Patrick Lynch - Mr. Lynch or Patrick - not yet a doctor/professor (April)Syllabus - online
Same book used in ME 400-
Use 6th edition of book - 5th has wrong questions
Office hours: Tuesday 4-5 in 1220 MEL, Thursday 3-5 in 336 MEBThermodynamics: how universe works with energy transferSterling engine: heat to workWeekly HMWK - due fridays - do it NEATLY, with computer generated plotsQuizzes on Fridays - every 2 weeks - bring text and calculatorMidterms (2) in class
First ClassWednesday, January 20, 2010
10:55
Notes Page 1
Control mass□
No mass exchange□
Ex: gas in cylinder□
Closed system
No mass transfer (mT) or heat transfer (HT)□
Isolated system (subset of closed system)
Control volume□
Mass can be exchanged□
Ex: pumps, turbines, people (food/air in, CO2/waste out)□
Open system
Systems are separated from surroundings by boundaries
boundaries can move
Selection of boundaries effects type of system
Systems - ignore everything else in universe-
What do we actually study?
Need to do lasers, chemical kinetics …
Microscopic view: Could use individual molecular behaviour, with quantum mechanics, statistics and total description (done in ME 404)
-
Macroscopic view (we will use it): overall behaviour -
Set of properties is a state
Properties: macroscopic characteristics (ex: variables in pV=nRT)-
This change is called a process (a series of states in a system)
Change states: T, n, V: want P to go to 2P-
Cycle: a series of states that repeat themselves and come back to where they were (like cupstacking cycle)
-
Describing systems
The capacity to produce an effect
Translational energy
Rotational
Potential
Adding energy but not going into motion□
Internal energy
In bonds□
Chemical energy
Total energy = E = Ekinetic+Epotential +IE
Energy-
Properties
Correspondence principle: limit of microscopic into macroscopic
These 3 are energy related to motion - in each they are related to the mass
Lecture 1Wednesday, January 20, 2010
11:17
Notes Page 2
HMWK 1 on website (5 problems) - due Friday
Must be in a specific format --
Given-Find-Solution format, then box answer at end-
Graphs: use computer generated ones (can sketch too, but must have the computer one)
-
Homework format
Problems are in American units
Units are Joules (Internal energy is same as IE is same as U)
All depend on mass
KE and PE depend on motion
Energy: E = (KE)+(PE)+(IE)-
Sense of hotness/coldness
With thermometer (mercury/alcohol) - kindergarten way□
Thermocouple - higher temperatures, dissimilar metals - puts out changes in voltage (ME 360)
□
Pyrometry - uses radiation to determine what temperature is (ME 522)□
Measure
Intensive property
Temperature is a unit that tells us if there will be a change when two "blocks" at different "hotnesses" are brought together
Temperature-
Properties:
Intensive property: not dependent on massExtensive property: dependent on massSpecific property: an extensive property divided by mass
Lower case means it is the intensive property of the extensive property.
Bar means molar - this is molecular specific energyThese all have the same temperature
If A is in thermal equilibrium with B and B is in thermal equilibrium with C then A is in thermal equilibrium with C.
Zeroeth law of thermodynamics:
0 C is freezing point of pure water at 1 atm
100 C is boiling point of pure water at 1 atm
Celsius:-
F=1.8C+32
Farenheit scale-
Tabsolute zero = -273.15 C
Tabsolute=C+273.15
It is the Kelvin scale
Absolute scale-
Temperature Scales:
X degrees FahrenheitY degrees CelsiusZ Kelvin (no degrees)
Fahrenheit scale goes to Rankine for absolutes
Extensive property
Specific property: specific volume
Volume-
Mass and volume are easy to measure, so see a lot
Density in liquids (and sometimes solids) is often normalized to density of pure water
Specific gravity: ratio of density to density of water (Hg has specific gravity of 13.6 - more in tables in text)
Start with definition from Physics 211
Pressure-
PropertiesFriday, January 22, 2010
10:56
Notes Page 3
1 Newton is about the weight of an apple□
Units: base is Pascal, 1 Pa=1Nm-2 - extremely small unit
1 bar = 105 Pa
1 MPa = 106 Pa
English units: lbfin2=psi => 14.5 psi~= 1 bar
Often accepted to assume 1 atm ~= 1 bar = 0.1 Mpa
Alternate unit, 1 atm = 1.013 bar□
Pgage=Pabs-Patm◊
This can be negative:◊
Pvacuum=Patm-Pabsolute=-Pgage◊
Difference in pressure from atmospheric is called gage pressure:
Pressure measurements not easy to make on absolute scale - usually done as a difference, often from atmospheric pressure
□
Atmospheric air pressure: 1.013 bar, 101300 Pa (at sea level)
Gives the funny units of pressure: 1" water, 1"HG, 1 mm = 1 Torr□
1 Torr = 33.416 Pa□
Important application: liquid volume
Notes Page 4
Pure substance: has one chemical composition: ex: CO2, H2O, N2, O2 - air and gasoline and vodka are not pure substancesPhase: a quantity of matter that is homogeneous both chemically and physically EX: water (H2O) as solid, liquid and gas (we primarily deal with liquids and gases)
Recall that a state is a series of properties that define a system
Gibb's was the first PhD in America (Yale 1863)-
Number of degrees of freedom = number of compounds - number of phases + 2-
We can't prove until last week of class-
Degrees of freedom: # of variables that can change independently-
Usually written as: F = C - P +2-
C = 1, P = 1, F = 1-1+2=2
This means that if we know two properties the third will be defined
Example: water vapor-
C =1, P=2, F= 1-2+2=1
Therefore if you have one property the other two will be defined
Example: ice water-
C=1, P=3, F = 1-3+2=0
All of them are known at this point
Example: triple point-
Example-
Gibb's Phase Rule
Supercritical fluid: cannot tell the difference between vapor and liquid
If melting line has a negative slope the species expands on freezingIf melting line has a positive slope the species contracts on freezing
Isotherms: constant temperature lines
Isobars: lines of constant pressure on T-V diagram
More Properties: Description of P,V,T systemsMonday, January 25, 2010
10:58
Notes Page 5
Isobars: lines of constant pressure on T-V diagram
2-phase region also known as vapor dome
Points on diagram are states-
If you change something, that is a process - a line on the diagram-
Diagrams:
Process 1-2: compression with pV = constant from p1=1 bar, V1 = 1.0m^3, to V2 = 0.2m^3
Process 2-3: constant pressure expansion to V3=1.0m^3
Process3-1: constant volume
Problem 1: A gas contained within a piston-cylinder assembly undergoes three processes in series.-
Examples:
The critical point is an isobar inflection point
Problem 2: A closed system consisting of 5 kg of gas undergoes a process during which the relationship between pressure and specific volume is pv1.3=constant. The process begins with p1=1 bar and v1 =0.2m^3/kg and ends with p2 = 0.25 bar. Determine the final volume in m^3 and plot the process on a PV graph.
