations. These equations were converted into a dimensionless form and then solved following the SIMPLE algorithm. The computer program was compiled using Fortran language. In the analysis, the flow was two-dimensional and laminar. Two Reynold numbers (Re=50 and 100), and two flow-field contraction ratios (0.25 and 0.5) were considered respectively. The Prandtl number was 0.7 and the wall was considered to be isothermal. The result of the fluid analysis shows that, the smaller the contraction ratio (= 0.25), the larger the change of the flow direction. The result of the heat transfer analysis shows that, at the same contraction ratio, for an increase of the Re from 50 to 100, the local Nusselt number would increase. But the average Nusselt number does not increase two-folds. At the Re of 50, the contraction ratio almost does not have any effect on the average Nusselt number. At the Re of 100, as the contraction ratio decreases, the average Nusselt number value slightly increases.
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