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8/10/2019 Module 1 Lec 2 - THERMODYNAMICS 2nd Qtr SY1112.pdf
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11/15/201
THERMODYNAMICS
Prepared By:
Prof. Rene D. Estember
THERMODYNAMICS
• branch of physical science that treats various phenomena of energy and
the related properties of matter, especially of the law of transformation
of heat into other forms of energy and vice-versa.
Examples of everyday transformation:
• Process of converting heat into electrical work (electrical powergeneration)
• Process of converting electrical work into cooling (air conditioning)
• Process of converting work into kinetic energy (automotive
transportation)
THERMODYNAMIC S YSTEM (or simply a SYSTEM)
• refers to the quantity of matter or certain volume in space chosen for
study.
Surroundings - the mass or region outside the system.
Boundary – the real or imaginary surface that separates the system
from the surroundings. The boundary of the system can either be
fixed or movable.
Kinds of Thermodynamic System
1. Closed system (also known as control mass)
• a system in which there is no transfer of matter across the boundary. It
consists a fixed amount of mass, and no mass can cross its boundary.
That is, no mass can enter or leave a closed system.
2. Open system (also known as control volume)• a system in which there is a flow of matter through the boundary. It
usually encloses the device that involves mass flow such as
compressor, turbine, or nozzle.
Kinds of Thermodynamic System
3. Isolated System
• A system in which neither mass nor energy crosses the boundaries and it
is not influenced by the surroundings. (Δm = 0, W=0, Q=0)
PROPERTIES OF A SYSTEM
• Any characteristic of a system is called a property .
Types of Thermodynamic Properties
A. Static Properties
• refer to the physical condition of the working substance such as
temperature, pressure, density, specific volume, specific gravity, or
relative density.
B. Transport Properties
• refer to the measurement of diffusion within the working medium
resulting from molecular activity, like viscosities, thermal
conductivities, etc.
Classification of Thermodynamic Properties
A. Intensive Properties
• independent of the mass such as temperature, pressure, density, and
voltage.
B. Extensive Properties
• dependent upon the mass of the system and are total values such as total
volume and total internal energy.
The State Properties
1. Temperature
• An indication or degree of hotness and coldness and therefore a
measure of intensity of heat.
Absolute temperature – the temperature measured from absolute
zero.
Absolute zero – the temperature at which the molecules stop
moving. The absolute zero equivalent to 0oK (-273.15oC) or 0oR (-
460oF).
Conversion Formulas
The Temperature Interval (Change)• The difference between two temperature readings from the same scale,
and the change in temperature through which the body is heated.
• Note: The degree must be written after the temperature scale for it to
indicate that it is a change in temperature
ZEROTH LAW OF THERMODYNAMICS
• When any two bodies are in thermal equilibrium with the third body, they
are in thermal equilibrium with each other. (Note: the third body is
usually a thermometer)
325
9 C F o 32
9
5 F C o
460 F Ro 273 C K
o
oo C K T T oo
C F T T 5
9
OO F R
T T ooC F
T T 5
9
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2. Density (Specific Weight)
Mass density – the mass per unit volume.
where: m = mass (kgm, g, slug, lbm)
V = volume (m3, cm3, ft3)
ρ = density (kgm/m3, g/cm3, lbm/ft3)
Weight density (Specific Weight) – the weight per unit volume.
where: Fg = force due to gravity /weight (kgf ,N, g, lbf )
V = volume (m3, cm3, ft3)
γ = specific weight (kgf /m3, N/m3, g/cm3, lbf /ft
3)
3. Specific Volume
• The volume per unit mass
where: m = mass (kgm, g, lbm)
V = volume (m3, cm3, ft3)
υ = specific volume (m3/kgm, cm3/g, ft3/lbm)
V
F g
1
m
V v
V
m
4. Pressure
• The force exerted per unit area.
Absolute pressure - the true pressure measured above a perfect
vacuum.
Gage Pressure
• pressure measured from the level of atmospheric pressure by most pressure
recording measurement like pressure gage and ope-ended manometer.
Atmospheric pressure
• pressure obtained from barometric reading.
where: pabs = absolute pressure
pgage = gage pressure
patm = atmospheric pressure
atm psi patm
17.14
mmHg kPa patm 760325.101
inHg cm
kg patm 92.29032.1
2
610013.1013.1
cm
dyne xbar patm
atm gageabs p p p
atmabs p p )(
atmabs p p )(
h A Ah
AV
A F p g
gag e
v gh ghh p
g
g g gage
Critical Pressure
• Minimum pressure needed to liquefy gas at its critical temperature.
