卷 57, 编号 5 (2019)

The investigation results on development and testing of heat-resistant coating with experimental composition of Si–TiSi₂–MoSi₂–TiB₂–CaSi₂ for protection of C_f/SiC composites from oxidation and high-speed erosion in high-speed gas flows are presented. This coating was formed by slip firing technology out of slip compositions with filler based on heterophase powder of the system of interest. The results of fire bench tests of samples with coating under conditions of flow-around and off balance heating by air plasma streams with M = 5.5–6.0 and enthalpy of 40–45 MJ/kg. The effectiveness of the protective action of coating at surface temperature T_w = 1810–1820°C is kept not less than 920–930 s, at T_w ≥ 1850–1860°C—not less than 420–430 s. It was found that the effectiveness of coating is provided by structural-phase state of the base coating and formation of heterogeneous oxide film on its surface formed by borosilicate glass with titanium and calcium segregation heterogeneity and TiO₂ reinforce micro-needles in the form of rutile. The decrease in saturated steam pressure level in oxide film–coating system induced by increase in the level of external layer heterogeneity was experimentally confirmed.

Some practically important problems of unsteady heat conduction with a time-variable relative heat-transfer coefficient are considered. Various approaches to finding a solution to the analytical problem are systematized: decomposition of the generalized integral Fourier transform, the serial expansion of a sought temperature function, and the reduction of the problem to an integral, second-order Volterra equation. It is demonstrated that the solution is reduced to an infinite series of successive approximations of different functional form in all cases, and the main objective of each of these approaches is to find the most advantageous first approximation. Particular cases of the time dependence of the relative heat-transfer coefficients are considered: linear, exponential, power, and root cases. Analytical solutions and numerical experiments are described, and some specific features of the temperature curves of a number of mentioned dependences are revealed. It is established that the picture of the change in the temperature curve for the linear time law of the heat-transfer coefficient becomes appreciably different from the classic case of a constant coefficient, whereas the exponential dependence does not introduce any essential changes.

An unsteady two-dimensional heat-conduction problem without initial conditions is considered for a half-space under boundary conditions of the I, II, and III types. It is assumed that the boundary-value functions (including heat-transfer coefficient) are periodic functions of time and a spatial variable. Cyclic solutions to the problem are found in the form of double trigonometric Fourier series. The application of the obtained dependences is demonstrated with several specific examples. The conditions under which the quasi-one-dimensional solutions can be used are studied.

The results of the development and experimental studies of a loop heat pipe designed for the simultaneous cooling of several heat sources that dissipate different power amounts is presented. The pipe has a cylindrical evaporator and a serpentine-shaped condenser that is connected via the vapor and liquid lines. The evaporator was in thermal contact with the main heat source. Two additional, less powerful heat sources were in contact with the heat exchangers located on the vapor line and liquid lines. Freon-152a and ammonia were used as the working fluids. The efficiency of the loop heat pipe is estimated according to the ratio of the maximum heat load on the additional sources to the heat load of the main source. A temperature of 90°C was the limiting condition for each source. The tests were performed at a condenser coolant temperature of 20°C. The maximum power of the pipe with ammonia, without additional heat sources, was equal to 350 W, and that with freon-152a was130 W. We show that the maximum load on the additional source located on the liquid line was 34 W (57%) for a freon pipe under a nominal heat load on the main source of 60 W and 10 W (16%) on the source located on the vapor line. For an ammonia loop heat pipe under a nominal load on the main source of 220 W, the respective values for the additional sources were 60 W (27%) and 13 W (6%).

The dynamics of weak harmonic disturbances in a superheated water-air bubble medium are stu-died when, in addition to water vapor, the bubbles contain an inert gas in that does not participate in phase transitions. Maps of the stability zones of the considered systems are analyzed with respect to the degree of liquid overheating on the plane, and the volumetric content is analyzed with respect to the radius of bubbles with an increase in equilibrium pressure from 0.1 to 10 MPa. The effects of the initial overheating (from hundredths to one degree) and pressure increase on the dispersion of harmonic waves, as well as the dependence of the increment on the bubble radius for unstable systems, are studied.

The pharmaceutical substances of a group of local anesthetics, in particular, the drug lidocaine, are the object of the study. The influence of the thermodynamic parameters of the state on the process of lidocaine dispersion via the rapid expansion of a supercritical solution in an aqueous medium (RESAS) is studied in this work. The experiments were carried out at saturation temperatures of 40 and 60°C, a temperature of the expansion device of 40°C, and a system pressure of 15–35 MPa. The structure of the obtained particles was analyzed with a D8 ADVANCE diffractometer (before and after grinding). The melting points of the initial and processed lidocaine substance were determined.

