Flow and pressure measurements in low-head hydraulic turbines

Abstract: Hydropower is a clean and sustainable energy resource that has been developed and used since the late 19th century. In Sweden, power plants have been constructed over the entire 20th century, with a peak in the period of 1950-1970. Currently, many of the older plants are in need of refurbishment. A modern hydropower turbine can have efficiency up to 95%. By upgrading older turbines, substantial gain can be achieved. As part of this thesis, a detailed experimental investigation of the flow in a Kaplan turbine model has been performed with a main focus on the draft tube. Besides the goal of describing the flow, the result will serve as validation data for CFD simulations and for scale-up studies (corresponding prototype is available for similar measurements). The investigation is performed with time-resolved pressure measurements in which different periodic flow phenomena are captured. The turbine is investigated at three different loads: part load, best efficiency point and near full load. During the refurbishment of a turbine, site efficiency tests are common practice to verify the improvements. A key feature of many methods regarding efficiency tests is flow rate measurements. Several commonly used methods exist for this task, such as current meter, Gibson's and ultrasonic (acoustic) methods. These methods can provide trustworthy results, but the difficulty to obtain satisfactory accuracy for low-head machines, below 50 m, is common to most of them. Gibson's method is rather economical and is easily performed on site. However, it has some limitations that make it difficult to apply to low-head turbines, such as short and non-uniform water passages. This thesis also aims to extend Gibson's method for use on low-head machines, i.e., outside the criteria stated in the IEC 41 standard. The investigation is performed with both numerical and experimental methods. In the numerical investigation, the physical quantities in rapidly decelerated flows are studied in detail. Unsteady friction terms are implemented in Gibson's method to enhance the accuracy under non-preferred conditions. A test rig was developed at the Norwegian University of Science and Technology in Norway for investigation with Gibson's method outside the criteria stated in the standard, as well as the validation of the numerical model and the updated Gibson method.