email: [email protected]http://www5.uitm.edu.my/faculties/fsg/drjj1.h tml Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA Dynamic Energy Dynamic Energy Transfer – Heat, Transfer – Heat, Work and Mass Work and Mass Thermodynamics Lecture Series
40
Embed
Email: [email protected] hotmail.com Applied Sciences Education Research Group (ASERG)
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Phase Change of Water - Pressure ChangePhase Change of Water - Pressure ChangePhase Change of Water - Pressure ChangePhase Change of Water - Pressure Change
H2O:Sat. Vapor
H2O:Sat. Vapor
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
H2O:SuperVapor
H2O:SuperVapor
T = 30 C
P = 2 kPa
T = 30 C
P = 2 kPa
H2O:Sat. Liq.
Sat. VaporSat. Vapor
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
T = 30 C
P = 100 kPa
T = 30 C
P = 100 kPa
H2O:C. liquid
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
H2O:Sat.
liquid
Water when pressure is reducedWater when pressure is reduced
Phase Change of WaterPhase Change of WaterPhase Change of WaterPhase Change of Water
P, C
, m3/kg
101.35
g@
100 C
1,553.8
1.2276
200 C
10 C
100 C
f@
100 C
22,090
P- diagram with respect to the saturation
lines
P- diagram with respect to the saturation
lines
Saturated Liquid-Vapor Mixture
H2O:Sat. Liq.
Sat. VaporSat. Vapor
t
g
m
mx
Vapor Phase:, VVapor Phase:, Vgg, m, mgg, , gg, u, ugg, h, hgg
Liquid Phase:, VLiquid Phase:, Vff, m, mff, , ff, u, uff, h, hff
Mixture:, V, m, Mixture:, V, m, , u, h, x, u, h, xMixture’s qualityMixture’s qualityMixture’s qualityMixture’s quality
Divide by Divide by total mass, total mass,
mmtt
Divide by Divide by total mass, total mass,
mmtt
gft
gavg x
m
m
1
ggffavgt mmm
fgfg wherewherewherewhere
fgf xx fgf x
ggfgt mmm
Given the pressure, P, then T = Tsat, yf < y <yg
Given the pressure, P, then T = Tsat, yf < y <yg
Saturated Liquid-Vapor Mixture
H2O:Sat. Liq.
Sat. VaporSat. Vapor
t
g
m
mx
Vapor Phase:, VVapor Phase:, Vgg, m, mgg, , gg, u, ugg, h, hgg
Liquid Phase:, VLiquid Phase:, Vff, m, mff, , ff, u, uff, h, hff
Mixture:, V, m, Mixture:, V, m, , u, h, x, u, h, xMixture’s qualityMixture’s qualityMixture’s qualityMixture’s quality
Given the pressure, P, then T = Tsat, yf < y <yg
Given the pressure, P, then T = Tsat, yf < y <yg
fgfg wherewherewherewherefgf x
fgfg yyy wherewherewherewherefgf xyyy
If x is known or has been determined, use If x is known or has been determined, use above relations to find other properties. If above relations to find other properties. If
either either , u, h are known, use it to find quality, x., u, h are known, use it to find quality, x.
If x is known or has been determined, use If x is known or has been determined, use above relations to find other properties. If above relations to find other properties. If
either either , u, h are known, use it to find quality, x., u, h are known, use it to find quality, x.
y can be y can be , u, h, u, hy can be y can be , u, h, u, h
CHAPTER
3
Energy Transfer byHeat, Work, and MassGoal: Identify forms of energy interactions and ways of representing it in thermodynamics processes
IntroductionIntroduction
1. Identify the types of dynamic energies interacting with a system.
2. Distinguish the difference and relate between heat transfer and thermal energy.
3. Write the different symbols and the conventions used to represent heat transfer.
4. Differentiate between heat transfer and work done.
Objectives:Objectives:
IntroductionIntroduction
5. Write the symbols and convention used for work done.
6. Obtain a mathematical relation representing mechanical work done for any system.
7. Obtain the amount of work done from a P – V or P - graph.
8. Write down the relationship between mass and volume flow rate.
Objectives:Objectives:
IntroductionIntroduction
9. Obtain a mathematical relation representing mass flow rate in terms of the mass velocities and the system’s inlet or exit area.
10.Write the specific energy carried by a flowing mass.
11.Use all mathematical relations and graphing skills to solve problems involving interaction energies.
