Objectives
MAE 320- Chapter 1
Identify the unique vocabulary associated with thermodynamics
through the precise definition of basic concepts.
Review the metric SI and the English unit systems.
Introduction and Basic Concepts
Explain the basic concepts of thermodynamics such as system,
state, state postulate, equilibrium, process, and cycle.
Review concepts of temperature, temperature scales
Review concepts of pressure, absolutepresuure and gage
pressure.
The content and the pictures are from the text book: Çengel, Y. A. and Boles, M. A., “Thermodynamics:
An Engineering Approach,” McGraw-Hill, New York, 6th Ed., 2008
Thermodynamics and Energy
Thermodynamics and Energy
What topics are covered in this course?
• The name thermodynamics stems from the Greek
words therme (heat) and dynamis (power).
• Thermodynamics: The science of energy.
• Energy: The ability to cause changes.
What is the difference between thermodynamics and kinetics?
Thermodynamics and Energy
• Storage of energy
- internal energy, (T)
- potential energy, (gravity, h)
- kinetic energy (motion, V)
- chemical energy (reaction)
• Transfer of energy
- from one form to another form
- from one state to another state
• First law of thermodynamics
- Energy cannot be created
- Energy cannot be destroyed
- Energy can be transferred
• Second law of thermodynamics
- quantity
- quality
- direction
• Entropy
• Substances
- H2O (ice, water, vapor)
- ideal gas
Application Areas of Thermodynamics
Conservation of energy
principle: During an
interaction, energy can
change from one form to
another but the total amount
of energy
gy remains constant.
Heat Pump
How to save energy bills?
Energy cannot be created or
destroyed.
Which one do you choose between a
heat pump and an electric heater?
How does a heat pump work?
What is the efficiency of the heat pump?
Electric heater
1
Application Areas of Thermodynamics
Application Areas of Thermodynamics
Hybrid car
Conventional car
Double-layer glass window
ƒ What is the efficiency of the
hot water tank?
ƒ What is the optimal
temperature for the water tank?
ƒ What is minimum tank
capacity needed for your
house?
braking
Pad-drum friction
braking
Electric generator
Heat dissipation
battery
electric motor
accelerating
Mechanical energy
converted to heat
Application Areas of Thermodynamics
How Hybrid car works
http://www.youtube.com/watch?v=2HrQeGzI
ZoI&feature=related
Hybrid SUV-3D animation
http://tw.youtube.com/watch?v=Fk7xbcnzxks
Mechanical energy converted to
electric energy, converted to
mechanical energy
Concepts of Thermodynamics
• The second law of thermodynamics:
It asserts that energy has quality as
well as quantity, and actual processes
occur in the direction of decreasing
quality of energy.
• Classical thermodynamics: A
macroscopic approach to the study of
thermodynamics that does not require
a knowledge
o edge o
of the
e be
behavior
a o o
of
individual particles.
Conservation of energy
gy
principle for the human body.
• It provides a direct and easy way to the
solution of engineering problems and it
is used in this text.
• Statistical thermodynamics: A
microscopic approach, based on the
average behavior of large groups of
individual particles.
• It is used in this text only in the
supporting role.
Dimensions and Units
Heat flows in the direction
of decreasing temperature.
Review SI units
• Any physical quantity can be characterized by dimensions.
• The magnitudes assigned to the dimensions are called units.
SI Primary units (base units)
• Some basic dimensions such as mass m, length L, time t, and temperature
T are selected as primary or fundamental dimensions,
Quantity
Name of Unit
Symbol
• velocity V, energy E, and volume V are expressed in terms of the primary
dimensions and are called secondary dimensions, or derived
dimensions.
Length (l)
Mass (m)
meter
kilogram
m
kg
Time (t)
Electrical current (I)
second
ampere
s
A
Thermodynamic
temperature (T)
Kelvin
K
Amount of substance mole
mol
2
Dimensions and Units
• English system: It has no apparent systematic numerical
base, and various units in this system are related to each
other rather arbitrarily.
SI unit system will be adopted in our course:
• SI system has a systematic numerical base (clear physical meaning)
meaning).
Density and Specific Gravity
Density is mass per unit volume:
Density:
specific volume is volume per unit mass:
Specific volume:
(m3/kg)
• SI system is a decimal system (x10).
