Environmental Engineering Reference
In-Depth Information
CO +
1
2
O
O
(R1)
2
2
1
2
HOHO
2
+
(R2)
2
2
and an ordinary Arrhenius reaction rate expression is applied to the forward reactions,
they will proceed beyond the chemical equilibrium. Hence, to prevent this from
happening, a correction has to be made to the model, so that the rate of those two
forward reactions may become zero when the chemical equilibrium is reached.
3.2.4
R
EACTION
M
ODEL
FOR
H
IGH
T
EMPERATURE
A
IR
C
OMBUSTION
What is the most suitable combustion model for HiTAC simulation? Since HiTAC
is usually initiated by autoignition, it is totally a chemical controlled phenomenon
and it takes place in an atmosphere of high temperature and low oxygen concentra-
tion. So, the mixing-is-reacted model and the eddy-break-up model seem inappro-
priate for the purpose, because these models emphasize that the reaction rate is
totally affected by the mixing processes, particularly in turbulence. Therefore, it is
necessary here to discuss the potential of reaction models in predicting the balance
between reaction and flow as stated before.
If a simulation on a realistic furnace of complex geometry is carried out, only
the resultant temperature and concentration distributions in the furnace can be
obtained. Thus, it will be difficult to verify the validity of the model, except by
comparison with distribution results from experiments as they become available. To
examine the potential of reaction models and their applicability to numerical simu-
lation of combustion, a simpler flow system is preferable.
There are reports
4,5
of observations of lifted flames formed in preheated air or
diluted airflows in a square (7 × 7 cm) duct with a high speed fuel jet issuing from
a fuel nozzle at the center of the duct. A lifted flame will often be formed depending
on flow velocity, oxygen concentration, and temperature. The type of flame and its
stability limit vary with even minor changes in the influencing factors. Thus, this
seems the best geometry to compare the potential of reaction models. As represen-
tative reaction models applicable to engineering numerical simulations, the following
three models are discussed: a one-step global reaction model and two types of
reduced reaction model. The reaction rates are given in Arrhenius type expressions,
and the related empirical constants are retained here as originally proposed.
3.2.4.1
One-Step Global Reaction Model (Coffee)
This type of model is one of the most commonly used for numerical simulations of
combustion, in which the reaction rate [kg/(m
3
s)] of fuel is generally expressed as
follows:
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