Siaka Dembele

A method for modeling the mitigation of hazardous fire thermal radiation by water spray curtains

A radiative transfer model describing the interactions between hazardous fire thermal radiation and water sprays is presented. Both the liquid (water droplets) and gaseous (mainly water vapor and carbon dioxide ) phases of the spray are considered in the present work. Radiative properties of the polydisperse water droplets are derived from Mie theory. The gaseous phase behavior is handled by the correlated-k distribution method, where the k-distribution function is evaluated for Malkmus narrow-band statistical model. The radiative transfer equation, in its integral form, is solved by a discrete ordinates method. After a general description of the radiative model developed and the experimental task to validate it, some results are discussed on its accuracy and CPU time. A deeper analysis is also carried out to point out the influence of the main parameters involved in the problem.

Investigation of a spectral formulation for radiative heat transfer in one-dimensional fires and combustion systems

The radiative heat transfer in variable concentration, non-isothermal and sooty combustion products is investigated with a correlated-k gas method and discrete ordinates technique for the solution of equation of transfer. The spectrally formulated model is an approach for predicting radiative intensities and fluxes in mixtures of gaseous products (H2O, CO2, CO) and scattering soot particles. The reliability of this approach for fire and combustion applications is analysed by comparing its predictions with some theoretical results and measured radiative intensities for a natural gas flame. Use of this model for more accurate calculations and evaluation of simplified fire and combustion radiation models, are among its main advantages.

Experimental study of water sprays for the attenuation of fire thermal radiation

A laboratory experimental work is carried out to investigate the attenuation ability of water sprays subjected to thermal radiation. The objective is to analyze the key parameters involved in the mitigation properties of this fire protection technique. The spectral transmittances of two types of sprayers, TG03 and TG05, are measured with a Fourier infrared spectrometer under various conditions. The wavelength range varies from 1.5 to 12 μm. The influence on the transmittance of both the flow rate and the pressure ranging from 1 to 7 bars, as well as the effect of the number of spray nozzles are considered. The results clearly show the advantage of small drops with high concentration. An investigation on the multi-ramp curtain configuration also provides valuable information on the mitigation behavior of the whole spray. Key guidelines are provided for fire protection engineering.

Analysis of the two-flux model for predicting water spray transmittance in fire protection application

An investigation is carried out to assess the two-flux model for evaluating the attenuation ability of a water spray curtain in fire protection. Transmittances calculated with this model are compared with the exact discrete ordinates solutions for a range of water curtains under practical conditions. The results show the unsuitability of the two-flux method under a collimated incidence boundary condition, even if some improvements could be expected with very small droplets. Whereas the diffuse incidence type provides relatively better results, it is more reliable for transmittance calculations.

Investigation of Glazing Behavior in a Fire Environment Using a Spectral Discrete Ordinates Method for Radiative Heat Transfer

Radiative heat transfer plays a major role in the analysis of glazing behavior in fires, but its rigorous modeling has received little attention. In the present study, a spectral radiative heat transfer model, based on the discrete ordinates method (DOM), is developed and employed to analyze heat transfer and the transient temperature distribution in a glazing structure subjected to fire heat flux. Comparisons are made between model predictions and literature experimental data; acceptable agreements are found. The study also investigates the influence of the glass properties and geometry on the temperature and time to breakage.

Thermal and Stress Analysis of Glazing in Fires and Glass Fracture Modeling with a Probabilistic Approach

The aim of the study is to predict the thermal and stress behavior of a framed glass subjected to typical fire conditions, and the initial glass fracture time and locations using a probabilistic approach as an alternative to Pagnis deterministic criterion. Thermal stresses in glass have been little researched. The probabilistic approach has the advantage of taking into account some uncertainties such as the edge conditions. The model employed is based on stress and conduction heat transfer models, a spectral discrete ordinates radiation model, and a failure probability model. Some results of its verification and applications are reported here.

