Research On The Key Elements Of Three-Dimensional Thermal Design And Its Effect Mechanism On The Thermal Performance Of Large-Scale Wet Cooling Tower

The current thermal design methods for wet cooling towers in industry are the one-dimensional and two-dimensional design method.With the increase in the size of wet cooling towers,the three-dimensional distribution characteristics of thermal parameters of air and water have brought more and more design errors to the traditional design methods.Therefore,the study of three-dimensional thermal design theory and design method is of great significance for the advancement of cooling tower technology,as well as the improvement of engineering design accuracy.To clarify the key elements of three-dimensional thermal design,and to obtain the influence principle of these key elements on the three-dimensional thermal performance are the basis for accurately establishing the three-dimensional thermal design models of wet cooling towers.As a result,this study lays a foundation for the development of the basic theory of three-dimensional thermal design for wet cooling towers.By combining field measurements,numerical calculations,theoretical analysis and laboratory experiments,this dissertation systematically studies the main influence factors of the three-dimensional thermal performance of wet cooling towers,and then proposes the key elements of three-dimensional thermal design for large-scale wet cooling towers.For wet cooling towers,the key elements include the water droplet diameter distribution characteristics in the rain zone,the environmental crosswind effect,and the coupling of internal flow field,tower core elements and the heat and mass transfer;for mechanical draft wet cooling towers,the key elements also include tower type effect and tower group effect,etc.The action mechanism of the above key elements on the cooling tower thermal performance is analyzed,and the transformation law of these key elements in the three-dimensional thermal design is obtained.The main efforts of this dissertation are as follows:(1)Research on the difference between the actual operating performance and the design performance of large-scale wet cooling towers.Three-dimensional thermal performance tests are carried out on natural draft-and mechanical draft wet cooling towers,and the one-dimensional iterative calculation model for cooling tower design is established.The difference between actual performance and design performance of cooling tower was studied,and several evaluation indicators for the performance difference are proposed.The change rule of water droplet diameter distribution in the rain zone with the spatial position,the tower inlet air velocity and the water drenching density is revealed.The influence of water drenching density,environmental crosswind,geometrical space transformation on the difference between the actual operating performance and design performance of natural draft cooling towers,as well as the influence of environmental crosswind speed and direction on that of mechanical draft wet cooling tower are analyzed.The key elements of three-dimensional thermal design for large-scale wet cooling towers are proposed as follows:water droplet diameter distribution in the rain zone,crosswind effect,and the coupling of internal flow field,tower core elements and the heat and mass transfer.This effort can provide guidance for the subsequent exploration and verification of the effect mechanism of key elements on three-dimensional thermal performance of wet cooling towers.(2)Research on the influence of water droplet diameter distribution in the rain zone on the three-dimensional thermal performance of wet cooling towers.It is one of the key elements in the three-dimensional thermal design of large wet cooling towers,which is also the basis for accurate calculation of thermal and resistance of the rain zone.Based on the field measurement data,a three-dimensional distribution model,a Gauss distribution model and an equivalent diameter model are established to describe the diameter distribution characteristics of water droplets,and the modification method for three-dimensional distribution model in high wind speed region is discussed.Results indicate that the variation trends of tower performance calculated by different water droplet diameter distribution models are consistent with the alteration of operating parameters such as water drenching density,inlet water temperature,ambient air temperature and ambient humidity.The sensitivity of tower performance to the water droplet distribution characteristics increases with the rise of water drenching density and ambient air temperature,and decreases with the increase of inlet water temperature,while it is basically not affected by the environmental humidity.It is found that the calculation results of the three-dimensional distribution model are the closest to the measured value,which is recommended to be used in the three-dimensional thermal design of wet cooling towers to replace the equivalent diameter.(3)Research on the influence of crosswind effect on the three-dimensional thermal performance of large-scale wet cooling towers.Crosswind effect is one of the key elements in the three-dimensional thermal design of large wet cooling towers,through which the wind control measures can be incorporated into the thermal design of cooling towers to eliminate the longitudinal vortex in the fill zone and realize the uniformity of the three-dimensional flow field around the tower.Aiming at the longitudinal backflow on the windward side of the fill zone induced by crosswind,an experimental device is set up to measure and evaluate the thermal performance of the fill under different air-water flow modes.The numerical model of the cooling tower performance under crosswind conditions is further modified.By analyzing the performance parameters such as the ventilation correction coefficient Kvr of the conventional fill zone,the area ratio of the longitudinal backflow fill zone δSad and the longitudinal