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Comparison of Two Heat Transfer Enhancing Tubes for Boiling Heat Transfer
|關鍵字:||micro-carved cross helical threads|
heat transfer coefficient
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|摘要:||本研究探討內部具微刻紋線(with micro-carved cross helical threads)及內部具鰭片模組(with fin-module insert)之兩支圓管之熱傳特性，實驗中以R-22冷媒在管內沸騰蒸發時，對兩者之熱傳性能分別進行量測。此實驗以直徑3 mm之矽膠電熱線纏繞於測試銅管之外壁，以進行等熱通量之加熱，在0.15、0.25、0.35、0.43 l/min四種不同之冷媒體積流率下及蒸發溫度固定在-12℃~-13℃之範圍內，分別對每一支測試管進行管壁溫度之量測，然後以量測之結果計算冷媒與管壁間相變化熱對流係數之大小，經獲得此兩支測試管之熱傳性能後，並將其與內部光滑 (smooth) 圓管之熱傳性能進行比較。由量測之結果發現，無論是內部具微刻紋線之圓管或內部具鰭片模組之圓管，在不同之冷媒體積流率下，其管壁溫度與冷媒蒸發溫度之溫差，均比內部光滑圓管之溫差為低，且分佈較均勻，因此兩者之熱對流係數亦均比內部光滑圓管之熱對流係數為大。在所考慮之四種冷媒蒸發量下，內部具鰭片模組之測試管為內部平滑圓管之1.76～2.4倍，內部具微刻紋線之測試管為內部平滑圓管的1.36～1.73倍。由以上量測分析之結果可以得知，兩支測試管均具有熱傳增強之效果，兩者均可改善蒸發器之熱傳性能。|
This work investigated the heat transfer performance of two tubes. One is with a fin-module insert and the other is with internal micro-carved cross helical threads. In the experiment, R-22 refrigerant was the working fluid. The boiling heat transfer characteristics of the two test tubes were measured respectively. The external surfaces of the test tubes were wrapped with silicone electric-resistance heating wires for achieving a constant surface heat flux condition. The diameter of the wires is 3 mm. At the evaporation temperature in the range -12℃~-13℃and the volumetric flow rate of the refrigerant at 0.15、0.25、0.35、0.43 l/min, the heat transfer performance of the two tubes were tested. The acquired heat transfer data were compared to those of a smooth tube. The result shows that, at all the considered volumetric flow rates, the temperature differences between tube wall and refrigerant for the two test tubes are smaller than that for the smooth test tube. This means the heat transfer coefficients of both test tubes are higher than that of the smooth tube. The average heat transfer coefficients of the tube with fin-module insert are in the range 1.76～2.4 times of those of the smooth tube and the tube with internal micro-carved cross helical threads are in the range 1.36～1.73 times. Thus the heat transfer is enhanced for both test tubes. This implies that the two test tubes can be used to improve heat transfer performance of evaporators in refrigeration systems.
|Appears in Collections:||機械工程學系所|
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