Home – Home – Jornals – Thermophysics and Aeromechanics 2025 number 6

The results of a numerical study of laminar fluid flow and heat transfer in an annular gap between two cylinders with a rib on one of the walls are presented. A mechanism for intensifying heat transfer between two cylinders, when one is rotating, was studied. Intensification was achieved by installing a single rib on the inner or outer cylinder. The rib was oriented along the annular system radius and occupied half the height of the gap between cylinders. The problem was formulated in two dimensions. The Reynolds number was varied in the range from 100 to 1000. Air was the working medium. Four options for the location of the intensifying rib were considered. A fundamental difference in the structure of the recirculation zone was noted for different rib locations. Maximum heat transfer was achieved on the wall of the stationary cylinder opposite the rib mounted on the rotating cylinder. Its multiple increase was observed compared to the case of rotating smooth cylinders.

This paper examines interaction of propagating waves with gas bubbles contained in liquid and the behavior of gas bubbles in a gas-liquid mixture. A description of the experimental setup for studying the influence of a propagating wave on gas bubbles is presented and the observed phenomena are described.

The interaction of longitudinal vortices with a turbulent boundary layer and a mixing layer developing in the diffusion zone was experimentally studied. Vortices were generated using jet vortex generators located near the beginning of the model tail unit, forming a diffusion separation. The main measurements were performed using hot-wire anemometry (HWA) and PIV methods. Based on the experimental data, information was obtained on the flow velocity in the pre-separation and separation zones and the dynamic pressure magnitude; the scale of turbulent structures was calculated, and the influence of longitudinal vortices on these characteristics was estimated under various regimes of vortex generator operation. It was found that the effect of longitudinal vortices on the flow leads to a decrease in the separation zone and significantly affects the characteristic scale of turbulent structures.

Investigation results concerning the influence of fuselage and control surfaces deflection on various types of separation that occur in the flow past a swept-leading-edge wing model at subsonic flow velocities in a wind tunnel are presented. This work continues an experimental series on the study of separated flows and the possibilities of flow control around aircraft wing models at low Reynolds numbers. Using proven methods for visualizing the flow near the wing surface and hot-wire measurements, experimental data were obtained that provide a comprehensive understanding of the separation structures that arise on the leeward side of the wing model at various angles of attack and control surface deflections. The issue of flow separation control is also examined. It is shown that global flow separation can be eliminated using the method of local influence at specific points.

To generate a spray flow in confined spaces, specialized nozzles are required to disperse micron- and submicron-sized droplets at the nozzle edge. High-speed visualization of a gas-droplet flow from a custom-made microchannel nozzle device with a resolution of 2.5 μm/pixel was performed, allowing the sizes of the dispersed droplets to be determined. The nozzle device was a custom-made device with a 243 μm thick microchannel silicon membrane and a microchannel size of 10×10 μm². Measurements of the characteristic dimensions were conducted and the velocities of the dispersed droplets were determined at low liquid flow rates (0.05-2 ml/min) and air pressure differences from 1 to 6 atm. At HFE-7100 flow rate of 1 ml/min and an air pressure differential of 1 atm, the average droplet size was approximately 40 μm, while at a flow rate of 2 ml/min and a pressure differential of 2 atm, the average droplet size was 20 μm. A significant increase in velocity was observed with increasing pressure differential. At the minimum flow rate, very small droplets were dispersed, which were not detected at a resolution of 2.5 μm/pixel, but the overall flow was clearly visible as a "mist."

Using the numerical solution of the Navier-Stokes equations, the development of unstable disturbances and the transition to turbulence in a simulated supersonic jet flowing from a circular supersonic nozzle were studied. The simulation was performed in a three-dimensional formulation with resolution of the vortex structure of the flow. The results of numerical simulations were compared with available experimental data.

The transient effects that develop when supplying fresh air to a ventilated space through a jet oscillator were numerically studied. The results of parametric two-dimensional URANS simulations in the Reynolds number range of (5.3 - 53)×10³ showed that with an increase in the supply flow rate, the frequency of self-oscillations of the supply jet increases linearly, while the Strouhal number varies slightly: from 3.3·10^-3 to 5.4·10^-3. The use of an oscillator intensifies air mixing significantly compared to the basic option with a stationary air supply through a slot, which provides almost uniform temperature distribution in the room during cooling.

The results of hot-wire measurements of periodic controlled disturbances of the mass flow beyond the boundary layer on a flat plate are discussed, and their wave characteristics reflecting the acoustic properties of the supersonic boundary layer are estimated. The possibility of emitting subharmonic disturbances at a Mach number of 2.5 is demonstrated for the first time.

