Large Eddy Simulation Of Flow And Heat Transfer On Air-Side Of The Direct Air-Cooled Condensers

Air-cooled condenser is the key element of the direct air cooling system.The performance of direct air-cooled condensers is affected directly by air-side flow and heat transfer characteristics. The complex flow structures were generated among the wakes behind the bodies when the air flow past multiple wavy finned flat tubes, beacuse of the nonlinear interactions among the wakes. High thermal resistance of air side will lead to low heat transfer effieieney. The flow complexities at the exit of the axial flow fan result in the flow mal-distribution on the surface of the finned tube bundles. There exists the strong coupling relationship between the fan and A-shaped frame condenser cell. All of the influence factors will lead to nonlinear scale effect.The characteristics of the flow and heat transfer on air-side of the direct air-cooled condenser have been simulated using large eddy simulation (LES).The large eddy simulation results of flow around a circular cylinder are in well agreement with the experimental data. Using LES theory and method for the direct air-cooled condensers, can captures the complex turbulent motions of air-cooled condenser in detail. And the complex vortex motion in the wake region of the flat finned-tubes are predicted more precisely than RANS approach. It provides theoretical basis to study the mechanism of flow and heat transfer.Three-dimensional flow structure and heat transfer characteristics of the flat tube without of fins has been studied. The results show that the flat tube have lower air-side pressure drop and smaller wake region. When the aspect ratio L/D of flat tube is reduced, the flat tube have lower air-side pressure drop, smaller wake region and better heat transfer coefficients.When the wake interaction of the adjacent finned tube is considered,the separation points of the boundary layers, the range of wake region,vortex shedding pattern, flow structure and heat transfer have a notable deviation from the single tube. For double finned tube,the vortex shedding from one flat tube have a definite phase relationship with the other tube.At the spacing H/D=1.95and Re=4006, the vortices remain distinct and the interaction between the flat tubes is weaker. The degree of nonlinear wake interaction depends on the Reynolds number at the spacing.When the Reynolds number increases, the nonlinear interference is enhanced.The two side of the single flat finned-tube have the same distribution of local Nusselt number since they experience the same undisturbed fluid, whereas for the two side by-side flat finned-tubes the local Nusselt number distribution on both side of the tube is different, due to the mutual interference of the flat finned-tubes.The difference of the local Nusselt number between the two fins are larger when the Reynolds number increased. Furthermore, the local Nusselt number of the same fin in single finned-tube is lower than the two side-by-side finned tubes at same Reynolds number.The secondary frequency induced by the interactions of the flat finned-tubes have a better effect on heat transfer.The design of ACC should also account for wake interaction.The flow and heat transfer characteristics of flat tube wavy finned channel with punched six delta winglet pairs was studied by large eddy simulation. The results show that the average Nusselt number increase by49.68%-54.14%and the average friction factor increase by35.63%-37.35%.The results are in well agreement with the experimental correlations of Nusselt number and friction factor with Reynolds number by others.The head vortex system and horseshoe vortex are induced by oblique-cut semi-elliptic cylinder shells vortex generator.The combination of the two type vortex system plays a vital role in heat transfer enhancement. The contribution of reynolds stress in sweep process and ejection process are basically same for trapezoidal wing, and the Reynolds stress distribution is in the Ⅱ quadrant and IV quadrant.The reynolds stress contribution of oblique-cut semi-elliptic cylinder shells is in the II quadrant, and the contribution of Reynolds stress is mostly in ejection process. The drag is large when the distribution of Reynolds stress in the sweep process.So the semi-elliptic cylinder shells have low flow drag. Based on the theory of turbulent coherent structures, the oblique-cut semi-elliptic cylinder shells vortex generator can achieve the aim that heat transfer enhancement and flow drag reduction.Large eddy simulation has been employed to flow and heat transfer over wavy fin surface for different amplitude:ao/λ=0.01、0.02、0.04、0.06. The smaller wave amplitude of wavy surface in this study does not form the recirculation in the valley.With the almplitude increasing,the range of the recirculation and the quantity of the vortex is significantly increased.The high and low speed streaks are obvious increased. A number of high wall sheer stress and Nusselt number spindle-shaped spots are observed. The bigger wave amplitude surface have better heat transfer performance.A three dimensional coupled air flow field calculation is carried out for the rotational fan and the stock-still A-shaped frame-work, which can provide an accurate description of the detailed flow filed in air-cooled condenser cell. And the air mal-distribution on the finned tube bundles due to the interaction between the fan and air cooled condenser.With the ambient temperature increasing, the average inlet air velocity of the finned tube bundles is decreased, the variation range is less than10%. Furthermore, the inlet air velocity of the left side and right side of the finned tube bundles are not the same,which is related to the biased flow that the non-symmetric velocity distribution on the top of the exhaust steam pipe even under the no-wind condition.