Heat Transfer Augmentation in the Entry Region of Circular Channels
Open Access
- Author:
- Lundburg, Evan
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 12, 2023
- Committee Members:
- Robert Kunz, Professor in Charge/Director of Graduate Studies
David Hall, Outside Unit & Field Member
Tamy Guimaraes, Major Field Member
Karen Thole, Major Field Member
Stephen Lynch, Chair & Dissertation Advisor
M Eric Lyall, Special Member - Keywords:
- Heat Transfer
Pin Fin
Fin
Turbulator
Entry Region
Circular Channel
Pipe
Inlet
Flow-field
Boundary layer development
Distortion
Inlet distortion
Thermal Performance
Total pressure loss
Infrared thermography
Heat transfer augmentation - Abstract:
- Due to the small thickness of the thermal boundary layer, entry region heat transfer in a smooth circular channel is known to be significantly higher than at fully developed conditions. This presents an opportunity to create short length high heat transfer heat exchangers at an inlet of a duct. With the velocity boundary layer being small, any feature disturbing the flow will either trip the boundary layer, cause higher turbulence, or divert flow from the sidewalls into the core flow, all of which will affect downstream velocity distributions and boundary layer development. One such application of this concept is turbomachinery, in which the effect of heat transfer augmentation features on a downstream fan is of particular interest due to the sensitivity of fan performance to the incoming velocity profile. Heat transfer enhancement using small features in the entrance region of a circular channel and the effect on the downstream boundary layer has not been studied at high Reynolds numbers. In addition, the impact of inlet distortions on circular channel entry region heat transfer has also not been investigated. Thus, the following studies analyze the viability of entry region heat transfer augmentation and the impacts it has on the entry region boundary layer development. Three geometries are tested: pins, fins, and turbulators, all of which have been widely studied in rectangular channels, with a few investigated in circular channels at low Reynolds numbers. All augmentation feature arrays are installed in the first diameter of the channel, with the heat transfer augmentation of a constant heat flux boundary condition measured using infrared thermography. The effect of feature spacing and height on developing velocity boundary layers is also investigated using total pressure and static pressure measurements immediately downstream and three diameters downstream of the augmentation features. All features are tested at Reynolds numbers ranging from 1*10^5≤ ReD ≤5*10^5. Overall, each type of geometry is capable of producing augmented heat transfer relative to a smooth entry region with minimal pressure losses by utilizing specific feature height and array spacing configurations. Fin arrays provide the highest performance with lowest boundary layer impacts, but also exhibit a sensitivity to Reynolds number. Distortion effects are also investigated, with the boundary layer impact of augmentation features overshadowing the near wall effects of a distortion screen. Thus, the heat transfer augmentation feature sensitivity to distortion screens is low, indicating that the viability of augmentation features in asymmetric flow is still acceptable.