Fire-induced Flows Research Articles

Fire-induced flows for complex fire scenarios in a mechanically ventilated two-storey structure

This work deals with smoke propagation through a multi-compartment assembly in case of a fire event in a nuclear installation. The scientific issues are the understanding of flows involving two modes of propagation (vent and doorway), together with the role of mechanical ventilation and oxygen backflows to the fire. The study is based on the analysis of two scenarios reproduced experimentally at large scale and simulated numerically. The main outcomes concern the comparison of the flow at a doorway and at a vent, the consequence of the smoke propagation for thermal stratification and the combined effect of the fire heat release rate and mechanical ventilation. The results highlight the performance of computational fluid dynamics simulations in predicting these complex scenarios. Low-velocity flow zones are identified, enabling the structure of these flows and their amplitudes to be quantified. This information provides new insights to improve fire risk assessment in nuclear facilities.

Large eddy simulation of fire-induced flows using Lattice-Boltzmann methods

Large-eddy simulations (LES) of the near-field region of large-scale fire plumes are performed for the first time with a pressure-based Lattice Boltzmann method (LBM) with low-Mach number approximation. Two scenarios are considered: the large-scale non-reactive helium plume and the 1 m methane pool fire, both investigated experimentally at Sandia. In the second scenario, a simplified modeling of the combustion and radiation processes is introduced involving a one-step irreversible reaction eddy-dissipation concept-based combustion model and a radiant fraction model, respectively. In both scenarios, a quantitative agreement is observed with the experimental data and model predictions are consistent with previously-published numerical studies. Our simulations demonstrate the computational efficiency of the proposed LBM solver to tackle fire-induced flows, suggesting that LBMs are a good alternative candidate for the modeling of fire-related problems.

On the interaction of wind, fire intensity and downslope terrain with implications for building standards in wildfire-prone areas

Background Wildfires can have detrimental impacts on the environment and urban structures when they spread from wildland areas. Aims In this work, a numerical study was performed to investigate the effect of downslope terrain on fire-induced flows in the presence of a building structure. Fires with intensities of 4 and 15 MW m−1 were considered on inclined terrain with downslope angles varying from 0° to −30°, and wind speeds of 6 and 12 m s−1. Methods Simulations were conducted using a large eddy simulation (LES) solver, implemented in the open-source platform FireFOAM. Key results The results were validated with experimental measurements of a full-scale building model. Results show that at a wind velocity of 12 m s−1, structures on steeper downslope terrains are at higher risk of wildfire damage, whereas at a constant wind velocity of 6 m s−1, these structures are at a lower risk. Conclusions The outcomes of the study highlight the physical effect of sloped terrain on buildings downwind of a line fire. Implications The results from this study can be used to evaluate the validity of risk management measures including building standards and asset protection zones and can better inform ways of improving these measures.

Scaling analysis of downstream heating and flow dynamics of fires over an inclined surface

Fire spread is known to accelerate uphill over inclined surfaces due to an intrinsic coupling between geometry and fire-induced flows, especially when the slope exceeds a critical angle. The fundamental relationships which govern this close coupling, however, are not yet fully understood. To investigate these relationships, propane-fueled fires were produced over a tilt table at the Missoula Fire Sciences Lab. Large-scale fire tests with heat-release rates ranging from 81 kW to 2.25 MW were conducted. The angle of inclination, θ, was varied between 0∘ and 30∘. A micro-thermocouple array along the centerline of the table was used to measure downstream gas temperatures. Flames were seen to start attaching to the inclined surface at θ = 18∘, independent of the fireline intensity. The centerline temperatures under attached flame conditions are consistent with McCaffrey's buoyant flame temperature correlation, suggesting the buoyancy-driven nature of the inclined fires. The local gas velocity was measured using temperature-correlation velocimetry through the streamwise temperature signals. Results show that the local flow was accelerated in the attached flame region driven by buoyancy before reaching a peak. Velocity of the flow slowed down after the peak due to weaker buoyancy within the intermittent and plume regions. The mean surface velocity of the attached flame scales directly with the angle of inclination (sinθ) and the fireline intensity (I2/3), providing a promising method to evaluate convective heat transfer using the geometry of the inclined fire.

