VENTCF2: an algorithm and associated FORTRAN 77 subroutine for calculating flow through a horizontal ceiling/floor vent in a zone-type compartment fire model

An algorithm and associated FORTRAN 77 subroutine, called VENTCF2, for calculating the effects on two-layer compartment fire environments of the quasi-steady flow through a circular, shallow (i.e. small ratio of depth-to-diameter), horizontal vent connecting two spaces is presented. The two spaces can be either two inside rooms of a multi-room facility or one inside room and the outside ambient environment local to the vent. The flow is determined by consideration of standard orifice-type flows driven by cross-vent pressure differences and, when appropriate, the combined pressure- and buoyancy-driven flows which occur when the density configuration across the vent is unstable, i.e. a relatively cool, dense gas in the upper space overlays a less dense gas in the lower space. The algorithm calculates rates of flow exchange between the two spaces based on previously reported model equations. Characteristics of geometry and the instantaneous environments of the two spaces are assumed to be known and specified as inputs. Outputs calculated are the rates and properties of the vent flow at the elevation of the vent as it enters the top space from the bottom space and/or as it enters the bottom space from the top space. Rates of mass, enthalpy and products of combustion extracted by the vent flows from upper and lower layers of inside room environments and from outside ambient spaces are determined explicitly. VENTCF2 is an advanced version of the algorithm /subroutine VENTCF in that it includes an improved theoretical and experimental basis. The subroutine is completely modular and it is suitable for general use in two-layer, multi-room, zone-type fire model computer codes. It has been tested numerically over a wide range of input variables and the results of some of these tests are described.     

Transient pollutant flushing of buoyancy-driven natural ventilation

The transient flushing of neutrally-buoyant pollutants from a naturally ventilated enclosure is investigated. A simplified transient model for buoyancy-driven natural ventilation produced by a point source of heat is presented to describe the ventilation development from the plume generation to its steady state. The instantaneous thermal stratification interface height and ventilation flow rate and the time taken for the flow to reach the steady state are then examined by the transient model. The results indicate that the decrease of the thermal stratification interface height with dimensionless time, the steady-state interface height and the dimensionless time taken for the flow to reach the steady state are only determined by the dimensionless effective area of the vents. The ventilation flow rate can be increased by decreasing the enclosure floor area or increasing the effective vent area, enclosure height or source buoyancy flux. Accordingly, for rooms with smaller floor area, larger effective vent area or larger source buoyancy flux, ventilation airflow provides more effective flushing of neutrally-buoyant pollutant. Nevertheless, increasing the enclosure height is only beneficial to flush the pollutant from the lower layer rapidly and is disadvantageous to reduce the pollutant concentration of the upper layer.     

Comparison of different combustion models in enclosure fire simulation

In this study, three combustion models, the volumetric heat source (VHS) model, the eddy break-up model and the presumed probability density function (prePDF) model, are examined in enclosure fire simulation. The combustion models are compared and evaluated for their performance in predicting three typical enclosure fires, a room fire, a shopping mall fire and a tunnel fire. High Reynolds number turbulence k–ε model with buoyancy modification and the discrete transfer radiation model (DTRM) are used in the simulation. Corresponding experimental data from the literature are adopted for validation. The results show satisfactory prediction in flow patterns and features in the complex enclosure fires. However, it is shown that these combustion models are not able to show consistent performance over the different locations and enclosure fires. The needs for adequate turbulent combustion models in enclosure fires are discussed.     

A two-dimensional numerical investigation of the oscillatory flow behaviour in rectangular fire compartments with a single horizontal ceiling vent

This paper presents a comparison of fire field model predictions with experiment for the case of a fire within a compartment which is vented (buoyancydriven) to the outside by a single horizontal ceiling vent. Unlike previous work, the mathematical model does not employ a mixing ratio to represent vent temperatures but allows the model to predict vent temperatures a priori. The experiment suggests that the flow through the vent produces oscillatory behaviour in vent temperatures with puffs of smoke emerging from the fire compartment. This type of flow is also predicted by the fire field model. While the numerical predictions are in good qualitative agreement with observations, they overpredict the amplitudes of the temperature oscillations within the vent and also the compartment temperatures. The discrepancies are thought to be due to three-dimensional effects not accounted for in this model as well as using standard ‘practices’ normally used by the community with regards to discretization and turbulence models. Furthermore, it is important to note that the use of the turbulence model in a transient mode, as is used here, may have a significant effect on the results. The numerical results also suggest that a linear relationship exists between the frequency of vent temperature oscillation (n) and the heat release rate ( ) of the type , similar to that observed for compartments with two horizontal vents. This relationship is predicted to occur only for heat release rates below a critical value. Furthermore, the vent discharge coefficient is found to vary in an oscillatory fashion with a mean value of 0.58. Below the critical heat release rate the mean discharge coefficient is found to be insensitive to fire size.     

