Research On Water Saving By Active Inhibition Mechanism Of Evaporative Mass Transfer In Wet Cooling Tower

During the cooling process of the cooling tower,there are two kinds of heat exchange,including evaporation heat transfer and convection heat transfer.When designing a cooling tower,too much attention is paid to the cooling significance of the cooling tower,and the cooling driving force of convection heat transfer and evaporation heat transfer is summarized as difference of enthalpy.In fact,this treatment has diluted the two different heat exchange processes of the cooling tower,which is not conducive to the understanding and in-depth study of the cooling mechanism.In fact,the cooling tower will cause great evaporation loss during the process of evaporation heat transfer.Therefore,in terms of studying the water-saving mechanism of wet cooling towers,active control of evaporation mass transfer and has theoretical significance.In this paper,the idea of water-saving in wet cooling tower is put forward by actively restraining evaporation mass transfer.The realization mechanism,theory and technology of this idea are systematically studied.Two key parameters,mass transfer coefficient and mass transfer driving force,which affect evaporation mass transfer process,are comprehensively analyzed.It is proposed that the wet cooling tower can be constructed by actively restraining evaporation mass transfer by changing the relative humidity of inlet air circulation channel,to change the cooling tower air inlet relative humidity.On the basis of actively restraining evaporation mass transfer,it is proposed to increase convective heat transfer to make up for the decrease of evaporation mass transfer heat and ensure the cooling efficiency of cooling tower.First,the influence of various factors on evaporation heat transfer is theoretically analyzed,and then the influence of mass transfer coefficient changes on heat transfer coefficient and the influence of evaporation heat transfer changes on convection heat transfer are studied;Then,natural draft counterflow wet cooling towers,wet cooling tower with water collecting devices and mechanical draft cooling towers are the research objects,a three-dimensional numerical model of the gas-water two-phase heat and mass transfer of wet cooling towers is established,and the influence of different factors on the evaporation loss of three types of cooling towers is analyzed.Based on the controllability of the influencing factors and the degree of influence which reducing evaporation loss on the convection heat transfer of the cooling tower,for three different types of cooling towers,the relative humidity of the inlet air is selected as the control parameter to actively inhibit the evaporation loss.Taking the natural draft counterflow wet cooling tower as the key point of research,the influence mechanism of the relative humidity of the inlet air on the evaporation loss of the natural draft cooling tower is studied in detail,and validated the established numerical calculation model by combining with the laboratory thermal model test and the field test;Then,a technical route of establishing a wet air circulation channel between the rain area inlet and the saturated wet air outlet of the cooling tower is proposed to change the humidity of the inlet air of the cooling tower.By changing the humidity of the inlet air of the cooling tower,the purpose of actively inhibiting evaporative mass and heat transfer is achieved.Analyzed the influence of the wet air circulation channel on the mass transfer driving force,aerodynamic field,ventilation,mass transfer coefficient and evaporation water loss to reveal the mechanism of the wet air circulation channel to actively control evaporation heat transfer.And established a thermal model test bench with wet air circulation channel,verified the accuracy of the numerical calculation model of the cooling tower with the wet air circulation channel through the thermal model test;Finally studies the use of the coupling effect of the air duct and the wet air channel to optimize the rain area flow field,and analyzes the critical value of the cooling tower's water saving to ensure the cooling performance of the tower.The main research includes the following aspects:(1)First,the theoretical research on the influencing factors of evaporation heat transfer is carried out,and the factors that dominate evaporation heat transfer are summarized as the mass transfer coefficient and the driving force of mass transfer.Through research,it is found that the three specific factors of the relative humidity of the inlet air,the ambient temperature and the temperature of the inlet water can inhibit evaporation heat transfer by inhibiting the driving force of the mass transfer,while the two specific factors of the cooling water volume and the air volume entering the tower can inhibit the mass and heat transfer by inhibiting mass transfer coefficient.Based on this,there are two technical routes for the study of water saving in cooling towers:the study of the inhibition of mass transfer coefficient and the study of the inhibition of the driving force of evaporation heat transfer.Inhibition of evaporation loss will inevitably lead to a decrease in evaporation heat transfer,at this time,if convection heat transfer is also greatly reduced,it is difficult to maintain the cooling performance of the cooling tower even through efficiency measures.Therefore,it is necessary