Thermal-fluid performance degradation of turbulators in additively manufactured turbine cooling

Abstract

Ribs, or turbulators are an effective method for enhancing cooling in internal passages in a wide range of applications like microfluidics, gas turbine engines, and nuclear fusion reactors, as they can typically double the heat transfer through the generation of secondary vortices and zones of high heat transfer coefficient. However, in future additive manufactured high-temperature turbine parts for aviation gas turbine engines, the sub-mm scale ribs within the cooling passage can sometimes be manufactured with geometry differences compared to the design intent. Here, we investigated the impact on thermal performance of potential turbulator manufacturing variation due to shrinkage of the rib center region and broadening of the rib side. Local heat transfer coefficients on all surfaces, including design-deviated curved ribs, were experimentally measured in a large-scale rig by applying two transient methods using liquid crystal thermography and high-conductive material ribs. Pressure measurements were used to evaluate the friction factor and to validate the numerical simulations. 5 cases, including the design intent case of 45° angled round-edged rib arranged in a staggered configuration, were tested under engine-representative high Reynolds number in the range from 60,000 to 155,000. The numerical simulations provided an understanding of the flow patterns around the ribs, enhancing insight into the flow mechanisms caused by the shape deviations. Depending on the extent of the shrinkage, the local vortex structure in the inter-rib region changed, resulting in complex heat transfer characteristics that decreased or increased. Broadening of the rib caused reduced heat transfer and increased friction factor. With the combination of shrinkage and broadening, heat transfer on the ribbed wall was reduced by up to 29 %, and the thermal performance factor was reduced by up to 17 %. The largest reduction in heat transfer caused by the potential manufacturing variations occurred in the rib itself. The work has quantified the degradation of thermal performance caused by potential turbulator manufacturing variability away from design intent geometry, and provides insight into the relationship between thermal-fluid performance and deviations from the ideal geometry caused by manufacture.