fluid characteristics

8/6/2019 Fluid Characteristics

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Engineering Fluid MechanicsFluid characteristics

Dr. Amirhossein MalakahmadCivil Engineering Department

Universiti Teknologi PETRONAS

LO1: Identify fluid properties associated withfluid flow problems


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Unit objectives

At the end of this session you will be able to:

Identify basic properties of fluids such asdensity, elasticity, viscosity, etc.

Describe viscosity and the consequences ofthe frictional effects it causes in fluid flow.

Calculate the capillary rises and drops due tothe surface tension effect.


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Density and Specific gravity Density is defined as mass per unit volume

Specific volume is defined as v = 1/ r = V/m .

The density of a substance depends on temperature andpressure.

The density of most gases is proportional to pressure andinversely proportional to temperature.

The variation of liquids and solids density with pressure isusually negligible.


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Specific gravity Sometimes the density of a substance is given relative to

the density of a well-known substance. Then it is calledspecific gravity or relative density and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature.

Usually water at 4C is used as standard substance forwhich water = 1000 kg/m 3.

What is the unit for specific gravity?


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Example 1Table below contains specific gravity of some substancesat 0C. Predict which one would float on water?


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222111/ / T V PT V P

Density of Ideal GasesThe simplest and best-known equation of state in the gas phase isthe ideal-gas equation:

whereP = absolute pressure (kPa)

= density (kg/m 3)v = specific volume (m 3 /kg)R = gas constant (kPa. m 3 /kg.K)T = absolute temperature (K)

the properties of an ideal gas at two different states are related toeach other:

222111 / / T V PT V P


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Example 2:

Determine the density, specific gravity, and mass of the airin a room whose dimensions are 3 m x 4 m x 5 m at 100kPa and 35C. The gas constant of air is R = 0.287 kPam3 /kg . K.


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Vapor pressureSaturation temperature ( T sat ): at a given pressure, the temperature atwhich a pure substance changes phase.

Saturation pressure ( P sat ): at a given temperature, the pressure at whicha pure substance changes phase.

The vapor pressure (P v )of a pure substance is defined as the pressure exerted by its vapor in phase balance with its liquid at a given temperature.

P v is a property of the pure substance, and turns out to be identical to the

saturation pressure P sat of the liquid ( P v = P sat ).

At an absolute pressure of 1standard atmosphere (1 atm),the saturation temperature ofwater is 100C.


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Cavitation

Cavitation is defined as the phenomenon of formationof vapor bubbles of a flowing liquid in a region wherethe pressure of the liquid falls below its vapor pressure.

Cavitation is a common cause for drop in performance

and even the erosion of impeller blades and is animportant consideration in the design of hydraulicturbines and pumps.

Cavitation must be avoided (or at least minimized) inflow systems since it reduces performance, generatesannoying vibrations and noise, and causes damage toequipment.


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Example 3: SYABAS (Syarikat Bekalan Air

Selangor Sdn Bhd) wants to distribute water inKlang Valley area. Based on Malaysian climate, thetemperature of water is observed to be as high as40C. Determine the minimum pressure allowed in

the system to avoid cavitation.


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Coefficient of CompressibilityFluids usually expand as they are heated or depressurized and contract asthey are cooled or pressurized.

The amount of volume change is different for different fluids, and someproperties should be defined that relate volume changes to the changes inpressure and temperature.

Where: = coefficient of compressibility or bulk modulus of elasticity

A large value of indicates that a large change in pressure is needed tocause a small fractional change in volume, and thus a fluid with a large isessentially incompressible.


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Water hammer phenomenaSmall density changes in liquids can still cause interesting

phenomena in piping systems such as the water hammer,which characterized by a sound that look likes the soundproduced when a pipe is hammered.

This occurs when a liquid in a piping network encountersan immediate flow restriction (such as a closing valve) andis locally compressed.

The acoustic waves produced strike the pipe surfaces,bends, and valves as they spread and reflect along thepipe, causing the pipe to vibrate and produce the sound.


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ViscosityViscosity is a property that represents the internal resistanceof a fluid to motion.

Dynamic viscosity unit is kg/m s, or equivalently, N s/m 2 (orPa s). A common viscosity unit is poise, which is equivalentto 0.1 Pa s (or centipoise , which is one-hundredth of a poise).

Where: = shear stress (N/m 2)

= dynamic viscosity (N.s/m 2)du/dy = velocity gradient (rate ofdeformation)


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Fluids for which the rate of deformation isproportional to the shear stress are calledNewtonian fluids.

Most common fluids such as water, air,gasoline, and oils are Newtonian fluids.Blood and liquid plastics are examples ofnon-Newtonian fluids.

For non-Newtonian fluids, the relationshipbetween shear stress and rate ofdeformation is not linear.

But, the shear force acting on aNewtonian fluids layer is linear and isequal to:


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Kinematic viscosity

In fluid mechanics and heat transfer, the ratio of dynamicviscosity to density appears frequently. This ratio is given thename kinematic viscosity ( ) and is expressed as = / .Two common units of kinematic viscosity are m 2 /s and stoke.

In general, the viscosity of a fluid depends on bothtemperature and pressure, although the dependence onpressure is rather weak. For liquids , both the dynamic andkinematic viscosities are practically independent of pressure.

The viscosity of liquids decreases with temperature, whereasthe viscosity of gases increases with temperature.


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Surface tensionLiquid droplets behave like smallspherical balloons filled with the

liquid.

The surface of the liquid acts like astretched elastic membrane undertension.

The pulling force that causes thistension acts parallel to the surfaceand is due to the attractive forcesbetween the molecules of the liquid.

The magnitude of this force per unitlength is called surface tension s and is usually expressed in the unitN/m.


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Capillary effectCapillary effect is the rise or fall of aliquid in a small-diameter tube insertedinto the liquid.

The strength of the capillary effect is quantified by the contact(or wetting) angle .

A liquid is said to wet the surface when < 90and not to wet

the surface when > 90.

The phenomenon of capillary effect can be explainedmicroscopically by considering cohesive and adhesive forces.


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Capillary effect calculation

The capillary rise is inversely proportional to the radius of the tube.Therefore, the thinner the tube is, the greater the rise (or fall) of theliquid in the tube.


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Home work 1 (due date: 31.01.2011)

2. A 0.5-mm-diameter glass tube is

inserted into mercury at 25C in a cup.The contact angle of mercury with aglass surface is 110and surfacetension of mercury is 0.440 N/m.Determine the capillary fall of mercury

in the tube.

1. Define the terms below:

i. Equation of stateii. Meniscus

fluid characteristics

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