本研究是設計於兩平行板間加裝一可忽略熱阻之隔板並加入迴流裝置，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，本研究中應用分離變數法、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 (Nusselt number)。此外，亦探討於改變不同迴流型式、迴流比值大小與隔板位置等參數下的熱傳效率之改善率，並與單行程熱交換器作一番比較。 於迴流系統中，熱傳效率主要是受到兩競爭效應的影響：預混效應及滯留效應，其中預混效應會降低熱傳驅動力(溫度差降低)，而滯留效應則影響對流熱傳係數。結果顯示本系統的熱傳效率會隨著格拉茲數的改變而有所影響；此外迴流系統與隔板位置的改變，亦會造成熱傳效率明顯改變，同時並與單行程來做比較，以求出改善熱傳效率之最佳操作設計條件。 The study on the new device of parallel-plate heat exchangers with uniform wall fluxes and external refluxes has been developed theoretically. The analytical solutions to the resultant mathematical formulations for such conjugated Graetz problems were achieved by using the superposition principle and an orthogonal expansion technique in extended power series. The theoretical esults of heat transfer efficiency are represented graphically and compared with those in single-pass devices (without an impermeable plate inserted and without recycle) of the same working dimension. Analytical predictions show that the preheating and residence-time can affect the heat transfer efficiency in designing parallel-plate heat exchangers with uniform wall fluxes. The influences of the location of the recycle and Graetz number on the heat transfer enhancement as well as power consumption increment have also been discussed.