Smoke concentrations inside and outside of a corridor-like enclosure

This work presents smoke measurements and correlations inside and outside of a corridor-like enclosure fires in order to determine the effects of burning on smoke concentrations inside and outside the enclosure. Thirty eight experiments were performed in a three metre long corridor-like enclosure having a cross section 0.5 m×0.5 m, door like openings in the front panel and a gaseous burner located near the closed end. Smoke concentrations were measured at two locations inside the enclosure and also in the exhaust duct of a hood collecting the fire gases from the enclosure. It was found that smoke concentration in the exhaust duct decreased whereas smoke concentration inside the enclosure increased after the flames started moving towards the opening and external burning occurred. This increased smoke concentration inside the enclosure was caused by reversion of the flow pattern inside the enclosure after the flames moved past a point towards the opening. Namely, the flow pattern changed direction behind the flame front in the sense that hot gases in the upper layer were travelling backwards towards the closed end of the corridor thus contributing to smoke increase inside the enclosure. This change of flow pattern was confirmed in all experiments by bidirectional probe velocity measurements in the upper and lower layer as well as by oxygen concentrations and temperature measurements inside the enclosure. These results are useful for CFD validation and specifically applicable for assessing smoke hazards in corridor fires in buildings where smoke concentrations can be much larger than anticipated owing to leakage to adjacent rooms behind a moving flame front.     

Definition and experimental evaluation of the smoke “confinement velocity” in tunnel fires

An experimental study is carried out on a reduced scale tunnel model (scale reduction is 1:20). The main objective is to evaluate the longitudinal velocity induced into a tunnel when a fire plume continuously released is confined and extracted between two exhaust vents located on both sides of the fire source. For the experimental simulations, fire-induced smoke is simulated by an air and helium mix release. Smoke flow is symmetrical as regards the fire location and experiments are realized for an half tunnel with only one vent activated downstream the source. The vent extraction flow rate is step by step increased and the length of the stratified smoke layer downstream the vent as well as the longitudinal fresh air flow induced, are measured. A confinement velocity is then associated to the minimum value of the longitudinal air flow needed to prevent the smoke layer propagation downstream the vent. This velocity is evaluated for several values of the fire heat release rate and finally compared with the corresponding critical velocity obtained for a longitudinal ventilation system.     

Calculating fire plume characteristics in a two-layer environment

Methods are developed to determine axial gas flow conditions within a weakly buoyant plume that passes from an ambient quiescent environment, in which the plume originates, to an upper layer at elevated temperatures. The methods are appropriate for inclusion in two-layer analysis of enclosure fire. In particular, they are first steps in developing a prediction of actuation time for thermally activated automatic sprinklers exposed to an enclosure fire. Results obtained with various methods are compared with measurements in a 1.22 m diameter cylindrical enclosure. National Bureau of Standards Reference: David D. Evans, “Calculating Fire Plume Characteristics in a Two Layer Environment”, Fire Technology, Vol. 20, No. 3, August 1984, p. 39. Note: This paper is a contribution of the National Bureau of Standards and is not subject to copyright.     

Enclosure Smoke Filling and Management with Mechanical Ventilation

Enclosure smoke filling and management are addressed from the standpoint of the volumetric flow rates commonly used for mechanical ventilation system design. In this context, fire-induced gas expansion is treated as a volumetric source term. A two-layer analysis developed previously for enclosure smoke filling without mechanical ventilation is extended to consider the impact of mechanical ventilation on smoke layer descent rates and conditions within the smoke layer. A spreadsheet-based model of enclosure smoke filling developed in conjunction with the previous unventilated analysis is also extended to consider both mechanical extraction and injection systems. Some implications of mechanical ventilation on the development and descent of a smoke layer in an enclosure fire are discussed.     

Numerical study of water spray interaction with fire plume in dual chambers connected to tall shaft

The interactions between water droplets and fire plume in a two compartmental enclosure connected to tall shaft are numerically investigated. The cooling and drag effects of water particles on thermal plume characteristics are analyzed under natural and forced ventilation conditions. Numerical study is performed by Large Eddy Simulation (LES) using Fire Dynamic Simulator (FDS) code. The water droplets oppose the smoke buoyancy force and reduces the ceiling vent discharge rate. For higher sprinkler operating pressure the drag force dominates buoyancy force and stops the plume propagation through horizontal passage. The critical sprinkler operating pressure that leads to smoke logging is identified. The horizontal vent mass flow rate decreases linearly with water spray discharge rate. The forced air stream supplied at low velocity assists buoyancy force and eliminates smoke logging. However, higher ventilation velocities intensify cooling effect by increasing the interactions between water droplets and thermal plume. The model employed has been validated with the existing experimental results available in the literature.     

