Home – Home – Jornals – Thermophysics and Aeromechanics 2024 number 6

The paper presents a method and results of simulation for turbulent air flow and heat transfer in a U-shaped square duct with straight sections and with two ribbed opposite sides. For a flow with Reynolds number of 6×10⁴ and Prandtl number of 2.5, the numerical solution method was validated by comparing the calculated and experimental data on local heat and mass transfer in geometrically identical U-ducts; the ribs inclined to the duct axis by 45 and 60 deg. For one of these ducts with the rib inclination angle of 45 deg (at the same Reynolds number and Prandtl number of 0.71), the flow pattern and heat transfer were calculated for a stationary state and for a mode with rotation (with the rotation parameter values of 0.1 and 0.25). The rotation axis is perpendicular to the duct axis and parallel to the duct bended plane. The calculation data for the U-shaped duct are compared with the calculation data for a fully developed (periodic) flow and heat transfer for a straight-and-ribbed duct. It is shown that for a U-duct, the effect of rotation has a positive influence on heat transfer enhancement.

To study the aerodynamics of a pulsating flow in a flat channel of 20×150×600 mm, an experimental setup has been constructed. Gas flow pulsations were generated by rotating a damper that completely blocked the channel. The damper was located behind the test section, and the setup operated in the regime of sucking air from the surrounding space. These studies were carried out using the PIV method, synchronizing the measuring system with the damper rotation angle. The experiments were carried out at pulsation frequency F = 8 Hz. The experimental results are directly compared with the case of a steady flow with a fixed damper, but with the same rotation angle as in the case of phase averaging. It has been established that under these conditions, superimposed flow pulsations weakly affect the longitudinal velocity profiles, integral characteristics of the wall layer and friction coefficient, which indicates a quasi-steady flow in the channel.

A pioneering study of the influence of longitudinal slots (grooves) with different depths h (corresponding to the Reynolds numbers Re_h = 900 and 1300) on the stability of a supersonic (Mach number М = 2) boundary layer on a flat plate to controlled disturbances of the first (vortex) mode of instability determining the laminar-turbulent transition at М = 2 is performed. At the Reynolds numbers Re _h = 900 and 1300, the growth rates of three-dimensional disturbances at the frequency ƒ = 14 kHz are found to be smaller than the corresponding value for the smooth plate. Thus, it is shown that controlled disturbances of the first mode can be stabilized by longitudinally aligned slots with a small depth (Re_h < 1500).

The synthesis of diamond coatings on silicon substrates using a gas-jet method, which uses a high-speed jet to transport a gas mixture activated in a microwave discharge, is studied. The coatings are synthesized from a mixture of hydrogen, methane and argon. The possibility of maintaining the integrity of the silicon substrate under conditions of gas-jet deposition at high temperatures is shown. The rate of diamond synthesis on a silicon substrate exceeded the one, achieved earlier in gas-jet experiments with activation in microwave plasma without adding argon, as well as during deposition from a hydrogen-methane-argon mixture at lower substrate temperatures.

Data on the structure of seeding a flow in a supersonic wind tunnel with electric arc heating of the test gas by particles are reported. Velocity profiles near the plate surface are obtained. It is shown that the spectral characteristics of particle seeding inhomogeneity depends on the Reynolds number and flow regime. The possibility of identifying the anisotropic component of the particle distribution inhomogeneity field in the laminar boundary layer on the plate is demonstrated.

A simple CFD modeling method is presented adapted for problem of liquid spraying by a centrifugal nozzle. The method is based on the VOF method and several assumptions; this approach enables calculating the spraying characteristics during a reasonable computation time. The primary testing results confirmed the possibility of using this simulation method.

Results of experimental measurements of aerodynamic forces acting on a finite-size cylindrical body in a compressible flow are reported. The flow around a transversely aligned cylinder and the influence of yaw angle variation in the interval β = 0 - 9° are studied. For each position of the cylinder, several locations of the blowing hole along the cylinder radius are considered. The force measurements are performed with the use of strain gauges. The changes in the drag force, lift force, yaw moment, and roll moment are investigated. New experimental data are obtained in the study on the influence of blowing on changes in aerodynamic cha-racteristics of the finite-size cylindrical body in a compressible flow. The data are analyzed and interpreted.

