R-134a冷媒於板式熱交換器中飽和與過冷流動沸騰熱傳特性之實驗研究

Abstract

本篇論文係以實驗研究R-134a冷媒於板式熱交換器中飽和與過冷流動沸騰的熱傳特性，並藉由流場觀測的方式來瞭解氣泡的特徵。實驗用的板式熱交換器是由三片具有山型角60 度結構的板片所形成兩個逆向流的流道，向上流動的R-134a冷媒接受向下流動熱水的熱量而產生沸騰。實驗控制的參數包括: 冷媒質通量、熱通量、系統壓力及過冷度。 飽和流動沸騰實驗的結果指出熱傳係數與壓降會隨著熱通量的增加而增加，在較高的質通量，壓降增加會比熱傳係數明顯。系統壓力從0.6增加到0.8 MPa會減少壓降但對熱傳係數沒有影響，值得注意的是壓力在0.5 MPa時會導致最大壓降與最低熱傳係數。由飽和的實驗結果求得熱傳係數與壓降的經驗公式。 過冷流動沸騰實驗的結果以沸騰曲線和熱傳係數來表示。沸騰曲線會受質通量與飽和溫度的影響，而遲滯的現象只會發生在低質通量。熱傳係數明顯受質通量的影響，受飽和溫度與過冷度的影響就較些微。 由流場觀測的照片顯示較大的熱通量會在板片的表面上產生更多的氣泡且產生的頻率也會增快，氣泡離開板片表面後會與鄰近的氣泡結合成大氣泡往下游移動，因板片特殊結構的流道安排會再破裂成小氣泡，此劇烈的氣泡行為會增強熱傳的效果。氣泡的成長則是受到質通量與過冷度的抑制，在較高的熱通量條件下會使得流譜由氣泡流轉換成環狀流，最後根據過冷實驗結果提出熱傳係數與氣泡離開表面直徑大小的經驗公式。

The saturated and subcooled flow boiling heat transfer of refrigerant R-134a in a plate heat exchanger are measured in this study. Besides, the associated bubble characteristics in the plate heat exchanger are also inspected by visualizing the boiling flow. Two vertical counterflow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the imposed heat flux, mass flux, system pressure and subcooling of R-134a on the saturated and subcooled boiling heat transfer are explored in detail. The experimental data for the saturated flow boiling showed that both the boiling heat transfer coefficient and pressure drop increase almost linearly with the imposed heat flux. At a higher mass flux the pressure drop is substantially higher but the boiling heat transfer coefficient only shows slight improvement. Raising the refrigerant pressure from 0.6 to 0.8 MPa, the frictional pressure drop is significantly lower but the change in the heat transfer coefficient is small. Furthermore, it is noted that at the lowest pressure tested here for P=0.5 MPa the boiling heat transfer coefficient is lowest, but the associated rise in pressure drop is highest. Based on the present data, empirical correlations for the saturated boiling heat transfer coefficient and friction factor are proposed. Next, the results in the subcooled flow boiling are presented in terms of the boiling curves and heat transfer coefficient. The results for the boiling curves show significant change in the slopes of the boiling curves during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, the inlet subcooling and saturation temperature have little effect on the boiling heat transfer coefficient. The photos from the flow visualization for the saturated and subcooled flow boiling revealed that at higher boiling heat flux the plate surface is covered with more bubbles and the bubble generation frequency is higher, and the bubbles tend to coalesce into big bubbles. But these big bubbles are prone to break up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also noted that the bubbles nucleated from the plate were suppressed to a larger degree for higher inlet subcooling and mass flux. At high heat flux the boiling is so intense that there is a two phase flow pattern transition from a bubbly flow to an annular flow in the channel. Finally, empirical correlations for the heat transfer coefficient and the bubble departure diameter in the subcooled flow boiling are proposed.