Graphics Reference
In-Depth Information
-7-
Fire
This is a short chapter: while the physics and chemistry of combustion can
be extraordinarily complicated, and to this day aren't fully understood,
we will boil it down to a very simplified model. Our two main sources in
graphics are the papers by Nguyen et al. [Nguyen et al. 02] for thin flames,
and those by Melek and Keyser [Melek and Keyser 02] and Feldman et
al. [Feldman et al. 03] for volumetric combustion. There are of course many
other approaches that avoid fluid simulation and directly model flames
procedurally, but we will not cover them in this topic.
Combustion is simply a chemical reaction
1
triggered by heat where an
oxidizer (like oxygen gas) and a fuel (like propane) combine to form a vari-
ety of products, giving out more heat in the process. Thus, at a minimum,
our fluid solver will need to be able to track fuel/oxidizer versus burnt prod-
ucts along with temperature—and if it's not a clean flame, we also need
smoke concentration to track the resulting soot. In the following sections
we'll take a look at two strategies for tracking the fuel/oxidizer appropriate
for different classes of combustion.
Just to clarify before proceeding: throughout this chapter we are assum-
ing that both fuel and oxidizer are either themselves gases, or suspended
in the air. If you analyze even a wood fire or a candle, you'll find that the
flames are the result of gaseous fuel—pyrolyzed from the wood as it heats
up, or evaporated wax—not the solid itself directly. Thus when we model a
solid (or even liquid) object that is on fire, we treat it as a source emitting
gaseous fuel, which then burns. Part of the emission is enforcing a velocity
boundary condition
u
n
+
u
emission
similar to the usual moving
solid wall boundary condition: gaseous fuel is injected into the grid at this
relative velocity
u
emission
. In addition, fields describing the presence of fuel
(see the following sections) should have the appropriate boundary condi-
·
n
=
u
solid
·
1
One of the complexities hidden here is that in most situations in reality, there are
actually hundreds of different chemical reactions in play, with a multitude of different
chemical species. For example, we all know that the gasoline burned in a car engine
doesn't simply combine with the oxygen in the air to produce water and carbon dioxide:
a vast range of other chemicals from carbon monoxide to nitrous oxide to all sorts of
carbon structures in soot particles end up in the exhaust.
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