• Better for communication. All the industrial country use the SI system.
Only USA is employing a dual-unit system. However, all the car
manufactures use the SI system. You need a (wrench) kit for metric bolts
instead English sockets
• Easy to remember
Density and Specific Gravity
Specific gravity: The ratio of the density of a substance to the density of
some standard substance at a specified temperature (usually water at 4°C).
Specific weight: The weight of a
unit volume off a substance.
mg m
γS =
= ⋅ g = ρg
V
V
Dimension and Units
In daily life, people usually use weight to express mass.
In science, weight is a force:
W = mg
Where m is the mass, g is the local gravitational acceleration,
g=9.807 m/s2 or g =32.174 ft/s2 at sea level and 45o latitude.
Dimension and Units
In SI, one newton (1 N) is the force required to cause a mass of one kilogram
(1 Kg) to accelerate at a rate of one meter per second squared (1 m/s2)
In the English system, the force unit, 1 lbf, is the force required to cause a
mass of 32.174 lbm to accelerate at a rate of one foot per second squared (1
ft/s2)
Unit Conversion Ratios
All equations must be dimensionally homogeneous.
To be dimensionally homogeneous, all the terms in an equation must have
the same unit system.
All non-primary units (secondary units) can be formed by combinations of
primary units. Force units, for example, can be expressed as
On the earth, at sea level, a mass of 1 kg weighs 9.8 N
W = mg = 1 kg x 9.8 m/s2 = 9.8 kg.m/s2 = 9.8 N
They can also be expressed more conveniently as unity conversion ratios
as
On the moon, a mass of 1 kg weighs 1.63 N
W = mg = 1 kg x 1.63 m/s2 = 1.63 kg.m/s2 = 1.63 N
Mass does not change with location, but weight does
Unity conversion ratios are identically equal to 1 and are unitless, and
thus such ratios (or their inverses) can be inserted conveniently into any
calculation to properly convert units.
3
Dimension and Units
Dimension and Units
How much is the weight of a mass of 1 lbm?
Work, a form of energy:
Work (W) = Force × Distance
Apply Newton’s second law:
1 J = 1 N·m
1 cal = 4.1868 J
1 Btu = 1.0551
1 0551 kJ
In SI system:
1 lbm= 0.454 kg and g =9.8 m/s2
power = W/t
Unit: J/s or watt, w
W = mg = 0.454 kg x 9.8 m/s2 = 4.45 kg.m/s2 = 4.45 N
A mass of 1 lbm weighs 4.45 N
specific energy (e), e=E/m
Review SI units – Derived Units
Quantity
Description
Symbol
(J/kg)
Review SI units – Derived Units
Expression in
other SI
units
Expression in SI
base units
Area (A)
square meter
m2
m2
Volume (V)
cubic meter
m3
m3
velocity (or speed) (V)
meters per second
m/s
m s-1
Symbol
Weight (W), W=mg
Newton
N
kg•m/s² m kg s-2
pressure (P), P=F/A
pascal
Pa
N/m2
Acceleration (a)
meter per second
d
squared
m/s2
m s-2
density (ρ), ρ=m/V
kilogram per cubic
meter
kg/m3
kg m-3
specific energy (e), e=E/m joule per kilogram
specific volume (v),
v=V/m
cubic meter per
kilogram
m3/kg
m3 kg-1
specific gravity (SG),
SG=ρ/ρH2O
Dimensionless
force (F), F=ma
Newton
kg•m/s²
Expression in
terms of SI base
units
Description
work ((energy
gy heat)) (W),
( )
W=F*d
N
Expression
in other SI
units
Quantity
Joule
J
N⋅m
m-1 kg s-2
m2 kg s-22
J/kg
m2 s-2
m-1 kg s-2
energy density
joule per cubic
meter
J/m3
power
Watt,
W
J/s
m2 kg s-3
m kg s-2
Continuum
Systems and Control Volumes
• Matter is made up of atoms that
are widely spaced in the gas
phase. disregard the atomic
nature of a substance and view it
as a continuous, homogeneous
matter with no holes, that is, a
continuum.
• System: A quantity of matter or a region in space chosen for study.
• Surroundings: The mass or region outside the system
• Boundary: The real or imaginary surface that separates the system
from its surroundings.
• The boundary of a system can be fixed or movable.
• Systems may be considered to be closed or open.
