Home – Home – Jornals – Thermophysics and Aeromechanics 2023 number 6

The paper presents the results of numerical simulations of high-velocity turbulent air flows in a plane channel with a variable cross section exhibiting sudden expansion with allowance for coupled heat transfer with copper plates modeling the sensitive elements of heat flux probes. The simulations are performed for conditions of a high-enthalpy short-duration wind tunnel whose specific features are the short duration of the test regime and unsteady “falling” conditions at the model channel entrance. The wave structure of the supersonic flow, which affects the heat fluxes at the walls, is analyzed for various Mach numbers at the model channel entrance. The numerical algorithm is validated on the basis of experimental data on heating of the sensitive elements of heat flux probes for unsteady input conditions at the channel entrance. The influence of the Mach number, static parameters, and stagnation parameters on the rate of heating of the sensitive elements located at various points in the channel is studied numerically. The heat fluxes calculated under constant and “falling” conditions at the channel entrance are compared. It is shown that the accuracy of heat flux modeling can be increased by taking into account the intensity of fluctuations of the flow parameters and their changes along the channel.

Fuel rod assemblies with dense packing are promising from the point of increasing the conversion rate and heat transfer in small modular reactors. The main feature of the flow in the dense packing is the formation of quasi-periodic large-scale velocity pulsations in a gap between fuel rods, which intensify mixing between the subchannels and greatly increase heat transfer between the fuel rods and the coolant. The large-scale pulsations relate directly to the pitch-to-diameter (P/D) ratio of the rod bundle and the Reynolds number (Re). In this work, the unsteady flow structure in a gap between a flat wall and three rods with a relative pitch P/D = 1.077 is experimentally studied using the PIV method with high time resolution. The averaged flow characteristics, including the three-dimensional ones, are presented. The influence of the Re number on flow oscillations in the gap was studied. The spatial most energy-intensive flow modes were analyzed using the POD method. The results obtained indicate the presence of several traveling waves propagating along the flow. Modulation of flow oscillations in the gap was detected. These investigation results are in good agreement with the results of other authors.

Turbulent flows in a flat diffuser are characterized by the presence of two local maxima in the profiles of longitudinal velocity pulsations. The mechanism of formation of a turbulent flow structure in a flat diffuser was experimentally studied. For this purpose, the parameters of the flow kinematic structure in a diffuser with an opening angle of 2.5° were measured. The profiles of velocities and turbulent characteristics of the flow in the typical cross-sections of the channel were obtained using the optical measurement method; based on these profiles the secondary flows in the diffuser were identified. A physical model of formation of the turbulent flow structure is proposed. Within the framework of this model, a high degree of turbulent pulsations far from the wall is associated with the convection of turbulence from the near-wall region into the flow core by a secondary flow in the form of an averaged spiral motion of the medium in a flat diffuser.

The paper presents the results of a numerical study of flow gas dynamics and integral parameters of the flow at the channel entrance located behind a conical or plane shock wave. The free-stream Mach number range is М = 2 - 4 and the range of the slopes of the compression surfaces of the wedge and cone is δ= 10 - 90°. Data on the flow structure at the channel entrance, mean-mass Mach numbers, total pressure loss, and flow rate coefficients are obtained. A comparative analysis of these parameters is performed, and advantages and drawbacks of the channel entrance positions in various types of the flow are noted.

The paper presents a method of molecular modeling of fluid transport coefficients, which is an alternative to the method of molecular dynamics. The transport coefficients are determined using fluctuation-dissipation theorems. The dynamics of molecules is calculated stochastically, with intermolecular forces being set using the appropriate created database. A distribution function of intermolecular forces is constructed and a formula is obtained for its analytical approximation. The method effectiveness is demonstrated by the example of calculating the viscosity and thermal conductivity coefficients of liquid argon and benzene. The obtained data are compared with the data of experimental and molecular dynamic modeling and their good agreement is established. With the same modeling accuracy, the developed method is shown to be significantly more time-efficient compared to the molecular dynamics method.

Supersonic gas-liquid jets of a coaxial atomizer at high liquid concentrations are studied experimentally. A complex of optical techniques is used for studying the droplet sizes: visualization and particle image velocimetry, laser Doppler anemometry, and Malvern Spraytec instrument. The research shows that the velocity and concentration profiles change with flow rate growth: an extended region with small droplet velocities appears behind the bow shock wave; in this case, the concentration decreases significantly slower than that at low liquid flow rates. A small increase in the jet energy at liquid flow rates greater than 100 l/h and a noticeable increase in the droplet size testify that the gas jet capabilities for breaking up the liquid in the described regimes are exhausted.

Rarefied gas flow into a vacuum through short linearly diverging and converging channels has been examined with the direct simulation Monte Carlo method. Solution to the problem has been suggested using complete geometric setup, with quite large areas on inlet and outlet of a model channel in examined geometry. A mass flow rate through the channel and flow field both inside the channel and upstream and downstream have been calculated in a wide range of gas rarefaction. These calculation results are comparable to corresponding data for the channel with constant cross section. A strong impact of channel geometry and gas rarefaction has been proved.

Experimental study was performed on dynamics of vapor bubble rising in the annular channel at subatmospheric pressure. The gas bubble is formed during boiling of an overheated degassed liquid in an annular channel restricted by two glass tubes with the diameters of 25 and 16 mm. It was demonstrated that the dynamics of vapor cavity while rising the vapor bubble in the annular channel demonstrates a qualitative difference from the dynamics for an ascending gas bubble. The behavior is similar to a Taylor vapor bubble behavior in a round tube with a small diameter. One of typical features of vapor cavity behavior in an annular channel is possibility of vapor cavity decay after bubble collapse during the pulsation flow mode.

