| Air cooling technology of power station has been widely adopted in water shortage areas of China.The cooling medium of air cooling system is ambient air.Due to the frequent change of environmental conditions,the performance of air cooling system will be easily affected.Air cooling system contains a large number of heat exchange equipment and working fluids,which have the heat storage effect.When the ambient conditions change,the variation of the parameters of the air cooling system is a dynamic process.At present,most researches focus on the steady state condition of air cooling system,besides the renewable energy such as solar energy and wind energy will see increasing development in the nearby future that the thermal power units will undertake more and more load regulation tasks with partial power load.Therefore,it is necessary to explore the dynamic characteristics under various working conditions,which may provide the theoretical guidance for the performance optimization of air cooling system during the dynamic process.Firstly,the dynamic characteristics for indirect air cooling system are studied.The dynamic heat transfer differential equations of circulating water side,tube wall side and air side of heat exchanger are established respectively.When the wind speed,circulating water flow rate and inlet water temperature change,the dynamic response characteristics of heat exchanger are solved by MATLAB.Furthermore,the effects of heat transfer area,heat transfer coefficient and working fluid volume on the dynamic characteristics of heat exchanger are also disclosed.The results show that,the dynamic response curve of heat exchanger varies more obviously if these parameters change aggravated.The dynamic response time of cooling air becomes shorter when the face wind speed and circulating water flow rate are higher.The inlet temperature of circulating water has fairly small impacts on the dynamic response time of heat exchanger.The dynamic response time of circulating water side is longer than that of air side.When the heat transfer area of air side increases,the dynamic response time of air side increases,while the circulating water side remains unchanged.The change of circulating water side heat transfer area has little effects on the dynamic characteristics of heat exchanger.The effect of heat transfer coefficient on dynamic characteristics of heat exchanger is similar to that of heat transfer area.With the increase of circulating water volume,the dynamic response time of air side and circulating water side increases.Secondly,based on the macro heat exchanger model,the steady-state calculation model of indirect air-cooled system is developed with considering the heat transfer process of dry cooling system and condenser.The dynamic heat transfer differential equations of condenser and dry cooling system are established respectively,besides the dynamic response mathematical model of indirect air cooling system is established by using steady-state calculation results as boundary conditions.The results show that,the water inlet and outlet temperature of condenser side and back pressure of steam turbine increase with the step increase of ambient wind speed,and the change speed decreases with time.With the increase of ambient wind speed,the dynamic response time of indirect air cooling system becomes longer.The dynamic response time of the cooling system is shorter when the ambient wind speed decreases than that when the ambient wind rises.For the indirect air cooling system with two towers,the dynamic characteristics of the two towers are similar to that of the cooling system with single tower when the wind direction is constant and the wind speed is increased.When the wind speed is constant and the wind direction angle is changed,the water outlet temperature of the sectors with improved flow performance first decreases and then rises,while that of the sectors with poor flow performance after changing wind direction keeps rising.The dynamic characteristics of indirect air cooling system with redistribution of the circulating water are investigated.The average net power of the unit is higher than that of the uniform distribution scheme,and the average net power of scheme 2 is the highest,the average net power can be increased by 0.901 MW compared with the uniform distribution scheme of circulating water.In view of the adverse effect of the air cooling system on the indirect air-cooled system,the auxiliary heat exchanger is proposed to improve the cooling performance of cooling system.When the wind speed is 8 m/s,12 m/s and 16 m/s,the flow and heat transfer performance of air cooling system can be improved by the auxiliary heat exchanger.Finally,the steady-state calculation of direct air cooling system is carried out based on the radiator and fan models.A set of dynamic heat transfer differential equations for the exhaust side,tube wall side and air side of the direct air-cooled condenser are established respectively.Taking the steady-state calculation results as the boundary conditions,the dynamic response mathematical model of the direct air-cooled system is established.The effect of ambient wind speed change degree,the change form of the wind and the environmental wind direction on dynamic characteristics of the direct air cooling system are studied.The results show that the greater the wind speed changes,the faster the change rate of back pressure,the greater the degree of change,and the longer the dynamic response time.When the wind speed increases in the form of ramp function and exponential function,the change rate of back pressure first changes from slow to fast,then from fast to slow,and the dynamic response time is longer than the step change of wind speed.After the change of wind direction,although the back pressure changes little,the heat exchange rate of condenser in different regions changes greatly.The optimal fan speed and the dynamic characteristics of the direct air cooling system under variable load conditions are studied.The results show that,when the unit load is high,the net power will not appear the extreme value with the increase of fan speed.When the load is low,there is an optimal value of fan speed.When the load is 30%,the net power of the unit can be increased by 1.67 MW at the optimal speed.In the process of variable load,the dynamic response time becomes shorter with earlier regulation of fan speed.The maximum average net power can be obtained with the optimal fan speed at the beginning of variable load. |
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