Large Eddy Simulations of Fires: From Concepts to Computations.
Large Eddy Simulations of Fires: From Concepts to
Computations.
(5992 K)
Baum, H. R.
Fire Protection Engineering, No. 6, 36-38,40,42, Spring
2000.
Keywords:
simulation; convection; fire plumes; industrial fires;
tank fires
Abstract:
The idea that the dynamics of a fire might be studied
using digital computers probably dates back to the
beginnings of the computer age. The concept that a fire
requires the mixing of a combustible gas with enough air
at elevated temperatures is well known to anyone
involved with fire. Graduate students enrolled in
courses in fluid mechanics, heat transfer, and
combustion have been taught the equations that need to
be solved for at least as long as computers have been
around. What is the problem? The difficulties revolve
about three issues: First, there are an enormous number
of possible fire scenarios to consider. Second, we do
not have either the physical insight or the computing
power (even if we had the insight) to perform all the
neccessary calculations for most fire scenarios.
Finally, since the "fuel" in most fires was never
intended as such, the data needed to characterize both
the fuel and the fire environment may not be available.
In order to make progress, the questions that are asked
have to be greatly simplified. To begin with, instead of
seeking a methodology that can be applied to all fire
problems, we begin by looking at a few scenarios that
seem to be most amenable to analysis. Hopefully, the
methods developed to study these "simple" problems can
be generalized over time so that more complex scenarios
can be analyzed. Second, we must learn to live with
idealized descriptions of fires and approximate
solutions to our idealized equations. These idealized
descriptions have to be based on the kind of incomplete
knowledge of fire scenarios that is characteristic of
real fires. Finally, the methods should be capable of
systematic improvement. Thus, as our physical insight
and computing power grow more powerful the methods of
analysis can grow with them. The "Large Eddy Simulation"
(LES) technique developed at NIST over a nearly two
decade period is our attempt to carry out the conceptual
program outlined above. The phrase refers to the
description of turbulent mixing of the gaseous fuel and
combustion products with the local atmosphere
surrounding the fire. This process, which determines the
burning rate in most fires and controls the spread of
smoke and hot gases, is extremely difficult to predict
accurately. This is true not only in fire research but
in almost all phenomena involving turbulent fluid
motion. The basic idea behind the use of the LES
technique is that the eddies that account for most of
the mixing are large enough to be calculated with
reasonable accuracy from the equations of fluid
mechanics. The hope (which must ultimately be justified
by appeal to experiments) is that small scale eddy
motion can either be crudely accounted for or ignored.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899