Notes Page 6
DiagramsMonday, January 25, 2010
18:42
Notes Page 7
Notes Page 8
Notes Page 9
Notes Page 10
Notes Page 11
Notes Page 12
Notes Page 13
Notes Page 14
Notes Page 15
Notes Page 16
Only part of homework that needs to be on computer is graphs
dw=F.ds=Fcos(phi)ds-
Work is force times distance-
Work:
Differential notation because (1) they apply at very small changes and (2) they are then path dependent
W=J/shP=745WPS (german horsepower) = 736W
Units of Power
Electric work
Thermodynamic definition of work: Energy exchange equivalent to lifting a weight - requires organized motion of boundaries of a system
System: gas inside the room
Work going into system is negative (done on system)
-
Work going out of system is positive (done by system)
-
Sign notation:
Heat: Energy exchange from a system at one temperature to a system with a lower temperature (note: not equivalent to the lifting of a mass)
-
Heat:
ProcessesWednesday, January 27, 201010:56
Notes Page 17
Heat transfer is microscopic changes in molecular motion - disorganizedAll heating up will never make the cold one move away
(more detail in ME 320)-
Conduction-
Convection-
Radiation-
Three forms of heat transfer
Heat is not shown on pV diagram
Units: Joules, Bru, calories (4.184 Joules, 1 calorie is the heat needed to raise 1g of water from 14.5 to 15.5 C)
Add heat to a system, positive-
Pull heat out then negative-
Sign convention:
Heat transfer by two objects in contact with each other-
Conduction
Heat transfer by bulk movement of a fluid-
Convection
Heat transfer that doesn't depend on anything between the two bodies - can happen in a vacuum -
Strong temperature dependence (difference thereof)-
Everything radiates (presuming it's above 0K)-
Radiation
Work Heat
Organized motion of system boundaries Disorganized molecular motion
W: extensivew: intensive
Q: extensiveq: extensive
Many modes 3 modes
Can see on diagram Not explicit on P-v diagram
Sign: +ve if extracted, -ve if provided Sign: +ve if provideded, -ve if extracted
Summary:
Note: for radiation must use Kelvin, as have T4 (could use rankine, but uncommon -just needs to be absolute)
Notes Page 18
Path dependent Path dependent
Notes Page 19
Work (W) Heat (Q)
Organized motion of system boundaries Disorganized molecular motion
W: extensive (J)w: intensive (J/kg or J/kmol)
Q: extensive (J)q: intensive (J/kg or J/kmol)
Many modes: all of form Only three modes:conduction, convection, radiation
Can see explicitly on P-v diagram Does not appear explicitly on P-v diagram.
Sign: (+) extracted from the system (expansion)
(-) provided to the system (contraction)
Sign: (+) provided to the system(-) extracted from the system
Path dependent Path dependent
Heat always flows from hot to cold
Pick a direction when solving equations, if Q comes out positive the heat transfer direction is correct - sign accordingly, positive if provided to system, negative if extracted from system
Work-Heat comparisonFriday, January 29, 201011:01
Notes Page 20
1st quiz next week in class - 15-20 minutes, will cover HMWKS 1 and 2
Axiom: The amount of work produced in a closed cycle is the same as the amount of heat provided to that cycle.
Clockwise positive workAnticlockwise negative work
Note: unlike Q and W, Q-W does not depend on the path taken, but just on the initial and final states = energy
As in E0=0, then ΔES0=Es-E0=ES-
Can have a reference state of energy
Is dV all that really matters for work?
Balance of heat and workFriday, January 29, 2010
10:59
Notes Page 21
H=U+PV (J)-
H=u+Pv (J/kg)-
Therefore: u=h-pv-
Enthalpy: accommodates the fact that changes in pressure are important
Notes Page 22
In class quiz on Friday - open book, need calculator - chapters 1/2 and a bit of 3
dQ=dW+dU-
dW=pdV, thus dQ=pdV+dU-
Enthalpy, H=U+pV-
dQ=dH-VdP, -VdP is technical work-
First law of thermodynamics:
dq=du+pdV=du
dq=du - additions of heat go directly into internal energy changes
The internal energy changes with Temperature
ΔV=0, constant volume-
a
Note: may also see in per mole forms, (dH)p=ncp(dT)p
Δp=0, constant pressure-
Solids and liquids: cp and cV are approximately constants, and are approximately the same
Substance Specific heat (J/kgK)
Aluminium 900
Steel 480
Copper 385
Water 4180 = 1kcal/kgK
□
Examples (pg850 in text)
Gases,
cp and cV are functions of T and p-
Special cases:
Addition of heat under constant pressure situations go directly to changes in enthalpy.
First law continuedMonday, February 01, 201010:59
Notes Page 23
d
dq=du+pdv => dq=cvdT+pdVdq=dh-vdp => dq=cpdT-vdp
When cp and cv are only dependent on temperature the first law becomes:
Notes Page 24
Notes Page 25
Make sure to do computer generated graphs on homework
Concepts: identifying governing equations, assessing work term, heat term, change in internal energy term…
-
Quiz Friday in class - open book, closed notes, bring calculator
pV=nRT, R is universal gas constant, R=8314 J.kmol-1K-1, can also use mass based unitsi.Equation of state: p,V,T1.
All ideal gasses have this property (converse is false)i.For ideal gasii.
cv and cp (and consequently U and h) are only functions of T2.
Ideal gas is an assumption we often use
Helium (sunny gas)
Neon (new gas)
Ar (slow gas)
Kr (hidden gas)
Xe (foreign gas)
Inert gases (noble gases)-
O2,N2, CO, CO2
Components of air-
In principle gases not close to vaporization (not always true though)-
Gaseous fuels: CH4 (methane), C2H2 (ethyne, but better called acetylene), C2H4 (ethene, better ethylene), C3H8 (propane)
-
Note: not water - it is not an ideal gas - will use steam tables-
What gases are ideal?
Ideal GasWednesday, February 03, 2010
10:58
Notes Page 26
Must use close points to match slope well (eg don't use 650K and 750K)
-
Can do 2D as well, values at T and P can be averaged
-
Caveats:
Notes Page 27
HMWK 3 ready - plots/sketches… must be computer generatedQuiz is 15 minutes
Isopleth, Isochore-
Constant volume process:
Isobar-
Constant pressure process
Isothermal process-
Constant temperature process:
Consequences of Ideal gas approximationFriday, February 05, 2010
10:53
Notes Page 28
No heat transfer-
Adiabatic process
On Monday we will discuss polytropic process, PVn=constant
Notes Page 29
1lbf = 32.2 lb.ft.s-2-
'lb' is same as 'lbm'-
Torr is mm Hg-
Units: will be on website - a description thereof
pVn=constant-
Polytropic process
Constant V-
Constant p-
Constant T-
Adiabatic - constant pV, dQ=0-
Generalized Polytropic -
Consequences of ideal gas assumption
n=0
Isobaric (constant p)-
n=1
Isothermal (constant T)-
n=gamma
Adiabatic (no ΔQ)-
n=infinity
Isochoric (constant V)-
All cases we have gone over are special cases of polytropic process
Units, Quiz, Polytropic ProcessMonday, February 08, 2010
10:54
Notes Page 30
3.75 not on homework anymore
Example: 10g of air at 250kPa, 300 C Piston has a mass of 75 kg, d=0.1mAtmosphere is 20 C and 100 kPa
What is the total heat transfer?a.What is T when the piston detaches from the stops?b.
Cylinder cools to room temperature
2 processesWednesday, February 10, 201011:00
Notes Page 31
Notes Page 32
Pv=RTIsotherms: P=1/v*constant
IG EOS:Non-Ideal Gases:
In this upper region have ideal gas behaviour, that is T>>TC
ΔU? ΔH?How to deal with other properties?
