Session: 05-16 Instrumentation IV: AI-based Improvements

Paper Number: 128565

128565 - An Active Turbulence Grid for Turbomachinery Flow Experiments 

The assessment of the aerodynamic performance degradation for turbine passage flows had and continues to have a key role in performance and efficiency predictability. One prominent effect that plagues this predictive capability is the role of free-stream turbulence, as it plays a major role in the fluid dynamic processes. Still today, quantifying the time-dependent and multi-scale interactions between turbulence and flow physics represents the main sources of uncertainty both in experiments and turbulence-modeled CFD simulations, causing significant discrepancies between measurements and predictions of global flow phenomena and entropy generating mechanisms. Although there have been only few literature evidences in which there has been a systematic variation of the free-stream turbulence intensity and length/time scales, the results consistently show the importance of both these two parameters on the flow physics of turbine passage flows. However, the individual role of turbulence length/time scales is still scarcely investigated in open literature. The isolated effects of scales are too often ignored or poorly modeled when not considered as a concurrent/dependent effect of the turbulence intensity, as nowadays the turbulence scales are difficult to generate and measure both in experiments and numerical simulations. The rationale behind the scope of this paper is to enable future experimental campaigns in a low-speed turbine cascade rig with a novel turbulence generator capable to expose turbine flows to a wide range of free-stream turbulence, characterized by values of turbulence scales that can be systematically varied from the intensity levels, so that the effects of these two key parameters are isolated/ decoupled and studied independently each other.

This paper discusses the development of a concept for generating free-stream turbulence in a low-speed turbine cascade rig that provides the authority for the generation of a wide range of turbulence intensities, however, allowing for an independent control of turbulence scales. The turbulence generator is designed for: generating different combinations of free-stream turbulence regimes representative of values encountered by a broad class of turbine systems, ensure homogeneous and isotropic inlet turbulence fields and be adaptable to a broad range of flow regimes. The proposed concept includes suppling momentum in the mainstream flow by using continuously, adjustable and distributed compressed jet injections. An experimental characterization is conducted first in an auxiliary low-speed test bench with a small cross-sectional area, testing at low Mach number (~0.05) and ambient conditions. Based on the results, a scaled-up version is designed and tested in an open-loop low-speed turbine cascade rig.

This paper presents measurements of pressure, flow velocity and turbulence features (turbulence intensity, length and time scales, turbulence isotropy and homogeneity degrees and turbulence decay, energy and spectra) at multiple planes downstream and upstream the generator. Multi-wire hot-wire anemometry in stream- and cross-wise directions is used to measure the three velocity components and characterize the turbulence flow fields. 2D time-resolved optical PIV measurement is applied for a fundamental study of the turbulence structures generated by the mixing and interaction of the jet injection with the bar wake’s. The results indicate that the turbulence generator is capable of generating a wide range of homogeneous and isotropic turbulence flow fields, using the same geometry and adjusting the jet to mainstream velocity ratio and the position. Values from low to high turbulence levels (2-10%) with minimum pressure losses and a low degree of non-uniformity (5%) have been obtained with yet the ability to tune independently the integral length scale within the range of 6-25 mm.

Presenting Author: Federico Bertelli von Karman Institute for Fluid Dynamics

Presenting Author Biography: Federico Bertelli received his MsC in Space Engineering from the University of Pisa in Italy, where he graduated “cum laude” in 2019. In July 2020, he obtained the Research Master degree with honors at the von Karman Institute for Fluid Dynamics in Belgium and, in January 2021, he started the PhD research jointly in the turbomachinery and propulsion department of the von Karman Institute for Fluid Dynamics and the Aerospace & Mechanical department of the University of Liege.

Authors:

Federico Bertelli von Karman Institute for Fluid Dynamics
Mizuki Okada von Karman Institute for Fluid Dynamics
Sergio Lavagnoli von Karman Institute for Fluid Dynamics
Koen Hillewaert University of Liege