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

Abstract

The surface roughness over a serviced turbine airfoil is usually multiscaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low free-stream 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% to 6.0%. 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 to 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%-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 free-stream turbulence 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.

The Influence of Thermal Barrier Coating Surface Roughness on Spark-Ignition Engine Performance and Emissions

“…The introduction of free-stream turbulence over a rough surface resulted in even greater increases in heat transfer (~100%). Similar results were seen by Wang and Rice [94], Turner et al [65] and Bons [59] when increasing turbulence intensity over a rough surface. These studies also showed that the effects of surface roughness and turbulence intensity on heat transfer seemed to be roughly cumulative.…”

Modification of the Algebraic Transition Model for Wall Roughness Effect Including a Rough Strip

“…from available experiments as a function of wall roughness expressed by the ratio k s / at the rough strip end. This difference represents the correction of the criterion for the transition onset.The estimation of this relation was derived using experimental data of Pinson and Wang[21] and Wang and Rice[22] related to the transition on the flat plate with smooth and/or rough walls. Further results of the investigation of flow over the airfoil NACA0012 with a rough strip downstream of the leading edge according to Kerho and Bragg[23] were used.…”