Showing papers by "Bart Merci published in 2009"

State of the art in vision-based fire and smoke dectection

Abstract: Video processing techniques for automatic fire and smoke detection have become a hot topic in computer vision during the last decade. The several vision-based detection algorithms that have been proposed in literature have lead to a large amount of techniques that can be used to detect the presence of fire at an early stage. In this paper, we perform a thorough evaluation of the existing techniques and propose an improved smoke detection algorithm. The novel algorithm is based on a chromaticity-based background subtraction method with back-step correction, wavelet-based energy analysis, and boundary disorder analysis. These steps are chosen because they are mainly independent of the burning material and the fire circumstances. Experiments with a large number of fire and non-fire video sequences show that our method is fast and capable of accurately detecting smoke.

Application of FDS to Adhered Spill Plumes in Atria

Abstract: In a recently published article (Poreh et al., Fire Saf J 43(5):344-350, 2008), Poreh et al. carried out a number of experiments in a small-scale atrium. They investigated the mass flow of the spill plume in case of fire emerging from an adjacent room or corridor. Based on these experiments, the equation for the mass flow rates of adhered spill plumes in atria was adjusted. In our article, we repeat the experiments in a computational fluid dynamics (CFD) program. The results agree well, both with the experiments and the suggested formula. After this first validation, large-scale CFD-simulations are carried out. It appears that the equation suggested by Poreh et al. is only valid in the case of a uniform smoke layer depth. If the smoke layer has a more complex configuration, the formula is no longer reliable for the design of the smoke and heat exhaust ventilation system.

Application of a simple enthalpy-based pyrolysis model in numerical simulations of pyrolysis of charring materials

Abstract: A new, simple pyrolysis model for charring materials is applied to several numerical and experimental test cases with variable externally imposed heat fluxes. The model is based on enthalpy. A piecewise linear temperature field representation is adopted, in combination with an estimate for the pyrolysis front position. Chemical kinetics are not accounted for: the pyrolysis process takes place in an infinitely thin front, at the ‘pyrolysis temperature’. The evolution in time of pyrolysis gases mass flow rates and surface temperatures is discussed. The presented model is able to reproduce numerical reference results, which were obtained with the more complex moving mesh model. It performs better than the integral model. We illustrate good agreement with numerical reference results for variable thickness and boundary conditions. This reveals that the model provides good results for the entire range of thermally thin and thermally thick materials. It also shows that possible interruption of the pyrolysis process, due to excessive heat losses, is automatically predicted with the present approach. Finally, an experimental test case is considered. Copyright © 2009 John Wiley & Sons, Ltd.

Joint Scalar versus Joint Velocity-Scalar PDF Simulations of Bluff-Body Stabilized Flames with REDIM

Abstract: Two transported PDF strategies, joint velocity-scalar PDF (JVSPDF) and joint scalar PDF (JSPDF), are investigated for bluff-body stabilized jet-type turbulent diffusion flames with a variable degree of turbulence–chemistry interaction. Chemistry is modeled by means of the novel reaction-diffusion manifold (REDIM) technique. A detailed chemistry mechanism is reduced, including diffusion effects, with N 2 and CO 2 mass fractions as reduced coordinates. The second-moment closure RANS turbulence model and the modified Curl’s micro-mixing model are not varied. Radiative heat loss effects are ignored. The results for mean velocity and velocity fluctuations in physical space are very similar for both PDF methods. They agree well with experimental data up to the neck zone. Each of the two PDF approaches implies a different closure for the velocity-scalar correlation. This leads to differences in the radial profiles in physical space of mean scalars and mixture fraction variance, due to different scalar flux modeling. Differences are visible in mean mixture fraction and mean temperature, as well as in mixture fraction variance. In principle, the JVSPDF simulations can be closer to physical reality, as a differential model is implied for the scalar fluxes, whereas the gradient diffusion hypothesis is implied in JSPDF simulations. Yet, in JSPDF simulations, turbulent diffusion can be tuned by means of the turbulent Schmidt number. In the neck zone, where the turbulent flow field results deteriorate, the joint scalar PDF results are in somewhat better agreement with experimental data, for the test cases considered. In composition space, where results are reported as scatter plots, differences between the two PDF strategies are small in the calculations at hand, with a little more local extinction in the joint scalar PDF results.

