Transcript
THERMODYNAMICS
CHAPTER 1 BASIC CONCEPTS
1.0 INTRODUCTION
• THERMODYNAMICS IS THE SCIENCE THAT
DEALS WITH HEAT AND WORK AND
PROPERTIES OF SUBSTANCES RELATED TO
HEAT AND WORK
• THERMODYNAMICS DEALS WITH THE
NECESSARY METHODS FOR CONVERTING
HEAT ENERGY FROM AVAILABLE
RESOURCES (SUCH AS CHEMICAL FUEL) TO
MECHANICAL WORK
• RELATIONSHIPS BETWEEN HEAT, WORK AND
THE PROPERTIES OF SUBSTANCES ARE
EXPRESSED IN THE FORM OF BASIC LAWS
KNOWN AS THE LAWS OF THERMODYNAMICS
• THE BASIS OF THERMODYNAMICS IS
EXPERIMENTAL OBSERVATION
• IT CAN BE OBSERVED THAT HEAT AND WORK
ARE TWO FORMS OF ENERGY THAT ARE
CLOSELY RELATED TO EACH OTHER AND
THIS OBSERVATION PROVIDES THE BASIS
FOR THE FIRST LAW OF THERMODYNAMICS
• IT CAN ALSO BE OBSERVED THAT HEAT
FLOWS FROM A HIGH TEMPERATURE OBJECT
TO A LOW TEMPERATURE OBJECT AND THE
REVERSE PROCESS IS ONLY POSSIBLE WITH
SOME HELP (IN THE FORM OF WORK). THIS
CONCEPT IS IMPORTANT IN THE
DEVELOPMENT OF THE SECOND LAW OF
THERMODYNAMICS
• EXAMPLES OF ENGINEERING PLANT WHICH
INVOLVES THERMODYNAMIC ANALYSIS ARE
THE STEAM POWER PLANT, THE INTERNAL
COMBUSTION ENGINE, REFRIGERATION
SYSTEMS, ETC.
1.1 BASIC CONCEPTS AND DEFINITIONS
• SYSTEM:
o A thermodynamic system is defined as a
quantity of matter or a region in space chosen
for study
• SURROUNDINGS:
o The mass or region outside the system
• BOUNDARY:
o The surface that separates the system from its
surroundings
o Boundaries can be fixed or movable
o Boundaries can be real (cylinder walls and
piston surfaces in an internal combustion
engine)
o Boundaries can also be imaginary (cross
sections of pipes at the entrance and exit of
turbines)
• CLOSED SYSTEM
o Consists of a fixed amount of mass
o No mass can cross its boundary
o No mass can enter or leave a closed system
o Volume does not have to be fixed
o But energy, in the form of heat or work can
cross the boundary
o Example as shown in Figure 1.1 where the gas
in the cylinder is considered the system
o If a Bunsen burner is placed under the
cylinder, the temperature of the gas will
increase and the piston will rise
o As the piston rises, the boundary of the system
moves
o Heat and work crosses the boundary of the
system during this process but not mass
Weights/load
Piston
Gas System’s boundary
(system)
Figure 1.1 Example of a closed system
o In the special case where even energy is not
allowed to cross the boundary, that system is
called an ISOLATED SYSTEM
• OPEN SYSTEM OR CONTROL VOLUME
o A properly selected region in space
o Used when the analysis involves devices with
mass flow into and/or out of the device such as
a compressor, turbine, nozzle, heat exchanger,
etc.
o The procedure in such an analysis is to specify
a control volume that surrounds the device
under consideration
o The boundaries of a control volume are called
a control surface
o Mass as well as heat and work can flow across
the control surface
o Figure 1.2 shows an example of an open
system.
