lecture 01 segi

7/30/2019 Lecture 01 SEGi

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EAT 106: Thermodynamics & Fluid

Mechanics

Lecture 1

Introduction and basic concepts


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Chapter Overview

Units system.

System, state, state postulate, equilibrium,

and process.

Temperature, and temperature scale.

Pressure: absolute and gage pressure.

Manometer and barometer.


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Thermodynamics and energy

Thermodynamics science of energy,

macroscopic approach, classical

thermodynamics.

Energy is the ability to cause changes.

Thermodynamics: Heat => power (Greek).

Today, all aspect of energy and itstransformation.

Law: Conservation of energy.


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Conservation of energy:

Energy can change from one form to

another but the total amount of energy

remains constant, it cannot be created or

destroyed.Energy in (food)

5 units

Energy storage4.5 units

Energy out0.5 units


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The change of energy content of a body or any other

system is equal to the difference between the energy

input and the energy output.

Energy balance: Ein

Eout=E

Energy in (food)

5 units

Energy storage4.5 units

Energy out0.5 units


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Thermodynamics and energy - (1850s)

1st law of thermodynamics: conservation of energy,

energy is a thermodynamic property.

2nd

law of thermodynamics: energy has quality, andquantity, actual processes occurs in the direction of

decreasing quality of energy.

Heat out

Cool coffee in the same room never getshot by itself. The high temperature energy

of the coffee is degraded, as the energy is

transferred to the surrounding air.


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Area of application

The warming ofhuman body: a balancebetween metabolism and heat rejection.

Household: heating and air-conditioningsystems, refrigerator, pressure cooker, waterheater, computer.

Industry: rockets, jet engine, conventional ornuclear power plants, solar collectors, thedesign from cars to airplanes.


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Dimensions

For a given dimension such as length can be

expressed in a number of different units such as

meters, millimeters, or kilometers.

DIMENSIONS is different from a unit. The principle of dimensional homogeneity states

that all physical relations must be dimensionally

homogeneous.

Based on L, M, and T.

Force is a derived quantity in SI units: F = MLT-2


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Dimensions and Units

Four primary dimensions of mechanics:

Quantity Dimensional

Symbol

SI Units

Unit SymbolMass M Kilogram kg

Length L Meter m

Time T Second s

Force F Newton N

Derived dimension: velocity and acceleration: ms-1, and ms-2.


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Dimensions

One important use of the dimensional homogeneityprinciple is to check the dimensional correctness ofsome derived physical relation.

Dimensional homogeneity is a necessary condition forcorrectness of a physical relation, but is not sufficient.

It is possible to construct an equation which isdimensionally correct but does not represent a correctrelation.

2122

]][[]][[2

1 LTMLMLTmvxF


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Gravitation

Newtons law of gravitation:

F: the mutual force of attraction between two particles.

G: a universal constant called the constant of gravitation,

6.673(10-11) m3kg-1s-2.

m1, m2: the masses of the two particles r: the distance between the centers of the particles

2

21

r

mmGF


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Gravitation

Newtons law of gravitation:

2

2

2

21

81.9:

msgWhere

mgWeightmR

mGF

r

mmGF

particleparticleE

me: mass of earth, 5.976(1024) kg;R: 6.371(106) m


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Weight and Mass

The mass of a body remains the same

regardless of its location in the universe.

The weight changes with a change in

gravitational acceleration.

A body weighs less on a mountain asg

decreases, and it is 1/6lesser on the moon.

Vm


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System and Control Volumes

System is defined as a quantity of matteror a region in space chosen for study.

Boundary can be real or imaginary, fixedor moveable, zero thickness (no mass andvolume).

System

Surroundings

Boundary


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Closed system: mass is fixed, no mass can cross the

boundary; energy can as heat and work; the volume

does not has to be fixed.

Isolated system: closed system + no energy allow tocross the boundary.

Closed system

m = constant

Energy


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Piston and cylinder.

Gas, 2 kg, 1m3

Heating

Closed system

Boundary

Surrounding

Gas, 2 kg, 2m3

Closed system

Moving boundary


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Open system: both mass and energy can

cross the boundary of the system.

Water heater, turbine, compressor

involve mass flow, thus a open system.

Nozzle

Real Boundary

Imaginary

Boundary


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The boundaries of an open system are

called control surface.

