Research On Test Method For Evaluating Three-dimensional Thermal Performance Of Wet Cooling Towers

Natural draft counter flow wet cooling tower is widely used as cooling device in power plant.Our country's cooling tower technology is developing rapidly because of the increasing demand for electric power.A series of numerical simulations and laboratory experiments is used to study how the cross wind affects cooling tower's performance,apparently full-scale field experiment is a more direct way to figure out thermal performance of a tower.However,ambient and operating conditions are uncontrollable during field experiment;besides,flow field and temperature field are influenced by cross wind.Therefore,a new model to evaluate cooling tower's performance in three-dimension is much needed.The influence of cross wind is analyzed and divided into three aspect:increasing air resistance at the tower entrance,causing additional draft at the tower exit,affecting heat and mass transfer intensity directly.Based on numerical simulation,it is indicated that the relationship between air resistance and wind rate follows quadratic regression and the correction coefficient for Merkel number is proportional to wind rate,which provides a theoretical basis for calculating performance parameters of a tower under specific ambient and operating condition.Full-scale experiment study is carried out and a new calculation method is used to figure out the relationship between performance parameters and cross wind velocity.Although ambient and operating parameters vary stochastically,air resistance of cooling tower and correction coefficient for Merkel number are only related to cross wind velocity rate.Based on experiment data,fitting formula of air resistance and correction coefficient can be acquired,which can be used to compute cooling tower's performance parameter under specific ambient and operating condition.At the typical condition in autumn,outlet water temperature by calculating is 20.05? when the wind speed is 2m/s,and outlet water temperature by calculating is 21.52? when the wind speed is 4m/s.The contrast between before and after installing air deflectors shows that air deflectors have little influence on outlet water temperature under low wind speed,but as the wind speed increases,air deflectors decrease outlet water temperature gradually.Based on analysis of temperature and flow field computed by numerical simulation,average outlet air temperature and ambient wind velocity shall be measured at more representative points.Wind velocity affect temperature field in tower,the maximum margin of error is 0.14? when measured in accordance with acceptance test specification.Test error can be reduced by changing the position of measuring points.The maximum error can be reduced to 0.06? with mi-zi type test method.Increasing the height of measuring points is another to reduce measuring error.Similar analysis is done to study measuring method of water temperature in tower.Wind velocity shall be measured in a location beyond the influence of tower.The stronger wind is,the larger influence area by tower is.So the wind velocity shall be measured at a distance greater than twice the base diameter from the cooling tower.Qualitative analysis of temperature field over drift eliminator and flow field around tower inlet reveals thermal performance of tower in three dimension.Over drift eliminator,outlet air temperature in the center area of tower is lower than that in outer area,which proves that central catheter pipes and fill non-uniform layout are beneficial to heat and mass transfer in tower.Vector diagrams show flow field around tower inlet visually.