St and cf Augmentation for Real Turbine Roughness With Elevated Freestream Turbulence

Abstract

Experimental measurements of skin friction cf and heat transfer (St) augmentation are reported for low speed flow over turbine roughness models. The models were scaled from surface measurements taken on actual, in-service land-based turbine hardware. Model scaling factors ranged from 25 to 63, preserving the roughness height to boundary layer momentum thickness ratio for each case. The roughness models include samples of deposits, TBC spallation, erosion, and pitting. Measurements were made in a zero pressure gradient turbulent boundary layer at two Reynolds numbers (Rex=500,000 and 900,000) and three freestream turbulence levels (Tu=1%, 5%, and 11%). Measurements at low freestream turbulence indicate augmentation factors ranging from 1.1–1.5 for St/Sto and from 1.3–3.0 for cf/cfo(Sto and cfo are smooth plate values). For the range of roughness studied (average roughness height, k, less than 1/3rd the boundary layer thickness) the level of cf augmentation agrees well with accepted equivalent sandgrain ks correlations when ks is determined from a roughness shape/density parameter. This finding is not repeated with heat transfer, in which case the ks-based St correlations overpredict the measurements. Both cf and St correlations severely underpredict the effect of roughness for k+