Systematic experiments and complementing numerical simulations have been reported for the first time to understand the spread of a ground fire over pine needle bed of the Siberian boreal forests (Pinus silvestris) in still air. Using equipment and instrumentations specifically developed for the purpose, careful experiments have been conducted to reveal the effects of the bed width, fuel moisture content, fuel load, and the packing ratio on flame spread rate, temperature distributions in both gas and condensed phases. Temperatures are measured using fine thermocouples fixed at various locations from the bed surface. The surface temperature of the bed during flame propagation has been measured using a micro-thermocouple inserted in a single pine needle in the bed as well as using an infrared (IR) camera. Further, for the first time, the total and radiant heat fluxes from the flame to the bed surface have been measured using compact cooled sensors placed inside the needle bed, over which the flame propagates. In order to understand more about the flow field and flame spread process, a 3D numerical model based on the Fire Dynamics Simulator (FDS) has been used to simulate few of the experiments. The processes governing pine needle pyrolysis, char oxidation, gas phase combustion and radiation have been modeled using simplified approaches reported in literature. The model is capable of predicting experimentally measured flame propagation velocities for most of the cases quite well.

For the first time, on the basis of a systematic experimental study of the propagation of a model ground fire over pine needles bed at a low wind speed (in the range of 0.1 m/s-0.4 m/s), in which there have been no measurements reported so far, a nonlinear dependence of flame spread rate on the wind speed has been established. Further, the total heat flux and its radiation counterpart have been measured using compact cooled sensors placed in the needles bed. Furthermore, the influences of the bed width and its moisture content on the flame propagation rate are studied. A three-dimensional numerical model based on Fire Dynamics Simulator (FDS) is used to investigate the processes governing the pyrolysis of pine needles, oxidation of char, gas phase combustion and radiation, using parameters from literature. The model, with simplified moisture release and pyrolysis sub-model, is able to predict the experimentally measured flame spread rates for most of the cases well within the measurement uncertainties. A sensitivity analysis is done to demonstrate the importance of pyrolysis chemistry over char oxidation rate. The predicted flow, temperature and species fields are presented to bring out the physics involved in the flame propagation. The validated model, coupled with detailed turbulence and radiation models, can be used as a first-hand predictive tool for scaled up ground fire scenarios.

Results of a comparative analysis of spectral-luminescent properties of crystalline and glassy benzophenone are presented. The main spectral characteristics (frequencies of purely electronic transitions in phosphorescence spectra, band half-widths, relative quantum yields) of glassy benzophenone have been found to possess a number of significant features as compared to the crystalline phase in the temperature range 4.2–220 K. Temperature-dependent structural changes in benzophenone have been studied by differential scanning calorimetry. The ability of phase transitions to appear in benzophenone and their sequences were shown to differ during cooling and heating of the samples. The relationship between the spectral characteristics of various benzophenone phases and their transition temperatures has been demonstrated. Model concepts describing the transport of charge carriers and the transfer of electronic excitation energy in disordered amorphous and glassy molecular systems and information on the glass structural features have been used to explain the experimental results.

The effect of triphenyl phosphate (TPP) retardant inhibition on flame propagation over the horizontal surface of polymethyl methacrylate (PMMA) has been studied experimentally and numerically. Regarding the flame spread behavior over the surface of PMMA (pure and inhibited by TPP), the following parameters were measured: thermal decomposition with TG / DTG analyzer, the flame spread rate, the pyrolysis zone length, the mass loss rate and spatial distribution of temperature by thermocouples and species concentration in the gas-phase flame by probing mass spectrometry. The previously developed coupled heat and mass transfer mathematical model describing the feedback interaction between flame and solid fuel, as well as volatilization of pyrolysis products, was modified to resolve the effect of TPP on flame spread by introducing the correcting factor of the gas-phase combustion reaction rate relating to the inhibitor concentration in the solid material. Good agreement between the measured and calculated flame spread parameters (flame spread velocity, mass burn-out rate, pyrolysis zone length), as well as a detailed flame structure (gas phase temperature and species concentration), has been obtained for pure PMMA and PMMA+10%TPP. It has been shown that the proposed approach describes a satisfactory retardant effect of TPP on the flame spread over PMMA surface by inhibiting the gas-phase combustion reaction.

