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

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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

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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

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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

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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

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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).

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