5. Specific Gravity (Relative Density)
• Also known as relative density. It is the ratio of the density of a certain
gas/substance to the density of air/water at the same temperature.
CONSERVATION OF MASS
• The law of conservation of mass states that the mass is indestructible. Mass
(m1) entering the system is equal to the sum of the stored mass (Δm) and
the mass (m2) that leaves the system.
Where: A = cross sectional area of thestream
υ = average speed
ρ = density
gas
air
water air
subs gas
water air
subs gas
water air
subs gas
R
R
MW
MW GS
/
/
/
/
/
/..
222111
21
A A
mm
CONSERVATION OF ENERGY
• The law of conservation of energy states that energy is neither created nor
destroyed.
• The fist law of Thermodynamics states that one form of energy may be
converted into another.
Gravitational Potential Energy – is its energy due to its position or elevation.
Where: z = height
Fg = weight
m = mass
g = acceleration due to gravity
P = Potent ial energy, ΔP = change in potential energy
Kinetic Energy – the energy or stored capacity for performing work possessed
by a moving body, by virtue of its momentum.
Where: m = mass
υ = velocityK = kinetic energy
ΔK = change in kinetic energy
)( 1212 z zmg P P P
mgz z F P g
2
1
2
212
2
2
2
m
K K K
m
K
CONSERVATION OF ENERGY
Internal Energy – is energy stored within the body or substance by virtue of the
activity and configuration of its molecules and of the vibration of the atoms
within the molecules.
u = specific internal energy (unit mass): Δu = u2 – u1
U = mu = total internal energy (m mass): ΔU = U2 - U1
Work (W) – is the product of the displacement of the body and the component
of the force in the direction of the displacement. Work is energy in
transition; that is, it exists only when a force is “moving through a
distance.”
• Work of a Nonflow System
Work done by the system is positive (outflow of energy).
Work done on the system is negative (inflow of energy).2
1
pdV W
CONSERVATION OF ENERGY
Flow Work (Wf ) – of work flow energy is work done in pushing a fluid across a
boundary, usually into or out of a system.
Where: ΔWf = change in flow
work
Heat (Q) – is energy in transit (on the move) from one body or system to
another solely because of temperature difference between the bodies or systems.
Q is posit ive when heat is added to the body or system.
Q is negative when heat is rejected by the body or system.
112212 V pV pW W W
pV W pAL FLW
f f f
f
f
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CONSERVATION OF ENERGY
Steady Flow Energy Equation
Characteristics of steady flow system
1. There is neither accumulation nor dimunition of mass within the system.
2. There is neither accumulation nor dimunition of energy within the system.3. The state of the working substance at any point in the system remains
constant.
Energy Entering the System = Energy Leaving the System
W U W K P QU W K P f f 22221111
CONSERVATION OF ENERGY
Enthalpy (H, h) - is a composite property applicable to all fluids . It is the
heat energy transferred to a substance at a constant pressure process. It is
defined by:
Thus, the steady flow energy equation becomes:
pV U H mh H
pvuh
W H K P Q H K P 222111
THE IDEAL GAS
• An ideal gas is ideal only in the sense that it conforms to the simple perfect
gas laws.
Boyle’s Law
• If the temperature of a given quantity of gas is held constant, the volume of
a gas varies inversely with the absolute pressure during a change of state.
2211
1
V pV p
C pV
p
C orV
pV
THE IDEAL GAS
Charles’ Law
(1) If the pressure on a particular quantity of gas is held constant, then, with
any change of state, the volume will vary directly as the absolute
temperature.
or
or
(2) If the volume of a particular quantity of gas is held constant, then, with any
change of state, the pressure will vary directly as the absolute temperature.
or
or C T
p
T p
2
2
1
1
T
p
T
p
CT p
C T
V
T V
2
2
1
1
T
V
T
V
CT V
THE IDEAL GAS
Equation of State or Characteristic Equation of a Perfect Gas
Combining Boyle’s and Charles’ Laws,
, a constant
where: p = absolute pressure
V = volume
v = specific volume
m = mass
T = absolute temperature
R = specific gas constant or gas constant
(unit mass) = u niversal gas constant
n = no. of moles
M = molecular weight
mRT
pV
mRT pV
RT pv
T Rn pV
R
M
R R
M
mn
THE IDEAL GAS
Equation of State or Characteristic Equation of a Perfect Gas
The values of Universal Gas constant:
= 8.314 kJ/moloK
= 1545 ft. lb./mol oR
= 1.986 BTU/mol oR
= 0.0821 L. atm/mol o K
Gas constant of diatomic oxygen:
= 0.2598 kJ/kg.K
= 48.28 ft.lbf /lbm.oR
Gas constant for air:
R w = 0.287 kJ/kg.K = 53.34 ft.lbf /lbm.oR
R
)()(
2
2O M
RO R
mol kg
K mol kJ O R
/32
./314.8)( 2
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THE IDEAL GAS
Specific Heat
• The specific heat of a substance is defined as the quantity of heat required
to change the temperature of unit mass through one degree.
