Fire-induced Flow Research Articles

Jet entrainment effect on fire-induced flow in isothermal model applied to reduced-scale tunnel

Based on the entrainment rate of isothermal jet, an application method of the isothermal model in tunnel fire was presented through the more reasonable estimation of total smoke flow rate and its density at jet exit. Acetone LIF measurement was performed to identify the entrainment of ambient air qualitatively, and the entrainment constants were provided with the results of a previous study of which jet velocity corresponded to that of this study. The entrainment effect was more remarkable for the smaller fire, because the normalized axial distance to ceiling was increased with decreasing the fire size. These results suggested that the present model considering the jet entrainment might enhance the previous isothermal model in tunnel fire.

Quantitative Salt-Water Modeling of Fire-Induced Flows for Convective Heat Transfer Model Development

This research provides a detailed analysis of convective heat transfer in ceiling jets by using a quantitative salt-water modeling technique. The methodology of quantitative salt-water modeling builds on the analogy between salt-water flow and fire induced flow, which has been successfully used in the qualitative analysis of fires. Planar laser induced fluorescence and laser doppler velocimetry have been implemented to measure the dimensionless density difference and velocity in salt-water plumes. The quantitative salt-water modeling technique has been validated through comparisons of appropriately scaled salt-water measurements, fire measurements, and theory. This analogy has been exploited to develop an engineering heat transfer model for predicting heat transfer in impinging fire plumes using salt-water measurements along with the adiabatic wall modeling concept. Combining quantitative salt-water modeling and adiabatic wall modeling concepts introduces new opportunities for studying heat transfer issues in basic and complex fire induced flow configurations.

Quantitative salt-water modeling of fire-induced flow

The dispersion of salt water carefully introduced into fresh water closely simulates the dispersion of hot exhaust gases in fire plumes. The salt concentration downstream of the source and exhaust gas temperature downstream of the flame zone behave like passive scalars and are transported by similar turbulent convective and diffusive processes. Transport properties of buoyant plumes are investigated through quantitative visualization of the distinctive turbulent structures responsible for dispersion in these flows. Turbulent mixing patterns are visualized via Planar Laser Induced Fluorescence (PLIF). This technique uses a laser light sheet to excite a fluorescent dye tracer suspended in the salt-water solution allowing visualization of a particular plane of the flow. The intensity of the fluorescence is proportional to the concentration of the salt water providing quantitative instantaneous field measurements of flow dispersion. Canonical flows including the unconfined plume and the impinging plume are carefully characterized and compared with fire scaling laws and fire measurements to validate the quantitative implementation of the PLIF salt-water technique.

Evaluation of the Field Model, Fire Dynamics Simulator, for a Specific Experimental Scenario

The new version of the fire dynamics simulator, FDS version 3.01, is verified by a series of full-scale fire tests. Experiments are carried out in a compartment similar in size to the ISO-9705 room calorimeter. Gasoline pool fires of different diameters are used to give different heat release rates. The ventilation factors of the compartment are adjusted to produce flashover. Fire-induced flow, temperature, and pressure fields in these different physical scenarios are calculated using the FDS software. The predictions for temperature and radiative heat flux of a post-flashover fire in the specific configuration tested are in reasonably good agreement with experimental results even when the flame occupies most of the room volume.

Numerical simulation of fire-induced flow through a stairwell

Smoke movement and ambient airflow in a stairwell under fire scenarios are studied numerically using large eddy simulation. Numerical investigation is performed on a typical two-storey confined stairwell, with an open door on the top floor and a fire source on the ground floor. Results show the existence of fairly distinct layers of hot smoke and ambient air under different fire scenarios. It is found that heat release rate has a remarkable effect on distributions of smoke temperature, velocity and oxygen concentration. This paper indicates that detailed patterns of velocity, temperature and species concentration and their evolutions can be predicted by numerical simulation of a stairwell during a fire.

SFPE Classic Paper Review: Fire-induced Flow Through an Opening by Joseph Prahl and Howard W. Emmons

SFPE Classic Paper Review: Fire-induced Flow Through an Opening by Joseph Prahl and Howard W. Emmons

A REVIEW ON STUDYING EXTINGUISHING ROOM FIRES BY WATER MIST

There have been numerous studies on extinguishing room fires by water mist fire suppression system (WMFSS) in the past decade. Interactions between the fire-induced flow and the discharging water mists were studied. Carrying out experiments with physical models to understand more on the physics of experiment is expensive. Therefore, well-validated numerical models by fire tests are also required. These models are commonly used as an engineering tool for designing water mist fire suppression system. In addition to taking the room as a homogeneous zone, two-layer zone approach is also used. Results based on these approaches were applied to evaluate the performance of water mist system. The application, advantages, limitations, and key results of the theories on predicting the performance of water mist fire suppression system will be reviewed in this article. Language: en

Simple Estimation Model on Ceiling Temperature and Velocity of Fire Induced Flow under Ceiling.