control surface AIR COMPRESSOR MOTOR
Figure 1.2 Example of a Control Volume
• MICROSCOPIC POINT OF VIEW
o Considers behaviours of individual molecules
o Involves a large number of equations to
explain the behaviour of a system
• MACROSCOPIC POINT OF VIEW
o Concerned with the gross or average effects of
many molecules
o These effects can be perceived by our senses
and measured by instruments such as a
pressure gauge
Heat
Air out
(high pressure) Air in
(low pressure)
Work
• CONTINUUM
o Concerned with systems that contain many
molecules
o Since we are not concerned with the behaviour
of individual molecules, we can treat the
substance as being continuous and this is
called a continuum
o This is valid as long as the size of the system
we deal with is large relative to the space
between the molecules
o Not valid in high-vacuum technology
1.2 PROPERTIES AND STATE OF A
SUBSTANCE
• If we consider a given mass of water, we recognize
that this water can exist in various forms
• If it is a liquid initially, it may become a vapour
when it is heated, or a solid when it is cooled
• Thus we speak of the different phases of a
substance
• A phase is defined as a quantity of matter that is
homogeneous throughout
• When more than one phase is present, the phases
are separated from each other by the phase
boundaries
• In each phase the substance may exist at various
pressures and temperatures or to use the
thermodynamic term, in various states
• The state may be identified or described by certain
observable, macroscopic properties
• Examples of these properties are temperature,
pressure and density
• Each of the properties of a substance in a given
state has only one definite value, and these
properties always have the same value for a given
state
• A property is defined as any quantity that depends
on the state of the system and is independent of the
path by which the system arrived at the given state
• The state is specified or described by the properties
• The number of properties required to fix the state of
a system is given by the STATE POSTULATE:
The state of a simple compressible system is
completely specified by two independent, intensive
properties
• A system is called a simple compressible system
when surface effects, magnetic effects, and
electrical effects are not significant
• Thermodynamic properties can be divided into 2
general classes, intensive and extensive properties
• Intensive properties are those that are independent
of the size of the system, such as temperature,
pressure and density
• Extensive properties are those whose values depend
on the size of the system. Mass(m), Volume(V) and
total Energy(E) are extensive properties
• If a quantity of matter in a given state is divided
into 2 equal parts, each part will have the same
value of intensive properties as the original, and
half the value of the extensive properties
• Extensive properties per unit mass are called
specific properties
• Examples are specific volume ( v = V/m) and
specific total energy (e = E/m)
• Thermodynamics deals with equilibrium states
• The word equilibrium implies a state of balance
• In an equilibrium state there are no unbalanced
potentials (or driving forces) within the system
• There are many types of equilibrium
• A system is in thermal equilibrium if the
temperature is the same throughout the entire
system
• A system is in mechanical equilibrium if there is no
change in pressure at any point of the system with
time
• A system is in chemical equilibrium if its chemical
composition does not change with time
• A phase equilibrium is when the mass of each phase
of the system (if the system involves two phases)
reaches an equilibrium level and stays there
• When a system is in equilibrium as regards all
possible changes of state, we say that the system is
in thermodynamic equilibrium
1.3 PROCESSES AND CYCLES
• Any change that a system undergoes from one
equilibrium state to another is called a process
• The series of states through which a system passes
during a process is called the path of the process
• To describe a process completely, one should
specify the initial and final states of the process, the
path it follows and the interactions with the
surroundings
Property A
State 2
Process path
