buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Fig. 2 – The closed system the
related availability streams.
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 131
with this environment. This state of
equilibrium is defined as the dead state of
the system and it depends on the pressure
P0, the temperature T0 and the composition
of the environment. In the present study,
the environment is dry atmospheric air
with P0=1 bar and T0=298 K. Thermal
equilibrium is achieved when there is no
heat exchange between the system and the
environment, i.e. when the system is at
temperature T0. Similarly, mechanical
equilibrium is achieved when there is no
work exchange between the system and its
environment. Chemical equilibrium is
achieved when no system component can
react with the environment. For the present
case, this means that in the dead state all
the 11 species of the working medium have been either oxidized or reduced to
N2, O2, CO2, and H2O. It is shown in Refs. (Bejan, 1988) that the availability of
a closed system is equal to
Φ = U + P V −T
S − G . (4)
0
A direct consequence of the above definition is that when a differential
amount of reversible work dW=(P-P0)dV is extracted from a system, its
availability is reduced by exactly that amount. Similarly, when a differential
amount of heat dQ is supplied to the system at temperature T, the system
availability is increased by an amount dQ(1-T0/T).
The system under consideration is shown in Fig. 2. It is a closed system,
since only the part of the cycle from inlet valve closing to exhaust valve
opening is considered. The only mass influx to the system is the prepared fuel,
which takes place after injection.
The differential availability fluxes can also be seen in Fig. 2. On the basis
of the availability balance, the differential variation of the working medium
availability is given by
d Φ=−( P− P)d V + [dQ+ d N ( h −h )](1 − T / T) + dN a −dI
. (5)
0
0 pr fl fv 0
pr fv
On the right hand side, the first term is availability out flux from the
system in the form of work. The second term is availability input through heat
flux and the third term is availability added to the system by the fuel vapor. The
term dI makes Eq. (5) different from a conservation equation.
During combustion working medium availability is destroyed and a
differential amount dI of combustion irreversibility is produced. Similarly,
0