Heat Transfer Augmentation Surfaces Using Modified Dimples/Protrusions

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Virginia Tech

This work presents direct and large eddy simulations of a wide range of heat augmentation surfaces roughened by modified dimples/protrusions. The dissertation is composed of two main parts: Part I (Chapters 2-4) for compact heat exchangers and Part II (Chapter 5) for internal cooling of rotating turbine blades. Part I consists of three phases: Phase I (Chapter 2) investigates flow structure and heat transfer distribution in a channel with dimples/protrusions; Phase II (Chapter 3) studies the application of dimples as surface roughness on plain fins; and Phase III (Chapter 4) considers a new fin shape, the split-dimple fin, that is based on modifying the conventional dimple shape.

Chapter 2 presents direct and large eddy simulations conducted of a fin bank over a wide range of Reynolds numbers, ReH=200-15,000, covering the laminar to fully turbulent flow regimes and using two channel height geometries. While the smaller fin pitch channel has better performance in the low to medium Reynolds number range, both channel heights show similar trends in the fully turbulent regime. Moreover, analysis of the results shows that vortices generated in the dimple cavity and at the dimple rim contribute substantially to heat transfer from the dimpled surface, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side.

Chapter 3 considers applying dimples as surface roughness on plain fin surfaces to further enhance heat transfer from the fin. Three fin geometries that consider dimple imprint diameter effect and perforation effect are considered. The dimple imprint diameter has a minimal effect on the flow and heat transfer of the fin. However, the introduction of perforation in the dimple significantly changes the flow structure and heat transfer on the dimple side of the fin by eliminating recirculation regions in the dimple and generating higher intensity vortical structures.

Chapter 4 presents a novel fin shape, the split-dimple fin, which consists of half a dimple and half a protrusion with an opening between them. The split dimple provides an additional mechanism for augmenting heat transfer by perturbing continuous boundary layer formation on the fin surface and generating energetic shear layers. While the protruding geometry of the split dimple augments heat transfer profoundly, it also increase pressure drop. The split dimple fin results in heat conductance that is 60–175% higher than a plain fin, but at a cost of 4–8 times the frictional losses.

Chapter 5 studies the employment of dimples/protrusions on opposite sides for internal cooling of rotating turbine blades. Two geometries with two dimple/protrusion depths are investigated over a wide range of rotation numbers, Rob=-0.77 to 1.10. Results show that the dimple side is more sensitive to the destabilizing forces on the trailing surface, while both react similarly to the stabilizing effect on the leading side. It is concluded that placing the protrusion on the trailing side for low rotation number, |Rob|<0.2, provides better performance, while it is more beneficial to place the dimple side on the trailing side for higher rotation numbers.

Turbine Cooling, Compact Heat Exchangers, Dimples, Fins, Large Eddy Simulation