Cavitation Performance Analysis of Tip Clearance in a Bulb-Type Hydro Turbine

Bulb-type hydro turbines are commonly used in small- to medium-scale hydropower stations due to their compact design and adaptability to low-head conditions. However, long-term operation often results in wear at the runner rim, increasing tip clearance and triggering leakage flow and cavitation. These effects reduce hydraulic efficiency and accelerate blade surface erosion, posing serious risks to unit safety and operational stability. This study investigates the influence of tip clearance on cavitation performance in a 24 MW prototype bulb turbine. A three-dimensional numerical model is developed to simulate various operating conditions with different tip clearance values (3.0, 4.5, and 6.0 mm) and cavitation numbers (σ = 1.20–1.33). Internal flow characteristics—including pressure distribution, velocity fields, hydraulic efficiency, and pressure pulsation—are analyzed to elucidate the impact of tip clearance on cavitation development. Results show that under σ = 1.2 and a 4.5 mm tip clearance, the pressure pulsation amplitude at the blade tip reaches 4870 Pa—approximately 1.5 times higher than that near the hub. At partial flow conditions, turbine efficiency decreases by up to 6.8% compared to the rated condition. Increasing the tip clearance from 1.5 to 6.0 mm expands the low-pressure area near the blade tip by around 32%, significantly intensifying cavitation. Frequency domain analysis reveals dominant pulsation frequencies between 10–20 Hz, characterized by blade-passing features and a wave-clipping effect. These findings provide theoretical insight and quantitative evidence to support the optimization of tip clearance design and cavitation mitigation strategies in bulb turbines, aiming to improve both efficiency and operational stability.