State 1
Property B
Figure 1.3 A Process
• When a process proceeds in such a manner that the
system remains infinitesimally close to an
equilibrium state at all times, it is called a quasi-
static or quasi-equilibrium process
• a quasi-equilibrium process can be viewed as a
sufficiently slow process that allows the system to
adjust itself internally so that properties in one part
of the system do not change any faster than those at
other parts
• All the states the system passes through during a
quasi-equilibrium process may be considered as
equilibrium states
• Referring back to Figure 1, if the weights on the
piston are small and are taken off one by one, the
process could be considered as quasi-equilibrium
• If all the weights were removed at once, the piston
would rise rapidly until it hits the stops. This would
be a non-quasi-equilibrium process
• For a non-quasi-equilibrium process, the system
would not be in equilibrium at any time
• For a non-quasi-equilibrium process, we are limited
to a description of the system before the process
occurs and after the process is completed and
equilibrium is restored
• For a non-quasi-equilibrium process, we are not
able to specify each state through which the system
passes and this process is denoted by a dashed line
between the initial and final states instead of a solid
line
• The prefix iso- is used to designate a process for
which a particular property remains constant
• An isothermal process is a process during which the
temperature T remains constant
• An isobaric process is a process during which the
pressure P remains constant
• An isochoric/isometric process is a process during
which the specific volume v remains constant
• When a system in a given initial state goes through
a number of different changes of state or processes
and finally returns to its initial state, the system has
undergone a THERMODYNAMIC CYCLE
• At the end of a cycle, all the properties have the
same value they had at the beginning
• Steam(water) that circulates through a steam power
plant undergoes a cycle
Property A
2
1
3
Property B
Figure 1.4 A Thermodynamic cycle
1.4 UNITS
• In SI, the units for mass, length and time are as
follows:
• mass : kilogram (kg)
• length : meter (m)
• time : seconds (s)
• In SI, the force unit is the newton (N):
o It is derived from Newton’s second law
o F = m x a
o Force = mass x acceleration
o It is defined as the force required to accelerate
a mass of 1 kg at a rate of 1m/s2
o 1 N = 1 kg.m/s2
o Weight is the gravitational force applied to a
body and is given by, W = mg (N)
o The weight of a unit volume of a substance is
called the specific weight w and is determined
from w = ρg where ρ is the density
o Prefixes
Prefix Symbol Multiple
Factor
Example
Kilo K 103 1KN=103N
Mega M 106 1MW=106W
Giga G 109 1 Gpa=109Pa
Tera T 1012 1 TW=1012W
• In engineering, all equations must be dimensionally
homogeneous. That is, every term in an equation
must have the same unit
• Checking units can serve as a valuable tool to spot
errors/mistakes
1.5 DENSITY AND VOLUME
• Density is defined as mass per unit volume
ρ = m/v (kg/m3)
• Density is the reciprocal of specific volume v
v = V/m = 1/ρ (m3/kg)
• ρ is an intensive property
• Unit for v is m3/kg
• Unit for ρ is kg/m3
• Unit for Volume (V) is m3 or another frequently
used unit for volume is litre (L)
• 1 Litre = 10-3 m3 or 0.001 m3
1.6 TEMPERATURE AND THE ZEROTH
LAW OF THERMODYNAMICS
• It is not easy to give an exact definition for
temperature
• Fortunately, several properties of materials change
with temperature in a repeatable and predictable way,
and this forms the basis for accurate temperature
measurement
• The commonly used mercury-in-glass thermometer,
for example, is based on the expansion of mercury
with temperature
• The equality of temperature is the only requirement
for thermal equilibrium
• The zeroth law of thermodynamics states that 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
Temperature Scales
• Temperature scales enable us to use a common basis
for temperature measurement
• Temperature scales are based on some easily
reproducible states such as the freezing and boiling
points of water, which are also called the ice point and
the steam point, respectively.