Compressibility, Z = Pv/(RT)Compressibility is the difference from ideal gasZ=1 for Ideal GasZ is a function of how far or close you are from the critical pointReduced pressure: PR=P/PC
Reduced temperature: Tr=T/TC
Compressibility:
Tr>>1, T>>TC-
Pr<<1, P<<PC-
To be an ideal gas:
Only relates P,v,T - doesn't mention ΔU or Δh-
Can't deal with the two phase region-
Compressibility is not the whole story -
Must rely on tabulated values
Use interpolation, pressures 6kPa to 320 barFor each P, lowest T is saturated temperatureFor each row have v,u,h,sBelow saturated temperature must go to compressed liquid tables
Find v, v=1.112e-3 m3.kg-1, therefore 180 C in the tableIf given pho is 900 kg.m-3
, p=20 bar, find T
EX: Start as superheated water vapor (vapor hotter than saturated temperature) - tables on page 821 (A-4)
Tables for this too, table A-2, can look up with either T or P2 phase region:
P and T are not the only inputs, also have phase
BRING TEXT ON FRIDAY
Book has water, refrigerant 134A, ammonia, propane
Non Ideal gasesWednesday, February 10, 2010
11:16
Notes Page 33
Q=W+UEasy: take the whole box
-W1=W2=Q2=-Q1, Q1=W1, therefore ΔU is zeroHard: choose the two parts separately, W1=-W2, ΔU2=U2-U1, W2=Q2, Q1=-Q2,
3.122: could identify system in two ways
For a given P there is a Tsaturated, if T>Tsat then vapor, if T<Tsat then compressed liquidFor a given T, then if P>Psat then liquid, then P<Psat in vapor
Properties in compressed liquid/superheated vapor
=100 bar
Tsaturated is the lowest in saturated vapor table, and highest in compressed liquid
At Tsat and Psat-
Gibb's Phase rule: F=C-P+2-
Also kind of sets v: the properties of the liquid are set and the properties of the gas are set
In this region there is a mixture of saturated vapor and saturated liquid
New property: quality x=mvapor/(mvapor+mliquid)
F=1-2+2=1, so only 1 dof in two phase region - if you choose a T that sets P -
What happens in 2 phase region?
This allows properties to change continuously in the two phase region
Sudden expansion of a non ideal gas1.Example:
2 phase regionFriday, February 12, 201010:58
Notes Page 34
Sudden expansion of a non ideal gas1.
Isochoric heating and cooling of a real gas1.
Find mass and T and P after partition bursts
Notes Page 35
Notes Page 36
Quiz FridayProbably keep a copy of homework
2 phase region-
x=mvapor/mtotal=mvapor/(mvapor+mliquid)-
Quality: the extent to which a substance vaporizes-
x=0 saturated liquid
x=1 saturated vapor
Can be anywhere in between
(Psat,Tsat),-
Given v, u find T, P and x - most difficult case, requires iteration
If in 2 phase region probably need quality to find anything (unless can be calculated)
If state is 'saturated vapor' or 'saturated liquid' then use x - just use the values in the column
Finding properties in 2 phase region-
v=xvgas+(1-x)vliquid
Cancel vliquid since it is essentially 0 (vg>>vf), then x=v/vg
Good approximation far from critical point
Approximate way□
h=xhg+(1-x)hf
h=xhg+hf-xhf
x=(h-hf)/(hg-hf)
This is because hg-hf=hfg, and is in the table as well - in between the saturated liquid and saturated vapor values
◊
This is easiest to do with enthalpy (but can be done with any property)
A more rigorous way□
T=200 C, therefore 852.45=>2793.2 is 2 phase, so this is 2 phase, so P=15.54 bar
h=hf+hfgx, 1730 kJ/kg =852 kJ/kg +x 1941 kJ/kg, therefore x=45%
v=xvg+(1-x)vf= 0.45 * 0.1274 m^3/kg+0.55*0.0115 m^3/kg= 0.058 m^3/kg
Example: T=200 C, h=1730 kJ/kg. Find P, x, v if saturated□
Find quality
Interpolation-
Quality:
5 basic steam table cases online
This is very important for quiz
QualityMonday, February 15, 2010
10:58
Notes Page 37
So far we've studied processes in a control mass system. It is much more useful to study scenarios in open systems - control volumes.
Mass flow rate is conserved-
Mass balance:
Energy is extensive-
Change in energy in a control volume-
Energy balance
All heat sourcesAll work sourcesEnergy flowing in and out of cv (control volume)
Need to separate work terms
1st law analysis of a control volumeMonday, February 15, 2010
11:17
Notes Page 38
CV cannot move1.No accumulation terms and dWcv/dt and dQcv/dt are constant2.
Two things:-
Steady state operation-
Steady State flow processes Passive system = no work done
Wednesday will do passive systems
Notes Page 39
Friday HMWK due + quiz - BRING BOOK, can use1st test is next Friday
Primarily chapter 3-
Inclusive-
Quiz Friday:
Open systems/control volumes
Open SystemsWednesday, February 17, 2010
10:58
Notes Page 40
Notes Page 41
Notes Page 42
Notes Page 43
Many of these - engines, pumps - we will do compressors and turbines-
Recall control mass:-
Active devices: consume or produce work
Put names on assignments (your name) Test next Friday: everything through today - won't test system integrationWill put some problems upMust bring book
Driving force is mass flow rate => does work, turbine-
Can force water with a pump/compressor-
Add power (work) to increase the pressure of a fluid-
Schematic:-
Special case: fluids of constant density (liquid) - called a pump-
Compressor:
Remove work to decrease the pressure of fluid-
Schematic:-
Turbine
Of both turbines/compressors-
Ideally adiabatic devices, but in reality there is heat transfer - still good assumption-
Analysis:
TurbinesFriday, February 19, 2010
10:58
Notes Page 44
Notes Page 45
Format similar to quizzes, conceptual based questions and then problems
Hard way: ΔQ=ΔW+ΔU - can then use coupling (but difficult) - solve for x from Q=m(u2-u1)+mP(v2-v1) => use x eqns
-
Medium way (book): collect terms Q=m(u2+Pv2)-m(u1+Pv1)=m(h2-h1), h2=h1+Q/,=465kJ/kg, therefore in 2 phase therefore T=41 C, get that x=0.85 (piston is ΔV/A)
-
Easy way (class): notice constant pressure, dQ=dH-Vdp, dp=0, dQ=dH-
Identifying systems on quiz
Enthalpy is for no pressure change systemsConstant volume: Q=ΔU+pdVConstant pressure: Q=ΔH-Vdp
Test/QuizMonday, February 22, 2010
10:59
Notes Page 46
Online problems for review - solutions on Wednesday evening
Exam informationMonday, February 22, 201011:00
Notes Page 47
Notes Page 48
Compressor Turbine
hout>hin hout<hin
Pout>Pin Pout<Pin
Power plant has 4 components
Even if we know P4 we cannot yet fully determine state 4 because we cannot yet solve for adiabatic expansion of non-ideal gas. For now will also be given that state 4 is a saturated vapor and state 1 is a saturated liquid and state 2 is close to a saturated liquid.
More active systems - power plantMonday, February 22, 201011:21
Notes Page 49
Notes Page 50
Test Friday - need book, strict time limit - at 11:50 no more writingWill have more office hours tomorrow
Power plant has 4 components
Figure 8.1 in text
Pump increases pressureBoiler creates steamExtract work from turbine to make work
There is a separate condenser loop here tooLow pressure, high enthalpy steam goes into condenser
Back to pump
Integrating of control volume systems:
Important ObservationsWednesday, February 24, 201011:00
Notes Page 51
Notes Page 52
Transient analysis: terms change with timeWe use a special case - uniform state, also called a uniform flow process
Can't move, can't expand (Not a balloon)i.A rigid box is an exampleii.
Control volume is fixed1.
Change in properties is allowed, but has to be uniform - must assume you fill the entire box2.
If there's a moving stream (to fill a box) it does not change. But if you fill something up, it changes.
i.
Example: filling a boxii.
Constant state in areas of flow3.
Assumptions:
Filling a box-
Example:
Final internal energy is streams enthalpy
If ufinal=hinitial, then, T=0 reference state-
Cv(Tf-Ti)=cp(Ti-0)-
Tf=Ticp/cv=kTi-
Ideal Gasses:
Transient analysisWednesday, February 24, 2010
11:27
Notes Page 53
Friday, 12 March will meet in 160 English BuildingAlso will have Quiz 3 then
Exams back on Friday likely
Hot water will not get even hotter in a room temperature roomBalloon will not blow up by itselfBook will not fall upwards1st law: conserve energy
Predicts direction of a process-
Determine theoretical limits of engineering systems-
2nd law of thermodynamics
EX: rivers, lakes, oceans, atmosphere-
Thermal reservoir: special hypothetical body that always remains the same temperature even though is added or removed by heat transfer
Clausius statement: It is impossible to construct a device that operates in a cycle and produces no other effect other than the heat transfer of heat from a lower temperature body to a higher temperature body.
