Journal of Applied Mechanics and Technical Physics最新文献

Ceramic composites based on B₄C with a CrB₂ mole fraction of 0–30% were obtained by reactive hot pressing of a mixture of B₄C, Cr₂O₃, and nanofibrous carbon at a uniaxial pressure of 17.5 MPa and a temperature of 2000 °C for 10 min. The possibility of boron carbide reduction of metal oxide during hot pressing was studied. During the synthesis of CrB₂, the density of B₄C was found to increase due to the formation of the CrB₂–B₄C eutectic liquid phase. The relative density of all B₄C–CrB₂ composites obtained under these conditions exceeds 90%. Experimental dependences of the microhardness and elastic modulus of samples on the concentration of the CrB₂ plastic phase were obtained. The dependences of the elastic modulus of the heterogeneous material on the volume fraction of chromium diboride were determined taking into account porosity by sequentially using the Reuss and Voigt averaging schemes.

A problem of mathematical simulation of a fuel element region, including many fuel pellets and a cladding fragment, is considered in an axisymmetric formulation. It is assumed that the cladding is a thermoelastic-plastic body and that the pellet is a thermoelastic body with account for cracking of the material. Different variants of the domain decomposition method are used to numerically simulate the thermal and mechanical contact of pellets with each other and with the cladding. Calculation results are presented, in which the region containing ten pellets reaches a nominal power and the effect of pellet cracking on the thermomechanical state of the fuel element is estimated.

Investigations (mainly those performed by the authors) of air blowing through a perforated section on a body of revolution with a large aspect ratio in an axisymmetric incompressible flow are summarized. Result of numerical and experimental studies of the flow properties, efficiency of the turbulent boundary layer control and prospects of using it for a body of revolution at low subsonic velocities equivalent to the take-off and landing regimes for a modern subsonic cargo aircraft are analyzed.

This paper presents the results of a study of the influence of various sources of magnetic field on the size of droplets formed in microfluidic flows. Direct and reverse emulsions in a microfluidic flow focusing were obtained using magnetic fluids based on oil and water which are a continuous phase. Non-magnetic inclusions of various volumes were formed depending on the selected parameters: continuous phase flow rate, magnetic field configuration, and the position of the magnet relative to the axis of the device.

The sizes and rise velocity of bubbles in a stationary liquid in an inclined channel with a circular cross-section at different gas flow rates through a capillary were determined (3.0–5.5 ml/min). The size and velocity of gas bubbles were studied by shadow photography. It is shown that in the range of channel inclination angles 40–60°, the formation of stable bubble structures—clusters consisting of bubbles of the same size (1.5–1.8 mm) — is possible. In regimes without the formation of chain clusters, the average diameter of gas bubbles increased (2.0–2.2 mm) due to their coalescence.

This study touches upon the characteristics of liquid (diesel) fuel combustion in a high-velocity jet of superheated steam in the combustor of a hot water boiler, the thermal power of the burner being approximately equal to 40 kW. It is shown that the burner based on the steam fuel atomizing technology meets modern technical and environmental standards and is quite advantageous in relation to its analogs. It is revealed adding steam makes it possible to cause a severalfold decrease in CO and NO(_x)concentrations in combustion products. At the same time, the amount of carbon monoxide and nitrogen oxide emissions when using this burner is quite smaller (2.5- and 1.4-fold, respectively) than when using a Weishaupt burner. Recommendations for optimizing the operation of such devices are presented.

The breakup of a jet consisting of two coaxial jets of immiscible liquids has been studied experimental with varying phase flow rates. Distilled water and a mixture of polymethylsiloxanes were used as working liquids. Jet breakup regimes, the types of capsules formed, and the sizes of single-core capsules formed under the influence of capillary instability were determined. The natural frequencies of surface instability were measured.

The efficiency of fluid atomization by an atomizer is studied as a function of the spray energy. The dependence of the maximum values of energy on the fluid flow rate is analyzed. A linear dependence is obtained for flow rates smaller than 80 g/s, which testifies to a high efficiency of fluid atomization. For flow rates greater than 80 g/s, the droplet energy is seen to decrease drastically, leading to an increase in the spray droplet size, which shows that the atomization quality is deteriorated. This behavior is observed in all regimes considered in the study.

Direct numerical simulation is used to study the effect of wall heating on the characteristics of a near-wall reverse flow during a turbulent stream of various coolants in a duct. A temperature field is considered both in a passive scalar approximation and in a low Mach number approximation. Qualitative and quantitative results are obtained that characterize the probability of occurrence of a near-wall reverse flow in all the cases considered at a Reynolds number (mathrm{Re}= 3150) based on mean flow velocity and duct half-height. It is revealed that, in the cases considered, the wall heating causes a two- or a threefold increase in the probability of near-wall reverse flow formation .

A heat conduction equation within the framework of the coupled dynamic theory of thermoelasticity is considered. Coupling in the heat conduction equation is estimated for a space with a constant initial temperature. This space contains a flat semi-infinite crack propagating at a constant velocity, and a constant temperature lower than an initial one (thermal shock) is instantly established on the edges of this space. The movement of the crack and the thermal shock on its shores determine dynamic effects that should be taken into account to estimate coupling in the heat conduction equation. It is demonstrated that, under real conditions of a thermal shock on massive bodies with cracks, one may ignore dynamic effects and coupling for materials that satisfy certain conditions imposed on their thermomechanical constants. This significantly simplifies the process of solving thermoelasticity problems for such bodies.