Moving

boundary

Fixed

boundaryFlow in

Flow in

Flow out

Water Heater

How much

heat is

needed to

maintain asteady

stream of

hot water?

Control surface


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Properties of a System

Any characteristic of a system is called a property.

PressureP, temperature T, Volume V, and mass m.

Viscosity, thermal conductivity, modulus of elasticity,thermal expansion coefficient, electric resistively, velocityand elevation

Intensive properties: independent of massP, T,

density; uppercase. Extensive properties: depend on mass mass,

volume; lowercase. Specific propertis: v=V/m, e=E/m.


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Continuum

Matter is made up of atoms.

Size of the system deal is large relative to the

space between molecules.

Mean free path: distance travels before the nextcollision, O2 at 1 atm 20

oCis 6.3x10-8 m.

The substance can be viewed as continuous,

homogeneous matter with no holes. The density of water is the same at any point.


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Density and Specific Gravity

Density: mass per unit volume.

The reciprocal of density: specific volume.

Density is a function of temperature and

pressure. Liquids and solids are incompressible, thus only

depend on temperature.

Specific gravity: the ratio of the density of a

substance to the density of some standardsubstance at a specified temperature.

COH o

SG4,2


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Density and Specific Gravity

SG < 1, will float on water.

Specific weight: weight per unit volume.

g


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State and Equilibrium

If a system does not undergoing any

change, all the properties can be

measured and completely describe the

condition, or the state of the system.

At a given state, all the properties have

fixed values.

2kg, 20oC,

1.5m3

2kg, 20oC,

2.5m3

State 1 State 2


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Thermodynamics deals equilibrium state.

Equilibrium implies a state of balance.

There are many types of equilibrium. A system is not in

thermodynamic equilibrium unless the conditions of all the relevanttypes of equilibrium are satisfied.

Thermal equilibrium: temperature are the same throughout the entiresystem.

Mechanical equilibrium: pressure are the same.

Phase equilibrium: the mass of each phase reach equilibrium andwill not change.

Chemical equilibrium: chemical composition will not change withtime.


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The state of a system is described by itsproperties.

We do not need to specify all the properties to fixa state.

Once a sufficient number of properties arespecified, the rest of the properties assumecertain value automatically.

The number of properties required to fix the

state is given by the state postulate. A simple compressible system: temperature, and

specific volume. Thus 2 independent intensityproperties.


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Processes and Cycles

Any change that a system undergoes from one

equilibrium state to another is called a process.

The series of state a system passes during the

process is called the path.

State 1

State 2

Process path

Property A

Property B


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Process proceed in a manner that the system remain infinitesimallyclose to an equilibrium state at all the times, quasi-static, orquasi-equilibriumprocess.

A sufficiently slow process that allows the system to adjust itself

internally so that properties in one part of the system do not changeany faster than those at other parts.

Slow compression

Fast compression

Uniform pressure, and

raise at same rate

Non-uniform

pressure


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Final state

Initial state

P

V

System

Quasi-equilibriumprocess

Non-quasi-equilibrium process

Isothermal process: aprocess during whichtemperature remainsconstant.

Isobaric process:

pressure remainsconstant.

Isochoric process:specific volume remainsconstant.

A system is said to have undergone a cycle if it returns to its initial stateat the end of the process. The initial and final states are identical.


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The Steady-Flow Process

Steady: no change with time.

Thus, the opposite is unsteady and transient.

Uniform: no change with location over a specified region.

Under steady-flow conditions, the mass and energy contents of a control

volume remain constant.

Mass in

Mass out

300oC 250oC

200oC 150oC

Time: 1 pm

Mass in

Mass out

300oC 250oC

200oC 150oC

Time: 3 pm


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Temperature and the Zeroth Law of Thermodynamics

Zeroth law of thermodynamics (Fowler,

1931): if two bodies are in thermal

equilibrium with a third body, they are also

in thermal equilibrium with each other.

The temperature are the same.

K (Kelvin), SI temperature unit.

T(K) = T(oC) + 273.15

T(K) =T(oC)


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Pressure

Pressure: normal force exerted by a fluid

per unit area.

When dealing with gas or liquid.

For solid: normal stress.

Unit: 1 N/m2 = 1 Pascal, Pa.

1 bar = 105

PaP = F/A


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Pressure

Absolute pressure, is a measure relative to

absolute vacuum (i.e. absolution zero pressure).

