| Cooling towers are one of the important cool-end components in thermal power plants,and their thermal performance affects the power generation efficiency they are serving.Natural draft dry cooling towers(NDDCTs)have advantages of low water consumption,low operating cost and low maintain cost,and they have been widely used in power plants which are constructed in dry climates.However,due to the cooling limitation of NDDCTs,the cooling performance of NDDCTs would particularly be reduced when the ambient air is hot,which is difficult to match with the peak power consumption period.To solve this problem,the spray evaporative pre-cooling technology of inlet air was proposed.The evaporation of water droplets can cool the inlet air of the tower and enhance the heat transfer between the air and the heat exchange at the radiator.Firstly,the evaporation process of spray droplets was analyzed,while a 3 D model was developed to investigate two-nozzle spray cooling.The numerical model was validated based on comparison of existing research and simulation results.The flow and heat and mass transfer in the process of two-nozzle spray cooling were studied by numerical simulation.A modified cooling efficiency was proposed to effectively evaluate the cooling performance with considering both the cooling affected region and the air temperature drop.The effects of two-nozzle distance,inlet air speed,inlet air dry-bulb temperature and humidity on the evaporative cooling performance were investigated.Secondly,considering the coupled characteristics of flow and heat and mass transfer in the NDDCT,a model of the NDDCT pre-cooled with nozzle spray was established.The effect of water droplets on flow filed of the NDDCT was reported.The feasibilities of nozzle positions and spray direction of a typical NDDCT were studied.The effects of spray direction and multi-nozzle's layout on the evaporative cooling performance were explored.Finally,the cooperative control mechanism of spray direction and layout on the heat transfer performance of the NDDCT pre-cooled with nozzle spray was studied.The optimal multi-nozzle arrangement relationship between spray direction and multi-nozzle's layout was obtained.In the meanwhile,Water saving control strategy of the NDDCT pre-cooled with nozzle spray under the condition of low water flow was studied.The mechanism of improving the heat transfer performance of NDDCTs pre-cooled with nozzle spray was revealed,which lays a theoretical foundation for the optimal design of energy-saving of NDDCTs.This study found that:(1)The nozzle distance affects the spray cooling performance by the temperature and area of cooling affected region,and the cooling performance is related to the distance between nozzles and the surface to be cooled.The modified cooling efficiency of tangent region is similar to the separating region,and it is 2.5%higher than the partially overlapping region at the wind tunnel exit.The better cooling performance is obtained at low wind speed.The modified cooling efficiency at the wind tunnel exit increases from 29%to 83%when the inlet air speed declines from 3 to 1m/s.The better cooling performance of two-nozzle system generally occurs at a higher air temperature and a lower air humidity.(2)The feasible nozzle positions are different in different air inlet heights and radii,both of which affect the feasible nozzle positions.In the low inlet height zone,the feasible positions of nozzles are concentrated in the middle and outer region of the tower.When the nozzle is injected horizontally to the center of the tower,the radiator can obtain better cooling performance.In the middle and high inlet height zones,the feasible positions of nozzles are concentrated in the outer area of the tower.When the spray direction is between horizontal and vertical or when the nozzle is injected vertically,the radiator can obtain better cooling performance.As the height of nozzle location increases,the cooling center of the radiator would gradually move from the center area to the outer area of the tower at the same radius.This height of the cooling center of the radiator moving outward increases with the increase of radius.(3)Heat transfer performance of the NDDCT affected by multi-nozzle's layout and spray direction.In comparison of different layouts of nozzles at the low and medium height zones,better solution is:the nozzles with the lower height should be located inside of the tower inlet region,while the nozzles with the higher height should be located outside the tower inlet region.At this time,the modified cooling efficiency of the radiator surface can be up to 5.9%higher than other layouts,and the average temperature of radiator surface is 0.8?lower than that of other layouts.In the study of optimized combination scheme of multi-nozzle's layout and spray direction,when the nozzle is set to a better spray direction,nozzles should be arranged at a radius of 45m at the high height zone,while the selection of height should be 11 and 13 m in order to obtain the best cooling performance.The modified cooling efficiency at radiator of this arrangement scheme are 1.4%and 8.0%higher than that of nozzles are located at 9 and 11 m and 10 and 12 m,respectively.The results show that the heat transfer of the NDDCT pre-cooled with nozzle spray is 42.38%higher than that of the traditional NDDCT.(4)In the study of water saving operation regulation,there are two methods to adjust water flow of each nozzle under the condition of low spraying water flow.Each nozzle should have the same water flow,or a larger water flow rate should be used at a higher spraying position.The same water flow should be chosen at 75%of base spraying water flow,and the scheme of higher nozzles positions having a larger flow should be chosen at 50%of base spraying water flow,the modified cooling efficiency of radiator surface is 4.8%and 2.9%higher than that of other schemes,respectively.In summary,the nozzle arrangement of the NDDCT pre-cooled with nozzle spray should follow this principle:the nozzles with the lower height should be located inside of the tower inlet region,while the nozzles with the higher height should be located outside the tower inlet region.The nozzle location with higher arrangement height should be close to the radiator,and it is not suitable to be placed at the outer edge of the tower inlet region.When the multi nozzle system operates under low spraying water flow conditions,each nozzle should have the same water flow,or a larger water flow rate should be used at a higher spraying position. |
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