Thermodynamics

772 | ThermodynamicsAssumptions 1 This is a steady-flow combustion process. 2 The combustionchamber is adiabatic. 3 There are no work interactions. 4 Air and the combustiongases are ideal gases. 5 Changes in kinetic and potential energiesare negligible.Analysis (a) The balanced equation for the combustion process with thetheoretical amount of air isC 8 H 18 12 12.5 1O 2 3.76N 2 2 S 8CO 2 9H 2 O 47N 2The adiabatic flame temperature relation H prod H react in this case reduces tosince all the reactants are at the standard reference state and h – f ° 0 for O 2and N 2 . The h – f ° and h values of various components at 298 K areh – f °h– 298 KSubstance kJ/kmol kJ/kmolC 8 H 18 () 249,950 —O 2 0 8682N 2 0 8669H 2 O(g) 241,820 9904CO 2 393,520 9364Substituting, we havewhich yieldsa N p 1h° f h h°2 p a N r h° f,r 1Nh° f 2 C8 H 1818 kmol CO 2 231393,520 h CO2 93642 kJ>kmol CO 2 4 19 kmol H 2 O231241,820 h H2 O 99042 kJ>kmol H 2 O4 147 kmol N 2 2310 h N2 86692 kJ>kmol N 2 4 11 kmol C 8 H 18 21249,950 kJ>kmol C 8 H 18 28h CO2 9h H2 O 47h N2 5,646,081 kJIt appears that we have one equation with three unknowns. Actually we haveonly one unknown—the temperature of the products T prod —since h h(T )for ideal gases. Therefore, we have to use an equation solver such as EES ora 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 bythe total number of moles, which yields 5,646,081/(8 + 9 + 47) 88,220kJ/kmol. This enthalpy value corresponds to about 2650 K for N 2 , 2100 K forH 2 O, and 1800 K for CO 2 . Noting that the majority of the moles are N 2 , wesee that T prod should be close to 2650 K, but somewhat under it. Therefore,a good first guess is 2400 K. At this temperature,8h CO2 9h H2 O 47h N2 8 125,152 9 103,508 47 79,320 5,660,828 kJThis value is higher than 5,646,081 kJ. Therefore, the actual temperature isslightly under 2400 K. Next we choose 2350 K. It yields8 122,091 9 100,846 47 77,496 5,526,654