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Combined Buoyancy- and Pressure-Driven Flow Through a Horizontal Vent.


pdf icon Combined Buoyancy- and Pressure-Driven Flow Through a Horizontal Vent. (1828 K)
Cooper, L. Y.

NISTIR 5384; 48 p. April 1994.

Available from:

National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161.
Telephone: 1-800-553-6847 or 703-605-6000;
Fax: 703-605-6900.
Website: http://www.ntis.gov
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.