Simulation of hydrogen auto-ignition in a turbulent co-flow of heated air with LES and CMC approach

Abstract: Large-Eddy Simulations (LES) with first order Conditional Moment Closure (CMC) of a nitrogen-diluted hydrogen jet, igniting in a turbulent co-flowing hot air stream have been performed. A detailed kinetic mechanism (9 species, 19 reactions) is used. Our study covers the following aspects: CFD mesh resolution; CMC mesh resolution; inlet turbulence boundary conditions; modeling of the conditional scalar dissipation rate. In all studied cases, the hydrogen auto-ignites and a diffusion flame is formed. The influence of the terms in physical space in the CMC transport equations (convection and diffusion) is small.

Hybrid RANS/PDF calculations of Sydney Swirling Flames

Abstract: In this work, we perform steady 2D axisymmetric RANS and hybrid RANS/PDF calculations to predict the turbulent flow and mixing fields of swirling inert flows and flames. The cases studied, N29S054 and SM1 respectively, are bluff body burner flows, studied experimentally at Sydney University. Turbulence is modeled with a non-linear k-e type model, taking into account effects of rotation and streamline curvature on the turbulence. Flow field predictions are in reasonable agreement with experimental data. For the reacting flow, agreement for mean mixture fraction and mixture fraction variance with experimental results is less satisfactory. Yet, the mean temperature field is quite well reproduced. We compare presumed and transported scalar PDF simulation results, with the same laminar flamelet model for chemistry. The influence of the micro-mixing model is small in our case. The mixing model constant Cφ, has a stronger influence, through the mixture fraction variance.

Numerical simulations of some possible fire scenarios in a closed car park with RANS and LES

Abstract: Prior to any CFD (‘Computational Fluid Dynamics’) calculation of a fire in a car park, a realistic fire scenario must be provided as input data. With this scenario, CFD results allow the evaluation of several characteristics of the performance of a smoke and heat control system, including: - effective limitation of the propagation of smoke (and heat) in the car park; - effective creation of a smoke-free access route from the public road on the fire storey for fire fighters to approach the fire source, i.e. facilitation of active fire suppression. - the pressure difference created by the smoke control system in the car park, which should not exceed 60Pa for the sake of opening doors. This paper presents CFD calculation for some possible fire scenarios. We do not consider the possible phenomenon of fire spread in the car park. Rather, we impose a localized fire, modeled as a heat source.

Hybrid RANS/PDF calculations of a swirling bluff body flame ('SM1')

Abstract: Steady 2D axisymmetric RANS and hybrid RANS/PDF calculations are performed to predict the turbulent flow and mixing fields of a swirling bluff body stabilized flame ('SM1'), studied experimentally at Sydney University and Sandia National Laboratories. Turbulence is modeled with a non-linear k-epsilon type model, taking into account effects of rotation and streamline curvature on turbulence. Flow field predictions are in reasonable agreement with experimental data. However, the agreement for mean mixture fraction and mixture fraction variance with experimental results is less satisfactory. The influence of the chemistry model, i.e. the use of a single laminar flamelet or the REDIM approach, is studied in transported PDF calculations with the EMST micro-mixing model. The amount of scatter is limited and observed differences are small. The combination REDIM/EMST does not reproduce the local extinction seen in the experiments. The direct comparison with the coalescence/dispersion (CD) mixing model could not be done with the same model parameter settings, as the flame extinguishes when using the REDIM approach together with the CD mixing model. Therefore, a final calculation with CD is performed with an increased value C-phi=3 and a steady solution is found. There is more scatter than with EMST, resulting in lower values for temperature and Y(CO2). Still, with the combination REDIM/CD, the level of local extinction is still under-estimated.

