| Inlet air pre-spraying can improve the cooling performance of natural draft drycooling towers in high temperature periods,but the ambient crosswind will change the aerodynamic field inside and around natural draft dry cooling towers.Research on the inlet air pre-spraying system that adapts to the ambient crosswind is of great significance to further improve the cooling performance of dry cooling tower.This article focused on the influence of crosswinds,and explored the control mechanism and strategy of inlet air pre-spraying on the cooling performance of natural draft dry cooling towers.Firstly,based on the theoretical analysis of the gas-liquid two-phase heat and mass transfer process,a three-dimensional numerical simulation model of the two-nozzle horizontal spray evaporation process was established,coupling calculations of heat and mass transfer in the spray evaporation process were carried out,and the heat transfer rate Qs was proposed combining the temperature and area of the cooling affected zone,to evaluate the evaporative cooling performance when the surface to be cooled was not completely covered by spray droplets;The effect of two-nozzle arrangement distance,inlet air velocity,dry bulb temperature,and relative humidity on the evaporative cooling performance was studied,which laid a foundation for the subsequent optimization of multi nozzle arrangement;Subsequently,a half-tower three-dimensional numerical simulation model of dry cooling tower with spray evaporation pre-cooling under crosswind was established,and the feasible pre-spray height,pre-spray angle and pre-spray radius under crosswind were explored,the feasible zone for pre-spraying under different crosswind speeds was obtained;Finally,taking a typical crosswind speed of 4 m/s as an example,the control mechanism of inlet air evaporative pre-cooling under nozzle cross arrangement and the whole tower nozzle arrangement on the performance of the 120 m high-altitude dry cooling tower was studied.The control strategy of natural draft dry cooling towers inlet air pre-spraying was obtained and the optimization direction of the nozzle arrangement was put forward,which laid the foundation for the design and optimization of dry cooling tower with spray evaporative pre-cooling under the influence of crosswind.Through the research,the main conclusions are as follows:(1)The arrangement distance of two-nozzle affects the maximum temperature drop after evaporative cooling and the area of cooling affected zone;high dry bulb temperature and low relative humidity are conducive to evaporative cooling,and the airflow speed is recommended to be 2～3 m/s,but it is limited to the application environment;considering the cooling object and application environment comprehensively,the appropriate nozzle arrangement distance should be selected combined with the goals of good cooling uniformity,large cooling temperature drop,and small amount of nozzles,to achieve efficient and water-saving cooling;(2)When exploring the feasible pre-spray zone of natural draft dry cooling towers under crosswind,the spray height,spray angle and spray radius should be considered comprehensively;when the pre-spray height changes from low to high,the droplet evaporation rate first increases and then decreases,the effective evaporation ratio continues to increase,that is,the nozzle should be placed at a higher position as far as possible under the premise of ensuring that the droplet evaporation rate is greater than 99%;the pre-spray angle has less influence on the droplet evaporation rate and the effective evaporation ratio;the pre-spray radius has almost no effect on the droplet evaporation rate,but it has a great influence on the effective evaporation ratio,the larger the pre-spray radius,the smaller the effective evaporation ratio;taking 4 m/s crosswind speed as an example,the selected appropriate pre-spray height is 9 m,the suitable pre-spray angle is 0°,the range of pre-spray radius that can be controlled and explored is 21.5～41.5 m for the dry cooling tower with a radius of 41.5 m;(3)At a typical crosswind speed of 4 m/s,a half-tower numerical simulation was performed on the inlet air evaporative pre-spraying of a 120 m high-altitude dry cooling tower.It is found that the cooling performance of dry cooling tower can be improved when the included angle between nozzle cross arrangement and crosswind is 0°,22.5°,45° and 67.5°,and the optimal effect appears when the included angle is 45°,compared with no spray,the heat transfer rate of dry cooling tower can be increased by 8.5%(The number of nozzles in the entire dry cooling tower under this arrangement is 33,with a total water consumption of 6.6 kg/s);the whole tower nozzle arrangement can greatly improve the cooling performance of dry cooling tower,through the active control of nozzle position and nozzle opening number,the heat transfer rate of dry cooling tower can be increased by 35.2%compared with no spray(The number of nozzles in the entire dry cooling tower under this arrangement is 160,with a total water consumption of 32.0 kg/s).In summary,when designing the dry cooling tower inlet air pre-spraying system under crosswind,the feasible pre-spray zone should be determined first according to droplet evaporation rate and effective evaporation ratio.Specifically,the pre-spray height with droplet evaporation rate greater than 99%and the highest effective evaporation ratio should be selected,the pre-spray angle with the highest effective evaporation ratio should be adopted,the range of pre-spray radius for sharp decrease of effective evaporation ratio on leeward side should be preliminarily determined;Secondly,according to the speed and direction of the ambient crosswind,the nozzle opening number and nozzle position of the inlet air pre-spraying system should be controlled actively,that is,the nozzle at the position with lower unit heat transfer rate should be closed to save water resources,and the nozzle at the position with higher unit heat transfer rate should be set to improve the cooling performance of the dry cooling tower;Finally,the application environment conditions and optimization objectives should be considered and the water consumption and the improvement of cooling performance of dry cooling tower should be balanced,to make a reasonable choice of spray system arrangement. |
