Home – Home – Jornals – Thermophysics and Aeromechanics 2024 number 5

The results of an experimental study of the flow around a sharp cone with an opening angle of 10° in two large-scale TsAGI wind tunnels are presented. The Mach and Reynolds numbers of the free-stream flow varied in a wide range of M=0.2÷6 and Re=3÷34 million. The drag, lift and side force coefficients were measured. The behavior of the pitching, rolling, and yawing moment coefficients was studied. The distribution of static pressure on the cone surface and pressure fluctuations were analyzed. A physical model of the cone streamlining was constructed. An ordered system of data was prepared to contain both the results of experiments and test calculations, intended for the validation of computer programs and models.

(To the 110th anniversary of the birth of Academician S.S. Kutateladze) The paper presents the contribution of Academician S.S. Kutateladze to the substantiation and development of engineering methods for calculating electric-arc plasmatorches based on the approaches of the similarity theory and dimensional analysis. Generalized volt-ampere and thermal characteristics of a single-chamber plasmatorch with both conventional water and porous anode cooling are presented, as well as some investigation results on high-temperature, underexpanded supersonic low-density jets, including the plasma ones.

The influence of the change in the velocity profile on stability of the Tollmien-Schlichting waves in the Blasius boundary layer is studied experimentally. The experiment is performed with a flat plate model in a low-turbulence wind tunnel T-324 at the freestream velocity U_∞ = 9 m/s. Hot-wire anemometry is applied for measurements. The results show that a distributed action on the Blasius boundary layer through a hydrodynamically smooth surface leads to attenuation of the amplitude of the naturally excited Tollmien-Schlichting wave almost by two orders of magnitude. This distributed action leads to changes in flow stability. The experimental results reveal that the distributed action is more effective than the action onto the flow through a slot, other conditions being identical.

The results of experimental and computational studies for the coolant flows mixing at different temperatures for standard operational parameters of a nuclear power unit (NPU) are presented in this paper. Experiments were performed using an upgraded version of measurement model. This study defines a temperature filed in the mixing zone and offers the optimal set of operation parameters calculated from the condition of maximum temperature pulsation within the mixing zone. Simulation was performed using a software kit Ansys Fluent. The grid model is based on the block structure in Ansys ICEM code. Numerical simulation was performed for unsteady problem statement based on the LES WALE turbulence model. Analysis of simulation and experimental fields for flow velocity and temperature confirmed the validity of the developed numerical method. A qualitative compliance for the probability density distribution and the spectral-correlation characteristics for temperature signals in the mixing zone was observed. The spectral power density for calculated temperature and velocity distributions fits the case of 1D energy spectrum of developed isotropic turbulence.

The issue of the existence of a reverse flow behind the Mach disk in the first shock cell of a supersonic underexpanded jet is numerically studied on the basis of solving the Navier-Stokes equations. It is shown that the reverse flow existence depends on the Reynolds number: it arises at low Reynolds numbers and vanishes when the Reynolds number exceeds a certain value. Thus, for experimental observations of this paradoxical phenomenon, it is necessary to perform experiments with jets of a sufficiently rarefied gas.

Passive intensification of heat transfer in a tube is achieved by using detachable intensifiers or changing the shape of the heat transfer surface. In this work, the intensification process was studied using spherical turbolizers located in a horizontal stainless steel channel. Experimental data were obtained during circulation of an alcohol-water mixture with a concentration of 30 wt. % at a pressure in the vessel of 0.03 - 0.04 MPa. It is shown that the efficiency of spherical intensifiers is commensurate with the efficiency of spiral intensifiers at significantly lower pressure drops.

The paper investigates the geometry and droplet size distribution of a fuel spray produced by an injection system with air-assisted (pneumatic) atomizer. Two optical research methods, based on the processing of images obtained through shadow photography, were employed. The evolution of a pulsed spray was studied using a high-speed video camera with a macro lens. Droplet size distributions were studied using a camera equipped with a long-distance microscope and backlighting comprising a luminescent background screen and a pulse laser. The study was conducted for air-to-fuel mass flow rate ratios (ALR) of 1 and 0.16. The Sauter mean diameter for atomized droplets was 13 and 18 microns for the ratio values of 1.0 and 0.16, respectively.