A quasi-steady-state model for predicting fire suppression in spaces protected by water mist systems

A quasi-steady-state model was developed to predict the effectiveness of a water mist system for extinguishing fuel spray and pool fires. The model was developed for obstructed fires where extinguishment primarily occurs as a result of a reduction in oxygen concentration due to the consumption of oxygen by the fire and due to dilution of the oxygen with water vapor. Interactions between the mist and the flame are neglected resulting in limiting case predictions. The model is based on conservation of energy and requires the following input parameters: fire size, compartment geometry, vent area, and water flow rate. The steady-state temperatures and oxygen concentrations predicted by the model can be used to determine the smallest fire that can be extinguished. The predictions made by the model compared favorably to the results of three full-scale test series conducted for the US Coast Guard. These tests were conducted in shipboard machinery spaces with compartment volumes ranging from 100 to 500 m³ with a wide range of ventilation rates and openings. The model was able to accurately predict the compartment temperatures during the tests where steady-state conditions were produced. The model was also able to accurately predict the extinguishment times for a wide range of fire sizes and was used to identify the smallest fire that could be extinguished for a given set of conditions.     

A Novel Multiscale Methodology for Simulating Tunnel Ventilation Flows During Fires

This paper applies a novel and fast modelling approach to simulate tunnel ventilation flows during fires. The complexity and high cost of full CFD models and the inaccuracies of simplistic zone or analytical models are avoided by efficiently combining mono-dimensional (1D) and CFD (3D) modelling techniques. A simple 1D network approach is used to model tunnel regions where the flow is fully developed (far field), and a detailed CFD representation is used where flow conditions require 3D resolution (near field). This multi-scale method has previously been applied to simulate tunnel ventilation systems including jet fans, vertical shafts and portals (Colella et al., Build Environ 44(12): 2357–2367, 2009) and it is applied here to include the effect of fire. Both direct and indirect coupling strategies are investigated and compared for steady state conditions. The methodology has been applied to a modern tunnel of 7 m diameter and 1.2 km in length. Different fire scenarios ranging from 10 MW to 100 MW are investigated with a variable number of operating jet fans. Comparison of cold flow cases with fire cases provides a quantification of the fire throttling effect, which is seen to be large and to reduce the flow by more than 30% for a 100 MW fire. Emphasis has been given to the discussion of the different coupling procedures and the control of the numerical error. Compared to the full CFD solution, the maximum flow field error can be reduced to less than few percents, but providing a reduction of two orders of magnitude in computational time. The much lower computational cost is of great engineering value, especially for parametric and sensitivity studies required in the design or assessment of ventilation and fire safety systems.     

Physical scaling of water mist fire extinguishment in industrial machinery enclosures

Froude-based scaling relationships had previously been theoretically extended to, and experimentally validated in the laboratory for, water mist suppression of fires in open environment and in enclosures, which were shown applicable to gas, liquid and solid combustible fires. Before applying these relationships to real-world settings, their applicability needs to be further evaluated for the intended protection. This paper presents such an evaluation on scaling water mist fire extinguishment in an industrial machinery enclosure. In this evaluation exercise, a full-scale water mist protection set-up tested for a 260-m³ machinery enclosure was selected as the benchmark. A ½-scale machinery enclosure test replica was then constructed, together with a ½-scale nozzle whose orifices were geometrically similar to those of the full-scale nozzle. Spray measurements indicated that the ½-scale spray closely met the scaling requirements, in terms of discharge K-factor, water mist flux, droplet velocity and droplet size distribution. Two spray fires and one pool fire, which were scaled with the respective full-scale fires, were used to challenge the water mist protection in the ½-scale enclosure. At least five replicated tests were conducted for each of the four tested fire scenarios. Overall, the instantaneous local gas temperature and oxygen concentration measured inside the ½-scale enclosure for each fire scenario agreed reasonably well with those measured at the corresponding locations inside the full-scale enclosure, meeting Froude modeling's requirement that scalar quantities be preserved in different scales. The fire extinguishment times obtained from the ½-scale tests for each fire scenario were also statistically consistent with that observed in the corresponding full-scale test. Based on the obtained results, it is concluded that, for machinery enclosures and other similar occupancies, the previously laboratory-validated scaling relationships for water mist fire suppression can be used to determine the fire extinguishing performance of a full-scale water mist protection in a ½-scale test facility.     

