The Design of an axial flow fan for air-cooled heat exchanger applications.

[摘要] ENGLISH ABSTRACT: Large air-cooled heat exchangers are used to cool process fluids in thermodynamiccycles in a range of industrial processes. These systems consume less water than wet-cooled systems and thus have major environmental and economic benefits. Axial flow fans are used to force air through the heat exchanger bundles of mechanical draft air-cooled heat exchangers, allowing heat to be transferred from the process fluid to the ambient air. Distorted inflow conditions can occur in large air-cooled heat exchangers reducing fan performance and therefore heat transfer. Recent trends of using high volume flow rate, lowpressure rise fans in these applications have led to a requirement for a fan designed for the specific conditions encountered in these systems.An axial flow fan for a forced draft air-cooled heat exchanger is designed with the aim of attaining as high as possible total-to-static efficiency at a given operating point. The design procedure is carried out by means of an algorithm which optimises both the flow distribution through the fan, as well as the blading for maximum total-to-static efficiency. Aerofoil selection and its effect on total-to-static effociency is also taken into account using XFOIL, a program which provides a range of aerofoil design and polar prediction tools.The procedure results in the M-fan. The performance of the M-fan is assessed using computational fluid dynamics (CFD) by means of a periodic threedimensional numerical model (P3DM). An actuator disk model (ADM) is also developed with the aim of providing a simplified numerical model for future use, as well as to assess to possibility of using the ADM as a design tool.Results obtained from the P3DM and ADM indicate that the design assumptions made are reasonable and that the fan meets the design requirementsunder ideal conditions. The P3DM predicts a fan total-to-static efficiency of 59:4% at the design point, with the ADM predicting 63:1%.Further numerical results obtained using the P3DM indicate that the introduction of a tip gap between the M-fan blade tip and shroud has a detrimentaleffect on fan performance. This results in the M-fan underperforming when operating with the recommended tip gap and a 7:9% drop in total-to-static efficiency. These results also show that the introduction of a tip gap alters the flow field in the vicinity of the entire blade span, not just the tip region, with implications for blade loading and specific work distributions. Reductionof the tip gap is shown to significantly improve fan performance. Further it is also noted that the aerofoil optimisation process may have resulted in pooraerofoil geometry at the blade root.Based on these results it is recommended that the M-fan is operated with the minimum tip gap reasonably possible. It is also recommended that the blade setting angle be adjusted to ensure the total-to-static pressure specification is met. The results of this study also indicate that further work should be undertaken to improve the aerofoil optimisation process and account for tipgap related effects in the design procedure.

[发布机构] Stellenbosch University