Accurate models to predict large-scale flame propagation are crucial to assessing the consequences of accidental explosions. A freely expanding spherical flame can experience significant acceleration due to the growth of the Darrieus–Landau instability, increasing the severity of the explosion hazard, and must be accounted for when modeling large-scale flames. Recent large-scale experiments have demonstrated an oscillatory rate of flame acceleration, consistent with self-similar flame propagation and the formation and growth of cells with discrete length scales. In this work, an analytical model describing the growth of multiple generations of cells on a flame surface is developed. Each generation is treated independently with a criteria for cell splitting based on a critical stretch rate. Superposition of the length scales is used to determine the global flame surface wrinkling and propagation velocity of the flame. The model is compared with experimental results and the effect of upstream flow disturbances on the development of the instability is also discussed. It is found that the model reproduces a number of features observed in the experiments, including the overall rate of flame acceleration and the frequency of oscillation. The model also captures the correct trends of flame behavior for the effect of elevated initial turbulence and the transition from positive to negative Markstein length. These results support the self-similar argument of spherical-flame acceleration and can be used in future studies to develop new models describing the behavior of large-scale flames.

Fire suppression tests with ceiling sprinkler protection in a rack storage fuel configuration are simulated using a Computational Fluid Dynamics tool. The fuel is arranged in a double-row, six pallet-load wide and three-tier high (2×6×3) rack storage array. Each pallet load consists of three nested double-wall corrugated cardboard boxes surrounding a metal liner. Two types of ceiling sprinklers are used in this study: a pendent quick response sprinkler designated as K14, and an upright standard response sprinkler designated as K11.2. The tests are simulated using FireFOAM, which couples necessary sub-models for fire growth, sprinkler response, and fire suppression. Numerical results are compared with experiments for both free burn tests under a 20-MW calorimeter and sprinkler suppression tests under a 7.6 m high ceiling. For the free burn case, the model results show good quantitative agreement of heat release rates in all three phases, from ignition to fire growth and steady burning. For the suppression cases, the model reproduces the suppression effectiveness of the two sprinkler protection designs: K14 sprinklers suppress the fire rapidly with only one sprinkler activation, while with K11.2 sprinklers, both in the tests and simulation, the fire spreads to the pallets on the end of the fuel array with multiple sprinkler activations. The modeled sprinkler activation times are within the estimated experimental uncertainty following three repeat tests. Quantitative results characterizing sprinkler suppression performance obtained from the simulations, such as the actual delivered density (ADD) and water evaporation rate, are also reported.

The relation between flame radiation, smoke yield, and smoke point of a practical solid fuel, namely corrugated cardboard, is studied experimentally. Experiments are performed using an ASTM E 2058/ISO 12136 Fire Propagation Apparatus (FPA). Corrugated cardboard flames are established in the FPA under external heat fluxes representative of those found in a large-scale fire scenario. The heat release rates for these flames are on the order of 7 to 10 kW based on calorimetry analyses. Radiation is measured using a heat flux gage located in the near field of the flame. In order to better interpret calorimetry data, effort is placed on the characterization of the chemical composition and thermodynamics of the corrugated cardboard used both in its virgin and charred states. A novel smoke point measurement system based on the FPA is also described and demonstrated. It is shown that the specific heat of combustion of volatiles released from the pyrolysis process increases with pyrolysis progress. Furthermore, flame radiant fraction, smoke point, and smoke yield are also shown to vary during pyrolysis and combustion. The variations of both the smoke point and radiant fraction with pyrolysis progress at different heating rates indicate that the volatile chemical composition continuously varies during pyrolysis. These observations are explained by faster release rates of fuel oxygen and hydrogen than that of carbon during pyrolysis.

Systems is the reference meeting focusing specifically on dynamic aspects of reactive systems.

