Session: 35-02/10-06 Joint Session Fan & Turbomachinery noise and operations

Submission Number: 176219

Aeroacoustic Noise Reduction of Large-Diameter Axial Flow Fans Through Vortex Behaviour Manipulation 

Thermal power plants in arid and semi-arid regions that operate on the Rankine principle utilize dry cooling technologies to minimize water usage. Examples of these types of plants include coal-fired, nuclear, and concentrated solar power (CSP) plants. Recently, the use of supercritical carbon dioxide (sCO₂) as a heat transfer fluid (HTF) further emphasises the importance of dry cooling technology. These dry cooling technologies utilize low-pressure-rise, large-diameter axial flow fans with diameters of 7-10 m to either force or draw air through heat exchanger bundles that condense the HTF. These units are known as air-cooled condenser (ACC) units. These fans are generally rotor-only, having no nose cone, flow straighteners, or guide vanes.

The ACC fans are a major source of thermal power plant noise emissions. Studies have shown that noise emissions of these fans may exceed 88 dB(A). Attempts to reduce the noise of these fans include adding silencers, modifying blade geometry or its surface, and/or adding blade tip appendages. These attempts proved to be expensive or detrimental to the fan’s performance or the power plant system's performance. This necessitates an in-depth understanding of the aeroacoustic behavior of these fans to find an innovative solution that reduces their noise emissions while maintaining fan efficiency and power plant system performance.

In this paper, the aeroacoustic behaviour of a large-diameter axial flow fan with various tip gaps, blade numbers, and flow rates is investigated numerically and experimentally. The numerical model includes a computational fluid dynamics (CFD) framework composed of two parts: (1) a RANS-based, three-dimensional, double-precision periodic model (P3DDPM) that uses the realizable k–ε turbulence model, and (2) a large eddy simulation (LES) model that simulates blade segments to capture airfoil surface pressure fluctuations. The turbulence quantities and pressure fluctuations are then post-processed using an internal acoustic prediction tool to predict the total noise emissions of the fans. Experimental tests are conducted in a 1.542 m diameter ISO 5801, Type A fan test facility at Stellenbosch University. The microphone is placed two fan diameters downstream from the fan in the free-field at a 45^o  angle from the shaft. The measured signal is corrected post-testing for acoustic reflections from the facility.

 A vortex trajectory comparison method, together with the internal acoustic prediction tool and experimental measurements, establishes a correlation between the vortex trajectory and the acoustic emissions of large-diameter axial flow fans. The vortex trajectory comparison method is applied to identify a trailing-edge appendage that can manipulate the vortex trajectory of the fan. The identified TE-appendage increased the baseline fan’s total-to-static pressure rise by 30 Pa, kept the total-to-static efficiency constant, and reduced total sound pressure level (SPL) by 4.55 dB and 5.13 db(A), at a flow rate of 12.9 m³/s. To match the pressure rise of the baseline fan, the tip speed of the modified fan (M2-fan) is reduced by reducing its rotational speed. A reduction in total-SPL of 7.07 dB and 7.31 dB(A), and a loss of 1.1% efficiency points, is obtained through this method. The pressure rise of the baseline fan is replicated by lowering the M2-fan’s blade angle by 1^o,  yielding a reduction in total-SPL of 5.93 dB and 6.29 dB(A) while only losing 1.46% total-to-static efficiency points.

References

[1] C. du Plessis, “Scaling of Axial Fan Noise,” Department of Mechanical and Mechatronic Engineering, Stellenbosch University, Stellenbosch, 2022.

[2] M. B. Wilkinson, S. J. van der Spuy and T. W. von Backström, The design of an axial flow fan for air-cooled heat exchanger applications., Stellenbosch: M.Eng Thesis, Department of Mechanical and Mechatronic Engineering, Stellenbosch University, 2017.

[3] P. C. Swanepoel, T. M. Biedermann and S. J. van der Spuy, “Experimental noise reduction (aeroacoustical enhancement) of a large diameter axial flow cooling fan through a reduction in blade tip clearance,” International Journal of Aeroacoustics, pp. Volume 22, Issue 3-4, 8 June 2023.

Presenting Author: Sybrand J. Van Der Spuy Stellenbosch University

Presenting Author Biography: Associate Professor and Chairman in the Department of Mechanical and Mechatronic Engineering, Stellenbosch University. He holds a PhD in Mechanical Engineering from the same university. He specializes in the design and development of turbomachinery, specifically large-diameter axial flow fans and micro-scale centrifugal compressors. He is registered as a professional engineer with the Engineering Council of South Africa. Above all, he has a passion to work with students and is a man of integrity and conviction.

Authors:

Pieter Swanepoel Department of Mechanical and Mechatronic Engineering, Stellenbosch University
Sybrand J. Van Der Spuy Stellenbosch University
Hanno c.r. Reuter Stellenbosch University