R-410A冷媒於垂直板式熱交換器與水平雙套鰭管中流動沸騰與冷凝熱傳特性之實驗研究

Abstract

本論文係以實驗方式研究環保冷媒R-410A(R-32與R-125之混合冷媒)在垂直板式熱交換器中之飽和、蒸發與凝結之熱傳與摩擦壓降特性，與其在水平雙套鰭管中之過冷、飽和與蒸發之熱傳特性及加熱過程中氣泡生成與發展機制。 實驗中包含兩套測試段分別是垂直板式熱交換器與水平雙套鰭管，板式熱交換器係由三片不鏽鋼片經衝壓而組成兩個流道，其溝槽之幾何形狀近似正弦函數，山形紋與側邊成60°。於飽和與蒸發實驗時，冷媒R-410A由下而上流動吸收另一流道中由上而下熱水側之熱量而蒸發。在凝結熱傳實驗時，冷媒與冷水之流動方向恰好相反。另一測試段為一雙套管，內管由商用鰭管所組成，外管使用Pyrex玻璃製成並以法蘭與螺栓組裝，以方便觀測流場，鰭管內部裝置加熱器以作為提供冷媒加熱蒸發之用。在垂直板式熱交換器之飽和熱傳實驗部分，實驗首先以Modified Wilson plot方法求取單相水與冷媒之熱傳係數及摩擦壓降，其結果並與R-134比較，結果顯示冷媒R-410A之單相熱傳係數高於R-134a約25%到35%，而摩擦壓降卻相對低R-134a，此歸因於R-410A冷媒具有較佳的熱力性質。其次，飽和熱傳實驗結果用以說明測試段冷媒平均乾度、冷媒流量、加熱量與系統壓力對飽和熱傳係數與摩擦壓降之影響。實驗數據顯示，飽和熱傳係數與摩擦壓降幾乎與加熱量成線性增加，此外在高加熱量區域內冷媒流量對熱傳係數有明顯提升之效果。而升高系統壓力對降低摩擦壓降有明顯助益，但對熱傳係數之影響有限。 第二部分探討冷媒R-410A之蒸發熱傳與摩擦壓降，結果顯示熱傳係數與摩擦壓降隨冷媒流量與平均乾度之增加而遞增，然而在低乾度區域內熱傳係數受冷媒流量之影響較不明顯；整體而言，摩擦壓降隨乾度之增加率大於熱傳之增益效果，此外升高加熱量對熱傳係數有重大之增加，相對地，加熱量對摩擦壓降影響較不顯著，增加系統壓力將使熱傳係數與摩擦壓降降低。最後並依據實驗數據提出熱傳係數與摩擦壓降之經驗公式。 實驗之第三部分探究R-410A冷媒於垂直板式熱交換器中之凝結熱傳與摩擦壓降實驗，其結果顯示熱傳係數與摩擦壓降均隨平均乾度之增加而呈線性遞增，但系統飽和壓力對兩者之影響相對較小，而增加冷媒流量與加熱量對熱傳係數有提升之效果，但將伴隨摩擦壓降之增加。此外，在低乾度區域內，加熱量對凝結熱傳係數與壓降之影響遠大於冷媒流量之影響。 第四部分主要探討在水平雙套鰭管中過冷流動沸騰之熱傳特性，實驗結果以沸騰曲線、熱傳係數與流場觀測照片說明冷媒在加熱表面氣泡形成與成長之機制，由實驗結果顯示在單相區域中，熱傳係數與加熱量無關，而隨冷媒流量之增加而增加，然而在沸騰區域中，熱傳係數隨加熱量之增加而上升，流動中之遲滯現象在低冷媒流量受冷媒流量之影響較輕微，而飽和溫度與入口過冷度對遲滯現象之影響則不明顯。此外，較低的飽和溫度與入口過冷度則可輕微地提升熱傳熱數，藉由流場觀測得知，升高冷媒流量與入口過冷度將使氣泡生成受到抑制，另外加熱量對氣泡之碰撞、結合與產生頻率有較大的影響。 最後部分則以實驗探討冷媒R-410A在水平雙套鰭管中之飽和與蒸發熱傳特性與氣泡生成、成長與脫離過程，由飽和沸騰曲線中發現，遲滯現象與壁面過熱度非常不明顯，而飽和沸騰曲線主要受加熱量與冷媒流量所左右。增加加熱量與冷媒流量將有助於飽和熱傳係數之提升，此外較高的冷媒流量將造成氣泡脫離加熱表面之直徑降低，而較高的加熱量將使氣泡於加熱表面成長更加迅速。在蒸發部分，蒸發熱傳係數受乾度、加熱量與冷媒流量影響較大，而飽和壓力之影響較不顯著。