Influence of current direction in longitudinal ventilated road tunnels on the backflow of combustion Produits

The results of numerical modeling in longitudinal ventilated sloping road tunnels are given. The slope of the tunnels varies in the range of 0-6%. The geometry of the tunnel is as follows: length: 100 m; width: 8 m; height: 6 m; area of the seat of fire: 16 m2. The seat of fire sized: 2.75x5.8x1.5 m is in the central part of the tunnel. The scenarios of development of 5, 10, 20, 30, 50 MW fires are studied in the case of positive and negative directional ventilation flows. The time of modelling was 120 seconds. The numerical problems were modelled with a volumetric grid method. The grid cell dimensions were: 0.5*0.5*0.5 m. Virtual point and volumetric measuring equipment was used to record the modeling results. The modelling used 4 groups of measuring devices that measured and recorded air velocity, temperature, and air and smoke densities. The paper discusses cases of algebraically summarizing the ventilation and fire-induced flows. Based on the results of numerical modeling, we can point out that the widely accepted indices of critical velocity and back-layering length in inclined road tunnels often give erroneous results. Therefore, in strategies for emergency ventilation, indicators such are critical velocity and back-separation should be used with caution.

Analyzing fire-induced dispersion and detector response in complex enclosures using salt-water modeling

The current investigation examines suitability of the hydraulic analog for analysis of fire-induced dispersion within a complex enclosure. This analog has been implemented using salt-water modeling and planar laser induced fluorescence (PLIF) diagnostics providing quantitative visualization of simulated fire-induced flows. The non-intrusive PLIF diagnostics are used to temporally and spatially characterize dispersion from a buoyant source within a 1/7th scale room–corridor–room enclosure. This configuration is geometrically similar to a full-scale fire test facility, where local fire conditions were characterized near five ionization type smoke detectors placed throughout the enclosure. The full-scale fire and salt-water model results were scaled according to the dimensionless fundamental equations that govern source dispersion.An evaluation of the local conditions and dispersive event times for both the systems was used to explore the ability of the hydraulic analog to predict smoke detector response times. The dispersive event (front arrival) times predicted by the salt-water model, which represents a necessary event for detector activation, were in excellent agreement with the fire test data. A methodology using these front arrival times along with local conditions at the detector location is introduced in this paper. However, the complex nature of detector response and fidelity limitations of the analog make precise predictions of detector response time challenging. The predicted dimensionless response times were within 25% for all detector locations, with the exception of the first-room ceiling detector location. For this latter, a shorter dimensionless response time by less than 40% of that in the actual fire was predicted.

On the extrapolation of CFD results for smoke and heat control in reduced-scale set-ups to full scale: Atrium configuration

It is a common practice to use reduced-scale experiments to develop formulae for the design of smoke and heat exhaust ventilation systems. Implicitly, up-scaling of results is assumed justified. A similar approach can be adopted with numerical simulations, i.e. a reduced-scale setup can be up-scaled to compare results to full-scale observations. However, both in numerical simulations and in experiments, scaling must be done in a proper way. The classical method for up-scaling results obtained in fire related experiments is based on preservation of the Froude number only. The present paper, focusing on the up-scaling of results by means of a series of CFD simulations of fire-induced flows in an atrium configuration, confirms this to be justified as long as the flows in both the reduced-scale and full-scale configurations are sufficiently turbulent. If this is not the case, it is illustrated that other dimensionless numbers must also be preserved in scaling, in addition to the Froude number.

Calculation and design of tunnel ventilation systems using a two-scale modelling approach

This paper develops a novel modelling approach for ventilation flow in tunnels at ambient conditions (i.e. cold flow). The complexity of full CFD models of flow in tunnels or the inaccuracies of simplistic assumptions are avoided by efficiently combining a simple, mono-dimensional approach to model tunnel regions where the flow is fully developed, with detailed CFD solutions where flow conditions require 3D resolution. This multi-scale method has not previously been applied to tunnel flows. The low computational cost of this method is of great value when hundreds of possible ventilation scenarios need to be studied. The multi-scale approach is able to provide detailed local flow conditions, where required, with a significant reduction in the overall computational time. The coupling procedures and the numerical error induced by this new approach are studied and discussed. The paper describes a comparison between numerical results and experimental data recorded within a real tunnel underlining how the developed methodology can be used as a valid design tool for any tunnel ventilation system. This work sets the foundations for the coupling of fire-induced flows and ventilation systems where further complexities are introduced by the hot gas plume and smoke stratification.