c
or dQ = mcdT
And for a particular mass m,
If the mean or instantaneous value of specific heat is used,
) _ _ )((
) _ (
etemperatur of changemass
unitsenergy Heat
mdT
dQc
2
1
cdT mQ
12
2
1
T T mcdT mcQ
THE IDEAL GAS
Constant Volume Specific Heat (cv)
Constant Pressure Specific Heat (cp)
12 T T mcQ
U Q
vv
v
12 T T mcQ p p
2
1
pdV U W U Q p
12
112212
12
H H Q
V pV pU U Q
V V pU Q
p
p
p
THE IDEAL GAS
Ratio of Specific Heats
Internal Energy of an Ideal Gas
• Joule’s law states that “the change of internal energy of an ideal gas is a
function of only the temperature change.”
Therefore, ΔU is given by the formula,
whether the volume remains constant or not.
1v
p
c
ck
12 T T mcU v
THE IDEAL GAS
Enthalpy of an Ideal Gas
• The change of enthalpy of an ideal gas is given by the formula,
whether the pressure remains constant or not.
Relations between cp and cv
From h = u + pv and pv = RT
dh = du + R dT
12 T T mc H p
RdT dT cdT c v p
Rcc v p
1
1
k
kRc
k
Rc
p
v
THE IDEAL GAS
Entropy (S, s)
• Entropy is that property of a substance which remains constant (if no heat
enters or leaves the substance, while it does work or alters its volume, but
which increase or diminishes should a small amount of heat enter or leave.
• The change of entropy of a substance receiving (or delivering) heat is
defined by
Where: dQ = heat transferred at the temperature T
ΔS = total change of entropy
(constant specific heat)
2
1 T
dQS
T
dQdS
1
2
2
1
2
1
lnT
T mc
T
dT mcS
T
mcdT S
THE IDEAL GAS
Temperature – Entropy Coordinates
dQ = TdS
Other Energy Relations
(Reversible steady flow,ΔP=0)
2
1
TdS Q
K W Vdp s 2
1
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PROCESSES OF IDEAL GAS
Thermodynamic Processes
• Thermodynamic process is any change that a system undergoes from one
equilibrium state to another. It can be reversible or irreversible.
Path is the series of states through which a system passes during a
process.
a) Reversible Process (Quasi-equilibrium process)
• It is the process that can be reversed without leaving any trace on the
surroundings. That is, both the system and the surroundings are returned o
their initial states at the end of the process.
b) Irreversible Process
• It is the process that proceed spontaneously in one direction but the other.
Once having taken place, the process cannot reverse itself and always
results in an increase of molecular disorder.
PROCESSES OF IDEAL GAS
Constant Volume Process (Isometric Process)
• An isometric process is a reversible constant volume process. A constant
volume process may be reversible or irreversible.
Process Formula Process Formula
p, V, T relations
n
(Wn) 0 (reversible)
Q – Δ U (irreversible)
Specific heat
c
cv
(Ws) V(p1 – p2) H2 – H1 mcp(T2 – T1)
U2 – U1 mcv(T2 – T1) S2 – S1
Q mcv(T2 – T1)
1
2
1
2
p
p
T
T
1
2lnT
T mcv
2
1
pdV
2
1
Vdp
PROCESSES OF IDEAL GAS
Isobaric Process
• An isobaric process is an internally reversible process of a substance during
which the pressure remains constant.
Process Formula Process Formula
p, V, T relations
n 0
P(v2 – V1)
Specific heat
c
cp
0 H2 – H1 mcp(T2 – T1)
U2 – U1 mcv(T2 – T1) S2 – S1
Q mcp(T2 – T1)
2
1
pdV
2
1
Vdp
1
2
1
2
V
V
T
T
1
2lnT
T mc
p
PROCESSES OF IDEAL GAS
Isothermal Process
• An isothermal process is an internally reversible constant temperature
process of a substance.
Process Formula Process Formula
p, V, T relations
n 1
Specific heat
c
H2 – H1 0
U2 – U1 0 S2 – S1
Q
2
1
pdV
2
1
Vdp
2211 V pV p
1
211 ln
V
V V p
1
211 ln
V
V V p
2
1
1
2 lnln p
pmR
V
V mR
2
1
1
211 lnln
p
pmRT
V
V V p
PROCESSES OF IDEAL GAS
Isentropic Process
• An isentropic process is a reversible adiabatic process. Adiabatic simply
means no heat. A reversible adiabatic is one of constant entropy.
