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

770 | Thermodynamics

Insulation

Tmax

Combustion

chamber

Air

Products

Fuel

FIGURE 15–25

The temperature of a combustion

chamber becomes maximum when

combustion is complete and no heat

is lost to the surroundings (Q
0).

(b) Noting that the process involves no work interactions, the heat transfer

during this constant-volume combustion process can be determined from the

energy balance EinEout
Esystemapplied to the tank,

Since both the reactants and the products are assumed to be ideal gases, all

the internal energy and enthalpies depend on temperature only, and the Pv–

terms in this equation can be replaced by RuT. It yields

since the reactants are at the standard reference temperature of 537 R.

From h

• f° and ideal-gas tables in the Appendix,

h

• f° h
  
  
  • 537 R h
    
    
    • 1800 R
      Substance Btu/lbmol Btu/lbmol Btu/lbmol
    
    
    
    
  
  
  
  

CH 4 32,210 — —

O 2 0 3725.1 13,485.8

CO 2 169,300 4027.5 18,391.5

H 2 O(g) 104,040 4258.0 15,433.0

Substituting, we have

Discussion On a mass basis, the heat transfer from the tank would be

308,730/16 
19,300 Btu/lbm of methane.

15–5
ADIABATIC FLAME TEMPERATURE

In the absence of any work interactions and any changes in kinetic or poten-

tial energies, the chemical energy released during a combustion process

either is lost as heat to the surroundings or is used internally to raise the

temperature of the combustion products. The smaller the heat loss, the

larger the temperature rise. In the limiting case of no heat loss to the sur-

roundings (Q
0), the temperature of the products reaches a maximum,

which is called the adiabatic flameor adiabatic combustion temperature

of the reaction (Fig. 15–25).

308,730 Btu/lbmol CH 4

¬ 1 1 lbmol O 22310 13,485.83725.11.986 18002 Btu>lbmol O 24

¬¬Btu>lbmol H 2 O 4

¬ 1 2 lbmol H 2 O 231 104,04015,433.04258.01.986 18002

¬¬Btu>lbmol CO 24

¬ 1 1 lbmol CO 2231 169,30018,391.54027.51.986 18002

¬ 1 3 lbmol O 22310 1.986 5372 Btu>lbmol O 24

Qout
 1 1 lbmol CH 4231 32,2101.986 5372 Btu>lbmol CH 44

Qout
aNr 1 h°f RuT (^2) raNp 1 h°fh1800 Rh537 RRuT (^2) p
Qout
aNp 1 h°fhh°Pv (^2) paNr 1 h°fhh°Pv (^2) r
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