Cavitation

Propulsor model cavitating in a water tunnel.

Cavitation or vacuum aspiration is a hydrodynamic effect that occurs when vapor cavities are created inside water or any other fluid in a liquid state in which forces that respond to differences in pressure act. pressure, as can happen when the fluid passes at high speed along a sharp edge, producing a decompression of the fluid due to the conservation of Bernoulli's constant. It can happen that the vapor pressure of the liquid is reached in such a way that the molecules that compose it immediately change to a vapor state, forming bubbles or, more correctly, cavities. The bubbles formed travel to areas of higher pressure and implode (the vapor suddenly returns to the liquid state, abruptly "squashing" the bubbles) producing a trail of high-energy gas on a solid surface that can crack on impact.

The implosion causes pressure waves that travel in the liquid at speeds close to those of sound regardless of the fluid where they are created. These can dissipate in the liquid stream or they can strike a surface. If the area where the pressure waves collide is the same, the material tends to weaken structurally and erosion begins which, in addition to damaging the surface, causes it to become an area of greater pressure loss and therefore greater focus of formation of steam bubbles. If the vapor bubbles are near or in contact with a solid wall when they implode, the forces exerted by the liquid crushing the cavity left by the vapor give rise to very high localized pressures, causing pitting on the solid surface. Note that depending on the composition of the material used, oxidation could occur with the consequent deterioration of the material.

The phenomenon is usually accompanied by noise and vibrations, giving the impression of gravel hitting different parts of a machine.

Cavitation can also occur in other processes, such as ship and airplane propellers, pumps, and vascularized tissues of some plants.

The phenomenon by which cavitation tears off the oxide layer (result of passivation) that covers the metal and protects it is usually called corrosion by cavitation, in such a way that this area enters (anode) and the one that remains passivated (covered by oxide) forms a galvanic pair in which the anode (the one that corrodes) is the area that has lost its oxide layer and the cathode that maintains it.

Introduction

The physical process of cavitation is almost exactly the same as that which occurs during boiling. The biggest difference between the two is due to pressure and temperature within the phase diagram. Boiling raises the vapor pressure of the liquid above the local ambient pressure to cause the change to the gas phase, while cavitation is caused by a local pressure drop below the vapor pressure causing suction.

For cavitation to occur, the "bubbles" need a surface where they can nucleate. This surface can be the wall of a container or tank, impurities in the liquid or any other irregularity, but normally the temperature of the fluid must be taken into account, which to a large extent will be the possible cause of cavitation.

The determining factor in cavitation is the temperature of the liquid. As the temperature of the liquid varies, the vapor pressure also varies significantly, making it easier or more difficult for a given local ambient pressure to drop the vapor pressure to a value that causes cavitation.

The position that the hydraulic machines must have, with respect to the water level, in the suction tank or in the restitution channel to avoid cavitation is also studied. Focusing is done taking a determined point of the pump or turbine as a reference level. Different parameters and concepts associated with cavitation are analyzed, such as: the net positive charge in the suction and the limitations that these concepts and parameters have in the sense of extrapolating results following the laws of similarity are presented. The need to further investigate the limitations of the cavitation coefficient in turbines is highlighted and a new relationship is proposed that allows estimating the static head of a reversible machine as a pump, based on the values of the cavitation coefficient of the machine operating as a turbine.

Problems

Cavitation damage of a Francis turbine.

Cavitation is, in most cases, an undesirable event. In devices like propellers and pumps, cavitation can cause high noise, component damage, and loss of performance.

This phenomenon is widely studied in naval engineering during the design of all types of ships because it shortens the useful life of some parts such as propellers and rudders.

In the case of submarines, this effect is even more studied, avoided and unwanted, since it makes it impossible for these warships to maintain their operational characteristics of silence and undetectability due to the vibrations and noises that cavitation causes in the hull and the propellers.

The collapse of the cavities involves a lot of energy that can cause enormous damage. Cavitation can affect almost any material. Pitting caused by collapsing cavities causes enormous wear on the different components and can greatly shorten the life of the pump or propeller.