-
Kelvin-Planck statement: It is impossible to construct a thermodynamic cycle to produce work only interacting with a single thermal reservoir.
-
This restricts efficiency to less than 100%
2nd law of thermodynamics statements
Doesn't work
Perpetual motion machine-
Examples
Also, only one thermal reservoir.
2nd law: Spontaneous processesMonday, March 01, 201010:58
Notes Page 54
Could put balloon back together after popping it-
There are no reversible processes (at least not real ones)-
Reversible process: process that can be reversed without leaving any trace on its surrouundings
Friction1.Heat transfer through a finite temperature difference2.
Break a partition in a box, gas spreads, cannot go back to have gas in only halfi.Unrestrained expansion of a gas or liquid3.
Spontaneous mixing4.Spontaneous chemical reactions5.Electric current flow through a resistance6.Magnetism or polarization with hysteresis 7.Inelastic deformation8.
Irreversabilities
Power cycle
Refrigeration cycle
Efficiency cannot be one
Restrict our efficiency -
Effect of irreversibilities
Easy to analyze-
Serve as an idealized model to which we compare real cycles-
Carnot cycle - highest maximum efficiency of every cycle we know of
Can define the Kelvin temperature scale
By combining 4 reversible processes, we can make a reversible cycle and come up with maximum theoretical efficiency.
-
Why discuss reversible processes?
Notes Page 55
It is reversible
1 to 2 Adiabatic compression - very fast
2 to 3 Isothermal expansion, ΔT=0 - very slow
3 to 4 Adiabatic expansion - very fast
4 to 1 Isothermal compression - ΔT=0 - very slow
Efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.
-
Efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
-
The Carnot corollaries
Efficiency is independent of the working fluid, type of cycle, or type of reversible engine, since the two reservoirs are characterized by temperature.
Efficiency of Carnot engine
This function can only be satisfied if the function has the form
The Carnot Cycle (or engine)Wednesday, March 03, 201010:58
Notes Page 56
This function can only be satisfied if the function has the form
Must use absolute temperature scale here
Have a diesel cycle operating between TH=2000K and TL=300K.Examples:
Example 2: Find thermal efficiency and amount of heat rejected
Example 3:
Find an expression for the efficiency for a single power cycle operating between T H and TC in terms of the efficiencies of the two cycles here.
Notes Page 57
85+ very good:75+ good understanding50-75 average
Problem mine Out of
1 16 16
2 9 9
3 10 10
4 6 10
5 17 25
6 24 30
total 16+9+10+6+17+24=82 16+9+10+10+25+30=100
My:
4. don't use ΔU, but use ΔH5b. Don't overcomplicate - just do graphically5d. Just use Q=U-W and tables6. read the table correctly
ExamFriday, March 05, 2010
11:02
Notes Page 58
The direction that processes go - if they only go one way, they are irreversible -
Just because something satisfies the first law doesn't mean it can happen:-
Heat flows from hot to cold, not the other way□
Heat transfer through a finite temperature difference
Bursting of a diaphragm □
Unrestrained expansion
But p2<p1, gas/liquid always flows towards lower pressure□
Throttling valve, where Δh=0
Examples-
Irreversibility:
Cycle that has no irreversibilities-
The best such cycle-
No heat transfer => no irreversibility -
dT=0, isothermal heating, no finite temperature difference for heat transfer, so no irreversibility-
Efficiency of Carnot cycle - maximum efficiency-
Only dependent on temperature
Only works for Carnot cycles
-
Carnot cycle
Works when heat is added or removed reversibly, infitesimly small amounts at a time
Arbitrary cycle:
Based on geometry, one can break a reversible cycle into Carnot parts as long as the cycle is also reversible.
ReviewFriday, March 05, 2010
11:12
Notes Page 59
These are numerically integrated and tabulated
Easy to calculateFor an ideal gas?
Entropy is not just a function of T, but also has a log dependence on v. (unlike u, h, c p, cv.
Only puts the part of entropy that depends on temperature, but not the part on specific volume or pressure.
-
's0' is called absolute entropy and is only based on T.-
Table A-22 has ideal gas entropies.
Notes Page 60
HMWK due Friday 16:45 in mailbox - 158 MEBClass Friday cancelledQuiz is take-home, will get it on Wednesday - due same time
Related to disorganized heat transfer and not work: EntropyEntropy:
Calculated and tabulated using first law
Example:-
Entropy and irreversible processes:
Disorganized energy transferMonday, March 08, 201010:59
Notes Page 61
Notes Page 62
Saturated liquid line on left, saturated vapor on right.Has a vapor dome - CP at top
Carnot cycle
Specific entropy
Temperature
2 adiabatic and reversible expansions/compressions2 isothermal heatings/coolings Carnot cycle appears as a BOX on the diagram
Carnot cycle
Notice:
Reversible heat is area under T-S curveFor a cycle, Q=W
Just as pV diagram did - but easier to calculate here for Carnot cycle
So, for a cycle work is area under curve too
Power producingQrev>0,W>0
Clockwise:
Refrigeration cycleQrev<0, W<0 - consumed work
Anticlockwise:
T-S diagram also has enthalpy, pressure, volume, and quality - can calculate internal energy from those -has everything necessary to solve a problem
Very important Topic: TS diagramMonday, March 08, 201011:32
Notes Page 63
Put HMWK in mailbox - 158 MEB, 16:45, as well as take home quiz
Carnot cycle appears as a squareArea under curve gives us Heat (which, in a cycle, gives us Work)Has almost everything
Constant temperature
Isothermal process-
Constant pressure
Isobaric process-
Constant volume processes
Isochoric process-
Different processes on the diagram:
Processes on T-S diagramWednesday, March 10, 2010
10:58
Notes Page 64
Constant enthalpy, decreasing pressure
Much easier on TS diagram
Isenthalpic throttling (Joule-Thompson throttling)-
NOTE: if something is both adiabatic and reversible then it is isentropic
Isentropic expansion/compression-
Also important for passive devices - heat was mass flow rate times change in enthalpy - chart with enthalpy as an axis! - mollier diagram-
Notes Page 65
Mollier: German Changes in enthalpy very important - so this is a useful diagram.Shows relatively high enthalpies/entropies - vapour regionsThere is a vapour dome - but peak is NOT critical point - whole thing is saturated vapour lineIsobars/isotherms parallel in 2-phase region, isochors are NOT
Isotherms-
Isobaric-
Isochor-
Isenthropic throttling-
Active devices (turbine / compressor)
Isentropic expansion/compression-
Processes:
Enthalpy/Entropy diagram (Mollier Diagram)Monday, March 15, 2010
10:58
Notes Page 66
d-
Notes Page 67
HMWK 7: state 2 is a saturated vapour not liquid
Tables1.T-s diagram2.h-s diagram3.
