| In recent years,waste heat recovery technology has been one of the most significant technologies in promoting energy conservation and emission reduction and energy sustainable development in China.As a key link of energy production and utilization,the recovery of low-grade waste heat(80~200?)plays an important role in the efficient utilization of energy.However,due to the weakness of energy conversion theory and technology research,there are many technical bottlenecks in the development of waste heat recovery equipment technology,and the existing large-scale application of smooth tube heat exchangers cannot meet the requirements of waste heat recovery.Therefore,it is extremely imminent to develop efficient waste heat recovery technologies.Because of its special rough surface,three-dimensional(3-D)finned tube produces transverse and longitudinal flow near the wall surface,which destroys the boundary layer and then enhance fluid disturbance and convection heat transfer.Elliptical tube can contribute to guiding the fluid flow owing to its special streamlined structure,which can reduce the flow resistance.Hence,mixed 3-D finned-tubes heat exchanger with high heat transfer efficiency and low flow resistance is developed based on combining 3-D finned ellipticaltubes with 3-D finned circular-tubes,which is of great significance for improving the conversion rate of waste heat energy and achieving the strategic goal of energy saving and emission reduction in industry.Few studies have been done on the mechanism of convection heat transfer outside tube,flow field and temperature field of 3-D finned tube,and the research of 3-D finned tube based on special-shaped tubes is still blank.In this paper,the numerical simulation models of flow and heat transfer of 3-D finned circular-tube and 3-D finned ellipticaltube with air swept were established.Firstly,Taguchi method was used to investigate the contribution ratio and main effect of single fin parameters on flow and heat transfer performance.The optimal fin geometric parameters combination was obtained based on the Thermal Hydraulic Performance.Then,on the basis of the optimal 3-D finned tube,the effects of tube bundle arrangement,fin height and tube bundle pitch on flow and heat transfer characteristics were studied.Finally,the mixed 3-D finned tubes bundle heat exchangers with the combination of 3-D finned circular-tubes and 3-D finned ellipticaltubes were proposed,and the Thermal Hydraulic Performance and field synergy of mixed 3-D finned tubes bundles were researched,which provided theoretical guidance for the optimization design of 3-D finned tubes bundles heat exchanger.Through the above research work,the main conclusions of this paper are as follows.(1)According to numerical simulations of the influence of fin geometric parameters on the flow and heat transfer characteristics of 3-D finned tubes,the study showed that the influences of fin height and the circumferential fin pitch on the flow and heat transfer performance were the largest(about 65%)and the smallest(about 1.57%),respectively.Multi-parameter coupling predictive correlations of friction factor and Nusselt number were achieved.Moreover,it was found that the heat transfer performance of 3-D finned tube was1.7~2.96 times that of smooth tube,and the flow resistance of 3-D finned elliptical-tube was 16.7%~44.2% lower than that of 3-D finned circular-tube.(2)The influences of the bundle arrangement,fin height and tube pitches on the flow and heat transfer characteristics of 3-D finned tubes bundles were studied.The results showed that compared with 3-D finned circular-tubes bundle,the fins had greater influence on the flow resistance of 3-D finned elliptical-tubes bundle.The flow resistance and heat transfer performance of the staggered 3-D finned tubes bundle were about 2.5 times and 1.2 times of that of the aligned 3-D finned tube bundle,respectively.Increasing fin height and decreasing tube pitches were helpful to improve the Thermal Hydraulic Performance of 3-D finned tube bundle.The flow resistance of 3-D finned elliptical-tubes bundle was about 35% smaller than that of the 3-D finned circular-tubes bundle.(3)Through the study of flow and heat transfer characteristics of the mixed staggered 3-D finned tubes bundles,it revealed that the average flow resistance of the mixed 3-D finned tubes bundles with good heat transfer performance was about 22% lower than that of the 3-D finned circular-tubes bundles.Moreover,the mixed 3-D finned tubes bundle where the 3-D finned circular-tube was arranged in the last row had better Thermal Hydraulic Performance,which was 8% higher than that of 3-D finned circular-tubes bundle and 29.5% better than that of 3-D finned elliptical-tubes bundle.This conclusion was also proved in the analysis of field synergy.The results indicated that the mixed 3-D finned tubes bundle heat exchanger has great performance improvement in obtaining efficient heat transfer and reducing flow resistance,which can contribute to promoting the waste heat recovery technology. |
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