This article presents the implementation of a Particle Image Velocimetry (PIV) into the high-speed short-duration rotating turbine facility of the von Karman Institute. The advantage of PIV as a whole field measurement is emphasized in such circumstances for which the use of optical technique can drastically reduce the number of tests and the need for multiple intrusive expensive probes, ultimately lowering the testing cost. Practical solutions were demonstrated that address various challenges for the effective application of PIV. An endoscope delivered the laser sheet to the region of interest and a planoconcave window provided optical access for the measurement in the annular test section. A high-speed laser system and a high-speed camera were synchronized at 1 kHz sampling rate. Complementary measurements and dedicated image processing were performed to ensure the synchronization of the PIV images with the rotor position that was monitored through an encoder. The region of interest was the blade-to-blade plane at the 58% span turbine exit on a rectangular field of view covering approximately one rotor pitch and 0.15 rotor axial chord from the rotor trailing edge. Phase-locked-average velocity fields are obtained from PIV and compared against steady-state Reynolds Averaged Navier Stokes (RANS) simulations along with four-hole probe measurement results. Together with an uncertainty analysis, the results demonstrate the promising robustness and accuracy of PIV. A practical guideline for PIV application in such kind of turbine test rigs is provided as a conclusion of the paper.
The paper addresses the study of the flow established in a HPT with rim seal purge. The test section, operated at engine-relevant flow conditions in the high-speed turbine rig of the von Karman Institute, is instrumented for high-bandwidth aerothermal measurements. The rainbow rotor allows the simultaneous testing of six different sectors, each hosting a specific tip and platform geometry. This paper focuses on the baseline sector, equipped with an axisymmetric hub platform and a squealer tip, tested at purge flow rates of 1.74% and 1% of the stage mass flow. At the rotor shroud, the maximum heat transfer is measured along the front pressure side rim, whereas the squealer cavity generates a region of low static pressure and heat flux. The experimental adiabatic wall temperature is derived to quantify the thermal contribution to the shroud heat flux, demonstrating a temperature increase of up to 20% with respect to the stage inlet. An Euler-based model is proposed to evaluate the temperature-driving work-exchange mechanism in the gap. The peak in casing heat transfer coefficient (750 W/m2) is found above the tip leading edge. Unsteady flow measurements at the stage outlet confirm the phase and intensity of the tip leakage and upper passage vortex predicted by RANS computations performed with experimentally-calibrated boundary conditions. In the lower 50% of the span, the numerical calculations show significant limits in predicting the interaction between rim seal purge and turbine main flow.
Previous experiments were conducted and reported in a safety relief valve. It was noticed that in presence of a cavitating two-phase flow, the mass flux tends to be reduced due to the two-phase mixture compressibility. Moreover, the forces acting on the valve are dependent on the dynamic pressure and therefore, characteristics may be affected by the presence of a gas phase. The goal of this study is to propose a mathematical model capable of predicting the mass flux and forces acting on a safety relief valve experiencing cavitation at initial high subcooling conditions. For the mass flux prediction, an extension of the actual recommended sizing equations of IEC 60534-2-3 is proposed, including a formulation of the semi-critical region based on the hypothesis that compressibility of a two-phase mixture may be considered as an ellipse. It is verified that at chocking flow conditions, the critical section is partially filled by vapor as the fluid velocity equals the local speed of sound. Finally, a theoretical analysis is proposed to estimate the hydrodynamic fluid forces acting on the disk of a safety relief valve, using a simplified axisymmetric system of a plate over a nozzle. Results show a good agreement against experimental data and underline the influence of the backpressure in the SRV flow characteristics.
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