Environmental Engineering Reference
In-Depth Information
are commercially available
and is used for defining an upper limit of reaction rate in combination with the eddy-
dissipation model, which will be explained later. Although the one-step global
reaction model can easily handle complex reactions of combustion through simpli-
fication, no consideration is given to any of the intermediates emerging during the
combustion processes, and hence generation and emission of CO and NO
This model is often combined with the generic softw
cannot
x
be estimated in practical terms by the model.
It is difficult to introduce the effects of fluctuating properties of turbulent com-
bustion into a chemically controlled reaction model, such as the Arrhenius model.
Since fluctuations of temperature and species concentrations in HiTAC are relatively
small compared with ordinary combustion, as described before, the use of a chem-
ically controlled reaction model for HiTAC seems reasonable from a practical point
of view. However, the validity of recommended empirical constants in the Arrhenius
expression has not been examined for HiTAC.
3.1.2.2
Mixing-Is-Reacted Model
ustion reactions is often far shorter than the time for
mixing between fuel and air. This holds true with diffusion combustion where fuel
and air are supplied separately and turbulence is relatively weak. In this case, it can
be assumed that combustion takes place at the instant of mixing, that is, at an infinite
reaction rate. Thus, a simulation is possible simply by calculating the mixing pro-
cesses, without considering calculation of reaction rate. As shown in
Figure 3.1
, the
mixture fraction
The time required for comb
f
at any point in a furnace is defined by the following equation:
mm
mm
fu
fu
,
0
f
=
(3.2)
fu
,
1
fu
,
0
where subscripts 1 and 0 indicate fuel mass fractions at each inlet of fuel and of air,
respectively. Thus defined, the value of
for any point in the furnace expresses the
mass fraction of fuel existing there in a non-combusting case. Once combustion
f
x mm
0
200
400
600
800
1000
1200
1400
1600
100
f = 0.05
0.03
80
0.06
60
f = f
st
40
Inle
t 2
Air
0.075
20
0.15
Inlet 1
0.087
0.1
0.2
Fuel
0
FIGURE 3.1
Distrib
ution of mixture fraction
f
(
f
: stoichiometric
f
).
st
Search WWH ::
Custom Search