A new simple estimation model was described on excess temperature and velocity of ceiling jet under the flat, horizontal and sloped ceiling of an unconfined room from a t²-fire. This simple model was composed of ceiling height (H), distance along ceiling from the impinging point (r), and heat release rate (Q) which was given as a t²-fire. This model was verified by the experimental data which were conducted using at² model fire with ceiling heights of 2m, 2.1m, 4m, 8m, 15m and 20m. Flat and sloped ceiling with smooth surface was also used as a model ceiling, and those experimental data were used to verify the model. This model can deal with the cases of a corner fire and a wall fire for its early stages, and also verified by the data of corner and wall fires. The model for excess temperature and velocity with suitable RTI gave the estimation on the operation time of heat detector and sprinkler.

Numerical Study of the Turbulent Burning between Vertical Parallel Walls with a Fire-Induced Flow

A numerical study is conducted to investigate the fire structure, heat transfer and pyrolysis rate between vertical parallel burning surfaces with a fire-induced flow. The strong coupling of the two initially unknown important parameters, such as the burning and fire-induced mass flow rates, is modeled using a parubolized numerical technique which takes account the effects of the streamwise pressure gradient in parallel configuration. Transport equations for mass, momentum, gas-phase chemical species, enthalpy are solved using a finite volume method. The turbulent flow field is solved using a standard k - φ turbulence model in conjunction with a wall function. A two-dimensional adaptation of the discrete ordinates method is used for estimating the flame radiation energy to the burning wall. Soot model is also included in order to permit application to radiative heal transfer within a flame. The results indicate that with decrease of the wall spacing/height ratio (L/H), convection flux decreases slightly, whereas, contribution by radiation increases considerably from 70 to 90 percent of the total heat feedback to the pyrolyzing surface. Of particular interest is a maximum local burning rate for a wall spacing/height ratio (UH≈0.1) due to enhanced convection and radiation fluxes.

Numerical Studies on Fire-Induced Flow in a Highrise Residential Building

Numerical Studies on Fire-Induced Flow in a Highrise Residential Building

A numerical study of atrium fires using deterministic models

The smoke filling process for the three types of atrium space containing a fire source are simulated using the two types of deterministic fire model; zone model and field model. The zone model used in this simulation is CFAST (Version 3.1) developed at the Building and Fire Research Laboratories, NIST in the USA. The field model is a self-developed CFD model based on full consideration of the compressibility and k– ε modeling for the turbulence. This article is focused on finding out the smoke movement and temperature distribution in atrium spaces. A computational procedure for predicting velocity and temperature distribution in fire-induced flow is based on the solution of three-dimensional Navier–Stokes conservation equations for mass, momentum, energy, species etc. using a finite volume method and non-staggered grid system. Since air is entrained from the bottom of the plume, total mass flow in the plume continuously increases. Also, the ceiling jet continuously decreases in temperature, smoke concentration and velocity; and increase in thickness with increasing radius. The fire models, i.e. zone models and field models, predicted similar results for the smoke layer temperature and the smoke layer interface heights. This is important in fire safety, and it can be considered that the required safe egress time in three types of atrium used, in this paper is about 5 min.

Modeling on burning of large-scale vertical parallel surfaces with fire-induced flow

Abstract An efficient numerical technique is developed to predict the burning rate in a large-scale vertical parallel PMMA walls fire with a buoyancy-induced flow. The strong coupling of the pyrolysis rate and wall fire-induced flow, in parallel configuration, is modeled by including the effects of the streamwise pressure gradient for the first time. Transport equations for mass, momentum, gas-phase mixture fraction and enthalpy are solved using a finite volume method. A two-dimensional adaptation of the Discrete Ordinates Method is used for estimating the flame radiation energy to the burning wall. Soot model is also included in order to permit application to radiative heat transfer within a flame. The results indicate that with increase of the wall spacing/height (L/H) ratio, convection flux increases slightly, and however, contribution by radiation decreases considerably from 90 to 70% of the total heat feedback to the pyrolyzing surface. It appears clearly that when the wall spacing/height ratio becomes so large (L/H>0.3) that the interaction of the two diffusion flames between the opposing burning walls is unimportant, the predicted burning rate decreases dramatically and follows closely to the experimental data from a single 3.56 m high PMMA slab. Moreover, the analysis claims a maximum local burning rate for a wall spacing/height ratio (L/H≈0.1).

