Cavitation number

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There are three dimensionless numbers that may be referred to as the cavitation number in various scenarios: the cavitation number for hydrodynamic cavitation, the Thoma number for cavitation in pumps, and the Garcia-Atance number for ultrasonic cavitation.

Contents

Hydrodynamic cavitation

The cavitation number (Ca) can be used to predict hydrodynamic cavitation. It has a similar structure as the Euler number, but a different meaning and use:

The cavitation number expresses the relationship between the difference of a local absolute pressure from the vapor pressure and the kinetic energy per volume, and is used to characterize the potential of the flow to cavitate.

It is defined as [1]

Application of the cavitation number to a propeller, a hydrofoil and a constriction. Cavitation number for hydrodynamic cavitation.png
Application of the cavitation number to a propeller, a hydrofoil and a constriction.

where

The cavitation number serves as one of the primary methods for characterizing cavitation within a fluidic system. Low cavitation number indicates a higher probability of cavitation, while high cavitation number indicates no cavitation.

Within a fluid conduit, a pipe or a constraint, as the upstream pressure rises, so does the velocity of the working fluid. However, it's important to note that the increase rate of the square of the velocity greatly surpasses that of the pressure increase. Consequently, the cavitation number exhibits a declining trend with increasing upstream pressure, that is the situation where the system approaches the cavitation.

The inception of cavitation occurs when cavitating bubbles first appear within the system, marking the inception cavitation number. This value represents the highest cavitation number observed within the system in presence of cavitation. Researchers often aim to record cavitation inception at relatively low upstream pressures, particularly when they are pursuing non-destructive applications of this phenomenon.

As the development of cavitating flow progresses, the cavitation number steadily decreases until the system reaches the point of supercavitation, characterized by the highest achievable velocity and flowrate. Lower cavitation numbers are indicative of more intense cavitating flow.

Following supercavitation, the system reaches its fluid-handling limit, even as upstream pressure continues to rise. Consequently, the measured cavitation number embarks on an upward trajectory. This trend is a recurring observation in numerous published articles within the literature. [2]

Cavitation in pumps

The Thoma number () is a dimensionless quantity that can be used to predict cavitation in the suction of a pump. It is defined as [3]

Application of the Thoma number to a pump for prediction of cavitation Thoma number for cavitation in pumps.png
Application of the Thoma number to a pump for prediction of cavitation

Where is the net positive suction head and is the hydraulic head developed by the pump. The fluid will cavitate in the suction of the pump if the Thoma number is smaller than the critical cavitation parameter or the critical Thoma number defined as

Where is the net positive suction head required to prevent cavitation. It is a parameter found experimentally for each pump model.

See also

Related Research Articles

<span class="mw-page-title-main">Cavitation</span> Low-pressure voids formed in liquids


The term Cavitation in fluid mechanics and engineering normally refers to the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial cavitation and non-inertial cavitation.

<span class="mw-page-title-main">Navier–Stokes equations</span> Equations describing the motion of viscous fluid substances

The Navier–Stokes equations are partial differential equations which describe the motion of viscous fluid substances, named after French engineer and physicist Claude-Louis Navier and Irish physicist and mathematician George Gabriel Stokes. They were developed over several decades of progressively building the theories, from 1822 (Navier) to 1842–1850 (Stokes).

In fluid mechanics, the Grashof number is a dimensionless number which approximates the ratio of the buoyancy to viscous forces acting on a fluid. It frequently arises in the study of situations involving natural convection and is analogous to the Reynolds number.

In fluid dynamics, the Darcy–Weisbach equation is an empirical equation that relates the head loss, or pressure loss, due to friction along a given length of pipe to the average velocity of the fluid flow for an incompressible fluid. The equation is named after Henry Darcy and Julius Weisbach. Currently, there is no formula more accurate or universally applicable than the Darcy-Weisbach supplemented by the Moody diagram or Colebrook equation.

In continuum mechanics, the Froude number is a dimensionless number defined as the ratio of the flow inertia to the external field. The Froude number is based on the speed–length ratio which he defined as:

In fluid dynamics, the pressure coefficient is a dimensionless number which describes the relative pressures throughout a flow field. The pressure coefficient is used in aerodynamics and hydrodynamics. Every point in a fluid flow field has its own unique pressure coefficient, Cp.

<span class="mw-page-title-main">Hydraulic head</span> Specific measurement of liquid pressure above a vertical datum

Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum.