Experimental and numerical studies of downward flame spread over polymethyl methacrylate (PMMA) with and without addition of triphenyl phosphate (TPP) are reported. Using the micro-thermocouple technique and molecular beam spectrometry, detailed flame structures of PMMA and PMMA+10%TPP were measured. From the experiments and quantum chemistry calculations, the retardancy capability of TPP on gas-phase reaction is proposed. Addition of flame retardant (10%, 20% TPP) results in reduction of the flame spread rate, the mass burning rate and conductive heat flux from the flame to the polymer surface. Numerical calculation was carried out to simulate the downward flame spread over PMMA and PMMA-TPP slabs. Based on the assumption of the TPP gas phase retardancy performance, a modified one-step reaction rate constant with pre-exponent dependent on the TPP mass content in the polymer and TPP retardancy effectivity is proposed. The predicted results have been compared with the data from sophisticated experimental measurement on thermal and chemical structures of both PMMA and PMMA+TPP flames.

Introduction. Polymer materials are widely used, however the actual object is to provide polymers combustion ­model to predict their behavior under fire, and reducing flammability. The work is devoted to the experimental ­study and numerical simulation of flame propagation over the surface of horizontally and vertically placed slabs of polymer in still air. Methods. The object of the investigation was cast polymethylmethacrylate (PMMA). The experiment was focused on measurement of the spatial distributions of the temperature and species concentrations of the PMMA pyrolysis and combustion products in the gas-phase over the surface of PMMA. Temperature was measured by micro­thermocouple with diameter of 50 microns. Probe mass-spectrometry was used for the measurement of the spa­tial distribution of species concentrations in the flame. Results and discussion. The main species (mehylmethacrylate (MMA), O2, CO2, H2O, N2, C2H4 (ethylene), C3H6 (propylene)) were identified and their concentration profiles were measured on the different distance from the flame front. The chemical structure of the flame was established to be in good agreement with the thermal one. The size of the “dark zone” of the flame, in which the temperature near the surface of the polymer is minimal, correlated well with the size of the oxygen-free zone. The mass burning rate, the velocity of flame propagation, the width of the pyrolysis zone and the temperature distribution in the condensed phase were also measured. Based on the experimental results, densities of conductive and radiation heat fluxes from the flame to the fuel surface were deter­mined. Calcula­tion of the radiation heat flux density was carried out under the assumption of an optically thin ­model. Modeling of the horizontal flame propagation over the PMMA surface was carried out using a two-dimen­sional conjugated laminar combustion model that takes into account one-step reactions — the decomposition reaction of PMMA in the condensed phase and the oxidation of decomposition products in the gas phase. Modeling of the vertical flame propagation over the PMMA surface was carried out using economical model of FDS. Conclusion. The model was shown to describe satisfactorily the experimental data such as the mass burning rate, flame propagation velocity, as well as the temperature distribution and concentration of species near the flame front.

Experimental and numerical investigations of burning of horizontal surfaces of poly methyl methacrylate and methyl methacrylate are presented. A burner used in this study allows for the fuel surface to be oriented horizontally at a given distance from the burner rim. One of the aims of this study is to understand the effects of ullage (distance between burner rim and fuel surface) on the burning rate of the fuel and the flame structure. In the case of poly methyl methacrylate, the surface at an initial ullage regresses during its burning, and in the case of methyl methacrylate, the pool level is maintained at the given ullage by supplying the fuel at the rate of its burning. Careful repeatable measurements of temperature and species fields are carried out. These reveal the structure of a small-scale pool flame established over a polymeric fluid such as methyl methacrylate, and such data are scarce in literature. In order to complement the experimental results, fire dynamics simulator is employed to simulate the experimental cases. Flame structure and flow field in the gas phase have been presented and discussed. As the ullage increases, the burning rate decreases. This trend is explained using surface convective heat flux results.

