# Sediment Erosion Characteristics and Mechanism on Guide Vane End-Clearance of Hydro Turbine
## 1 Introduction  
China is rich in water energy resources. The country's outstanding features include precipitous watercourses with a concentrated distribution for climate, its topography and other factors, which are very beneficial to the development of hydropower [1]. According to statistical data from the National Energy Administration, by the end of 2018, the total hydropower installed capacity and annual energy output in China were up to 350 GW and 1200 TWh, respectively. All natural water resources carry impurities such as suspended sediment and solid particulates. For hydro turbines working in the natural environment, flow problems can be classified as complex multiphase flow. In the China's Yellow River and northwest inland basin, the running hydroelectric project suffers from maintenance challenges as sediment causes erosion of the flow passage components of the turbines (see Figure 1). The energy loss caused by the outage or maintenance of the hydro turbine flow passage parts due to erosion is about 2-3 TWh each year, and the annual maintenance cost and renewal cost of equipment is hundreds of millions of yuan. Guide vanes direct the sediment-laden water onto the runner blades at a suitable angle, thus providing a tangential velocity and an angular momentum to the sediment-laden water. Furthermore,a small end-clearance is designed at the upper and lower surface of the guide vane, allowing them to be pivoted to regulate the flow (see Figure 2) [2]. Turbines experience several unsteady sediment-laden flows where such flows contribute to erosion in the flow-passage walls of the guide vane clearance. The enlarged clearance would lead to changes in the flow pattern, increases in water leakage, losses in efficiency and output power, vibration, and even the final breakdown of the turbine components [3; 4]. A number of detailed studies of the flow field around guide vanes and end-clearance flow have been undertaken by many scholars. A theoretical and experimental study of flow through the double cascade of a Francis turbine was conducted by [PERSON] et al. [5] who observed the flow around guide vanes with different sizes of end-clearance, where a crosswise leakage flow developed from the end-clearance due to the pressure gradient between the guide vane surfaces. An experimental apparatus with a cascade of guide vanes in a Francis turbine was developed by [PERSON] et al. [6; 7; 8] who observed leakage flow as a jet flow, which was mixed with the mainstream in the transverse direction and then formed a vortex. Therefore, the inlet flowrate of runner blades will be uneven. In the aspect of numerical simulations studies, [PERSON] et al. [9] used the Tabakoff erosion model to analyze the erosion in Francis turbines and studied the effect of the guide vane clearance gap on Francis turbine performance. A very thorough numerical investigation detailed by [PERSON] et al. [10] showed the influence of clearance size on the leakage flow, vortex, and secondary flow. [PERSON] et al. [11] used the k-omega based Shear-Stress-Transport (SST) model and the Finnie and Tabakoff erosion models in ANSYS CFX to studied flow field and erosion wear on Francis Turbine components due to sediment flow, who found that erosion and efficiency loss both decreases with increase in particle shape factor. [PERSON] et al. [12] used a volume of fluid (VOF) model for simulating the free nozzle jet, a Lagrangian particle tracking model for the trajectories of the solid particles, and Oka and det norske veritas (DNV) models for estimating the mass removal. A previously validated multiscale model of erosion is presented by [PERSON] et al. [13; 14] to obtain erosion distribution and the  
Figure 1: Sediment erosion in the flow passage components of the turbine: (**a**) Guide vane; (**b**) runner; and (**c**) hydropower station in a sand-laden river. Figure 2: Schematic diagram of end-clearance of the guide vane. erosion process of Pelton bucket impacted by a sediment-laden water jet. This model consists of two coupled sub-models.