• The continuum idealization allows
us to treat properties as point
functions and to assume the
properties vary continually in
space with no jump
discontinuities.
• This idealization is valid as long
as the size of the system is large
relative to the space between the
molecules.
Despite the large gaps between
molecules, a substance can be
treated as a continuum because of
the very large number of molecules
even in an extremely small volume.
• In this course we will limit our
consideration to continuum.
4
Systems and Control Volumes
Coke Can
• Closed system (Control mass): A fixed amount of mass, and no
mass can cross its boundary.
• Isolated system: in such a close system, even energy is not allowed
to cross the boundary
• Demonstrate
Concept of system,
boundary and
surroundings
• Demonstrate
closed system
• Demonstrate open
system
Properties of System
Systems and Control Volumes
• Open system (control volume): A properly
selected region in space.
• It usually encloses a device that involves mass
flow such as a compressor, turbine, or nozzle.
• Both mass and energy can cross the boundary of
a control volume.
• Control surface: The boundaries of a control
volume. It can be real or imaginary.
• Property: Any characteristic of a system.
• Some familiar properties are pressure P,
temperature T, volume V, and mass m.
• Properties are considered to be either
intensive or extensive.
• Intensive properties: Those that are
independent of the mass of a system,
such as temperature, pressure, and
density.
density
• Extensive properties: Those whose
values depend on the size or extent of the
system.
• Specific properties: Extensive
properties per unit mass.
In an open system, the
volume can vary with time!
An open system (a
control volume) with
one inlet and one exit.
State and Equilibrium
Thermodynamics deals with equilibrium states.
• Equilibrium: A state of balance. there are no unbalanced
potentials (or driving forces) within the system.
• Thermal equilibrium: If the temperature is the same
throughout the entire system.
Criterion to differentiate intensive
and extensive properties.
The Zeroth Law of Thermodynamics
• The zeroth law of thermodynamics: If two bodies are in thermal
equilibrium with a third body, they are also in thermal equilibrium with
each other.
• By replacing the third body with a thermometer, the zeroth law can be
restated as two bodies are in thermal equilibrium if both have the same
temperature reading even if they are not in contact.
Two bodies reaching
thermal equilibrium
after being brought
into contact in an
isolated enclosure.
A closed system reaching thermal equilibrium.
5
State and Equilibrium
Thermodynamics deals with equilibrium
states.
• Mechanical equilibrium: If there is
no change in pressure at any point of
the system with time.
• Chemical equilibrium: If the
chemical composition of a system
does not change with time, that is, no
chemical
h i l reactions
ti
occur.
State and Equilibrium
• Phase equilibrium: If a
system involves two
phases and when the
mass of each phase
reaches an equilibrium
level and stays there.
State: a certain condition
that can be completely
described a set of properties
a phase is a region of space (a
thermodynamic
h
d
i system),
)
throughout which all physical
properties of a material are
essentially uniform.
A system at two different states.
A closed system reaching thermal equilibrium.
The State Postulate
Processes and Cycles
Process: Any change that a system undergoes from one equilibrium state to
another.
Path: The series of states through which a system passes during a process.
To describe a process completely, one should specify the initial and final
states, as well as the path it follows, and the interactions with the
surroundings.
• The number of properties
required to fix the state of a
system is given by the state
postulate:
9 The state of a simple
compressible system is
completely specified by two
independent, intensive
properties.
• Simple compressible system: If
a system involves no electrical,
magnetic, gravitational, motion,
and surface tension effects.
The state of nitrogen is
fixed by two independent,
intensive properties.
• Process diagrams plotted with
thermodynamic properties as coordinates
are use to visualize the processes.
• Some common properties as coordinates
are temperature T, pressure P, and
volume V (or specific volume v).
• The prefix iso- is often used to designate
a process for which a particular property
remains constant.
• Cycle: A process during which the initial
and final states are identical.
Processes and Cycles
Processes and Cycles
• Isothermal process: A
process during which the
temperature T remains
constant.
Quasi-static or quasi-equilibrium process:
When a process proceeds in such a manner that the system
remains infinitesimally close to an equilibrium state at all
times.
• Isobaric process: A process
during which the pressure P
remains
e a s co
constant.
sta t
• Isochoric (or isometric)
process: A process during
which the specific volume v
remains constant.