Three ways to solve problems:
Can solve for real devices-
In general, devices are not perfect and deviate from perfectly reversible behavior.-
Heat transfer in turbine/compressor
EX: near the wall□
Leaks do not pass through the turbine blades
Friction in the fluid flow
Other irreversibilities (dead cat)
Irreversibilities:-
Example: -
Imperfect devices:
Imperfect devicesWednesday, March 17, 2010
11:02
Notes Page 68
Notes Page 69
6.40, 6.59, 6.85, 6.115, 6.132 - make sketches on print off of T-s diagram
Carnot-like refrigeration cycle on water vapor. There is a turbine to recover some of the compressor work, but neither the compressor nor the turbine is necessarily isentropic (although they are adiabatic)State 3: 530 C and 50 barState 4: 5 barState 1: same temperature as state 4, 50 kPaState 2: 530 CSeveral parts long
Additional Problem
HMWK 8 - look for email:
He will not be here on Wednesday after break - will be acceptable on Monday afterwards so as to go to office hoursQuiz on Monday (1 week after break - see syllabus)
Can still be adiabatic - loss from leaks, friction, general irreversibilities (dead cats)If still adiabatic not isentropic anymore
Actual processes and devices have irreversibilities
Actual ProcessesFriday, March 19, 2010
10:58
Notes Page 70
Because of fluid mechanics, the isentropic efficiency of a turbine is generally greater than that of a compressor.
Notes Page 71
Differential form of entropy
Even in steady state there is generation of entropy with the production term ENTROPY IS NOT CONSERVED
Entropy balance for control volumes (open systems)Friday, March 19, 2010
11:28
Notes Page 72
Notes Page 73
HMWK 8 due on 2 April, but will be accepted until MondayQuiz will be had - will be slightly less involved than the HMWK
Efficiency - irreversibilities reduce efficiencyControl mass
Entropy and the Second LawMonday, March 29, 2010
10:58
Notes Page 74
Notes Page 75
Notes Page 76
Office hours: Friday, 2 April in 336 MEB, 14:00-17:00-
HW8: due 2 or 5 April-
Quiz: 2 April-
Exam: 9 April-
Announcements:
Efficiency of a Carnot cycle-
Change in entrooy-
For a control volume-
Key concepts of 2nd law and entropy analysis
Review of EntropyWednesday, March 31, 2010
10:53
Notes Page 77
Notes Page 78
Notes Page 79
Quiz today
Next Friday - midtermNext Thursday 3-7pm office hours 336 MEBExam will be through chapter 6 - not chapter 7Turn in HMWK Monday in classOffice hours today 2:15 to 5 pm
3 ways: tables, hs diagrams, Ts diagramsSolving problems with real fluids
Electric motor problem
Friday, April 02, 2010
10:58
Notes Page 80
Heat exchangers□
Throttling devices□
Nozzles and Diffusers□
Mixers□
Passive devices (no work)
Turbines□
Compressors□
Pumps□
Active devices (work involved)
System integration (also using entropy considerations to solve these)-
Chapter 4:
Thermal reservoirs-
Clausius (cannot have a system with only result of HT from cold to hot)
Kelvin-Planck (cannot produce work from interacting with one reservoir)
Second law statements-
Causes of irreversibility
Reversible process
Reversible vs. irreversible-
How you make a thermal Carnot cycle (2 adiabats, 2 isotherms)
Maximum efficiency (for any cycle at the two reservoir temperatures)
What this means for real cycles (lower efficiency)
Real substance, ideal gas, ideal gas + const. specific heat
Power, refrigeration, heat pump
Carnot Cycle-
Chapter 5
Two sources (reversible heat transfer and irreversibilities)
Definition of Entropy Change-
Area representation of HT (and when not appropriate)
Carnot cycle on these
Solving processes
T-s and h-s diagrams-
Chapter 6
Will be practice problems online tonight - solutions WednesdayExam on Friday
Exam 2 Key ConceptsMonday, April 05, 201010:59
Notes Page 81
Rankine cycleSteam cycle (today)
Otto (gasoline)DieselBrayton (gas turbines)
Gas cycles (next week)
Inverted Carnot-Rankine cycleRefrigeration
Overview of cycles (Power generating, Refrigeration)
Referred to as Rankine cycle-
Steam cycle
Note: direction of arrow is positive
Power cycle, therefore clockwise on T-s diagram-
QH>QL - heat adding at boiler much higher than heat losing at condenser, makes sense as QH-QL=Wnet
-
Note importance of enthalpy of vaporization - it impedes our ability to go up in temperature (or down) as we must surpass it
-
The amount of work needed to pump a liquid from 3 to 4 is very low - the specific volumes are very low due to high density and Wpump=vΔp, v is small so not much work -incompressible fluid
-
Liquids aren't good for turbines - want it to run with vapor
Greatly increases QH, but does not increase QL, thus
We need superheat - must super heat the steam to gain a lot of work
-
Notes:
CyclesMonday, April 05, 201011:06
Notes Page 82
increasing work and efficiency
How to make better use of QH-
Sketch -
How to improve the efficiency?
Increase the temperature keeping pressure constant1.
Increase pressure in the burner2.
Costly, but can be done-
2a. Put both together - high temperature and pressure - supercritical operation
Lower the condenser pressure3.
Decreases QL, but very difficult to actually do - would necessitate vacuum pressures
Reheat - If too much QH goes unused4.
Notes Page 83
Notes Page 84
Exergy/availability analysis (chapter 7)-
Thermodynamic equilibrium-
Rudimentary combustion-
Novel automotive trends (increasing efficiency/decrease emissions)-
Climate change-
Which topic for extra time teaching (question on exam, Friday, in class):
Office hours 3 to 7 tomorrow
Relatively inefficient cycle
Use superheati.Increase boiler temperature1.
Somewhat lowers QL rejected at condenseri.Increase boiler pressure2.
May have to operate at least partway in a vacuum - not easy but possible
i.Lower the condenser pressure3.
Taking two passes through the boiler - we like the superheated region here
i.
Can do multiple reheatsii.
Reheat4.
Used when running a power plant and use steam for other purposes too
i.Cogeneration5.
Preheat some of the water that enters into the boiler using some of the steam bled off of the turbine
i.
Requires less heat at the boiler1)Heat transfer is occurring at a higher temperature, and this is inherently more efficient
2)
Increases efficiency for two reasonsii.
Regeneration6.
Increase efficiency by:
1 and 2 are limited by the metal used, can operate supercritically (very high T and p)
throttle it and put it back in the condensera.
Use a double condensate pumpi.Overall, reduce the pumping powerii.Pump the condensate liquid up to intermediate pressure, then combine and pump them both up to the boiler
iii.
If you have a liquid coming out of the cogeneration (at high pressure)b.
