| With the growing shortage of water resources world-wide,the traditional wet cooling heat exchange equipment with evaporation and high water consumption can’t satisfy the rapid economic and social development any longer.The air cooling technology of the cold end system has become popular and been widely applied in power generating units owing to its significant water-saving superiorities.The heat rejection of the air cooling system depends generally on various factors,among which the complex and changeable ambient conditions as well as the inherent structure and layout deficits of the multi-scale heat transfer surface are the dominated ones incurring the decreased cooling performance of air cooling system.Based on the transport characteristics of the small-scale heat transfer unit,the methods of modeling simulation and experimental verification are adopted for the multi-scale heat transfer process of the typical air cooling system in power plants.The air flow and heat transfer performances of the air-cooled heat exchanger are deeply explored,and then the structure and layout deficits of the air cooling heat transfer surface are disclosed.In order to improve the cooling efficiency of air cooling system under various ambient conditions,this research proposes the heat transfer surface construction principle as well as the air flow guidance and control strategy,which may contribute to the theoretical foundation for the development of air cooling system in power generating units and provide the corresponding technical supports.In practical engineering,the extended fins are used for the air-cooled heat exchanger,so the air flow resistance in the finned-tube channel and its heat transfer characteristics on the stretched heat transfer surface are key influencing factors of the thermo-flow performances of the air-cooled heat exchanger.The thermal wind tunnel experimental platform of the fined tube is set up to obtain the air-side flow and heat transfer variables,and then the accurate numerical model is developed.The impacts on the transport performance of the tube bundle through the air inlet velocity,the base tube parameters(arrangement mode,tube pitch,diameter of base tube),as well as fin parameters(fin angle,fin thickness and fin pitch)are analyzed.The effects of the equal and the non-equal heights of the fin slots on enhancing the air-side heat transfer are studied for the four-row tube bundle.The results show that the rectangular slots disturb the air flow boundary layer periodically,which improves the thermo-flow performances of the finned-tube bundle directly.Furthermore,the comprehensive performance evaluation standard for convection heat transfer of finned tube bundles is proposed to characterize its heat rejection per unit of the power consumption,besides the optimal finned-tube and extended heat transfer surface structures are designed.Through the multi-scale simulation,the transport performances of large scale air cooling system and the ambient wind effects are revealed.In order to weaken the adverse wind effects on the conventional forced draft air cooling system(CACS)with the rectangular layout of the condenser cells,a circular layout is proposed.The results show that this recommended air-cooled condenser strengthens the aerodynamic performance of the windward fans and reduces the hot air recirculation of the lateral condenser cells,thus improving the overall cooling efficiency apparently.As also been pointed out,CACS adapts itself hardly to the complex ambient conditions and has a high power consumption in power plants,so in this research the natural draft direct air cooling system(NACS)and the hybrid ventilation direct air cooling system(HACS)of are numerically designed and investigated,besides the principle of heat transfer surface construction for air cooling system is recommended.For NACS,the V-type fin tube bundles of the vertically arranged condenser cells outside the tower usually suffer from the aerodynamic inadequacy.In such a case,the annularly arranged finned tube bundles are proposed to cripple the air flow interaction between the adjacent tube bundles,therefore the transport performance of NACS is improved greatly.For NACS with the horizontally arranged condenser cells,the inflow air appears deviation when crosses through the finned tube bundles.So fins with the incline angle of 3 0° along the air flow direction are designed for the condenser cells,which reduce the air flow pressure drop directly by lowering down the steering resistance because the air inflow channels parallel to the dry-cooling tower suction.Cooling air in HACS flows through the heat exchanger by the combined drive of the natural ventilation dry-cooling tower and the axial flow fans,so the power consumption of the fans is greatly reduced compared with the CACS.The thermal performance parameters of air cooling system at various wind directions are illustrated,and the results present that CACS with the circularly arranged condenser cells significantly decreases the turbine back pressure on the premise of satisfying the heat load of the cold end system.Following the principle of reorganizing and fully utilizing the wind energy resource,the air flow guiding strategy is carried out in this research,which improves the transport performances of air cooled equipment and realizes the useful utilization of wind energy.The results show that,with the air guidance for the upwind condenser cells of CACS,the adverse wind impacts are slashed obviously which lift the thermal efficiency of the direct air cooling system remarkably.By wind guiding inside and outside the dry-cooling tower of NACS,the flow and heat transfer performances of the side condenser cells increase apparently,besides the head loads non-uniform distribution recedes among the air-cooled sectors,thus promoting the overall transport performance of the air cooling system.Conclusively,the air flow guidance and control technology of air cooling system builds up its resistance to ambient winds dramatically which reduces the turbine back pressure evidently for the power generating unit. |
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