Effect of Elevated Free-Stream Turbulence on Transitional Heat Transfer Over Dual-Scaled Rough Surfaces

Abstract

The surface roughness over a serviced turbine airfoil is usually multi-scaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low freestream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2–6.0 percent. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 ∼ 40 (Ra = 37 ∼ 119 μm). Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30 ∼ 40 percent over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low FSTI cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

Effects of Surface-Roughness Geometry on Separation-Bubble Transition

“…Modification of the blade-surface boundary layer by surface roughness has been shown to reduce turbine aerodynamic efficiency ͑e.g., Kind et al;Boynton et al;, and increase the surface heat-transfer rate ͑e.g., Bons and McClain;Blair,Pinson,and Wang;Wang and Rice ͓2,[8][9][10]. Blade-surface heat transfer rates may also be affected by changes in the surface material properties when the roughness is due to hydrocarbon deposits, or when a protective coating is eroded.…”

Boundary layer transition in attached and separated flows at low Reynolds numbers

“…The combined influence of surface roughness and free-stream turbulence has been studied recently by Wang & Rice (2003). This study demonstrated that the process of attached flow boundary layer transition remains sensitive to surface roughness at elevated levels of free-stream turbulence, although no models to predict these effects were proposed.…”

“…The more recent studies of Addison & Hodson (1990-a,-b) and Hodson (1991) have documented the time-averaged and ensemble-averaged shear stresses on both the suction and pressure surfaces of turbine blades. Schobeiri & Radke (1994) Senyitko, 2003), and also potentially in thermal damage to the blades due to changes in the surface heat transfer rates (Bons & McClain, 2003;Blair, 1994;Pinson & Wang, 1997;Wang & Rice, 2003). Surface heat transfer rates may also be affected by changes in the surface material properties when the roughness is due to hydrocarbon deposits, or when a protective coating is eroded.…”

On Turbulence and its Effects on Aerodynamics of Flow through Turbine Stages

“…The roughness over streamed surfaces is nonuniform and characterized by significant variations in streamwise and spanwise directions [73,74]. Modifications in the behavior of the boundary layer due to the presence of definite values of surface roughness can decrease the aerodynamic efficiency [75][76][77] and can also increase the heat transfer [78][79][80]). Levels of heat transfer may also be affected by changes in material properties or in the case of eroded or broken protective coatings.…”