# Analysis of Bulb Turbine Hydrofoil Cavitation
## 2 Hydrofoil  
Bulb turbine (Figure 1) blade hydrofoils are optimized for a mix of parameters, among which are maximum hydraulic efficiency, high output power, minimum turbine vibrations, high strength, reduced fatigue, and reduced cavitation phenomena. Reduced cavitation enables operators to reduce the service intervals required for cavitation damage repair and welding costs, as well as loss of money for nonoperation. In this study, we focus on a range of volumetric flows, which appear on a bulb turbine at a 25\({}^{\circ}\) blade angle (Figure 2). Here cavitation frequently occurs at approximately 15% higher volumetric flows than at the best efficiency point [6]. The 25\({}^{\circ}\) blade angle was chosen because it lies in the middle of the angle interval usually agreed between the producers and customers of bulb turbines. The blade angle 25\({}^{\circ}\) is often reinforced during performance or acceptance tests [18], because it corresponds to both common operating conditions and features increased probability for cavitation. The bulb runner blade hydrofoil's shape determines the flow conditions in the runner. Well-shaped 3D blade hydrofoils reduce the inception and the growth of cavitation on the bulb turbine runner hydrofoils. Turbine hydrofoil profiles were used in designing the bulb runner blade's 3D shape, spanning from the runner hub toward the blade tip (Figure 2). For reliable 3D characterization of the turbine runner blade, and thus the entire bulb runner, several hydrofoils are usually selected. Later, a smooth surface is drawn over all hydrofoils to form the entire 3D geometry of the runner blade, further enabling CFD or experimental analysis. Figure 1: Bulb turbine runner. Although the bulb turbine blade can consist of many hydrofoils as explained above, in this paper we focus on a single hydrofoil. The most potential for energy conversion in bulb turbines is available near the tip, where tangential blade velocity and flow velocities are high. Due to high velocities, large pressure gradients, and the vicinity of the tip gap flow, this region is highly susceptible to cavitation occurrence and surface erosion. Here, several cavitation structures are usually found during acceptance tests. To focus on the cavitation properties of the hydrofoils, we have selected for further analysis the hydrofoil at 95% bulb turbine diameter D, measured from the runner rotation axis to the blade tip. The selected hydrofoil is thus located near the tip as shown in Figures 2 and 3. We made this selection on the basis of the energy transfer. Most of the energy is transferred where the turbine blade has a large diameter and the blade has a high surface area, and where the blade and flow velocities are high. Hydrofoils have been studied extensively, including profile families such as NACA (among them 4-8 series), Gottingen, Munk, NPL, and others [19; 20; 21]. The available families of hydrofoils facilitate the turbine engineer's selection of the blade's hydrofoil to suit the required operating intervals and conditions, inter alia, for operation at high volumetric flows and runner blade angles. To enable operation in specific operating conditions, including cavitation properties, the turbine designer selects hydrofoil properties like leading  
Figure 3: Bulb turbine blade design from individual 2D hydrofoils. Figure 2: Selection of a 25\({}^{\circ}\) angle on a modified bulb turbine hydrofoil. edge radius, camber, location of maximum thickness, maximum curvature, and trailing edge design. The locations of cavitation occurrence on bulb turbine blades are well documented in [22]. In the hill diagram, we will focus on the high discharge region. Here, the cavitation is found in four regions [22]: the blade suction side, blade pressure side, fillet, and tip gap. The flow properties in and near the gap are dominated by turbulence and may appear in the entire hill diagram.