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

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 duct 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. 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) were also deduced.; Turbulent flow characteristics of the rib-roughened triangular duct were visualized using Particle Image Velocimetry (PIV) technique. 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.; Numerically, there were three configurations considered: (1) a two-dimensional flow in a channel formed by two parallel plates with ribbed bottom surface; (2) a threedimensional triangular duct with smooth internal surfaces; (3) a three-dimensional triangular duct with internal ribbed surfaces. 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 - epsilon 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 - epsilon 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.