Cavi ta tion

Exper im ent fo r bo i l ing o f water a t reduced pres s ure

An Insight into Cavitation Phenomenalearncax.com /blog/2014/01/24/an- insight- into-cavitation-phenomena-introduction/

Ganesh Visavale

We all must have heard about the term ‘Cavitation’ sometime, more certainly if we have dealt with pumps or similarfluid application and probably might have also seen some effects of ‘Cavitation’. Cavitation is very common term inengineering domain especially if we are dealing with fluids. So let us have a detailed insight into this ‘Cavitation’phenomenon through this blog.

What is ‘Cavitat ion’…?

‘Cavitation’ is defined as thephenomenon of formation of vaporphase of a liquid when it is subjectedto reduced pressures at constantambient temperature. So basicallycavitation is a boiling process in aliquid as a result of pressure reductionrather than heat addition.

So what exact ly happens during‘Cavitat ion’…?Let us see what happens duringcavitation by taking the simpleexample of water. We know waterboils on addition of heat. So if weimagine atmospheric pressure of 101and we heat water to 100 deg we aregoing to see boiling of water.Now consider a condition where we aregoing to keep temperature constant, letus assume it as at 15.5 deg and then weare going to reduce pressure. In thiscase also at specific low pressure valuewater does boil. In the condition wheretemperature is 15.5 deg water will boil at1.7 . This is what happens duringcavitation. Liquid boils by loweringpressure rather than increasingtemperature or heat addition.

As a simple demo example we cantake the case of boiling water in vacuumas shown in the above figure. When theair is pumped out of the small container,the pressure is reduced, resulting inlowering of the boiling point temperatureand ultimately the water boils at roomtemperature. The boiling process canbe observed by placing the containeron the overhead projector. (This is partof very informative demo series byDept. of Physics, Univ. of Illinois at Urbana-Champaign)

Why does liquid boil when pressure is lowered…?So to answer this question we must understand three basic concepts. Those are:

atmospheric pressure,

vapor pres s ure-tem perature d iagram for water

vapor pressure and

the relation between temperatures and vapor pressure.

First let us discuss about the atmospheric pressure. We know that earth is surrounded by a layer of gas. Why doesthis gas stay in contact with earth ? This is because it is held in contact by gravity. When we define pressure we saypressure is force per unit area. This layer of air above us on earth is exerting a pressure on the surface of planetand this pressure is about 760 mm column of mercury at sea level. We refer to this pressure as atmosphericpressure. Now let us discuss about the concept of vapor pressure. All liquids exert a certain vapor pressure at anytemperature. This is the pressure value at the instant at that temperature when the liquid molecules escape intovapor phase. The vapor pressure increases with temperature. This is because at higher temperature the moleculesare moving faster and capable to overcome the attractive intermolecular forces that bind them. So boiling occurswhen the vapor pressure reaches or exceeds the surrounding pressure from the atmosphere or the pressure due towhatever else is in vicinity of the liquid. Standard atmospheric pressure is 0.101325 which is defined as 1atmosphere. At this pressure water boils at approximately 100 deg .

In other words, one can say that the vapor pressure of water at that temperature (100 deg ) is 1 atmosphere. Whenwe increase pressure, temperature required is higher before the vapor pressure reaches the surrounding pressure.Due to this water under pressure boils at a higher temperature. When the surrounding pressure is lower, vaporpressure reaches that pressure at a lower temperature. Hence water can boil at lowered pressure instead oftemperature increase. There is a strong relation between vapor pressure and temperature. For example if weconsider water, the variation of vapor pressure is as shown below:

At a higher temperature, moremolecules have enough energyto escape from the liquid orsolid. At a lower temperature,fewer molecules have sufficientenergy to escape from the liquidor solid. Vapor pressureincreases with increasingtemperature.

Boiling and Cavitat ion… arethey similar or dif ferent…?The physics of cavitationinception is similar to boiling.The major difference betweenthe two is the thermodynamicpaths followed when vapor isformed. Boiling occurs when thelocal vapor pressure of the liquidrises above its local ambientpressure and sufficient energy ispresent to cause the phasechange to a gas. Cavitation inception on the other hand occurs when the local pressure falls sufficiently far below thesaturated vapor pressure.

What we see during ‘Cavitat ion’…?During cavitation when the liquid encounters local regions of low pressure, the vapor bubbles start growing. Thesebubbles then flow along with the liquid and when they are in regions of higher pressure downstream, bubblescollapse on solid walls resulting in high local pressure.

How the word ‘Cavitat ion’ came into picture…?Why this phenomenon was named ‘Cavitation’ & how the word originated ?The word ‘Cavitation’ comes from the Latin word “cavus” meaning hole or cavity. The vapor bubbles are like cavityin liquid flow.

Who discovered ‘Cavitat ion’…?

