Fluid Properties

FLUID PROPERTIES

FLUID PROPERTIES

1.1FLUID CHARACTERISTICS

Fluid Mechanics is a section of applied mechanics, concerned with the static and dynamics of liquids and gases.

Knowledge of fluid mechanics is essential for the chemical engineer, because the majority of chemical processing operations are conducted either partially or totally in the fluid phase.

The handling of liquids is much simpler, cheaper, and less troublesome than handling solids.

Even in many operations a solid is handled in a finely divided state so that it stays in suspension in a fluid.

1.1.1 Fluids

In everyday life, we recognize three states of matter: solid, liquid and gas. Although different in many respects, liquids and gases have a common characteristic in which they differ from solids. Both are fluids, but lacks the ability of solids to offer a permanent resistance to a deforming force.

A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be. Conversely, if a fluid is at rest, there can be no shearing forces acting and, therefore, all forces in the fluid must be perpendicular to the planes upon which they act.

Fluid (Bendalir) adalah bahan yang sentiasa berubah bentuk (aliran) di bawah tegasan ricih yang digunakan. Bendalir adalah termasuk cecair dan gas. Bendalir ialah sesuatu yang boleh mengalir di mana partikel-partikel yang terkandung di dalamnya boleh berubah kedudukan diantara satu sama lain. Bendalir mengambil bentuk bekas yang mengandunginya dan terbahagi kepada dua bentuk, iaitu cecair dan gas.

Di antara sifat-sifat penting bendalir ialah :

Ketumpatan, Berat tentu, isipadu tentu, ketumpatan bandingan, kebolehmampatan, kelikatan,rerambut , tekanan permukaan dan tekanan wap.

1.1.2 Fluid Terms

Solid (Pepejal) - zarahnya mempunyai tenaga kinetik yang sedikit dan kekal dalam pembentukan tetap, hanya bergetar. lukiskan satu kotak segi empat tepat dan isi dengan bulatan kecil dalam lajur dan baris - padat.
Liquid (Cecair) – zarah2nya mempunyai lebih banyak tenaga kinetik dan bergerak sedikit, pembentukan biasa yang wujud dalam suatu pepejal yang hilang. lukiskan satu kotak segi empat tepat dengan bulatan kecil tetapi setiap bulatan dijarakkan lebih daripada kotak pepejal, tidak mengikut pembentukan baris atau lajur, hanya dalam susunan rawak.
Gas – zarah2nya mempunyai tenaga kinetik yang kuat dan boleh bergerak bebas dan lebih dijarakkan walaupun daripada cecair. lukiskan satu kotak segi empat tepat dengan bulatan lebih kurang daripada kotak cecair, dengan keluar bulatan masing-masing lebih jaraknya.

1.1.3 Compare the characteristics between liquid, gas and solid

Bandingan ciri-ciri antara gas & cecair

Liquid / Gases
Sukar dimampat/Mampatan terlalu kecil / Mudah dimampat
Mengambil bentuk bekas yang mengandunginya. / Sentiasa mengembang dan memenuhi ruang bekas.
Permukaan bebas terbentuk / Tidak mempunyai permukaan bebas

1.2TYPES OF PRESSURE GAUGE

Fluid will exert a normal force on any boundary it is in contact with. Since these boundaries may be large and the force may differ from place to place it is convenient to work in terms of pressure, p, which is the force per unit area.

Units : Newton’s per square metre, Nm-2,kgm-1s-2.

(The same unit is also known as Pascal, Pa i.e 1 Pa = 1Nm-2)

Also frequently used is the alternative SI unit the bar,

where 1 bar = 105Nm-2

Absolute Pressure, Gauge Pressure, and Vacuum

In a region such as outer space, which is virtually void of gases, the pressure is essentially zero. Such a condition can be approached very nearly in a laboratory when a vacuum pump is used to evacuate a bottle. The pressure in a vacuum is called absolute zero, and all pressures referenced with respect to this zero pressure are termed absolute pressures.

Many pressure-measuring devices measure not absolute pressure but only difference in pressure. For example, a Bourdon-tube gage indicates only the difference between the pressure in the fluid to which it is tapped and the pressure in the atmosphere. In this case, then, the reference pressure is actually the atmospheric pressure. This type of pressure reading is called gauge pressure. For example, if a pressure of 50 kPa is measured with a gauge referenced to the atmosphere and the atmospheric pressure is 100 kPa, then the pressure can be expressed as either p = 50 kPa gauge or p = 150 kPa absolute.

Whenever atmospheric pressure is used as a reference, the possibility exists that the pressure thus measured can be either positive or negative. Negative gauge pressure is also termed as vacuum pressure. Hence, if a gauge tapped into a tank indicates a vacuum pressure of 31 kPa, this can also be stated as 70 kPa absolute, or -31 kPa gauge, assuming that the atmospheric pressure is 101 kPa .

a) Atmospheric Pressure, Patm or Po

- The earth is surrounded by an atmosphere many miles high. The pressure due to this atmosphere at the surface of the earth depends upon the head of the air above the surface.

- The air is compressible, therefore the density is different at different height.

- Due to the weight of atmosphere or air above the surface of earth, it is difficult to calculate the atmospheric pressure. So, atmospheric pressure is measured by the height of column of liquid that it can support.

