Home – Home – Jornals – Thermophysics and Aeromechanics 2024 number 3

The work is devoted to modeling the disturbance propagation in viscous incompressible laminar boundary layers, using linearized equations for disturbance amplitudes. Along with the numerical model based on original linearized equations, the article considers three models based on equations derived from the original ones by neglecting the streamwise pressure gradient, or the streamwise viscous terms, or both. The models are compared numerically by the example of generation and propagation of disturbances in the boundary layer over a slightly concave plate. Conclusions are drawn about the feasibility of the same simplified models to adequately simulate both Tollmien-Schlichting waves and Görtler vortices in a range of practically important parameters.

The paper presents results of a theoretical study for parameters of a compressible boundary layer for the case of a re-entry space vehicle flying in atmosphere with the free flow at Mach 6≤M≤10 with sublimation of the carbon coating (graphite). Since a high flight velocity results in a higher wall temperature and a higher mass loss rate, the effect of wall material evaporation decreases the wall temperature as compared with the case of zero-sublimation flow. All that increases the gas mixture density in the sublimation vapor cloud nearby the wall; this is beneficial for stability of the high-speed boundary layer in the response to the first-mode disturbance. As for the second mode disturbances, the lower values of spatial amplification rate with increasing Mach number is observed due to the surface material sublimation. The position of laminar-turbulent transition was evaluated using the e^N method. Our computations demonstrated that (for a flow with M = 6) the surface sublimation has no influence for laminar-turbulent transition; this transition is governed by the growth of three-dimensional (3D) first mode disturbances. At higher Mach numbers (for M = 8 and higher) we observe that the disturbance amplification rate in the downstream direction becomes smaller. The transition is driven by a 2D second mode. The graphite coating sublimation has destabilizing influence for the second mode; that accelerates the boundary layer transition to turbulence.

Velocity fields and air flow structures have been experimentally established in a local working area with a horizontal panel heated by a gas infrared emitter in the modes of free and mixed (during operation of the air exchange system) convection using the optical SIV (Smoke Image Velocimetry) method. The analysis of the influence of the structure of the air flow, resulting from the heating of surfaces located in the local working area, on the formation of air velocity fields in the operating conditions of a gas infrared emitter was carried out. The extent of the influence of the convection regime on the aeromechanics of the local working area heated by a gas infrared emitter has been established.

A pioneering experimental study of the influence of distributed suction of a supersonic boundary layer on a flat plate on its stability to controlled (artificial) disturbances at the freestream Mach number М_∞ = 2 is performed. Experimental results are compared to numerical predictions, and good quantitative agreement is observed. The conclusions of the linear stability theory that suction of the type considered in the study stabilizes the flow in a supersonic boundary layer are experimentally validated; moreover, the stabilizing effect of suction is more pronounced than the destabilizing effect of surface porosity.

Tests of a streamlined cone with a rear hemispherical part were carried out in a supersonic wind tunnel using an installation of free oscillations along the pitch angle at Mach number M = 1.75 and for several values of the body inertia moment (calculated relative to the axis of rotation). In all tests, after the completion of the transient process, undamped oscillations of the cone were recorded with an amplitude depending on the reduced frequency. The dependence for the of undamped oscillation amplitude on the reduced frequency exhibits a pronounced resonant behavior.

The paper presents the results of the study of effective turbulent heat transfer along a rod bundle at its transverse streamlining, performed using the CONV-3D code based on the DNS approach. To determine the effective heat conductivity coefficient, the mixing of two plane-parallel coolant flows, moving at the same velocities and having different inlet temperatures in the working area was simulated. Comparison of the calculation results with experimental data for water has confirmed the usability of numerical modeling instead of real experiments. Such a replacement of a real experiment with its computational analogue is relevant for investigation of liquid metal coolants.