Experimental and numerical investigations of upward and downward flame spread over flat polymethyl methacrylate (PMMA) slabs are presented here. Experiments have been carried out using PMMA slabs of different thickness in the range of 1.6 mm-5,4 mm. Downward and upward flame spread processes have been recorded under atmospheric pressure and normal gravity conditions. Careful repeatable high resolution measurements of temperature and species fields have also been carried out, to fill the scarcity of such data in literature. These data illustrate the structure and spread rates of flames established over PMMA slabs. A simple numerical model, used widely to simulate flame spread over condensed surfaces, called Fire Dynamics Simulator (FDS), has been employed to numerically simulate the experimental cases. Infinite rate chemistry and sublimation based interface model have been used. FDS is economical when compared to CFD tools such as FLUENT and OpenFOAM. It provides predictive results when compared to theoretical models. Results from FDS have been validated against numerical and experimental data from literature, by comparing quantities such as mass loss rate, flame spread velocity and flame structure. FDS is seen to capture essential transport processes of a spreading diffusion flame. Even though discrepancies have been observed between the numerical and experimental results near the fuel surface, the overall comparison of the trends has been quite reasonable. Numerical model is capable of predicting the unsteady and steady features of downward spread as well as transient rapid upward flame spread, as observed in the experimental results. Detailed structure and flow field have been presented and discussed. (C) 2017 Elsevier Ltd. All rights reserved.

The spectral-luminescence properties of the 4-cyano-4'-pentylbiphenyl CH 3 (CH 2 ) 4 (C 6 H 4 ) 2 CN (5CB) liquid crystal has been studied in the temperature range 4.2-297 K. It is shown that at increasing temperature the fluorescence spectra are shifted to the red side. The spectral long-wavelength shifts are also analyzed. A comparison of the temperature behaviour position of the emission of the band maxima l max and their half-widths Δl/2 in the fluorescence spectra and the results of the DSC-investigation show that phase transitions occur in 5CB liquid crystals at T ~ 230 and 260 K.

This study presents the results of a comprehensive experimental investigation and numerical simulation of the downward flame spread over PMMA slabs. For the first time, in the case of downward flame spread over PMMA slab 9.6mm thick, temperature and species concentration fields in the gas-phase flame, temperature profiles in the condensed phase and dependence of the heat flux to the burning surface on the distance from the flame front were obtained. A coupled model of heat and mass transfer involving two-dimensional elliptic conservation equations both for gas phase and solid fuel has been used with the fuel surface approximation of the samples burnout. This allowed us to state, for the indefinite intermediate mode (in terms of the sample thickness, which are not neither thermally thin nor thermally thick), a mathematical model ensuring good agreement between the experimental and calculated macro parameters of combustion. The results of comparing the experimental and calculated data allowed us to determine a number of facts, which, despite the satisfactory agreement between the simulation and the experimental data in the main macro parameters, indicate the necessity of further improvement of the model derived. Such facts are: the increasing disagreement between the calculation and the experiment in the position of the maxima of the temperature in the gas phase as the distance from the flame front grows; essential difference in the width of the MMA and O-2 consumption zone between the calculation and the experiment; identification in the experiment of CO as an intermediate product. Further improvement of the model should be aimed m more detailed development of the combustion reaction mechanism, which should consider at least two steps. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The spectroluminescence properties of 4cyano4'pentylbiphenyl CH 3 (CH 2 ) 4 (C 6 H 4 ) 2 CN (5CB) were studied in the temperature range 4.2–297 K. A red shift of the fluorescence spectrum was noted with increasing temperature. The long-wavelength shifts in these spectra were also analyzed. Comparison of the temperature dependence curves for the emission at the band maxima λ max and their halfwidths Δλ/2 in the fluorescence spectra and the results of a differential scanning calorimetry (DSC) study showed that phase transitions occur in the 5CB liquid crystals at ~230 and ~260 K.