<span class="mw-page-title-main">Weber number</span> Dimensionless number in fluid mechanics

The Weber number (We) is a dimensionless number in fluid mechanics that is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It is named after Moritz Weber (1871–1951). It can be thought of as a measure of the relative importance of the fluid's inertia compared to its surface tension. The quantity is useful in analyzing thin film flows and the formation of droplets and bubbles.

<span class="mw-page-title-main">Centrifugal pump</span> Pump used to transport fluids by conversion of rotational kinetic energy

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from which it exits.

In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation:

  1. The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to flashing, and so to cavitation. Technically it is the absolute pressure head minus the vapour pressure of the liquid.
  2. The Required NPSH (NPSHR): the head value at the suction side required to keep the fluid away from cavitating.

The Euler number (Eu) is a dimensionless number used in fluid flow calculations. It expresses the relationship between a local pressure drop caused by a restriction and the kinetic energy per volume of the flow, and is used to characterize energy losses in the flow, where a perfect frictionless flow corresponds to an Euler number of 0. The inverse of the Euler number is referred to as the Ruark Number with the symbol Ru.

<span class="mw-page-title-main">Multiphase flow</span>

In fluid mechanics, multiphase flow is the simultaneous flow of materials with two or more thermodynamic phases. Virtually all processing technologies from cavitating pumps and turbines to paper-making and the construction of plastics involve some form of multiphase flow. It is also prevalent in many natural phenomena.

Specific speedNs, is used to characterize turbomachinery speed. Common commercial and industrial practices use dimensioned versions which are of equal utility. Specific speed is most commonly used in pump applications to define the suction specific speed —a quasi non-dimensional number that categorizes pump impellers as to their type and proportions. In Imperial units it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot of hydraulic head. In metric units flow may be in l/s or m³/s and head in m, and care must be taken to state the units used.

<span class="mw-page-title-main">Shallow water equations</span> Set of partial differential equations that describe the flow below a pressure surface in a fluid

The shallow-water equations (SWE) are a set of hyperbolic partial differential equations that describe the flow below a pressure surface in a fluid. The shallow-water equations in unidirectional form are also called Saint-Venant equations, after Adhémar Jean Claude Barré de Saint-Venant.

In fluid dynamics, the Morton number (Mo) is a dimensionless number used together with the Eötvös number or Bond number to characterize the shape of bubbles or drops moving in a surrounding fluid or continuous phase, c. It is named after Rose Morton, who described it with W. L. Haberman in 1953.

The Cauchy momentum equation is a vector partial differential equation put forth by Cauchy that describes the non-relativistic momentum transport in any continuum.

<span class="mw-page-title-main">Reynolds number</span> Ratio of inertial to viscous forces acting on a liquid

In fluid mechanics, the Reynolds number is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers, flows tend to be turbulent. The turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow. These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation.

The Stuart number (N), also known as magnetic interaction parameter, is a dimensionless number of fluids, i.e. gases or liquids.

Cavitation modelling is a type of computational fluid dynamic (CFD) that represents the flow of fluid during cavitation. It covers a wide range of applications, such as pumps, water turbines, pump inducers, and fuel cavitation in orifices as commonly encountered in fuel injection systems.

The Bodenstein number is a dimensionless parameter in chemical reaction engineering, which describes the ratio of the amount of substance introduced by convection to that introduced by diffusion. Hence, it characterises the backmixing in a system and allows statements whether and how much volume elements or substances within a chemical reactor mix due to the prevalent currents. It is defined as the ratio of the convection current to the dispersion current. The Bodenstein number is an element of the dispersion model of residence times and is therefore also called the dimensionless dispersion coefficient.

References

  1. Eisenberg, P.; David Taylor Model Basin Washington DC (1947). "A Cavitation Method for the Development of Forms Having Specified Critical Cavitation Numbers". David Taylor Model Basin Report. 647.
  2. Gevari, Moein Talebian; Ghorbani, Morteza; Svagan, Anna J.; Grishenkov, Dmitry; Kosar, Ali (2019-10-01). "Energy harvesting with micro scale hydrodynamic cavitation-thermoelectric generation coupling". AIP Advances. 9 (10): 105012. Bibcode:2019AIPA....9j5012G. doi: 10.1063/1.5115336 .
  3. Manderla, M.; Kiniger, K.; Koutnik, J. (2014). "Improved pump turbine transient behaviour prediction using a Thoma number-dependent hillchart model". IOP Conference Series: Earth and Environmental Science. 22 (3): 032039. Bibcode:2014E&ES...22c2039M. doi: 10.1088/1755-1315/22/3/032039 . ProQuest   2534467902.