Combined Buoyancy- and Pressure-Driven Flow Through a Horizontal Vent.
Combined Buoyancy- and Pressure-Driven Flow Through a
Horizontal Vent.
(1828 K)
Cooper, L. Y.
NISTIR 5384; 48 p. April 1994.
Available from:
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(NTIS), Technology Administration, U.S. Department of
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Order number: PB94-210077
Keywords:
vents; building fires; compartment fires; computer
models; fire models; mathematical models; zone models;
ceiling vents; oxygen concentration
Abstract:
Combined buoyancy- and pressure-driven (i.e., forced)
flow through a horizontal vent is considered where the
vent-connected spaces near the elevation of the vent are
filled with fluids of different density in an unstable
configuration, with the density of the top space larger
than that of the bottom space. With zero-to-moderate
cross-vent pressure difference the instability leads to
a bi-directional exchange flow between the two spaces.
For relatively large cross-vent pressure difference the
flow through the vent is unidirectional, from the high-
to the low-pressure space. An anomaly of a standard
vent flow model, which uses cross-vent pressure
difference to predict stable unidirectional flow
according to Bernoulli's equation (i.e., flow-rate is
proportional to [equation], where [equation] is an
orifice coefficient), is discussed. Such a model does
not predict the expected bi-directional flow at small to
moderate [equation] or non-zero flow at [equation].
Even when cross-vent pressure difference exceeds the
critical value which defines the onset of unidirectional
or "flooding" flow, it has been determined
experimentally that until cross-vent pressure difference
exceeds many times [equation] there is a significant
dependence of [equation] on the relative buoyancy of the
upper and lower fluids. Also, it has been shown
theoretically that the location of the high-pressure
side of the vent, i.e., the top or bottom, can be
expected to influence vent flow characteristics.
Previously published experimental data and results of an
analysis of the relevant boundary value problems are
used to develop a flow model which takes all of these
effects into account. The result is a uniformly valid
algorithm to calculate flow through shallow (small
depth-to-span ratio), horizontal, circular vents under
the high-Grashof number conditions. This is suitable
for general use in zone-type compartment fire models
(e.g., an ambient temperature environment above the vent
and a hot smoky environment below). The algorithm is
used in example applications where steady
rate-of-burning in a ceiling-vented room is estimated as
a function of room temperature, vent area, and oxygen
concentration. Results of the analysis are seen to be
consistent with previously-published data involving
ceiling vented fire scenarios.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899