Home – Home – Jornals – Thermophysics and Aeromechanics 2023 number 5

The article considers the development of a near-wall separation vortex, arising at a supersonic flow around the external dihedral angle due to the pressure drop between its faces, in the range of angles of attack α = 0.5° - 6°. The processes of origin and development of separation and secondary vortices at the angle increase are investigated in detail. Particular attention is paid to the flow structure change in the vortex location zone. A clear violation of the flow self-similarity in the front part of the model in the zone of vortex system formation is shown.

The results of experimental and computational studies of the processes of hydrodynamics and heat transfer of a swirling flow, heated by the counter coolant flow in the heat exchange channel under standard technological parameters of a nuclear power plant are presented in this paper. The temperature field of the heat exchange channel as a whole and the coefficient of hydraulic resistance of channels with twisted tapes of a constant swirl pitch were obtained in experiments. The numerical study was carried out using the domestic software package LOGOS and the Ansys CFX complex. The simulations were performed using the k-ω SST turbulence model, corrected for streamline curvature and rotation. Two versions of the calculation grid were developed. A comparative analysis of the calculated and experimental values of the hydraulic friction coefficient, the swirling flow temperature, and the heat transfer coefficients from the wall to the swirling flow was carried out. The analysis allowed identification of the strengths and weaknesses of the calculation methodology implemented in the domestic package. Deviations of the obtained values were compared. There is good agreement between the calculated and experimental data, as well as with the data based on generalized dependences. One of the most important conclusions of the study is the need to modernize the process of solving the coupled heat transfer problem in the LOGOS package to expand the range of problems to be solved

Results of studying the process of air blowing through a perforated section of the surface on an axisymmetric body with an aspect ratio of 25.3 in an incompressible flow with the Reynolds number Re_L = 4.36·10⁶ are reported. The blowing coefficient C_b is varied in the interval from zero to 0.00885. It is shown that distributed blowing through a perforated wall with improved geometry ensures a significant gain in friction drag as compared to that for the base configuration. Beginning from the input boundary of this section and further downstream, stable reduction of local friction is observed, which reaches 72% directly in the region of blowing with the maximum intensity. In view of the energy expenses on the blowing process, for the blowing region being located on the cylindrical part of the model, the degree of energy saving can reach 1.4 to 6.1%. The efficiency of this method of boundary layer control can be refined by more accurate determination of the contribution of the drag component induced by the pressure and friction forces on the frontal part of the body. The importance of estimating the possibility of using the proposed approach in the case of air blowing through a surface section on the frontal part of the body is noted.

The effect of wall treatment on the performance of κ-ε model in incompressible, turbulent, separated flows with and without heat transfer has been evaluated in this study. We have simulated two benchmark cases: (i) flow past a circular cylinder at Re = 3900, and (ii) flow past a heated square cylinder at Re = 21 400 using the open source CFD package: OpenFOAM. We have compared three variants of the κ-ε model namely, Launder-Sharma κ-ε model (Yap corrected) (LSKEY), Lam-Bremhorst κ-ε model (Yap corrected) (LBKEY) and two-layer κ-ε model (TLKE) along with the available experimental and direct numerical simulation (DNS) data. Comparisons are made in terms of the models' capability to predict the mean flow variables, surface integral quantities and heat transfer characteristics at different wake locations. On the basis of the presented study, we conclude that LSKEY performs better than the other models in predicting the wake and surface flow and heat transfer parameters. Further our comparisons show that, while LSKEY and LBKEY require comparable clock time per flow-through cycle, the computational time needed by TLKE is almost twice as compared to LBKEY or LSKEY. These results call for more attention from the CFD community onto the LSKEY model, in particular, so that, it can be incorporated in various other flow fields, especially the scale resolving methodologies like the partially-averaged Navier-Stokes (PANS), wherein a superior wall treatment along with a shorter computational time could be of immense advantage. In authors' opinion, these benefits of the LSKEY model have largely been overlooked, perhaps because of a biased preference to the TLKE model, which enjoys the default presence in popular commercial computational fluid dynamics (CFD) packages.