The results of correlating the emissivity of pure metals in the liquid phase with the atomic number of the element in the periodic system and the properties, which include the coefficients of thermal expansion and surface tension, are presented. The results obtained fit the periodic system. The emissivity has a periodicity with traceable translational properties.

The effect of light gas injection on the linear stage of Görtler vortex development in a compressible boundary layer was studied. The problem is solved in the framework of linear stability theory and using the direct numerical simulation for Navier-Stokes equations. The data on the vortex growth and vortex profiles are in agreement for these two methods. It was demonstrated that helium injection into a compressible boundary layer upon a convex surface can reduce the growth rate for Görtler vortices.

A lattice model of the pore space of a microsphere packing and a method for calculating the microstructure of a two-phase flow, taking into account the topology of pores, are proposed. The sequence of filling the pore constrictions and expansions with drainage and impregnation phases and the dependence of capillary pressure on saturation of phases and the coefficient of longitudinal dispersion for transfer of a dissolved substance were obtained under conditions of the determining influence of capillary forces.

The results of an experimental study of the rising process of a cluster of monosize drops of isoparaffin oil in an water-glycerol solution in the presence and absence of surfactant at the interface are presented. The flow for the Reynolds numbers range Re=0,01÷1 was studied. The effect of initial volume concentration of drops in a cluster on the rising dynamics was evaluated. New experimental data on the drag coefficient for a cluster of monosize drops uprising in a liquid with/without surfactant at the interface were obtained.

The effect of peripheral flow swirling in a cylindrical channel on the local flow structure and heat transfer in a turbulent gas-droplet flow was numerically studied with variations in the initial mass concentration of water droplets in the range M_L1 = 0 - 0.1 and the initial droplet diameter d₁ = 10 - 100 μm. It is shown that adding droplets leads to a significant increase in heat transfer (more than 2.5 times) in comparison with a single-phase swirling flow. The heat transfer patterns in gas-droplet flows with and without swirling are qualitatively similar. The effect of heat transfer intensification due to gas-droplet flow swirling reaches 50% at M_L1 = 0.1. Initially, with an increase in the droplet size, a significant intensification of heat transfer in the swirling flow occurs. The maximum heat transfer is obtained at d₁ ≈ 50 μm for all swirling intensities of the two-phase flow. Then the value of heat transfer decreases and its dependence on the droplet size becomes more gradual.

Laminar flows of a gas-dust medium with chemical reactions in an axisymmetric tube are studied in much detail in numerical experiments. The study is performed for non-oxidative reactions of catalytic synthesis of hydrocarbons from methane. The hot walls of the tube warm up a two-phase flow of methane and catalytic nanoparticles. Heating of this flow ensures endothermic conversion of methane into hydrocarbons and hydrogen. It is shown that the presence of catalytic nanoparticles in the tube, which activate heterogeneous-homogeneous reactions, leads to significant deviation of the two-phase medium flow from the Poiseuille flow of a single-component gas due to changes in the medium volume and its viscosity. The change in the heat regime along the tube with a change in the thermal conductivity of the reactive medium is investigated. Calculations are performed for various values of the wall temperature, particle concentration, laminar flow velocity, and other parameters. It is found that the medium velocity and the gas component distributions in the volume and at the tube outlet are most strongly affected by the methane activation energy on the surface of the catalytic nanoparticles, particle diameter, and particle concentration.

In a bubble flow, both proper turbulence of liquid and pseudo turbulence induced by the bubbles are present. Total two-phase turbulence of the flow is simultaneously determined by the interaction of these two components. The experimental values of the average friction stress and its pulsations obtained by eight double electrodiffusion friction sensors located uniformly in one section along the pipe perimeter are presented. The gas phase is represented by monodisperse bubble mixtures with average diameters of 1 and 2 mm and a volumetric gas content of up to 20%. To modify the Reynolds numbers for a bubble, two working solutions with different viscosities were used. The average flow velocity of liquid did not exceed 1 m/s. Flow regimes with deviations from the Sato hypothesis on the additivity of pseudo turbulence and proper turbulence of liquid were detected. At transitional Reynolds numbers of the flow and small Reynolds numbers of a bubble, total turbulence is determined by pseudo turbulence, and there is no the influence of singe-phase turbulence.

