In: Chemistry
A small gas turbine uses C8H18(l) for fuel and it
enters the combustion
chamber at 25◦C whereas air coming from the high pressure stage is
at 287◦C . The products of combustion leave the combustion chamber
at 1167◦C . The fuel is injected at a rate of 0.95 kg/min where it
is mixed and burned with 400 % excess air. An analysis of
combustions reveals that all the hydrogen in the fuel burns to H2O
but only 90% of the carbon burns to CO2, with remaining 10% forming
CO. Determine:
(a) the phase of the water in the products
(b) the mass flow rate of the air
(c) the rate of heat transfer from the combustion chamber.
Solution
here
C8H18-------->CO2 +H2O+O2+N2 .......... in combustion chember
here we assume that combution gases are ideal
C8H18 +b(1+a)(O2+3.76N2) -------->xCO2 + yH2O + abO2 + zN2
(1+a) is percent theoretical air a is excess air
a =3
after putting these value we get
C ---->x =8
H----> 18=2y so y =9
O ----->22b =2xY2b b =12.5
N ----->3.76 (1+a) =188
BALANCE REACTION
C8H18 +50(O2+3.76N2) -------->8CO2 + 9H2O + 37.5O2 + 188N2
TO DETERMINE HEAT TRANSFER
Qin +Win Nr(h0f +h-h0)r =Qout +Wout Np(h0f +h-h0)p
heat transfer and work done assumed to be out of these system is zero
enthalpies of reactants will be determined
for reactants
Enthalpy of formation
for stable moleculs enthalpy is zero
h0f o2 =0
h0f N2=0
The enthalpy of formation for C8H18 (l)
,h0f = –249950 kJ/kmol