Cavitation, is a physics phenomenon occurring in nature. Osborne

Os borne Reynolds

Cavitation, is a physics phenomenon occurring in nature. OsborneReynolds was the scientist who first discovered cavitation. In 1894 hepresented a short paper at a meeting of the British Association atOxford. This paper was titled `Experiments concerned with the boiling ofwater in an open tube at ordinary temperatures’. The paper began with adescription of the processes involved in the boiling of water by heatingit. But it also highlighted the phenomenon of cavitation. An experimentwas shown in the meeting. In the experiment a glass tube of internaldiameter half an inch and length six inches was used. The tube had aneck in the middle, to less than a tenth of an inch bore. One end of thetube was coupled to water main. There was an inclination towards theopen end downwards into a vessel filled with water. Then the watersupply was turned on very slowly to fill the tube. As the velocity of flowwas increased until a particular velocity a distinct sharp hiss washeard. Reynolds provided the following explanation for thisphenomenon:`As the bubbles of air and vapor would be carried with great velocity fromthe low pressure at the neck, where they formed, into the higher pressurein the wider portion of the expanding tube; so that the pressure beinggreater than the vapor tension, condensation would ensue and thebubbles would collapse…’. It is interesting to note that in the paper thereis never a mention of the cavitation, but Oscar Reynolds was actuallydemonstrating the very understanding of it.

Is ‘Cavitat ion’ good or bad…?The effects of ‘Cavitation’ phenomenon are sometimes a problem but as a concept it is also utilized for manypositive outcomes.‘Cavitation’ is a problem in processes like:1. In pump or fluid flow instruments ‘Cavitation’ has negative effects.2. Cavitation leads to noisy operation3. It can cause pitting, accelerated erosion and damage to components4. Cavitation can lead to imbalance and vibration which in turn damages components5. Overall effect of cavitation is loss of efficiency of equipments like pumps

Cavitation is also used for many significant purposes like: 1. It is used in ultrasonic[1] 2. It is used in particle breakingequipments 3. It is used in water purification devices 4. It is also used in treatment of kidney stones

Closer look at cavitat ion’ around hydrofoil…Cavitation can occur in various situations and in various equipments and processes involving liquid. An interestingexample to study cavitation is the cavitation around hydrofoil. Let us have a closer look at this study.

What is a hydrofoil?Hydrofoil is very similar to an airfoil. Foil has its meaning origin from the word wing. Hydrofoil is like a wing in liquid orwater. The basic purpose of hydrofoils is to make the boat move faster. It can do this by taking hull out of the water.During normal operation of a boat, most of the energy is spend in moving the water in its path away. The hull doesthis job by pushing through the water in front of the boat. The hydrofoil basically reduces the energy spend inovercoming the drag on the hull by using the generated lift, to lift it above water.

How does a hydrofoil work…?When the speed of the boat is low, the hull of the boatis in water and the hydrofoil which is below the boat issubmerged in water. As the speed of boat increasesthere is lift created on the hydrofoil. This is similar tothe lift created by wings on an airplane. At a certaincritical speed the lift of hydrofoil will be same as theweight of boat and this will lift the boat up. Due to this,the hull of the boat comes above water. Now the boathas to counter the drag on the hydrofoil than the dragon the hull which is a much efficient way of cruising.

Let us see an interesting video from America’s Cup

Hydrofo i l working pr incip le

In i tia tion o f cavi ta tion

which shows the impact of hydrofoil.

‘Cavitat ion’ around a hydrofoil…

Many experiments have been performed to studycavitation around hydrofoils. These experiments are

usually performed in circulating water channels. The velocity, pressure and temperature can be changed duringexperiments separately of each other. A symmetrical hydrofoil is then suspended in the channel with angle of attackso that lift is produced upwards. Now the ambient pressure is held constant at below atmospheric value and the flowvelocity is increased till cavitation occurs. It is observed that cavitation first starts at the intersection of the strut andhydrofoil.

This is because, the presenceof strut causes a pressurereduction compared to otherregions of the hydroifoil. On thesurface of the foil cavitation firstoccurs in the low pressureregion of the laminar boundarylayer separation. If the velocityis further increased cavitationbegins near the leading edge.This is because as the velocityis high the Reynolds number isalso high and transition fromlaminar to turbulent boundaryoccurs and a minimum pressureline is formed at the leadingedge.

If the same phenomenon of

Cavi ta tion near the lead ing edge

Cavi ta tion under s trobos cop ic l igh t

If the same phenomenon ofcavitation is observed understroboscopic light[2] it isobserved that the cavitatingregion is actually made up ofbubbles. Until the bubbles arein low pressure region, theygrow. As they are swept alongthe flow to higher pressureregion, the bubbles startcollapsing near the trailingedge.

Let us see this phenomenon indetail in a short video.

Coming up in next part…This is the point where we endthis Part 1 of An Insight intoCavitation blog. We willcontinue to study Cavitation indetail and well as its modelingmethodologies further. Comingup in next blog we will try tocover below topics:1. Types of Cavitation2. Cavitation number3. Flow around propeller andCavitation4. Effects of Cavitation5. Why and how noise isproduce by Cavitation

In later parts we will also see how to model cavidation using CFD. So stay connected to this LearnCAx blog to getmore insight into ‘Cavitation’!

Def init ions:1. Ultrasonic – Ultrasonics is the application of ultrasound. Ultrasound can be used for medical imaging, detection,measurement and cleaning.2. Stroboscopic light – A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produceregular flashes of light. It is one of a number of devices that can be used as a stroboscope

References:1. Dept. of Physics, Univ. of Illinois at Urbana-Champaign Department of Chemistry, Purdue University2. Handbook of Fluid mechanics3. web.mit.edu4. Ocean Engineering Group, EWRE, Civil, Architectural and Environmental Engineering, UT AustinMarine Hydrodynamic Laboratory at MIT5. Hydrofoil ‘Cavitation’ Experiment at École Polytechnique Fédérale de Lausanne (EPFL)America’s Cup youtube channel

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