This document provides an introduction to fluid mechanics for chemical engineers. It defines a fluid and discusses the continuum hypothesis. It describes key fluid properties like density, pressure, viscosity, surface tension and vapor pressure. It also classifies fluid motions as steady or unsteady, uniform or non-uniform, viscous or inviscid, compressible or incompressible, and laminar or turbulent. The document provides examples of areas where fluid mechanics is applied, like process units, pipelines, fluid machinery and environmental applications.

1. Fluid Mechanics For
Chemical Engineers
Course Code: ChEg2083
Chapter One
Introduction

2. Chapter Contents
Introduction
1.1 Definition a Fluid
1.2 Definition and Scope of Fluid Mechanics
1.3 The Continuum Hypothesis
1.4 Fluid Properties
1.4.1 Density, Specific Weight, and Relative Density
1.4.2 Pressure
1.4.3 Bulk Modulus of Elasticity and Coefficient of Compressibility
1.4.4 Viscosity
1.4.5 Surface Tension
1.4.6 Vapor Pressure
1.4.7 Other Thermodynamic Properties
1.5 A Classification of Fluid Motions

3. Objectives
• Define fluid and basic fluid properties
• Identify the relationships between specific
weight, specific gravity and density, and solve
problems using their relationships.
• Classify fluid motions and distinguish their
difference

4. Introduction
Fluid mechanics is a study of the behavior of
fluids, either at rest (fluid statics) or in motion
(fluid dynamics).
The analysis is based on the fundamental laws of
mechanics, which relate continuity of mass and
energy with force and momentum.
An understanding of the properties and behavior
of fluids at rest and in motion is of great
importance in engineering.

5. 1.1 Definition of Fluid
• Fluid mechanics is a division in applied
mechanics related to the behaviour of liquid or
gas which is either in rest or in motion.
• The study related to a fluid in rest or stationary
is referred to fluid static, otherwise it is referred
to as fluid dynamic.
• Fluid can be defined as a substance which can
deform continuously when being subjected to
shear stress at any magnitude. In other words,
it can flow continuously as a result of shearing
action. This includes any liquid or gas.

6. Fluid
A fluid is a substance, which deforms
continuously, or flows, when subjected to
shearing force
In fact if a shear stress is acting on a fluid
it will flow and if a fluid is at rest there is
no shear stress acting on it.
Fluid Flow Shear stress – Yes
Fluid Rest Shear stress – No

7. Fluid
• Thus, with exception to solids, any other
matters can be categorised as fluid. In
microscopic point of view, this concept
corresponds to loose or very loose bonding
between molecules of liquid or gas,
respectively.
• Examples of typical fluid used in
engineering applications are water, oil and
air.

8. Behavior of solid and fluid
Figure 1.1. Behavior of (a) solid and (b) fluid under the action of a constant shear force

9. Scope of fluid mechanics
Examples of areas of applications
include:
• Fluid and fluid-solid flows in process units
• Reactors
• Mechanical unit operations (e.g., settling,
sedimentation, filtration, fluidization, pneumatic
conveying)
• Thermal and mass transfer unit operations (e .g.,
heat exchangers, evaporators, drying, distillation,
absorption, extraction, leaching)
• Fluid and fluid-solid flows in pipelines
• Fluid machinery: pumps, compressors, blowers,
fans, turbines
• Fluid flow in humans (e.g. blood flow); design of
artificial organs and fluid flow in animals and plants
• Power generation and transmission
• Environmental fluid mechanics
• Lubrication
• Water handling for agriculture, drinking and washing
• Transportation means: automobiles, trains, aircraft,
ships, space flight systems: Rockets, Jet propulsions
• Buildings and structures
• Heating and ventilating systems
• Meteorology, oceanography, aeronautics, astronautics
• Plasma flows; magneto hydrodynamics

10. The Continuum Hypothesis/(The
Continuum Model)
• In most engineering applications, the interest is on
the average or means values of the fluid
properties over many molecules
• Instead of the actual aggregations of separate
molecules, we assume that the fluid is a
continuum
• We assume a continuous distribution of matter
with no empty space. This is called the continuum
hypothesis or the continuum model.
• Fluid properties are considered to be continuous
functions of position and time.

11. Fluid Properties
Density, specific weight &specific gravity

12. Pressure
The fluid pressure at a point is defined as the
normal force per unit area subject to the
continuum hypothesis.

13. Pressure

14. Bulk Modulus of Elasticity and Coefficient of
Compressibility
The degree of compressibility of a substance is characterized by its bulk modulus of
elasticity, β, which is defined as:

15. Bulk Modulus of Elasticity and Coefficient of Compressibility
For the solution of practical problems involving finite changes in
pressure and volume, the differentials may be replaced by finite
differences to calculate average values .
The reciprocal of the bulk modulus of elasticity is called the
coefficient of compressibility.
Liquids have very large values of β which means that
is very small.
liquids are essentially incompressible. In contrast, gases are
easily compressible.

16. Viscosity
• We have defined a fluid as a substance that deforms continuously
under the action of a shearing stress.
• Fluids in which the shear stress is directly proportional to the rate of
deformation are called Newtonian fluids.
– Most common fluids of simple structure such as water, air, and oils are
closely Newtonian under normal conditions.
• All fluids in which the shear stress is not directly proportional to the
rate of deformation are classified as non-Newtonian fluids.
– Examples of non-Newtonian fluids are toothpaste, paints, ink, mud, dough,
polymer melts and solutions, suspensions, emulsions, foams.

17. Non-Newtonian Fluid

18. Non-Newtonian Fluid
Shear stress versus shear rate for various fluids

19. Non-Newtonian Fluid
Viscosity versus shear rate for fluids in the above figure

20. Surface Tension
• The tension force per unit length of the surface is called the surface
tension.
• The unbalanced forces tend to drown the molecule at the surface.
This causes the liquid surface to seek a minimum possible area by
developing a force at the surface whose effect is to pull the surface
molecules upwards. This force per unit length is called the surface
tension
• Surface tension plays an important role in the formation droplets
(drops) and bubbles and in the rise or fall of a liquid in a capillary
tube.

21. Vapor Pressure
• If a liquid and its vapor coexist in equilibrium, the
vapor is called a saturated vapor, and the pressure
exerted by this saturated vapor is called the vapor
pressure, psat. Note: psat = f(T) for a given liquid,
increasing as temperature increases.
• Saturated means rate of vaporization = rate of
condensation

22. Other Thermodynamic Properties
• The thermodynamic properties closely interact with
the motion characteristics (velocity) of a fluid. The
three most important ones are:
1. Pressure
2. Density
3. Temperature

23. Classification of Fluid Motions
• There is no a universally accepted way of classifying fluid
motions. We may introduce some possible classifications on
the basis of commonly encountered flow behaviors.
• A fluid flow may be
– Steady or unsteady
– Uniform or non uniform
– Viscous or inviscid
– Compressible or incompressible
– Viscous one could be laminar or turbulent