The paper presents the results of experimental study for rising a cluster of monodisperse gas bubbles in viscous liquid with/without surfactant for the Reynolds number in the range Re = 0.01 ÷ 1. The influence of surfactant type on the dynamics the bubble cluster rise has been analyzed. The qualitative pattern of monodisperse bubbles cluster rise was defined as a function of initial volumetric concentration in the range С_V = 0,001 ÷ 0,04. New experimental data were obtained on velocity and drag coefficient for a compact cluster of monosize bubbles rising in a liquid with/without surfactant (both for contact and contactless type of bubble rising.

Reflections of hydraulic jumps on shallow water are studied. Theoretical criteria of the transition between the regular and Mach reflections are derived, and it is shown that there is a domain of wave incidence angles where both types of reflection are possible. Numerical simulations reveal a hysteresis of this transition, which is consistent with theoretical predictions. It is shown that the hysteresis can be obtained by smoothly varying both the angle of the wedge generating the hydraulic jump and the free-stream Froude number.

This work examines the transition from kinetic to diffusion combustion using optical diagnostic methods. Experimental data were obtained on the temperature fields, composition and velocity of gas near the leading edge of a hydrogen flame flowing from a 2×20 mm slit into the air. The distribution of the rate of combustion products formation, intensity of heat release and pressure was obtained using the method of balances in equations of energy, momentum and matter transfer. It is shown that during the transition to diffusion combustion, heat release along the flame length decreases more slowly than the rate of water formation.

The problem of hydrodynamic stability of a boundary layer with diffusion combustion is formulated in the Dan-Lin-Alekseev approximation and at constant Prandtl and Schmidt numbers; it is reduced to solving a system of the tenth-order ordinary differential equations with homogeneous boundary conditions. With Lewis numbers equal to unity, it may be lowered to the eighth order. In the inviscid approximation, the stability problem is reduced to the integration of a single second-order differential equation. Based on the obtained stability equations and calculations of stationary flow parameters, the stability of a supersonic boundary layer with diffusive combustion on a permeable plate with hydrogen supply through its pores is studied for the first time by direct numerical modeling. With the Mach number M = 2, the possibility of flame flow stabilization is established using calculations. It is shown that within the framework of the inviscid theory of stability, it is possible to obtain quite reliable data on the maximum degrees of the growth of disturbances.

The paper studies the quality of water spraying by a pneumatic injection system designed for water injection into the input system of a piston internal ignition engine. The system consists of a two-cylinder piston compressor with compression of water-air mixture phase in the compressor’s cavities. The piston compressor has connection to the atomizer through long channels. The droplet sizes were measured through automatic image processing by the Shadow Photography method. The velocity field of droplets was measured by the 2D-PIV method . Experimental results demonstrated that the injection system offers a high quality of spraying for the air/water mass ratio higher than 0.46. The value of Sauter mean diameter was less than 31.1 µm.

The research considers the microwave treatment of snow-and-ice mass with the stages of heating and melting. A nonlinear mathematical model for two-phase Stephan problem was developed for the case of laminar set of dielectrics. We offer approximate analytical solutions for taking into account the thermophysical and electrophysical properties of layers; this approach allows parametric analysis.

The papers analysis the flow patterns for air flow around cubic-shaped buildings. Experimental and simulation data were compared for the flow problems with different scales. Geometry parameters for the models can be varied from 0.025 to 6 m, meanwhile the range of Reynolds number for considered data is from 10⁴ to 10⁶. The study proves that the problem is scalable one: this creates a foundation for running the lab-scale wind tunnel experiments.

Using an ultrasonic interferometer in the temperature range from 293 to 393 K at pressures from 0.13 to 1.5-2.8 MPa, the speed of sound U was measured in helium-xenon gas mixtures with a helium content of 60.34, 71.72, and 85.32 at. %. The measurement errors of temperature, pressure and speed of sound were ±20 mK, ±4 kPa and ± (0.15-0.30) %, respectively. By approximating the experimental data for each composition, equations were obtained to describe changes in the speed of sound as a function of pressure and temperature over the entire measurement range. The existing reference and experimental data on the speed of sound in inert gases and He-Xe mixtures were analyzed. A method for calculating U of mixtures with a helium content above 71.7 at. % He to a temperature of 1500 K and a pressure of up to 7 MPa was developed.

Numerical simulation was applied to the processes of heat and moisture transfer and for ice-water phase transition in a season-thawed soil layer. Analysis was performed for consequences of natural wildfire on the soil temperature and the thawing depth as a function of water retention by soil for the condition of Siberian permafrost zone. Calculations demonstrate that the permafrost thawing depth increases due to burnout of the top organic horizon. The quantitative indexes of natural wildfire impact depend on water retention properties of the upper organic horizon of soil.

Using isothermal drop calorimeter, the enthalpy increment of the Na₁₅Pb₄ and Na₅₀Pb₅₀ alloys was measured and the heat capacity was determined in the temperature range of 420-1075 K, including the solid and liquid states. It has been established that the values of the heat capacity of melts significantly exceed the calculations of this value according to the laws for an ideal solution, and this difference decreases with increasing temperature. The obtained results confirm the currently known fact that various complexes with a partially ionic character of the interatomic interaction are formed in melts of alkali metals with lead systems.