The P-V diagram of a compression
process.
6
Temperature Scales
The Steady-Flow Process
• Ice point: A mixture of ice and water that is in equilibrium with air
saturated with vapor at 1 atm pressure (0°C or 32°F).
• Steam point: A mixture of liquid water and water vapor (with no air) in
equilibrium at 1 atm pressure (100°C or 212°F).
• Celsius scale: in SI unit system, ice point=0°C, steam point=100°C
• The term steady implies no change with
time. The opposite of steady is unsteady,
or transient.
During a steady-flow process, fluid properties
within the control volume may change with
position but not with time.
• Fahrenheit scale: in English unit system, ice point= 32°F, steam point
212°F.
T (oF) =1.8 T(oC) +32
• Rankine scale:
T (R) = T(oF) +459.67
Under steady-flow conditions, the mass and
energy contents of a control volume remain
constant.
• Steady-flow process: A process during
which a fluid flows through a control
volume steadily.
• Steady-flow conditions can be closely
approximated by devices that are intended
for continuous operation such as turbines,
pumps, boilers, condensers, and heat
exchangers or power plants or
refrigeration systems.
• Thermodynamic temperature scale: A temperature scale that is
independent of the properties of any substance.
• Thermodynamic temperature scale is the Kelvin scale (SI);
T (K) = T (°C) + 273.15 (K)
Temperature Scales
Temperature Scales
Thermodynamic temperature scale
The International temperature
Scale of 1990 (ITS-90):
Tools for temperature measurement:
Ice point: 0 oC (273.15 K)
• Alcohol Thermometer
Steam point: 99.975 oC
• Thermocouple
Atomic-resolution STM image
of reconstructed Si(111)(7×7), Frame size: 31nm
STM (scanning tunneling microscope)
Temperature arises from the random
submicroscopic vibrations of the
particle constituents of matter. These
motions comprise the kinetic energy in
a substance
• Infrared sensor
0 K = - 273.15 oC
Thermodynamic temperature at 0
K, absolute zero, is the
temperature at which the particle
constituents of matter are as close
as possible to complete rest
Pressure
Gage Pressure and Vacuum Pressure
Pressure: A compressive force per unit area
• Absolute pressure: The actual pressure at a given position. It is
measured relative to absolute vacuum (i.e., absolute zero pressure).
• Gage pressure: The difference between the absolute pressure and the
local atmospheric pressure. Most pressure-measuring devices are
calibrated to read zero in the atmosphere, and so they indicate gage
pressure.
• Vacuum pressure: Pressures below atmospheric pressure.
68 kg
136 kg
Afeet=300cm2
0.23 kgf/cm2
0.46 kgf/cm2
P=68/300=0.23 kgf/cm2
Some basic
pressure gages.
The normal stress (or “pressure”)
on the feet of a chubby person is
much greater than on the feet of a
slim person.
Throughout
this text, the
pressure P
will denote
absolute
pressure
unless
specified
otherwise.
7
Gauge Pressure and Vacuum Pressure
Pascal’s Law
A pressure gauge connected to a tank displays 75 kPa in Morgantown (the
atmosphere pressure is 100 kPa in Morgantown WV). After the tank was
moved to Denver, CO, the pressure gauge shows a pressure of 92 kPa.
What is the atmosphere pressure in Denver?
Pascal’s law: The pressure applied to a confined fluid increases the
pressure throughout by the same amount.
75 kPa
Pgauge = Pabs -Patm
Pgauge, morgantown = Pabs –Patm, morgantown
Pabs
75 KPa = Pabs – 100 kPa
100 kPa in
Morgantown
The area ratio A2/A1 is called the
ideal mechanical advantage of the
hydraulic lift.
Pabs = 175 kPa
92 kPa
Pgauge, denver = Pabs –Patm, denver
Pabs
92 kPa = 175 kPa – Patm, denver
??? kPa in
Denver
Patm, denver = 83 kPa
Variation of Pressure with Depth
∑F
z
= 0:
Lifting of a large weight by a
small force by the application
of Pascal’s law.
Pascal’s Law
P2 ΔxΔy − P1ΔxΔy − mg = 0
P2 ΔxΔy − P1ΔxΔy − ρΔxΔyΔxΔzg = 0
P2 − P1 = ρgh
P2 − P1 − ρΔzg = 0
In a room filled with a
gas, the variation of
pressure with height is
negligible.