A numerical simulation of the NBS cup furnace toxicity test

A numerical simulation of the NBS cup furnace toxicity test (Potts' Pot) is presented. The Navier-Stokes equations describing the hydrodynamics are solved numerically using the general-purpose computational fluid dynamics code, PHOENICS. In this initial study, the behaviour of the potentially toxic material is represented by introducing a tracer gas into the flow from the furnace fire model. This approach gives some idea of typical toxicity lifetimes inside the enclosure attached to the furnace.

The results are displayed in contour and vector plots. They clearly show the recirculatory flow within the furnace pot and enclosure. This recirculation may play an important role in maintaining the toxicity of the particulate fraction that has been implicated in producing ultra-high toxicity products in this apparatus. An upper limit of 2–5 min is estimated the lifetime of the particulate toxicity during the oxidative pyrolysis of polytetrafluoroethylene (PTFE).     

CALFIRE: An interactive model for fire calculations

CALFIRE, the acronym for CALculate Fire In Room and Enclosure, is a knowledge-based mathematical formulation of analytical and numerical procedures to predict the consequences of a fire in a room or enclosure. CALFIRE is a well-knit and integrated computer model that offers menu items such as heat release rate (HRR), flame height, vent size, and room temperatures of closed rooms, and rooms with natural and forced ventilation. Warnings and checks have been provided to prevent the misuse of the model. Care has been taken to require minimal keyboard responses in order to make CALFIRE a truly user-friendly, interactive fire model.     

Numerical study of under-ventilated fire in medium-scale enclosure

In an enclosure, as all the air inflow is consumed in burning with the excess fuel, the internal fire enters the decay phase, and such process is said flame exhaust. The complicated multistage process from an initial fire growth up to a flame exhaust followed by an external burning is investigated by means of a Large-Eddy-Simulation (LES). Turbulent combustion process is modelled by an Eddy Break-Up concept by using two sequential, semi-global steps for CO prediction. The numerical model solves three dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The critical fuel supply rate needed for flame to exhaust and the time period from the fuel ignition to the appearance of an external flaming in medium-scale facilities are previously obtained experimentally by Chamchine AV, Graham TL, Makhviladze GM, et al. [Experimental studies of under-ventilated combustion in small and medium-scale enclosures. In: Proceedings of the fourth international seminar on fire and explosion hazards; 2003. p. 97–107.], and the general trends predicted by the numerical model follow closely their experimental observation. This model is capable of adequately describing the essential simultaneous phenomena (flame height, soot generation, CO production, convection and radiation) occurring in a room fire. The distinct transient stages of fire development prior to flame exhaust and scenarios of the exhaust are analysed. An external burning is followed after the flame exhaust inside enclosure, and the flame height, H_f, past the ceiling is approximately in an order of the opening height. Even though the flame exhaust takes place under the critical conditions, the heat transferred from the hotter gases and the external fire source poses significant threat to people inside enclosure, and potentially induces an ignition of fuel package exposed near the opening of an enclosure.     

A general formula for the prediction of vent flows

A new formula for fire-induced wall vent flow rate is developed based upon a theoretical derivation and mathematical fit to data. Previous research had developed a formula of mass flow rate for fire-induced doorway flows only. Here it is extended to include window flows. A theoretical model based on an ideal point source fire plume is used to guide the form of the empirical correlation. A thorough examination concerning the difference between the window and doorway flow modes is conducted. Both sill height and width of the windows pose key influence on the formula. The two vent configurations are merged into one equation. The results were compared to available flow data and shown to be within 15% accuracy for a wide range of fire conditions.     

Statistical Characterization of Heat Release Rates from Electrical Enclosure Fires for Nuclear Power Plant Applications

