Heat Transfer And Pressure Drop Of R410A-Oil Mixture Flow Boiling Inside Tubes

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.