Vol 56, No 1 (2018)

An experimental set-up with a unified system of synchronized control signals was constructed to study the interaction of high-enthalpy plasma jet with the surface of heat-resistant materials. The analysis of air–plasma stream interaction with technical (isotropic) graphite is performed. The air–plasma jet has a temperature of 7000–8000 K in the interaction zone, this interaction is accompanied by destruction of the graphite material and lasts for 100–200 s. Using methods like two-positioning visualization, micropyrometry and spectroscopy the following data are obtained: temperature fields on the graphite sample’s surface and their temporal and spatial changes, dynamics of the sample’s mass loss rate, changes in the parameters (temperature and concentration of electrons, heavy particles temperature) of the incoming plasma stream.

This study is devoted to the development of an analytical theory for calculating the spatial distribution of the energy release upon propagation of a high-energy (1–100 keV) electron beam in a gas (based on the example of air). Based on the analysis of data on the cross sections of elastic and inelastic interactions between electrons and molecules of gases that are in the air composition, it was suggested that inelastic interaction causes energy relaxation, whereas elastic interaction leads to momentum relaxation. The model cross section of inelastic collisions of electrons with molecules is used for solving the Boltzmann kinetic equation for electrons; this cross section provides adequate description of the experimentally found energy dependence of the mass stopping power of electrons. The results for the dependence of the mean electron energy on the number of inelastic collisions are in good agreement with the results of calculation based on expansion of the distribution function in the number of collisions and solution by the Monte Carlo method. The calculations we performed show that the consideration of elastic collisions increases the spatial density of the energy release due to narrowing of the region where the main part of the energy of fast electrons is released, in comparison with calculations where only inelastic deceleration is taken into account.

Premixed flames under different levels of gravity were studied experimentally and numerically. The experiments were carried out in the Bremen Drop tower. The object of investigation were conical premixed rich, lean, and stoichiometric CH₄–air flames with wide range of flow regimes, at Reynolds numbers of 600–2000, which were generated on specially designed cone nozzle and premixed cylinder chamber with grids and beads. Planar laser-induced fluorescence OH radicals and high-speed video recording was performed under microgravity, terrestrial conditions, and inverted gravity. All the experiments were performed at atmospheric pressure. Our experiments confirm that gravity has a complex influence on laminar and weakly turbulent premixed flames. Gravity causes flame flickering, while under reduced gravity flames are stable and almost not flicker. Based on the experimental data and numerical simulation, a correlation of flickering frequency with different mixture equivalence ratio and gravity is formed.

The results of numerical simulation of the structure of a two-phase flow of a gas–liquid bubble mixture in a vertical ascending flow in a pipe are presented. The mathematical model is based on the use of the Eulerian description of the mass and momentum conservation for the liquid and gas phases, recorded within the framework of the theory of interacting continua. To describe the bubble-size distribution, the equations of particle-number conservation for individual groups of bubbles with different constant sizes are used for each fraction, taking the processes of breakage and coalescence into account. Comparison of the results of numerical simulation with experimental data has shown that the proposed approach enables the simulation of bubble turbulent polydisperse flows in a wide range of gas concentrations.

Two-dimensional Fokker–Planck type kinetic equations were derived, and some calculation results are presented to illustrate the principal distinction of the process of translational relaxation in a flow behind the front of a shock wave from the one-dimensional description that is valid for a stationary gas. In contrast to a Lorentz gas (a small admixture of light particles in a thermostat of heavy particles), the process of translational relaxation in a Rayleigh gas (a small admixture of heavy particles in a thermostat of low-weight gas particles) has an obvious two-dimensional character.

In the given brief communication, new experimental data on superheated water atomization are presented. It is shown that in contrast to the case of short cylindrical nozzles, which provide bimodal water–droplet sprays, the application of divergent nozzles makes it possible to obtain one-modal water atomization with droplets of about micrometer diameter. This is explained by the changes in the mechanism of superheated water jet fragmentation.

We performed experimental studies of the phase-frequency and the amplitude-frequency characteristics of the electrical conductivity of asphalt-resin-paraffin deposits in oil storage reservoirs within the frequency range of 4 Hz–1 kHz and the temperature range of 20–70°C. We found two domains in the obtained temperature dependences. For the temperature range below the phase-transition temperature, the activation energy was equal, on average, to 0.52 eV, while above this it was approximately 1.00 eV. The peculiarity of the frequency dependences of the phase-frequency characteristics results from the multi-factor mechanisms of the polarization, which resulted in “smearing” of their extremes over these frequencies.

A finite difference method was used for modeling heat transfer in a pulsating laminar flow in rectangular channels with different aspect ratios for the wall boundary condition q_w = const. The specific frequency ranges were found where the channel perimeter-averaged and the oscillation period-averaged Nusselt number varies in a different manner depending on the channel length and the channel aspect ratio. The predictions for the first and second type boundary conditions are compared. In both cases, there is a frequency range where the average Nusselt number can increase considerably over its steady-state value.

We investigated the enthalpy of liquid bismuth within the temperature range of 580–1325 K in a massive isothermal drop calorimeter using the mixture method. We obtained the approximation equations and determined the isobaric heat capacity. The estimated errors of the data on the enthalpy and the heat capacity are equal to 0.2% and 0.5%, respectively. The results are compared with the literature data. We confirmed the existence of a heat-capacity minimum of liquid bismuth of approximately 800 K. We show that above 940 K the heat capacity depends linearly on the temperature. We developed tables of the recommended values of the caloric properties within the range from the melting point to 1325 K.

The optical parameters of quartz ceramics from a previously proposed identification method and simulation using different optical models of the material are compared. The identification method is based on deliberately measuring hemispherical spectral reflectances for layers of different thicknesses and solving the inverse problem using asymptotic formulas. Mathematical models are constructed based on the Mie theory on the assumption of independent scattering of electromagnetic radiation by fragments of the material. The material is considered as a polydisperse packing of spheres, the sizes of which are determined by data on the material structure. Both a grain surrounded by gas and a pore in monolithic material are considered as a scatterer. Data on the material structure were gathered using optical microscopy, static laser scattering, and mercury porosimetry. The best agreement with the results of the identification method is demonstrated by the model of ceramics in the form of a glass monolith with spherical voids. Comparative analysis eliminates uncertainty in the form of the scattering phase function and shows that the scattering is close to isotropic.

The layout of an absorption spectrometer with diode lasers for contactless measurement of the temperature and water-vapor concentration in gas flows with mixture pressures of up to 3 atm and temperatures of 300–2000 K has been designed. The technique is based on the rapid tuning of the radiation wavelength of two lasers, the registration of the absorption lines of water molecules that are located in the tuning range, and the fitting of the experimental absorption spectra by theoretical ones that have been simulated using spectroscopic databases. The original components of the spectrometer and different algorithms of the processing of experimental spectra are described. The performance of the spectrometer and processing methods were tested in the laboratory with a cuvette at a pressure of 1 atm and temperatures of 300–1500 K. The different processing algorithms give a reasonable coincidence of data on hot zone parameters that were obtained by the method of diode laser absorption spectrometry, and the temperature that was measured using standard sensors. The designed layout of the spectrometer passed the first tests on the T-131 experimental stand at the TsAGI (Central Aerohydrodynamics Institute).

We analyze modern methods for calculating heat and hydrodynamic flow parameters in a boundary layer during the laminar–turbulent transition. The main approaches for describing the phenomenon of laminar–turbulent transition are examined. Each approach is analyzed. The manner in which different factors influence the laminar–turbulent transition is studied. An engineering model of the laminar–turbulent transition in a high-velocity flow is presented.