entropy change p m v subbarao professor mechanical engineering department a single reason for every...
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Entropy Change
P M V SubbaraoProfessor
Mechanical Engineering Department
A Single Reason for Every Thing That Happens!!!
The Thermodynamics of Temperature Creation
• The Gibbsian equation,defines the change in specific entropy of any substance during any reversible process.
vdpdhpdvduTds • Consider a control mass
executing a constant volume process:
pdvduTds
constant
vs
uT
The relative change in internal energy of a control mass w.r.t. change in entropy at constant volume is called as absolute
temperature.
The Thermodynamics of Temperature Creation
vdpdhTds
• Consider a control volume executing a reversible constant pressure process:
constant
ps
hT
The relative change in enthalpy of a control volume w.r.t. change in entropy at constant pressure is called as absolute
temperature.
Entropy change of an ideal gas
• From the Gibbsian equations, the change of entropy of a substance can be expressed as
dPT
v
T
dhdsdv
T
P
T
duds or
For an ideal gas, u=u(T) and h=h(T), du=cv(T)dT and dh=cp(T)dT and Pv=RT
dP
T
v
T
dTTCdsdv
T
P
T
dTTcds pv or
By Integration, the change in the entropy is
1
22
1
12 lnv
vR
T
dTTcss v
1
22
1
12 lnp
pR
T
dTTcss p
or
Ideal Gas with constant specific heats
• When specific heats are constant (calorically perfect gas), the integration can be simplified:
• If a process is isentropic (that is adiabatic and reversible), ds=0, s1=s2,
1
2
1
212 lnln
p
pR
T
Tcss p
1
2
1
212 lnln
v
vR
T
Tcss v
Isentropic Process with idea gas
0lnln1
2
1
2
p
pR
T
Tcp0lnln
1
2
1
2
v
vR
T
Tcv
1
2
1
2 lnlnv
vR
T
Tcv
1
2
1
2 lnlnp
pR
T
Tcp
1
2
1
2 lnlnv
vcc
T
Tc pvv
1
2
1
2 lnlnp
pcc
T
Tc vpp
1
2
1
2 ln1lnv
v
c
c
T
T
v
p
1
2
1
2 ln1lnp
p
c
c
T
Tc
p
vp
1
2
1
2 ln1lnv
v
T
T
1
2
1
2 ln1
1lnp
p
T
T
1
1
2
1
2
v
v
T
T
1
1
2
1
2
p
p
T
T
1
1
2
1
1
2
p
p
v
v
1
1
2
1
2
p
p
v
v
1
1
2
1
2
p
p
v
v
1
2
1
2
v
v
p
p
2
1
1
2
v
v
p
p 1122 vpvp
Isentropic Process by an idea gas with constant propeties
1
1
2
1
2
v
v
T
T
1
1
2
1
2
p
p
T
T
2
1
1
2
v
v
p
p
1
11
Cv
T
2
1
CT
p
3Cpv or or
Are the reversible Process practicable?100% perfection is possible but may not ne practicable..!?!!?!
Practical Processes are influenced by Irreversibilities
• Fluid friction
• Solid friction
• Electrical resistance
• Thermo-chemical Reactions (Combustion)
• Unrestrained motion
• Heat Transfer with a finite temperature difference
Solid Friction is an Irreversibility
PE KE
Q
Solid Friction is an Irreversibility
PE KEQ
Solid Friction is an Irreversibility
PE KE
Q
Solid Friction is an Irreversibility
ReverseTHIS IS NOT POSSIBLE.
Q?
Solid Friction is an Irreversibility
Q
1 2
43
Irreversible and Reversible engines
LTR
QHER
QLER
ER
WnetR
Assume that an irreversibleEngine is more efficient than
the reversible engine.
QHEI
QLEI
EI
WnetI
QHE
QLE
EWnetI
revirr
HER
Rnet
HEI
Inet
Q
W
Q
W ,,
HTR
• For same Wnet, QHEI < QHER
• Implies that, |QLEI | < |QLER|
• But a reversible engine can be completely reversed and it will work as a heat pump.
Wnet,I Wnet,R>QHEI QHER
revrev
1
HPRHEIrev
irr QQ
1
Let us construct a compound machine using an irreversible engineand reversed reversible engine (reversible Heat Pump).
For same |Wnet |,
QHPR
QLPR
QHEI
EIR
WnetR
QHEI < |QHPR |
QLE
LTR (Source)
HTR (Sink)
|QLEI| < QLPR
QLPR - |QLEI |
|QHPR | - QHEI
Irreversible Machines
• The efficiency of an irreversible heat engine will always less than the efficiency of a reversible engine working between the same reservoirs.
• The COP of an irreversible heat pump will always less than the COP of a reversible heat pump working between the same reservoirs.
Further Discussions
irrrev
irrrev but, 1/revrev
irrrev (Mathematically possible but thermodynamically impossible).
• Similarly, irrrev 1/irrrev
irrrev (Mathematically possible but thermodynamically impossible).