Effect of scale and mixture properties on behaviour of turbulent flames in obstructed areas Results for the study on the effect of scale and mixture properties on the behaviour of turbulent flames in obstructed areas are presented. A set of dimensionslass parameters was chosen which were defined by the laminar flame speed SL, the flame thickness eS, the integral length scale L, and thermodynamic mixture properties. The experiments were focused on the study of the effect of these parameters. Two tubes (174 and 520 mm id and similar geometry of obstacles with blockage ratio BR = 0.6) were used in the tests. Different hydrogen mixtures were chosen in order to provide (1) a wide range of the scaling parameters, and (2) combinations with similar values of the parameters at different scales. lt was shown that the mixture properties and scale have a mutual effect on the behavior of the turbulent flame. The resulting regime of flame propagation was found to depend mainly on the values of parameter U8 and expansion ratio cr. lt was found that the range of the scaling parameters Uö < 500, cr < 3.75 resulted in slow combustion regimes with global quenching. The range Uö > 500, cr < 3.75 corresponds to relatively slow and unstable flames. For cr > 3.75, fast combustion regimes (chocked flames and quasi-detonations) were observed. The expansion ratio cr was found to be the main parameter which defined a border between "weak" (unable to support effective flame acceleration) and "strong" mixtures. The critical value cr "" 3.75 found in the present tests should not be considered as universal, but may be a function of Zeldovich and Lewis numbers. Zusammenfassung Einfluß von Skala und Mischungseigenschaften auf das Verhalten von turbulenten Flammen in versperrten Geometrien Der Bericht beschreibt die Ergebnisse einer experimentellen und theoretischen Studie zum Verhalten von turbulenten Flammen in versperrten Geometrien. Für die theoretische Skalierung wurden dimensionslose Parameter abgeleitet, die die laminare Brenngeschwindigkeit SL, die laminare Flammendicke 8, das integrale Längenmaß der Geometrie L und thermodynamische Größen der Gasmischung enthalten. Die Experimente konzentrierten sich auf die Untersuchung des Einflusses dieser Parameter. Dazu wurden zwei rohrförmige Versuchsanlagen mit 174 bzw. 520 mm Innendurchmesser und einem Versperrungsgrad von 60 % benutzt. Verschiedene Wasserstoff-Inertgasgemische wurden untersucht um 1.) einen großen Bereich für die Skalierungsparameter abzudecken, und 2.) ähnliche Parameterkombinationen auf unterschiedlichen geometrischen Skalen zu erhalten. Es wurde gezeigt, daß die Mischungseigenschaften und die Abmessung der Umschließung einen wechselseitig abhängigen Einfluß auf die Entwicklung von turbulenten Flammen haben. Das sich einstellende Verbrennungsregime hängt im wesentlichen von den Parametern U8 und dem Expansionsverhältnis cr der Gasmischung ab. ln dem Bereich U8 < 500 und cr < 3.75 traten bei Anfangstemperaturen um 300 K nur langsame Verbrennungen mit globalen Löschvorgängen auf. Der Parameterbereich U8 > 500, cr < 3.75 entsprach langsamen und instabil brennenden Flammen. Für cr > 3.75 wurden dagegen nur schnelle Verbrennungsformen wie Überschallflammen und Quasi-Detonationen beobachtet. Das Expansionsverhältnis cr (= Volumen des verbrannten GasesNolumen des unverbrannten Gases bei konstantem Druck) erwies sich als der Haupteinflußparameter, der die Grenze definiert zwischen "schwachen" Mischungen die nur langsam brennen, und "starken" Mischungen die auf hohe Verbrennungsgeschwindigkeiten beschleunigen. Der hier in Versuchen bei etwa 300 K gefundene kritische Wert von cr = 3.75 stellt keinen universellen Wert dar. Es ist zu erwarten, daß er von der Zeldovichund der Lewis-Zahl abhängt.

This paper reviews the state of knowledge on flame acceleration and deflagration-to-detonation transition (DDT) in smooth ducts and ducts equipped with turbulence-producing obstacles. The objective is to bring to light the basic understanding of the phenomenon and its application to explosion safety. The scope of the review is restricted to homogeneous gas-phase combustion with emphasis placed on experimental investigation.