One-dimensional analysis of the global chimney effect in the case of fire in an inclined tunnel

A one-dimensional computational analysis is presented of the global chimney effect due to a fire source in an inclined tunnel. Fire-induced flows are not accounted for in the present study: the computed velocity, due to global heating, is to be superposed as an initial and/or boundary condition in detailed computational fluid dynamics (CFD) simulations in a region of interest. The results must not be used as a replacement for such detailed simulations in e.g. network problems. Two calculation options are explained for the global chimney effect. The first method is based on the global heating of the entire tunnel part between the fire source and the higher positioned tunnel outlet. In the other method, the initial fire-influenced region is restricted to a certain distance and, as soon as a prescribed temperature rise is met, the fire-influenced region length increases until the tunnel outlet is reached. The treatment of the enthalpy flow rate through the fire-influenced region boundaries is discussed. In order to identify the most important parameters and to quantify their influence on the global chimney effect, a sensitivity study is presented with respect to the different model ingredients. The following quantities are examined: fire source heat release rate, tunnel inclination angle, tunnel length, tunnel wall roughness, ambient temperature, ambient pressure conditions, tunnel cross-section area, and fire source position. Attention is focused onto the fire-influenced region mean temperature and the air velocity at the tunnel inlet. The inlet air velocity is quantified by means of a correlation, taking into account the parameters mentioned.

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.

Salt water modeling of fire induced flows in multicompartment enclosures

Salt water modeling is used to study fire-induced flows in multicompartment structures. Scaling laws relating salt water flows and hot gas flows are developed. Results from 1/20 scale salt water simulations of fire-induced flows in a single-story multiroom structure are shown to be in good agreement with available full-scale results. Experiments involving a 1/20 scale model of a U.S. Navy ship demonstrate the feasibility of using the technique to study hot gas flows in compartmented structures too complex to study economically by other means.

Doorway flow induced by a propane fire

A series of full-scale steady-state experiments was conducted to study the high temperature fire-induced flows through doorway openings connecting a burn room to a second room containing a hot gas layer. A propane diffusion burner served as the energy source. Fire strength and doorway width were varied. Measurements included two-dimensional velocity and temperature profiles within the opening and vertical gas temperature profiles within the rooms. Opening mass flows were determined from the opening data. These mass flows are explained in terms of a static pressure model based on actual gas temperature profiles in the two rooms and an orifice coefficient of 0.68. It is also shown that the simple relationship between mass flow rate through the opening, M a , and ventilation parameter, WH 3 2 (W, opening width; H, opening height) holds even when the opening is in the wall between two rooms. The proportionality constant for this simple relationship may depend on the configuration.

Fire induced flows through room openings-flow coefficients

A full-scale experimental and theoretical study was made of steady-state fire-induced flows through doorway and window openings. Measurements included two-dimensional temperature and pressure-difference profiles within the opening and vertical temperature profiles within the rooms connected by the openings. Gas burners served as the energy source. Mass flow rates through the openings were calculated from the opening data. A static-pressure flow model was used to establish ideal orifice flows. The opening and ideal flow results were combined to form room-opening flow coefficients as a function of fire energy release rate, opening geometry, and fire location. An irrotational jet model for the flow coefficients was developed and found to be in reasonable agreement with these and other measurements. Measured flow coefficient results show no significant dependence on fire strength, opening geometry, or fire location. The mean value of the inflow coefficient is 0.68 and 0.73 for the outflow coefficient. However, the theory indicates a significant variation in flow coefficient with opening widths larger than those used in the experiments.

An Assessment of Fire Induced Flows in Compartments

An Assessment of Fire Induced Flows in Compartments

Erratum ; An Assessment of Fire Induced Flows in Compartments

Erratum ; An Assessment of Fire Induced Flows in Compartments