Design of a Highly Loaded Turbine Exit Case Airfoil With Active Flow Control

Abstract

In this paper a design process of a highly loaded profile for a turbine exit case (TEC) application is described. The profile has an increased pitch to chord ratio which is approximately 50% higher compared to conventional airfoils. For the design of the airfoil a two-dimensional (2D) computational fluid dynamics (CFD) prediction method was used in addition to in-house design rules and low Reynolds number experience from previous experiments. Furthermore, common knowledge from turbine and compressor design as well as turbine exit guide vane studies was evaluated and taken as basis for the new design. To verify the highly loaded design, the profile was tested over a wide Reynolds number range in the high speed cascade wind tunnel of the Institute of Jet Propulsion (ISA) at the University of the German armed forces in Munich. The experiments showed a very good agreement between the CFD predictions and the measurements for high Reynolds numbers. In the low Reynolds number regime the tendency to massive flow separation was slightly underestimated by the CFD predictions. It is particularly challenging as the CFD predictions still have problems to calculate open separation bubbles. Active flow control (AFC) by fluidic oscillators was also part of the design process and successfully applied on the profile.

Active Boundary Layer Control on a Highly Loaded Turbine Exit Case Profile

“…Interaction between the fluidic oscillators and the static pressure taps acting as resonant devices as for example shown by Yang and Spedding [13] can be neglected. On one hand, the measured data agrees very well with numerical CFD data shown in Kurz et al [2]. On the other hand, the tube length between the taps and the pressure transducers indicates resonant frequencies that are far below the operating frequencies of the fluidic oscillators.…”

“…The chord length is l = 67.2 mm which results together with the pitch of t = 97.0 mm in a rather big pitch to chord ratio of t/l = 1.44. More details on the cascade geometry, the operating conditions and the design intent can be found in Table 1 and in Kurz et al [2].…”

“…In total there are 26 outlet holes of the fluidic oscillators with a diameter of d AFC = 0.7 mm and a spanwise distance of 5.6 mm on the suction side of each profile covering approximately 88% of the blade height. Detailed information on the integration and positioning of the actuators into the profile was reported by Kurz et al [2]. Fluidic oscillators have a manufacturing advantage as they do not depend on moving or electrical parts to produce pulsed outflow.…”

“…The design process of such a highly loaded turbine exit case profile with active boundary layer control was presented by Kurz et al [2]. The paper describes in detail the profile pressure distribution which was chosen for the highly loaded TEC profile according to computational fluid dynamics (CFD) predictions with the numerical code MISES.…”

High Frequency Boundary Layer Actuation by Fluidic Oscillators at High Speed Test Conditions

“…To improve the efficiency, the latest investigations at the Institute of Jet Propulsion (ISA) prove for an aerodynamically highly loaded LPT exit casing profile that the mass flow investment of the oscillator can be decreased as low aṡ m osc.,T EC = 2.1 • 10 −4 •ṁ pas.,T EC . The key for this achievement is to consider the AFC in the design process with an optimized position and the right trigger frequency [10][11][12].…”