The Characteristics Of Air Side Secondary Flow And Heat Transfer Enhancement In Circle Tube Bank Fin Heat Exchanger

With the rapid development of modern industry and the unceasing enhancement of human production and living standards, the global energy situation is becoming tenser and the energy crisis has become a worldwide problem. Chinese total energy consumption accounts for 20% of the world total energy consumption and China becomes the world’s largest energy consumer. To save energy, China clearly stated that in the "12th five-year plan" period, we will cut energy consumption per unit of GDP by 20% than the end of "11th five-year plan". So, the "energy saving" work has been widely carried out in all walks of life. Building energy consumption in China has accounted for 20%~30% of the total energy consumption, and building energy consumption gradually increases to more than 1/3 with the acceleration of urbanization process and the improvement of people’s living standard in china. The energy consumption of air-conditioning system accounts for about 50 ~ 60% of building energy consumption. So, reducing the energy consumption of air-conditioning system has become an important part of "energy saving" work. A variety of heat exchangers in air-conditioning system are the main part of energy consumption, the heat exchanger performance will directly affect the energy consumption level. Therefore, studying on the technology of enhanced heat transfer in heat exchanger is not only an important issue that must to be solved during the modern industrial development, but also an urgent task to carry out the "energy saving" work.For the tube bank fin heat exchangers used in air-conditioning system, usually, air flow in the fin side channel and water or refrigerant flow in the tube. The airside resistance generally comprises 70%~90% of the total thermal resistance in tube bank fin heat exchangers. And the flow resistance loss of the air is 5~8 times higher than the water or refrigerant flow resistance loss. It can be said that heat transfer performance of the air side directly determines the overall heat transfer performance of the heat exchanger. Therefore, how to reduce the thermal resistance of the air side, effectively improve the air side heat transfer performance of tube bank fin heat exchanger and reduce the flow resistance loss of air has become a focus of the study.In this paper, the numerical method with the isotherm boundary condition is used to solve the air side heat transfer and flow problems of circular tube bank fin heat exchanger. In order to strengthen the heat transfer performance of tube bank fin heat exchanger and improve the heat transfer coefficient of air side, winglet type vortex generators are mounted on the fin surface of circular tube bank fin heat exchanger. The characteristics of air side secondary flow and heat transfer enhancement in circle tube bank fin heat exchanger with vortex generators are numerically investigated. The research shows that:(1) The numerical method with the isotherm boundary condition can be well used in the study of air side heat transfer and flow problems in circular tube bank fin heat exchanger, thus greatly simplify the physical model and calculation process.(2) For the parameters of geometrical morphology and arrangement of vortex generator studied, when the delta winglet type vortex generators with attack angle θ =35? and height H = 1.75 mm are placed on the intersection position of the two tangent lines that perpendicular and parallel to the flow direction at the tube tail, the best effect of comprehensive heat transfer can be obtained.(3) Under the same air flow rate and the same front area of a heat exchanger, the optimal fin spacing changes with different front inlet velocity ufront. When front inlet velocity ufront = 1.75 m/s, the optimal fin spacing is 2.25 mm, when front inlet velocity ufront = 2.5 m/s, the optimal fin spacing is 2.0 mm, and when front inlet velocity ufront >2.5 m/s, the optimal fin spacing is 1.75 mm.(4) Under the same air flow rate and the same front area of a heat exchanger, the optimal transverse tube pitch changes with different front inlet velocity ufront. When front inlet velocity ufront ≤ 3.5 m/s, the larger transverse tube pitch is, the better comprehensive heat transfer performance is. When front inlet velocity ufront > 3.5 m/s, the optimal transverse tube pitch is S1 = 23 mm. For the longitudinal tube pitch, the smaller longitudinal tube pitch is, the better comprehensive heat transfer performance is.(5) When the shape and the starting position of vortex generators are determined, for all the cases studied with mounted VGs and without mounted VGs, the correlation of Nu and f with Re,Tp,H,?,S1 and S2 is obtained, respectively.(6) The absolute vorticity flux Jn ABS and its non-dimensional parameter Se can be used to quantify the secondary flow intensity of fin side channel in circle tube bank fin heat exchangers.(7) For the studied circle tube bank fin heat exchangers, the volume average non-dimensional intensity of secondary flow has one-to-one correlation with Nu for all the cases studied and the correlation of Nu with Sem is obtained. But there is no such correlation between Sem and the friction factor f. These results reveal that secondary flow intensity determines the heat transfer characteristics of the fin surfaces of circle tube bank fin heat exchanger only.