【作者】 胡海涛；

【导师】 丁国良；

【作者基本信息】 上海交通大学 ， 制冷及低温工程， 2008， 博士

【摘要】 环保制冷剂R410A为近共沸混合物,温度滑移微小,具有优良的传热特性和流动特性,是R22的理想替代物。采用小管径换热管是降低R410A空调蒸发器成本和改善能效的重要手段之一。目前外径为7.0 mm和5.0 mm的小管径换热管正在被广泛地应用于R410A空调蒸发器,且有采用更小管径换热管的趋势。润滑油的混入增加了R410A在这类换热管内流动沸腾换热的复杂性。如何计算R410A-油混合物在小管径换热管内的流动沸腾换热系数和压力损失,定量评价润滑油的混入对蒸发器换热管内换热与压降性能的影响,对于设计开发紧凑式蒸发器,促进小管径换热管的工程应用,推动环保制冷剂R410A替代R22的进程,具有重要价值。本文从R410A-油混合物在单根换热光管和强化管内的流动沸腾换热特性和压力损失特性等基础性问题的研究入手,以实验为手段,以开发工程应用的换热及压降模型为目的,为预测润滑油对蒸发器换热系数、压力损失以及蒸发器性能等的影响奠定了一定基础。取得了以下几方面的成果:1.对R410A-油混合物在7.0 mm和5.0 mm光管内的流动沸腾换热特性进行了实验研究,并对流动型态进行了观测。研究发现润滑油的混入可以促进间歇流的提前形成,并延迟流型从环状流向干涸流的转化,从拍摄的流型可以发现,R410A-油混合物流动沸腾过程中存在发泡现象,这是润滑油增强换热的一个影响因素。基于R410A-油混合物的物性,开发了R410-油混合物在光管内流动沸腾的流型图,并将实验中观测的流型与开发的流型图进行验证,结果表明,实验观测的流型与流型图吻合较好。开发了基于混合物的流型和物性的R410A-油混合物在光管内的两相流动沸腾换热关联式。对于7.0 mm光管内的换热数据,新的关联式预测值与96%的实验数据的误差在±20%以内;对于5.0 mm光管内的换热数据,新的关联式预测值与92%的实验数据的误差在±30%以内,可以很好的预测R410A-油混合物在光管内流动沸腾的换热特性。2.对R410A-油混合物在7.0 mm和5.0 mm光管内流动沸腾的压降特性进行了实验研究,研究结果表明,R410A-油混合物的摩擦压降随平均油浓度的增大而增大,而且随着质流密度和干度的增大而增大。随着质流密度的增大,油的存在对摩擦压降的影响减小。与低干度和中等干度工况下相比,高干度工况时润滑油对压降的影响会增大。开发了基于混合物物性的R410A-油混合物在光管内的压降关联式,对于7.0 mm光管,新关联式的预测值与92%的实验数据误差在±20%以内;对于5.0 mm光管,新的关联式预测值与95%的实验数据误差在±25%以内。新的关联式能够很好地预测R410A-油混合物在光管内流动沸腾的压降特性。3.对R410A-油混合物在7.0 mm和5.0 mm强化管内流动沸腾的换热特性进行了实验研究,研究结果表明,在低干度工况下,换热系数随平均油浓度的增大而增大,在高干度时,随着干度和平均油浓度的增大,换热系数迅速降低;随着管径的减小,润滑油对强化管内换热系数的恶化作用减小;与光管内的换热特性相比,润滑油的存在对强化管内换热的影响较小,尤其在高干度工况下,润滑油会积聚在强化管的螺纹之间,会弱化强化管内强化结构对换热的扰动,因此在高干度工况下,润滑油的存在会恶化换热。开发了基于混合物物性的R410A-油混合物在强化管内的换热关联式,考虑了螺纹强化结构对换热的增强作用,同时考虑了润滑油的存在对混合物物性的影响。对于7.0 mm强化管内的换热数据,新的关联式预测值与87%的实验数据的误差在±30%以内;对于5.0 mm光管内的换热数据,新的关联式预测值与85%的实验数据的误差在±30%以内。新的关联式可以很好的预测R410A-油混合物在强化管内流动沸腾的换热特性。4.对R410A-润滑油混合物在7.0 mm和5.0 mm强化管内流动沸腾过程中的压降特性进行了实验研究,研究结果表明,润滑油的存在会增大压降,在高干度工况下,这种增强作用更明显。润滑油对5 mm强化管内压降的增强作用大于润滑油对7 mm管内压降的增强作用,表明随着管径的减小,润滑油对压降的增强影响变大。与光管内的压降相比,强化管内润滑油对压降的影响较小。开发了基于混合物物性的R410A-油混合物在强化管内的压降关联式,考虑了螺纹强化结构对压降的增强作用,同时考虑了润滑油的存在对混合物物性的影响。新的关联式与95%的实验数据误差在±20%以内;对于5.0 mm强化管,新的关联式预测值与93%的实验数据误差在±20%以内。新关联式能够很好的预测R410A-油混合物在强化管内流动沸腾的压降特性。最后给出了由于时间关系本文尚没有深入研究的问题,以及将来应重点关注的相关研究方向。更多还原

