本研究運用冪次流體為工作流體，設計一可忽略熱阻之隔板於兩平行板間並加入迴流裝置，使之成為於板壁固定熱流通量下，具迴流之二行程平板型熱交換器。此系統所推導之統制方程式屬於共軛格拉茲問題 (conjugated Graetz problem)，為求其解析解，本研究應用分離變數法、重疊原理 (superposition)及正交展開法(orthogonal expansion technique)，求得於板壁固定熱流通量下，兩平板間流體的溫度分佈與平均納塞數 (average Nusselt number)。此外，亦探討於不同流體種類、迴流比值、隔板位置和迴流型式等參數下的熱傳現象之改善效率，並與單行程熱交換器作比較。結果顯示，本研究之實驗數據與理論模擬結果趨勢相符合，並且皆指出於格拉茲數大時，冪次指數愈小的流體具有較好的熱傳效率。 Considerable improvement of heat transfer efficiency for power-law fluids in flat-plate heat exchangers is obtainable by inserting in parallel an impermeable resistless sheet to divide an open duct into two channels for conducting double-flow operations. The influence of the power-law index on Nusselt numbers in double-pass parallel-plate heat exchangers with uniform wall fluxes has been investigated theoretically and experimentally. The analytical solutions were obtained using the separation of variables, superposition principle and an orthogonal expansion technique. The theoretical results of the power-law fluids in the temperature distribution and Nusselt number are compared with those in an open conduit. The effects of power-law index, recycle ratio, impermeable-sheet position and recycle configuration have also been presented graphically. The results show that the good agreement between the experimental results and theoretical prediction. The smaller the power-law index is, the greater is the improvement of heat transfer efficiency, especially for high Graetz number.
