Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

1500 K, determine (a) the mass flow rate of air and (b) the rate of heat

transfer from the combustion chamber.

Solution Liquid propane is burned steadily with excess air. The mass flow

rate of air and the rate of heat transfer are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Air and the combustion

gases are ideal gases. 3 Kinetic and potential energies are negligible.

Analysis We note that all the hydrogen in the fuel burns to H 2 O but 10

percent of the carbon burns incompletely and forms CO. Also, the fuel is

burned with excess air and thus there is some free O 2 in the product gases.

The theoretical amount of air is determined from the stoichiometric reac-

tion to be

O 2 balance:

Then the balanced equation for the actual combustion process with

50 percent excess air and some CO in the products becomes

(a) The air–fuel ratio for this combustion process is

Thus,

(b) The heat transfer for this steady-flow combustion process is determined

from the steady-flow energy balance EoutEinapplied on the combustion

chamber per unit mole of the fuel,

or

Assuming the air and the combustion products to be ideal gases, we have

hh(T), and we form the following minitable using data from the property

tables:

h

• f° h
  
  
  • 280 K h
    
    
    • 298 K h
      
      
      • 1500 K
        Substance kJ/kmol kJ/kmol kJ/kmol kJ/kmol
      
      
      
      
    
    
    
    
  
  
  
  

C 3 H 8 (
) 
118,910 — — —

O 2 0 8150 8682 49,292

N 2 0 8141 8669 47,073

H 2 O(g) 
241,820 — 9904 57,999

CO 2 
393,520 — 9364 71,078

CO 
110,530 — 8669 47,517

QoutaNr 1 h°fh
h° (^2) r
aNp 1 h°fh
h° (^2) p
QoutaNp 1 h°fh
h° (^2) paNr 1 h°fh
h° (^2) r
1.18 kg air/min
 1 23.53 kg air>kg fuel 21 0.05 kg fuel>min 2
m#air  1 AF 21 m#fuel 2
25.53 kg air>kg fuel
AF
mair
mfuel

1 7.54.76 kmol 21 29 kg>kmol 2
1 3 kmol 21 12 kg>kmol 2  1 4 kmol 21 2 kg>kmol 2
C 3 H 81
2 7.5 1 O 2 3.76N 22 S2.7CO 2 0.3CO4H 2 O2.65O 2 28.2N 2
ath 3  2  5
C 3 H 81
2 ath 1 O 2 3.76N 22 S3CO 2 4H 2 O3.76athN 2
768 | Thermodynamics