Session: 7.2.1 - Cavitation I

Paper Number: 156675

156675 - Study on the Mechanism of Cavitation Detachment in a Centrifugal Pump 

Abstract: 

Pump is one of the widely used and indispensable mechanical equipment in the national economy. According to incomplete statistics, pump machinery alone accounts for 20% -25% of the total electricity consumption in China, with centrifugal pumps accounting for about 80% of the market share and consuming over 80% of the electricity in pump products. At present, China has become the world's largest producer of civilian centrifugal pumps. At the same time, China has strict energy-saving and emission reduction requirements for the pump industry.

Cavitation, as an important hydrodynamic problem faced by hydraulic machinery, its occurrence and development will seriously affect the efficient, safe, and stable operation of mechanical equipment. Cavitation is also a cutting-edge issue in the field of hydrodynamics, and is one of the fluid mechanics challenges that have not yet been fully solved. At present, the research on the cavitation mechanism of hydrofoils is relatively mature, and it is generally believed that the return jet or condensation shock wave is the main mechanism that affects the periodic shedding of bubbles. However, due to the existence of rotational effects, the cavitation evolution process and mechanism of hydraulic machinery are different from non-rotating hydrofoil cavitation flow. Its essence lies in the existence of rotation effect, which causes acceleration of fluid micro clusters. Therefore, the NACA0015 hydrofoil is used to replace the cylindrical blades in a single-suction centrifugal pump, and the DES turbulence model and Zwart Gerber Belamri cavitation model are combined to study the spatiotemporal evolution process of cavitation in hydraulic rotating impellers.

Based on the evolution characteristics of cavitation in numerical and experimental results, the development process of cavitation is divided into three typical stages: the initial stage of cavitation, the weak stage of cavitation, and the unstable stage of cavitation. In the analysis of numerical simulation results, the initial stage of cavitation(4.26<σ≤6.06), the volume of attached bubbles on the leading edge of the blade is small, and the wandering bubbles are continuously affected by vortex motion, gathering at the inlet of blade, and not easily leaving the impeller. In the weak cavitation stage (0.57≤σ≤4.26) includes two types: attached cavitation and moving cavitation, the cavitation zone is mainly concentrated at the inlet of the blade, and the external characteristic parameters are almost not affected by cavitation. The cavitation inside the impeller periodically generates, falls off, and collapses. The unstable stage of cavitation (0.26≤σ<0.57), the bubble shape only includes attached bubbles, and the volume of the bubbles changes periodically. According to the visualization experimental results, the weak cavitation stage (0.64≤σ≤4.26), the cavitation in the same channel undergoes a process of initiation, development, detachment, regeneration, and growth. The development of cavitation induces the generation of more obvious unsteady dynamic characteristics. By analyzing the phenomenon of bubble shedding in various flow channels of the impeller under different cavitation numbers and combining numerical simulation results, it was found that the weak cavitation stage, the fluid near the inlet channel of cavitation bubble continuously pulls the cavitation bubble away from the back of the blade, and the return jet perpendicular to the surface of cavitation bubble continuously impacts the root of the cavitation bubble, ultimately leading to cavitation bubble detachment. The fluid motion velocity near the inlet channel of cavitation bubble, return jet velocity, shape of cavitation bubbles, and the randomness of cavitation bubbles collapse in different flow channels results in differences in shape of cavitation bubbles detachment and detachment time. During the unstable cavitation stage(0.27≤σ<0.64), the fluid velocity near the inlet channel of cavitation bubble and return jet both decreases. Due to the influence of the high-speed jet between the tail of cavitation bubble and the working surface of the blade on the vapor-liquid interface of cavitation bubble, there is a larger volume of cavitation bubble shedding in the flow channel.

Presenting Author: Weiguo Zhao Lanzhou University of Technology

Presenting Author Biography: