Physical scale modelling of adhered spill plume entrainment

This work provides new experimental data to characterise entrainment of air into adhered thermal spill plumes using physical scale modelling. For the two-dimensional plume, the rate of entrainment with respect to height of rise is approximately half that of an equivalent two-dimensional balcony spill plume. For the three-dimensional plume, the rate of entrainment appears to be linked to the plume behaviour, which has been characterised in terms of the width and depth of the layer flow below the spill edge. In general, a layer flow below the spill edge that is shallow compared to its width will tend to adhere to the wall above the opening compared to flows whose depth approaches its width. This work proposes new empirical entrainment design formulae that have been developed on a more general basis compared to existing methods.     

The Balcony Spill Plume: Entrainment of Air into a Flow from a Compartment Opening to a Higher Projecting Balcony

In the design of smoke ventilation systems, a crucial input parameter for balcony spill plume calculations is the mass flow rate of gases at the spill edge. In some cases, depending upon the specific geometry, these calculations can require the entrainment of air into a flow from a compartment opening to a higher projecting balcony to be determined. Entrainment of air into these smoke flows are not well understood and the current guidance available to the designers of smoke ventilation systems is crude. This work presents a simple empirical correlation to predict the entrainment of air into these flows, and hence, the subsequent mass flow rate of gases at the spill edge. A combination of computational fluid dynamics modelling and physical scale modelling was used in the analysis. In general, this work has demonstrated that the current guidance on the entrainment of air into these flows is conservative.     

A phenomenological model of near-field fire entrainment

A phenomenological model of air entrainment into combusting regions of axisymmetric fire plumes is developed and compared with some existing experimental entrainment data. The model considers engulfment of ambient air into a fire plume due to the action of the toroidal vortices that are formed periodically close to the source of an axisymmetric fire. It is shown that the diffusive entrainment rates of air towards a wrinkled flame sheet representing the turbulent diffusion flame severely underpredicts the entrainment rates by at least two orders of magnitude, suggesting that the turbulent large-scale engulfment is the primary mechanism of entrainment. By describing the entrainment process as periodic engulfment of ambient air around the toroidal vortex rings and the frequency of formation and passage of these vortex rings in combusting regions of fire plumes, it is shown that the modeled entrainment process gives good agreement with the experimentally determined fire plume entrainment or plume mass fluxes. This model accounts for the physical processes of unsteady fire plume dynamics, including the effects of pulsations for the first time. The model predicts the plume mass flux to grow linearly with height above the fire source and to be independent of the fire heat release in the visible flame region. The predicted scaling with respect to the source diameter appears to be in the range suggested by the experimental data. Overall, this new model provides a better foundation for the scaling of entrainment rates in the near-field of fire plumes. The methodology can be extended to predict entrainment rates in other periodic unsteady vortex dominated flows, such as pulsating buoyant non-reacting plumes.     

Entrainment by adhered two-dimensional plumes

Measurements are presented of the variation with height of the mass flux of two-dimensional, adhered smoke plumes created by horizontal smoke layers that flow below a ceiling and spill into a hall near one of its walls, and of the effect of the Froude number (Fr) of these horizontal layers on the shape of the rising plumes in the hall. The measurements suggest that such smoke layers would usually rise vertically; either adhered to the wall, when Fr is roughly <1, or as a free plume, when Fr is roughly >1, and that the entrainment of ambient air to these plumes is approximately proportional to their free perimeter. It is also shown that in both cases, the variation at large heights of the mass flux of these plumes varies approximately as that of a similar, weak plume from a virtual line source, whose location can be estimated by a previously suggested simple model. Possible effects of downstands in the ceiling and of boundary conditions in the hall are also discussed.     

Comparison of plume rise models against water tank experimental data for neutral and stable crossflows

The evaluation of the trajectories of plumes rising into a crossflow is relevant for the correct modelling of pollutant dispersal in the atmosphere. Plume rise models are therefore an important part of several dispersion models. There is a general consensus on the formulation of plume rise through solution of equations describing the conservation of energy and momentum in the plume closed with an empirical formulation of the entrainment rate of fresh air into the developing plume. However, there are differences in the entrainment coefficients found in different studies. In this work the trajectories of sixty plumes, simulated at small scale in a towing water tank, have been analysed in order to test the performance of widely used dispersion codes and to find statistically the best entrainment coefficients for the different models. The plumes simulated cover a wide range of scenarios from pure jets to buoyant plumes developing in both neutral and linearly stable stratified crossflows. A new analytical model for stable crossflows, representing an extension of an existing model, has been presented and tested. Results show that the entrainment coefficients are different for neutral and stable crossflows, especially for approximate analytical codes. In contrast, the coefficients of the integral model seem to be less sensitive to the stability of the crossflow. The entrainment coefficients found by the fitting of the analytical models are significantly lower than the measured spread rate of the plumes. In neutral crossflows the generalised Briggs model and the integral model give statistically similar performances. In stable crossflows the new analytical model as well as the integral model are able to predict the oscillation of the plumes around their equilibrium height; however, there is an underestimation of both the oscillation frequency and the downwind position of the maximum height. The use of an added mass coefficient allows, with almost the same entrainment coefficients, improved prediction of the oscillation frequency and of the maximum rise position. Measured plume height oscillations are more strongly damped than predicted ones.     

Experimental studies of mechanically exhausted smoke within the transport passage of the main transformer of an underground hydropower station

Scale modeling experiments were conducted in a 1/12th scale model of the underground transport passage of the main transformer of a hydropower station model. The influence of heat release rate and smoke exhaust rate on the smoke exhaust efficiency was investigated. Results showed that the smoke exhaust process was less efficient when the heat release rate was larger, and that better smoke control may be obtained with larger smoke exhaust rates. The results of the scale modeling experiments have been compared with full-scale experimental results in other studies. Through comparison, it can be concluded that the results of the scale modeling experiments may be extrapolated to full scale, and the spill plume has similar characteristics to the smoke generated directly from fires that had spread along the passage.     

Tenability conditions and filling times for fires in large spaces

Motivated by recent demands on regulatory reform, closed form solutions are developed for the filling times and upper layer temperatures for fires in large spaces including the volume expansion term that was neglected in previous similar efforts. The solutions evolve from (a) utilizing the air entrainment to a buoyant plume from a point source having the same convective heat release as the fire and (b) applying an energy balance for the hot layer. Heat losses to the surfaces of the enclosure and provisions for smoke control by natural ventilation are also considered in an approximate way. Although analytic solutions for the filling times exist in the literature if the volume expansion term is neglected, this work is the first to (a) present analytic solutions for the upper layer temperature including the volume expansion term and (b) incorporate heat losses and smoke control by natural ventilation. The present predictions agree with recent numerical results (Fire Sci. Technol. 19(1) (1999) 27), which agree with experimental data and consequently, the present results in turn agree well with experimental data (Fire Sci. Technol. 19(1) (1999) 27). They are also corroborated by asymptotic analysis worked out in Appendix A. For certain large spaces, the results show that critical times for evacuation or rescue operations from fire brigade depend on the upper layer temperature reaching high enough values to cause harm by radiation to occupants or fire fighting rescuers. Thus, critical times in large spaces do not result from the smoke layer descending below a critical height (e.g. 2.1 m from the floor), as they do for small spaces. The present results for large spaces having high ceiling clearance do not agree with CFAST calculations because the entrainment equation for the fire plume in CFAST is different from the one in this work.     

Two-layer zone model including entrainment into the horizontally spreading smoke under the ceiling for application to fires in large area rooms

A model for smoke filling in a large room was developed that considers entrainment into the horizontally spreading smoke under the ceiling. This was accomplished by incorporating a simple model for the spreading ceiling jet into the two-layer zone model. Here, the proposed model was validated by focusing on the horizontal smoke spread phase on the basis of the previous experimental data of smoke spread in a large office room. The calculated results were in good agreement with the experimental data. In addition, a case study on the smoke filling in a large room was conducted to clarify the characteristics of the proposed model. As a result, the proposed model can predict the underside of the smoke layer to be lower than the two-layer zone model.     

Experimental studies on natural smoke filling in atrium due to a shop fire

How smoke spilling out of a shop fire would fill up an atrium has been studied experimentally in this paper. Full-scale burning tests were carried out in the PolyU/USTC Atrium constructed in Hefei, China. Since balcony spill plume expressions appeared in the literature might not be applicable for a shop fire with finite width, two plume models for smoke spilling out of a shop fire inside an atrium were proposed and assessed. An equation on studying the smoke layer interface height with a two-layer approach was derived.     

