| Recently air-cooling technology has been actively developed in north China where water resource is of shortage, to solve water crisis for power plant. To compensate for the low cooling capacity of the air-cooling technology, finned flat tube is employed. As a main direction to improve the design, the resistance loss among the fins and the heat transfer of the extended surface are the main factors.According to the low Renolds number, the physical model regarding flow and heat transfer characteristic between fins were analyzed. Based on it, the air-side heat transfer characteristics of oval tube with rectangular fins, the oval tube with deviating-geometric-center rectangular fins, the wavy finned flat tube, the wavy finned flat tube with longitudinal vortex generators, a new staggered short fins and discontinuous short wavy fins were investigated in detail.The numerical simulations concerning deviating-geometric-center rectangular fins show that when deviating for the air inlet direction, the average air-sided heat transfer coefficient increased by4.2%. The more distance away for the center is, the bigger the heat transfer coefficent is. Four various types of punched longitudinal vortex generators were employed to enhance air-side heat transfer on the fin surface of flat tube used in direct air-cooled condenser. It was found that the delta winglet pair with attack angle25°could reach the greatest performance evaluation criteria (PEC) under the conditions of the inlet air flow velocity varied from2m s-1to5m s-1. The influences of locations on the fin surface and the row number of longitudinal vortex generator were also discussed. One delta winglet pair at the middle of fin surface and minimum row number, n=1, with the average PEC being1.182, has the best heat transfer performance of all conditions, which can be recommended for practical applications. A visualization experimental study on flow field in wind by Particle Image Velocimetry (PIV) was conducted. The numerical simulations verified that the delta winglet pairs can generator obvious longitudinal vortex pairs at the down-sweep zone.A further experimental and numerical investigation was conducted into flat tube wavy finned channel flows with and without punched six delta winglet pairs. The experimental correlations of Nusselt number and friction factor vs Reynolds number in the experimental region were obtained. Through comparisons between experiments and numerical simulations, the numerical feasibility was verified in the air-side heat transfer of wavy finned flat tube with and without punched six delta winglet pairs. Experimental and simulation results showed that a substantial increase in the heat transfer with six delta winglet pairs generators. It was observed that average Nusselt number increase by20.5%-59.8%and the average friction factor increase by12.8%-82.6%in experiment. And, the performance evaluation criteria, PEC, is greater than1. By employing IR thermography, temperature distribution information about wavy fin surface with and without punched six delta winglet pairs were obtained.It has been proved that the serrated fins can enhance heat transfer obviously by breaking the development of boundary layer periodically. In the present study, the original continuous wavy fins were disconnected to create a new structure, namely, discontinuous short wavy finned. The results showed that, compared with the. original continuous structure, the discontinuity short-wavy-fin flat tube effectively improved the air-side heat transfer at the air velocity of engineering field, and because the discontinuous fins structure reduces the contact area of air and wall, the increase in pressure loss is suppressed. |
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