Isotope Half-Life DecayHe-3 Stable N/A
He-4Stable N/A≈ 0.5 x 10-21 sec - 1 x 10-21 sec p or n
He-5 1 x 10-21 sec n
He-60.8 sec β-5 x 10-23 sec - 5 x 10-21 sec n
He-7 3 x 10-22 sec - 4 x 10-21 sec n
He-80.1 sec β-0.5 x 10-21 sec - 1 x 10-21 sec n/α
He-9 unknown unknown
Increase of Entropy Principle
Entropy change
Entropy Generation
•The principle states that for an isolated Or a closed adiabatic Or System + Surroundings;•A process can only take place such that Sgen 0 where Sgen = 0 for a reversible process only and Sgen can never be less than zero.
Entropy Transfer
(due to heat transfer)
Increase of Entropy Principle
Define entropy generation Sgen as,
For a general Process
Implications of Increase of Entropy Principle
• Entropy, unlike energy, is non-conservative since it is always increasing.
• The entropy of the universe is continuously increasing, in other words, it is becoming disorganized and is approaching chaotic.
• The entropy generation is due to the presence of irreversibilities.
• Therefore, the higher irreversibilities lead to the higher the entropy generation and the lower the efficiency of a device.
• The above is Engineering statement of the second law
Second Law & Entropy Balance
• Increase of Entropy Principle is another way of stating the Second Law of Thermodynamics:
• Second Law : Entropy can be created but NOT destroyed
• In contrast, the first law states: Energy is always conserved.
• Note that this does not mean that the entropy of a system cannot be reduced, it can.
• However, total entropy of a system + surroundings cannot be reduced.
Entropy of Universe
A quantity of heat Q is spontaneously transferred from the surroundings at temperature T0 to the control mass at temperature T. Let the work done during this process be W. For this process by control mass and write
For the surroundings at T0, Q is negative, and we assume a reversible heat extraction so
The total net change of entropy is therefore
Since T0 > T, the quantity [(1/T) - (1/T0)] is positive, and we conclude that
Net Change in Entropy of Universe
If T > T0, the heat transfer is from the control mass to the surroundings
It should be noted that the right-hand side of above equation represents an external entropy generation due to heat transfer through a finite temperature difference.
+
The Third Law of Thermodynamics
The entropy change of a system during a reversible isothermal process tends
towards zero when the thermodynamic temperature of the system tends towards
zero. In the neighbourhood of absolute zero, all
reactions in a liquid or solid in internal equilibrium take place with no change in
entropy. [Nernst 'principle'].
Planck’s statement of the 3rd law
• In 1911, Planck one step further and made the hypothesis that not only does the entropy difference vanish as T → 0, but that:
• Planck’s statement of the Third Law: The entropy of every solid or liquid substance in internal equilibrium at absolute zero is itself zero.
• Planck is just saying:
0lim0
ST
Engineering Relations from Second Law
Entropy as A Rate Equation
• The second law of thermodynamics was used to write the balance of entropy for a infinitesimal variation for a finite change.
• Here the equation is needed in a rate form so that a given process can be tracked in time.
• Take the incremental change and divide by t.
• We get
• For a given control mass we may have more than one source of heat transfer, each at a certain surface temperature (semi-distributed situation).
The rate of entropy change is due to the flux of entropy into the control mass from heat transfer and an increase due to
irreversible processes inside the control mass.
The Second Law Of ThermodynamicsFor A Control Volume
• The rate of change of property B of the system .
inoutCVCM smsm
dt
dS
dt
dS
• Let B = Entropy of the system, S = ms.
inoutCVCM BB
dt
dB
dt
dB
genCM S
T
Q
dt
dS
Entropy Rate Equation for CV
Rate of change in entropy of a CV = Entropy in flow rate –Entropy out flow rate + the flux of entropy into the control mass from heat
transfer + Rate of Entropy generation
The Steady State Steady Flow Process
• For the steady-state process, which has been defined before, we conclude that there is no change with time of the property (entropy) per unit mass at any point within the control volume.
• That is,
so that, for the steady-state process,
• If in a steady-state process there is only one area over which mass enters the control volume at a uniform rate and only one area over which mass leaves the control volume at a uniform rate,
• we can write
and dividing the mass flow rate out gives
Since sgen is always greater than or equal to zero, for an adiabatic process it follows that
where the equality holds for a reversible adiabatic process.
Geometry of Turbine Blades for High Efficiency
Transient Process
• For the transient process, the second law for a control volume, it can be written in the following form:
If this is integrated over the time interval t, we have
Therefore, for this period of time t, we can write the second law for the transient process as
Since in this process the temperature is uniform throughout the control volume at any instant of time, the integral on the right reduces to
and therefore the second law for the transient process can be written
Mechanical Engineering Inventions
• Carnot Cycle
• Lenoir Cycle
• Otto Cycle
• Stirling Cycle
• Atkinson Cycle
• Diesel Cycle
• Brayton cycle
• Rankine Cycle
• Vapour Compression Refrigeration Cycle
• Vapour Absorption Refrigeration Cycle