On the basis of a complete system of acoustic equations, the evolution of acoustic disturbances in a high-temperature reactive mixture of carbon dioxide, carbon monoxide, and oxygen CO₂/CO/O in the dissociation-recombination reaction, as well as in the nonequilibrium endothermic reaction of CO₂ dissociation and exothermic reaction of afterburning of carbon monoxide in oxygen, is investigated. A realistic single-temperature Arrhenius model of chemical kinetics is used. The values of thermodynamic parameters on the trajectory of hypersonic flight in the Martian atmosphere are taken as stationary environmental conditions. For the dissociation-recombination reaction with a negative thermal effect near chemical equilibrium in the asymptotic high-frequency limit, attenuation of acoustic disturbances is shown. Under the same conditions, acoustic disturbances amplify in the nonequilibrium exothermic reaction and attenuate in the nonequilibrium endothermic reaction.

This paper presents the results of numerical simulation of a submerged jet leaving an orifice of diameter D with velocity U, when the nozzle is subjected to mechanical oscillations (vibrations) of frequency ƒ. Vibrational excitation leads to jet bifurcation at Reynolds number Re = DU/ν>50 in wide ranges of oscillation amplitude Z₀, Strouhal number St = ƒD/U , and it is similar to acoustic forcing. The obtained estimates of the characteristic jet thickness and its expansion angle α performed to study the influence of Z₀, Re, and St parameters allow identification of the optimal values St_opt of the Strouhal number, at which the splitting process is most pronounced. The previously noted effect of an increase in angle α with an increase in the excitation amplitude and saturation after some threshold values of amplitude Z₀ are confirmed. These results also indicate that with a decrease in Re, the threshold amplitude (above which saturation in a occurs) increases, and the maximum possible values of a drop significantly. The influence of Re on the optimal Strouhal number was found: with a decrease in Re from 3000 to 100, the St_opt values drop significantly.

Numerical modeling of the local flow structure and mixing process during injection of a gas-droplet wall jet into a turbulent heated air flow has been performed. The numerical solution is based on a system of axisymmetric Reynolds-averaged Navier-Stokes equations (RANS) taking into account the two-phase character of the flow. To describe the dynamics and heat and mass transfer in the dispersed phase, the Euler two-fluid approach is applied. A significant effect of the liquid mass concentration on the distribution of parameters over the channel cross-section and thermal efficiency is shown. An increase in thermal efficiency reaches 30% in the initial part of the channel and 200% in its end part in comparison with a single-phase wall air jet.

This papers presents the results of experiments and simulation on the regularities of a high speed gas-particle flow impinging a complex target under conditions of cold spry deposition. The target consists of a substrate and an installed in front partition with conical converging/diverging perforation. Numerical simulation was performed using the ANSYS Fluent code in the mode of single particles motion (without the influence of particles on the gas flow parameters). Also, the results obtained were compared with previously investigated penetration of gas-particle flow through a mask with a cylindrical perforation.

The characteristics of three-component slurry fuels, including coal, water and pyrogenetic liquid (a by-product of thermal processing of wood waste) have been experimentally studied, and the dispersion of the obtained slurries has been investigated as applied to the technology of their combustion in energy boilers. Samples of slurries for the study were obtained using a technology that includes coal mass grinding in a ball drum mill, metered mixing of components and processing of the resulting mixture on a rotary hydrodynamic cavitation generator, ensuring additional grinding of the coal mass, slurry homogenization, and mechanochemical activation of the fuel. The slurries were dispersed using a pneumatic injector with an external mixing of the slurry and the spraying agent. The average slurry droplet size in the jet after spraying was determined using the Interferometric Particle Imaging (IPI) method. The droplet velocity was recorded using Particle Image Velocimetry (PIV). The characteristics of the gas-liquid flow created by the nozzle have been obtained from the initial breakup of the liquid jet to the final state of the spray.

It is demonstrated in the paper that the range of an equilibrium SF₆ jet normalized to the nozzle diameter is independent of the dimensional stagnation temperature if the values of four dimensionless parameters (jet pressure ratio, Reynolds number based on the nozzle diameter, and two temperature factors) remain unchanged. In contrast to the equilibrium case, the range of a vibrationally excited SF₆ microjet is an essentially nonmonotonic function of the dimensional stagnation temperature.