Δy
Δx
Δz
If take Point 1 at the free surface of the liquid
Where P1=Patm.
(1). Pressure in a liquid at rest is independent
of the shape or cross section of the container.
(2). It changes with the vertical distance by
remains in other directions.
Free-body diagram of a
rectangular fluid element in
equilibrium.
The pressure is the
same at all points on
a horizontal plane in
a given fluid
regardless of
geometry, provided
that the points are
interconnected by
the same fluid.
Pascal’s Law
The Manometer
It is commonly used to measure small and moderate pressure differences.
A manometer contains one or more fluids such as mercury, water, alcohol,
or oil.
Patm
P2 = Patm + ρgh
Pgas = P1 = P2
Pgas = P1 = P2 = Patm + ρgh
Pgas
P1
The basic manometer
P2
The gage pressure measured by
manometer:
Pgage = ρgh
In stacked-up fluid layers, the pressure change across a fluid layer of
density ρ and height h is ρgh.
8
The Manometer
The Manometer
Measuring the pressure drop across a
flow section or a flow device by a
differential manometer.
PA= PB
P1 + ρ1g(a+h)= P2 + ρ1ga + ρ2gh
PA
PB
A
B
C
Alternatively
D
F
Measure pressure with multi-fluid manometer
Other Pressure Measurement Devices
•
Bourdon tube: Consists of a hollow metal tube
bent like a hook whose end is closed and
connected to a dial indicator needle.
•
Pressure transducers: Use various techniques to
convert the pressure effect to an electrical effect
such as a change in voltage, resistance, or
capacitance.
The Barometer and Atmospheric Pressure
• Atmospheric pressure is measured by a device called a barometer; thus,
the atmospheric pressure is often referred to as the barometric pressure.
• A frequently used pressure unit is the standard atmosphere, which is
defined as the pressure produced by a column of mercury 760 mm in
height at 0°C (ρHg = 13,595 kg/m3) under standard gravitational
acceleration (g = 9.807 m/s2).
Pressure transducers are smaller and faster, and
they can be more sensitive, reliable, and precise
th their
than
th i mechanical
h i l counterparts.
t
t
•
Strain-gage pressure transducers: Work by
having a diaphragm deflect between two
chambers open to the pressure inputs.
•
Piezoelectric transducers: Also called solid-state
pressure transducers, work on the principle that an
electric potential is generated in a crystalline
substance when it is subjected to mechanical
pressure.
The length or the
cross-sectional area
of the tube has no
effect on the height of
the fluid column of a
barometer, provided
that the tube diameter
is large enough to
avoid surface tension
(capillary) effects.
Various types of Bourdon tubes
used to measure pressure.
The basic barometer.
Dimension and Units
The Barometer and Atmospheric Pressure
All equations must be dimensionally homogeneous.
To be dimensionally homogeneous, all the terms in an equation must have
the same unit system.
On a day when the barometer reads 760 Torr, a tire pressure gage reads 204
kPa. What is the absolute pressure in Pascals in the tire?
Pabs = Patm + Pgauge = 760 + 204 = 764kPa
Pabs = Patm + Pgauge = (760torr ) ×
?
133.3Pa
+ 204 ×1000 Pa = 305308Pa
torr
The barometer (with a diameter 3 mm in diameter) reads 10 inch H2O. The
density of water is 1 g/cm3. What is this pressure in kiloPascals?
Attention: the pressure reading is independent on the diameter of the
barometer.
P = ρgh = (1g / cm3 )(9.8m / s 2 )(10inch)
P = ρgh = [
(unit?)
1 ×10 −3 kg
](9.8m / s 2 )(10 × 0.0254m)
(1 ×10 −2 )m3
= 2489 Pa
1 mmHg = 1 torr
= 2.489 kPa
9
Summary
• Importance of dimensions and units
9 Some SI and English units, Dimensional homogeneity, Unity
conversion ratios
• Systems and control volumes
• Properties of a system
• Density and specific gravity
• State and equilibrium
9 The state postulate
• Processes and cycles
9 The steady-flow process
• Temperature and the zeroth law of thermodynamics
9 Temperature scales
• Pressure
9 Variation of pressure with depth
• The manometer and the atmospheric pressure
10