| Due to its superior water saving,air-cooled technology is a common cooling method for coal-fired power generation in coal-rich and water-scarce regions in China.The direct air cooling system uses mechanical ventilation forced convection to achieve direct heat exchange between the cooling air and the steam turbine exhaust.Because air is used as cooling medium,direct air cooling systems are highly susceptible to environmental factors such as ambient temperature and ambient wind.In this work,the flow and heat transfer characteristics of air-cooled condensers(ACCs)in extreme environments are studied,including summer high temperature condition and winter heating condition.High condensing pressure of ACC at high environmental temperature in summer is a common problem of air-cooling power generating units.The wavy finned flat tube bundle is the key element of ACC.Experiments on heat transfer enhancement of wavy finned flat tube bundle by water spray cooling was carried out by wind tunnel with a visualized window for temperature distribution measurement of fin surface.The mechanisms of heat transfer enhancement,including that of the cooling of air due to upstream droplets evaporation and the evaporation of water film formed by droplets impacting on the finned tubes,were investigated,respectively.The cooling efficiencies for different inlet air velocities were required.While the air flow Reynolds number ranges from 210 to 680,the experimental correlations of flow and heat transfer were obtained.As compared to the case without spray,the Nusselt numbers of three water flow rates are increased by 48%,56%and 68%on average,respectively.The respective contributions of both pre-cooling and water film evaporation were evaluated,which were almost equal as the Reynolds number was about 500.The results can benefit the optimization of spray cooling technique used in air-cooled condenser.In order to further study the heat transfer enhancement of finned tube by spray cooling,a single fin unit was built as physical model.The numerical simulation method was used to analyze the process of droplets evaporation between the fins and the formation of liquid film.The results showed that spray cooling can effectively reduce the fin surface temperature and improve the convective heat transfer coefficient.The heat transfer enhancement by increasing spray water rate is limited.The droplet size and air face velocity have a significant effect on the dispersion of droplets,thereby changing the cooling effect of finned tube.In practical operation,the appropriate droplet size should be selected.If the droplet size is too small,it is beneficial to dispersion of droplets,but some droplets pass through the fins region and are not utilized.However,if the droplet size is too large,the droplets cannot fully diffuse.Only part of the fins is affected by droplets and liquid film evaporation,which makes the overall heat transfer enhancement effect poor.Taking the air-cooled 300MW power generating unit as object,the flow and temperature fields of ACC with water spray cooling were analyzed by numerical simulation with experimental validation.Based on the air flow field inside ACC cell,an improved water spray nozzles arrangement was proposed to enhance the cooling capacity.The inlet air temperature and back pressure of turbine were compared with that under the original nozzles arrangement.The water consumptions including that of water droplets pre-cooling evaporation in air flow,liquid film evaporation on the finned tube bundles,and drainage of redundant water were obtained.The influences of water spray rate,spray direction and nozzles distance on cooling performance were investigated.The results showed that the improved nozzles arrangement is expected to provide efficient cooling performance and significant reduction in back pressure of the objective power generating unit.The improved nozzles arrangement with upward spray direction and nozzles distance of 0.8m can be applied to the design of water spray system in ACC.In addition,the obtained redundant drainage water under various ambient conditions provided the evidence for spray water rate optimization.Combined heat and power(CHP)generation is a major heating method in winter.The exhaust steam from steam turbine is directly supplied as heat source of the heating network.At the meanwhile,the abundant exhaust steam is cooled by ACCs.For a CHP generation unit,because the amount of steam entering the air-cooled island is reduced,the axial flow fans can be operated with quite low speed or even stopped.The influences of natural wind on the thermo-fluid performance of 300MW air-cooling CHP generating unit were investigated both numerically and experimentally.The CHP unit operated with only partial ACC cells in work under the winter chill environment.The physic-mathematical model was established for air flow and temperature fields of air-cooling island,in which the natural convection was considered while the axial flow fans of ACC cells stopped.Variations of heat rate of ACC cells with natural wind direction and speed were acquired.The results indicated that the heat rate varied significantly both with hot plume recirculation and reversed air flow to ACC cells caused by natural wind.However,different from the normal air cooling power generating unit,the total heat rate of ACC cells might increase due to the reversed flow for all axial flow fans stopped.Based on the observations,a case study was executed for the CHP unit with 280MW power generating loading under natural wind conditions.The optimal operating schemes were obtained under the conditions to satisfy heat supply of urban heating network and maintain economic operation of the CHP unit. |
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