Fire-induced flow of smoke and hot gases in open vertical enclosures

An experimental study on the flow and heat transfer in open vertical enclosures, representing elevator shafts, warehouses, and atriums, due to a building fire is carried out, using a scale model. Smoke and hot gases are injected into the enclosure at a lower opening and the resulting downstream flow and temperature fields are studied. The inlet temperature and flow rate of the hot gases are varied over wide ranges to simulate the flow due to fire in multi-leveled buildings with vertical open shafts or atriums under natural ventilation. The conditions at the outlet, which is located on the same wall as the inlet, are also monitored to determine the effects of entrainment into the flow and heat transfer to the walls. Typical values of the operating conditions have been investigated, ranging from high buoyancy levels, for which the flow stays close to the vertical wall of the enclosure, to much lower levels, at which the flow enters the enclosure with a significant flow velocity and spreads outward very quickly. With increasing temperature at the inlet, the buoyancy effect is larger, resulting in higher velocities and shorter time to reach the top. The measured temperature at the outlet depends on heat transfer to the walls as well as on the flow velocity. Detailed measurements of the velocity and temperature fields have also been taken. It is found that a wall plume is generated which conveys the hot fluid rapidly along the vertical wall containing the inlet and the outlet. A recirculating flow arises away from this wall and this flow affects the heat transfer and flow in the wall plume. This feature, in turn, affects the entrainment into the flow, decay of the temperature level and the evolution of mean flow. Therefore, horizontally uniform conditions cannot be assumed here, as employed in several studies of tall enclosures. The wall plume has to be modeled in this case, considering the entrainment into the boundary layer flow and the effect of the recirculating flow.

Fire spread through a porous forest fuel bed: a radiative and convective model including fire-induced flow effects

A simplified physical model for the steady-state propagation of an infinite fire front through a uniform forest fuel bed in still air is derived from a mechanistic approach that considers a forest fire as a compressible, reactive and radiative flow through a multiphase medium. This model, named the PIF97 model for shortness, includes the effects of the buoyancy induced gas flow on the preheating of the unburned fuel. Fuel is composed of one type of motionless particles uniformly distributed in a fuel bed of constant depth. The conservation equations used in the model are integrated over the fuel bed depth. The spatial domain is divided into the preheating zone ahead of the fire front and the flaming combustion zone. In the preheating zone (model A), pyrolysis and chemical reactions are neglected, and the gas flow is assumed to be one-dimensional. In the flaming combustion zone (model B), some average parameters over this zone are given in order to simplify the description of physical and chemical processes. Models A and B are coupled to form the PIF97 model. The predictions of this model are compared with experimental rates of spread measured during laboratory fire experiments in pine needle fuel beds. That shows the accomplished progress by comparison to the predictions of a purely radiative model and also the limits of the PIF97 model. Under the present experimental conditions, this model correctly predicts the effect of surface-to-volume ratio, and may predict the effect of fuel load, but the quantitative effect of slope is clearly underestimated. Possible reasons for the remaining discrepancies between predictions and experimental results are investigated through an analysis of separate predictions of model A and model B.

Experimental estimation of thermal expansion and vorticity distribution in a buoyant diffusion flame

The flow induced by buoyant diffusion flames results from thermal expansion caused by heat release and vorticity generated because of density gradients in the flame. The thermal expansion source induces a potential velocity field, and the vorticity induces a solenoidal velocity field. Baum and McCaffrey's technique for the calculation of fire-induced flow field requires specifications of thermal expansion and vorticity source terms. We estimate the thermal expansion using the laminar flamelet method in conjunction with measurements of major gas species concentrations. Mixture fractions are calculated based on the major species concentration data, and gradients of mixture fraction are obtained from the curve fits to the radial profiles of mixture fractions. The temperature, density, and mass diffusivity of the gases are determined using laminar flamelet state relationships from OPPDIF simulations of a natural gas/air diffusion flame. These quantities are needed for estimating the source term for thermal expansion. The mean velocity field is measured using particle imaging velocimetry. The mean vorticity is obtained by differentiating this field using a finite difference. Near the burner surface, the vorticity components based on the axial and radial velocity gradients are approximately equal. A few centimeters from the burner surface, the component involving the axial velocity becomes dominant. We have computed the fire-induced flow field using the method of Baum and McCaffrey in conjunction with the present source term measurements. The results of these computations agree reasonably well with experimental data.

