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  • 學位論文

微熱交換器熱傳與流體流動現象之研究

A Study on the Heat Transfer and Fluid Flow Phenomena of Microchannel Heat Exchangers

指導教授 : 鄧治東

摘要


數值模擬及實驗測試係被用於研究具流體流動現象及熱傳特徵之矩形微熱交換器,本論文 探討於微熱交換器中流體之熱傳與流體流動行為,並以實際效能值(actual effectiveness)和效能值 (effectiveness, 亦即-NTU) 探討五類微熱交換器之熱傳與流體流動性能。 接著,本研究計算了微熱交換器之流動排列對熱交換功能之影響(以熱傳率與壓降比表示 之),此功能值(performance index)獲自數值模擬結果及實驗數據。對於所有個案之研究,反向流 動熱交換之功能值係高於自平行流動熱交換所得值:從反向流得到之值為從平行流得到之值的 1.13 倍至1.2 倍。 本文藉實驗方式探討幾何結構參數對微熱交換器於其性能值上之影響。幾何結構參數包括 基板厚度、截面積和進出口的位置。為評估此類影響,乃進行兩類流體流動之條件(熱端的不同 入口溫度與冷端的不同質量流率)。於此研究中,熱交換器功能值於其整體最薄厚度中最高達 21.67 W/kPa;熱通量在熱交換器之最淺微流道中最高則達18.7 W/cm2,而與S 型整體結構作比 較,最低之壓降在I 型整體結構之熱交換器達506 Pa。 本文亦藉數值的與實驗的方式探討熱傳與壓降於微熱交換器與minichannel 之比較,於本 研究之流道在相同的平均流速中,從微熱交換器所得之效能值為minichannel 所得之效能值的 1.2 至1.53 倍。 此外,本文進行實驗研究,以探討重力效應對熱傳與壓降行為在微熱交換器上之影響。結 果顯示,重力對微熱交換器之影響是微乎其微的。 再者,微熱交換器於熱流行為上之數值模擬係藉COMSOL 軟體,該軟體具處理穩態與暫 態條件之功能。模擬結果顯室於0.2 秒之內,微熱交換器的熱傳與流體流動行為可自暫態解獲 得,而自0.2 秒之後基本上其暫態解趨向於其穩態解。 整體來看,本文於探討微熱交換器於熱傳與流體流動之行為,所進行之數值模擬所得之結 果與自流體流動實驗數據所得之結果兩者相比較,兩者間係屬吻合,此兩結果之最大百分比差 別係小於10%。

並列摘要


Numerical simulations and experimental tests were carried out to study the fluid flow and heat transfer characteristics for rectangular-shaped microchannel heat exchangers. The present study firstly investigated the influences of fluid properties on behaviors of heat transfer and fluid flow of a microchannel heat exchanger. The results obtained for the actual effectiveness and for the effectiveness (the so-called effectiveness-NTU method) were determined. Next, the study evaluated the effects of flow arrangement on the performance index (expressed as the ratio of the heat transfer rate to the pressure drop) of a microchannel heat exchanger. For all cases done in this study, the performance index obtained from the counter-flow arrangement is always higher than that obtained from the parallel-flow one: the value obtained from the counter-flow is 1.13 to 1.2 times of that obtained from the parallel-flow. The influences of configurations on the performance index of microchannel heat exchangers were presented experimentally. The effects of geometrical configuration consist of substrate thickness, crosssectional area, and inlet/outlet location. In this study, the highest performance index of 21.67 W/kPa was achieved for the heat exchanger with the thinnest thickness; the highest heat flux of 18.7 W/cm2 was achieved for the heat exchanger with the shallow channel; and the lowest pressure drop of 506 Pa was achieved for the heat exchanger with the I-type configuration, as compared to those with the Stype ones. The present study also investigated comparison of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, both numerically and experimentally. At the same average velocity in the channels used in this study, the effectiveness obtained from the microchannel heat exchanger was 1.2 to 1.53 times of that obtained from the minichannel heat exchanger. Moreover, the dissertation presented an experimental study of the effects of gravity on heat transfer and pressure drop behaviors of the microchannel heat exchanger. The results showed that the influence of gravity in microchannel heat exchanger was negligibly small. Furthermore, the numerical simulations of microchannel heat exchangers using solver (COMSOL software package) with the capability of dealing with steady-state and time-dependent conditions were carried out. It was observed that after 0.2 s, the heat transfer and fluid flow behaviors obtained by timedependent solver for microchannel heat exchanger become essentially the same as those obtained by steady-state solver. On the whole, good agreements were achieved for the behaviors of heat transfer and fluid flow between the results obtained from numerical simulations and those obtained from experimental data for fluid flowing in the microchannel heat exchangers used in this study, with the maximum percentage difference between the two results of less than 10%.

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