Process Formula Process Formula
p, V, T
relations n k
Specific heat
c 0
H2 – H1 mcp(T2 – T1)
U2 – U1 mcv(T2 – T1) S2 – S1 0
Q 0
2
1
pdV
2
1
Vdp
k k V pV p 2211
k
k k
p
p
V
V
T
T 1
1
2
1
2
1
1
2
k
T T mR
k
V pV p
11
121122
k
T T mRk
k
V pV pk
1
)(
1
)(121122
PROCESSES OF IDEAL GAS
Polytropic Process
• A polytropic process is an internally reversible process during which
and
Process Formula Process Formula
p, V, T
relations n - to +
Specific heat
c
H2 – H1 mcp(T2 – T1)
U2 – U1 mcv(T2 – T1) S2 – S1
Q mcv(T2 – T1)
2
1
pdV
2
1
Vdp
nn
n
V pV p
C pV
2211
nn V pV p 2211
n
nn
p
p
V
V
T
T 1
1
2
1
2
1
1
2
n
T T mR
n
V pV p
11121122
n
T T mRn
n
V pV pn
1
)(
1
)( 121122
n
nk cc vn
1
1
2
T
T mcn
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General Equation for Thermodynamic Curves
The general equation of any process is:
If
n = 0 ; Isobaric process
n = 1 ; Isothermal process
n = k ; Isentropic process
n = - to + ; Polytropic process
n = ; Isometric process
Note: pVk is steeper than pV curve.
C pV n
OTHER DEFINITION OF TERMS
1) Saturation temperature
• Saturation temperature is the temperature at which liquids start to boil or
the temperature at which vapors begin to condense.
• The saturation temperature of a given substance depends upon its pressure.
• It is directly proportional to the pressure, i.e., it increases as the pressure is
increased and decreases as the pressure is decreased.
Examples:
Water boils at 100oC at atmospheric conditions (101.325 kPa).
Water boils at 179.91oC at a pressure of 1000 kPa.
Steam condenses at 311.06oC at 10 MPa.
Steam condenses at 39oC at 0.0070 Mpa.
OTHER DEFINITION OF TERMS
2. Subcooled Liquid
• A subcooled liquid is one which has a tempeature lower than the saturation
temperature corresponding to the existing pressure.
Example:
Liquid water at 60oC and 101.325 kPa is a subcooled liquid. The saturation
temperature at 101.325 kPa is 100oC. Since the actual temperature of liquid
water of 60oC is less than 100oC, therefore, it is a subcooled liquid.
3. Compressed Liquid
• A compressed liquid is one which has pressure higher than the saturation
pressure corresponding to the existing temperature.
Example
Liquid water at 110 kPa and 100oC is a compressed liquid since the actual
liquid water pressure of 110 kPa is greater than the saturation pressure of
101.325 kPa at 100oC.
OTHER DEFINITION OF TERMS
4. Saturated Liquid
• A saturated liquid is a liquid at the saturations (saturation temperature or
saturation pressure) which has temperature equal to the boiling point
corresponding to the existing pressure. It is a pure liquid, i.e., it has no
vapor content.
Examples:
Liquid water at 100oC and 101.325 kPa.
Liquid water at 333.90oC and 3 Mpa.
Liquid water at 324.75oC and 12 Mpa.
5. Vapor
• Vapor is the name given to a gaseous phase that is in contact with the liquid
phase, or that is in the vicinity of a state where some of it might be
condensed.
OTHER DEFINITION OF TERMS
6. Saturated Vapor
• A saturated vapor is a vapor at the saturation conditions (saturation
temperature and saturation pressure). It is 100% vapor, i.e., has no liquid or
moisture content.
Examples:
Steam (water vapor) at 100oC and 101.325 kPa.
Steam at 212.42oC and 2 Mpa.
7. Superheated Vapor
• A superheated vapor is a vapor having a temperature higher than the
saturation temperature corresponding to the existing pressure.
Examples:
Steam at 200oC and 101.325 kPa. (tsat at 101.325 kPa= 100oC)
Steam at 300oC and 5 Mpa (t sat at 5 Mpa = 263.99oC)
OTHER DEFINITION OF TERMS
8. Degrees of Superheat, oSH
• The degrees of superheat is the difference between the actual temperature of
superheated vapor and the saturation temperature for the existing pressure.
