Effects of a Realistically Rough Surface on Vane Heat Transfer Including the Influence of Turbulence Condition and Reynolds Number

Heat transfer distributions are experimentally acquired and reported for a vane with both a smooth and a realistically rough surface. Surface heat transfer is investigated over a range of turbulence levels {low (0.7%), grid (8.5%), aero-combustor, (13.5%), and aero-combustor with decay (9.5%)} and a range of chord Reynolds numbers {Re_C = 500,000, 1,000,000, and 2,000,000}. The realistically rough surface distribution was generated by Brigham Young University’s accelerated deposition facility. The surface is intended to represent a TBC surface which has accumulated 7500 hours of operation with particulate deposition due to a mainstream concentration of 0.02 ppmw. The realistically rough surface was scaled by 11 times for consistency with the vane geometry and cast using a high thermal conductivity epoxy (k = 2.1 W/m/K) to comply with the vane geometry. The surface was applied over the foil heater covering the vane pressure surface and about 10% of the suction surface. The 958 by 573 roughness array generated by Brigham Young on a 9.5 by 5.7 mm region was averaged to a 320 by 191 array for fabrication. The calculated surface roughness parameters of this scaled and averaged array included the maximum roughness, Rt = 1.99 mm, the average roughness, Ra = 0.25 mm and the average forward facing angle, α_f = 3.974°. The peak to valley roughness, Rz, was determined to be 0.784 mm. The sand grain roughness of the surface (k_s = 0.466 mm) was estimated using a correlation offered by Bons [1]. Based on estimates of skin friction coefficient using a turbulence correlation with the vane chord Reynolds numbers representative values for the surface’s roughness Reynolds number are 23, 43, and 80 for the three exit condition Reynolds numbers tested. Smooth vane heat transfer distributions exhibited significant laminar region augmentation with the elevated turbulence levels. Turbulence also caused early transition on the pressure surface for the higher Reynolds numbers. The rough surface had no significant affect on heat transfer in the laminar regions but did cause early transition on the pressure surface in every case.