Home – Home – Jornals – Thermophysics and Aeromechanics 2026 number 3

This article presents experimental results for determining the heat transfer intensity under aerodynamic suspension (free levitation) conditions for a spherical body in an air flow in a diffuser duct. Surface temperatures during heating and the sphere's rotational velocity were measured using contactless methods. It was established that over a wide range of airflow velocities, rotational motion of the sphere along two coordinates occurs, leading to a fundamental change in the hydrodynamics of the flow around it, manifested in the absence of stationary separation zones in both the front and rear sections. Heat transfer coefficient studies were performed using a non-stationary method at various initial sphere surface temperatures and airflow Reynolds numbers. For comparison, a study was conducted on a sphere fixed in the flow under the same conditions. It was found that the free aerodynamic suspension of the sphere allows for equalization of the surface temperature field and significantly intensifies heat transfer compared to a fixed sphere. Experimental heat transfer results show that cooling time decreases and the Nusselt number increases with increasing sphere rotation speed by 300-600%.

An experimental study was conducted to examine the effect of heat flux density on the pressure drop of two-phase gas-liquid flows in a flat microchannel with local heating at a fixed liquid mass velocity and varying gas mass velocities. It was shown that at low gas mass velocities, the pressure drop increases due to enhanced interphase interaction; at high gas mass velocities, it decreases due to intense evaporation, wall drying, and reduced liquid-wall friction; and at moderate gas mass velocities, it remains virtually unchanged due to the competition between the two mechanisms.

An experimental study of the dynamics of liquid droplet formation and detachment in a periodic drip regime was conducted using tubes with an outer diameter ranging from 0.5 to 4 mm. Water, ethanol, a 40% aqueous ethanol solution, and glycerol were used as working fluids, allowing for a wide range of physicochemical properties (viscosity, surface tension, density) to be covered. Analysis of the droplet shape dynamics revealed a three-stage cyclic process. Dimensionless parameters were used for the analysis, allowing for a comparison of experimental data obtained under different operating conditions. It was found that throughout the entire period of droplet formation, a jet flow region with a liquid flow rate lower than that through the nozzle develops in the central region. This leads to the droplet separation into two parts-an upper and a lower part. The lower droplet is separated directly from the forming jet. However, the characteristics of this process differ significantly from similar droplet detachment processes under chaotic droplet conditions.

A detailed systematic experimental study of four types of high-frequency and medium-frequency secondary instability of a swept-wing boundary layer has been carried out under conditions of predominance of the stationary primary cross-flow instability modes. The measurements were carried out with a hot-wire anemometer in a low-turbulence wind tunnel at a low subsonic velocity of the incident flow under conditions of excitation of controlled primary stationary disturbances and controlled non-stationary secondary disturbances. The structure of wave fronts of several types of secondary disturbances in a frequency range from 100 Hz to several kilohertz has been studied in detail. A new type of medium-frequency secondary instability, called type IV, has been found. The wavenumbers, wavelengths, phase velocities, and propagation angles of all four types of secondary disturbances are obtained along three spatial coordinates. The evolution in time of the instantaneous spatial structure of secondary disturbances in a broad frequency range is visualized.

Numerical modeling of the process of deposition of water droplets in a turbulent pulsed two-phase flow in a cylindrical channel with peripheral swirl was carried out when varying the initial mass concentration of water droplets using the Euler description. The gas phase is described by a system of 3D URANS equations taking the influence of particles on transport processes in the carrier phase. Gas phase turbulence is calculated using an elliptical model of Reynolds stress transfer, also written taking into account the influence of the dispersed phase. The maximum concentration of the precipitated liquid was obtained in the pulse frequency range ƒ = 100 - 150 Hz (Strouhal number Sr = 0.4 - 0.6).

This paper numerically studies the propagation of temperature waves during a phase transition in a substance completely filling a plate, cylinder, or sphere. At the boundaries of the regions, the temperature varies harmonically. The phase transition temperature lies within the temperature variation range of the boundaries. The enthalpy method was used to model the phase transition. To validate the results, they were compared with analytical solutions and the results of other authors. The influence of dimensionless parameters and geometric characteristics on the temperature behavior at the center of the region and the penetrating power of temperature waves was studied. A correlation between the temperature extremum at the center of the region and the thermophysical parameters was determined. When this extremum is reached, the difference between the average temperature at the center of the region and at the boundaries is greatest. It was found that at the extremum point, the average temperature at the center of the region is equal to the phase transition temperature. The type of extremum depends on the ratio of the thermal conductivities of the phases. A qualitative dependence of the heat wave attenuation coefficient on thermophysical parameters was established. Conditions under which the penetrating power of the temperature wave is minimal were determined.

The analysis of the measurement and calculation results for the main characteristics of the thermoelectric cooling system is carried out. Using temperature and electrophysical parameter measurements for various temperature conditions and operating modes, the values of thermal resistances of heat exchangers, cooling capacity, coefficient of performance and temperature differences on the structural elements were determined. During modeling, heat transfer processes were described by integral relationships, taking into account the measured operating characteristics of the thermoelectric module and the thermal resistances of the heat supply and removal devices. Based on the results of the analysis, a range of power supply current of the thermoelectric module is recommended, which ensures high cooling capacity and efficiency of the cooling system.

