Research On Heat Transfer And Resistance Characteristics Of A Novel Type Of Heat Transfer Element-semi-elliptical Tube

As universal equipments for the purpose of heating or cooling, heat exchangers play an important role in effective energy utilization. Cylinder tubes are widely applied in heat exchange equipments for their fine processability and pressure bearing property. However, when fluid flows across cylinder tubes, boundary layer separation occurs in large scale, which engenders a broad separation region at the tail of tubes, leading to considerable pressure drop, and vibration in severe conditions, which might cause direct damage. For energy saving and some special requirements, streamline tubes with slender shapes have been drawing people’s attention. This thesis proposes a novel type of heat transfer tube with excellent streamlined shape—the semi-elliptical tube, and detailed research is carried out on its flow and heat transfer characteristics with the method of numerical calculation and experiment.Research is carried out by numerical simulation on the flow and heat transfer characteristics of air flowing across semi-elliptical tubes of six different shapes with the length to diameter ratio L/D ranging among1(cylinder tube),1.5,2,3,4, and5. The results show that the optimum heat transfer tube shape varies with Re number: for Re>8000, the average convection heat transfer coefficient is the highest for the shape L/D=2; while for Re<8000, the highest for L/D=1.5. For the same Re number, the resistance coefficient reduces with the increase of L/D, but the heat transfer performance of tubes with too high L/D gets worse. The formulae of Nu number with Re number for different semi-elliptical tube types are fitted, which provide reference for engineering practice and theoretical research. By analysis of the variation of the comprehensive evaluation factor j/f for flow and heat transfer with L/D and Re number, the optimum heat transfer tube shape is obtained for different Re numbers: for Re=3984, the value of j/f is highest for the semi-elliptical tube with L/D≈3.5, which implies the best comprehensive performance for convection heat transfer; while for Re=47809, the tube shape with the best comprehensive performance is that with L/D≈2.2. Within the range of Re number in numerical simulation of this thesis, the convection heat transfer coefficients outside the semi-elliptical tube with L/D=2are higher than those of corresponding cylinder tube by7%-44.3%, while the resistance coefficients are only32%-53%of those of cylinder tube. According to the field synergy theory, the distribution of average field-synergy-angle along heat transfer surface in the wall boundary layer of the semi-elliptical tube (L/D=2) is calculated and contrasted with that of cylinder tube under identical conditions. The results show that for Re=31872, the average synergy angle for the heat transfer surface of semi-elliptical tube with L/D=2is88.6°, while that of the cylinder tube is89.3°, indicating that the average synergy level of velocity vector and temperature gradient on the surface of semi-elliptical tube is better, and this further explains the heat transfer enhancement mechanism for the semi-elliptical tube.A wind tunnel experiment table is designed and set up, and the average convective heat transfer coefficient and resistance coefficient outside the semi-elliptical tubes for the semi-elliptical tubes with L/D=1,2,3,4are studied and compared with the results of numerical simulation. The result shows that the variation trends of Nu number and f factor with Re obtained by experiment are consistent with those obtained by numerical simulation, and error of the results are within allowable error range, which verifies the reliability of numerical simulation.Analysis is carried out by numerical simulation on the flow and heat transfer characteristics of air flowing across smooth bundle of semi-elliptical tube with L/D=2, and a contrast is made with cylinder tube bundle and elliptical tube bundle with identical blockage ratio. The results show that within the range of Re number studied in this thesis, the heat transfer performances of both semi-elliptical and elliptical tube bundle are better than that of corresponding cylinder tube bundle; for Re<1.2X104, the heat transfer coefficient of staggered semi-elliptical tube bundle is lower than that of elliptical tube bundle with identical arrangement, but for Re≥1.2X104, the former is higher, and the difference between them grows larger with the increase of Re number. Within the range of Re number of numerical simulation in this thesis, the outside convection heat transfer coefficient of staggered semi-elliptical tube bundle is larger than that of corresponding cylinder tube bundle by8.3%-29%, while the resistance coefficient is only53%-63%of the latter. This shows that the adoption of semi-elliptical tube with streamlined shape could help enhance heat transfer and reduce flow resistance simultaneously. The distribution of average field synergy angle in boundary layer is calculated for the semi-elliptical tube bundle(L/D=2), elliptical tube bundle and cylinder tube bundle, and the heat transfer enhancement mechanism for semi-elliptical tube bundle is further analyzed according to the field synergy theory. For Re=23904, the average field synergy angle for the semi-elliptical tube bundle, elliptical tube bundle and cylinder tube bundle are respectively88.1°,88.4°and88.6°, which shows that the synergy level of fluid flow velocity vector and temperature gradient in the boundary layer outside each row of semi-elliptical tube bundle is best, explaining the reason for its excellent heat transfer performance.