Investigation of the tip clearance flow in a compressor cascade using a novel laser measurement technique with high temporal resolution
Andreas Fischer, Lars Büttner, Jürgen Czarske, Marcel Gottschall, Ronald Mailach, Konrad Vogeler
abstract:
A primary goal of the development of jet engines is to increase the pressure ratio and the efficiency, while reducing the number of blades and stages. It is one of the design challenges to include beneficial unsteady flow effects and to account for secondary flows to improve the engine parameters. This requires a detailed physical understanding of the time-resolved 3D flow pattern and the resulting effects on performance and aerodynamic stability. In compressors the tip leakage flow is of crucial importance for loss production and flow stability. CFD-simulations are not able to capture all details of the complex flow field in the endwall region with sufficient accuracy yet. Therefore, to enhance the physical understanding, measurements in the blade tip region and within the tip clearance are necessary. Especially the latter is challenging due to the small tip gap dimensions.
The present work describes the non-intrusive measurement of the tip clearance flow in a linear compressor cascade providing planar velocity fields of all three velocity components. This was accomplished with a novel technique denoted as FM-DGV (Doppler global velocimetry with laser frequency modulation) applied to this task for the first time. Since FM-DGV is capable of measuring all components with high measurement rates up to 100 kHz, not only the mean velocity field, but also flow turbulence characteristics such as velocity standard deviation, velocity spectra and spatial velocity correlations can be evaluated. Its principle is based on measuring the Doppler frequency shift of scattered light by using a molecular absorption cell and a fast detector array. In comparison with conventional DGV, the frequency modulation allows to omit the additional reference camera, so that image misalignment errors do not occur.
The focal point of this paper composes the application in a tip gap flow investigation in the compressor cascade. Due to three illumination directions, all three velocity components were acquired in a plane perpendicular to the blade axis at middle clearance height. Measurements with a hot-wire and a 5-hole-probe behind the cascade supplement the dataset. The mean velocity field illustrates the structure of the tip clearance vortex. The velocity standard deviations show higher turbulence intensities in the roll-off section of the vortex. The observed unsteady oscillations were discussed with the imitation of a mainly stochastic flow behavior in the clearance region revealed by the analyzed velocity spectra. Hence, the measurements to be presented will open promising perspectives of the FM-DGV technique for the turbulent flow analysis in turbomachines.
reference:
Fischer, A., Büttner, L., Czarske, J., Gottschall, M., Mailach, R., and Vogeler, K., 2011:
"Investigation of the tip clearance flow in a compressor cascade using a novel laser measurement technique with high temporal resolution"
Paper No. GT2011-45176, ASME Turbo Expo 2011, Vancouver, Canada, 2011
and Journal of Turbomachinery 134:051004, 2012 [DOI: 10.1115/1.4004754].