Two options:
Increasing efficiency of Rankine cycleWednesday, April 07, 2010
10:55
Notes Page 85
Notes Page 86
ExampleWednesday, April 07, 2010
11:27
Notes Page 87
Notes Page 88
Exam problem 1: most answers for automotive trends - so we will do that for the last few daysExams back Wednesday or Friday - are graded already
Similar to other cycles-
Closed Feedwater Heater
Closed Feedwater heater
Rankine cycleMonday, April 12, 2010
10:58
Notes Page 89
IC = Internal Combustion
Reciprocating engines-
Gas turbines-
www.animatedengines.com-
2 types
Spark plug for gasFuel injector for dieselBore: diameter of piston
At top is clearance volume - least amount possible, called top dead centerAt bottom called bottom dead center, that volume is called the displacement volumeDisplacement volume = Area*stroke
Stroke: distance piston travels
Crank angle: measured from top dead center - CAD (crank angle degrees) - degrees from top dead center
Each stroke is half a rotation-
Power stroke is the one where spark plug ignites-
4 stroke engine:
Gasoline air mixture then ignited-
Delivers power at every rotation-
Intake and exhaust at same time-
2 stroke cycle
Intakes air - at top dead center you inject liquid diesel which expands itself-
Diesel engine
Allowed to do so because gasoline is highly volatile - goes to vapor very easily
Otto: control ignition very well as its spark ignition - fast propagation through the cylinder
-
Called "auto-ignition"
Much more difficult to control
Very active area of research
Diesel: not very volatile - inject it and it burns when it's ready -based on appropriate conditions for T,p, oxidizer concentration
-
4 stroke operation - 2 rotations per power cycle-
2 stroke operation - every rotation gives a power cycle-
Fundamental differences between gasoline (Otto cycle) and diesel (diesel cycle):
Not really a thermodynamic cycle-
Has components but not really a cycle - we continuously replenish gasoline/air and release exhaust
-
We can model it as a cycle because of periodically replenishing the air-
Practically (for the purposes of this class) the working medium is almost entirely air-
Even though Mwfuel=170 kg/kmol, and Mwair= 29 kg/kmol, the mass ratio of mfuel/mair
is approximately 1/15□
Therefore we commonly assume working medium is all air□
We also tend to assume it is IG (for the purposes of this classes we assume cp, cv, and γare constant
□
C12H26+37/(2*.21)*(0.21O2+0.79N2)=12CO2+13H2O+37*0.79/0.42*N2
EXAMPLE: dodecane: C12H26 - typical average formula for a diesel-
Thermodynamic Considerations
Constant cp, cv, and γ - referred to as cold air analysis-
P-v diagram can be used-
Our 2 assumptions:
V2: clearance volumeV1-V2: displacement volumeAt V2: top dead centerAt V1: bottom dead center
2 isochors and 2 adiabats-
Rankine cycle had 2 isobars and 2 adiabats-
For a 2 stroke part merely cut off the A->1 part-
A to 1: intake stroke
1 to 2: compression stroke
3 to 4: power stroke
1 to A: exhaust stroke
Strokes:-
Otto Cycle:
At A open inlet valves-
Go from A to 1 by letting in fuel and air mixture into cylinder (assume constant p)
-
At 1 close the intake valve-
Go from 1 to 2 by compressing adiabatically-
At 2 hit the spark plug-
Burn the gas, and it releases the heat isochorically -
Go 3 to 4 by expanding adiabatically-
At 4 open the exhaust valves-
Go 4 to 1 by isochoric-
1 to A is the exhaust stroke-
At A close the exhaust valves and then repeat-
Cycle:
IC EnginesMonday, April 12, 201011:13
Notes Page 90
Knocking: early autoignition of gasoline
If gasoline goes off by itself, that is bad
High compression ratios give us knock-
BUT lead is hazardous to your health
Limit knocking: add lead to the gas, which limits the propensity to burn-
Switching to unleaded gas means we need lower compression ratios, maximum compression ratio is typically about 8-10
-
87, 91 and 93
Octane tells the propensity of a fuel to resist knocking
Bigger cars tend to need higher octane rated gas - it allows it to reduce the knocking more
Octane rating:-
But there are problems with this:
Notes Page 91
High displacement volume gets high workEg: to get 200HP it is much easier with a 4L engine than a 1.5L engineNeed a metric that factors this size out
Other important calculationsWednesday, April 14, 2010
11:46
Notes Page 92
Diesel, Otto, and Atkinson cyclesHW10 assigned - do number 1 with excel
-
How do we get from work in cycle to power?
How do we get to power?Friday, April 16, 2010
11:01
Notes Page 93
If it is cold outside it may not work as wellThey used to use glow plugs - it's a bit of heat that helps it to ignite when it is cold (10-20 years ago)Now we vary the injection - though some use glow plugs
Here we want and need high compression ratios so that state 2 is high temperature and high pressure - this is NECESSARY - once it's ignited it must burn.
Diesel EngineFriday, April 16, 2010
11:12
Notes Page 94
Otto cycle is limited to compression ratios of 8-10Diesel cycle compression ratios are typically around 20 or moreTherefore the efficiency of a diesel engine tends to be higher as a higher compression ratio lets this happen
Note that real ones are not completely perfect
Notes Page 95
4 stroke 4 cylinder diesel engine. Total displacement of 2000cc and operates at 2000 RPM. qH=1800kJ/kg airAir intake has p1=1 bar, T1=15 C, r=20, cutoff ratio is 2.88Find: net work per cycle, the power, thermal efficiency, MEPHint: cold air standard, constant cp, cv and gamma
Example Diesel ProblemMonday, April 19, 201010:59
Notes Page 96
Compresses the incoming air to high pressure
Compressor-
Burns the fuel and produces high pressure, high velocity gas
Combustion area-
Extracts the energy from the high pressure, high velocity gas flowing from the combustion chamber
Turbine-
Three parts:
It is not strictly a cycle - replenishment like in others
Interested in high T, high P gas - need momentum equations for that (ME 310)-
Used on jet planes
We analyze them for power
Modeled as:
Brayton: two isobars, two adiabatsSame as Rankine cycle, BUT we are compressing a gas rather than a liquidRankine had a phase change, which made liquids very easy to pumpBrayton cycle deals with gas, which is much more costly to pressurize from an energy standpoint
Compare to Rankine cycle
Used as backups, occasionally in small power plants-
Even though they are inefficient, you can get a lot of power-
Turbine just recovers the work from the compressor, BWR ~= 1, maybe less□
Interested in the high T, high p exhaust gas for thrust□
Jet engine
Put on car□
Don't really want a lot of work, just enough to compress air□
Turbocharger
Can also use to convert power
Used in applications which demand high power-
Due to this, gas turbines are expected to be inefficient
Gas Turbines (Brayton cycle)Monday, April 19, 2010
11:18
Notes Page 97
d
Notes Page 98
Make it look more like a Carnot cycle1.Increase the temperature at which we add heat2.Decrease the temperature at which we exhaust3.
Brayton cycle (continued)Wednesday, April 21, 201011:01
Notes Page 99
Notes Page 100
Example 1: Regenerator aloneWednesday, April 21, 2010
11:19
Notes Page 101
Notes Page 102
Example 2: Regenerator + Reheat + IntercoolingWednesday, April 21, 2010
11:39
Notes Page 103
Notes Page 104
Stirling cycle with regeneration
Kind of have two adiabatic processes - heat is transferred through regeneratorHowever, it only works properly with low temperature differences, so not particularly practical
Stirling EngineFriday, April 23, 2010
11:15
Notes Page 105
\\ //\||/
Also called inverted Rankine cycle-
Vapor-compression refrigeration cycle
Therefore it consumes work
Note: this is a refrigeration cycle, therefore it goes anticlockwise (as opposed to clockwise for power cycles)
Fluid inside refrigerator must operate at lower temperature than desired temperature - if operating at 100 C, cannot cool to more than 100 CMust reject heat to a temperature lower than the high temperatureThese are refrigerator design constraints
The important thing about refrigeration cycles are the working fluids, called refrigerants
Not on earth - would have to operate at at least 100 C - at typical temperatures it would be nearly a solid, nearly impossible to move
-
Is water an appropriate refrigerant?
A boiling point below the target temperaturea.Relatively high enthalpy of vaporization, h fg=hg-hf.b.High density, as then it has low specific volume, so lower pumping/compressing requirementsc.High critical temperature - makes peak temperature closer to average temperatured.
Best refrigerants have certain properties
For refrigerants, play with pressures to make it work
B and C mean you can have lower mass flow rates
This required very high pressures
One of first refrigerants was CO2,-
A very good refrigerant - wonderful
Low boiling point, relatively large h fg
Ammonia smells awful at 5ppm□
It could ruin your lungs if you stayed in it for a long time□
Also flammable□
Why not use it?
Later on NH3 (ammonia)-
Chlorofluorocarbons (chlorofluoronates)
EX: freon
Don't use them anymore because of the chlorine - contributes to ozone depletion
Work pretty well as a refrigerant
Then developing refrigerants, first were CFC's-
Hydrofluorocarbons (hydrofluoronates)
Still have chlorine in them, but less
Still have ozone effects
Will we use these in the long term? Probably not.