Results of a series of tests on the deflagration of methane-air mixtures in a large vented enclosure are presented. Experiments were made in FM Global’s 63.7 m3 chamber. The chamber was 4.6 x 4.6 x 3.0 m with a vent opening on one side. Vent areas of either 2.7 or 5.4 m2 were used. Tests were performed with ignition either at the center of the chamber or at the center of the wall opposite the vent. Methane-air mixtures with methane concentrations close to 9.5% vol. were used in the tests. Pressure data, as function of time, and flame time-of-arrival data were obtained both inside and outside the chamber near the vent. Detailed experimental data is used in the paper to test a three-dimensional gasdynamic model for the simulation of gaseous combustion in vented enclosures. The model is based on a Large Eddy Simulation (LES) solver created using the OpenFOAM CFD toolbox using sub-grid turbulence and flame wrinkling models. Results from the calculations are compared with the experimental data. The capabilities and deficiencies of the model are discussed.

A three-dimensional gasdynamic model with constant burning rate is applied for the prediction of the maximum pressure rise from gaseous combustion in vented enclosures. A series of calculations for an enclosure with aspect ratio close to unity are presented. Both cases with and without obstacles in the enclosure are considered. Results of calculations are compared with a simple 0D solution for spherical vessels. It is shown that, in cases without obstacles, the 0D solution for the maximum reduced overpressures is close to the predictions of the detailed modeling. In cases with obstacles, the detailed simulation gives significantly higher overpressures than those from the 0D model. However, in all the cases the reduced pressures are correlated well with the maximum flame surface area.

A three-dimensional gasdynamic model with constant burning velocity is applied for the prediction of the maximum pressure rise from gaseous combustion in vented enclosures with variable aspect ratios and ignition locations. A series of calculations for vented enclosures with aspect ratio varied between 0.5 and 8 are presented as well as a series of calculations for three different ignition locations. In addition to the aspect ratio and ignition location, the flame speed and the vent size were also parameters of the study. A solution for the evaluation of the flame surface area from the model output is developed. This solution is used for the determination of a quantity known as the “vent parameter”, which is widely used in analytical models for prediction of the maximum reduced explosion pressure. Results of calculations are compared with an ideal analytical solution for spherical vessels, with results from a previous study using the same 3D gasdynamic model and with the NFPA 68 guidelines for vent sizing. It is shown that both changes in aspect ratio and changes in the ignition location result in shifts along the same reduced pressure rise - vent parameter curve. The results of the analysis have been summarized by a formula for the practical evaluation of the effects of aspect ratio on the maximum explosion pressure.

To evaluate the explosion hazard of ITER-relevant dusts, a standard method of 20-l-sphere was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The effect of dust particle size was studied on the maximum overpressures, maximum rates of pressure rise, and lower explosive concentrations of graphite dusts in the range 4μm to 45μm. The explosion indices of 1μm tungsten dust and its mixtures with 4μm graphite dust were measured. The explosibility of these dusts and mixtures were evaluated. The dusts tested were ranked as St1 class. Dust particle size was shown to be very important for explosion properties. The finest graphite dust appeared to have the lowest minimum explosion concentration and be able to explode with 2kJ ignition energy.

Results of experimental study on DDT in a smooth tube filled with sensitive mixtures having detonation cell size from 1 to 3 orders of magnitude smaller than the tube diameter are presented. Stoichiometric hydrogen–oxygen mixtures were used in the tests with initial pressure ranging from 0.2 to 8 bar. A dependence of the run-up distance to DDT on the initial pressure is studied. This dependence is found to be close to the inverse proportionality. It is suggested that the flow ahead of the flame results in formation of the turbulent boundary layer. This boundary layer controls the scale of turbulent motions in the flow. A simple model to estimate the maximum scale of the turbulent pulsations (boundary layer thickness) at flame positions along the tube is presented. The largest scale of the turbulent motions at the location of the onset of detonation is shown to be 1 order of magnitude greater than the detonation cell widths, λ, in all the tests. It is suggested that the onset of detonation is triggered during flame acceleration as soon as the maximum scale of the turbulent pulsations increases up to about 10 λ. The model to estimate the maximum size of turbulent motions, δ, and the correlation δ≈ 10λ, give a basis for estimations of the run-up distances to DDT in tubes with internal diameter D > 20λ.