Objectives:Objectives:
3-1
Energy Transfer -Heat Energy Transfer -Heat TransferTransfer
Energy Transfer -Heat Energy Transfer -Heat TransferTransfer
Source of thermal energySource of thermal energySource of thermal energySource of thermal energy
QQinin, , kJkJ
QQinin, , kJkJ
kg
kJ
m
Qq in
in , ,t
QQ in
in
kW
s
kJor
Symbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transfer
Total heat entering, QTotal heat entering, Qinin = 100 kJ, m = 100 kJ, m = 5 kg= 5 kg
Total heat entering, QTotal heat entering, Qinin = 100 kJ, m = 100 kJ, m = 5 kg= 5 kg
Rate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 seconds
Symbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transfer
Total heat leaving, QTotal heat leaving, Qoutout = 20 kJ, m = 20 kJ, m = 5 kg= 5 kg
Total heat leaving, QTotal heat leaving, Qoutout = 20 kJ, m = 20 kJ, m = 5 kg= 5 kg
Rate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 secondsRate of heat transfer if heat is allowed to interact for 50 seconds
Symbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transferSymbols and Convention – Heat transfer
Net heat Net heat transfertransferNet heat Net heat transfertransfer
kWs
kJ
s
kJ
t
QQQQ outin
innet 2.1 2.1 50
60,
kg
kJ
kg
kJqqqq outininnet 12
5
)2080(,
Net specific heat transferNet specific heat transferNet specific heat transferNet specific heat transfer
Net rate of heat transfer if heat is allowedNet rate of heat transfer if heat is allowedto interact for 50 secondsto interact for 50 seconds
Net rate of heat transfer if heat is allowedNet rate of heat transfer if heat is allowedto interact for 50 secondsto interact for 50 seconds
J 60 20 80, kkJkJQQQQ outininnet
H2O:SuperVapor
H2O:SuperVapor
QoutQout
Qin
Qin
Example: A steam power cycle.Example: A steam power cycle.
SteamTurbine
Mechanical Energyto Generator
Heat Exchanger
Cooling Water
Pump
Fuel
Air
CombustionProducts
System Boundaryfor ThermodynamicAnalysis
System Boundaryfor ThermodynamicAnalysis
Steam Power Plant
Qou
tQ
ou
t
Win
Win
WoutWout
QinQin
Energy Transfer – Work Done
ii
Voltage, VVoltage, V
No heat transferT increases
after some time
No heat transferT increases
after some time
H2O:SuperVapor
H2O:SuperVapor
Mechanical work:Piston moves up
Boundary work isdone by system
Mechanical work:Piston moves up
Boundary work isdone by system
Electrical work is done on systemElectrical work is done on system
H2O:Sat.
liquid
Wpw ,kJWpw ,kJ
We = Vit, kJWe = Vit, kJ
ViW e
Symbols and conventions for Work – Symbols and conventions for Work – Isothermal ProcessIsothermal Process
Symbols and conventions for Work – Symbols and conventions for Work – Isothermal ProcessIsothermal Process
H2O:Sat. Vapor
H2O:Sat. Vapor
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
H2O:SuperVapor
H2O:SuperVapor
T = 30 C
P = 2 kPa
T = 30 C
P = 2 kPa
H2O:Sat. Liq.
Sat. VaporSat. Vapor
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
T = 30 C
P = 100 kPa
T = 30 C
P = 100 kPa
H2O:C. liquid
T = 30 C
P = 4.246 kPa
T = 30 C
P = 4.246 kPa
H2O:Sat.
liquid
Water when pressure is reduced – no thermal energy or heatno thermal energy or heatWater when pressure is reduced – no thermal energy or heatno thermal energy or heat
System expands, volume increases-boundary work is doneSystem expands, volume increases-boundary work is doneSystem expands, volume increases-boundary work is doneSystem expands, volume increases-boundary work is done
Final, nFinal, nFinal, nFinal, nFIGURE 3-19A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds. InitialInitialInitialInitial
nbbb
f
ibb WWWWW ,2,1, ...
Total work done by system toTotal work done by system toexpand from initial to final stateexpand from initial to final stateTotal work done by system toTotal work done by system toexpand from initial to final stateexpand from initial to final state
nn
f
ibb dsFdsFdsFWW ...2211
WorkWork done involves done involves force moving an objectforce moving an objectin the direction of movementin the direction of movementWorkWork done involves done involves force moving an objectforce moving an objectin the direction of movementin the direction of movement
Pressure exerted on a surface is the ratioPressure exerted on a surface is the ratioof force applied w.r.t. the area of surfaceof force applied w.r.t. the area of surfacePressure exerted on a surface is the ratioPressure exerted on a surface is the ratioof force applied w.r.t. the area of surfaceof force applied w.r.t. the area of surface
kPaA
FP ,
f
i
i
i
bb dsFWW
As ds approaches zeroAs ds approaches zeroAs ds approaches zeroAs ds approaches zero
Final, nFinal, nFinal, nFinal, n
FIGURE 3-19A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds.
