59465 - Optimization of a Tip Appendage for the Control of Tip Leakage Vortices in Axial Flow Fans 

Large air-cooled heat exchangers (ACHEs) find extensive application throughout the power generation industry; specifically in dry, arid regions such as South Africa. This is due to these types of heat exchangers (HEs) offering a significant advantage over competing technologies as they minimize water consumption. ACHEs work by using large industrial scale axial flow fans (some up to 10 m in diameter) to motivate the movement of large volumes of air over a set of HE bundles thereby rejecting heat to the atmosphere. Due to the required size and quantity of these units, these fans require a significant amount of power in order to be effective. This motivates their development as a small improvement in fan performance would scale to a more substantial savings in power consumption at a reduced operational cost.

It is well known within literature that the tip region plays an important role in dictating the performance of axial flow fans. Tip clearance tests conducted by Venter (1992) indicate increased performance characteristics with a reduction in tip clearance. However, a reduced tip clearance has the inherent disadvantage of increased risk of blade strikes which makes maintaining small tip clearances impractical. Research conducted by Corsini et al. (2007) indicates that, through the introduction of a tip end-plate to control the chord-wise tip leakage flow distribution, improved fan performance and noise characteristics can be achieved. However, this design was found only to improve the fan’s performance at below peak efficiency flow rates.

Considering the current investigation, a novel tip end-plate design is introduced with the aim of improving the performance characteristics of the 1.5m MFan. This fan was specifically designed for application in large scale ACHEs, however, was found unable to meet its design specifications. The introduced end-plate design works by controlling the vortex-to-vortex interaction between the blades, thereby improving their loss behaviour and consequently improving fan performance. The numerical optimization of such a design is also considered using the efficient global optimization algorithm of Jones et al. (1998). Finally, two of the optimized end-plate designs are experimentally tested using the standard BS848 type A test facility at Stellenbosch University.

The findings indicate improved fan performance characteristics for both tested end-plate designs when compared to that of the datum fan. The end-plate designs are found to, more specifically, improve the fans performance at higher than peak efficiency flow rates with an improve efficiency plateau towards the rotor choke margin.

References

Corsini, A. and Sheard, A.G. (2007). Tip end-plate concept based on leakage vortex rotation number control. Journal of Computational and Applied Mechanics, vol. 8, no. 1, pp. 21–37.

Jones, D.R., Schonlau, M. and Welch, W.J. (1998). Efficient global optimization

of expensive black-box functions. Journal of Global optimization, vol. 13, no. 4,

pp. 455–492.

Venter, S. and Kröger, D. (1992). The effect of tip clearance on the performance of an axial flow fan. Energy conversion and management, vol. 33, no. 2, pp. 89–97.