engineering applications of control volume-2 p m v subbarao professor mechanical engineering...
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Engineering Applications of Control Volume-2
P M V SubbaraoProfessor
Mechanical Engineering Department
More Innovations for Extrasomatism…..
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Steam Power Plant: A series of CVs in SSSF
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outlet
inlet
inoutinpump dpvmhhmW
inoutinoutin ppvmpvupvum
inoutinininoutoutoutin ppvmvpvpmuum
Pumping of Incompressible Fluids
0
outin uum
outin uu
outin TT
Adiabatic pumping of a liquid is almost an Isothermal process!!
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2 – 3 : Steam Generation : Isobaric Heating
No work transfer, change in kinetic and potential energies are negligible
QCV
CVout
outin
inCV WgzVhmgzVhmQ
22
outoutininCV hmhmQ
inoutSG hhmQ
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Oil Fired Steam Boiler
inoutSG hhmQ
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Turbine : Adiabatic Process
No heat transfer. Change in kinetic and potential energies are negligible
3
4
T
outin mmCMSSSF
::
CVout
outin
inCV WgzVhmgzVhmQ
22
CVoutoutinin Whmhm
ssuming a single fluid entering and leaving…
43 hhmhhmW outinturbine
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Diagram of Large Power Plant Turbine
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HP Turbine Rotor
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LP Turbine Rotor
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Highly compressible flow through Turbine
outinturbine hhmW
outlet
inlet
outin vdpmhhm
inout vv
ininoutoutininoutoutoutin vpvpn
nmvpvpmuum
1
!!! 0 outin uu
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Steam Power Plant: A series of CVs in SSSF
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4 – 1 : Condenser : Isobaric Cooling : p4 = p1
No work transfer, change in kinetic and potential energies are negligible
QCV
CVout
outin
inCV WgzVhmgzVhmQ
22
outoutininCV hmhmQ
ssuming a single fluid entering and leaving…
inoutCondenser hhmQ
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Schematic for PC Power Plant with cooling Water from A River
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Windcatcher (Bagdir)
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Schematic for PC Power Plant with cooling tower
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Air Cooled Condenser System
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Net Heat and work Actions
• First law for a cycle:
WQ
11 i
i
i
i
wmqm
11 iii WmQm
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Turbojet Engine: A Heat Engine with Single Phase –Non Pure Substance
Qin
Qout
Wout
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Structure of A Domestic Refrigerator
1: Evaporator/Freezer 2: Condenser3: Compressor4 : Throttling Device
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Thermodynamic Cycle of A Refrigerator
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Simplified Diagram of A Refrigerator
Throttling Valve
Compressor
All the parts are CVs working in USUF processes.
Condenser : Transient Constant Pressure Cooling.
Compressor : Transient adiabatic Compression.
Throttling Device: Homogeneous and Transient Isoenthalpic process.
Evaporator: Transient Constant Pressure Cooling.
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Analysis of Uniform State Uniform Flow Devices
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First Law for CV:Uniform State Uniform Flow
• Conservation of mass:
outinCV
inoutCV mm
dt
dmmm
dt
dm 0
• Conservation of energy:
Wdt
dEQ out
CVin
Properties of CV are variant:
Continuous Accumulation or/and depletion of mass of a CV.
Continual Addition or removal of energy for a CV.
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Salient Features of CV @USUF Process
• Rate of mass inflow Rate mass outflow.
• The states of inflows and outflows are invariant although the mass flow rates may be time varying.
• Rate of Work done is variant.
• Rate of Heat transfer is variant.
• Change of state or process is both for the CV and Flows!
• The incoming fluid changes its state from inlet(at one time t0) to exit (at time t0+t) condition.
• A CV with USUF process is approximates as a homogeneous but variant device.
• The importance of time is very high!
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CV following A USUF Process for time t
• A change of state occurs in a CV with USUF due to change in time.
• A total change in a CV over time t can be calculated using:
tt
t
outin
tt
t
CV dtmmdtdt
dm 0
0
0
0
tt
t
outinCVCV dtmmtmttm0
0
)( 00
Total change in mass of A CV during a time interval t
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Total change in energy of A CV during a time interval t
dtWdtdtdt
dEdtdtQ
tt
t
tt
t
out
tt
t
CV
tt
t
in
tt
t
0
0
0
0
0
0
0
0
0
0
in
inin gzV
hm
2
2
out
outout gzV
hm
2
2
gz
VumE CVCV 2
2
All parameters mentioned above are homogeneous and variant.
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Let us now integrate this equation over time t, during which time we have
dtQQtt
t
CV
0
0
dtWWtt
t
CV
0
0
dttt
t
inin
0
0
dttt
t
outout
0
0
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1
21
112
22
22 22
0
0
gZV
umgZV
umdtdt
dEtt
t
CV
First Law for A CV executing USUF for finite time
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Throttling Devices
• Throttling devices are any kind of flow restricting devices.
• They cause large pressure drop in the fluid.
• The pressure drop in fluid is often accompanied by a large drop in temperature and rarely a raise in temperature.
• The magnitude of temperature drop or rise during a throttling process is governed by a property called Joule-Thomson Coefficient.
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Throttling Valves• Throttling: Reduces Pressure• Common Assumptions:
– SSSF– No work or heat transfer– Neglect changes in PE and KE
• Energy Balance:Throttling Valve
CV
out
out
in
inCV WgzV
hmgzV
hmQ
22
220 0
outin
gzV
gzV
22
22
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outoutininoutin pvupvuhh
• Isenthalpic (h = constant) Process
Internal energy + Flow energy = Constant
The fluids whose pv increases during throttling generate cooling effect.The fluids whose pv decreases during throttling generate cooling effect
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The Joule-Thomson Experiment
constant
H
p
T