Two-Phase Heat Transfer of Refrigerants HFC-134a and HFO-1234yf in Small Channels

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Title:	Two-Phase Heat Transfer of Refrigerants HFC-134a and HFO-1234yf in Small Channels
Authors:	聶翰明;Abhar, Hamid Nalbandian
Contributors:	機械工程學系
Keywords:	流動沸騰;流動冷凝;微小管;微流道;兩相流;熱傳係數;flow boiling;flow condensation;small tube;microchannel;two-phase flow;heat transfer coefficient
Date:	2021-07-01
Issue Date:	2021-12-07 13:15:15 (UTC+8)
Publisher:	國立中央大學
Abstract:	因應全球暖化問題，各國訂定法規將逐漸淘汰氫氟碳化合物的使用(hydrofluorocarbon，簡稱HFC，為常見冷媒)。其中1,1,1,2-四氟乙烷（HFC-134a）是用於小型冰箱與汽車空調中的冷媒，更是已經在歐盟被全面禁用，因此尋找替代性冷媒迫在眉睫。近年來一種新型冷媒問世—2,3,3,3-四氟丙烯（HFO-1234yf），其為熱力性質和HFC-134a相近的氫氟烯烴（Hydrofluoroolefins，簡稱HFO），且其全球暖化潛勢（Global warming potential，簡稱GWP）又在規範之內，是有望取代HFC-134a的冷媒。然而目前關於HFO-1234yf的熱傳性能研究卻少之又少，應用在空調系統內微流道熱交換器中的更是寥寥無幾，如果是水力直徑小於0.9mm的微流道研究則是完全沒有。本研究針使用直徑4mm的圓管及水力直徑0.5mm的正方形平行流道扁平管作為測試段，量測HFC-134a及HFO-1234yf的沸騰及冷凝熱傳性能、壓降，並使用冷媒熱力性質、實驗條件差異、流譜等因素對實驗結果進行詮釋。由於不同的實驗條件下會造成不同的流譜，進而導致不同熱力性質對熱傳、壓降結果的影響比重改變，因此HFC-134a及HFO-1234yf的結果差異也會相應的做出轉換。此種假設同時也可以解釋過去文獻中的不一致之處。本研究的另一個目的是探討流道幾何形狀對於微流道熱傳表現的影響。實驗結果顯示表面張力對於方形微流道的熱傳影響甚鉅，表面張力會將液體聚集至方形流道的角落，使得管壁的液膜變得非常薄，進而提升熱傳性能。而除了表面張力之外，液體黏滯性、液體熱傳導係數也都是影響方形微流道熱傳性的的主要原因。 ;The hydrofluoroolefin (HFO) HFO-1234yf has similar thermodynamic properties to ?HFC-134a but a considerably lower global ?warming potential?. HFO-1234yf is a favorable ?candidate to replace the refrigerant HFC-134a in the near ?future. However, few studies have ?examined the ?flow boiling and condensation heat transfer performance of ?HFO-1234yf. ?Microchannel heat exchangers have been commonly used in air conditioning ?systems in vehicles due to their low weight and low air-side pressure drops. However, no ??experimental study has been conducted on the flow boiling and condensation heat ?transfer performance of HFO-??1234yf in microchannels with a hydraulic diameter of less than ??0.9 mm.? In this study, an experimental analysis was conducted on the flow boiling and condensation heat ?transfer, and pressure drops of HFO-1234yf and HFC-134a in a small ?circular tube ?with an inner diameter of 4mm and a square parallel microchannel with a hydraulic ?diameter of 0.5 mm. ?The test results indicate that the pressure drop, flow boiling, ?and condensation heat ?transfer performance depend on the fluid properties, flow ?conditions, and flow patterns. The ?pressure drops and heat transfer coefficients of HFO-??1234yf and HFC-134a are strongly affected by their two-phase flow patterns at various flow ?conditions. Flow pattern ?analysis can explain the inconsistency between the results of this ?study and those of previous studies on the flow boiling and condensation heat transfer and pressure of HFC-??134a and HFO-??1234yf.? One purpose of this study was to determine the effects of channel geometry on the flow ?boiling and condensation inside a microchannel. The experimental results indicate that the? ?surface tension drainage force has crucial effects on the flow boiling and condensation heat ??transfer in square channels, especially for small channels. The surface tension drainage ?force ?pulls the liquid at the wall to the corner of square microchannels. This drainage ?force causes a ?very thin liquid film to remain on the tube wall. Surface tension, liquid viscosity, ?and liquid ?conductivity are the major factors affecting the flow boiling and ?condensation heat transfer in ?square microchannels.?
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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