Large eddy simulation of fire dynamics with the improved eddy dissipation concept

The eddy dissipation concept (EDC) for turbulent combustion modeling was originally proposed by Magnussen in the Reynolds averaged Navier-Stokes (RANS) context. This study has extended it to the large eddy simulation (LES) framework. Since the fine structures in EDC are still not resolved in LES, they are modeled with SGS turbulent kinetic energy and its dissipation rate instead of the mean quantities in RANS. A new expression is proposed for the reacting fraction of the fine structures to alleviate the limitations of the original formula. Three fire cases including a 7.1 cm methane fire, a 30 cm heptane fire and a 30.5 cm methanol fire are simulated to verify the improved EDC (M-EDC), which has been implemented in the FireFOAM solver. The predictions from the M-EDC are found to be in reasonably good agreement with the measurements, while the original EDC tends to under-predict temperature and velocity.

Investigation of Turbulence Models for CFD Simulations of Gas and Liquid Pool Fires

The effect of turbulence models on CFD predictions of gas and liquid pool fires is investigated. A buoyancy-modified four-equation (4EQ) model and two modified versions of the standard two-equation(2EQ) k-ε model are employed for simulating three pool fire scenarios. The 4EQ model includes all the important source terms in the turbulent heat flux expressions and emphasizes the anisotropy of turbulence due to buoyancy effects by adding an algebraic term to the eddy-viscosity expression of Reynolds stresses. Predicted results show that the 4EQ model predicts far larger values of buoyancy production of turbulent kinetic energy than the 2EQ models, and consequently achieves better agreement with experimental temperature and velocity data.

Simulation of glazing behavior in fires using computational fluids dynamics and spectral radiation modeling

The performance and behavior of glazing systems have significant impact on fire growth and development. In the typical scenario of a glass pane exposed to fire, radiation is the predominant mode of heat transfer. In previous studies, the present authors have developed and validated a spectral radiation heat transfer model based on the Discrete Ordinates Method (SDOM) which accounts for the glass spectral properties (e.g. emissivity, transmissivity) and the diffuse nature of radiation incident on the glazing. In order to model the dynamic interaction between a glass pane and fire, the SDOM has been implemented in the CFD code FDS 5.0 and the new code (FDS-SDOM) is evaluated. The first part of FDS-SDOM validation study reports a comparative analysis between FDS-SDOM, the original (unmodified) version of FDS (OFDS) and the exact solutions for varying absorption coefficients typical of glass. The comparative study, in terms of radiative heat flux, shows that FDS-SDOM provides results closer to the exact solutions in comparison to OFDS when the absorption coefficient is varied from 0.1 to 100 m -1 (maximum error less than 1% for FDSSDOM against 13 % for FDS). This provides further justification for the need to account for varying spectral properties of material such as glass and the diffuse nature of radiation in calculations. In the second part of the validation study, FDS-SDOM is applied to two experimental fire and glass scenarios with the aim of predicting the transient temperature distribution in the glass. Relatively good agreements are found between the code’s predictions and the experimental data. The work demonstrates the good potential of combining the CFD approach with advanced spectral radiation modeling for fire and glazing studies.

Assessments of Spectral Narrow Band and Weighted-Sum-of-Gray-Gases Models for Computational Fluid Dynamics Simulations of Pool Fires

The statistical narrow band (SNB), correlated-k (CK), and weighted-sum-of-gray-gases (WSGG) models have been incorporated into a computational fluid dynamics (CFD) code. Their accuracy and computing times are evaluated for three pool fires scenarios. CFD_CK and CFD_SNB yield similar predictions, and the former is three times more CPU demanding than the latter. CFD_CK is unrealistic for practical fire applications. Temperature and velocity predictions with CFD_WSGG and CFD_SNB agree better in the persistent and plume regions, but significant discrepancies are found in the intermittent region. The overall predictions of the two approaches show reasonable agreement. The WSGG model should not be discarded for CFD fire simulations.