Enclosure smoke filling revisited

Building fires go through a series of stages. They start with a fire plume/ceiling jet period during which buoyant fire gases rise to the ceiling and spread radially beneath the ceiling. A second stage, the enclosure smoke-filling period, follows; this second stage is the subject of this paper. It has been more than 20 yr since Zukoski first addressed the smoke filling stage of enclosure fires in terms of thermodynamic control volume concepts and fire plume entrainment, yet his analysis remains pertinent. This paper reviews and extends fire modeling concepts related to enclosure smoke filling developed by Zukoski. The mass-based analysis of Zukoski is recast in terms of the volumetric flow rates typically used for ventilation system design; it is extended to consider global average temperature rise and the effects of oxygen consumption on the maximum global average temperature rise that can be achieved in a closed-room fire. A spreadsheet template is developed to compare hand calculations based on a global analysis with numerical smoke filling calculations. Results of this comparison suggest that there is little difference in conditions predicted with the global hand calculations and the numerical smoke filling calculations; consequently, the hand calculations are suitable for preliminary fire hazard analyses.     

On the upward movement of smoke and related shopping mall problems

The data of Morgan and his co-workers for the movement of smoke in 1/10 scale shopping malls are re-examined. This has been done to see how far a simple theoretical treatment based on plume theory and virtual sources (a basis for many calculation methods) can be exploited instead of the more complicated theory of sources of finite sizes. The work follows and extends the work of Law.

A number of theoretical relationships are used to correlate data and, in particular, they confirm Law's view that the data can be used to give plausible values for the positions of the virtual sources. Some of the complexities of the theory for finite strip sources may be bypassed if experiments are used for estimating the position of effective virtual sources. Experiments to locate the effective virtual source and the limiting asymptotic form of the theory (appropriate to line sources) can correlate and scale data to an accuracy satisfactory for fire engineering purposes. Various aspects of theory are discussed including the method of allowing for the ‘ends’ of finite strip sources, and the use of the theory of weak plumes for strong plumes.

There remains however a problem in reconciling the ‘near’ field of the plume rising around the balcony edge to the ‘far field’, but it is shown that the introduction of an exceptionally high local entrainment could — for these data — be avoided if: (a) the mass at any level in the plume is about 45% larger than expected from other laboratory experiments on plumes; or (b) there is extra entrainment due to the deflection and consequent disturbance as the plume strikes the ceiling; or (c) there is extra upward flow owing to radiation heating up the lower walls and floor as suggested by recent investigations. These matters await further investigation.     

建筑倾斜外立面火溢流沿程温度分布研究

建筑火灾是火灾科学领域的重要研究方向之一,外立面火溢流温度分布对于建筑火灾风险评估与建筑防火设计具有重要意义.虽然已有众多学者对竖直外立面火溢流温度分布特性进行了研究,但是对倾斜外立面火溢流沿程温度分布特性的研究还较少.因此,本文对建筑倾斜外立面火溢流沿程温度分布特性进行了研究,以求建立适用于倾斜外立面火溢流沿程温度分...     

Numerical study on smoke movement driven by pure helium in atria

The hot smoke test is often used for commissioning fire smoke management system in atrium buildings, in which liquid fuel is burnt to generate a buoyant plume mixed with artificial tracer smoke to model a fire smoke. The method is usually costly and often causes safety concerns. This paper studied an alternative method of using a cold smoke test, in which pure helium is used to create the buoyant plume. A method was developed to determine the supply rate of pure helium necessary to achieve the same buoyancy effect as that of the corresponding hot smoke test. Computational fluid dynamics (CFD) simulations of the helium smoke tests were conducted and compared to the measured hot smoke tests in a full-scale naturally ventilated atrium and a sub-scale atrium with mechanical ventilation. A new method was added in the CFD model to track the smoke layer height for the simulations of helium smoke based on the concentrations of smoke and helium. It is found that the predicted smoke layer heights based on the mass fractions of the tracer smoke are generally close to the measured ones in the hot smoke tests of different heat release rates. A non-dimensional temperature in the hot smoke test is also found closely related to the dimensionless helium concentrations in the helium smoke test for the atria modeled. Although the consumption of pure helium for a full-scale helium smoke test can be very high, it is promising to use the pure helium smoke test in the lab-scale experiments as the preliminary tests of full-scale and/or lab-scale testing of real fires.     

Application of FDS to Adhered Spill Plumes in Atria

In a recently published article (Poreh et al., Fire Saf J 43(5):344–350, 2008), Poreh et al. carried out a number of experiments in a small-scale atrium. They investigated the mass flow of the spill plume in case of fire emerging from an adjacent room or corridor. Based on these experiments, the equation for the mass flow rates of adhered spill plumes in atria was adjusted. In our article, we repeat the experiments in a computational fluid dynamics (CFD) program. The results agree well, both with the experiments and the suggested formula. After this first validation, large-scale CFD-simulations are carried out. It appears that the equation suggested by Poreh et al. is only valid in the case of a uniform smoke layer depth. If the smoke layer has a more complex configuration, the formula is no longer reliable for the design of the smoke and heat exhaust ventilation system.     