The presented research has made progress in describing sea spray and understanding the associated processes in terms of its physical properties, as well as the resulting fluxes from the ocean to the atmosphere, its influence on the development of ocean storms, and related problems and issues. The main results were obtained during laboratory experiments on wind-wave channels using high-speed photography. Descriptions of studies related to the identification of the dominant type of spray generation (the "bag-breakup" type) and the statistics of these phenomena in a wide range of conditions depending on the wind-wave Reynolds number characterizing small-scale processes are given. Convolution of this statistics with the droplet size distribution from a single phenomenon of spray generation of the specified type, obtained from data from a special experiment, yields the spray generation function.

Based on the previously developed method for constructing the equation of state (EOS) for liquid and vapor, the thermodynamically consistent analytical equation of state for liquid and gaseous nitrogen (in the molecular phase) is obtained. While constructing the equation of state, the Mie-Gruneisen form was used as a sum of potential and thermal components for pressure and internal energy. The potential components are described by a potential of the Born-Meyer type. For the thermal components, a simplifying approximation is adopted that follows from the condition for the constant average heat capacity and the dependence of Gruneisen function on the volume. When deriving the equation of state for nitrogen vapor and liquid phases, the tables calculated from equations approximating experimental data on isothermal compressibility, adiabatic speed of sound (including the critical region and phase equilibrium line) were used. The obtained equation of state for nitrogen can be useful in studying the phenomena associated with the processes of nitrogen evaporation and condensation in multiphase flow. This modeling takes into account the interfacial heat and mass transfer under conditions of low pressures and cryogenic temperatures.

The density and thermal expansion coefficients of the Li₂Ca intermetallic alloy were measured in the temperature range of 284 - 906 K for the solid and liquid states by means of sample irradiation a narrow gamma-ray beam. The density jump of the Li₂Ca intermetallic alloy was also measured during the solid - liquid phase transition. Data on the thermal properties of the alloy above room temperature were apparently obtained for the first time. A table of recommended values of the thermal properties of the Li₂Ca alloy over the entire measurement range is presented and their errors are estimated. It is confirmed that the Li - Ca melt with a calcium content of 33.33 at. % is an almost ideal solution.

A direct stastistic modeling method was applied for simulation of dynamics for a “gas - dust particle” system within the problem of study for near-core gas-dust atmopshere of a comet. The paper considers the gas-dynamic mechanism of dust particle ejection from the comet core surface. This assumes the esistance of a dust-coated cavity which is filled by sublimation products from the ice-core components. Calculation offers the data on dust particle velocity as a fuction of key parameters. The ejection velocity for a dust particle is described by one general parameter.

Results of an experimental study of gold-induced crystallization of thin films of amorphous germanium (a-Ge) are reported. The Raman spectroscopy and optical microscopy are applied to study the influence of the initial thickness of the a-Ge film on the morphology of the material obtained by means of annealing of bilayer Au/a-Ge structures. The a-Ge/Au thickness ratios equal to 1.1 and 2.3 are considered. Low-temperature annealing in quartz/Au/a-Ge samples is performed in a high-vacuum atmosphere (10^-4 Pa) at a temperature of 300 °С for 24 h. The Raman spectroscopy results show that annealing of the samples results to complete crystallization of a-Ge with formation of polycrystalline germanium (poly-Ge) in both lower and higher layers of the samples. For the sample with a-Ge/Au = 2.3, a continuous poly-Ge film is obtained on the substrate owing to implementation of the layer exchange mechanism. In this case, poly-Ge hillocks are formed in the upper layer of the sample due to a-Ge crystallization in the presence of Au. For the sample with a-Ge/Au = 1.1, a non-continuous poly-Ge film with a coverage ratio of 30% is formed in the lower layer. The poly-Ge material is formed in the upper layer owing to the processes of “explosive” crystallization and also secondary crystallization induced by Au.

A possibility of using non-monochromatic photodetectors capturing radiation in a wide range of wavelengths as sensors of the brightness channel in spectral brightness pyrometry is considered. A definition of the effective wavelength of the pyrometer is introduced on the basis of the dependence of the detected signal on the temperature of the heat source. Experimental results on determining the effective wavelength of the photodetector with the sensitivity in the interval of 400 - 1100 nm in recording the temperature lamp radiation in the temperature range from 1557 to 2494 K are reported.

The paper presents the results of dilatometric measurements for thermal expansion of monocrystal silicon for the temperature range of 100 - 1373 K. The temperature dependencies for thermal properties of material are calculated. The look-up tables for a broad range of temperatures for a solid material are calculated.