Riblets for Reducing Turbine-Duct Pressure Loss by Influencing the Boundary Layer

A turbine center frame (TCF) is part of the hot section of a modern High-Bypass Turbofan engine. It is located between the high-pressure and the low-pressure turbine. The TCF guides and decelerates the flow from the HPT to the LPT, typically located at significantly higher radius, at minimum pressure losses. As the flow through the duct is primarily governed by secondary flow structures and unsteady flow effects induced by the HPT, the prediction of the flow phenomena, especially the pressure loss, is a difficult task. To address this challenge, the Institute of Thermal Turbomachinery and Machine Dynamics investigated several TCFs in previous years to shed new light on the mechanisms of loss generation and help designers to develop more efficient TCFs. These tests were carried out in the Transonic Test Turbine Facility (TTTF), reaching engine-relevant flow conditions at the inlet of the TCF.

One of the main goals for in the development of future  aircraft is to lower fuel consumption and increase engine efficiency, which can be achieved by reducing pressure losses in the main engine components. One possibility for decreasing TCF total pressure loss is to reduce the friction losses by applying riblets to the TCF surface. Riblets are surface structures consisting of grooves arranged parallel to each other. Riblets have demonstrated the potential for viscous drag reduction in turbulent boundary layers. This poster presents the aerodynamic effects of riblet foils applied on the struts, as well as on the casing and hub endwalls of the TCF. The tested riblet structure is trapezoid-shaped, with 45 µm tip distance and a height to tip distance ratio of 0.45. This type of riblet design was used in the entire TCF. The structure of the riblets was approximately oriented to the surface streamlines throughout the TCF. Detailed pre-test CFD simulations were carried out to identify and quantify the dominant surface streamline structures.

A large set of measurement data was taken at engine representative flow conditions. To experimentally investigate and quantify the potential of riblets, a back-to-back comparison of the same geometry with and without riblets was performed. Five-hole-probe and hot-wire-probe area traverses were obtained up- and downstream of the TCF. The flow within the TCF was visually examined by oil flow visualizations.