Interaction between water spray and smoke in a fire event in a confined and mechanically ventilated enclosure

International audienceThis work deals with the interaction between water droplet flows and smoke in a fire event in a confined and ventilated enclosure. The objective is to identify the specific effect of water spray in the specific environment of a confined and ventilated enclosure. The study is based on 17 large-scale fire tests performed in one room of 165 m3 ventilated at a renewal rate of 15.4 h−1. The fire source is a propane gas burner with a heat release rate of between 140 and 290 kW. The water spray system consists of two Deluge nozzles with a nozzle coefficient of 26 l/min/bar0.5. The test parameters are the fire heat release rate, the water flow rate, from 50 to 124 l/min, and the activation time. The study focuses on three topics, the interaction of the droplets with the smoke, the droplet evaporation process and the energy transferred to the droplets. The water spray significantly modifies the smoke stratification by mixing and cooling the gas phase. The rate of droplet evaporation has been determined from the water mass balance and is of the same order of magnitude as the rate of water vapor production by the combustion reaction. Heat transfer from the smoke to the droplets has been investigated using the energy balance equation. For a fire scenario in a confined and ventilated enclosure, the energy released by the fire is mainly transferred to the walls and extracted by the ventilation network. In the event of water spray activation, a significant share, up to 65%, is transferred to the droplet flows. © 2017 Elsevier Lt

Periodic puffing instabilities of buoyant large-scale pool fires in a confined compartment

International audienceThis article presents an experimental study of the pulsating period of pool fires in a confined and ventilated compartment. Based on large-scale pool fire experiments, two modes of flame pulsation are identified. The first one is the common mode of flame puffing observed in an open atmosphere, and the second one is a perturbed pulsation mode induced by a hot vitiated environment. A description of these modes and their spectral analysis based on image processing are described. The second new mode differs from the common mode by a lower puffing frequency of the reacting gas phase. The effect of the pool diameter is analysed for the two modes. © 2012, SAGE Publications. All rights reserved

New developments in data regression methods for the characterization of thermal stratification due to fire

International audienceThe study deals with the data regression methods used for validating zone code modeling in comparison with experimental fire tests. These methods aim at determining both the temperatures of upper and lower zones and the interface location calculated from experimental vertical temperature profiles. A new data regression method is proposed to improve the determination of upper and lower temperatures and position of the interface. This new method merges both the thermal stratifications showing constant temperature in the upper layer as often encountered for well ventilated fire (approach of Quintiere et al. [4]) and those showing constant gradient as often observed for fire in closed and forced ventilated compartment (approach of Audouin et al. [8,9]). These two latter existing methods and the new former one are then applied to large scale fire tests experiments performed in IRSN facilities. The major outcomes indicate that the most appropriate regression method depends on the basic shapes of the experimental vertical temperature profiles. The interface height is the most sensitive variable to the choice of the regression methods. Indeed, discrepancy higher than 100% can be found concerning this last variable. In contrast, the temperature of lower zone appears to be the less sensible variable. The discrepancy on the upper temperature is found also significant in some cases. From these investigations, guidelines are proposed to improve the data regression process needed for validating zone codes and are supported by a thorough analysis of these three data regression methods using experimental data from large-scale fire tests. © 2015 Elsevier Ltd. All rights reserved

Doorway Flows Induced by the Combined Effects of Natural and Forced Ventilation in Case of Multi-compartments Large-Scale Fire Experiments

International audienceThis paper deals with the combined effects of fire and mechanical ventilation on the bi-directional flow occurring at a doorway for a fire scenario involving a three-compartment assembly. Based on large-scale fire tests, the analysis focused on three issues the velocity profiles at the doorway, the location of the neutral plane, and the inflow and outflow rates entering and leaving the fire compartment. The combined effect of fire and mechanical ventilation leads to different flow regimes depending on whether the smoke outflow (or the fresh air inflow) and the forced ventilation are directed in the same direction or in opposite directions. The theoretical description based on the Bernoulli approach for natural convection flow is applied in case of the combined effect of buoyancy and inertia. Five configurations are identified one-directional flow of smoke, bi-directional flow with upper smoke flow as dominant, natural convection case with equal smoke and fresh air flows, bi-directional flow with lower fresh air flow as dominant, and one-directional flow with only fresh air flow. These regimes are observed experimentally and clearly defined theoretically. A satisfactory agreement is obtained between the theory and the experiments. This approach permits identifying the critical conditions under which one-directional flow is encountered at the doorway. © 2015, Springer Science+Business Media New York

Experimental study on low-frequency oscillating behavior in mechanically-ventilated compartment fires

