|The Complexity of Smoke Control Design, Theory and Practice|
The Complexity of Smoke Control Design, Theory and Practice
By Gabriele Vigne
Smoke management in atria and large spaces
Smoke management in atria and large-volume spaces poses separate and distinct challenges from well-compartmented spaces. In particular, smoke control strategies using pressure differences and physical barriers become infeasible . Without physical barriers, smoke propagation is unimpeded, spreading easily throughout the entire space. The tall ceiling heights in many large-volume spaces create additional challenges in terms of substantial quantities of smoke production and delayed detection times. However, on the positive side, the large-volume space and tall ceiling height permit the smoke to become diluted and cooled as it spreads vertically and horizontally. Dilution acts to reduce the level of hazard posed by the smoke. In atriums, hazard development is moderated by the large volume typically associated with the space .
However, there is still a need to ensure that dangerous concentrations of smoke are prevented. In addition to atria and covered malls, there are many other examples of large-volume spaces, including convention centers, airport terminals, sports arenas, and warehouses. The engineering principles governing the design of smoke management systems for these various large-volume spaces are the same. However, differences in smoke management system designs for these large-volume spaces may occur as a result of different fire scenarios and design goals reflecting changes in function, shape, and connection to other spaces, among other factors . When a fire takes place in an atrium or a large enclosure, the smoke can travel along vertical distances, affecting multiple floors simultaneously and threatening the life safety of occupants far away from the fire origin. Also, atrium layout intrinsically does not allow for vertical compartmentation and, thus, fire can spread to interconnected floors. Moreover, detection, control, and extinction of fires in atria differ significantly from those in small enclosures. These complex and non-conventional architectonical elements, like many others in modern buildings, can lead to fire environments that diverge significantly from those assumed in current codes and standards and most engineering calculation methods. With such remarkable architectural features, the fire dynamics in one building does not necessarily correspond to the fire dynamics in another building. Thus, a proper understanding of fire dynamics and smoke movement for each particular enclosure is needed to provide the scientific understanding required in the correct design of buildings .
In the last 10 years, smoke modelling assessments have been performed by several researchers, the most noteworthy being the research undertaken by Harrison and Spearpoint 
Smoke control design
The smoke control design process often relies on fire modelling. However, there are gaps in the state-of-art knowledge in fire dynamics and smoke movement in atria and large spaces. One example of this is the differing approaches used to calculate the required extract rate in an atrium. This presents the problem of different answers to the same smoke control problems around the world and consequently different designs for similar buildings.
Engineers have a number of options available for evaluating the performance of a smoke management system and, depending on the country where he/she is based, different design guides are used. Often national guides are considered the only one valid in a given country, no matter if the approach used is out-of-date or simply wrong.
It may happen that the same designer needs to make similar calculations in different countries encountering a non-homogenous approach to the smoke modelling. Given the fact that every country has its history and traditions and that in some countries fire engineering is a very young discipline while in others it is an established discipline, it is not surprising to see this happening.
Large-scale fire tests
Researchers from Spain and the United Kingdom have undertaken several full-scale fire tests in the Fire Atrium test facility in Murcia (Spain).
Through large-scale fire tests, it was possible to compare and contrast the tools available to the fire engineering community and compare them with real data obtained in the Fire Atrium test facility in Murcia, Spain.
Several fire tests have been performed in the last 10 years; the most recent ones have been done in January 2013, with a more sophisticated instrumentation, by researchers from the University of Jaén (Spain), The University of Comillas (Spain) with the collaboration of the Imperial College of London (UK), MAPFRE (Spain) and the fire engineering consultancy JVVA.
This unique test facility is a full-scale facility consisting of a prismatic structure of 19.5 m x 19.5 m x 17.5 m and a pyramidal roof raised 2.5 m at the center. The walls and roof are made of thick steel sheets and the floor is made of concrete. The atrium is provided with four exhaust fans installed on the roof.
