微鰭片和平滑管應用於R-1234ze和R-134a冷媒兩相熱對流性質研究

Abstract

全球暖化成為環境保育的主要議題，而其中一項與冷媒排放到大氣相關。本研究目的為探討第四代冷媒R-1234ze的特性。R-1234ze用來取代R-134a，在於它有更低的GWP。探討上述兩種冷媒的壓降與熱傳系數，應用於兩相流，水平對流，沸騰在外徑為9.52mm，70片鰭片，鰭片高度為0.17mm的微鰭管中與相似幾何的平滑管中。不鏽鋼微鰭管的管壁是用黃銅，直接使用電流去加熱而成。T型的熱電偶嵌入於管壁，作為相位改變時量測溫度分佈之用。整個實驗量測操作在 10 - 200C的溫度區間，透過調整不同的冷媒質量流速 ，而流速屆於 50kg/m2s - 300kg/m2s 之間。蒸氣的乾度從 0.1 到 0.9。熱通量從 5kW/m2 到 11kW/m2。實驗結果透過比較兩種冷媒的兩相熱傳係數和壓降得知。結果顯示R-134a無論在微鰭管或平滑管的熱傳表現都比R-1234ze來的好。特別是質量流速G大於50kg/m2s 時。然而，在特定的低質量流速 ，G小於100kg/m2s，R-1234ze 與 R-134a 有相似的熱傳係數。 在所有的質量流速 下R-1234ze的壓力梯度更是顯著的高於 R-134a 。此外在低質量通量 G = 50kg/m2s ，R-1234ze 在更高的熱通量下，有些微的增加壓降 。在高質量通量下飽和溫度沒有影響兩種冷媒的熱傳。Murata 等人，展示最好的熱傳表現預測關係式，預測資料顯示 R-1234ze，在微鰭管 E%=28.9 ，σ% = 37.2 。此外 Kandlikar 等人，預測量好的實驗數據 。R-134a在相同的微鰭管 最低的 E%= 24.9。為了比較 R-1234ze 在相似幾何的平滑管，Shah 的關係式 在平均百分誤差 有最好的熱傳表現 E%= 19.1, 此外 Thome 和 Chisholm 的關係式，在低壓降的 實驗數值中 分別有些微的高估 和低估 。

This research is based on investigations of the fourth generation refrigerant R-1234ze that is considered to substitute R-134a due its low GWP which is as low as 7 in a microfin tube with outer diameter 9.52 mm, number of fins 70, and fin height 0.17 mm. In comparison, a smooth tube with similar geometries was used to study the pressure drop and the enhancement of heat transfer coefficients related to the two fluids. The microfin tube was brazed inside a stainless steel tube and heated electrically with direct current. T-type thermocouples were inserted in the wall to measure the temperature distribution during the phase change process. The experimental measurements were carried out at constant saturation temperatures of 10 – 200C by varying the refrigerant mass velocities between 50 kg/m2s and 300 kg/m2s, the vapor quality from 0.1 to 0.9, and heat flux range from 5 kW/m2 to 11 kW/m2. Results from the two refrigerants and tubes were analyzed and compared basing on the values of the two-phase heat transfer coefficient and pressure drop. The results showed that heat transfer performance of R-134a in both microfin and smooth tube was better than R-1234ze especially at mass velocities above G = 50 kg/m2s. However, at certain low mass velocities below G = 100 kg/m2s R-1234ze yield better heat transfer coefficients than R-134a. The pressure gradient of R-1234ze was markedly higher than that of R-134a at all mass flow rates and heat flux had no strong effect on friction pressure drop for R-134a whereas at low mass flux G = 50 kg/m2s, R-1234ze slightly registered an increase in pressure drop for higher heat flux q = 5 kW/m2. Saturation temperatures had no effect on heat transfer for both refrigerants at high mass flux whereas at lower mass flux G = 50 kg/m2s, R -134a dry-out inception was a bit longer for about x = 0.9 compared to that of R-1234ze. Murata et al. correlation showed the best thermal performance prediction of the data with E%= 28.9 and σ% = 37.2 for R-1234ze in a microfin tube whereas Kandlikar et al. predicted well the experimental data with the lowest E%= 24.9 for R-134a in the same microfin tube. For comparison purpose with R-1234ze in a smooth tube of similar geometries, Shah Correlation had the best thermal performance with an average percentage error of E%= 19.1 whereas Thome and Chisholm correlations slightly over-predicted and under-predicts respectively the experimental data at low pressure values.