| dc.description.abstract | Concerning management of smoke following an accidental fire within a building it is
desirable to be able to estimate, within some understood, acceptable magnitude of error, the
volume of smoke resulting from the combustion process of a predefined design fire scenario.
Traditionally a range of first principle-based and empirically derived correlations are used to
estimate the mass flow of smoke at a height of interest within the fire plume and are based
upon the understanding that the mass flow of smoke at that height is a function only of the
gravitational vector within the fire system, that is to say, that induced by the pressure
differential between the naturally occurring hot plume gases and the surrounding quiescent
bulk fluid. The statement that the fire plume is surrounded by a quiescent bulk fluid is in
itself a significant simplification and is a key assumption required to facilitate the relative
simplicity of the Froude-based entrainment correlations.
It is of course quite intuitive to imagine that in real accidental fire scenarios in the built
environment and across an array modern infrastructure, rarely does a fire exist submerged in
a passive, quiescent atmosphere. This disconnect between the natural mechanics of the
buoyant fire mechanism and the surrounding fluid in which it exists was necessary when the
problem of entrainment by the fire plume was first described in the mainstream engineering
literature around the middle of the twentieth century. Some 25 years later as ideal
entrainment mechanics were beginning to be discussed specifically for application by a field
of engineering in its infancy, a few researchers in the field of fire safety engineering
published data that suggested that the addition of a relatively weak cross flow to the fire
plume could have a significant impact upon the rate of air entrained by the plume, and by
extension, the resultant smoke mass flow rate. The data published appeared more as a brief
comment on an observation made during testing. It would be easily missed, nuzzled away in
the middle of a lengthy doctoral thesis. Said thesis however happens to be one of the primary
pieces of work that may be cited in reference to the formulation of perhaps the best known
form of the axis-symmetric fire plume entrainment correlation, that of the so-called Zukoski
correlation. It is perhaps curious then that the mention of a 3-fold increase in entrainment
measurements following “small disturbances” in the atmosphere during the experimental
work has seemingly been ignored by researchers, probably never-learned by students, and
apparently forgotten by an industry.
In a fire situation smoke can limit way-finding ability, severely irritate critical soft tissue like
the eyes, trachea and oesophagus, impair cognitive function, contribute to significant
property damage, facilitate the transfer of heat and carcinogens to locations remote to the fire
source and it is well understood that most deaths due to fire are caused by asphyxiation
following smoke inhalation. Significant portions of project budgets may be spent on
designing, validating, installing and maintaining smoke management systems including the
use of active systems such as extraction and pressurisation, passive curtains/reservoirs and
detection such aspirating, video and beam detectors. Turbulent atmospheres may arise in any
manner of situations such as modern buildings with large open spaces (airports, museums),
hotel foyers and those with atriums spanning many floors, hangars and storage
facilities/warehouses. Strong winds are normal on offshore oil platforms, outside the window
on most floors of super-tall buildings or quite simply, anywhere on a blustery day. In specific
cases the extraction systems designed to remove smoke and even normal HVAC systems can
cause substantial air flow over large areas. In fact, a simple compartment with an uneven
distribution of ventilation points (windows/doorways) has been shown to result in a
directional fire flow that results in a significantly tilted flame, essentially inducing a cross
flow scenario using the natural fire alone.
With the coming-of-age of computational fluid dynamics models which are now a standard
tool in all commercial fire engineering design offices, and probably in every smoke
modelling report, it might be argued that there is little need to revisit the hand calculations
from the ground up. Accepting, however, that a cross flow may increase the rate of
entrainment of a fire plume and that this challenges the fundamental principles that all
previous entrainment correlation knowledge is based on, and demonstrating the outcome (in
terms of plume mass flow rate) with the use of a computational model, is an entirely
different thing to understanding why this happens.
Smoke management is one of the core design criteria, or questions at least, in practically all
fire engineering design projects. In the literature there appears to be; no work quantitatively
investigating cross flow fire plume entrainment rates; no work qualitatively describing the
behaviour of the flame / fire plume under the influence of a cross flow (with respect to
entrainment); and certainly no work framing this paradigm in the theoretical or practical
context of the impact upon modern smoke control systems.
This work aims to venture into these areas in the hope of beginning to piece together the
overarching story of entrainment in the cross flow fire plume. The fundamental paradigm
here is the addition of cross flow inertia (a horizontal pressure differential) to the axis-symmetric
case where buoyancy (a zero initial momentum, vertical pressure differential) is
the sole driver of the fluid flow system. How these flows then interact in a mixed convection sequence is investigated and described in terms that are useful for practical consideration by
fire safety engineers. It is hoped that the concepts postulated and the questions raised will
inspire further investigation into this poorly understood, but fundamental fire safety problem. | en |
