Forced convection and fluid friction in a horizontal triangular duct with uniformly ribbed or grooved internal surfaces

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Forced convection and fluid friction in a horizontal triangular duct with uniformly ribbed or grooved internal surfaces

 

Author: Luo, Dandan
Title: Forced convection and fluid friction in a horizontal triangular duct with uniformly ribbed or grooved internal surfaces
Degree: Ph.D.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations.
Fluid dynamics.
Surfaces (Technology)
Department: Dept. of Mechanical Engineering
Pages: xxv, 216, [17] p. : ill. ; 31 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2093871
URI: http://theses.lib.polyu.edu.hk/handle/200/2636
Abstract: Fully developed turbulent flows in equilateral-triangular ducts with or without internal roughened surfaces were investigated from both experimental and numerical approaches in the present study. Better understanding of the thermal performance as well as flow characteristics of the forced convective turbulent flow was achieved, which is of importance for nowadays engineering applications. The inner surfaces of the triangular ducts were either plane or fabricated with uniformly spaced square-ribs or V-grooves. It was aimed to identify the effects of the duct geometry, fabricated ribs and V-grooves, as well as flow conditions on forced convection and fluid friction of the turbulent flow in the triangular duct. For the present investigation, the rib size to hydraulic diameter ratio (e/D) ranged from 0.11 to 0.21 and the rib-to-rib spacing to rib size ratio (Sr/e) ranged from 3.41 to 13.93. The apex angle (0) of the V-groove varied from 30o to 150o, corresponding to a relative apex angle (I=0/180 * pi) varying from 0.78 to 2.62, and the groove-to-groove spacing to hydraulic diameter ratio (Sv/D) varied from 0.54 to 1.23. The measurements were performed under steady-state turbulent conditions, with hydraulic-diameter-based Reynolds number (Re) ranging from 4,000 to 23,000. Enhancement in heat transfer efficiency of the triangular duct was observed with either ribbed or V-grooved internal surfaces, though the ribbed surface obtained a much better performance than the V-grooved surface. Optimum rib size, rib-to-rib spacing, V-groove apex angle and groove-to-groove spacing corresponding to maximum forced convections were proposed, respectively. Non-dimensional expressions for the determinations of average Nusselt number (Nu) and average friction factor (f) in terms of (Re), (e/D), (I), (Sr/e) and (Sv/D) were also deduced. In addition, the turbulent flow characteristics of the rib-roughened triangular duct were particularly investigated through experimental visualization method by using Particle Image Velocimetry (PIV) technique. The aspect ratio of rib size to hydraulic diameter of the triangular duct was kept constant at e/D = 0.2 at a fixed Reynolds number of Re = 10,800. Development of the secondary flow, detachment and reattachment of the main flow, as well as formation of the vortex around the ribs and in the duct corners were studied, respectively. In the numerical simulation studies, the assumptions of steady-flow of incompressible Newtonian fluid were taken into consideration under turbulent-flow conditions. There were three configurations considered: 1) a two-dimensional flow in a channel formed by two parallel plates with a uniformly heated and ribbed bottom plate; 2) a three-dimensional triangular duct with smooth internal surfaces; 3) a three-dimensional triangular duct with internal ribbed surfaces. A finite volume code, FLUENT 6.0, was applied to perform the calculations. The governing equations (i.e., continuity, momentum and energy) were solved by the pressure correction algorithm SIMPLE. Two semi-empirical turbulence models, namely, the Standard k - e Model and Reynolds Stress Model (RSM), were used for the present simulations. It was found that, in the prediction of a two-dimensional flow, the former model had superiority over the latter one. However, to predict a three-dimensional channel flow, application of the RSM became necessary instead of the Standard k - e Model. Comparisons between the numerical-predicted and experimental-measured turbulent flows in the triangular duct were also conducted. Good agreements were observed from both visual and metrical aspects. Moreover, it was proposed that a suitable two-dimensional numerical model could be applied more effectively instead of the complicated and expensive three-dimensional model in simulating the turbulent flow characteristics in a triangular duct with ribbed internal surfaces. In conducting the present research project, I have made the following contributions towards this topic area: 1. Provide general summarizations of the thermal performance of the triangular duct with internal uniformly ribbed or grooved surfaces; 2. Explore the flow characteristics of the rib-interrupted turbulent flow in the triangular duct through visualization study; 3. Achieve a better understanding of the heat transfer mechanism of such a dynamic thermal system via numerical simulation approach; 4. Suggest the use of a suitable simple two-dimensional model to tackle a complicated three-dimensional problem; 5. Publish several journal papers and conference proceedings with the findings obtained, which are listed detailedly in Section: PUBLICATIONS ARISING FROM THE THESIS.

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