The action of a pair of weak shock waves on a supersonic boundary layer on a swept flat plate with a bluntness radius of the leading edge equal to 2.5 mm at the Mach number 2 is experimentally studied. Transverse hot-wire measurements are performed in the boundary layer with a fixed distance of the probe from the model surface. It is found that a change in the sweep angle of the leading edge from 35 to 45 degrees reduces the intensity of the action of the weak shock waves on the boundary layer flow. As the sweep angle of the leading edge increases to 50º, the weak shock waves no longer affect the flow in the supersonic boundary layer on the swept plate.

The paper presents the results for simulation of heat transfer between the laminar axisymmetric methane/air flame and a cold flat surface; simulation was performed for the distance between the burner exit and the flat surface equal to the one, two, and three exit calibers. The study is based on the direct numerical simulation with a detailed kinetic mechanism with the GRI-MECH 3.0 code. Simulation for the configuration with the distance of three calibers demonstrated a significant reduction in heat flux nearby the front point; this is due to a local recirculation zone between the flame cone and the flat surface. This phenomena can explain the experiment-observed reduction in heart transfer.

A modified method of transfer units is developed for a countercurrent film cooling tower with a structured tubular packing with surface intensifiers in order to determine the thermal efficiency of the gas and liquid phases and the temperature of the cooled water at the output. The approach of presenting the number of transfer units, taking into account additional terms with reverse mixing coefficients is applied to indirectly consider the hydrodynamic structure of flows and a decrease in the heat and mass transfer efficiency, compared with the ideal displacement model. An experimental installation with a layout (column) of a Plexiglass cooling tower with a diameter of 200 mm and a height of 2 m is described. Experimental data for water cooling in a structured packing block in the form of a vertical bundle of tightly packed polyethylene pipes with a diameter of 0.05 m with an annular discretely structured surface roughness are presented. Generalized calculated empirical expressions for the drag of dry and irrigated pipes, as well as the dependence of the volumetric mass transfer coefficient on air velocity at different irrigation densities, are obtained. The parameters of expression of the modified number of transfer units are identified based on experimental data on thermal efficiency in the gas phase. As a result, the dependence of the thermal efficiency in the gas phase on the pressure and design characteristics of the structured packing is obtained taking into account the reverse mixing of the flows. Reverse mixing is shown to reduce thermal efficiency by 8 - 15 %, which must be taken into account in the calculations of film cooling towers. The calculation results for the SK-400 industrial cooling tower using the presented expressions are provided and the agreement of the thermal efficiency of the cooling tower with the calculation according to the proposed method is shown.

The effect of capillary-porous coatings obtained by the method of directed plasma spraying on the dynamics of evaporation and heat transfer during nitrogen boiling under conditions of steady-state heat release on copper tubular heaters with a diameter of 16 mm was experimentally studied. It is shown that the presence of coatings leads to an increase in the critical heat flux relative to a smooth heater and heat transfer intensification by the factor of up to 3.5 in the region of low heat fluxes. According to the analysis of high-speed video filming, intensification of heat transfer at low heat fluxes is associated with a significant activation of stably operating nucleation sites. With a subsequent increase in the heat load, intensification relates to a significant contribution of high-intensity heat transfer in the macrolayer zone in local areas between the ridges of structured coatings. It is shown that there is a decrease in the slope of boiling curves of the modified heaters in the pre-crisis regimes, determined by the pulsating behavior of interfaces and accompanied by significant fluctuations in the surface temperature.

The article presents the results of pulse heating measurements for the thermal resistances of contacts of liquid lead with Al₂O₃ ceramics and steel in gaps, modeling a heat conducting liquid-metal sublayer in the developed fuel rods of fast reactors of a new generation. The method of obtaining and processing experimental data is described, the results of estimating the measurement error are presented, and the dependence of thermal resistances of liquid lead contacts in model gaps on temperature and number of melting and crystallization of lead is investigated. Based on the experimental results, the thermal resistance of a heat conducting liquid-metal sublayer in fuel rods is evaluated.

