Environmental Engineering Reference
In-Depth Information
, computer software for analyzing thermofluid dynamics including
chemical reactions has become commercially available. However, the software can-
not predict whether combustion will start or not; it only predicts the burned properties
when combustion is ensured by forced ignition or a pilot flame. The onset of
combustion in HiTAC depends on the balance between chemical reactions and
physical processes, particularly mixing processes for the case. No available models
are suitable for use in HiTAC simulations. If they were used for this purpose, accurate
predictions could not be expected.
The following reviews the combustion models used for furnace simulations, and
considers a new direction of models that reflects the characteristics of HiTAC.
Recently
3.1.2
P
E
C
M
ROBLEMS
OF
XISTING
OMBUSTION
ODELS
ustion models incorporated in the
generic commercial software for thermofluid dynamics and their problems when
they are applied to HiTAC.
This section describes characteristics of comb
3.1.2.1
Arrhenius Type One-Step Global Reaction Model
In the comb
ustion processes of ordinary hydrocarbon fuels, various intermediate
species appear and disappear within a very short time, and correspondingly all these
take place within a very thin zone called a flame front or a flamelet. Thus, to simulate
precise distribution of chemical species and temperature inside the thin flame,
reaction rates have to be calculated regarding each of the species appearing during
the combustion processes, coupled with the flow field and the energy transport. It
is known that tens of chemical species and more than 200 elementary reactions are
involved in the combustion reaction scheme even for methane, the hydrocarbon fuel
of the simplest structure. The computers at present available are incapable of per-
forming all these calculations with the conditions for industrial furnaces of compli-
cated three-dimensional geometry.
Since we are interested in estimates of temperature distribution and flame length
in combustion equipment, it is appropriate to regard the entire combustion process
simply as a one-step reaction between fuel and oxygen to generate heat and com-
bustion product. The reaction when understood in this manner is called a one-step
global reaction. Its reaction rate is commonly expressed in the following Arrhenius
type formula.
E
RT
2
R
=−
p mm
exp
(3.1)
fu
fu
ox
where
T
is temperature,
m
and
m
are mass fractions of fuel and oxygen, respec
-
fu
ox
tively,
the universal
gas constant. Here, a stoichiometric relationship holds that 1 kg of fuel and
F
is a frequency coefficient,
p
pressure,
E
activation energy, and
R
r
kg of
oxygen react to generate (1 +
) kg of burned product. Accordingly, a similar
algebraic relationship is valid between the reaction rate of fuel and that of oxygen
or burned product.
r
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