ME 475/675 Introduction to Combustion Lecture 8. Announcements.
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Transcript of ME 475/675 Introduction to Combustion Lecture 8. Announcements.
Dissociation• Last time: CO2 as a function of temperature and pressure: • ; three unknowns: , , • ;
• For more complete analysis, add additional reactions• For add ; one more unknown: • One more constraint: ;
• + ,+ • …
Equilibrium Products of Combustion•Combine Chemical Equilibrium (2nd law) & Adiabatic Flame
Temperature (1st law) • For Example: Propane and air combustion • Ideal
• Four products for a range of air/fuel ratios: •Now consider seven more possible dissociation products:
•What happens as air/fuel (equivalence) ratio changes
Flame temperature and major mole-fractions vs • Equivalence Ratio • At , O2, CO, H2 all present due to
dissociation. Not present in “ideal” combustion• at .15• at .05
• and decrease for • For decreases faster• For decreases faster
Fuel RichGet CO, H2
Fuel LeanO2
%
Tad[K]
“Old”
“New”
Simple Product Calculation method
• No minor species
•
• Assume and are known
• What is a good assumption for lean or stoichiometric mixtures ?• • c = e = 0 (no CO or H2), but now include • 3 unknowns (b, d, f), 3 atom balances (C, H, O)
Atomic Balance for Lean combustion • • C: so • H: so • O: so
• • if
• Mole Fractions• ; ; ; • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/index.htm
Comparison
• Total number of modes decreases as increases• Does not include CO or H2 at • Not bad for a simple model for
0.6 0.7 0.8 0.925
30
35
40
4541.667
25.8
Ntot Phi( )
10.6 Phi
0.6 0.7 0.8 0.90
0.05
0.1
0.15
0.20.155
0
Xco2 Phi( )
Xh2o Phi( )
Xo2 Phi( )
Xn2 Phi( )
10
10.6 Phi
CO2
H2O
O2
N2/10
1000 1500 2000 2500 3000 35000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
T [K]
Kp
For Rich combustion
• ; no (or fuel)
• 4 unknows: b, c, d and e• 3 Atom balances: C, H, O• Need one more constraint
• Consider “Water-Gas Shift Reaction” equilibrium
• Not dependent on P since number of moles of products and reactants are the same• ;
• See plot from data on page 51• KP = 0.22 to 0.1635 for T = 2000 to 3500 K
Atomic Balances
• C: • (in terms of b and “knowns”)
• O:
• H:
• Plug into equilibrium constraint• • ; b is the only unknown!• Expand and collect terms to get
Solution
• Since , use “-” root
• Mole fraction can be calculated from b• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/index.htm
Comparison
• Total number of moles continues to decrease as increases• Does not include O2 at • Not bad for a simple model for • More accurate models may be developed by including more equilibrium reactions and constants
using computer programs• Computer Programs Provided by the Book
• Appendix F, pp. 713-4, for Complex Reactions
0.6 0.7 0.8 0.925
30
35
40
4541.667
25.8
Ntot Phi( )
10.6 Phi
Φ≤1
1 1.1 1.2 1.320
22
24
2625.8
20.429
Ntot Phi( )
1.41 Phi
1 1.2 1.4 1.6 1.80
0.05
0.1
0.15
Xco2 Phi( )
Xco Phi( )
Xh2o Phi( )
Xh2 Phi( )
Xn2 Phi( )
10
Phi
CO2
H2O
H2
N2/10
CO
Air Preheaters
• Preheating the air using exhaust or flue gas increases the flame temperature • Recuperators uses heat transfer across a wall• Regenerators use a moving ceramic or metal matrix