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ME 475/675 Introduction to Combustion

ME 475/675 Introduction to Combustion. Lecture 8. Announcements. HW 3 Ch 2 (57), Due Monday Modify MathCAD program from lecture notes with. Dissociation. Last time: CO 2 as a function of temperature and pressure: ; three unknowns: , , ;

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ME 475/675 Introduction to Combustion

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  1. ME 475/675 Introduction to Combustion Lecture 8

  2. Announcements • HW 3 Ch 2 (57), Due Monday • Modify MathCAD program from lecture notes with

  3. 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: ; • + ,+ • …

  4. 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

  5. Flame temperature and major mole-fractions vs Tad[K] • 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 % “Old” “New” Fuel Rich Get CO, H2 Fuel Lean O2

  6. Minor-Specie Mole Fractions • NO, OH, H, O • < 4000 ppm = 0.4% • Peak near ppm

  7. 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)

  8. Atomic Balance for Leancombustion • C: so • H: so • O: so • if • Mole Fractions • ; ; ; • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/index.htm

  9. Comparison • Total number of modes decreases as increases • Does not include CO or H2 at • Not bad for a simple model for H2O CO2 N2/10 O2

  10. 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

  11. 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

  12. Solution • Since , use “-” root • Mole fraction can be calculated from b • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/index.htm

  13. Comparison H2O • 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 CO CO2 N2/10 H2

  14. 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

  15. Exhaust or Flue Gas Recirculation • Inserting exhaust gas into the reactants reduces flame temperature, which can reduce pollution (oxides of nitrogen, NO, NO2)

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