Hydrodynamic Cavitation: Mechanisms and Applications

Key Takeaways

• Hydrodynamic cavitation is the breakdown of the fluid medium under very low pressure. 

• Hydrodynamic cavitation involves three mechanisms: nucleation, bubble growth, and bubble implosion.

• Hydrodynamic cavitation is used in polymerization and depolymerization, microbial cell disruption, and fatty acid hydrolysis. 

Turbine is an example of a hydrodynamic cavitation application

We use a variety of machines that induce pressure as well as velocity variations in fluids. Some common examples of these machines are turbines, propellers, pumps, and bearings. In these machines, cavitation occurs whenever the fluid used in the machine undergoes fluctuations in pressure and velocity. Hydrodynamic cavitation is a type of cavitation that occurs when a low-pressure region is developed in a fluid device and forms vapor cavities. 

Cavitation

Pumps, bearings, and propellers all use fluids in association with their operation. Whenever the pressure and velocity of the fluid fluctuate, cavitation develops in the fluid. In cavitation, the pressure drops to a low level at a constant ambient temperature.

Cavitation is a two-phase fluid flow phenomenon. Typically, cavitation is described using the free-stream cavitation number. This number is the ratio of the static head of the fluid flow to its dynamic head.

Types of Cavitation

Cavitation is used in wastewater treatment, drug delivery, rock cutting, steel plate cutting, marine propellers, and valves, among other things. In general, cavitation includes the generation, growth, and rapid collapse of bubbles or cavities. The collapse of cavities induces effects such as high shear forces, extreme temperatures, shock waves, turbulence, and extreme pressure in the fluid.

Among the four types (particle, optic, ultrasonic, hydrodynamic), the last two types of cavitation are widely in use.

1. Ultrasonic cavitation - Ultrasonic cavitation is produced due to pressure fluctuations induced by ultrasonic waves traveling through the fluid. Ultrasonic cavitation is sometimes called acoustic cavitation.
2. Hydrodynamic cavitation - In hydrodynamic cavitation, the fluid in motion undergoes a rapid change in the velocity profile, which leads to localized pressure drops. 

Hydrodynamic Cavitation

Hydrodynamic cavitation is the cavitation phenomenon that involves the development of vapor cavities inside a liquid medium. Unlike ultrasonic cavitation, which is due to the compression and expansion of the fluid on the passage of ultrasonic waves, hydrodynamic cavitation is induced by a drop in the static pressure of the flowing fluid.

Mechanisms in Hydrodynamic Cavitation

Hydrodynamic cavitation involves three mechanisms:

1. Nucleation
2. Bubble growth
3. Bubble implosion

Hydrodynamic cavitation can be described as the breakdown of the fluid medium under low pressure. When a fluid flow passes through irregular geometries or narrow orifices, the velocity of the fluid rises. The rise in velocity decreases the static pressure. Whenever the pressure becomes less than the local saturated vapor pressure, a large number of cavities are released (nucleation). Upon the pressure drop, the generated cavities expand and break down (growth and implosion). When the cavities collapse, they release sharp shock waves of energy into the surrounding fluids. The shock waves are capable of bringing the microscopic mixing effect, scale-free heating, and controllable rotor/liquid friction.

Hydrodynamic Cavitators

The mechanical rotation of the fluid using a specialized rotor (with holes) at a specific velocity can produce hydrodynamic cavitation. The specialized rotors used for generating hydrodynamic cavitation are called hydrodynamic cavitators.

In hydrodynamic cavitators, the rotation of the rotor produces hydrodynamic cavitation away from the metal surface within the holes. The hydrodynamic cavitation produced by hydrodynamic cavitators is completely controllable within the system, which prevents surface damage. The implosion of the bubbles discharge shockwaves that help blend and inhibit scaling. The hydrodynamic cavitators provide uniform distribution of temperature throughout the liquid without any heat transfer surfaces.

Hydrodynamic Cavitation Applications

Hydrodynamic cavitation is a promising cavitation technology that can be used as an effective tool for nanomaterial synthesis. Hydrodynamic cavitation is successfully harnessed in chemical or physical processes such as polymerization and depolymerization, microbial cell disruption, and fatty acid hydrolysis. It is also utilized for water decontamination. Applications of controlled hydrodynamic cavitation includ biodiesel synthesis, biomass pretreatment, ozonation, desulfurization of fuels, valve operation, marine propellers, and the food and beverage industry.

Cadence CFD simulation tools can help you analyze the hydrodynamic cavitation effects on a complex fluid-based machine system. When designing turbines, propellers, and pumps, these tools can support the study of the shear, shock, and turbulence effects.

Subscribe to our newsletter for the latest CFD updates or browse Cadence’s suite of CFD software, including Fidelity and Fidelity Pointwise, to learn more about how Cadence has the solution for you.