Measurements have conducted to investigate the saturated flow boiling, evaporation and condensation heat transfer, and associated frictional pressure drop of the ozone friendly refrigerant R-410A (a mixture of 50 wt% R-32 and 50 wt% R-125) in a vertical plate heat exchanger. Besides, the flow boiling heat transfer (including subcooled, and saturated flow boiling and evaporation heat transfer) and associated bubble characteristics in a horizontal annular finned duct with the integral low fins on the outside surface of the heated inner pipe were also carried out in the present study. There are two test sections in the present study including a vertical plate heat (PHE) exchanger and a horizontal annular finned duct. In the PHE two vertical counter flow 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-410A in one channel receives heat from the downflow of hot water in the other channel for the saturated flow boiling and evaporation heat transfer tests. In the condensation heat transfer test, upflow of the R-410A liquid-vapor mixture condenses in one channel and rejects heat to the downflow of cold water in another channel. The test section for the horizontal annular finned duct consists of an outer pipe made of Pyrex glass and an inner copper pipe, intending to measure the heat transfer coefficient and to facilitate the visualization of boiling processes. A cartridge heater is installed inside the finned pipe to supply the required heat flux to the refrigerant flow in the annular duct. In the first part of the present study, the saturated flow boiling heat transfer and the associated frictional pressure drop of refrigerant R-410A flowing in the vertical PHE are investigated experimentally. The effects of the mean vapor quality, refrigerant mass flux, imposed heat flux and system pressure on the saturated flow boiling heat transfer coefficient and associated frictional pressure drop are explored. At first, the measured data for the liquid water-to-water single-phase convection heat transfer test is collected and analyzed by the Modified Wilson plot. The obtained single-phase heat transfer coefficient and frictional pressure drop for R-410A are compared with those for R-134a. The comparison indicates that the single phase heat transfer coefficient for R-410A is about 25% to 35% higher than that for R-134a. In the saturated flow boiling heat transfer tests, the measured data show that both the boiling heat transfer coefficient hr and frictional pressure drop △Pf increase almost linearly with the imposed heat flux. Furthermore, the refrigerant mass flux exhibits significant effect on hr only at higher imposed heat flux. For a rise of the refrigerant pressure from 1.08 to 1.44 MPa, the frictional pressure drop is found to be lowered to a noticeable degree. However, the saturated temperature of the refrigerant has a very slight influence on hr. Finally, empirical correlations are proposed for hr and △Pf. In the second part of the present study, the evaporation heat transfer coefficient and associated frictional pressure drop of R-410A in the vertical PHE are measured. The results manifest that the evaporation heat transfer coefficient and pressure drop increase substantially with the refrigerant mass flux and vapor quality in most situation. It is further noted that the evaporation heat transfer coefficient is only slightly affected by the refrigerant mass flux at a low vapor quality. Furthermore, the increase of the frictional pressure drop with the vapor quality is more prominent than that in heat transfer enhancement. Moreover, a rise in the imposed heat flux results in a significant increase in the evaporation heat transfer coefficient. Nevertheless, the influence of the imposed heat flux on the frictional pressure drop is rather slight. Both the evaporation heat transfer coefficient and frictional pressure drop reduce as the system pressure increases. Finally, empirical correlations for the measured data are provided to facilitate the design of evaporators using R-410A. In the third part of this study, the condensation heat transfer coefficient and associated frictional pressure drop of R-410A in the vertical PHE are measured. The results indicate that the condensation heat transfer coefficient and associated frictional pressure drop almost increase linearly with the mean vapor quality but the system pressure exhibits relatively slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger pressure drop. Besides, the effects of the imposed heat flux on the condensation heat transfer coefficient and pressure drop is stronger than those of the refrigerant mass flux especially at low vapor quality. An empirical correlation for the measured data is also provided. Then in the fourth part of the study, the subcooled flow boiling heat transfer and associated bubble characteristics of R-410A in the horizontal annular finned duct are examined. The experimental results are presented in terms of boiling curves, flow boiling heat transfer coefficient and flow photos. The result indicates that the single-phase forced convection heat transfer coefficient is independent of the imposed heat flux but is dependent of the refrigerant mass flux. However, the subcooled flow boiling heat transfer coefficient increases with the imposed heat flux. The boiling hysteresis is slightly affected by the refrigerant mass flux, especially at a lower mass flux. The effects of the saturated temperature and inlet liquid subcooling on the hysteresis unnoticeable. Meanwhile, lowering the saturation temperature and inlet liquid subcooling of the refrigerant results in a slight improvement in the boiling heat transfer. Visualization of the boiling processes reveals that the bubbles are suppressed by raising the refrigerant mass flux and inlet liquid subcooling. Moreover, the imposed heat flux shows large effects on the bubble population, coalescence and generation frequency. Finally, the saturated flow boiling and evaporation heat transfer of R-410A in the horizontal annular finned duct are investigated. The saturated flow boiling curves show that no boiling hysteresis is detected in the experiments and the wall superheat needed for the onset of nucleation boiling is small. Besides, the boiling curves are mainly affected by the imposed heat flux and refrigerant mass flux. The measured saturated flow boiling heat transfer coefficient increases with the imposed heat flux and refrigerant mass flux. Furthermore, at a higher refrigerant mass flux the mean bubble departure diameter is small. And the bubble growth is substantially faster for a higher imposed heat flux. In the evaporation heat transfer experiments of R-410A in the annular finned duct, the heat transfer coefficient increases significantly with the mean vapor quality, imposed heat flux and refrigerant mass flux. However, the refrigerant saturated temperature shows negligible effects on the evaporation heat transfer coefficient.