• The temperature scales used in the SI and in the
English system today are the Celcius scale and the
Fahrenheit scale
• On the Celcius scale, the ice and steam point are
assigned the values of 0 and 100°C, respectively
• In thermodynamics, it is very desirable to have a
temperature scale that is independent of the properties
of any substance or substances
• Such a temperature scale is called a thermodynamic
temperature scale, which is developed later in
conjunction with the second law of thermodynamics
• The thermodynamic temperature scale in the SI is the
Kelvin scale
• The temperature unit on this scale is the Kelvin
• The lowest temperature on the Kelvin scale is 0 K
• The thermodynamic temperature scale in the English
system is the Rankine scale
• A temperature scale that turns out to be identical to the
Kelvin scale is the ideal gas temperature scale
• The Kelvin scale is related to the Celcius scale by
T(K) = T(°C) + 273.15
• The Rankine scale is related to the Fahrenheit scale by
T(R) = T(°F) + 459.67
• The temperature scales in the two unit systems are
related by
T(R) = 1.8 T(K)
T(°F) = 1.8 T(°C) + 32
• Therefore, when we are dealing with temperature
differences ∆Τ, the temperature interval on both scales
is the same
• Raising the temperature of a substance by 10°C is the
same as raising it by 10 K
∆T(K) = ∆T(°C)
∆T(R) = ∆T(°F)
• Some thermodynamic relations involve the
temperature T and often the question arises of whether
it is in K or °C
• If the relation involves temperature differences (such
as a = b∆Τ ) , it makes no difference and either can be
used
• However, if the relation involves temperatures only
instead of temperature differences (such as a = bΤ ) then K must be used
1.7 PRESSURE • Pressure is defined as the force exerted by a fluid per
unit area
• We speak of pressure only when we deal with a gas or
a liquid
• The counterpart of pressure in solids is stress
• Since pressure is defined as force per unit area, it has
the unit of newtons per square meter (N/m2), which is
called a pascal (Pa), that is
1 Pa = 1 N/m2
• Three other pressure units commonly used in practice
are bar, standard atmosphere, kilogram-force per
square centimeter
1 bar = 105 Pa = 0.1 Mpa = 100kPa
1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars
1 kgf/cm2 = 9.807 N/cm2 = 9.807 × 104 N/m2 = 9.807×104 Pa
= 0.9807 bar
= 0.96788 atm
• The actual pressure at a given position is called the
absolute pressure
• It is measured relative to absolute vacuum (i.e.,
absolute zero pressure)
• Most pressure-measuring devices, indicate the
difference between the absolute pressure and the local
atmospheric pressure. This difference is called the
gage pressure
• Pressures below atmospheric pressure are called
vacuum pressure and are measured by vacuum gages
that indicate the difference between the atmospheric
pressure and the absolute pressure
• Absolute, gage, and vacuum pressures are all positive
quantities and are related to each other by
Pgage = Pabs – Patm (for pressures above Patm)
Pvac = Patm – Pabs (for pressures below Patm)
• In thermodynamic relations and tables, absolute
pressure is almost always used
• Pressure is the compressive force per unit area, and it
gives the impression of being a vector
• However, pressure at any point in a fluid is the same
in all directions. That is, it has magnitude but not a
specific direction, and thus a scalar quantity
1.8 Variation of Pressure with Depth
• To obtain a relation for the variation of pressure with
depth, consider a rectangular fluid element of height
∆z, length ∆x, and unit depth (into the paper) in
equilibrium, as shown in the Figure 1.5 below:
z
x
Figure 1.5 Free Body Diagram of a rectangular fluid
element in equilibrium
W
P1
P2
∆z
∆x
• Assuming the density of the fluid ρ to be constant, a
force balance in the vertical z-direction gives
∑ FZ = maz = 0 : P2 ∆x – P1 ∆x – ρg ∆x ∆z = 0
where W = mg = ρg ∆x ∆z is the weight of the fluid
element. Dividing by ∆x and rearranging gives
∆P = P2 – P1 = ρg ∆z = γ ∆z
where γ = ρg is the specific weight of the fluid
• Pressure in a fluid increases linearly with depth
• For a given fluid, the vertical distance ∆z is sometimes
used as a measure of pressure, and it is called the
pressure head
• For small to moderate distances, the variation of
pressure with height is negligible for gases because of
their low density. The pressure in a tank containing a
gas, for example, may be considered to be uniform
since the weight of the gas is too small to make a
significant difference. Also, the pressure in a room
filled with air may be assumed to be constant (Figure
1.6 below)
Figure 1.6 Pressure in tank containing a gas
AIR
(a 5 metre high room)
Ptop = 1 atm
Pbottom = 1.006 atm
• Pressure in a fluid is independent of the shape or cross
section of the container. It changes with the vertical
distances, but remains constant in other directions
• Therefore, the pressure is the same at all points on a
horizontal plane in a given fluid
• The pressures are not the same if two points cannot be
interconnected by the same fluid, although they are at
the same depth
1.9 THE MANOMETER
• We notice that an elevation change of ∆z of a fluid
corresponds to ∆P/ρg, which suggests that a fluid
column can be used to measure pressure differences
• A device based on this principle is called a
manometer
• It is commonly used to measure small and moderate
pressure differences
• A manometer mainly consists of a glass or plastic U-
tube containing one or more fluids such as mercury,
water, alcohol or oil.
