Date of Award

Summer 8-27-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Dang, Thong

Keywords

Close-coupling interaction, Heat-exchanger, Turbomachinery

Subject Categories

Aerospace Engineering | Engineering | Mechanical Engineering

Abstract

This research investigates the fluid-dynamic interaction between close-coupled axial turbomachines and surrounding components, e.g., heat-exchangers, filters, or cooling fins in high-density computer servers, and its effects on turbomachine and system performance. The importance of this research is that the flow interaction effect occurs in every scenario that requires system compactness.

Firstly, a combined experimental and 3-D computational study on an off-the-shelf server cooling fan is performed, showing that the fan performance has little changed. However, the system performance can be very different since the server cooling has a large hub-to-tip radius ratio, which effectively blocks the high resistance media's (HRM) flow area. The studies also suggest that the interaction effect worsens performance at the system level when the HRM is placed downstream of the fan.

Next, 2-D studies on cascade-HRM interaction are carried out. A major finding is that conventional off-the-shelf fans are not affected significantly by the close-coupling between fan and HRM; the effect benefits only fan designs with a high diffusion factor (DF). In particular, the benefits can only be harvested when the fan's aerodynamic loading is very high (or high DF), and the fan blades stall when the fan operates in the fan alone mode. In this case, when a high DF fan is designed with the HRM close-coupling effect, the resulting fan design can operate efficiently (i.e., flow separation is removed) at a high-pressure coefficient.

Then, parametric studies on high DF cascade blade and HRM flow interaction are performed, including the sensitivity of fan-HRM separation gaps, HRM flow resistance K factor, and HRM flow resistance homogeneity for both draw-through and blow-through configurations. We find that the interaction effect significantly increases the fan static pressure rise and fan total pressure rise. Regarding energy usage, the fan's total efficiency benefits from the interaction, while static efficiency stays no change because most of the benefit goes to swirl dynamic pressure. The HRM resistance homogeneity study suggests that the fan and system performance can be very different when a fan rotates close to a conventional non-axisymmetric heat-exchanger configuration.

Also, a parametric study on high DF stator and HRM flow interaction is performed, including sensitivity to stator-HRM separation gaps, HRM flow resistance K factor, HRM flow resistance homogeneity. Finally, evaluation of stator blade loading types (leading-edge design and trailing-edge design) are carried out.

After investigating the close-coupling effect on rotors and stators, close-coupled fan stages (rotor + stator) are studied on fan performance and system performance. The stator blade row is introduced to convert the swirl dynamic pressure to useful static pressure. Taking advantage of the significant swirl production from the high DF rotor, the stage static efficiency can reach 80\%.

Finally, a new heat-exchanger design is evaluated. We investigate the feasibility of using curved-fin to replace the conventional straight-fin design. We find that the system performance can be significantly improved since a curved-fin heat-exchanger is capable of converting swirl dynamic pressure to static pressure. The overall pressure drop in the HRM is reduced.

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Open Access

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