Session: 32-05 Low Pressure Turbines 2

Paper Number: 121345

121345 - Impact of High Freestream Turbulence on LPT Endwall Flow: Part I – Loss Development and Time-Averaged Flow Field 

Front-loaded blades are more resistant to separation; however, they can experience high losses in the endwall region due to the secondary flow and complex vortical structures present. Realistic turbomachinery conditions include high levels of freestream turbulence but often this aspect of the flow is ignored in wind tunnel testing of turbomachinery hardware. Therefore, a better understanding of the impact of high freestream turbulence intensity on the endwall region is imperative. A mechanical actuator based active grid was implemented into a low-speed linear cascade of high-lift high-work blades in order to investigate the impact of freestream turbulence on losses within the endwall region and the temporal behavior of the associated flow structures. The impact of elevated turbulence on the temporal behavior of the endwall vortices is addressed in part II of this paper. In order to fully assess the impact of elevated turbulence, three different incoming turbulence levels, low (<1%), moderate (~3%), and high (~7%), were examined at three different Reynolds numbers, Re_ex = 50,000, Re_ex = 85,000, and Re_ex = 140,000.  Three planes of total pressure loss were measured downstream from the cascade to map loss development and surface pressure measurements were collected along the blade surface at midspan. In addition, three planes of high-speed Stereo Particle Velocimetry were acquired to investigate the endwall flow field of the turbine blade to determine the impact of flow interactions between vortical structures on loss development when the flow is subjected to varying levels of freestream turbulence. Increasing the turbulence to the highest level was found to reduce the overall losses at the exit of the passage. The integrated losses in the downstream plane were found to be greater for the high turbulence condition compared to the moderate at midspan. The losses in the endwall region were lower at the high freestream turbulence condition offsetting the increase at midspan. The time-averaged velocimetry measurements in the endwall region demonstrated that increasing the freestream turbulence intensity altered the locations of vortices at the leading edge, within the passage, and at the passage exit. The greatest impact of elevated freestream turbulence was observed at the lowest Reynolds number investigated. In order to more accurately predict total pressure losses when designing or simulating the flow surrounding LPT blades the freestream turbulence levels must be increased to higher levels, better matching real engine conditions. Otherwise, predictions likely overestimate passage losses and underestimate 2D losses. The prediction or measured time averaged locations of vortices would also be inaccurate at lower turbulence levels, potentially reducing the effectiveness of flow control methods developed.

Acknowledgements: Distribution Statement A: Approved for Public Release; Distribution is Unlimited. PA# AFRL-2023-4899. This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-21RQCOR016 Any opinions, finding, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Air Force.

Presenting Author: Molly Donovan U.S. Air Force Research Laboratory

Presenting Author Biography: Molly completed her both her bachelor's' and masters degrees in Mechanical Engineering from Wright State University in 2018 and 2019. She graduated with her Ph.D. in aerospace engineering from the University of Dayton in the Spring of 2023. Following graduation she began working as a Aerospace Engineering in the Turbine Engine Division at U.S. Air Force Research Laboratory completing experimental research in the aerodynamics of low pressure turbine blades.

Authors:

Molly Donovan U.S. Air Force Research Laboratory
Christopher Marks U.S. Air Force Research Laboratory
Nathan Fletcher U.S. Air Force Research Laboratory
Markus Rumpfkeil University of Dayton