| A typical air condenser unit of a direct air cooling unit in a power plant is taken as the research object.The air flow field and heat transfer were numerically simulated,and in order to improve the uniformity of velocity distribution and heat transfer effect between finned tube bundles,a guide plate was installed at the outlet of the air-cooled axial flow fan to optimize the flow field disorder and uneven velocity distribution among tube bundles.Firstly,the air flow field of an air-cooled axial fan is numerically simulated when it operates alone,MRF(multi-reference frame)model is used to deal with the dynamic and static coupling problems in the rotating region and the stationary region.The simulation results were compared with the data provided by the fan manufacturer,and the static pressure-flow performance curve of the fan was drawn.The error was compared with the performance curve given by the manufacturer,which fully verified the reliability of the fan model and the use of MRF model to deal with the rotation problem of the fan.Then the whole coupling model of Atype frame of air cooling unit and axial flow fan is simulated.Porous media is used to simplify the heat dissipation fin tube bundle,and the result of five turbulence models and four kinds of import and export conditions are compared.And the RNG k-ε model is determined as the final turbulence model,the final inlet and outlet conditions are hemispherical inlet with mass flow entrance and pressure outlet boundary conditions.Next,the simulation results are analyzed comprehensively.It is found that the existence of A-type frame causes backflow at the outlet of the fan,which leads to the reduction of the total pressure of the fan.The air flow characteristics at the outlet of the fan are analyzed,and the distribution rules of axial velocity,circumferential velocity,radial velocity and total velocity along the radius direction are obtained.Then the flow field in the frame of air cooling unit is analyzed.It is confirmed that the airflow is rotating and rising.Because of the sudden change of the flow area at the outlet of the fan,whirlpools are generated in the four corners of the unit,forming the dead space of flow.As the velocity in the outlet of the fan is low in the center and high in the sides,a large range of backflow is formed in the center area of the unit,which hinders the smooth of airflow.The disorders of the internal flow field directly leads to the uneven distribution of velocity between the the finned tube bundles,as well as the uneven distribution of temperature and heat transfer,which weakens the heat transfer effect.By comparing the results of the fan running at full speed with that at half speed,it is found that when the fan running at half speed,the air volume is small,and the vortex and backflow phenomenon inside the unit are weakened,thus making the low speed zone at the bottom of the tube bundle outlet shrink.Finally,in order to improve the flow uniformity between the tube bundles and enhance the heat transfer effect,a guide plate was installed at the outlet of the fan to optimize the flow field.Firstly,the models with different number of guide plates were simulated,and the velocity and temperature distribution and heat transfer at the outlet of each model tube bundle were compared,and the optimal number of guide plates was determined to be 8.It is found that the incidence Angle of the flow is not positively correlated with the heat transfer.Then,the arc-tochord ratio,installation Angle and width of the guide plate were optimized on the basis of the eight guide plates.The uniform velocity distribution and heat transfer at the outlet of the tube bundle were compared.The optimal arc-to-chord ratio is 1.02,the optimal installation Angle is60°,and the optimal width is 2.1m.At last,the simulation results of the optimal scheme and the original model are compared,and the optimization effect is remarkable. |
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