The hybrid nonuniform microwave-DC discharge in nitrogen at pressure 1–15 Torr is studied by the method optical emission spectroscopy with spatial resolution. Discharge is ignited between the microwave powered antenna and grounded disk electrode placed at the distance of 2.5 cm from antenna. Antenna was positively biased relative to the grounded electrode. The ratio of the intensities of the nitrogen band at 337 nm and the emission of the ion band at 391 nm is used to determine the microwave field strength. Second positive system of nitrogen emission prevails over others in near antenna region. Emission of the first negative nitrogen ion system has the highest intensity near flat electrode. The vibrational distributions of nitrogen molecules and their dependence on direct current and position between antenna and grounded electrode are determined.

Numerical simulation of the interaction of the current layer in the magneto-gasdynamic channel with the induction of external magnetic fields of 1, 2, and 4 T and various load resistance values has been carried out. Features of the energy balance and differences in the temperature profiles in the current layers, which were both streamlined by a nonconducting gas flow and completely covered the channel cross section and interacted with the flow like a piston, are revealed. The dependences on the load resistance are obtained for the current, coefficients of current-layer permeability, and enthalpy conversion.

A method based on spectral analysis of the radiation of a low-power high-voltage electric discharge is proposed for the estimation of the degree of mixing of hydrocarbon fuels with air and nitrogen. The transition bands N₂(C³P_u → B³P_g, 0–2) and CN (B²Σ⁺ → X²Σ⁺, 0–0) were selected for analysis. A series of calibration measurements was carried out for several model mixtures. The range of applicability of the method with respect to the concentration of carbon-containing gas is determined. The proposed approach is used to assess the effectiveness of the intensification of the mixing a jet of gaseous fuel with air by a powerful pulsed discharge.

We present the experimental results for the temperature dependences of the thermal diffusivity, resistivity, and thermoEMF of six iron-titanium binary-system alloys with a Ti content of up to 3 at % at high temperatures. The existence of the γ domain and, thus, presence of the α → γ and the γ → α structural phase transitions are clearly recorded in the thermal diffusivity polytherms. The Curie temperature, which is determined from the position of the λ minimum of thermal diffusivity, does not depend on the titanium concentration within the experimental accuracy.

The equilibrium thermodynamic properties of a simple fluid with an interatomic potential in the form of a superposition of double Yukawa potentials is calculated. The proposed constructive computational scheme can reduce the time-consuming task of the determination of the free parameters of the potential to a search for one variable parameter. It is shown that the thermodynamic functions and the interaction potential are converted to the corresponding one-parameter dependences, and the optimal parameter value is calculated based on the best agreement with the experiment. The results of calculations of the properties of various model systems are compared with the measurement data.

The ablation of gold and nickel with a single exposure to 80-fs laser pulses with a moderate intensity (10¹²–10¹³ W/cm²) is studied by intereference microscopy. The reflection coefficients were measured, and the threshold values of thermomechanical ablation were determined from the absorbed energy rate. The crater morphology and the dependence of their depth on the rate of the laser-pulse energy are studied.

A mathematical model has been developed and the electrophysical characteristics of combustion products in the flow path of a liquid-propellant rocket engine have been calculated taking into account the electrification of combustion products and solid metal particles resulting from the combustion of engine structural elements. Numerical methods have been used to calculate the trajectories, speed, and temperature of the metal particles and the total electric charge acquired by the particles as a result of interaction with electrons present in the combustion products. The influence of particle size on the electric charge is analyzed. The results can be used to develop an early diagnosis system for the ignition of heat-stressed structural elements of power plants.

The authors propose a model of the evaporation of iron impurities from microporous alumina granules into vacuum. The analogy between the solid-state diffusion with subsequent evaporation from the microparticle surface and the cooling of sphere is used.

Analytical formulas describing the non-stationary temperature field in inhomogeneous structures are obtained. In practice, such structures are most often multilayer, which makes it difficult to obtain a mathematically acceptable solution for practical calculations. A feature of this problem is the significant dependence of thermal conductivity on spatial coordinate. In the case where this dependence is exponential, it is possible to obtain a rigorous analytical solution.

In a long, plane-parallel channel with single-row, inclined, oval-trench dimples on the wall, a periodic section with a relative length of 8 (the distance between centers of neighboring dimples), a width of 9, a height of 1, and a centrally located, inclined, oval-trench dimple with a length of 7.05, a width of 1.05, a depth of 0.25, and with variation in the inclination angle from 1° to 89° is considered. An anomalous intensification of a separated turbulent air flow in an inclined dimple at the Reynolds number of 10⁴ is discovered. At an inclination angle of 60°, the maximum absolute value of negative friction in the middle longitudinal cross-section exceeds the friction in a plane-parallel channel by more than 2.5 times. The cause of this phenomenon is related to the formation of a high pressure drop between the closely located zones of stagnation and low pressure in the semispherical inlet segment of the dimple.