A flat methane-air mixture flame was numerically simulated in a longitudinal electric field at various fuel-to-fuel ratios. A computational code developed in the OpenFOAM based on the reactingParcelFoam solver was used for numerical calculations. The results showed that the predicted value of the ion current correlates with the experimental data. The best agreement is achieved in the lean (φ ≤ 0.8) and rich (φ ≥ 1.3) flames. The greatest discrepancy is recorded for the regime of fuel-air equivalence ratio φ = 0.9. The maximum ion current is achieved at φ = 1.1, which corresponds to the experimental results. The obtained data demonstrate the adequacy of the developed code in predicting the flame current value. Calculation data indicate an increase in CH concentration by 20.5% and O concentration by 2.0% when exposed to an electric field; the interaction of these components is the initiating reaction in the formation of charged particles in hydrocarbon flames.

Stationary perturbations of equations of the boundary layer stability are studied for subsonic and supersonic flow past a plate. For two-dimensional perturbations, the results obtained are in good agreement with existing literature data. Three-dimensional perturbations resembling longitudinal structures are studied for the first time. It is established that the amplitude of boundary layer perturbations of longitudinal velocity and temperature has a bell-shaped form, whose maxima position in the Dorodnitsyn-Howarth variables is conservative with respect to changes in the Mach number, Reynolds number, and wave number along the lateral coordinate. Moreover, the intensity of perturbation attenuation along the longitudinal coordinate and the phase shift of the boundary layer perturbation increase with an increase in the above quantities. The largest phase changes are observed for the lateral velocity.

Using a modified mean flux method, a numerical analysis was performed for highly asymmetric scattering phase functions. To solve the radiative transfer equation, the delta-Eddington approximation is used. Radiative-conductive heat transfer in a flat layer of an anisotropically scattering medium was calculated for the Henyey-Greenstein phase function and the transport approximation as an example.

T effect of wettability parameters of a porous heater material on heat transfer during boiling was investigated using a hybrid numerical model based on the lattice Boltzmann method and the heat transfer equation. By simulating the boiling process on a porous heater with a regular hexagonal structure of rectangular metal heat-conducting elements at various thermal heads, boiling curves and coefficients of heat transfer enhancement/degradation were obtained for lyophilic, lyophobic, and neutral heater materials. The results showed that at moderately low thermal heads, the heat transfer coefficient on the lyophobic heater is higher than that for a surface with neutral wettability due to earlier initiation of the vapor phase, while at high thermal heads, heat transfer on the lyophilic heater is the greatest. It was also shown that the temperature of boiling beginning is minimal for lyophobic heaters and maximal for lyophilic heaters, and the boiling crisis does not occur on a porous heater of any wettability even at the maximum heat heads considered.

The problem of ignition and combustion of a region expanding over time is considered. Combustion was initiated using a pulsed-periodic energy source. The influence of pulse frequency and flow velocity on ignition was revealed. Autoignition of a laminar hydrogen jet diluted with nitrogen in a co-current heated air flow was numerically simulated. A transient reacting mixing layer between two flows (air and fuel) with different velocities and temperatures was studied.

Percolation theory is widely used to study heat transfer and other kinetic processes in systems with disordered structures. This paper proposes a phenomenological approach combining methods of percolation theory and multifractal analysis to model a percolation cluster in a three-dimensional infinite medium with multifractal properties. Based on the developed model, unambiguous numerical values for topological critical indices and the critical conductivity index were obtained, previously unknown relationships between critical indices were revealed, threshold conditions that separate qualitatively different regimes of system behavior in the scales of correlation length were determined, and a relationship between the systems order parameter and the golden ratio was established, which indicates the universal nature of the identified dependences.

Percolation theory is widely used to study heat transfer and other kinetic processes in systems with disordered structures. This paper proposes a phenomenological approach combining methods of percolation theory and multifractal analysis to model a percolation cluster in a three-dimensional infinite medium with multifractal properties. Based on the developed model, unambiguous numerical values for topological critical indices and the critical conductivity index were obtained, previously unknown relationships between critical indices were revealed, threshold conditions that separate qualitatively different regimes of system behavior in the scales of correlation length were determined, and a relationship between the systems order parameter and the golden ratio was established, which indicates the universal nature of the identified dependences.

Experimental results on the temperature distribution in a jet of nitrogen flowing from a long pipe into a vacuum under laminar and turbulent flow conditions are presented. A temperature increase downstream was observed under turbulent flow conditions compared to laminar flow conditions. It is concluded that the temperature increase is associated with the turbulent energy relaxation.