FIGURE 3-19A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds.
InitialInitialInitialInitial11112222Pressure exerted on a surface is the ratioPressure exerted on a surface is the ratio
of force applied w.r.t. the area of surfaceof force applied w.r.t. the area of surfacePressure exerted on a surface is the ratioPressure exerted on a surface is the ratioof force applied w.r.t. the area of surfaceof force applied w.r.t. the area of surface
FIGURE 3-19A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds.
InitialInitialInitialInitial11112222
When the pressure is kept constant,When the pressure is kept constant,Isobaric process,Isobaric process,When the pressure is kept constant,When the pressure is kept constant,Isobaric process,Isobaric process,
if
f
i
b VVPdVPW ThenThenThenThen
iiff
f
i
b VPVPdVPW OrOrOrOr wherewherewherewhere PconstPP if
Specific Boundary Work, Specific Boundary Work, bb,,outout
Specific Boundary Work, Specific Boundary Work, bb,,outout
Final, nFinal, nFinal, nFinal, n
FIGURE 3-19A gas does a differential amount of work Wb as it forces the piston to move by a differential amount ds.
InitialInitialInitialInitial11112222
When the pressure is kept constant,When the pressure is kept constant,Isobaric process,Isobaric process,When the pressure is kept constant,When the pressure is kept constant,Isobaric process,Isobaric process,
FIGURE 3-22The net work done during a cycle is thedifference between the work done by thesystem and the work done on the system.
Work done - Cyclic process
Total work is area of A minus areaTotal work is area of A minus areaof B. of B. Total work is shaded areaTotal work is shaded areaTotal work is area of A minus areaTotal work is area of A minus areaof B. of B. Total work is shaded areaTotal work is shaded area
Flow work is energy required to overcome resistance at the Flow work is energy required to overcome resistance at the Boundary both while entering and leaving the systemBoundary both while entering and leaving the systemFlow work is energy required to overcome resistance at the Flow work is energy required to overcome resistance at the Boundary both while entering and leaving the systemBoundary both while entering and leaving the system
Work done – Flow work
kJPVW f ,
kgkJPf / ,
InletInletInletInlet
ExitExitExitExit
kJPVW f ,
kgkJPf / ,
Energy Transfer – Mass Conservation
Consider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area A
Mass must be conserved in any process. Mass must be conserved in any process. Mass must be conserved in any process. Mass must be conserved in any process.
kgmmm sysoutin ,
0
sysmInlet
ExitAA
Length,
orororor
For steadyFor steadyflowflow
For steadyFor steadyflowflow
s
kgmmm sysoutin ,
ThenThenThenThen
outin mm
Energy Transfer – Energy of Moving Mass
Consider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area A
Let the mass flow through for time Let the mass flow through for time interval of interval of t, such as 10 secondst, such as 10 secondsLet the mass flow through for time Let the mass flow through for time interval of interval of t, such as 10 secondst, such as 10 seconds
Let the mass Let the mass flowflowwith a velocity with a velocity
Let the mass Let the mass flowflowwith a velocity with a velocity
Since volume is Since volume is Since volume is Since volume is mV Then volume Then volume flow rate is flow rate is Then volume Then volume flow rate is flow rate is t
m
t
V
s
mmV
3
,
Inlet
ExitAA
Length,
orororor
s
kgVm ,
Then massThen massFlow rateFlow rate
Then massThen massFlow rateFlow rate
Energy Transfer – Energy of Moving Mass
Consider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area AConsider mass flowing through cylinder of length and area A
Let the mass flow through for time Let the mass flow through for time interval of interval of t, such as 10 secondst, such as 10 secondsLet the mass flow through for time Let the mass flow through for time interval of interval of t, such as 10 secondst, such as 10 seconds
Let the mass Let the mass flowflowwith a velocity with a velocity
Let the mass Let the mass flowflowwith a velocity with a velocity
Since volume is Since volume is Since volume is Since volume is mAV Then volume Then volume flow rate is flow rate is Then volume Then volume flow rate is flow rate is t