This study focuses on fire behavior in a mechanically-ventilated compartment, with a special emphasis on the low-frequency (LF) oscillatory behavior that has been occasionally observed. LF oscillations, typically in the order of a few mHz, can lead to large thermodynamic pressure variations, which in turn can cause fire safety issues (e.g. loss of confinement, mechanical damages). To address these issues, small-scale experiments are conducted varying the air renewal rate (ARR) in the compartment from 8 to 20 h-1 and the ventilation configuration. The fire source consists of heptane fuel loaded in a pan with a diameter of 18 cm. Depending on the ARR, LF oscillations are observed on the time evolution of the burning rate, and thus of all the other variables, with a frequency in the range of 16-26 mHz. As the ARR increases, there are three distinct regimes of burning behavior observed in this study: (1) rapid extinction due to smoke filling; (2) LF oscillating burning, followed by blow-off extinction after a few oscillations; and (3) LF oscillating burning, where extinction occurs because of the burning rate "runaway" due to an intensification of the heat transfer through the rim of the container. The oscillatory behavior and fire extinction result from the competition between oxygen supply and fuel vapor supply due to the heat feedback to the fuel tray. Both regimes (2) and (3) are accompanied by displacements of the flame out of the pan towards regions where oxygen is present. The influence of ARR and ventilation configuration (i.e. air inlet location and blowing direction) on the burning rate and LF oscillation properties (frequency, amplitude) is examined and discussed. © 2019, Isfahan University of Technology

Experimental and numerical study of low frequency oscillatory behaviour of a large-scale hydrocarbon pool fire in a mechanically ventilated compartment

International audienceThe contribution presents an experimental and numerical study of the oscillatory behaviour of a pool fire in a confined and mechanically ventilated enclosure. The fire safety issue concerns the loss of dynamic confinement of the enclosure due to large pressure variations and therefore the possible release of toxic products outside the compartment. The experimental analysis is based on large fire tests showing a periodic low frequency oscillatory behaviour of the burning rate. The frequency is 0.005-0.007 Hz (period of 150-200 s) with amplitude of about twice the mean level of the burning rate. A parametric analysis is performed to identify the most influential parameters. This oscillatory phenomenon is explained as a coupling process between the burning rate, the room pressure, the ventilation flowrate, the oxygen concentration and then a feedback effect on the burning rate. The phenomenon occurs for under-ventilated conditions for which the burning region moves within the room leading to the displacement of the flame. Numerical simulations with the fire field model ISIS is performed to check the ability of a standard CFD modelling to reproduce the flame oscillatory behaviour and to give perspective issues for numerical developments. The average values of fuel mass loss rate, compartment pressure, ventilation flow rate, oxygen concentrations and gas temperatures are well predicted. The oscillatory behaviour of the fuel mass loss rate is also obtained with a dominant low frequency although the amplitude of the fluctuations is underestimated due to a poor simulation of the flame displacement inside the compartment. The simulations points out the key effect of the pyrolysis model, the combustion model, the treatment of local extinction and the effect the ventilation flow rate. © 2016 Elsevier Ltd

Improvement of Correlative Approaches for Mixed Convective Flow Through a Horizontal Vent

International audienceThis study deals with the mixed convection flow through a shallow horizontal vent linking two compartments (one over the other). Depending on the temperature difference of gas as well as the ventilation flow rate between the two compartments, the flow through the vent can be bi- or uni-directional. A literature survey highlights that three correlations are used in safety engineering to calculate these upward and downward mixed convection flow rates. Unfortunately, for the same conditions, these correlations give very different results and, to date, there is no common agreement in the scientific community to identify quantitatively the most accurate model. This study proposes a new assessment of these correlations based on new experimental data obtained from the laboratory facility as well from the industrial apparatus. In addition, an improvement of the best model is proposed which better reproduced the experimental results. © 2019 by ASME

Doorway flow from a reduced scale isothermal air/helium approach

International audienceThis paper deals with an experimental study of buoyancy-induced gas flow at a doorway-like opening as part of a fire smoke propagation application. The experimental approach is based on a reduced scale air/helium protocol, which allows a wide range of density conditions to be studied. The flow at the opening has been studied for wide ranges of density (0.17-1.22) kg/m(3), flow rate (20-600) l/min and doorway width (0.4-14) cm usually associated with smoke propagation in life-size applications. The data set consists of measurements of flow thicknesses, spilling angles and laser tomography visualizations. The results show that flow behaviour is in harmony with the basic Bernoulli theory commonly applied to this type of flow. The value of the flow coefficient obtained with several density conditions corresponds to the behaviour found by Emmons for a given flow density. The magnitude of the spilling angle has been investigated and its dependency on the Froude number has been demonstrated. The opening width has a significant effect on flow behaviour and the associated spill plume. The reduction of the opening width enhances the mechanism of air entrainment and modifies the position of the transition height at which plume cross sections change from being rectangular to circular in shape. Published by Elsevier Masson SAS

Numerical method for determining water droplets size distributions of spray nozzles using a two-zone model

International audienceThe water spray systems are widely used for fire safety area and is a well-established technique for providing safety and protection of nuclear installations and also industrial facilities. One major challenge is to be able to properly determine the technical features of the water spray system that are required for predictive simulations. For that, a Phase Doppler Interferometer (PDI) device, that is a complex and challenging laser technique, is often used to measure the water droplets size distributions and the water droplets velocities. However, some usual water spray models can require as input parameters only an overall water droplets size distribution and water droplets initial velocity and some statistical methods are needed to determine them from local accurate measurements. In this paper, it is addressed a new calibration approach for assessing the input parameters of this modeling by using large-scale and well-controlled fire tests. Then, by introducing some correlations to take into account different operating conditions of the pressure at the spray nozzle head, this technique is validated on other large-scale fire tests. After discussing thoroughly the results, this new method shows that it can be a valuable and efficient tool for determining the overall features of water spray systems linked with the modeling of water spray system used in this study. © 2017 Elsevier B.V