Jean-Pierre Garo

Chemical and physical effects of water vapor addition on diffusion flames

Abstract The use of water mist to extinguish fires is a problem of particular interest since the banning of halogen-based agents for environmental reasons. This interest is reflected in the large number of studies performed on the main mechanisms of extinguishment: heat extraction, oxygen displacement and attenuation of radiant heat fluxes. In contrast, little is still known about the chemical and some other aspects of water vapor addition in hydrocarbon diffusion flames. In this paper, a phenomenological study is conducted of the effect of water vapor addition, through the base of a small-scale heptane pool fire. Heptane, supported on a pool of liquid water, is burned as a pool flame while the water underneath is heated to boiling. This is a unique and original method in that boiling the water under the fuel is used for adding water vapor at the base of a diffusion flame where chemical reactions and air entrainment take place. This region is the most interesting regarding flame reactivity and soot formation and, in spite of the perturbing influence of the injected water vapor, the major characteristics of the flame are conserved. The procedure permits measurements of exhaust gas composition by means of a flow through system. Measurements of temperature, CO and CO2 species concentrations, and monochromatic absorption coefficient which can be associated with soot concentration, are performed. It is seen that adding water vapor in such a way affects both physical phenomena and chemical reactions. That water vapor inhibits the soot formation and shifts CO to CO2 is further confirmed by injection of an inert gas instead of water vapor. However, the resulting heat release is not efficient enough to counteract the cooling effect of water vapor and the temperature is significantly decreased.

External heating of electrical cables and auto-ignition investigation

Electric cables are now extensively used for both residential and industrial applications. During more than twenty years, multi-scale approaches have been developed to study fire behavior of such cables that represents a serious challenge. Cables are rather complicated materials because they consist of an insulated part and jacket of polymeric materials. These polymeric materials can have various chemical structures, thicknesses and additives and generally have a char-forming tendency when exposed to heat source. In this work, two test methods are used for the characterization of cable pyrolysis and flammability. The first one permits the investigation of cable pyrolysis. A description of the cable mass loss is obtained, coupling an Arrhenius expression with a 1D thermal model of cables heating. Numerical results are successfully compared with experimental data obtained for two types of cable commonly used in French nuclear power plants. The second one is devoted to ignition investigations (spontaneous or piloted) of these cables. All these basic observations, measurements and modelling efforts are of major interest for a more comprehensive fire resistance evaluation of electric cables.

Influence of Ventilation on Ignition Risk of Unburnt Gases in the Extraction Duct of Underventilated Compartment Fire

Ignition risk of unburnt gases in the extraction duct of an underventilated compartment fire was studied from 8-cubic-meter room fire at the Laboratoire de Combustion et de Détonique, in France. A study of factors that have an influence on the ignition risk at the extraction is made. Two main factors appear: heat release rate and ventilation flow. These factors were studied by changing ventilation flow for different diameters of fire source. Fire tests were also made in order to understand the impact of closing the inlet vent on the production of unburnt gases. Criteria of heat release rate and ventilation flow are determined in order to predict conditions that lead to an ignition risk.

THIN-LAYER BOILOVER: PREDICTION OF ITS ONSET AND INTENSITY

Although in the burning of liquid fuel floating on water the fuel burning itself is similar to that of the simple fuel, the presence of the water underneath introduces a number of effects that are caused by the transfer of heat from the fuel to the water. One of the main effect is the disruptive burning of the fuel known as boilover that is caused by the water boiling and splashing, which results in the explosive burning of the fuel. From a practical point of view, it appears that there are two important aspects of the problem, one is the onset of boilover, the second its intensity. In the present work, the liquid heating is analyzed as a semi-infinite conduction problem with a surface suddenly increased to a constant temperature (boiling point), the penetration of the thermal wave being a function of time (of the order of , where α is the thermal diffusivity of the liquid). This analysis provides predictions for the times at which the nucleation of the water starts (assumed to occur when the water/fuel interface reaches a constant level of superheat ≃120°C). They are in satisfactory agreement with the measurements. These measurements were conducted in two laboratories and address the major issues of the thin layer boilover process by analyzing the effect of the variation of the key parameters of the problem (fuel layer thickness, pool diameter and fuel type). Knowing that the fuel layer remaining before nucleation is closely related to the thickness of the superheated water layer (water that gasifies), it is shown that the boilover intensity can be estimated on the basis of the pre-boilover fuel mass ratio. The thicker these layers, the more intense is the boilover. Fuels used as support of the proposed analysis include single-component fuels and multicomponent fuels.

Combustion Characteristics of p-Cymene Possibly Involved in Accelerating Forest Fires

A potential implication of volatile organic compounds (VOCs) emitted by vegetal species has been introduced in the literature to explain accelerating forest fires. The main purpose of this article is to determine the combustion characteristics of a VOC emitted by Rosmarinus officinalis shrubs, namely p-cymene. The emission of this compound is studied for the temperature range 353–475 K, and an emission peak is found at 448 K. Laminar burning speeds, Markstein lengths, and flame thicknesses are determined using outwardly propagating spherical flames in a combustion chamber at atmospheric pressure. The effects of equivalence ratio (0.8–1.4) and unburned gas temperature (358–453 K) are studied. A correlation is proposed to estimate laminar burning speeds as a function of equivalence ratio and temperature. Due to the lack of data concerning the combustion characteristics of p-cymene, our results are compared to experimental data of heavy molecules such as ethylbenzene, iso-octane, and α-pinene, and to computed data of JP-10 and n-decane. The computed laminar burning speeds of these last two molecules are obtained using the PREMIX code of the CHEMKIN package.

