Experimental setup for detailed secondary flow investigation by two-dimensional measurement of total pressure loss coefficients in compressor cascades

The department of Engine Acoustics of the German Aerospace Center in Berlin operates an especially designed high-speed subsonic cascade wind tunnel where secondary flow mechanisms in axial compressor flow are evaluated integrally. Instead of traversing a single 5-hole probe through the passage, a 26 + 5 probe wake rake is used in order to minimise the measurement duration. Easy access to the test section and cascade blades allows surface flow visualization. The long-term ambition of the experimental setup is to reduce total pressure losses resulting from corner separation. Passive and active flow control such as vortex generators or sidewall boundary layer aspiration is used. The test setup allows detailed investigation of the flow control effects. Total pressure loss coefficients are measured, the post-processing program is thereby able to rate active and passive secondary flow control mechanisms immediately. The gained data is recorded in a measurement computer. Due to a high degree of automation of the measurement string, the measuring period can be shortened to less than 10 minutes. NOMENCLATURE SYMBOLS SUBSCRIPTS c [mm] chord h [mm] height p [Pa] pressure u [m] peripheral coordinate t [mm] blade pitch w [m/s] absolute speed z [m] blade height coordinate Re [-] Reynolds number Ma [-] Mach number β [°] Flow Angle ρ [kg/m3] density ζ [-] total pressure loss coefficient 1 conditions at inflow 2 conditions at outflow g global m mass-flow averaged P Profile/ Airfoil W Wall BL Boundary Layer S Secondary Flow INTRODUCTION Sidewall effects are responsible for a substantial amount of the total compressor losses of modern aero engines or stationary gas turbines. Current compressor blades with an aspect ratio of about c/h = 1 are characterized by a loss breakdown of 1/3 airfoil loss and 2/3 sidewall and secondary flow loss [1]. In order to minimize the losses produced by a blade stage, further understanding of secondary flow phenomena is essential. The main effort is to understand and reduce the secondary flow losses in a compressor stage. The test setup provides all kinds of passive and active flow control such as suction or blowing in the corner region. A fast mounting time ensures the possibility to regard the cascade with different settings and inflow conditions. TEST SETUP In order to investigate secondary losses in compressor cascades, a special experimental facility has been developed [2]. The high-speed subsonic wind tunnel allows Mach numbers of up to 0.7 and Reynolds numbers of up to 0.65×10 (formed with a 40mm chord length). A 65kW electric engine drives the radial-blow fan. The experimental setup provides a settling chamber with flow speeds of less than w = 1m/s. Here total pressure and total temperature are measured with only minor deviations. The flow accelerates to its final Mach number in the 1 Rhode-St-Genèse, Belgium April 7-8, 2008 XIX Biannual Symposium on Measuring Techniques in Turbomachinery Transonic and Supersonic Flow in Cascades and Turbomachines Rhode-St-Genèse, Belgium nozzle with a contraction ratio of 1:218. The adjacent rectangular test section with a cross section area of 40×82mm carries cascades with five stator blades. The test facility is shown in Fig. 1. Fig. 1: Test Setup BOUNDARY LAYER ASPIRATION The rectangular test section provides boundary layer aspiration of all four tunnel side walls. By monitoring the static pressures along the cascade side walls with water columns and an independently adjustable aspiration rate of the upper and lower boundary layers, periodicity of the flow according to an “infinite blade cascade” is ensured. The sidewall boundary layer in the cascade inlet can be varied between 3 and 10mm. The length of the extendible inlet section varies boundary layer thickness between 4 and 10mm, by aspiration the thickness can be reduced by one mm. The boundary layer parameters are measured in a boundary layer rake upstream of the cascade. CASCADE DESIGN The cascades consist of 5 blades which are fixed at the sidewalls (Fig.2). Stagger angle correction is no longer required, as most airfoils are optimized for a fixed stagger angle. In order to alter the inlet angle β1, the whole passage can be rotated over a wide range of angles. Determination of full working ranges of the compressor cascade is possible. Although the flow is regarded as infinite blade row flow, only the middle blade is investigated. Hereby the influence of the upper and lower channel side wall is minimised. Active means of flow control can be implemented directly in the sidewalls. Blowing and suction slots and pressure connections can be integrated into the walls. Fig. 2: Cascade with wake rake DATA ACQUISITION Most common for axial compressor cascade wind tunnels is the method of traversing a 5-hole probe through a passage. This method takes a long time as every measurement point is measured alone. In the method used here 26 points are measured simultaneously with one rake. The measurement plane is located 16mm (40%