Browsing by Author "Du Plessis, Clarice"

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    Scaling of axial fan noise
    (Stellenbosch : Stellenbosch University, 2022-04) Du Plessis, Clarice; Van der Spuy, S. J.; Reuter, H. C. R.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: Axial flow fans play an important part in industrial cooling applications, such as air-cooled condensers (ACCs). Reliable methods of predicting the noise generated by a full-scale axial fan from a scale model at the design stage would result in less uncertainty and financial risk to cooling system suppliers. Axial fan noise is separated into its main mechanisms, namely trailing edge (TE), tip vortex formation and turbulence ingestion noise. TE noise of a 1.542 m diameter M-fan is predicted by dividing the fan blade into segments using strip theory. Large Eddy Simulation (LES) is performed for NACA 0012, LS 0409 and LS 0413 airfoils for a range of chord lengths, free stream velocities and angles of attack. Far field noise is predicted by implementing the Ffowcs-Williams Hawkings analogy with unsteady pressure fluctuations on the airfoil surface as source term. A method is developed with which the noise computed for an airfoil with a small modelled spanwise length can be corrected for an increased spanwise length if the main noise mechanism is not turbulence. Existing airfoil noise scaling methods are investigated, and a modified method is proposed. Free stream velocity, angle of attack and TE boundary layer displacement thickness are averaged over each fan blade segment and the LS 0409 and LS 0413 self-noise are scaled to represent each segment of the M-fan, which is logarithmically summed to represent the full fan. Simulations of the 4- and 8-bladed M-fans are performed using a steady state Reynolds Averaged Navier Stokes (RANS) turbulence model at various volume flow rates and tip clearances. From these simulations, the tip vortex formation noise is computed from a semi-empirical model. The tip vortex causes increased turbulence in the flow, resulting in turbulence ingestion noise generated at the blade leading edge. Assuming isotropic turbulence, turbulence ingestion noise can be calculated using an analytical model, validated on airfoils. For each fan blade segment, the inputs to this model are the relative tangential velocity, turbulent intensity, kinetic energy and dissipation rate at the relative velocity streamline reaching the airfoil leading edge. It is shown that the inputs from a scale model can be modified and applied to predict the turbulence ingestion noise for the same fan for a different diameter. Experimental measurements of the 1.542 m diameter M-fan noise are conducted in an ISO 5801, Type A (free inlet, free outlet) test facility. Corrections are made for the effect of reflections on the measured SPL spectrum. The predicted fan noise agrees well with the measured spectrum (within 0.06 dB). The noise for an M-fan with a 3.084 m diameter is also predicted. Finally, it is recommended by the frequency spectrum of the total noise be scaled with a factor proportional to the ratio of Dfan,1/Dfan,2 (Dfan,1 is the original fan diameter, Dfan,2 is the fan diameter to which the SPL is scaled). For a doubling in fan diameter, the total SPL in this case is found to scale well with the ratio of diameters to the power of 1.67.