- Atmospheric pressure at sea level is about 101.325 kN/m2, which is equivalent to a head of 10.35 m of water or 760 mm of mercury approximately, and it decreases with altitude.

b) Gauge Pressure, pG

- It is the pressure, measured with the help of a pressure measuring instrument, in which the atmospheric pressure is taken as datum; in other words the atmospheric pressure at the gauge scale is marked zero.

- The gauge pressure can be either positive or negative depending on whether the pressure is above atmospheric pressure (a positive value) or below atmospheric pressure (a negative value).

c) Absolute Pressure, pA

- It is the pressure equal to the algebraic sum of the atmospheric and gauge pressures.

d) Vacuum, pv

- In a perfect vacuum which is a completely empty space, the pressure is zero.

1.2.2Solve the problems

1) Define the following terms :

Pressure (p )
Atmospheric Pressure ( patm)
Gauge Pressure ( pG )
Absolute Pressure ( pA )
Vacuum ( pv )

Solution :

Pressure ( p ):

Pressure is force ( F ) per unit area ( A ).

Atmospheric Pressure ( patm) :

The pressure due to atmosphere at the surface of the earth depends upon the head of the air above the surface.

Gauge Pressure ( pG ) :

It is the pressure, measured with the help of a pressure measuring instrument, in which the atmospheric pressure is taken as datum.

Absolute Pressure ( pA )

It is the pressure that equals to the algebraic sum of the atmospheric and gauge pressures.

Vacuum ( pv )

A completely empty space where the pressure is zero.

2) What is the pressure gauge of air in the cylinder if the atmospheric gauge is 101.3 kN/m2 and absolute pressure is 460 kN/m2.

Solution :

pA = 460 kN/m2

patm = 101.3 kN/m2

pG = ?

With reference to the formula below :

Absolute pressure, pA = Gauge pressure, pG + atmospheric pressure, patm

Therefore ,

pG = pA – patm

= 460 – 101.3

ACTIVITY 1A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

1.1 Label the diagram below :

1.2 A Bourdon pressure gauge attached to a boiler located at sea level shows a reading pressure of 7 bar. If atmospheric pressure is 1.013 bar, what is the absolute pressure in that boiler (in kN/m2) ?

FEEDBACK ON ACTIVITY 1A

1.1

1.2

pA = ?

patm = 1.013 bar

pG = 7 bar

With reference to the formula below :

Absolute pressure,pA = Gauge pressure,pG + atmospheric pressure,patm

Therefore ,

pA = pG + patm

= 7 x 105 + 1.013 x 105

=801300 N/m2

1.3PHYSICAL PROPERTIES OF FLUID

Fluid properties are intimately related to fluid behaviour. It is obvious that different fluids can have grossly different characteristics. For example, gases are light and compressible, whereas liquids are heavy and relatively incompressible.

To quantify the fluid behaviour differences certain fluid properties are used. The fluid properties are mass density, specific weight, specific gravity, specific volume and viscosity.

1.3.1Mass density, ρ is defined as the mass per unit volume.

( SI units, kg/m3 )

1.3.2 Specific weight,  is defined as the weight per unit volume.

( SI units, N/m3 )

(where g = 9.81m/sec2)

In SI units the specific weight of water is 9.81 x 1000 = 9810 N/m3

1.3.3Specific gravity or relative density, s is the ratio of the weight of the substance to the weight of an equal volume of water at 4 ºC.

1.3.4Specific volume, v is defined as the reciprocal of mass density. It is used to mean volume per unit mass. (SI units, m3/kg ).

1.3.5Viscosity

A fluid at rest cannot resist shearing forces but once it is in motion, shearing forces are set up between layers of fluid moving at different velocities. The viscosity of the fluid determines the ability of the fluid in resisting these shearing stresses.

1)What is the mass density, ρ of fluid (in kg/m3) if mass is 450 g and the volume is 9 cm3.

Solution :

2)What is the specific weight,  of fluid (in kN/m3) if the weight of fluid is 10N and the volume is 500 cm2.

Solution :

= 20 000 N/m3

3)What is the specific gravity of fluid in Example 1.10.

Solution :

4)What is the specific volume, v of fluid in Example 1.9.

Solution :

ACTIVITY 1B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

1.1Match the following

FEEDBACK ON ACTIVITY 1B

1.1

SELF-ASSESSMENT

1.1Assume the density of water to be 1000 kg/m3 at atmospheric pressure 101 kN/m2. What will be:

a) the gauge pressure

b) the absolute pressure of water at a depth of 2000 m below the free surface?

1.2Determine in Newton per square metre, the increase in pressure intensity per metre depth in fresh water. The mass density of fresh water is 1000 kg/m3.

1.3Given specific weight of fluid is 6.54 kN/m3 and its mass is 8.3 kg, calculate the following:

a) volume of fluid

b) specific volume of fluid

c)density of fluid

1.4Given oil specific gravity is 0.89, find :

a)density of oil

b)specific weight of oil

c) specific volume of oil

Feedback ON Self-Assessment

Answers :

1.1a) 117.72 kN/m2 ,

b) 218.72 kN/m2

1.29.81 x 103 N/m2

1.3a) 0.072 m3

b) 0.0015 m3/kg

c) 691.67 kg/m3

1.4 a) 0.89 x 103 kg/m3

b) 8730.9 N/m3

c) 0.00112 m3/kg