• In equation form:
oSH = Actual superheated temperature – tsat at existing pressure
9. Degrees Subcooled, oSB
• The degrees subcooled of a subcooled liquid is the difference between the
saturation temperature for the given pressure and the actual subcooled
liquid temperature.
• In equation form:oSB = tsat at a given pressure – actual liquid temperature
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OTHER DEFINITION OF TERMS
10. Wet Vapor
• A wet vapor is a combination of saturated vapor and saturated liquid.
11. Quality, x
• The quality of wet vapor or wet steam is the percent by weight that is
saturated vapor.
12. Percent moisture, y
• The percent moisture of wet vapor is the percent by weight that is saturated
liquid.
13. Latent Heat of Vaporization
• The latent heat of vaporization of a pure substance is the amount of heat
added to/removed from the substance in order to convert it from saturated
liquid/saturated vapor to saturated vapor/saturated liquid with the
temperature remaining constant. It is inversely proportional to the
temperature or pressure of the substance.
OTHER DEFINITION OF TERMS
14. Critical point
• The critical point represents the highest pressure and highest temperature at
which liquid and vapor can coexist in equilibrium. The state of water at
critical conditions whether it is saturated liquid or saturated vapor is
unknown. Hence, the latent heat of vaporization of water at this condition
is either zero or undefined.
15. Sensible Heat
• Heat that causes change in temperature without a change in phase.
16. Sublimation
• The term used to describe the process of changing solid to gas without
passing to the liquid state.
17. Deposition
• The reverse of sublimation. It is the process of changing gas to solid
without passing to the liquid state.
OTHER DEFINITION OF TERMS
18. Latent heat of fusion
• It is the heat needed by the body to change from solid to liquid without
changing is temperature.
19. Second Law of Thermodynamics
• Heat cannot be transferred from cold body to a hot body without an input of
work. It similarly states that heat cannot be converted 100% into work.
The bottom line is that an engine must operate between a hot and a cold
reservoir. Also indicated is that energy has different levels of potential to
do work, and that energy cannot naturally move from realm of lower
potential to a realm of higher potential.
20. Third law of Thermodynamics
• The total entropy of pure substances approached zero as the absolutethermodynamic temperature approaches zero.
OTHER DEFINITION OF TERMS
21. Dalton’s Law of Partial Pressure
• The pressure exerted in a vessel by a mixture of gases is equal to the sum of
the pressures that each separate gas would exert if it alone occupied the
whole volume of the vessel.
22. Avogadro’s Law
• At equal volume, at the same temperature and pressure conditions, the
gases contain the same number of molecules.
23. The Carnot Cycle
• The Carnot Cycle is the most efficient cycle conceivable. It consists of two
isothermal processes and two isentropic processes.
24. Mean effective pressure
• It is the average constant pressure that, acting through one stroke, will do
on the piston the net work of a single stroke.
OTHER DEFINITION OF TERMS
25. Expansion ratio
• The ratio between the volume at the end of expansion and the volume at the
beginning of expansion.
26. Compression ratio
• The ratio between the volume at the beginnign of compression and the
volume at the end of compression.
27. Internal Combustion Engine
• It is a heat engine deriving its power from the energy liberated by the
explosion of a mixture of some hydrocarbon, in gaseous or evaporated
form, with atmospheric air.
28. Four-stroke cycle
• The four-stroke cycle is one wherein four strokes of the piston, two
revolutions, are required to complete the cycle.
OTHER DEFINITION OF TERMS
29. Heat engine or thermal engine
• It is a closed system (no mass crosses its boundaries) that exchanges only
heat and work with its surrounding and that operates in cycle.
30. Elements of a thermodynamic heat engine with a fluid as the working
substance:
A working substance, matter that receives heat, rejects heat, and does
work;
A source of heat (also called a hot body, a heat resevoir, or just source),
from which the working substance receives heat;
A heat sink (also called receiver, a cold body, or ject sink), to which the
working substance can reject heat; and
An engine, wherein the woking substance may do work or have work
done on it.
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OTHER DEFINITION OF TERMS
31. Thermodynamic cycle
• It occurs when the working fluid of a system experiences a number of
processes that eventually return the fluid to its initial state.
32. Available energy
• It is that part of the heat that was converted into mechanical work.
33. Unavailable energy
• It is the remainder of the heat that had to be rejected into the receiver (sink).
34. Otto Cycle
• It is the ideal prototype of spark-ignition engine.
35. Spark-Ignition engine
• It is also referred to as gasoline engine.
OTHER DEFINITION OF TERMS
36. Compression-Ignition Engine
• It is also referred to as diesel engine.