The Effect of Thermal Barrier Coating Roughness Magnitude on Heat Transfer With and Without Flowpath Surface Steps

An experimental study has been performed to investigate the convective heat transfer coefficients resulting from differing magnitudes of surface roughness for ceramic thermal barrier coatings (TBC) of the type used to protect gas turbine hot section components. A flat plate wind tunnel test is employed with zero freestream pressure gradient, and levels of freestream turbulence from 4.5 to 6.5%. The TBC applied in these tests is an air plasma sprayed, yttriastabalized zirconia-oxide. Average test surface Reynolds numbers are varied from 400,000 to 2,200,000. Average roughness values (RA) for the TBC surfaces are varied by degrees of polishing from 10 μm to 1 μm, resulting in roughness Reynolds numbers of up to 70. Baseline comparison tests use smooth metal surfaces. The changing physical and heat transfer character of the TBC surfaces with polishing is demonstrated. Results show a significant effect of polishing degree on the surface heat transfer coefficients. Such surfaces were also tested with varying magnitudes of forward facing flowpath steps to model the potential interruptions seen at component interfaces. Flowpath steps are shown to produce quite elevated levels of heat transfer for the smooth surfaces, with little or no further additional enhancement due to increased TBC roughness levels.