Experimental Study On CO 2 Residential Air-condition System And Numerical Simulation On Enhanced Heat Transfer Of Heat Exchangers

In recent years, the problems of environment and energy are more and more serious. Ozone depletion, greenhouse effect and energy crisis have become the main challenges which restrict the development of human being. The refrigerants used in present refrigeration and air-condition systems not only make ozonosphere destroyed seriously but also cause greenhouse effect. Looking for substitute refrigerant has become an exigent task for refrigeration industry.On the other hand, the requirement for energy is more and more increasing, and the combustion of fossil energy exhausts a large amount of greenhouse gas which aggravates the greenhouse effect. At the same time, the fossil energy is drying up day by day. So it is very important to study the enhanced heat transfer and decreased pressure drop characteristics of the heat transfer equipments which are widely used in many fields.The present dissertation contains experimental study and theoretical analysis. In the experimental study, test rig of the performance of CO2 residential air-condition system is set up; experimental study on the performance of the system is performed. In the theoretical analysis, grid generation and numerical simulation codes in body fitted coordinates are developed; numerical study on air side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchangers are performed. The major contents and findings are as follows.Experimental study on CO2 residential air-condition system:(1) Test rig of the performance of CO2 residential air-condition system is set up. The experimental results of COP and heat transfer and fluid flow characteristics of the heat exchangers show that COP of the CO2 system which generally locates between 2 and 3, is a bit lower than that of traditional air-condition system; enhancing the cooling effect of the gas cooler is an effective approach to improve the performance of the system; the COP of theThe present work is supported by the Key Project of National Natural Science Foundation of China (No.50736005), National Basic Research Program of China (973 Program) (2007CB206902) and the Key Grant Project of Chinese Ministry of Education (No.306014). CO₂ system can achieve the level of traditional system by cycle optimization and adopting proper enhanced heat transfer method to the gas cooler.(2) Throttling loss of CO₂ transcritical cycle, effect of throttling loss on COP and influencing factors on throttling loss are analyzed. The conclusions are derived that inner heat exchanger can decrease throttling loss of the cycle in a large degree, but to the cycle with inner heat exchanger, there is still 5% capacity loss by throttling, which decreases COP about 20%; cooling effect of gas cooler is the major factor which influences throttling loss.Numerical study on enhanced heat transfer and decreased pressure drop characteristics of wavy fin-and-tube heat exchangers:(3) Three-dimensional grid generation code and numerical simulation code based on SIMPLE algorithm are developed in body fitted coordinates. At present, heat exchangers used in air-condition system are mostly wavy fin-and-tube type. There are wavy channels with tubes between fin surfaces. Due to the complicated geometric configuration, the grid generation is very difficult, so three-dimensional numerical simulations on them have not been opened out until recent years. In present dissertation, body fitted coordinates and structured grid generation methods are proposed, three-dimensional grid generation code and numerical simulation code based on SIMPLE algorithm are developed, which can be used to simulate the heat transfer and fluid flow problems in complicated regions.(4) Effects of Reynolds number, wavy angle, fin pitch and tube row number on heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchangers are investigated. The numerical results show that with the increases of Reynolds number, wavy angle and the decrease of fin pitch, tube row number, heat transfer performance of the heat exchanger is enhanced, but at the same time the pressure drop is increased.(5) Correlation between unit volume heat transfer capacity and whole field average synergy angle is set up. The effects of the parameters on heat transfer characteristics of wavy fin heat exchanger with circular or elliptic tubes can be properly revealed by the correlation. The correlation can make the field synergy principle used in design optimization of heat exchanger configuration more conveniently.(6) Effects of Reynolds number, wavy angle, fin pitch, fin thickness and transverse tube pitch on fin efficiency of wavy fin surface are analyzed numerically. The results show that with the increases of Reynolds number, wavy angle, fin pitch, transverse tube pitch and the decrease of the fin thickness, average temperature on fin surface decreases which leads to the decrease of fin efficiency.(7) Local heat transfer coefficient and fin efficiency distributions on wavy fin surface are investigated numerically. The results show that local heat transfer coefficient decreases quickly along air flow direction; at the locations of wavy crest and wavy trough, local heat transfer coefficient presents fluctuation, the larger of the air velocity and wavy angle, the more obvious of the fluctuation. Local fin efficiency increases along air flow direction, its distribution characteristics basically depends on the distribution of temperature. Convective heat transfer mostly occurs at inlet region, so the corrugations at outlet region have little effects on heat transfer characteristics but greatly increase pressure drop.(8) New type of local wavy fin is designed and numerical validation is performed. On the basis of the numerical results of local heat transfer coefficient, a new type of wavy fin pattern is designed. The simulation results on the new fin pattern show that the heat transfer coefficient only decreases 4% compared with traditional wavy fin pattern, but friction factor decreases 18%; the heat transfer coefficient increases about 45% and friction factor only increase 26% compared with plain plate fin pattern. The enhancement of heat transfer is larger than the increase of pressure drop. So the new fin patter has better enhanced heat transfer and decreased pressure drop characteristics.(9) Wavy fin heat exchanger with elliptic tubes is proposed, numerical study on the heat transfer and fluid flow characteristics is performed. The results show that the elliptic tube arrangement can enhance heat transfer up to 30% and only increase friction factor about 10% compared with circular tube. Both the heat transfer coefficient and the friction factor of the elliptic tube heat exchanger decrease with the increases of ellipse centrality, fin pitch, transverse tube pitch and the decrease of fin thickness.(10) Effects of elliptic tube pattern on heat transfer and fluid flow characteristics are investigated. Contrastive study on the performances of five tube patterns are performed. The results show that heat transfer performances of all the four kinds of elliptic tube patterns are better than that of circular tube; the enhanced percentages are 17.0%, 16.9%, 14.6% and 12.3% respectively. The friction factor of the elliptic tube with the same minimum airside free-flow area as circular tube is about 7.1% larger than that of circular tube. However, the friction factors of the other three kinds of elliptic tubes are smaller than that of circular tube, the friction factors are decreased about 23.9%, 20.7% and 17.0% respectively.