to study the influence of inhibition of evaporation heat transfer on convection heat transfer,respectively,through studying the influence of inhibiting the mass transfer coefficient on the convection heat transfer coefficient and the influence of inhibiting the driving force of evaporation heat transfer on the driving force of convection heat transfer.The results show that:for the cooling tower problem studied,the ratio of heat transfer coefficient to mass transfer coefficient is almost unchanged,and the ratio ranges from 0.96 to 0.97,it can be seen that the convection heat transfer coefficient is almost reduced by the same amount while the mass transfer coefficient is reduced.It shows theoretically that the evaporation loss cannot be inhibited by inhibiting the mass transfer coefficient.Studies have found that when the ratio of heat transfer coefficient to mass transfer coefficient is constant,the ratio of evaporation heat transfer and convection heat transfer is the product of the ratio of evaporation heat transfer driving force to the convection heat transfer driving force and a certain constant.Therefore,inhibiting the influence of the driving force of evaporation heat transfer on the driving force of convection heat transfer is equivalent to inhibiting the influence of evaporation heat transfer on convection heat transfer,and obtains the influence factors of the ratio of evaporation heat transfer to convection heat transfer,which increases When increase the relative humidity of the inlet air to inhibites the evaporation mass transfer,the ratio of the evaporation heat transfer to the convection heat transfer decreases,and the law is not affected by other factors.This shows that when the method of controlling the relative humidity of the inlet air is used to control evaporation heat transfer,the convection heat transfer is not affected.Therefore,this law can be used to effectively control evaporation heat transfer to save water.At the same time,the method of improving convection heat transfer is studied to compensate for the reduction of evaporation heat transfer to ensure the cooling efficiency of the cooling tower.In order to obtain a method to improve convection heat transfer by changing the aerodynamic field in the rain zone,the air cooling tower under the utmost condition of zero evaporation heat transfer is taken as the research object,learn from the idea that changing the aerodynamic field of the air cooling tower can change the cooling efficiency,to study the method of controlling the aerodynamic field in the rain area and provides ideas for the efficiency increasing of the wet cooling tower.(2)On the basis of theoretical research,numerical research is carried out on the factors affecting the evaporation loss of natural draft counterflow wet cooling towers,wet cooling tower with water collecting devices and mechanical draft cooling towers.First,three-dimensional numerical models of three types of wet cooling tower gas-water two-phase heat and mass transfer are established.The grid independence verification of three types of cooling towers is carried out,and the numerical calculation results and design values of wet cooling towers under different working conditions are compared to verify the accuracy of the three-dimensional numerical calculation models;the natural draft counterflow wet cooling tower is the research object,at the same time,establish a one-dimensional numerical calculation model,use one-dimensional and three-dimensional numerical calculation models to study various factors affecting evaporation loss,and analyze the advantages of choosing a three-dimensional numerical calculation model;Then,a three-dimensional numerical calculation model was used to study the influence of relative humidity of the inlet air,ambient temperature,atmospheric pressure,crosswind,cooling water volume,and inlet water temperature on the three types of wet cooling tower's evaporation loss,the ratio of evaporation heat transfer to convection heat transfer,the ratio of evaporative transfer.heat to the total heat transfer and the outlet water temperature.Obtained that:the relative difference between the one-dimensional and three-dimensional numerical calculation results of the cooling tower outlet water temperature under different influencing factors is less than 3%,indicating that the one-dimensional numerical calculation model can calculate the cooling performance of the cooling tower,but the calculation result of the evaporation loss is rather large,and the calculation result of convection heat transfer is rather small,and it can only be studied under windless conditions.The three-dimensional numerical calculation model can not only accurately calculate the cooling performance of the cooling tower,but also accurately calculate the evaporation loss and convection heat transfer of the cooling tower.Therefore,the numerical study in this paper uses the three-dimensional numerical calculation model;Changing the relative humidity of the inlet air,the ambient temperature,the inlet water temperature and the inlet water volume can greatly inhibit the evaporation loss for all three types of cooling towers.When the evaporation loss of the cooling tower is inhibited under different influencing factors,the cooling performance of the cooling tower is also reduced;For the three types of cooling towers,when the evaporation loss is inhibited,the influencing factor for the reduction of the ratio of evaporation