In this paper, a deterministic analysis of one of the ITER reference accident scenarios, the so-called "Ex-Vessel Loss-of-Coolant Accident", and of its consequences for system integrity is given. Using inflow rate data from a preceding MELCOR calculation, the evolution of the gas state in the ITER facility (vacuum vessel (VV), pressure suppression system (VVPSS), and drain tank (DT)) is calculated for the first 21,000 s of accident time with the GASFLOW code. Combining these results with new experimental data, for low initial pressures, on flame acceleration and transition to detonation, it is shown that a detonation in the VVPSS is possible, whereas the gas mixture in the VV and the DT never even reaches the flammability limit. To get an upper bound on the possible accident consequences, a detonation in the VVPSS was therefore postulated and simulated with the DET3D code. Using the resulting pressure loads for a structure-mechanical analysis of the end plate of the VVPSS with the ABAQUS code, it turned out that this end plate will almost fully change into the plastic state. Since this would violate the design rules of the ITER project, a further study of this accident scenario seems advisable. (c) 2005 Elsevier B.V. All rights reserved.

Addressing the dust explosion hazard in ITER, a standard method with a 20-l-spherical combustion chamber was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The indices include maximum overpressure, maximum rate of pressure rise, and lower explosion concentration limit. The effect of dust particle size was studied on the explosion behaviour of graphite dusts in the range 4–45μm. The explosion indices of 1μm tungsten dust and its mixtures with 4μm graphite dust were measured. The graphite dust particle size is shown to have a profound effect on the explosion characteristics. The finest dust features the highest maximum overpressure and rate of pressure rise, and the lowest explosible concentration. Four tungsten/graphite dust mixtures with molar ratios of W/C=1/30, 1/4, 1/1, and 3/1 were tested at the concentrations at which the dust combustion consumed all the oxygen in air producing maximum overpressures and rates of pressure rise. The maximum overpressure decreases slightly with increasing tungsten content, while the maximum rate of pressure rise has a pronounced peak at W/C=1/1, i.e. this mixture burns faster than both pure graphite and pure tungsten dusts alone. All the tested dusts belong to the mildest explosion class.

A joint research project was carried out in the EU Fifth Framework Programme, concerning hydrogen risk in a nuclear power plant. The goals were: Firstly, to create a new data base of results on hydrogen combustion experiments in the slow to turbulent combustion regimes. Secondly, to validate the partners CFD and lumped parameter codes on the experimental data, and to evaluate suitable parameter sets for application calculations. Thirdly, to conduct a benchmark exercise by applying the codes to the full scale analysis of a postulated hydrogen combustion scenario in a light water reactor containment after a core melt accident. The paper describes the work programme of the project and the partners activities. Significant progress has been made in the experimental area, where test series in medium and large scale facilities have been carried out with the focus on specific effects of scale, multi-compartent geometry, heat losses and venting. The data were used for the validation of the partners CFD and lumped parameter codes, which included blind predictive calculations and pre- and post-test intercomparison exercises. Finally, a benchmark exercise was conducted by applying the codes to the full scale analysis of a hydrogen combustion scenario. The comparison and assessment of the results of the validation phase and of the challenging containment calculation exercise allows a deep insight in the quality, capabilities and limits of the CFD and the lumped parameter tools which are currently in use at various research laboratories.

Introduction Many practical applications involve handling of extremely sensitive gaseous combustibles in volumes or tubes with characteristic sizes, which exceed considerably typical chemical length scales of the mixture, such as detonation cell size. If these mixtures are ignited the combustion process should easily end up with the transition from deflagration to detonation (DDT). In these cases an important problem to be solved is where can the onset of detonation occur, rather than can it be expected or not. This problem is related to creation of conditions that are necessary for the onset of detonation during flame propagation. It has been shown by many investigators that a flame should reach high, generally supersonic speed, to create conditions necessary for the onset of detonations. The process of flame acceleration is affected significantly by obstructions along the flame passage. In many situations with dense obstructions, the growth of the flame surface can be the leading factor affecting the rate of flame acceleration. Different physical mechanisms are expected to play their roles in smooth tubes or channels. Thus processes of flame acceleration and DDT in smooth tubes filed with highly sensitive mixtures differ significantly from DDT in obstructed tubes at nearly critical conditions. Present study addresses the situation with highly sensitive mixture and smooth tubes. The objective of this work was investigation of DDT process for stoichiometric hydrogen-oxygen mixture. Run-up distances for the onset of detonations were studied in the series of tests as a function of initial pressure.