In addition to all of the above, the creation and subsequent collapse of bubbles creates friction and turbulence in the liquid. This contributes to a further loss of performance in devices subjected to cavitation.

Cavitation also occurs at the bottom of rivers where it is generated from irregularities in the bed, dissociating water and air. Both are subjected to pressure, giving rise, the latter, to bubbles that, with the force of the water, break down into microscopic sizes, shooting out at high speed. This causes a strong impact on the bed that can be up to 60 t/m². Its importance lies in the constancy and repetition of the phenomenon, which favors its performance. Cavitation is a common erosive process in bridge piers.

Although cavitation is an undesirable phenomenon in most circumstances, this is not always the case. For example, supercavitation has military applications such as supercavitation torpedoes in which a bubble surrounds the torpedo thus eliminating all friction with the water. These torpedoes can travel at high speeds underwater, even up to supersonic speeds. Cavitation can also be a positive phenomenon in ultrasonic cleaning devices. These devices make use of ultrasonic sound waves and take advantage of the collapse of bubbles during cavitation to clean surfaces.

Another application of cavitation is during the process of homogenization of a product (eg biotechnological process) that is carried out using a sonicator. This device has a tip that is immersed in the solution to be homogenized and vibrates at ultrasonic frequencies. These high frequency vibrations cause cavitation, which as already mentioned is the formation of small bubbles in the liquid medium. These bubbles collapse releasing their mechanical energy in the form of shock waves equivalent to several thousand atmospheres of pressure, breaking cells present in the suspension. The duration of ultrasound needed depends on the type of cells, the size of the sample, and the cell concentration. For bacterial cells like E. coli, 30 to 60 seconds may be sufficient for small samples. Yeast cells, may need 2 to 10 minutes.

Cavitation is thought to be the cause of 'joint cavitation'.

Pumps and propellers

It was worn by the cavitation in a round of a centrifugal bomb.

Another example of wear produced by cavitation in a round of a centrifugal pump.

The blades of a pump impeller or a ship's propeller move in a fluid. When the fluid accelerates through the blades, regions of low pressure are formed. The faster the blades move, the lower the pressure around them. When the vapor pressure is reached, the fluid vaporizes and forms small vapor bubbles which, when collapsed, cause audible pressure waves and wear on the blades.

Cavitation in pumps can occur in two different ways:

Suction cavitation

Suction cavitation occurs when the pump suction is under low pressure/high vacuum conditions which causes liquid to turn to vapor at the impeller inlet. This vapor is transported to the discharge zone of the pump where the vacuum disappears and the vapor of the liquid is again compressed due to the discharge pressure. At that moment a violent implosion occurs on the surface of the impeller. An impeller that has worked under suction cavitation conditions presents large cavities produced by pieces of material torn away by the phenomenon. This causes premature failure of the pump.

Discharge cavitation

Discharge cavitation occurs when the pump discharge is too high. This typically occurs on a pump that is running at less than 10% of its optimum efficiency point. The high discharge pressure causes most of the fluid to circulate inside the pump instead of exiting through the discharge zone. This phenomenon is known as slippage. As the liquid flows around the impeller it must pass at a very high velocity through a small opening between the impeller and the pump cutwater. This speed causes a vacuum in the cutwater (a phenomenon similar to that which occurs in a venturi) which causes the liquid to turn into steam. A pump operating under these conditions shows premature wear of the impeller, cutwater and blades. In addition, and due to the high operating pressure, premature failure of the pump seals and bearings is to be expected. Under extreme conditions, the impeller shaft may break.

Plants

Cavitation can appear in the xylem of plants when the potential of the water becomes so great that the dissolved air within the water expands to fill the plant cell. Plants are generally capable of repairing damage caused by cavitation, for example by root pumping pressure. In other types of plants such as vines, cavitation can lead to death. In some trees, cavitation is clearly audible.