heat transfer to convection heat transfer is the relative humidity of inlet air and atmospheric pressure;The ambient temperature,atmospheric pressure,ambient crosswind and tower water temperature can hardly be actively controlled,while the relative humidity of inlet air and the amount of cooling water can be controlled,but the ratio of evaporation heat transfer to convective heat transfer is gradually increases when evaporation heat transfer is inhibited by reducing the amount of cooling water,this indicates that while the evaporation heat transfer is reduced,the more convection heat transfer is also reduced.It is obtained finally that,only the relative humidity of the inlet air can be controlled,and the ratio of evaporation heat transfer to convection heat transfer is reduced when evaporation heat transfer are inhibited by reducing the relative humidity of the inlet air.(3)Based on theoretical and numerical research on the influencing factors of evaporation loss in wet cooling towers,choosing the relative humidity of the inlet air as the control factor to actively inhibit evaporation heat transfer for all of natural draft counterflow wet cooling towers,wet cooling tower with water collecting devices and mechanical draft cooling towers.Taking the natural draft counterflow wet cooling towers as the research object,the evaporation loss,the mass transfer driving force,the mass transfer coefficient of each zone and the change law of the air volume entering the tower when the relative humidity of the inlet air changes under two conditions of windless and cross wind are studied.The focus is on the analysis of the changing law of the mass transfer driving force along the radial and axial directions,and the mechanism of the relative humidity of the inlet air inhibiting evaporation heat transfer is revealed.It is pointed out that the relative humidity of the inlet air can effectively inhibit the driving force of mass transfer in both cases of windless and cross wind.(4)The mechanism of the relative humidity of the inlet air inhibiting evaporation heat transfer is revealed by three-dimensional numerical study.Then the thermal model test of the relative humidity of the inlet air inhibiting evaporation heat transfer was conducted to analyze the influences of relative humidity of inlet air on evaporation heat transfer in cooling tower under different cooling water volume,inlet water temperature,relative humidity of inlet air and cross wind.It was verified that the accuracy of the numerical analysis on the inhibition mechanism of evaporation heat transfer process of the three-dimensional numerical calculation model of wet cooling tower.Field tests were carried out on a mechanical draft cooling tower with water collecting devices in a power plant.It is analyzed the effects of relative humidity of the inlet air on evaporation loss and the ratio of evaporation heat transfer to convection heat transfer,and verified the accuracy of the theory of controlling evaporation heat transfer of wet cooling tower by reducing relative humidity of inlet air.(5)A three-dimensional numerical model of cooling tower with wet air circulation channel was established to study the variation of mass transfer driving force,mass transfer coefficient and evaporation loss of cooling tower before and after the establishment of wet air circulation channels,which revealed the mechanism of wet air circulation channel inhibiting evaporation heat transfer.Then the thermal model test bench of wet air circulation channel was established to study the variation rules of evaporation loss before and after the establishment of wet air circulation channel.The accuracy of the three-dimensional numerical model of cooling tower with wet air circulation channel is verified by comparing with the above variation rules.It is concluded that the evaporation heat transfer decreases by 15.1%and convection heat transfer increases by 33.4%after the establishment of wet air channel.The wet air circulation channel can not only reduce evaporation loss,but also increase the convection heat transfer.The paper studied the coupling of wet air circulation channel and cooling tower efficiency enhancing measures,and concluded that the air ducts are most effective by analyzing the maximum improvement of cooling efficiency of cooling tower adopting efficiency measures such as air duct,air guide plate and non-uniform packing in the current research.Then the same mathematical model as the air duct efficiency study was established to study the water-saving critical value of cooling tower while ensuring the cooling effect under the coupling of wet air circulation channel and air duct.The evaporative water loss is reduced by 4.6%,and the critical value of water saving in this coupling mode is 30.2t/h?The conclusion of this paper elaborates the influence of the relative humidity of the inlet air on evaporation heat transfer in detail.It is proposed that evaporation heat transfer process of the cooling tower can be inhibited by establishing the wet air circulation channel and controlling the relative humidity of the inlet air,in order to achieve the purpose of saving water.Combined with the flow field optimization technology in the rain area and the wet air circulation channel,the cooling tower can save water while ensuring the cooling performance of the cooling tower remains unchanged.The conclusion has directed significance for the water-saving design of cooling towers.