Award Date

12-15-2019

Degree Type

Thesis

Degree Name

Master of Science in Engineering (MSE)

Department

Mechanical Engineering

First Committee Member

Samir Moujaes

Second Committee Member

Hui Zhao

Third Committee Member

Mohamed Trabia

Fourth Committee Member

Samaan Ladkany

Fifth Committee Member

Kathryn Hausbeck Korgan

Number of Pages

101

Abstract

This numerical study investigates the thermal performance of internally grooved tubes used in heat exchangers. In order to enhance the performance of heat exchangers, turbulence promoters are inserted along the streamwise flow. The use of inserts, grooves, and augmentations along the axial length of the tube creates disturbances that improve the overall heat transfer due to enhanced fluid mixing and greater surface contact boundary walls. However, the implementation of tube wall augmentations creates an increase in pressure drop across the tube which must be compensated for with additional pumping power for the working fluid. Therefore, in order to analyze the heat enhancement benefits with respect to axial pressure drop, a CFD study was conducted.

The CFD study investigated three different groove geometries: circular, rectangular and trapezoidal grooves and these results were compared to a smooth tube. An analysis for Reynolds numbers between 10,000 to 50,000 were performed to model forced convection heat transfer in the turbulent region. The results obtained from this CFD study were compared with experimental work completed by Bilen et al. (2009). Bilen et al. (2009) conducted an experimental study for four different Reynolds numbers: 10,000, 22,000, 34,000 and 38,000. This CFD work analyzed three additional Reynolds numbers of: 42,000, 46,000 and 50,000.

For increasing Reynolds numbers in the turbulence region, the Nusselt number significantly increased. The Nusselt number for the grooved geometries was significantly greater than that of the smooth tube. The augmented grooves resulted in an enhancement of the heat transfer coefficient for the circular, rectangular and trapezoidal grooves of 64%, 53% and 61%, respectively. These results are consistent with the experimentally obtained results for the circular, rectangular and trapezoidal grooves of 63%, 47% and 58%; when compared to the smooth tube. It was observed that overall thermal enhancement was greatest for the augmented grooves in the Reynolds number range between 15,000 and 20,000. The friction factor for each grooved tube increased with higher Reynolds numbers, while the smooth tube showed a reduction in friction factor for the range 10,000 < Re < 38,000. All three grooves had greater axial pressure drop than the compared smooth tube. The rectangular groove exhibited the greatest axial pressure drop, while the circular and trapezoidal grooves were similar in their results. The heat enhancement factors for the circular, rectangular and trapezoidal grooves were determined to be: 1.24, 1.10 and 1.17, respectively. Therefore, all three grooved geometries proved to be thermodynamically advantageous when compared to the smooth tube. Fully developed turbulent flow was determined to be at x/D ≈ 5. This is comparable to the experimental results in which the fully developed flow was achieved at an axial distance of x/D = 4.6. Therefore, all three grooved geometries proved to be thermodynamically advantageous when compared to the smooth tube. From the obtained results, it can be stated that the CFD models are valid when compared to the experimental results.

Keywords

Grooved tubes; Heat transfer enhancement; Nusselt number; Reynolds number; Thermal enhancement factor; Turbulent kinetic energy

Disciplines

Mechanical Engineering | Thermodynamics

File Format

pdf

File Size

8.0 MB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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