Smoke management for modern infrastructure
Hilditch, Ryan Robert
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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.