The present study investigated the phenomenon of heat transfer when a liquid droplet evaporated on a heated structured surface made of black silicon and smooth glass with a graphite coating. Water and volatile dielectric liquid HFE-7100 were used as the working fluids. The temperature fields were studied when shooting with a thermal imager from the top and from the side. A convective flow inside the droplet caused by the Marangoni effect was also investigated. The structures formed inside the heated liquid droplet were analyzed depending on the type of liquid. In particular, for HFE-7100 droplets, the evolution of the flower-shaped convective cells was studied in detail, and the contact line movement velocities were measured during evaporation of microdroplets.

The paper presents the experimental results on capillary liquid spreading over superhydrophilic black silicon surfaces of various morphologies created by cryogenic plasma-chemical etching. Five samples of capillary surfaces with different microstructure characteristics were made, including two hybrid surfaces combining microstructuring of different heights. The morphology of surfaces and characteristics of their wetting with water were studied, including the Wi number, which characterizes the capillary absorption of liquid. It was found that a greater height of microstructures ensures better liquid spreading. The maximum values of the Wi number are achieved on hybrid surfaces with a greater density of “high” microstructures. The prospects of using hybrid capillary surfaces to increase the critical heat flux during water boiling are shown.

This study demonstrates a process of nanoparticle crystallization in an expanding jet of high-enthalpy in-equilibrium plasma carrying the titanium particles flow (as an example) and their deposition on a substrate. The plasma source is a disk-type MHD accelerator. The transport gas with a gaseous precursor is fed to the plasma accelerator input. The flow from this MHD accelerator gains the velocity about several km per second and emerges into vacuum chamber. The generated plasma jet flows around the substrate. When the substrate is placed at the distance from the MHD exit equal to the channel width size, this arrangement produces a smooth coating on a substrate. For one order higher distance to the sample, this gas-particle flow creates a coating of nanosized crystals.

The thermal diffusivity and the thermal conductivity of solid erbium in the temperature interval of 295-1475 K have been measured using the laser flash method. Approximation equations and a table of reference values for the temperature dependence of heat transfer coefficients of the studied metal have been received. The obtained results have been compared with the known literature data. The temperature dependence of the erbium thermal conductivity is shown to be mainly determined by the electronic contribution.

The paper presents experimental data on the effect of the morphology of the superhydrophilic black silicon surface structure, obtained by plasma-chemical etching, on heat transfer during pool boiling of water. Silicon surfaces with homogeneous and hybrid microstructures are investigated. Heat transfer experiments were carried out on pre-selected microstructured surfaces with the best characteristics of capillary absorption. It is shown that the critical heat flux (CHF) for a surface with a hybrid structure is approximately three times higher than the CHF for a smooth silicon surface (660 kW/m²), reaching a value of 1914 kW/m², while the CHF for a surface with a homogeneous structure exceeds the CHF for a smooth surface by the factor of 2.4, reaching a value of 1568 kW/m². At that, the maximum recorded heat transfer coefficient (HTC) of the surface with a homogeneous capillary structure, on the contrary, is the highest (77 kW/(m²K)), almost twice exceeding the heat transfer coefficients for the unmodified surface in the region of moderate heat fluxes. The surface with a hybrid structure demonstrates a delay in boiling incipience when compared with the results for a smooth surface, but with a further increase in the heat flux it significantly exceeds the HTC for the smooth reference surface, ultimately reaching a maximum value of 45 kW/(m²K) in the pre-crisis region.

The density of a liquid mixture of lithium, sodium and potassium fluorides of eutectic composition (FLiNaK) was measured using gamma-ray attenuation technique over the temperature range from liquidus to 1010 K. Based on experimental data, an equation for the temperature dependence of density has been proposed. The obtained results on the volumetric properties of the FLiNaK melt have been compared with literature data. Using the approximation of an ideal solution, the temperature dependence of the FLiNaK density in the solid state has been calculated and the density jump during melting has been estimated.

The processes of heat transfer and phase transformations in a freshwater reservoir during the cold season are considered. A non-stationary computational model applied for determining the thermal behavior of a reservoir and the ice cover dynamics is based on the numerical solution of the Stefan problem in a generalized formulation. Climatic conditions are considered in the form of dependences of the average daily values of air temperature, wind velocity, humidity and solar radiation flux. The influence of heat fluxes and snow cover height on the reservoir temperature and ice growth dynamics is analyzed.