The paper presents a comprehensive experimental study of flame spread over the surface of horizontally placed slabs of four types of PMMA specimens in still air. Temperature distributions in the gas phase near the solid fuel surface and in the condensed phase were measured using microthermocouples. Spatial variation of the species concentration in the gas -phase flame near the solid fuel surface was measured using probing mass spectrometry. Also flame spread rate over the polymer surface was measured. The experiments revealed differences in the combustion character of the specimens investigated. At the flame spread over surface of two (out of the four) specimens boiling and formation of large bubbles were discovered. The main flame components including MMA, O-2, CO2, H2O, N-2, C2H4 (ethylene), C3H6 (propylene) have been first identified, and their concentration profiles at different distances from the flame front have been measured. The data on the chemical flame structure have been shown to be in good agreement with the data on its thermal flame structure. The size of the "dark zone" of the flame, in which the temperature near the polymer surface is minimal, correlates well with the size of the oxygen-free zone, which is adjacent to the burning surface. Conductive heat feedback from the flames to the condensed fuel surface was estimated on the basis of the experimental results. The conductive heat flux averaged over the burning surface was estimated to be approximately 13.2 kW/m(2). It has been established that it is maximal in the flame front and decreases as the specimen burns out. The data obtained may be used for developing and validating a numerical model of flame spread over PMMA surface. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The paper addresses a comprehensive experimental laboratory-scale study of the characteristics and regularities of fire spread across a bed of pine needles of Siberian boreal forests (SBF) and the impact of wind velocity on these regularities. We used such precision physical and physicochemical methods as in situ mass spectrometry, PIV, microthermocouple technique, etc. We measured fire spread rates, spatial gas temperature distribution near the surface and inside the pine needle bed, the pine needle temperature distribution on the bed surface, and gas flow velocity fields before and behind the flame front. Data were obtained on concentration profile of ethanol as the main pine needle pyrolysis product, on the concentration profiles of О2 and СО2, on the angle of slope of the flame sheet, and on the impact of the wind velocity on these characteristics. It was established that as the wind velocity changes in the range of 0.15–0.2 m/s, the regularities and characteristics of fire spread drastically change. PIV measurements have demonstrated high turbulence near the flame front. It was established that at the wind velocity of 0.2 m/s, СО2 concentration grows, while О2 drops inside the pine needle bed before the flame front more significantly than at the wind velocity of 0.1 m/s. This is attributed to the increase of turbulent mass transfer before the flame front. At that, the pyrolysis rate of the pine needles slows down, and concentration of the pyrolysis products inside the bed in the flame front decreases. The data obtained throw light on the physicochemical processes taking place during fire spread across the bed of pine needles.

The combustion of ultrahigh molecular weight polyethylene (UHMWPE) in airflow perpendicular to the polyethylene surface (counterflow flame) was studied in detail. The burning rate of pressed samples of UHMWPE was measured. The structure of the UHMWPE–air counterflow flame was first determined by mass spectrometric sampling taking into account heavy products. The composition of the main pyrolysis products was investigated by mass spectrometry, and the composition of heavy hydrocarbons (C7—C25) in products sampled from the flame at a distance of 0.8 mm from the UHMWPE surface was analyzed by gas-liquid chromatography mass-spectrometry. The temperature and concentration profiles of eight species (N2, O2, CO2, CO, H2O, C3H6, C4H6, and C6H6) and a hypothetical species with an average molecular weight of 258.7 g/mol, which simulates more than 50 C7—C25 hydrocarbons were measured. The structure of the diffusion flame of the model mixture of decomposition products of UHMWPE in air counterflow was simulated using the OPPDIF code from the CHEMKIN II software package. The simulation results are in good agreement with experimental data on combustion of UHMWPE.