An analysis of compartment fire doorway flows

A mathematical study is made to compute the doorway flow behavior due to fire in a room. Two approaches were taken, first a model attempting to include the effect of fire entrainment and vent mixing; second was a model based on an ideal point source plume fire—both in the zone model concept. Limiting analytic results were found for the latter to give insight into the physics. The results were compared to available flow data, and an approximate formula was developed to predict the doorway mass flow rate to within 20% for a wide range of fire conditions. CFD computations were also explored using FDS. Results are compared from FDS and the zone model with experimental data for a wide range of variables.     

On smoke control for tunnels by longitudinal ventilation

Two aspects of smoke control by longitudinal ventilation systems in tunnels is analysed. The first is the increase in air entrainment rate of the plume after operating the system, and the second is the increase in heat release rate as a consequence of increase in air supply rate for combustion. Computational fluid dynamics (CFD) was applied for verifying the study. Keypoints to consider in the design of longitudinal ventilation systems are noted.     

The spill plume in smoke control design

Using recent data, obtained by Morgan, Poreh and colleagues, we produce correlations for the mass flow of a two-dimensional plume emerging normal to the straight edge of a flat horizontal surface—the balcony—and rising up into a uniform atmosphere (the spill plume). A comparison is made with an earlier correlation of the same data by Poreh et al. which required values of the layer depth, D_B, in addition to those of the layer flow per unit length of line plume, M^′_B. The treatment of Poreh et al. followed others assuming the linear relationship typical of far-field line plumes between the mass flow M^′ and the height z with a correction Δ—the virtual source. This linearity is a theoretical consequence of self-similarity (and a constant entrainment coefficient) in the velocity and temperature profiles across the plume, but recent, as yet unpublished, studies including some by computational fluid dynamics (CFD) cast doubt on the existence of self-similarity for these plumes at the low heights relevant in practice. However, a dimensional analysis of the flow does not require the assumption of self-similarity and we have demonstrated the linearity as a conclusion and not an assumption. The effective entrainment coefficient is, as found by Poreh et al., less than the value 0·16 found by Lee and Emmons and used in early work by Morgan and Marshall. The lower figure of 0·11 is consistent with other recent work on line plumes. The experimental values of D_B, the layer depth reported by Poreh et al., are in reasonable agreement with theoretical values for small increases in temperatures only. Experiments in model atria by Hansell, Morgan and Marshall which are not fully two-dimensional are discussed. Our correlation of them can be reconciled with that obtained by Law and subsequently used by the Chartered Institution of Building Service Engineers (CIBSE).     

中庭的形状系数与烟羽方程

分析了我国中庭空间几何形状的实地终统计调查结果，提出了形状系数的概念，并根据形状系数大小对中庭进行了分类。针对国内较普遍的瘦高型中庭(形状系数小于0．4)，按1／8比例建造了中庭火灾相似模型实验台，开展了烟气填充与机械排烟实验研究，得到了适用于这类中庭的稳态火灾烟气填充方程与反映受限烟羽特点的烟羽质量流量方程。     

Benchmarking the Single Item Ignition Prediction Capability of B-RISK Using Furniture Calorimeter and Room-Size Experiments

This paper benchmarks B-RISK’s capability to predict item ignition in multiple object compartment fire simulations. A series of fire experiments have been conducted which measured single item ignition times under the furniture calorimeter and in the ISO 9705 room. These experiments used mock-up armchair, TV and cabinetry furniture items created from three common materials found in most households in New Zealand exposed to a 100 kW gas burner flame. B-RISK uses the flux-time product (FTP) method as the criterion to predict ignition of items, based on radiation received using the point source model (PSM). This paper presents an analysis of the B-RISK predictions compared to the experimental measurements. Due to the mathematical formulation of the PSM and FTP method, it is found that the predicted ignition time is sensitive to the distance between the radiative source and the item. Predicted ignition times of armchairs constructed of polyurethane foam were within 14% of the ISO 9705 room experimental results. However, for the furniture calorimeter experiments it is found that to get reasonable predictions of the ignition times for the mock-up armchair and TV items there is a need to account for the burner flame movement by adjusting the radial distance by 10–30 mm. Direct flame contact was required to ignite the mock-up cabinetry items and B-RISK was unable to successfully predict this ignition time.