A. G. Tereshchenko

Perturbations of the Flame Structure Due to a Thermocouple. I. Experiment

In thermocouple measurements in flames of gaseous or condensed systems, it is usually assumed that due to the small sizes of thermocouples, the flame perturbations caused by thermocouples are negligible. It is shown, however, that these perturbations can be significant. Temperature measurements in a laminar methane flame at atmospheric pressure revealed a systematic overestimation of measured temperatures compared with the temperature of the unperturbed flame in the temperature gradient region and in the region of the maximum concentration of radicals. This overestimation was measured, and its causes were analyzed. Previously, such effects have not been studied in the literature.

Heat transfer between flame and probe in mass-spectrometric research on flame structure

The authors examine the heat transfer between the probe and flame which has been examined previously, where the first approximation was based on the assumption that constant and equal heat-transfer coefficients apply to the end of the probe and side surface. The authors compare the calculations with measurements for several quartz probes to derive the empirical quantities in the model for heat transfer in the probe. Particular interest as regards the thermal perturbations is represented by the temperature of the end of the probe. The corresponding calculations have been performed for various distances between the end and the combustion surface.

Structure of Ultrahigh Molecular Weight Polyethylene-Air Counterflow Flame

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.

Synthesis of mesoporous nanocrystalline TiO2 films in a premixed H2/O2/Ar flame

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.

The Velocity and Structure of the Flame Front at Spread of Fire Across the Pine Needle Bed Depending on the Wind Velocity

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.

Perturbations of the flame structure due to a thermocouple. II. Modeling

Flame perturbations caused by the presence of a thermocouple have been studied by modeling the external flow of a reactive gas mixture around a thermocouple using the complete system of unsteady Navier-Stokes equations modified to approximately account for the effect of heat release due to chemical reactions. The modeling was performed for a methane gas flame and for the flame of a condensed system exemplified by RDX under the experimental conditions described in the literature. It is found that the deceleration of the flow in the vicinity of the thermocouple leads to a local increase of heat due to chemical reactions and the corresponding increase in temperature relative to the thermocouple unperturbed value. This effect has not been previously discussed in the literature.