| The huge demand of water resource for the cold end system in traditional power plant restricts the development of electric power in water-scarce areas.The air cooling technology for power plant has been rapidly developed.The direct dry cooling technology has attracted wide attention and applications due to its outstanding water-saving ability,simple system,flexible adjustment and other advantages.The direct dry cooling system adopts large-scale axial flow fans to force the cooling air to take away the heat rejection of the exhaust steam from the steam turbine.Environmental meteorological conditions will disturb the aerodynamic characteristics of the axial flow fan array easily,and then affects the cooling performance of the direct dry cooling system.The flow and heat transfer performance of the direct dry cooling system can be effectively improved by the methods of design optimization and operating regulation.For the direct dry cooling power plants,installation of air diversion devices and operating optimization of the fan array have the advantages of small investment and quick results,which are of great significance for engineering application.This paper takes the mechanical draft direct dry cooling power station as the research object and reveals the distribution characteristics of the air flow field under the influence of ambient wind by numerical simulation.Guidance schemes of ambient wind and operating adjustment strategies of fan array are proposed to improve the flow status of cooling air and enhance the operating performance of the cold end system of the power station.The increase in the temperature and the decrease in the flow rate of the cooling air at the fan inlet are caused by the ambient wind.The direct dry cooling system usually adopts air deflectors installed under the air-cooled platform to guide the flow of cooling air at the fan inlet and suppress the adverse effects of environmental wind.This paper studied the single-layer arrangement of plane and arc deflectors firstly,and then proposed three lateral double-layered deflectors,and analyzed the changes in the flow and heat transfer performance of direct dry cooling systems under different ambient wind conditions.The results show that the main function of the deflectors is to reduce the air temperatures at the inlet of the windward air-cooled cells and increase the inlet pressures of the fans,thereby increasing the cooling air flow rate and improving the cooling capacity of the air-cooled cells.The increase of cooling air flow rate and the decrease of turbine back pressure are more obvious as the ambient wind speed increases.It can be concluded that the arc deflector and the lateral double-layer deflector with the inclined outer layer and the vertical inner layer can effectively improve the flow field of the cooling air,which provides a reference for the development of air diversion devices in the direct dry cooling system.For the problem of inefficient operation of the fan array caused by the ambient wind,this paper proposed partition adjustment strategies for the axial flow fan array under different environmental wind conditions to realize the potential.When the total power consumption of the fan array remains constant,increasing the speed of windward fans can effectively reduce the inlet air temperatures while the changes in speed of the downstream fans play unimportant roles in the inlet air temperatures.Generally speaking,the increase in the rotational speed of the windward fans is beneficial to increase the cooling air flow rate and reduce the turbine back pressure,and the partition adjustment strategy of the fan array should be determined based on the environmental meteorological conditions.Changes in environmental meteorological conditions and the adjustment of the fan array speed play key roles in the performance of the direct dry cooling system.The decrease in the rotational speed of the fan array under wind conditions will cause a significant increase in the air temperature at the fan inlet,and the deflection of the air flow at the outlet will be more serious.The back pressure of turbine increases with the decrease in the speed of the fan array,and the increase in the ambient temperature and wind speed.And the higher the temperature and wind speed or the lower the speed of the fan array,the faster the back pressure rises.Since the increase in the fan speed will reduce the back pressure and increase the power consumption,an optimal operating speed of the fan array exists under specific environmental conditions.By coupling the direct dry cooling system and the thermal cycle system of the power generating unit,it is found that the standard coal consumption rate of the unit varies very little with the fan array speed at low ambient temperature.When the ambient temperature or wind speed is high,the coal consumption rate can be reduced effectively at the highest fan array speed.At lower wind speeds and ambient temperatures,reducing the speed of the fan array appropriately is beneficial to the economic operation of the power generating unit.In previous studies,Radiator model is usually used for the finned tube bundles of direct dry cooling system to calculate the heat transfer between the cooling air and exhaust steam,and the changes in the steam temperature could not be considered.In order to make the flow status of steam closer to the actual situation,this paper adopts Heat exchanger model by the method of user-defined function(UDF)to take into account the changes in the steam state,and proposes two adjustment strategies for fan array to make the flow field of cooling air more uniform.The variation of the temperature field,flow field and power consumption of the fan array under different ambient wind conditions are obtained.The results show that adjusting the fan speed in the absence of wind can reduce the demand for cooling air and cut down the power consumption of the fan array.Under the windy conditions,the adjustment strategies of the fan array can also reduce the inlet air temperature,which is more obvious at high wind speeds.It can be seen that the cooling performance of the air-cooled condenser and the operational energy efficiency of the direct dry cooling power generating unit can be improved by adjusting the operation method of the fan array to make the air flow field more uniform. |
PDF Full Text Request