R-407C冷媒在水平小圓管內蒸發熱傳及壓降特性之實驗研究

Abstract

本論文是針對R-407C冷煤在28根(內徑2mm、外徑3mm)的水平小圓管內蒸發熱傳及壓降特性之實驗研究。實驗的目的是探討改變冷煤質通量、測試段的熱通量、測試段的進口蒸氣乾度及系統的飽和壓力(相對應之飽和溫度)對R-407C冷煤在本熱交換器的影響。實驗參數的範圍，冷媒質通量G從200到400 kg/m2s，測試段熱通量q從5到15 kW/m2，進口蒸氣乾度xin從0.15到0.8，系統壓力設定在667，776和900 kPa (Tsat = 5, 10 and 15℃)。 經由實驗的結果可知，蒸發熱傳係數與摩擦壓降係數除了在較低的冷煤質通量狀態下，皆會隨著冷煤質通量的加大與進口乾度的增加而上升。在熱通量的影響方面，除了在較低的熱通量情況下，蒸發熱傳係數也會隨著熱通量的加大而上升。但相較於熱通量對蒸發熱傳的影響，改變熱通量對摩擦壓降係數的影響則是很微弱的。而在飽和溫度的影響上，蒸發熱傳係數會隨著系統的飽和溫度上升而增加；但相反的情況則發生在降低系統的飽和溫度會造成摩擦壓降係數上升。因此，成核與對流效應對本實驗系統的熱傳機制皆有重要的影響。最後我們將蒸發熱傳係數與摩擦壓降係數的實驗資料作分析，求出經驗公式。

An experiment is carried out here to unravel the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-407C flowing in horizontal small circular tubes, each having an inside diameter of 2.0 mm. The effects of the imposed wall heat flux, mass flux, and vapor quality and saturated temperature of R-407C on the measured evaporation heat transfer coefficient and frictional pressure drop are examined in detail. In the experiment the refrigerant mass flux is varied from 200 to 400 kg/m2s, imposed heat flux from 5 to 15 kW/m2, inlet vapor quality from 0.15 to 0.8, and saturated temperature from 5 to 15℃. The experimental data clearly show that both the evaporation heat transfer coefficient and frictional pressure drop increase with the refrigerant mass flux and vapor quality except at low mass fluxes. A rise in the imposed heat flux results in a significant increase in the evaporation heat transfer coefficient except at low heat fluxes. Nevertheless the influence of the imposed heat flux on the frictional pressure drop is rather slight. At increasing saturated temperature of the refrigerant the evaporation heat transfer coefficient increases significantly. But an opposite trend is noted for the frictional pressure drop. As a result, both the convection and nucleation can be important. This is indeed the case according to the experimental data presented above. Finally, the empirical correlations for the evaporation heat transfer coefficient and friction factor are proposed.