Flame Speeds of α-Pinene/Air and Limonene/Air Mixtures Involved in Accelerating Forest Fires

Several researchers have reported that under certain conditions, forest fires with normal behavior suddenly start to propagate at unusual and very high rate of spread. Over the last decades, these accelerating forest fires were responsible for many fatalities in Europe. A thermochemical approach, based on the ignition of a volatile organic compounds (VOCs) cloud, has been proposed previously to explain this phenomenon. Indeed, some vegetal species emit volatile substances when they are heated. A typical Mediterranean plant, Rosmarinus officinalis, emits 14 components, mainly α-pinene and limonene. The acceleration of the rate of spread can be the consequence of the ignition of these emitted gases. The determination of α-pinene/air and limonene/air premixed flame speeds is essential to take into account this approach in forest fire modeling. It is the main purpose of this article. The spherical expanding flames methodology coupled with a nonlinear model was used to determine the unstretched premixed flame speeds of the two major compounds involved in accelerating forest fires. Experiments were performed in a spherical vessel at atmospheric pressure. The results for different equivalence ratios and unburned gas temperatures are given for the first time. The unstretched premixed flame speeds are discussed relatively to rates of spread of three real accidents.

Estimation of species concentrations during a fire in a reduced-scale room

An approach to estimate species production during a compartment fire is proposed. Semi-empirical correlations based on oxygen concentration are given. They give an estimate of the concentrations of carbon monoxide, carbon dioxide, hydrogen, and hydrocarbons with a carbon chain length lower than five. Three intervals of oxygen concentration are noted, and they correspond to sufficiently ventilated, underventilated, and very underventilated fires. In order to validate these correlations, fire experiments are performed in a reduced-scale room. Species concentrations predicted by the model are in good agreement with our experimental data and with those of the literature. Coefficients used in the correlations are obtained for heptane and dodecane fires.

DETERMINATION OF LAMINAR BURNING SPEEDS AND MARKSTEIN LENGTHS OF p-CYMENE/AIR MIXTURES USING THREE MODELS

The aim of this article is to determine the laminar burning speeds and Markstein lengths of p-cymene. This fuel is emitted by a typical vegetal species of the Mediterranean region often involved in forest fires. Experiments are performed in a spherical vessel for different equivalence ratios ranging from 0.7 to 1.4 at 180 °C and at atmospheric pressure. The effect of temperature (85 to 180 °C) at the stoichiometry is also studied. Three models (one linear and two nonlinear) are used in the extraction process. Depending on the equivalence ratio, the more accurate models are adopted to determine the laminar burning speeds and Markstein lengths of p-cymene. Results are compared favorably to experimental values of a similar fuel (α-pinene) and to numerical data of n-decane computed using the in-house code A-SURF.

Investigation on minimum ignition energy of mixtures of α-pinene–benzene/air

Minimum ignition energies (MIE) of α-pinene-benzene/air mixtures at a given temperature for different equivalence ratios and fuel proportions are experimented in this paper. We used a cylindrical chamber of combustion using a nanosecond pulse at 1,064 nm from a Q-switched Nd:YAG laser. Laser-induced spark ignitions were studied for two molar proportions of α-pinene/benzene mixtures, respectively 20-80% and 50-50%. The effect of the equivalence ratio (Φ) has been investigated for 0.7, 0.9, 1.1 and 1.5 and ignition of fuel/air mixtures has been experimented for two different incident laser energies: 25 and 33 mJ. This study aims at observing the influence of different α-pinene/benzene proportions on the flammability of the mixture to have further knowledge of the potential of biogenic volatile organic compounds (BVOCs) and smoke mixtures to influence forest fires, especially in the case of the accelerating forest fire phenomenon (AFF). Results of ignition probability and energy absorption are based on 400 laser shots for each studied fuel proportions. MIE results as functions of equivalence ratio compared to data of pure α-pinene and pure benzene demonstrate that the presence of benzene in α-pinene-air mixture tends to increase ignition probability and reduce MIE without depending strongly on the α-pinene/benzene proportion.

Biogenic volatile organic compounds emissions at high temperatures of common plants from Mediterranean regions affected by forest fires

Vegetal species emit biogenic volatile organic compounds at elevated temperatures. Because of their combustibility, biogenic volatile organic compounds can modify the wildland fires propagation dynamics, changing them from a moderate behavior to an explosive propagation. This phenomenon is known as an accelerating forest fire. The origin of such phenomena can be the accumulation of biogenic volatile organic compounds in concentrations close to their lower flammability limit in seasons where the plants are themselves very flammable. There is a lack of information on the biogenic volatile organic compounds emissions of vegetal species typically found in wildland fires at temperatures higher than ambient temperature. In this work, we used a flash pyrolysis device linked to a gas chromatograph/mass spectrometer to investigate experimentally the biogenic volatile organic compounds emissions of Thymus vulgaris, Lavandula stœchas, and Cistus albidus between 70°C and 180°C. High amounts of terpenoid compounds were found, except for C. albidus emissions, including thymol, l-fenchone, and 3-hexen-1-ol. The information provided in this work could help to improve the characterization of thermal degradation of vegetal fuels and to incorporate the biogenic volatile organic compounds combustion in physical forest fires models. They also show that under the right circumstances, biogenic volatile organic compounds from these vegetal species could contribute to the development of an accelerating forest fire.