Then HFC's (mostly used in USA) - still used-
Next step, probably back to CO2 or NH3 - a very active area of research-
A brief history of refrigerants (not time)
Coefficient of performance-
Efficiency of refrigeration cycles
Can be relatively high, 2 to 3
Compressor - compressing gas-
Throttling valve-
Relatively inefficient processes here
RefrigerationMonday, April 26, 201010:57
Notes Page 106
Will use R134a, has properties in book (along with R22 and NH3)Compressor has isentropic efficiency of 80%Want QC=30kWWant TC=4 CFind: all the states, the operating temperatures, the power requirement, the COP
Example: industrial refrigerator
Notes Page 107
Notes Page 108
1 cooling ton = 12,000 Btu/hour1 Btu = 1055 J1 hour = 3600s1 cooling ton = 12,000*1055/3600=3516.6667 J/s
Cooling ton: this is a unit of power
Units in coolingWednesday, April 28, 2010
11:10
Notes Page 109
Constant p in evaporator
Constant p in condenser
2 isobaric processes-
1 isenthalpic throttling-
1 adiabatic compression, possibly isentropic-
We have
Be careful - pressure is on a log scaleHas density rather than specific volume
T-s an dh-s diagrams do not have that problem
Note: the reference point is different than that used in the tables - therefore one cannot go between the two from the p-h diagram
Ideal system that operates between 10 bar and 0.5 bar.-
Use R-134a-
Minimum cooling temperature
Maximum temperature to reject heat into
COP
Find-
Coming out of condenser it is a saturated liquid at high p
Coming out of evaporator it is a saturated vapor at low p
3 to 4 is constant enthalpy
1 to 2 is constant entropy
Solution-
Example:
Easier way to solve refrigeration problemsWednesday, April 28, 2010
11:14
Notes Page 110
Next HMWK is due on 5 May 2010 - on refrigerationThere will be a 'concepts to know' thing onlineFinal exam is on the Monday, 7pm in normal classroom
Reheat: remember that it goes through the boiler several times - must include this in QH.
Same thing as a refrigerator, except you are interested in the heat you can reject in the condenser-
The Heat Pump
Want huge COP's
If operating at -20 C, then won't work in an area with lower temperatures than that - it wouldn't heat your home
This means you need moderate temperature differences to work
In climates such as Florida, this is a very attractive system - it acts as a heat pump and a refrigerator by merely twisting a valve (both heater and air conditioner)
But such a process wouldn't work in many climates (including IL)
The lowest temperature (outdoors) is TL, The highest indoor temp is TH. -
Throttling
Compressing gas - takes a lot more power than pumping a liquid
If we use absorption refrigeration, we can get around this - it generally uses ammonia, NH3
Inefficient processes-
Caveats of heat pumps
Pumping a liquid rather than compressing a vapor, so work requirement is much lower
-
Advantages
Only works with NH3 - there are problems with ammonia-
But this isn't too high, for the generator heat it could be solar or waste process heat or (portable system) use kerosene
For absorber can just use cooling water
Need to add heat to generator and take it out at absorber -this can be an issue
-
Much more complicated - have to pay the initial cost for all these parts - may not be worth it
-
Disadvantages
Working fluid is always a gas-
No phase change-
EXAMPLE: Brayton refrigeration cycle-
Gas refrigeration systemsCan get to really low Temperatures
Refrigerant: air
Have lower cooling capacity-
Dealing with air, so not taking advantages of a refrigerant-
BUT it isn't very efficient
Because that is when air condensesMain advantage is the possibility of extremely low temperatures (~70 K)
Finishing Refrigeration - Other refrigeration systemsFriday, April 30, 2010
10:59
Notes Page 111
Because that is when air condensesMain advantage is the possibility of extremely low temperatures (~70 K)
This is a closed system now, so it is actually a thermodynamic cycle.
Notes Page 112
There WILL be questions on the exam about this
3 important concerns: performance, safety, pollutants-
The bigger it is, probably safer, and higher power, but less fuel efficiency, and pollutes more
Over past 40 years needed to balance these competing pulls-
Huge driver of trends in automobiles and transportation
Often against market forces
Less pollutants and more performance are generally good□
Ideas that are used to be summarily dismissed now are needed to be researched, optimized and implemented
□
Challenges and opportunities (by government…)
Regulations have required the amounts to go down very low - about an order of magnitude multiple times - NOx, PM, Sulphur
Currently Europe is pretty strict, US pretty low - California higher, in between S. Korea, Canada, Australia, China, Japan close to EU in MPG converted to CAFÉ
Regulation-
C8H18+12.5O2=>8CO2+9H2O□
Take combustion of iso-octane:
BUT we live in air, have to add 47N2
What if we are not stoichiometric
Stoichiometric: have just enough fuel and oxidizer to complete combustion
Equivalence ratio:
Lean(Fuel lean): Φ<1: more oxidizer than fuel
Rich(Fuel rich): Φ>1: more fuel than oxidizer
Can control Φwith the carburetor in real life
If we have only 10 O2 molecules for every molecule of iso-octante□
C8H8+10O2+37.6N2=>3CO2+5CO+9H2O+37.6N2□
BAD - now have CO rather than CO2□
Could have other bad molecules□
Also haven't gotten all energy available□
What happens if we burn rich:
If we have 15O2 molecules for every molecule of iso-octane□
C8H18+15O2+56.4N2=>8CO2+2.5O2+9H2O+56.4N2□
Doesn't look bad BUT□
Combustion temperatures may be low□
Are going to end up getting NOx□
What happens if we burn lean
CO, Aldehydes, PAHs - all poisonous□
C2H4, - not harmful to humans, but harmful to plants - a ripening agent□
These come from burning too cool or too rich - not enough oxygen□
Unburned hydrocarbons
Increased respiratory symptoms
Decreased lung function
Asthma
Bronchitis
Irregular heartbeats
Heart attacks
~10microns
This is the actual smoke you see□
These are highly regulated, even those which are 2.5 microns (these are highly regulated too)
□
Formed from low temperature or rich conditions□
Particulate matter (PM)
NOx is the general term for NO, NO2, NO3□
NO is safe, but NO2 is a poison□
NO+O3=>NO2+O2
NO2+sunlight=>NO+O
NO2+O=>NO+O2
Contribute to ozone depletion□
Contribute to acid rain - like sulphur dioxide does□
Thermal NOx - high temperature, slow reaction, lean conditions
Prompt NOx - more quickly than thermal NOx, CN and HCN hydrocarbons, rich conditions, low T conditions
NNH
Bound to your fuels - interesting because companies are currently nitrogenating their fuels in an attempt to make it better (but it also produces NOx)
Formed by four mechanisms□
NOx
1970's and 1980's: LA catalyst for getting smog under control - much better today□
Pictures from outer space often a catalyst□
Secondary pollutant - formed by reactions of other pollutants□
NOx+unburnt HCs + sunlight => PM +O3□
PM is the hazy smoke, ozone near ground level is bad□
Smog
Pollutants-
Historical Perspective
Conflict final on Thursday 13 May 2010 at 13:30 in 256 MEB
Novel Automotive TrendsMonday, May 03, 2010
11:01
Notes Page 113
Exam: Monday 19:00 in 1310 DCL
NOx is the biggest deal now - four mechanisms on previous pageWhat does this mean for engines
Catalysts are surfaces that help reactions without being consumed-
Ceramic core, honeycomb structure, etc.
