Session: 34-12 Hydraulic turbines methods

Paper Number: 123823

123823 - Cavitation Measurements and Flow Visualization on a Hydrofoil With Different Tip Gaps 

Cavitation occurs in flows of water when, due to regions of high flow velocity, the local static pressure decreases below the vapor pressure and the water begins to boil. Cavitation on the hydraulic turbomachinery can lead to unwanted effects such as noise, vibrations, efficiency- and head drop, as well as erosion and damage on surfaces. One of the critical regions in axial turbomachinery where cavitation has major impact is the tip gap region (the clearance between the blade and side walls) with the tip leakage flow and vortex. Various types of cavitation on axial turbomachinery runners are observed in practice, including bubble cavitation, sheet cavitation, cloud cavitation, and tip vortex cavitation. In spite of considerable research, there are still many features of the cavitation problem that have not been explored or understood. For axial runner blades, it is necessary to identify location and growth of cavitation vapor bubbles and their collapses as a function of different fluid dynamic parameters such as Reynold number, angle of incidence, and cavitation number. On the other hand, the optimum size of the tip gap of axial turbomachines is mainly investigated focusing on the effects of the tip gap size on the tip leakage flow, losses, and efficiency. There is lack of information about the size of the tip gap on the cavitation onset and development on axial turbomachines. This paper tries to contribute to this area by investigating the effect of the variation in the size of the tip gap on the cavitation experimentally.

The behavior of cavitation is investigated using a hydrofoil (a NACA profile) representing an axial blade of a turbomachine. The goal is to analyze and find correlations between the fluid dynamic parameters mentioned above and the cavitation type, length, and area of cavitation region around the hydrofoil first without a tip gap. Cavitating flow around a NACA0012 profile hydrofoil with a maximum thickness 12% at 30% chord, are investigated at incidence angles ranging from 0° through 8° and with varying cavitation number (strength of cavitation), s, between 0.2 and 1.8. For these parameters, the pressure distribution around the hydrofoil, the length of cavitation zone, the dimensions of cavitation zone, and the cloud shedding Strouhal number are determined using the pressure measurements and flow visualization. The experimental research is carried out in the cavitation water channel of the Lucerne University of o Applied Sciences in Switzerland. The tested hydrofoil has a chord length of 95 mm, a width of 100mm and are equipped with pressure transducers at the leading edge, on the suction side, on the pressure side and in the tip gap region. The Reynolds number defined with the chord length was held constant at 10⁶. Flow visualization is carried out using a high-speed camera with recordings around the hydrofoil from the top and from one side. With flow visualization, the dynamics and spatial structures of the cavitation are investigated qualitatively. Based on image analysis of the video recordings, frequencies of changes are estimated and the characteristic features of the cavitating flow are explored.

The cavitation behavior around the NACA0012 profile hydrofoil is further investigated for different tip gaps in order to determine the effect of the tip gap size variation on the cavitation onset and development as well as to analyze the flow field such as the effect of the tip vortex. Tip gap sizes investigated are 0.25%, 0.50%, 1%, 2%, 3% and 5% of the hydrofoil width (span). With the pressure measurements and flow visualizations for different tip gaps, the influence of the tip gap size change on the hydrofoil cavitation characteristics are determined.

Presenting Author: Sabri Deniz Lucerne University of Applied Sciences

Presenting Author Biography: Sabri Deniz studied at the Technical University of Istanbul (ITÜ) Mechanical Engineering and graduated in 1983. This was followed by an MBA at the University of Istanbul, Faculty of Economies and a Master of Science degree in Mechanical Engineering (MSc) at the (ITÜ). In 1993 he earned his doctorate (Ph.D.) at the ETH Zurich, Department of Mechanical and Process Engineering. Until 1997 he worked at the Massachusetts Institute of Technology (MIT) in Cambridge, USA as a postdoctoral fellow at the Gas Turbine Laboratory of the Department of Aeronautics and Astronautics.

In 1986, he first came to Switzerland with a federal scholarship and carried out research at the ETH Zurich, Institute of Fluid Technology. From 1988 to 1994 he was employed at the ETH Zurich, as a research assistant and as a graduate student in the Laboratory of Hydraulics, Hydrology and Glaciology as well as at the Institute of Energy Technology, Laboratory for Turbomachinery. After a short stay in Istanbul as an Associate Professor, he worked in the Turbomachinery research and development group at the Praxair, Inc. Technology Center in Tonawanda, New York. Between 2006 and 2012 he was with Ramgen Power Systems in Bellevue, Washington, as a senior staff engineer, where he worked for the development and design of supersonic compressors and turbomachines.

Since August 2012 he is a faculty member at the Lucerne University of Applied Sciences (HSLU) School of Engineering and Architecture. He is working at the competence center for Fluid Mechanics and Hydraulic Machinery. His research interests include design of hydraulic and thermal turbomachines with focus on efficiency and operating range extension, flow instabilities, flow-induced vibrations, and active flow control and development of micro-engines.

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