• Heavy fluids such as mercury are used if large
pressure differences are anticipated
• Consider the manometer shown in the figure below
that is used to measure the pressure in the tank. The
pressure anywhere in the tank and at position 1 has the
same value
Figure 1.7 The Basic Manometer
• Since pressure in a fluid does not vary in the
horizontal direction within a fluid, the pressure at
point 2 is the same as the pressure at 1, P2 = P1. The
pressure at point 2 is
P2 = Patm + ρgh
where ρ is the density of the fluid in the tube
• For manometers with multiple fluids of different
densities stacked on top of each other, remember:
• (1) the pressure change across a fluid column of height
h is ∆P = ρgh ,
• (2) pressure increases downward in a given fluid and
decreases upward (i.e., Pbottom > Ptop), and
• (3) two points at the same elevation in a continuous
fluid at rest are at the same pressure
• The pressure at the bottom of the tank in the figure
below can be determined by starting at the free surface
where the pressure is Patm, and moving downwards
until we reach point 1 at the bottom, and setting the
result equal to P1. It gives
Patm + ρ1gh1 + ρ2gh2 + ρ3gh3 = P1
Figure 1.8 Stacked-up fluid layers
Fluid 1
Fluid 2
Fluid 3
1
P atm
h1
h2
h3
• Manometers are particularly well-suited to measure
pressure drops across a horizontal flow section
between two specified points due to the presence of a
device such as a valve or heat exchanger or any
resistance to flow
• The working fluid can be either a gas or a liquid
whose density is ρ1. The density of the manometer
fluid is ρ2, and the differential fluid height is h
• A relation for the pressure difference P1 – P2 can be
obtained by starting at point 1 with P1 and moving
along the tube by adding or subtracting the ρgh terms
until we reach point 2, and setting the result equal to
P2
P1 + ρ1g (a + h) – ρ2gh – ρ1ga = P2
P1 – P2 = ( ρ2 – ρ1 ) gh
• When the fluid flowing in the pipe is a gas, then ρ1<ρ2
and the relation above simplifies to P1 – P2 = ρ2gh
1.10 OTHER PRESSURE MEASUREMENT
DEVICE
• Another type of commonly used mechanical pressure
measurement device is the Bourdon tube, which
consists of a hollow metal tube bent like a hook whose
end is closed and connected to a dial indicator needle
• When the tube is open to the atmosphere, the tube is
undeflected, and the needle on the dial at this state is
calibrated to read zero (gage pressure). When the fluid
inside the tube is pressurized, the tube stretches and
moves the needle in proportion to the pressure applied
• Modern pressure sensors, called pressure
transducers, are made of semiconductor materials
such as silicon and convert the pressure effect to an
electrical effect such as change in voltage, resistance,
or capacitance
• Pressure transducers are smaller and faster, and they
are more sensitive, reliable, and precise than their
mechanical counterparts. They can measure pressure
from less than a millionth of 1 atm to several
thousands of atm
1.11 BAROMETER AND THE
ATMOSPHERIC PRESSURE
• The 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 0ºC
(ρHg = 13,595 kg/m3) under standard gravitational
acceleration (g = 9.807 m/s2)
• The standard atmospheric pressure for example, is 760
mmHg (29.92 inHg) at 0ºC. The unit mmHg is also
called the torr
• The standard atmospheric pressure Patm changes from
101.325 kPa at sea level to 89.88, 79.50, 54.05, 26.5,
and 5.53 kPa at altitudes of 1000, 2000, 5000, 10,000,
and 20,000 meters, respectively
• Remember that the atmospheric pressure at a location
is simply the weight of the air above that location per
unit surface area
• For a given temperature, the density of air is lower at
high altitudes, and a given volume contains less air
and less oxygen. So, we tire more easily and
experience breathing problems at high altitudes
top related