Experimental data are reported on the structure of laminar premixed methane/oxygen/argon flames stabilized over a flat burner at 1, 3, and 5 atm with different equivalence ratios phi (0.8-1.2). Mole fraction profiles of the reactants (CH4, O-2), major stable products (CO2, H2O, H-2, CO) and intermediates such as H, OH, CH3 radicals, as well as ethylene and acetylene, were measured by molecular-beam mass spectrometry. The temperature profiles in the flames were measured by thermocouples in the presence of a sampling probe to take into account the flame cooling effect due to the probe. The structures of stoichiometric flames at 1, 3 and 5 atm were compared to elucidate the effect of pressure on the mole fractions of the flame species. Fuel-lean (phi = 0.8) and fuel-rich (phi = 1.2) flames at 5 atm were also investigated in this work. All the experimental data were compared with the numerical simulations using the Premix code and three detailed chemical kinetic mechanisms for methane combustion available in the literature: the GRI-Mech 3.0, AramcoMech 1.3 and USC Mech II. The absolute mole fractions of CH4, O-2, H2O, CO, CO2, H-2, H, OH, CH3 in the flames and their dependences on pressure were captured by both mechanisms reasonably well. An analysis of the reaction mechanisms was performed to gain insights into the kinetics of methane combustion in stoichiometric conditions in the range of pressures from 1 to 5 atm and to explain the observed pressure effects on peak mole fractions of flame radicals. The decrease of peak mole fractions of acetylene and ethylene with pressure increase, which was observed in the experiments, was not reproduced by the mechanisms. Both mechanisms predicted the increase in their peak mole fractions with pressure (in the range from 1 to 3 atm). The kinetic analysis indicated the need to revise the pressure-dependent chemistry of acetylene and ethylene formation in the mechanisms. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

On the basis of the assumption concerning the symmetric properties of the free aromatic hydrocarbon molecules and those placed in host crystalline lattice cell, the analysis of the multiplet structure of the optical spectra of impurity centers of some the monosubstituted naphthalenes in naphthalene crystal was performed. The low-temperature fluorescence and absorption spectra of the 2-fluoronaphthalene, 2-chloronaphthalene, and 2-naphthol in crystalline naphthalene have been studied at 4.2K. The results of the symmetry group theoretical analysis of the spectra multiplet structure are suggested with experimental data. The developed approach was also applied for the analysis of the multiplet structure in optical spectra of rapidly frozen solutions of 3,4-benzpyrene, porphin and its derivative in normal alkanes.

Luminescence spectra of naphthalene and benzophenone doped with their monosubstituted derivatives at 4.2 K were studied. The multiplet structure of these spectra was analyzed using the symmetry properties of the free molecules and crystal structures. It was shown that fl uorescence and phosphorescence spectra of the studied objects had a doublet character.

The synthesis of mesoporous nanocrystalline TiO2 films in a premixed (in vol %) 12.9/14.4/72.7 H2/O2/Ar flame doped with 0.1% titanium tetraisopropoxide [Ti(OC3H7)4] was studied. Stabilization of the flame and deposition of TiO2 nanoparticles were carried on the plane of a rotating disk. Spatial distributions of temperature and concentrations of major species in the flame were investigated. The applicability of the employed kinetic reaction scheme and the calculation method to the description of the structure of the investigated flame was tested. It was established that this method can be used to synthesize spherical crystalline nanoparticles of the anatase polymorphic form of TiO2 with an average diameter of 12 nm and a narrow size distribution (σ = 1.45), which can be used in the production of solar cells and gas analyzer sensors.

In flame temperature measurements by a thermocouple, it is usually assumed that, due to its small size, the thermocouple produces negligible perturbations of the flame structure. Our studies show, however, that this assumption may be incorrect. The temperature of a premixed atmospheric methane/oxygen/argon flame measured by several thermocouples was found to be systematically higher than the theoretical temperature at small distances from the burner (in the region with a high temperature gradient). The external flow of the flame over a thermocouple was simulated using the full set of unsteady Navier–Stokes equations to explain the discrepancy between experimental and theoretical data. An approximate allowance for the heat release due to chemical reactions was made by adding a source term to the energy equation to provide a given temperature distribution in the unperturbed isobaric flame. The observed discrepancy was found to be related to deceleration of the flow in the vicinity of the thermocouple, resulting in additional heat release due to chemical reactions in the flow. In addition, significant additional heating of the thermocouple was observed, when it was placed in the zone with maximum concentrations of H and OH radicals.