Experimental methods and design of a Pelton bucket
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The Pelton turbine is a widely used technology for power generation first patented in 1880. Despite being a mature technology, there is still little published knowledge regarding the design method of Pelton turbines. There is need of a common and accessible Pelton turbine geometry as a basis for future research. The chaotic environment surrounding the turbine during operation makes quantifiable measurements of the flow in the turbine buckets challenging. Large velocity gradients and two phase flow dominate the environment, and the buckets of the turbine undergo enormous changes in pressure caused by periodic contact with a high velocity water jet. Since the flow in Pelton turbine buckets are dependent on the design of the bucket, the investigation of the flow phenomena can not be done without an understanding and availability of the bucket design. Numerical simulations are subject to an increased importance for the research of hydraulic machinery in the last decades. On-board pressure and water thickness measurements have to date been the only available quantifiable measurements of the flow in a rotating bucket. These measurements can be used to validate the pressure and, to some extent, water thickness found in numerical simulations. However, the flow pattern and behaviour of the flow lacks direct validation data that are essential for trustworthy simulation results. The main objective of this thesis is therefore twofold and interdependent. The first part is to develop a publicly available Pelton turbine geometry and design methodology. The second part is to produce quantifiable flow measurements of the flow in the turbine and to study these results. A design methodology for Pelton turbine buckets has been developed by the author which relies heavily on the use of Bézier curves and a high degree of user interaction during the design process. All the theoretical basis of the method are presented along with the design strategy and implementation of the theory. An experimental turbine with buckets, designed with the presented method, has been manufactured and tested. The flow in the designed bucket is analysed and improvements to the design method are presented. All experiments were carried out in the Waterpower Laboratory at the Norwegian University of Science and Technology. The flow in the turbine has firstly been studied using high speed filming from the stationary frame of reference. The focus of the early study was the effects of jet alignment on the flow in the turbine. Gradient based edge tracing is utilized to quantify the misalignment. Validation of the method based on the jet diameter compare well with analytical models and the method is, to the authors knowledge, the first time advanced image processing has been used for studying hydraulic turbines. To enabled an improved study of the flow, a visualization system has been developed to enable high speed filming from within the rotating frame of reference. A unique post processing technique for the results from the visualization system has been developed. The technique enables a study of the water front as it propagates through the bucket. The location of the front has been found as an absolute position in the physical realm for multiple angular positions. These results, combined with the publicly available turbine geometry, can be used for direct validation of water propagation through a rotating turbine bucket found by numerical simulations. Future research within the topic presented should focus on the improvement of the design method with regard to the definition of the lip area. Work within the automation of the water front tracing should also be conducted, where the focus should be on improving the contrast between the wetted and dry surface.