A Stable Pressure-Correction Scheme for Time-Accurate Non-Premixed Combustion Simulations

Abstract: An efficient time accurate algorithm is presented for numerical simulations of low-Mach number variable density flows in the context of non-premixed flames. The algorithm is based on a segregated solution formalism in the class of pressure-correction methods. It shows good conservation properties and returns stable results, regardless of the difference in density between neighboring cells. In the illustrative example, a flamesheet model is used to describe the combustion of fuel and oxidizer. The stability of the method is discussed for a 1D channel flow, containing both fuel and oxidizer. Results for a 2D testcase of a reacting mixing layer are also shown.

Numerical simulations of pyrolysis of wet charring materials using a one-dimensional enthalpy based model

Abstract: Solid phase modelling coupled to gas phase combustion of volatiles helps to investigate the flame spread over solids. The present work focuses on the solid phase. Numerical simulations of pyrolysis of wet charring materials are performed. When the solid is exposed to an external heat flux, the model is such that the moisture present first flows out, leaving behind dry solid. This solid then further heats up and finally generates a mass flow rate of combustible volatiles during the 'pyrolysis' process, i.e. the degradation of the solid. Evaporation and pyrolysis, possibly occurring at the same time, are assumed to take place in infinitely thin fronts. Pyrolysis is thus modelled as an infinitely fast, irreversible endothermic process at the 'pyrolysis temperature' (Tpyr) and as such, all kinetics are ignored. The cases considered are one-dimensional in nature, in order to focus on the principal model performance. Enthalpy is the basic model variable. The temperature distribution inside the material, along with the evaporation and pyrolysis front positions, are related to the distribution of enthalpy. A piecewise linear temperature field representation is adopted.

Study of hydrogen non-premixed auto-ignition in mixture fraction space using detailed mechanisms

Abstract: This work concerns numerical simulations of auto-ignition of hydrogen, diluted with nitrogen, in a co-flow of heated air, with conditions as in the experiment of Markides and Mastorakos [1]. Numerical simulations are performed with the Conditional Moment Closure model [2]. Combustion is at atmospheric pressure. In studies of auto-ignition phenomena, the use of detailed chemical mechanisms is important. We focus on the low temperature non-premixed auto-ignition behaviour of five different chemical mechanisms: Yetter et al [3], Mueller et al [4], Li et al [5], O'Conaire et al [6] and Konnov [7]. The temperature of the air stream (945K- 1100K) is higher than the temperature of the fuel stream (650-930K). The effect of the coflow temperature, the fuel temperature, the conditional scalar dissipation rate and the resolution in mixture fraction space is investigated. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model [8]. We compare to results when a constant scalar dissipation rate is applied over the entire mixture fraction range.

Numerical study of the adhered smoke plume for fire in an atrium

Abstract: In case of fire in an adjacent room to an atrium, one of the most important parameters in fire safety design is the smoke free height in the atrium. A recent article [1] reports on a series of small-scale experiments and the development of a new one-line equation for the mass flow rate of the spill plume. In our paper, we first describe a calculation method to determine the smoke layer interface height in CFD-simulations. A series of CFD-simulations is validated, based on the experiments of [1]. Large-scale atria are also simulated and discussed. The presence of a “more-dimensional” effect is detected and discussed.

Hybrid Rans/PDF calculations of a swirling bluff body flame (Sydney 'SM1')

