Browsing by Author "Meissner, Timo W."
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- ItemNumerical investigation of the effect of scaling on the performance of large scale axial flow fans.(Stellenbosch : Stellenbosch University, 2018-12) Meissner, Timo W.; Van der Spuy, S. J.; Meyer, Chris J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Large-scale axial flow fans are utilised in air-cooled heat exchanger systems in power plants. Due to the size of these fans, their performance cannot be experimentally tested. A common practice is to test a smaller, scaled-down version of the fan in a fan test facility and use the results to determine the performance of the large scale fan. Improving the accuracy of the scaled fan performance allows for a more accurate assessment of the fan performance and subsequently of the efficiency of the power plant. The parameters influencing fan performance and their variation in magnitude with scaling are investigated. The performance of the Pelz scaling method for up- and down-scale scenarios compared to experimental data is assessed. The results show that the scaling method over-predicts the change in efficiency. The accuracy of a CFD analysis compared to experimental results of the B2a-fan at different sizes is investigated, showing an over-prediction of the numerical results at low flow rates and a under-prediction at high flow rates. The numerical results of a 0.63 m, 1.542 m and 9 m diameter B2a-fan show an increase in fan static efficiency and -pressure with fan size. Due to the similarity set between the fans, the Reynolds number range over the blade span increases with an increase in fan size. An increase in fan size and thus Reynolds number over the fan blade results in a logarithmic increase in fan static efficiency. As a result the increase in efficiency between the 0.63 m and 1.542 m diameter B2a-fan is about the same as the increase in efficiency between the 1.542 m and 9 m diameter fan, even though the increase in size of the 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 turbulence models and the effect of Reynolds number on the lift and drag characteristics of an airfoil is conducted. The analysis showed an over-prediction in lift and drag by the Realizable k-" turbulence model. The Spalart Allmaras turbulence model produces results with a much smaller deviation to the experimental results. A numerical analysis of the B2a-fan using the Spalart Allmaras turbulence model does, however, not reduce the deviation between the numerical- and experimental results. It is observed that the change in liftto- drag ratio of the two-dimensional airfoil over a change in Reynolds number produces a similar trend than the results of the peak fan static efficiency over a change in Reynolds number in the three-dimensional analysis. The fan static efficiency is a function of the lift-to-drag ratio. A comparison showed that three-dimensional losses have a greater effect on the total losses at a high Reynolds number than at a low Reynolds number.