1_Introduction to Fluid Mechanics for Chemical Engineers

11/26/2011

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Dr. Udaya Bhaskar Reddy Ragula

Assistant Professor (SG)

Department of Chemical Engineering & Materials Science

Amrita School of Engineering

Coimbatore- 641 105


Amrita Vishwa Vidyapeetham 1

Fluid mechanics for Chemical Engineers - Introduction

Contents



What is a Fluid ?

What is Fluid Mechanics ?

Physical / Chemical process

Properties of fluid Density, Specific gravity & API gravity

Viscosity & Kinematic viscosity

Surface Tension & Capillarity

Vapor Pressure and

Compressibility

Difference between a liquid and a gas

Classification of fluids based on their shear properties

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What is a Fluid?



A substance which has no definite shape

Yields easily when subjected to external pressure

In simple terms, Which is able to flow easily (How easy ?)

What is Fluid Mechanics?



Mechanics – Brach of Physics that study the forces acting on a body and their effect on the motion of the body on which they are acting

Fluid mechanics – Study of forces acting on the fluid and their effect on the fluid motion

Basic Ideas in Fluid Mechanics

Principle of conservation of Mass

Principle of conservation of Energy (First law of Thermodynamics)

Conservation of momentum

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Physical / Chemical Process



Process is anything that under goes a change in the system

In fluid flow Change in the system = Volumetric flow rate Driving force = Pressure Drop / Change in Pressure Resistance = Fluid Viscosity (internal resistance),

Gravity (external resistance) etc.,

ResistanceSystemtheinChange

forceDrivingSystemtheinChange

1

Properties of Fluids



Density

Specific gravity

Viscosity

Kinematic viscosity

Surface tension and

Vapor pressure

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Fluid Density



Density is defined as the mass occupied by unit volume of the fluid

The Fluid (Liquid) density is measured by using a measuring jar or specific gravity bottle

The Solid density is measured by using the same technique

Solid bulk density is measured also in the same way

How to measure the density of the gas?

)1000

1(

1000

13 cc

g

ml

g

m

kgUnits

V

m

Volume

Mass

Fluid density – Gas density



Gas density is determined by using some gas laws called Equation of state (EOS)

Example – Ideal gas law

Ideal gas law is the simple form and may not be applicable for all the gases (You will come to know in detail when you study the Chemical Engineering Thermodynamics)

nRTPV

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Effect of Temperature and Pressure on Fluid density



Liquids

Temperature – Lowers the density down

Pressure – Very minimal effect (Not significant)

So we call the liquids to be incompressible fluids

Gases

Temperature – Lowers the density

Pressure – Increases the density

Fluid Specific gravity & API gravity



Specific gravity

Reference fluid Liquids – Water at ambient conditions (1 atm and 25oC)

Gases – Air at STP conditions (1 atm and 20oC)

API gravity

fluidreference

fluidgs

.

5.1315.141

sggravityAPI

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Fluid Viscosity



Viscosity is the property of the fluid that offers resistance to its own flow

It is denoted by the symbol (µ)

Units

Effect of pressure and temperature on viscosity Pressure has very less effect on viscosity Increase in temperature decreases the viscosity of the liquids Increase in temperature increases the viscosity of gases at normal

pressures and the behavior is similar to liquids at high pressure

Forces (Stresses) acting on the fluid Compressive Tensile and Shear

2

.10

.10

1100

.)(

m

secN

secm

kgcP

seccm

gmPPoise

Fluid viscosity – Shear Stress



Shear stress

Shear strain

Shear relation

For fluids

Newton’s law of

Viscosity

A

F

h

s

G

dy

du

dt

dx

dy

d

dy

dx

dt

d

dt

d

x

.

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Fluid Kinematic Viscosity



Kinematic Viscosity

sec

cm

sec

mUnits

24

2

10

Fluid Pressure



Pressure is the ratio of force per unit area Static pressure – It is the pressure exerted by the still

fluid (Both liquid & gas)

Dynamic pressure – Pressure caused by liquid flow (liquid velocity)

Total pressure = Static pressure + dynamic pressure

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Absolute pressure and Gauge pressure



Pressure is measure using a pressure gauge

If P > Patm

If P < Patm

levelseaMeanatkPaormmHgorpsiP

PPP

atm

atmgaugeabsolute

3.1017607.14

gaugeatmabsolute PPP

Absolute pressure and gauge pressure



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Fluid Surface tension



Surface tension is a property of the surface of a liquid that allows it to resist an external force

Units Force per length (N/m)

Question to students – How can we reduce the surface tension of water?

Vapor pressure of the fluid



At all temperatures, both liquid and vapor will be in equilibrium (The quantities may vary) The pressure exerted by the vapor on its own liquid is

called as vapor pressure A liquid boils when the vapor pressure is equal to the

surroundings (external) pressure

Vapor pressure is a function of temperature and composition only

Water has a vapor pressure of 760 mm Hg @ 100oC

So, Water boils at 100oC when the external pressure is equal to 1 atm (760 mm Hg)

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Liquid vs. Gas



Liquid Gas

Molecules of liquid are

together because of forces of

attraction

Molecules are relatively apart

(Low intermolecular forces of

attraction)

Higher densities Lower densities

It has vapor pressure No Vapor pressure

Difference between a Scalar, Vector and a Tensor



Scalar – Only Magnitude

Vector – Both Magnitude and direction

Tensor – Magnitude, Direction and variation of magnitude in other directions

dx

du

dy

du

y

xy

xyx

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Sign convention for Newton’s law of Viscosity



decreasesvelocitytheIfdy

duConventionNegative

increasesvelocitytheIfdy

duConventionPositive

xyx

xyx

1_Introduction to Fluid Mechanics for Chemical Engineers

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