The experimental results on the structure of the two-phase “liquid metal-gas” medium in vertical channels depending on the gas flow rate and channel diameter are presented. Lead-bismuth melt at a temperature of 160°С was used as a liquid medium, and argon was used as a gas phase. Data were obtained on the shape of gas bubbles, temporary changes in the gas content in channels, histograms of gas content distribution, and features of the slug flow of gas in the metal melt.

The results of an analytical solution to the problem of heat distribution inside a massive solid sample with concentrated heat supply to this sample surface are presented. Analytical expressions for the non-stationary temperature distribution inside the body are obtained using the integral cosine Fourier transform and the Hankel transform. Examples of solution application for estimating the characteristic times of reaching the Chernov points Ac₁ and Ac₃ in model hypoeutectoid steels under the effect of laser radiation are presented. The application of this solution to calculating the cooling dynamics of ceramic Al₂O₃ and SiO₂ samples, affected by the air and water jets, is demonstrated.

The structure of a gas-liquid flow and interfacial mass transfer during the transition from the slug to the bubbly flow of ethanol-CO₂ mixture in a horizontal straight microchannel were experimentally studied. The experiments were performed for a channel with a rectangular cross section of 141×385 μm in the range of superficial gas and liquid velocities J_G = 0.16 - 0.8 m/s and J_L = 0.22 - 0.5 m/s. To determine a change in the volume of the elongated bubble along the microchannel length due to CO₂ absorption, the method of high-speed visualization with subsequent image processing was applied. The bubble frequency, velocity, size, and change in the volume of gas slugs and bubbles along the channel were measured, and volumetric coefficient of mass transfer from liquid k _L a was calculated. The measured volumetric mass transfer coefficient was compared with the known correlations and a new correlation was proposed for the transition from the slug to the bubbly flow due to interfacial mass transfer.

Numerical studies of aerodynamics and heat transfer in a vortex burner during flame oxygen combustion of pulverized coal are presented. The proposed numerical method has been tested using experimental data on oxygen combustion of coal in a flow. The influence of oxygen concentration in the blast on the processes of ignition and combustion of coal dust in a nitrogen-free environment has been considered. It has been established that for the burner under study, an increase in oxygen concentration from 40.1 to 66.7 vol. % leads to a change in the flow structure, an extension of the flame size, and an increase in the average value of unburned solid carbon concentration from 0.00136 to 0.4 g/m³ at a distance of 1.5 m from the burner.

The flow around a stationary gas Taylor bubble at downward flow velocities from 0.15 to 0.3 m/s in a vertical tube with a diameter of 20 mm was experimentally and numerically studied. Three-dimensional calculations were performed using the VOF (volume of fluid) method in the OpenFOAM package with application of the unsteady κ-ω SST turbulence model. Hydrodynamic characteristics of the flow were experimentally studied using the electrodiffusion method. The effect of flow velocity on the change in the shape of the gas Taylor bubble nose was shown. The calculated and experimental data were compared and their good agreement was shown. The distribution of velocities in liquid and gas was studied as well as the distribution of the liquid film thickness around a gas Taylor bubble. It is shown that the wall shear stress in the liquid film around a gas Taylor bubble does not depend on the downward flow velocity.

The results of thermodynamic calculations and experimental studies on hydrocarbon gas pyrolysis at atmospheric pressure in a combined plasma-chemical reactor with the production of hydrogen and carbon black with nano-carbon structures are presented. The plasma pyrolysis technology consists of heating hydrocarbon gas in an electric arc combined reactor to a temperature that ensures its dissociation into hydrogen and carbon black in a single techno-logical process.

Gas-driven atomization is the major approach for production of metal powders. This method gives up to 70 % of the entire metal powder production. However, modern trends demonstrate new requirements to the particle size distribution. This drives the development of novel methods for power production. Here we present the plasma-jet atomization method.