This paper presents the results of a comprehensive study of the rheological and viscoelastic properties of aqueous polyacrylamide (PAA) solutions modified with single-walled (SWCNTs) and multi-walled (MWCNTs) carbon nanotubes. The study was conducted on a modular rheometer at 25°C. It was found that all studied compositions based on 0.05 wt.% PAA exhibit pseudoplastic behavior and are characterized by a predominance of elastic properties over viscous ones in the linear shear strain range. The addition of CNTs has been shown to significantly alter rheological properties. With increasing nanotube concentration, an increase in effective viscosity and consistency parameters is observed, along with a decrease in the flow index. The viscoelastic moduli (G′ and G′′) also increase, demonstrating a nearly linear dependence on the CNT content. A fundamental difference in the effectiveness of the two types of nanotubes was revealed. SWCNTs exhibit a significantly stronger modifying effect compared to MWCNTs. Thus, at the same concentration of 0.1 wt.%, the consistency parameter of the solution with SWCNTs is 2.2 times higher, and the elastic modulus G^, is 3.15 times higher than the values for the polymer solution with MWCNTs. This is due to the exceptionally high aspect ratio of SWCNTs, which ensures the formation of a strong percolating network at lower threshold concentrations. The results of the study demonstrate the high potential of carbon nanotubes for controlling the rheology of polymer solutions.

A gas-jet method for depositing diamond coatings with gas activation by a microwave discharge is considered. A distinctive feature of this method is the use of a high-speed jet to transport gases activated by microwave plasma in the discharge chamber to the substrate located in the deposition chamber. This approach opened up new possibilities in the field of gas-phase diamond synthesis, but its development required systematic research. The effectiveness of this method is demonstrated. The success of the study is due to a systematic numerical and experimental study of individual processes involved in the gas-jet synthesis of diamond coatings. Accumulated international experience in the development and optimization of the "classical" MPCVD method served as a starting point for choosing development and optimization paths for the gas-jet method. Methodologies were developed that allowed for studying the influence of individual processes and obtaining a number of new results in the field of gas-phase synthesis of diamond coatings. It was established that a gas-jet method using microwave discharge to activate precursor gases enables the deposition of diamond coatings on substrates made of various materials suitable for various applications. The possibility of producing diamond coatings with various structures was established. Gas-jet synthesis using a hydrogen-argon-methane mixture yielded diamond coatings with a growth rate of up to 200 μm/h. Diamond coating of molybdenum substrates significantly increased their erosion resistance compared to uncoated molybdenum and tungsten carbide. The analysis allowed us to summarize the results obtained during the development of this method and to understand the influence of various processes on the synthesis of diamond coatings.

This paper presents the results of numerical modeling of the effect of periodic volumetric heat release on the structure of supersonic turbulent flow in a flat channel with a step and the associated heat transfer with heat flux sensor elements embedded in the walls. The modeling is based on the Reynolds-averaged Navier-Stokes equations, supplemented by the k-ω SST turbulence model. The source power was varied harmonically with a period of 10 ms, and the oscillation amplitude was varied while the average power remained constant. It was established that the flow enters a periodic regime with a pronounced relaxation delay. During the phase of maximum heat input, an upstream shift in the wave structure and the formation of local subsonic zones in the flow core are observed. It is shown that periodic power variations influence local heat transfer. For a plate located in the expanding section of the channel, the periodic regime leads to more intense heating than with a constant source of the same average power. A nonmonotonic dependence of the integral heat flux on the oscillation amplitude was discovered. The maximum heat load is achieved at an intermediate, rather than maximum, heat input amplitude, which is explained by the dynamics of shock wave stabilization ahead of the plate.

This paper presents a comparative numerical simulation of supersonic plasma jets generated by a DC plasma torch using straight and angled nozzle configurations. A numerical analysis of the nozzle design developed by the authors is performed in the context of air-plasma powder spraying onto internal surfaces of small-sized components. The gas dynamics of mixing between the high-temperature jet of the plasma-forming gas and the flows of cold carrier and focusing gases is investigated within and downstream of the annular powder injection unit. Verification is carried out and comparative data on shock diamond positions are obtained for the case of a slightly under-expanded jet. The effect of wall cooling on jet enthalpy is calculated, demonstrating its significant influence in the case of the angled nozzle design. For this nozzle type, characteristic distances are determined at which the velocity and temperature profiles of the jet core become uniform and axisymmetric.

The ascent dynamics of a Taylor vapor bubble in a circular (cross-section diameter of 16 mm) and annular (cross-section diameters of 25 and 16 mm) channels at saturation pressure was experimentally studied. The bubble was formed by boiling a superheated degassed liquid. It was found that the velocities of its front and rear surfaces are variable and depend on the ascent mode, its position in the channel, and the previous ascent dynamics. It is shown that this behavior is determined by the spatiotemporal distribution of liquid temperature near the bubble boundary. Based on the obtained results, a map of ascent modes was constructed, linking the nature of the motion with the degree of liquid superheating near the vapor bubble surface.