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Title: The effect of surface roughness and free stream turbulence on the flow and heat transfer around a circular cylinder
Author: Al-Rubaiy, Ahmed Abed Ali Ghaidan
ISNI:       0000 0004 7231 771X
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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Given the significance of numerous practical applications in thermo-fluid mechanics for circular cylinders in cross-flow and the lack of research on the effect of surface's roughness and free-stream turbulence on heat transfer from cylinders in cross-flow, a study on these effects is necessary. This work aims to gain a deeper understanding of the individual and common effects of roughness and turbulence intensity on the boundary layers around the cylinder and their effects on local and total heat transfer enhancement. For that purpose, wind-tunnel experiments were used with four levels of surface's roughness (0 < ε/D ≤ 7.25x10⁻³) and different levels of turbulence intensity (2.2%-9.7%). The Reynolds number was varied at a range of 16000 6 Re 6 87000. The test cylinder had an outer diameter of 50 mm and a length of 100 mm, producing an aspect ratio (L=D = 2) in the cross-flow direction and a blockage ratio (D=B = 0:4). A hot wire anemometer system was used to perform the velocity field measurements and to obtain the mean and fluctuating velocity information. A micro-foil heat flux sensor was used to measure local heat flux. The study also extended to creating a surface temperature map using an IR thermal camera. From the results, it has been found that the boundary layers around the cylinder are most strongly influenced by free-stream turbulence and surface roughness which play an essential role in enhancing thermal performance by increasing turbulence in the boundary layer. The existence of these two factors augmented the heat transfer, but the surface roughness had a greater influence than the free stream turbulence. The value of the critical Reynolds number was also affected where it decreased according to surface roughness and/or free stream turbulence values. Combining the effects of roughness and turbulence intensity led to multiplying the heat transfer; an increase of turbulence intensity from 2.2% to 9:7% resulted in an 80% increase in the mean heat transfer at Re= 7.5x10⁴ at the high surface roughness (ε/D = 7.25x10⁻³).
Supervisor: Woolley, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available