Assumptions 1 This is a steady-flow combustion process. 2 The combustion
chamber is adiabatic. 3 There are no work interactions. 4 Air and the com-
bustion gases are ideal gases. 5 Changes in kinetic and potential energies
are negligible.
Analysis (a) The balanced equation for the combustion process with the
theoretical amount of air isThe adiabatic flame temperature relation HprodHreactin this case reduces tosince all the reactants are at the standard reference state and h- f°0 for O 2
and N 2. The h
f° and hvalues of various components at 298 K are
h- f° h
- 298 K
Substance kJ/kmol kJ/kmol
- 298 K
C 8 H 18 () 249,950 —
O 2 0 8682
N 2 0 8669
H 2 O(g) 241,820 9904
CO 2 393,520 9364Substituting, we havewhich yieldsIt appears that we have one equation with three unknowns. Actually we have
only one unknown—the temperature of the products Tprod—since hh(T)
for ideal gases. Therefore, we have to use an equation solver such as EES or
a trial-and-error approach to determine the temperature of the products.
A first guess is obtained by dividing the right-hand side of the equation by
the total number of moles, which yields 5,646,081/(8 + 9 + 47) 88,220
kJ/kmol. This enthalpy value corresponds to about 2650 K for N 2 , 2100 K for
H 2 O, and 1800 K for CO 2. Noting that the majority of the moles are N 2 , we
see that Tprodshould be close to 2650 K, but somewhat under it. Therefore,
a good first guess is 2400 K. At this temperature,This value is higher than 5,646,081 kJ. Therefore, the actual temperature is
slightly under 2400 K. Next we choose 2350 K. It yields
8 122,091 9 100,846 47 77,4965,526,6545,660,828 kJ8 hCO 2 9 hH 2 O 47 hN 2 8 125,152 9 103,508 47 79,3208 hCO 2 9 hH 2 O 47 hN 2 5,646,081 kJ 1 1 kmol C 8 H 1821 249,950 kJ>kmol C 8 H 182¬ 1 47 kmol N 22310 hN 2 86692 kJ>kmol N 24
¬ 1 9 kmol H 2 O 231 241,820hH 2 O 99042 kJ>kmol H 2 O 4
¬ 1 8 kmol CO 2231 393,520hCO 2 93642 kJ>kmol CO 24
aNp^1 h°fhh°^2 paNr^ h°f,r^1 Nh°f^2 C 8 H 18C 8 H 181 2 12.5 1 O 2 3.76N 22 S8CO 2 9H 2 O47N 2772 | Thermodynamics