Engineering Applications of Control Volume-2 P M V Subbarao Professor Mechanical Engineering...
Transcript of Engineering Applications of Control Volume-2 P M V Subbarao Professor Mechanical Engineering...
Engineering Applications of Control Volume-2
P M V SubbaraoProfessor
Mechanical Engineering Department
More Innovations for Extrasomatism…..
Steam Power Plant: A series of CVs in SSSF
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!!
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
Oil Fired Steam Boiler
inoutSG hhmQ
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
Diagram of Large Power Plant Turbine
HP Turbine Rotor
LP Turbine Rotor
Highly compressible flow through Turbine
outinturbine hhmW
outlet
inlet
outin vdpmhhm
inout vv
ininoutoutininoutoutoutin vpvpn
nmvpvpmuum
1
!!! 0 outin uu
Steam Power Plant: A series of CVs in SSSF
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
Schematic for PC Power Plant with cooling Water from A River
Windcatcher (Bagdir)
Schematic for PC Power Plant with cooling tower
Air Cooled Condenser System
Net Heat and work Actions
• First law for a cycle:
WQ
11 i
i
i
i
wmqm
11 iii WmQm
Turbojet Engine: A Heat Engine with Single Phase –Non Pure Substance
Qin
Qout
Wout
Structure of A Domestic Refrigerator
1: Evaporator/Freezer 2: Condenser3: Compressor4 : Throttling Device
Thermodynamic Cycle of A Refrigerator
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.
Analysis of Uniform State Uniform Flow Devices
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.
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!
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
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.
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
1
21
112
22
22 22
0
0
gZV
umgZV
umdtdt
dEtt
t
CV
First Law for A CV executing USUF for finite time
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.
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
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
The Joule-Thomson Experiment
constant
H
p
T