Alumina overcoat - increases surface area
Precious metals - gold, rhodium (most prevalent), platinum, palladium
There is an underground market for stealing these for the precious metals
Require high surface area structures coated with a special metal - typically-
1980's
2NOx=>xO2+N2□
Reduce NOx
2CO+O2=>2CO2□
Oxidize CO to CO2
Numerous reactions□
Oxidize unburned HC's
TWC works pretty effectively on 3 reactions□
Simple to install□
Advantages
Need to have tight control of temperature and concentration for optimal performance□
Need to run a little rich (wastes gas)□
Hard to take care of NOx on a diesel, but you need catalysts for the soot□
Catalyst poisoning□
Disadvantages
Three way catalyst-
Common now, will be more in future - on most cars
Widely used since 1970's
Some of the exhaust is recycled back into the intake
Instead of just air you now have CO2 and H2O as well, increasing specific heat and lowering temperatures
□
Lower exhaust temperatures - lowers thermal NOx□
Advantages
Inclusion of CO2 and H2O lowers mixture γ, which lowers efficiency□
Increases the amount of PM in diesels□
Smaller power density (can be an advantage, especially in Ottos)□
Disadvantages
Exhaust Gas Recirculation - EGR-
Otto's are great, but have low compression ratio
Diesels are great, but the fuel is injected and burns when it wants, thus little control of temperature or equivalence ratio. Also, typically burns rich and sooty.
Highly volatile□
Mixed well with air in Otto cycle□
Plenty of air everywhere to react□
Premixed□
Gasoline
Not mixed well, fuel has to meet up with air□
Local areas of high concentration fuel□
Diffusion flame□
Diesel
Automotive trends
Next step?-
Ways to fight these
More Novel Automotive TrendsWednesday, May 05, 2010
11:01
Notes Page 114
Higher compression ratio, higher efficiency
Otto to Diesel □
High degree of control
Accurate stoichiometric combustion
Low soot
Low NOx
Three way catalyst
Diesel to Otto□
Yes
Direct injection of gasoline (GDI)
HCCI and other LTC strategies
Are there regiments in between these that could be interesting?-
Gasoline Direct Inject
Instead of mixing gasoline up in the carburetor, only air is brought in and then injects gasoline in a hollow cone of injected fuel
Excellent control of injection and equivalence ratio□
Lower fuel consumption□
Multiple injections□
In theory can do higher compression ratios (in practice gains are lower than anticipated)
□
Less worry about knock□
Advantages
Cost□
Complex components, gasoline fuel injectors□
Higher pressure materials□
Disadvantages
GDI-
Homogeneous Charge Compression Ignition
Ignites in multiple points in the piston-cylinder assembly
Works with any fuel - gasoline, diesel, biofuel…
High compression ratios (r~15)□
Really lean conditions□
Lower fuel consumption□
Low temperature□
Low NOx - almost none□
Advantages
Knock□
Spotty ignition□
Low temperatures mean unburned hydrocarbons□
Higher pressure materials□
Really lean conditions can mean less power□
Lots of factors to control - like valve control□
Disadvantages
Being actively researched - many challenges, but increasingly useful
HCCI-
Notes Page 115
Chapter 1
– closed system – open system/control volume – surroundings – boundary
• Systems and identifying them
– extensive property – intensive property
• properties
– state– process (path)• SI unit system, conversion of units• Specific volume, density, specific gravity• Pressure
Kelvin, Celsius, Fahrenheit and Rankine scale–• Temperature (Zeroth Law of Thermodynamics)
Chapter 2
– kinetic energy– potential energy– internal energy
• Forms of energy
– heat transfer– work– sign convention
• Modes of energy transfer
• First Law of Thermodynamics – Energy conservation Various forms of 1st law:
Q = W + U (particularly useful for constant volume systems) why?
Q = - VdP (particularly useful for constant pressure systems) why?
Consumes or produces work–Direction–
• Energy analysis of thermodynamic cycles
Chapter 3• Phases of pure substances (solid, liquid, gas) • Phase, p-v and T-v diagrams• Enthalpy When to use it?• Specific heats
EOS–What does this say about enthalpy and IE?–
• Ideal gas model
• Closed system process relations (isothermal, adiabatic, const. volume, const. pressure)
Critical point properties, reduced temperature and pressure–• Compressibility factor
– quality– saturation temperature – saturation pressure– superheated vapor
• Two-phase, liquid–vapor mixture
• Using steam tables to determine properties
Chapter 4• Mass flow and volumetric flow rate• Conservation of mass – mass rate balance• One-dimensional flow
How control mass is a special case of general energy law–• Energy rate balance (THE most general eq. of the 1st Law)
Why enthalpy matters and not just internal energy–• Flow work
heat exchangers–throttling devices–nozzles and diffusers–mixers–
• Passive devices (no work)
– turbines– compressors– pumps
• Active devices (work involved)
• System integration (also using entropy considerations to solve these)
Chapter 5• Thermal reservoirs
Clausius (cannot have a system with only result of HT from cold to hot)
Kelvin-Planck (cannot produce work from interaction with one reservoir)
• Second law statements
Causes of irreversibility
reversible processes
• Reversible vs. irreversible
How you make a thermal Carnot cycle (2 adiabats, 2 isotherms)
Maximum efficiency (for any cycle at the two reservoir temperatures)
What this means for real cycles (lower efficiency)
real substance, ideal gas, ideal gas + const. specific heat
power, refrigeration, heat pump
• Carnot Cycle
Chapter 6
Two sources (reversible heat transfer and irreversibilities)
• Definition of Entropy Change
App. 1.5 times length of hour exams - time shouldn't be as much of an issue as on midtermsHMWKS will be available tomorrow in Office hours (10,11,12) -15:00All solutions are online (except tests…)
Final Exam InformationWednesday, May 05, 201010:41
Notes Page 116
Two sources (reversible heat transfer and irreversibilities)
Area representation of HT (and when not appropriate)
Carnot cycle on these
Particularly for active deviceso
Solving processes
• T-s and h-s diagrams
Incompressible substances
Real substance
Ideal gas
• Change of Entropy
• Entropy balance for closed system (entropy production/irreversibility)
directionality
Entropy transfer by flux of mass
• Entropy rate balance for open and closed systems (entropy production)
turbine, compressor, nozzle
• Isentropic efficiencies
Chapter 8
Analyzing Vapor Power systems
turbine, condenser, pump, boiler, heating rate, fuel consumption
thermal efficiency
Deviation between actual vapor power systems and ideal
• Ideal Rankine cycle
Superheat
Boiler pressure
Condenser pressure
reheat
regeneration (open/closed feedwater heaters)
Cogeneration
Integration with other cycles, topping cycle
• Improving performance of the Rankine cycle
Chapter 9
Diagram, displacement volume, stroke, bore, CAD, MEP, etc.
• Engine Terminology
• Thermodynamic model of reciprocating internal combustion engine
How is this not officially a thermodynamic cycle. Why?o
Efficiency, work, power, etc.
• Otto and Diesel Cycles
How is this not officially a thermodynamic cycle. Why?o
Adding non-ideal compressors, turbines, etc.o
• Performance improvements to gas turbines– Intercooling, regeneration and reheat
• Brayton Cycle - gas turbine
• Stirling cycles
• cold air-standard analysis, constant specific heats, ratio, etc.
Why we use throttling valves instead of a turbine?o
Refrigeration cycle vs. power cycle
Environment requirements on condenser and evaporator temperatures.
Loads, cooling tons, etc.
Historyo
Refrigerants
Adding non-ideal compressors, etc.o
P-h diagramo
Analysis of the ideal inverse rankine cycle
COP
Heat pumps, combined heat pump/refrigerators
Absorption refrigeration
Gas refrigeration cycles
Chapter 10
Power requirementso
NOx
Unburned hydrocarbons
PM
Pollution Remediationo
Regulation
TWC-three way catalysto
EGR-exhaust gas recirculationo
HCCI-homogeneous charge compression ignitiono
LTC-low temperature combustiono
Developments and combat strategies
Combining the best components of Otto and Diesel cycles.
Automotive trends:
Notes Page 117