Most pressure-measuring device are calibrated

to read zero in the atmospheric pressure thegage pressure.

Pressure below atmospheric pressure is called

vacuum pressure.

Absolute, gage and vacuum pressures are all

positive quantities.


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Pressure

Pgage = PabsPatm

Pvac = PatmPabs

In thermodynamics relations and tables,absolute pressure is almost always used.

Pwill denote absolute pressure unless

specified otherwise.


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Pressure

Patm

Patm Patm

Pabs Pabs

Pvac

Pgage

Pabs= 0Absolute vacuum

Pressure is a compressive force

per unit area, scalar quantity.

Pressure at any point in a fluid is

the same in all directions.


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Example

A vacuum gage connected to a chamber

reads 40 kPa at a location where the

atmospheric pressure is 100 kPa.

Determine the absolute pressure in thechamber.

Pabs = PatmPvac = 100 40 = 60 kPa


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Variation of Pressure with Depth

Pgage The pressure in a fluid at rest does not

change in the horizontal direction.

The pressure in a fluid at rest increases

with depth as a result of added weight.


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z

x

W

P1

P2

0 x

z

Unit depth (into the page), y = 1

Assuming the density of fluid, = constant

The force balance in thezdirection,

zPPP

zgPPP

zgPP

zxgxPxPmaF

s

zz

12

12

12

12

0

0

:0

s is the specific weight.

Pressure head

Free body diagram of a rectangular

fluid element in equilibrium


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1

h

P1 = Patm

P2 = Patm+gh

Liquids are incompressible, the variation ofdensity of negligible.

For small elevation change the gas densitycan be neglected.

Temperature will change with fluids.

For ocean and large elevation,

2

112 gdzPPP

2


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The Basic Barometer

h

The measure the atmospheric pressure.

Patm = gh, Hg = 13595 kgm-3

h = 760 mm = 1 atm, standard atmosphere.

At 0oC and g = 9.807 ms-2,W

PaP

ghP

APghAW

atm

atm

atm

9.101327

Patm


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Example: Manometer

A manometer is used to measure the pressure in a tank. The fluid used has

a specific gravity of 0.85, and the manometer column height is 55 cm, as

shown in the figure. If the local atmospheric pressure is 96 kPa, determine

the absolute pressure within the tank.

h = 55cm

Patm = 96 kPa

SG = 0.85

P = ?

Assumption:

The fluid in the tank is a gas whose density is

much lower than the density of manometer fluid

The gravitational effect of the gas is negligible,

the pressure anywhere in the tank and the

position 1 are the same.

kPaP

P

ghPPkgmSG

atm

OH

6.100

)55.0)(81.9(85096

850100085.0

3

2

1

2

3


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Manometer: Pressure Drop Measurement

ghPP

ghghPP

gaghPhagP

PP BA

1221

1221

12211

h

Fluid

1 2

A flow section or flow device

1

2

AB

a

Horizontal device.

Valve, heat exchanger, or any

resistance to flow.

a has not effect on the result.

If1


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Example: Effect of piston weight on pressure in a cylinder

The piston of a vertical piston-cylinder device containing a gas has mass of

60 kg and a cross-sectional area of 0.04m2. The local atmospheric pressure

is 0.97 bar, and the gravitational acceleration is 9.81ms-2. (a) Determine the

pressure inside the cylinder. (b) If some heat is transferred to the gas and

its volume is doubled, do you expect the pressure inside the cylinder to

change?

A = 0.04m2

Patm = 0.97 bar

m = 60 kg

Assumption: Friction between the

piston and the cylinder is negligible.Patm

W =mg barP

P

A

W

PP

WAPPA

atm

atm

12.1

1004.0

81.96097.0

5


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Example: Effect of piston weight on pressure in a cylinder

The piston of a vertical piston-cylinder device containing a gas has mass of

60 kg and a cross-sectional area of 0.04m2. The local atmospheric pressure

is 0.97 bar, and the gravitational acceleration is 9.81ms-2. (a) Determine the

pressure inside the cylinder. (b) If some heat is transferred to the gas and

its volume is doubled, do you expect the pressure inside the cylinder to

change?

The volume change will have no effect on the free-body diagram,

therefore the pressure inside the cylinder will remain the same.

If the gas behaves as an ideal gas, the absolute temperature doubles

when the volume is doubled at constant pressure.

lecture 01 segi

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