1. 1 MAE 5310: COMBUSTION FUNDAMENTALS Adiabatic Combustion Equilibrium Examples September 19, 2012 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk

2. 2 ADIABATIC COMBUSTION EQUILIBRIUM Previously we have considered: –Known Stoichiometry + 1 st Law (Energy Balance) → Adiabatic Flame Temperature Problems 1-4 –Known P and T + 2 nd Law (Equilibrium Relations) → Stoichiometry Problems 5-9 Now we can combine these: –1 st Law (Energy Balance) + 2 nd Law (Equilibrium Relations) → Adiabatic Flame Temperature + Stoichiometry Problems 10-14 Solution Scheme 1.Guess a T=T guess 2.Do equilibrium calculation to solve for species concentrations at Tguess 3.Plug into 1 st Law We want F(T guess )=0 If F(T guess ) > 0, then initial guess was too high If F(T guess ) < 0, then initial guess was too low 1.Increment T guess

3. 3 PRACTICAL APPLICATION: RECUPERATION A recuperator is a heat exchanger in which energy from a steady flow of hot combustion products, called flue gases, is transferred to the air supplied to the combustion process

4. 4 SOME COMMON TYPES OF RECUPERATORS http://www.hardtech.es/hgg_tt_hrt.0.html Tubes cage radiation recuperator working at 1,200ºC Double shell radiation recuperator Tubes cage radiation recuperator Installation consisting of a tubes cage recuperator and a double shell one, series-connected

5. 5 EXAMPLE: RECUPERATION (TURNS) A recuperator, as shown in figure, is employed in a natural-gas-fired heating-treating furnace. The furnace operates at atmospheric pressure with an equivalence ratio of 0.9. The fuel gas enters the burner at 298 K, while the air is pre-heated. 1.Determine the effect of air preheat on the adiabatic flame temperature of the flame zone for a range of inlet air temperatures from 298 K to 1,000 K. 2.What fuel savings result from preheating the air from 298 K to 600 K? Assume that temperature of flue gases at furnace exit, prior to entering recuperator, is 1700 K, both with and without preheat. Radiant-tube burner with coupled Recuperator for indirect firing. Note that All flue gases pass through the recuperator Source: Turns, An Introduction to Combustion

6. 6 NASA CEA PRACTICE PROBLEM Consider combustion of a methane-air mixture at 10 atm (both fuel and air are at 10 atm). 1.Plot mole fractions,  i, for the species CO 2, CO, H 2 O, H 2, OH, O 2, N 2, NO vs. temperature for  =1 for a temperature range from 1000 to 2500 K. 2.Calculate T flame and mole fractions as a function of  for adiabatic combustion. Plot these results vs.  and discuss at what value the peak flame temperature occurs. Comment on this value in light of the discussion found in Chapter 1, Part 2 of Glassman. 3.Compare T flame from part (2) to what would be obtained assuming complete oxidation (burning in only oxygen) and what would be obtained assuming complete combustion (burning in air).