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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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Thermodynamics and energy
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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Thermodynamics and energy
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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System and Control Volumes
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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System and Control Volumes
Piston and cylinder.
Gas, 2 kg, 1m3
Heating
Closed system
Boundary
Surrounding
Gas, 2 kg, 2m3
Closed system
Moving boundary
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System and Control Volumes
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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System and Control Volumes
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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State and Equilibrium
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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State and Equilibrium
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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Processes and Cycles
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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Processes and Cycles
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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Variation of Pressure with Depth
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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Variation of Pressure with Depth
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.