Research On Performance Analysis And Regulation Of Combined Natural Draft Hybrid (Dry/Wet) Cooling System

In many power systems of the thermal power generating units,more than half of the low-grade heat is released into environment by the cold-end devices,of which the energy-efficient operation with high heat dissipation capacity is expected.By using the evaporative cooling technology,natural draft wet cooling system is widely adopted in some areas with sufficient water resource owing to its outstanding cooling performance.While in North China deficient in water resource,natural draft dry cooling system with surface heat transfer is prevalent because of its remarkable water-saving advantage,however cooling performance of this dry cooling pattern is limited in summer.Therefore,the natural draft hybrid(dry/wet)cooling system has attracted more attention in recent years thanks to its double superiorities in water conservation and cooling efficiency.According to the main purpose of water evaporation,natural draft hybrid(dry/wet)cooling system can be categorized into the natural draft hybrid(dry/wet)cooling system with evaporation pre-cooling and the normal natural draft hybrid(dry/wet)cooling system.Based on these two types of cooling systems,the multi-scale three-dimensional numerical models are developed in this research.Following the effective experimental validation,the transport properties of working fluid are thoroughly explored.Then,some controlling strategies are proposed to offset the adverse impacts of ambient conditions.The results may provide the theoretical guidelines for the development and engineering application of the natural draft hybrid(dry/wet)cooling technology.The spraying evaporation pre-cooling system and the traditional natural draft dry cooling system constitute the combined natural draft hybrid(dry/wet)cooling system with spraying pre-cooling.The coupled simulation for the two subsystems is carried out to achieve more accurate flow,heat and mass transfer characteristics.Besides,the complicated effects of both environmental parameters(ambient wind and relative humidity)and pre-cooling system operating factors(water droplet size,spray distance and number of nozzle rows)on the thermo-flow performance of the hybrid cooling system are discovered.The results show that the pre-cooling system mainly reduces the airflow temperature at the inlet of air-cooled heat exchanger and then enhances the heat transfer in dry cooling section,but it plays a negligible role in the air mass flow rate.The ambient relative humidity dominates the performance of pre-cooling system.Under moderate ambient humidity,reducing the droplet size,extending the spraying distance,and increasing number of nozzle rows can-improve the cooling performance of the hybrid cooling system to some extent.The ambient wind will disturb the uniform airflow at inlet of the cooling unit and subsequently induce variation of the performance improvement of air-cooled heat exchanger.Generally,when the pre-cooling system is switched on,the heat dissipation of the windward air-cooled heat exchanger improves slightly,which whereas rises obviously of the leeward side.At the high wind speed condition,the hot air penetration in side heat exchanger will weaken the performance of the pre-cooling system.In such a case,spraying more water to the leeward cooling unit can reduce the turbine back pressure further.The counter-flow natural draft wet cooling system(wet cooling section)and the natural draft dry cooling system(dry cooling section)with air-cooled heat exchanger arranged horizontally inside the tower and vertically arranged outside the tower,compose the normal natural draft hybrid(dry/wet)cooling system with serial and parallel air inlet path respectively,in which the dry cooling section is above the wet one.For both normal hybrid cooling systems,the impacts of environmental parameters(ambient wind and relative humidity)and operating parameters(wet spraying ratio)on their thermo-flow performance are discussed and analyzed.The results indicate that the ambient wind mainly hinders the airflow motion of the windward area for the hybrid cooling system with serial air inlet path.When the wind speed is large enough,the reverse flow occurs in the upwind air-cooled heat exchanger and fill.With regard to the natural draft hybrid cooling system with parallel air inlet path,the side area of the dry part and the windward region of the wet one,are significantly affected by the ambient wind.If the wind is strong enough,the hot air penetration and reverse flow can both appear.Additionally,the cooling performance of both hybrid cooling systems will degrade further due to the eddy in the leeward side of the cooling tower and the airflow deviation at the cooling tower outlet.For both normal natural draft hybrid(dry/wet)cooling systems,the air temperature rise in the dry cooling section is much higher than that in wet one,but the heat dissipation of the later exceeds that of the former at the most working conditions.As for the hybrid cooling system with serial air inlet path,the air intake mass flow rate is insusceptible to ambient relative humidity.However,regarding the hybrid cooling system with parallel air inlet path,ambient relative humidity can bring about some apparent impacts on air flow rate except at the medium or high wind speeds,besides humid environment sees benefit to the performance of dry cooling section under windless condition.As a whole,reducing ambient humidity can conduce to the heat rejection for both hybrid cooling systems.Meanwhile,enlarging water spraying ratio can effectively enhance the wind-proof effect of the water droplets in the rain zone and the heat and mass transfer in the wet cooling section,thereby decreasing the outlet water temperature of the-hybrid cooling system.In the presence of wind,reverse flow in the upwind area of fill can be crippled by the wind leading in wet cooling section,and then the thermo-flow performance gets improved for both normal natural draft hybrid(dry/wet)cooling system.As for the hybrid cooling system with serial air inlet path,the performance improvement of dry cooling section is more considerable compared to the wet one.However,for the hybrid cooling system with parallel air inlet path,the heat rejection of the dry part drops sharply.Cover plate could be adopted as the preferred wind leading measure thanks to its simple structure,convenient installation and favorable wind-proof effect.