MAE 5310: COMBUSTION FUNDAMENTALS

1. MAE 5310: COMBUSTION FUNDAMENTALS Lecture 4: Introduction to Chemical Equilibrium August 29, 2012 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk

2. CHEMICAL EQUILIBRIUM • So far we have calculated adiabatic flame temperature assuming complete combustion • All fuel is completely oxidized to form CO2, H2O and excess O2 and N2 are carried through unaffected • Assumption reasonable for T < 1250 K, but most combustion systems operate at higher T • Species that are normally stable at ambient conditions dissociate • Concentration is determined by a balance between oxidation and formation • Balance is a function of T, P and concentration • Note: The chemical equilibrium relations we will use still only approximate the species concentrations in a combustion process. That is, they rest on the assumption that the conditions are constant for a sufficiently long time for all the reactions to reach equilibrium

3. ADDITIONAL PRODUCT FORMATION • NO Dissociation: Complete Combustion • Equivalence ratio less than or equal unity, f≤ 1 • The products formed are: CO2, H2O, O2, and N2 • Equivalence ratio greater than unity, f > 1 • The products formed are: CO2, CO, H2O, H2, and N2 • WITH Dissociation • Products formed include: CO2, CO H2O, H2, H, OH, O2, O, NO, N2, and N • Concentration is dependent on T, P and f

4. CHEMICAL EQUILIBRIUM FOR A FIXED-MASS SYSTEM • If final temperature of combustion reaction is high enough, CO2 will dissociate • Can calculate adiabatic flame temperature as function of a (a = fraction of CO2 dissociated) • Must consider second law: dS ≥ 0 • Composition of system will shift toward point of maximum entropy when approaching from either side, since dS is positive • Once maximum entropy is reached no further changes since would violate second law • (dS)U,V,m = 0

5. PROPANE-AIR COMBUSTION AT 1 ATMAdopted From: Turns, S.R., Introduction to Combustion