ME 475/675 Introduction to

CombustionLecture 4

Announcements

• Extra Credit example due now• HW 1 Due Friday• Tutorials• Wednesday 1 pm PE 113• Thursday 2 pm PE 113

• Please bring you textbook to class• Please turn in HW on white or engineering paper• Grading based on solution (not solely on answers)

Example:

• Last lecture (turned in today)• Problem 2.14, p 71: Consider a stoichiometric mixture of isooctane and air.

Calculate the enthalpy of the mixture at the standard-state temperature (298.15 K) on a per-kmol-of-fuel basis (kJ/kmolfuel), on a per-kmol-of-mixture basis (kJ/kmolmix), and on a per-mass-of-mixture basis (kJ/kgmix). • Find enthalpy at 298.15 K of different bases

• This time (turn in next lecture)• Problem 2.15: Repeat for T = 500 K

Standard Enthalpy of Isooctane

• Coefficients to from Page 702• ;

• Spreadsheet really helps this calculation

T [K] theta298.15 0.29815

a1 a2 a3 a4 a5 a6 a8-0.55313 181.62 -97.787 20.402 -0.03095 -60.751 20.232

h [kJ/Kmol] -224108.82 -0.16492 8.072412 -0.8639 0.040304 0.103807 -60.751

Enthalpy of Combustion (or reaction)

• How much energy is released from a reaction if the product and reactant temperatures and pressures are the same?• 1st Law, Steady Flow Reactor

• and Enthalpy of Reaction (< 0 for combustion)• Dependent on T and P of reaction

• Heat of Combustion

Reactants298.15 K, P = 1 atm

Stoichiometric

ProductsComplete Combustion

CCO2 HH2O298.15 K, 1 atm

�̇�𝐼𝑁<0 �̇� 𝑂𝑈𝑇=0

Stoichiometric Methane Combustion, CH4 • CH4 + __ (O2 + 3.76 N2) __ CO2 + __ H2O + ___ N2

•@ 25C and 1 kmol CH4

p 688 p 692 p 701 (Heat in to system for TR = TP)

Water Vapor

Other Bases• Per kg fuel

• Heat of Combustion• (Heat out for TR = TP)• See page 701, LHV = Lower Heating Value • Corresponds to water vapor in the products

• p 692 p 692

• • p. 701: Higher Heating Value = HHV = 55,528 (slightly larger due to dissociation?)

Per kg of reactant mixture

• LHV =

Adiabatic () Flame Temperature

• 1st Law, Steady Flow Reactor

• All chemical energy goes into heating the products• To find adiabatic flame temperature use• PP = PR and

StoichiometricReactants

TR PR

Complete Combustion ProductsCCO2 HH2OPP = PR, T = TAd

�̇�𝐼𝑁=0 �̇� 𝑂𝑈𝑇=0

Adiabatic Methane Combustion TR = 25°C

•CH4 + 2 (O2 + 3.76 N2) 1 CO2 + 2 H2O + 7.52 N2

•

Example (Turn in next time for Extra Credit)• Find TAd for a 25°C Stoichiometric mixture of Acetylene and air