Since the publication of NUREG/CR-6850/EPRI 1011989 in 2005, the US nuclear industry has sought to re-evaluate the default peak heat release rates (HRRs) for electrical enclosure fires typically used as fire modeling inputs to support fire probabilistic risk assessments (PRAs), considering them too conservative. HRRs are an integral part of the fire phenomenological modeling phase of a fire PRA, which consists of identifying fire scenarios which can damage equipment or hinder human actions necessary to prevent core damage. Fire ignition frequency, fire growth and propagation, fire detection and suppression, and mitigating equipment and actions to prevent core damage in the event fire damage still occurred are all parts of a fire PRA. The fire growth and propagation phase incorporates fire phenomenological modeling where HRRs have a key effect. A major effort by the Electric Power Research Institute and Science Applications International Corporation in 2012 was not endorsed by the US Nuclear Regulatory Commission (NRC) for use in risk-informed, regulatory applications. Subsequently the NRC, in conjunction with the National Institute of Standards and Technology, conducted a series of tests for representative nuclear power plant electrical enclosure fires designed to definitively establish more realistic peak HRRs for these often important contributors to fire risk. The results from these tests are statistically analyzed to develop two probabilistic distributions for peak HRR per unit mass of fuel that refine the values from NUREG/CR-6850, thereby providing a fairly simple means by which to estimate peak HRRs from electrical enclosure fires for fire modeling in support of fire PRA. Unlike NUREG/CR-6850, where five different distributions are provided, or NUREG-2178, which now provides 31, the peak HRRs for electrical enclosure fires can be characterized by only two distributions. These distributions depend only on the type of cable, namely qualified versus unqualified, for which the mean peak HRR per unit mass is 11.3 and 23.2 kW/kg, respectively, essentially a factor of two difference. Two-sided, 90th percentile confidence bounds are 0.0915 to 41.2 kW/kg for qualified cables, and 0.0272 to 95.9 kW/kg for unqualified cables. From the mean (~70th percentile) upward, the peak HRR/kg for unqualified cables is roughly twice that for qualified, increasing slightly with higher percentile, an expected phenomenological trend. Simulations using variable fuel loadings are performed to demonstrate how the results from this analysis may be used for nuclear power plant applications.     

Airflow and heat flux through the vertical opening of buoyancy-induced naturally ventilated enclosures

Characteristics of the mean and turbulent airflow and heat flux through the vertical opening of a buoyancy-induced naturally ventilated full-scale enclosure with upper and lower vents on one of the sidewalls were studied experimentally. The effect of the interaction between the mixing and the displacement ventilation modes on the airflow through the upper vent is explored. Measurements include vertical profiles of mean and turbulent air velocity and temperature through the upper opening using a three-dimensional sonic anemometer. The airflow appears to be inclined to the horizontal plane due to the effect of buoyancy. The level of the neutral plane at the upper vent, defined here as the plane separating between inflow and outflow, can be identified by the vertical profiles of both mean flow and turbulence intensity, with good agreement between the two approaches. The contribution of the turbulent to the total (mean and turbulent) heat flux through the vent decreases as ventilation transforms from the mixing to the displacement mode.     

Numerical simulation of fire in a tunnel: Comparative study of CFAST and CFX predictions

The mathematical modeling of fire growth and smoke movement in any enclosure is a formidable task. Two types of deterministic models are in vogue, zone models and field models (popularly known as CFD technique). CFAST is a popular zone model used for modeling of fires in enclosures. Likewise, CFX is a general purpose CFD code used for various purposes including modeling of fires. In the present paper, a tunnel of length 150 m having a rectangular cross-section of 80 m² has been considered for analyzing the temperature and velocity profiles generated by fire, placed at a distance of 20 m from one end of portal, by both CFAST and CFX. The simulation by CFAST has been carried out by dividing the tunnel into 1, 2, 5, 8, 10, 12 and 15 compartments of equal size, where these compartments are joined by openings or vents having same cross-section as that of the tunnel. In case of tunnel divided into 15 compartments the fire source position lies at the position of vent; CFAST predicted very high temperatures. The simulations have also been carried out by dividing tunnel into unequal sized compartments such that position of fire was at the center of the compartment. It was found that for accuracy of results, location of fire source inside compartment is an important factor. Computational difficulty was experienced when tunnel was divided into more than 15 compartments. In this paper, a comparative study of temperatures predicted by CFAST and CFX has been done. The CFX and CFAST predictions show that smoke temperature changes with a pattern roughly similar to that of heat release rate. The temperature profiles at selected positions cannot be predicted by CFAST unlike CFX. The detailed features like flame tilt, flow field can only be observed from CFX predictions.     

矩形充气膜结构的火灾温度场及烟气分布

针对有广泛应用前景的矩形充气膜建筑,利用火灾动态模拟软件FDS,模拟其用于公共建筑时的火灾温度场分布特性和烟气流动规律,为人员安全疏散和进一步结构力学性能研究提供依据。温度场模拟结果表明:疏散门和风扇打开可有效降低室内及膜面热环境温度,结构不易发生倒塌破坏,且通风口位置对膜面温度有一定影响。由烟气流动规律可知:通风口打开,热烟气非均匀下降,垂直平面内无明显层状分布特性,室内能见度迅速降低;通风口位置对风扇口附近及火源周边水平烟气流动有较大影响。