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MAE 5310: COMBUSTION FUNDAMENTALS. Lecture 3: Heat of Combustion and Adiabatic Flame Temperature August 27, 2012 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk. ADIABATIC FLAME TEMPERATURE.
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MAE 5310: COMBUSTION FUNDAMENTALS Lecture 3: Heat of Combustion and Adiabatic Flame Temperature August 27, 2012 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk
ADIABATIC FLAME TEMPERATURE • For an adiabatic combustion process, with no change in KE or PE, temperature of products is called Adiabatic Flame Temperature • Maximum temperature that can be achieved for given concentrations of reactants • Incomplete combustion or heat transfer from reactants act to lower temperature • Adiabatic flame temperature is generally a good estimate of actual temperature achieved in a flame, since chemical time scales are often shorter than those associated with transfer of heat and work • Most common is constant-pressure adiabatic flame temperature • Conceptually simple, but in practice difficult to evaluate because requires detailed knowledge of product composition, which is function of temperature
1st LAW FOR COMBUSTION PROBLEMS (GLASSMAN) • Most general form (rarely used, but know what each term means) • Note on sign of Q is negative. This is consistent with the 1st Law, but you must recall that Qp the is heat evolved, or the heat given off by the system, or the heat of combustion or heat of reaction. Sensible enthalpy change from T=298 to some reference Term is zero if reference T=298 Enthalpy of formation of products at T=298 K Sensible enthalpy change (kJ/mol) relative to some reference T Sensible enthalpy change relative to some reference T Term is zero if reactants enter at some reference T Sensible enthalpy change from T=298 to some reference Term is zero if reference T=298 Enthalpy of formation of reactants at T=298 K
1st LAW FOR COMBUSTION PROBLEMS (GLASSMAN) • Much more common, and what we will use in MAE 5310 Sensible enthalpy change relative to T=298 K Enthalpy of formation of products at T=298 K Sensible enthalpy change relative to T=298 K This term is zero if reactants enter the system at T=298 K Enthalpy of formation of products at T=298 K
ADIABATIC FLAME TEMPERATURE (GLASSMAN) • For an adiabatic combustion process, with no change in KE or PE, temperature of products is called Adiabatic Flame Temperature, Tad • Maximum temperature that can be achieved for given concentrations of reactants • Incomplete combustion or heat transfer from the reactants act to lower the temperature • The adiabatic flame temperature is generally a good estimate of the actual temperature achieved in a flame, since the chemical time scales are often shorter than those associated with transfer of heat and work
EXAMPLE: FUEL-LEAN OCTANE-AIR COMBUSTION • Calculate Tad of normal octane (liquid) burning in air at f= 0.5 • Assume no dissociation of stable products formed • All reactants are at 298 K and system operates at a pressure of 1 atm • Compare results with figure
