The Impact of Multiscale Ceramic Matrix Composite Roughness on Heat Transfer and Boundary Layer Behavior

Abstract Ceramic matrix composites (CMCs) can enable more efficient gas turbines relative to traditional nickel alloys, resulting from enabling higher turbine entry temperatures that, in turn, benefit cycle performance. One negative effect of adding CMCs to the hot section is the introduction of a unique surface roughness due to the underlying weave topology. This surface roughness is generally at a macroscale compared with traditional turbine roughness, such as deposits or erosion, which are well known to interact with the boundary layer development and increase convective heat transfer. In this study, scales representative of traditional turbine roughness in combination with macroscale weave roughness are investigated for convective heat transfer augmentation and boundary layer behavior. In addition to investigating the impact of the CMC roughness scales, 5-harness satin and twill weave patterns are studied to understand the differences between weaves. Heat transfer measurements are conducted in scaled-up wind tunnel tests using a conjugate steady-state analysis with freestream turbulence intensities of 0.5% and 24%. Boundary layer behavior is measured using particle image velocimetry to capture cross-stream and streamwise planes. Compared to the 0-deg 5-harness satin surface, the twill surface has a higher Stanton number augmentation, owing to the increased number and high density of flow-facing features that disrupt the boundary layer. Additionally, the large-scale weave roughness and traditional small-scale turbine roughness act largely independent of one another.