COMPARISON OF THE ALGORITHMS PISO AND SIMPLER FOR SOLVING PRESSURE-VELOCITY LINKED EQUATIONS IN SIMULATING COMPARTMENTAL FIRE

This article reports a comparison of three algorithms SIMPLER, PISO, and iterative PISO, for solving pressure-velocity linked equations in simulating fire using the field modeling technique. The set of equations describing transient three-dimensional turbulent fluid flow induced by afire was solved. The standard k-e turbulent model is employed for describing the turbulent effect. Numerical experiments were performed in simulating the fire-induced flow and temperature in the large test room reported by Ingason and Olsson [IS], The predicted results using the three algorithms are compared with the experimental data. The results show that the iterative-type solution method is good for fire field modeling. Solutions predicted from the original noniterative PISO algorithm might not satisfy the continuity equation, but do give preliminary estimation for fire engineering application, as the required computation time is significantly reduced. The treatment of free boundary conditions for the cases studied are important in achieving a reasonably steady state solution.

Numerical simulation of smoke plumes from large oil fires

A large eddy simulation (LES) model of smoke plumes generated by large outdoor pool fires is presented. The plume is described in terms of steady-state convective transport by a uniform ambient wind of heated gases and particulate matter introduced into a stably stratified atmosphere by a continuously burning fire. The Navier-Stokes equations in the Boussinesq approximation are solved numerically with a constant eddy viscosity representing dissipation on length scales below the resolution limits of the calculation. The effective Reynolds number is high enough to permit direct simulation of the large-scale mixing over two to three orders of magnitude in length scale. Particulate matter, or any non-reacting combustion ;product, is represented by Lagrangian particles which are advected by the fire-induced flow field. Background atmospheric motion is described in terms of the angular fluctuation of the prevailing wind, and represented by random perturbations to the mean particle paths. Results of the model are compared with two sets of field experiments.

Measurements and prediction of air entrainment rates of pool fires

Motivated by the various applications of entrainment rate correlations in fire research and the large uncertainty in the efficacy of existing correlations and experimental data, the first particle imaging velocimetry (PIV)-based measurements of fire-induced flow field around pool fires burning methanol, heptane, and toluene were obtained. Air entrainment rates for 15-cm and 30-cm pool fires burning the three different fuels were calculated based on the mean velocity field. The entrainment data for the six fires could be correlated well using the fire Froude number as the nondimensional parameter. An existing kinematic approach to the prediction of the fire-induced flow field was extended to the present fires. The driving processes for the entrainment flow, namely, the volumetric heat release and the baroclinic vorticity generation, were evaluated based on correlations of buoyant diffusion flame structure in the literature. The predicted entrainment velocities were substantially higher than the measurements but were in qualitative agreement with the data. On this basis the heat release rate and vorticity correlations used in the analysis were corrected by using a smaller radius for the 1/c point in the velocity profile. The modified predictions were in better agreement with the experimental data. Therefore, further evaluation of the kinematic approach with proper heat release rate and vorticity distributions is warranted.

A comparison of the use of fire zone and field models for simulating atrium smoke-filling processes

The smoke filling process for the three types of atrium spaces commonly built in Hong Kong are simulated using the two types of deterministic fire model: zone models and field models. The zone models used are the FIRST, CFAST, and CCFM.VENTS models developed at the Building and Fire Research Laboratories, NIST, USA and the NBTC one-room model of FIRECALC developed at CSIRO, Australia. The field model is a self-developed fire field model based on computational fluid dynamics theories. The results predicted by the two approaches are very similar. Simulation using a field model requires much more computing time compared with the use of a zone model, but it can give more detailed information on the fire-induced flow and temperature fields.

Use of Computational Fluid Dynamics for Simulating Enclosure Fires

This paper reports on comparing results on the fire environment predicted by using the technique of computational fluid dynamics (CFD) (or field modelling) on simulating an enclosure fire at the preflashover stage with those results reported experimentally in the literature. The theory behind this is to solve a system of partial differential equations describing conservation of mass, momentum and heat with the k-ε turbulence model. The computer pro gram PHOENICS is used as the simulation tool. By specifying the geometrical configurations of the enclosure and the location, size and thermal power of the fire source, it is possible to predict the fire-induced flow and temperature fields. The package can be executed in an IBM personal computer at or above 486 level. Experiments reported by Steckler et al. at the National Bureau of Stan dards, U.S.A. on a single fire chamber; Nakaya et al. at the Building Research Institute, Japan on a double room fire chamber; the corner fire plume by Tran and Janssens; the two-room fire experiment reported by Hagglund at the Na tional Defence Research Establishment (abbreviation: FOA), Sweden; and in the full-scale burning hall by Ingason and Olsson at the Swedish National Test ing and Research Institute (abbreviation: SP), Sweden are considered. Some of the results on the temperature and interface height of the smoke layer are com pared with the popular fire zone model CFAST.