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To study the mechanism of flame retardancy, counter-flow flames of air and ultrahigh-molecular-weight polyethylene (UHMWPE) with triphenylphosphate (TPP) added, and also without it, were studied at atmospheric pressure. Burning rates were measured. Also the thermal and chemical structure of these counter-flow flames (with and without TPP added) was investigated. The concentrations of the heavy products from the polymer's thermal degradation were measured by sampling the flame at 0.8 mm from the polymer's surface. Temperature profiles in both the flame and the condensed phase were measured, as well as the temperature of the polymer's burning surface, the concentration profiles of 8 species (N 2 , O 2 , CO 2 , CO, H 2 O, C 3 H 6 , C 4 H 6 , C 6 H 6 ) and finally the concentration profile for a hypothetical species, whose molecular weight was the average of more than 50 hydrocarbons containing C 7 –C 25 . The effect of adding TPP to UHMWPE on all the above characteristics indicates that TPP changes what is occurring in both the condensed phase and in the flame. Chromatography and FTIR spectroscopy revealed the formation of phosphorus-containing compounds (phosphates, ethers, and carbonates) on the surface of the burning polymer. Elemental analysis showed half the TPP additive remains in the condensed phase and half goes into the flame. Adding 5 wt% of TPP to UHMWPE reduced the burning rate; also the composition of the heavy products from the destruction of this polyethylene was changed markedly by the additive. In fact, the maximum in the distribution of heavy hydrocarbons shifted towards lighter masses. All these facts indicate that TPP does have a real effect in the condensed phase. However, the lowering of the polymer's flammability was ascribed to the effect of TPP on gas-phase processes, as manifested by: a widening of the flame front, a decrease in the maximum flame temperature and a reduction of the extinction strain rate. Overall, these observations do indicate that TPP (a retardant of hydrocarbon flames) reduces the flammability of UHMWPE by a gas-phase mechanism. [ABSTRACT FROM AUTHOR]

The popularity of organic polymers despite their high flammability forces the introduction of flame retardants (FR) such as metal phosphinates into the combustible material. The thermal behavior of aluminum diethylphosphinate (AlPi) as FR in the widely used polymer ultra-high molecular weight polyethylene (UHMWPE) is investigated here. The study focuses on the effect of the FR on the gas phase activity when a polymer is pyrolyzed or burned. For this purpose, the fast pyrolysis of AlPi was investigated by differential mass-spectrometric thermal analysis (DMSTA). Also, the thermal and chemical structures of diffusion flames of UHMWPE + AlPi specimens were investigated using micro thermocouples and molecular beam mass spectrometry, respectively. Small amounts of AlPi (2.5 wt.%) decrease the gas temperature significantly by a maximum of 155 K related to FR-free polymer flames, indicating a retardancy effect of the additive on the flame. From the results of subsequent limiting oxygen index (LOI) tests, it is obvious that a PE burn-up cannot be achieved in a self-sustained flame when an additive content above 10 wt.% is used as FR. In the mass-spectrometric studies, the phosphorus-containing species produced in the pyrolysis experiments (DMSTA) of the neat AlPi as well as the species which are formed in flames during combustion experiments can be detected. In the flames, the concentration of the phosphorus containing compounds peaks at low heights above the polymer surface which indicate a gas phase activity of AlPi or its pyrolysis products. Besides a charring layer on top of the burning surface could be noticed. The use of AlPi as a FR for UHMWPE shows flame retardant effects in both the condensed and the gas phase. [ABSTRACT FROM AUTHOR]

The burning behavior of polyoxymethylene in the counterflow of oxidizing air has been studied experimentally and numerically. Measurements have been performed using a specially designed burner. The temperature profiles were measured in gas phases by a microthermocouple technique. The burning surface temperature and the mass burning rates of solid fuel were determined. The chemical structure of counterflow flame was investigated using the mass spectrometric sampling by microprobe. The composition of the gas sample was analyzed on-line with a mass spectrometric complex (Hiden HPR 60), based on a quadrupole mass spectrometer. The species CH 2 O, CO, CO 2 , H 2 O, O 2 , N 2 were identified and their concentration profiles were measured. The composition of products and kinetic parameters of POM thermal degradation have been determined by mass spectrometry and thermogravimetric analysis. Quasi one-dimensionality of considered diffusion flame has been established experimentally, which allows analysis of flame parameters in relation to the only coordinate normal to the solid fuel's burning surface and substantially simplifies the theoretical description. Predictions have been performed by a coupled model describing feedback heat and mass transfer between gas-phase flame and polymeric solid fuel. A two-dimensional elliptic equation in axisymmetric formulation has been employed to simulate heat transfer in solid fuel, and a set of one-dimensional hyperbolic equations has been used to determine the solid-to-gas conversion degree of the pyrolysis reaction. Gas-phase formulation is presented by one-dimensional conservation equations for multi-component flow with a detailed kinetic mechanism of combustion. Numerical results showed good agreement with the measurements for temperature and species concentration profiles, as well as for the mass burning rate and the surface temperature of solid fuel. [ABSTRACT FROM AUTHOR]

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 ММА, О 2 , СО 2 , Н 2 О, N 2 , С 2 Н 4 (ethylene), С 3 Н 6 (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. [ABSTRACT FROM AUTHOR]

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 ϕ (0.8–1.2). Mole fraction profiles of the reactants (CH 4 , O 2 ), major stable products (CO 2 , H 2 O, H 2 , CO) and intermediates such as H, OH, CH 3 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 ( ϕ = 0.8) and fuel-rich ( ϕ = 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 CH 4 , O 2 , H 2 O, CO, CO 2 , H 2 , H, OH, CH 3 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. [ABSTRACT FROM AUTHOR]

The spatial distributions of naturally occurring cations in near-stoichiometric (equivalence ratios ϕ = 0.8 and 1.2) burner-stabilized methane/oxygen/argon flames at atmospheric pressure are studied by molecular beam mass spectrometry (MBMS) and kinetic modeling. The disturbing effect of a nickel sampling cone used in the measurements on the flame was comprehensively modeled using a two-dimensional direct numerical simulation of reacting flow near the axially symmetric probe along with a reduced chemical kinetic mechanism. The use of one-dimensional numerical simulation for predicting the flame structure perturbed by the metallic probe was firmly justified. We also proposed and applied a procedure to correct the measured spatial profiles of cations on the contribution of signals from the hydrates formed slightly upstream the reaction zone due to the sampling probe effects. With all these methodological advantages, we validated the ion chemistry mechanisms proposed earlier in the literature against the novel experimental data on the spatial distributions of the following cations: HCO+, CH 3 +, H 3 O+, C 2 H 3 O+, CH 5 O+, C 3 H 3 +. The kinetic mechanism published recently by Chen et al. [Combust. Flame 202 (2019) 208] for the charged species formed in methane flames was revised in order to improve its predictive ability. To this end, the highly accurate W2-F12 quantum chemical calculations were used to obtain the reliable formation enthalpies of all cations considered in the mechanism. In the case of C 2 H 3 O+ and C 3 H 3 + cations, the calculated values turned out to be profoundly lower than those reported before. Apart from this, the theory also predicted another exit channel for H 3 O+ + acetylene reaction yielding HCO+ + CH 4 instead of C 2 H 3 O+ + H 2 proposed earlier. The corrections significantly improved the predictive ability of the ion chemistry mechanism. We also considered the most important directions for the further refinement of the mechanism. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]