The atrium was equipped with temperature, pressure, and velocity sensors in order to study the thermal and flow fields induced by the fire. Up to 61 sensors have been installed.
Measurements of walls and roof metal temperature, and air temperature at several locations (next to the walls, at a central section, through the exhaust fans and through the inlet vents) have been recorded. Differential and absolute pressure sensors at the exhaust fans also were installed to check fan performance curves, mass and volume flow rate evacuated.
The smoke layer interface was measured by an aspiration detection system installed at different heights in the atrium.
In order to estimate the smoke layer interface for previous tests, the N-percent method  was used. The smoke layer interface was then identified as the position where the temperature rise dropped to N% of the maximum temperature rise. In large spaces with a relatively small fire, temperature rise of smoke is relatively low. Therefore, the smoke layer interface was determined at the position where the temperature rise started to be larger than 10%, 20%, and 30% of the highest temperature rise along the vertical direction.
The buoyant axi-symmetric plume caused by a diffusion flame formed above the burning fuel is commonly used in fire safety engineering. An axis of symmetry is assumed to exist along the vertical centerline of the plume and air is entrained horizontally from all directions .
The most diffused and used plume correlations that were developed more than two decades ago and then slightly optimized by another developer are shown below. Zukoski et al.  in 1980-1981 measured entrainment rates for diffusion flame methane fires stabilized on porous-bed burners of 0.10 to 0.50 m diameter with fire magnitudes ranging from 10 to 200 kW. Heskestad  in 1984 developed a correlation for entrainment in fire plumes splitting the flow into two regions: a non-reacting plume and a flaming region. McCaffrey  in 1983 developed a correlation based on relatively small fires, splitting the flow field into three regimes (a continuous flame region, an intermittent flame region, and a plume region). A most recent work was undertaken by Thomas in 1987 .
This equation is also commonly shown in the form below where the ambient air properties are assumed to be T∞ = 293 K, ρ∞ = 1.1 kg/m3, cp = 1.0 kJ/kg·K and g = 9.81 m/s2.
The plume mass flow rate above the flame height (z > Lfl,) is given by:
The Thomas plume model is intended for entrainment in the near field or flame region, when the mean flame height is considerably smaller than the fire diameter. In this region, the entrained air is less influenced by the heat release rate than by the fire perimeter, and therefore the fire diameter, and can be expressed as:
Three different zone models also have been used in the benchmark, and are described in this section. In these models, it is possible to choose different plume entrainment correlations (Table 2) (Heskestad and McCaffrey in CFAST, Heskestad, McCaffrey, Thomas and Zukoski in OZone and MacCaffrey and Delichatsios in BRANZFIRE) resulting in a great variety of results that not always help to find the right answer for a given scenario.
The analysis through comparison between real data and models has shown a significant variability in the smoke layer height using different approaches. The average range of uncertainty is about ± 35% with a peak of almost 100% (Thomas, MFA1). Most of the results are not conservative, i.e. the predicted smoke layer is higher than the measure one.
Nowadays the international trend in fire safety is towards the use of fire models. These models are commonly used both in fire protection system design and in fire research. There are three main approaches to study fire-induced conditions -- analytical algebraic models, zone models, and field models. In addition, these models implement different plume entrainment correlations and make different assumptions to simplify the phenomena that appear in the case of fire. Therefore, different predictions can be obtained for the same problem, depending on the model used. Furthermore, these approaches have been widely tested within small enclosures but their use and accuracy in large enclosures such as tunnels or atria is not clear; there is a real need to perform fire tests and fire models validations to set bounds to their use, accuracy, and limitations within these structures.
The present work studies the suitability and applicability of these approaches to properly predict the fire-induced conditions within an atrium fire. The results obtained from the models have been compared with full-scale experimental data.
Although the general trend of the “best” formulae is in line with results proving that those formulae can be used for a first approximation of the phenomena, the average range of uncertainty is about ±35% with a peak of almost 100% (Thomas, MFA1).
Gabriele Vigne is with JVVA Fire & Risk.