Thermophysics and Aeromechanics最新文献

Experimental study was performed for the dynamics of vapor bubble rising in the annular channel at subatmospheric pressure. The gas bubble is formed during boiling of superheated 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 the possibility of vapor cavity decay after the bubble collapse during the pulsation flow mode.

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

The paper presents the results of experimental study for rising a cluster of monodispersed gas bubbles in viscous liquid with/without surfactant for the Reynolds number in the range Re = 0.01–1. The influence of the surfactant type on the dynamics of bubble cluster rising has been analyzed. The qualitative pattern of monodispersed bubble cluster rising was defined as a function of initial void fraction in the range C_V = 0.001–0.04. New experimental data were obtained on velocity and drag coefficient for a compact cluster made of monodispersed bubbles rising in a liquid with/without surfactant (both for contact and contactless type of bubble rising).

The paper considers the aerodynamics of flow around a cubic model building. 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, while the range of Reynolds number for considered data is from 10⁴ to 10⁶. The scalability of the modeling is confirmed, which is beneficial for validity of laboratory aerodynamic experiments.

Fuel rod assemblies with tight lattice bundles are considered promising for increasing the conversion rate and heat transfer in small modular reactors. The main feature of the flow in a tight lattice rod bundle is the formation of quasi-periodic large-scale velocity oscillations in the gap between fuel rods. These oscillations enhance mixing between the subchannels and significantly increase heat transfer between the fuel rods and the coolant. The large-scale oscillations are directly related to the pitch-to-diameter (P/D) ratio of the rod bundle and the Reynolds number. In this study, we experimentally investigate the unsteady flow structure in a gap between a flat wall and three rods with a relative pitch P/D = 1.077 using time-resolved particle image velocimetry (TR-PIV) technique. The obtained TR-PIV velocity vector fields were used to analyze flow characteristics, including two- and three-dimensional mean velocity, velocity fluctuations, and Reynolds stress profiles. We also examined the influence of the Reynolds number on flow oscillations in the gap. The spatial most energy-intensive flow modes were further analyzed using the proper orthogonal decomposition (POD) method. Our results indicate the presence of several traveling waves propagating along the flow. Modulation of flow oscillations in the gap was observed. These findings are consistent with those of other researchers.

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 burnouts in the top organic horizon. The quantitative indexes of natural wildfire impact depend on water retention properties of the upper organic horizon of soil.

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.

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.

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

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 the 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 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 the 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.