backflow intensityηad,the performances of the conventional fill zone and the longitudinal backflow fill zone of cooling towers in different sizes under variable operating conditions are studied.Besides,the effective utilization factor of the fill zone is proposed to describe the overall flow and heat transfer characteristics.At the same time,the change law of thermal-resistance characteristics of rain zones in different sizes with the environmental wind speed is obtained under variable working conditions.Finally,the regulation mechanism and effect of wind deflectors on the crosswind effect are discussed.Results show that with the increase of the crosswind velocity,the Kvr first decreases and then increases,mainly affected by the ratio of inlet water temperature to inlet air temperature,RT;both the δSad and the ηad increase continuously,mainly influenced by the water drenching density,q.A larger tower size and a greater RT leads to a higher crosswind which can restore the overall thermal performance of fill zone.The pressure drop of rain zone increases with the rise of q and RT.The resistance coefficient of rain zone increases and decreases with the rise of q and RT,respectively.The water temperature drop and the Merkel number of rain zone increase with the rise of q and RT.Wind deflectors can improve the uniformity of the air intake,decrease the degree of longitudinal backflow,and enhance the cooling tower performance,which is more effective under high-speed crosswind conditions.The positive effect of wind deflectors is not only due to the increase in ventilation,but also the optimization on the aerodynamic field and the intensity of heat and mass transfer.(4)Research on the coupling of internal flow field,tower core elements and the heat and mass transfer on the three-dimensional thermal performance of natural draft wet cooling towers.It is also one of the key elements in the three-dimensional thermal design of wet cooling towers.In view of the unreasonable distribution of the aerodynamic field inside the cooling tower and the difference in heat and mass transfer in different regions caused by tower structure and environmental crosswind,the coupling researches on the flow field and the heat and mass transfer are carried out,which makes the layout of tower core elements match with the flow field,the heat and mass transfer reach the maximum in all the spaces,and the cooling effect be improved.Based on the above analysis,the coupling optimization of flow field and heat and mass transfer in geometric spaces such as the air distribution zone,the fill zone and the water distribution zone is carried out,including the forced distribution of air in rain zone through air ducts,the proactive macrocontrol of draft and resistance as well as heat and mass transfer through non-uniform fill and partitioned water distribution.Numerical calculations are performed to study the variable-condition characteristics of various optimization measures,the optimal non-uniform fill arrangement and partitioned water distribution are proposed,and the applicability of various measures is further discussed.In addition,the coupling of wind deflectors and other measurement including air duct,partitioned water distribution,non-uniform fill on the tower performance is explored.The results show that the coupling effect of non-uniform fill and air deflectors performs the maximum cooling effect,which is confirmed by thermal state model experiment.(5)Research on the key elements of three-dimensional thermal design for mechanical draft cooling tower and their effect on tower performance.The key elements of three-dimensional thermal design for mechanical draft cooling tower such as tower type,crosswind and tower group are proposed.The effect of variable operating conditions on the three-dimensional thermal characteristics of single air inlet and double air inlet mechanical tower is studied,and the formation mechanism of low-efficiency zones and their influence on the tower performance are investigated.The variation of the low-efficiency zone with the environmental crosswind is analyzed,and the influence mechanism of the environmental crosswind on the ventilation and heat dissipation performance of the mechanical draft cooling tower is obtained.Taking the 10-tower double-row tower group as the object,the performance differences of the cooling towers in the tower group under windless and crosswind conditions are studied,the influence of interference effect and the hot air recirculation characteristics under crosswind conditions is discussed,and the effect mechanism of tower group effect on the three-dimensional thermal performance of mechanical draft cooling tower is obtained.Through the above research,the key elements of the three-dimensional thermal design for large-scale wet cooling tower and the relative mechanism on tower thermal performance are obtained.Based on the three-dimensional distribution model of water droplets in the rain zone,a design calculation method of replacing the conventional equivalent diameter with the diameter distribution of water droplets is proposed.The influence of crosswind effect on the three-dimensional thermal performance of each zone in wet cooling towers is studied,and the evaluation method for the influence of crosswind-induced longitudinal backflow on heat and mass transfer is obtained.The variable condition characteristics of the coupling optimization measures of flow field and heat and mass transfer in the air distribution zone,fill zone and water distribution zone are studied,and the optimal coupling optimization method under crosswind conditions is obtained.The mechanism of the key elements in three-dimensional thermal design on the thermal performance of mechanical draft cooling tower is studied.This research has important guiding significance for the establishment of three-dimensional thermal design theory and design method for large-scale wet cooling towers,and the conclusions obtained have high theoretical value and engineering application prospects.