Numerical investigation of the effect of scaling on the performance of large scale axial flow fans.

[摘要] ENGLISH ABSTRACT: Large-scale axial flow fans are utilised in air-cooled heat exchanger systemsin power plants. Due to the size of these fans, their performance cannot beexperimentally tested. A common practice is to test a smaller, scaled-downversion of the fan in a fan test facility and use the results to determine theperformance of the large scale fan. Improving the accuracy of the scaled fanperformance allows for a more accurate assessment of the fan performance andsubsequently of the efficiency of the power plant.The parameters influencing fan performance and their variation in magnitudewith scaling are investigated. The performance of the Pelz scalingmethod for up- and down-scale scenarios compared to experimental data is assessed.The results show that the scaling method over-predicts the change inefficiency. The accuracy of a CFD analysis compared to experimental resultsof the B2a-fan at different sizes is investigated, showing an over-prediction ofthe numerical results at low flow rates and a under-prediction at high flowrates. The numerical results of a 0.63 m, 1.542 m and 9 m diameter B2a-fanshow an increase in fan static efficiency and -pressure with fan size. Due tothe similarity set between the fans, the Reynolds number range over the bladespan increases with an increase in fan size. An increase in fan size and thusReynolds number over the fan blade results in a logarithmic increase in fanstatic efficiency. As a result the increase in efficiency between the 0.63 m and1.542 m diameter B2a-fan is about the same as the increase in efficiency betweenthe 1.542 m and 9 m diameter fan, even though the increase in size ofthe later is more than double the size increase from 0.63 m to 1.542 m.A two dimensional analysis investigating the accuracy of the turbulencemodels and the effect of Reynolds number on the lift and drag characteristicsof an airfoil is conducted. The analysis showed an over-prediction inlift and drag by the Realizable k- turbulence model. The Spalart Allmarasturbulence model produces results with a much smaller deviation to the experimentalresults. A numerical analysis of the B2a-fan using the SpalartAllmaras turbulence model does, however, not reduce the deviation betweenthe numerical- and experimental results. It is observed that the change in liftto-drag ratio of the two-dimensional airfoil over a change in Reynolds numberproduces a similar trend than the results of the peak fan static efficiency over achange in Reynolds number in the three-dimensional analysis. The fan staticefficiency is a function of the lift-to-drag ratio. A comparison showed thatthree-dimensional losses have a greater effect on the total losses at a highReynolds number than at a low Reynolds number.

[发布机构] Stellenbosch University