【Abstract】 R410A is one kind of environmentally-friendly refrigerants, and it is an ideal substitute refrigerant for R22 for its small temperature glide and good heat transfer and flow characteristics. It is an important way to utilize small diameter tubes to reduce the cost and improve efficiency of evaporator, and recently, tubes with outside diameter of 5 mm and 7 mm have been used widely in evaporator of R410A air-conditioner. Smaller and smaller tubes will be used into evaporator in the near future. Heat transfer of refrigerant flow boiling in such scale tubes with oil presence is more complex than that with oil-free. How to predict the heat transfer and pressure drop characteristics of R410A-oil mixture flow boiling inside such scale tubes? How to estimate quantitatively influence of oil on performance of evaporator? To give the reply to these issues is important for promoting practical application of mini-scale tubes in compact evaporator design and promoting the substitute of R22 by R410A.This paper investigated influence of oil on characteristics of flow pattern, heat transfer, and pressure drop of R410A-oil mixture flow boiling inside small smooth and enhanced tubes by experimental and theoretical methods. Performance analysis platform is built for this investigation, which is a technical and theoretical foundation to analyze influence of oil on performance of evaporator. The main results and finding are summarized as following.1.Experimental study for the heat transfer characteristics of R410A-oil mixture flow boiling inside small smooth tubes was performed, and the flow pattern of R410A-oil was studied. The test results show that the presence of oil promote the the transformation of flow pattern from“Slug”to“Intermittent”, while it delays the transformation of flow pattern from“Annular”to“Dryout”and from“Dryout”to“Mist”; the flow patterns observed during the experiment also show that the foaming was notable for nearly all test conditions, which is one of effect factors for oil to increase the heat transfer coefficient of refrigerant-oil mixture. A new flow pattern map for R410A-oil mixture flow boiling inside small smooth tubes was developed based on the properties of R410A-oil mixture, and the observed flow patterns match well with the flow pattern map. A new correlation to predict the local heat transfer of R410A-oil mixture flow boiling inside small smooth tubes was developed based on flow patterns and local properties of refrigerant-oil mixture, and it agrees with 96% of the experimental data within the deviation of±20% for 7.0 mm smooth tube and with 92% of the experimental data within the deviation of±30% for 5.0 mm smooth tube, respectively. The new correlation can provide satisfied predictions to the heat transfer characteristics of R410A-oil mixture flow boiling inside smooth tubes.2.Experimental study for the pressure drop characteristics of R410A-oil mixture flow boiling inside small smooth tubes was performed. The test results show that the frictional pressure drop of R410A-oil mixture flow boiling inside smooth tubes increases with the increase of nominal oil concentration, mass flux and vapor quality; the effect of oil on frictional pressure drop decreases with the increase of mass flux, and the effects at low and intermediate vapor qualities is higher than that at high vapor qualities. A new correlation to predict the frictional pressure drop of R410A-oil mixture flow boiling inside small smooth tubes was developed based on the experimental data and the local properties of R410A-oil mixture, and it agrees with 92% of the experimental data within the deviation of±20% for 7.0 mm smooth tube and with 95% of the experimental data within the deviation of±25% for 5.0 mm smooth tube, respectively. The new correlation can provide satisfied predictions to the pressure drop characteristics of R410A-oil mixture flow boiling inside smooth tubes.3.Experimental study for the heat transfer characteristics of R410A-oil mixture flow boiling inside small enhanced tubes was performed. The test results show that the heat transfer coefficient of R410A-oil mixture flow boiling inside small enhanced tubes increases with the increase of nominal oil concentration at low nominal oil concentration, while it decreases rapidly with the increase of vapor quality and nominal oil concentration; the effect of oil on the heat transfer coefficient for enhanced tubes is small than that for smooth tubes, especially at high vapor quality, the oil may be retained between the microfins, and then reduce the disturbing effect of microfin on fluids, which reduces the convective heat transfer at high vapor quality. A new correlation to predict the local heat transfer of R410A-oil mixture flow boiling inside small enhanced tubes was developed based on the local properties of refrigerant-oil mixture, and it agrees with 87% of the experimental data within the deviation of±30% for 7.0 mm enhanced tube and with 85% of the experimental data within the deviation of±30% for 5.0 mm enhanced tube, respectively. The new correlation can provide satisfied predictions to the heat transfer characteristics of R410A-oil mixture flow boiling inside enhanced tubes.4.Experimental study for the pressure drop characteristics of R410A-oil mixture flow boiling inside small enhanced tubes was performed. The test results show that the presence of oil increase the frictional pressure drop, and the enhanced effect at high vapor quality is evidence than that at low and intermediate vapor qualities; the effect of oil on frictional pressure drop for 5 mm enhanced tube is higher than that for 7 mm enhanced tube, which means that the effect of oil on frictional pressure drop increases with the decrease of the tube diameter; the effect of oil on frictional pressure drop for enhanced tubes is small than that for smooth tubes. A new correlation to predict the frictional pressure drop of R410A-oil mixture flow boiling inside small enhanced tubes was developed based on the experimental data and the local properties of R410A-oil mixture, and it agrees with 95% of the experimental data within the deviation of±20% for 7.0 mm enhanced tube and with 93% of the experimental data within the deviation of±20% for 5.0 mm enhanced tube, respectively. The new correlation can provide satisfied predictions to the pressure drop characteristics of R410A-oil mixture flow boiling inside enhanced tubes.At the end of this dissertation, the author presented the main weakness and the further key points should be focused on in the near future.更多还原