板式熱交換器中波浪型通道熱傳之實驗及因次分析

Abstract

本研究係針對板式熱交換器內之波浪型流道，進行局部與平均的熱傳遞性能實驗測試與分析。本實驗中共測試四組具有不同幾何形狀之波浪型流道，而流道表面以大量均勻等溫的熱水維持等壁溫的邊界條件。 流道的幾何參數包括流道波長、波狀角度、波狀部分曲率半徑、直線部分長度與流道寬度等。 本研究以空氣為工作流體，流體流經流道內被加熱，並以T型熱電偶線量測流場內大約數百點的局部溫度。 實驗之控制變因包括流體的雷諾數、波狀角度、流道寬長比、流道直波比等。 實驗所探討流道的熱傳遞性能則包括流體局部溫度與各截面平均溫度、局部與平均紐塞數。 研究結果顯示，局部與平均紐塞數均隨著雷諾數的增加而上升，但局部紐塞數則隨著離流道入口距離的增加而下降。 值得注意的是，熱傳遞在靠近波峰的區域有增強的效果，而局部紐塞數在靠近波峰的區域也有躍升的現象。 局部與平均紐塞數均隨著流道寬長比的增加而微幅增加。 一般而言，局部與平均紐塞數均隨著波狀角度增加而增加，但隨流道直波比的增加而下降。 本研究亦提出在不同操作範圍下之經驗式並與實驗數據做比較。 第二部分是利用柏金漢π理論，針對波浪型彎曲通道之熱傳性能，定義出流道之無因次幾何參數，再透過實驗數據統計分析，最後歸納出無因次參數方程式，藉此找出影響熱傳性能的主要參數。 研究結果顯示，影響Nux參數依次為Re、R/Dh、x/Dh、β、局部溫差及局部普蘭托數，若省略影響Nux較小之參數，得到影響Num的參數依次為Re、R/Dh及β。

This study performed an experimental investigation into the heat transfer characteristics of the corrugated channels within plate heat exchangers (PHEs). Four flow channels with the different geometry were constructed, and a series of experiments were performed in which air was passed through the channels at flow rates corresponding to Reynolds numbers in the range 300~7000 while the channel surface was maintained at a constant temperature. The channel geometry includes the corrugated angle, the width, the curvature radius of the corrugation part, and the length of straight section of the channel. Starting from the entrance, the air temperatures within the test section monitored with T-type thermocouples. Results indicate that both the local and average Nusselt numbers increase with the Reynolds number. The phenomenon that the local Nusselt number decreases with increasing the distance from the entrance commonly occurs in channel flows. It is worth to notice that the heat transfer is greater at sections near the crest after the entrance region. In general, the local and average Nusselt numbers increase with the increased corrugated angle but decrease with the ratio of the length of the straight part to the curvature radius of corrugation of the channel. Finally, the empirical relations are then developed to describe the correlation between the Nusselt number and the Reynolds number in the laminar and turbulent regimes. In Part II, based on the Buckingham Pi theorem, this study derives dimensionless correlations to characterize the heat transfer performance of the corrugated channel in a plate heat exchanger. The experimental data are substituted into these correlations to identify the flow characteristics and channel geometry parameters with the most significant influence on the heat transfer performance. Simplified correlations by omitting the factors with less influence are then obtained. The results show that Nux is affected primarily by Re, R/Dh, x/Dh, and β. Neglecting the minor effect of factors on Nux, it is shown that Num is determined primarily by Re, R/Dh and β.