Abstract: In this work, we perform steady 2D axisymmetric RANS and hybrid RANS/PDF calculations to predict the turbulent flow and mixing fields of a swirling bluff body flame (Syndey SM1). The case studied, SM1, is a bluff body stabilized flame, studied experimentally at Sydney University and Sandia National Laboratories. Turbulence is modeled with a nonlinear k-e type model, taking into account effects of rotation and streamline curvature on the turbulence. Flow field predictions are in reasonable agreement with experimental data. However, the agreement for mean mixture fraction and mixture fraction variance with experimental results is less satisfactory. Yet, the mean temperature field is quite well reproduced. We first compare presumed and transported scalar PDF simulation results, with the same laminar flamelet model for chemistry. The influence of the micro-mixing model is small in this case. The mixing model constant Cφ has a stronger influence, through the mixture fraction variance. Finite rate chemistry effects are studied in transported scalar PDF calculations using REDIM. For Cφ = 2, a steady solution is obtained with EMST, but with CD the flame extinguishes. The combination REDIM/EMST is not able to predict the local extinction seen in the experiments. Therefore, a final calculation with the CD model with Cφ=3 is performed and a steady solution is found. With CD, there is more scatter than with EMST, resulting in lower values for temperature and Y(CO2). Still, this combination REDIM/CD is not able to predict the correct level of local extinction, though.

Stability of Pressure-Correction Algorithms for Low-Speed Reacting and Non-Reacting Flow Simulations

Abstract: A promising technique for reliable and accurate simulations of turbulent nonpremixed flames is large-eddy simulation (LES) With this technique timeaccurate flow field predictions are imperative and efforts for improvement of algorithms in terms of computational cost are of great value One way of reducing computing time is to solve the Navier-Stokes equations in their low-Mach number formulation, and to apply a segregated solution method to the set of equations Such methods are called pressure-correction methods, since the kinematic part of the pressure in low-Mach number flows acts as a variable whose magnitude is determined by imposing a constraint on the velocity field in a corrector step In flows where the density remains constant in time and space, these methods have been extensively used [1] and do not suffer from inexpected stability problems However, when these methods are applied to variable density flows in the context of non-premixed combustion simulations, instabilities arise

Elimination of fast modes in the coupled process of chemistry and diffusion in turbulent nonpremixed flames: an application of the REDIM approach

Abstract: A computational study has been made of bluff-body stabilized turbulent jet flames with strong turbulence-chemistry interaction (Sydney Flames HM1 and HM3). The wide range of scales in the problem is described using a combination of a standard second moment turbulence closure, a joint scalar transported probability density function (PDF) method and the Reaction-Diffusion Manifold (REDIM) technique. The latter provides a reduction of a detailed chemistry mechanism, taking into account effects of laminar diffusion. In an a priori test it is evaluated to what extent the single shot experimental data are located on the reaction-diffusion manifold. Next, computed spatial profiles of mean and variance of independent and dependent scalar variables and profiles of conditional averages and variances (conditional on mixture fraction) are compared to the experimental results. The quality of these predictions is interpreted in relation to the a priori analysis. In general, simulations using the REDIM approach for reduction of detailed C2-chemistry confirm earlier findings for micro-mixing model behavior, obtained with a skeletal Cl-mechanism. Nevertheless it is concluded that the experiments show important features that are not described by the currently used REDIM.

Analysis of Hydrogen Non-Premixed Auto-Ignition in Mixture Fraction Space using Detailed Mechanisms

Abstract: Auto-ignition is a problem of great fundamental importance and practical interest. In many combustion devices it involves complex interaction between chemistry and turbulence. Our research objective is the performance of Large-Eddy Simulation (LES) with first order Conditional Moment Closure (CMC) of the test case experimentally studied by Markides and Mastorakos [1]. The experiment concerns auto-ignition of hydrogen, diluted with nitrogen, in a co-flow of heated air. In order to better understand auto-ignition phenomena, use of detailed chemical mechanisms is important. Therefore, we focus here on the low temperature non-premixed auto-ignition behaviour of different chemical mechanisms: Li et al [2], O’Conaire et al [3], Konnov [4] and Yetter et al [5]. The temperature of the air stream is much higher than temperature of fuel stream. The effect of the co-flow temperature, conditional scalar dissipation rate and resolution in mixture fraction space is investigated. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model [6] with imposed maximal scalar dissipation rate (at mixture fraction equal 0.5). Combustion is only considered at atmospheric pressure.