Title

An investigation of the fluid-dynamics aspect of the fan and HRM (High Resistance Medium) interaction

Date of Award

2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Thong Q. Dang

Keywords

Fan, High-resistance medium, Blades, Heat exchangers, Filters

Subject Categories

Engineering | Mechanical Engineering

Abstract

An investigation of the fluid-dynamic interaction between a closely coupled fan and High Resistance Medium (HRM), e.g. heat exchangers or filters, is presented. The current study uses simplified 2D airfoil/HRM and cascaded-blade/HRM models. Using both CFD (Computational Fluid Dynamics) and experimental works, the airfoil/HRM model reveals that the presence of the HRM upstream (draw-through system) or downstream (blow-through system) of the airfoil significantly delays stall and drastically enhances the lift capacity of the airfoil, although the flow-interaction mechanisms between these two configurations are different. For flow-resistance magnitude typical of heat exchangers and filters (K-loss factor of 10 or higher), it was found that the extent of fan/HRM flow-interaction is controlled primarily by the gap distance between the fan and the HRM.

Next, a CFD study was carried out for the cascaded-blade/HRM model to study the influence of blade solidity and stagger-angle on blade performance. Similar to the airfoil/HRM study, blade performance enhancements in terms of stall delay and increased flow-turning capacity was also observed, although a large portion of the increase goes into dynamic pressure rather than the useful static pressure. The study indicates that conventional design criteria such as Howell's nominal flow turning, Lieblien's diffusion factor limit, and deviation-angle rules are much too conservative for highly-coupled fan/HRM configuration. One important finding of the investigation is that, in the blow-through configuration where the gap distance between the fan and the HRM is very small (5% chord or less), there is no sign of "conventional" blade stall over a very large range of flow-incidence. Moreover, at high flow-incidence, a "virtual" re-cambering of the blade is created by viscous effect, resulting in flow-deviation angle less than zero (i.e. flow turning is higher than the ideal case of inviscid-flow and infinite number of blades!). As the blade stagger angle and/or the blade solidity are increased, the study shows that blade performance enhancements are reduced. In the draw-through arrangement, the drop in overall blade performance enhancement is not only due to reduction in turning-capacity improvement, but also due to a presence of a more non-uniform flow field in the core region, which results in higher dynamic pressure and hence lower static pressure rise.

Based on the above findings, several practical design arrangements are presented for closely coupled fan/HRM systems. For problems where static pressure rise is most critical, the current investigation recommends placing HRM's both upstream and downstream of the blade in order to take maximum advantage of the beneficial flow-interaction effects. The study also recommends using de-swirling devices to convert the excessive amount of dynamic pressure associated with the larger swirl velocity produced by the highly-loaded blades to useful static pressure. Finally, fans designed for highly coupled fan/HRM system should employ new design criteria that are more aggressive than current correlations such as Howell's nominal deflection curve and Lieblien's diffusion factor limit. For HRM with large K-loss factor (10 or higher), the new design rules should incorporate the dependency of the blade-loading limit on the gap distance between the fan and the HRM.

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