An experimental study of the process of air outflow from a container with a characteristic duration of 0.6 to 9 s through throttling tubes of various configurations was carried out. The equivalent area of the outlet orifice is determined depending on the ratio of the length of the throttling tube to its nominal diameter. It was found that during the outflow process, the temperature of the gas inside the container decreases by 10-15%, which value differs significantly from the theoretical estimate made under the assumption that the process is adiabatic (60 %). Based on the results of measuring the pressure and temperature of the gas in the container, a method is proposed for calculating the heat flow from the walls of the container to the outflowing gas.

A novel thermal lattice Boltzmann approach is proposed for the implementation of the thermal boundary conditions at the fluid-solid interface. The numerical code, developed on the basis of the new approach, was validated against reliable numerical data from the literature in the cases of both square and circular conducting blocks. Analytical and experimental validations were also performed in the case of a circular block. The numerical tests show that the adopted approach allows to handle interface problems with large thermal conductivity ratios. In the present study, this approach is validated first in the case of a square conducting block and used to simulate a conjugate convection-conduction problem in a square cavity enclosing a circular block. The novel developed TLBM approach reduces computational memory as well as numerical programming issues associated with the use of a hybrid method that combines the lattice Boltzmann method and classical methods.

The study is aimed at measuring the gas flow temperature by thermocouples whose time of reaching the equilibrium temperature is comparable with the measurement time, and heat release to structural elements of the transducer can be rather large. Results of numerical simulations of the gas flow in the temperature transducer used for measuring the stagnation temperature in high-enthalpy hotshot wind tunnels are presented. A coupled problem of the air flow around the temperature transducer is solved, and the flow field inside the stagnation chamber is calculated with allowance for heat losses to input wires and structural elements of the transducer. The data obtained are considered as results of a virtual experiment and are treated by methods of experimental aerodynamics. The retrieved results are compared with the initial numerical values of the stagnation temperature in the flow impinging onto the transducer. Sources of uncertainties arising in temperature measurements are determined, and the applicability of experimental methods for determining the stagnation temperature in short-duration wind tunnels, including those with parameters decreasing during the run, is justified. It is shown that the method of “two thermocouples” can be successfully used to determine the stagnation temperature even if the heat losses to transducer elements are comparable with heat input from the gas flow. The values of the retrieved stagnation temperature correspond to the flow temperature in the transducer within 1.2 - 3% depending on the initial temperature of the thermocouple.

With the aim to model the self-excited oscillations of a body, a hypothesis is proposed for the formation of periodic vortex structures in the bottom wake whose frequency coincides with the natural frequency of oscillations of the body, and the force effect of the oscillations on the body is mathematically described with a harmonic function of time. Analytical formulas for aerodynamic derivatives and equivalent aerodynamic derivatives are obtained. It is shown that the mathematical model satisfactorily describes the dependence of the pitch angle on time and the dependence of the equivalent aerodynamic derivatives on the amplitude of oscillations for two moments of inertia of the body. The mathematical model predicts the hyperbolic law of dependence of the amplitude of self-excited oscillations on the reduced frequency.

The morphology of carbon material formed in an arc discharge in a mixture of i-butane, n-butane and propane when spraying a graphite-nickel electrode was studied. The experiments were carried out with changing the gas medium pressure. Carbon globules, graphene structures and carbon nanotubes have been discovered. It was found that at pressures of 75 and 400 torr, carbon globules predominate in the resulting materials. At gas pressures of 200 torr, the material collected from the cold screen surface contains both graphene-like structures and significant amounts of carbon nanotubes. The physical reasons influencing the observed phenomena are discussed.

The paper presents the calculation estimates for efficiency of regenerative cooling for a model cylinder-shaped duct using a suspension of heat-conductive metal nanoparticles in n-decane as fuel/coolant. We adapted a standard mathematical model of conjugated heat transfer that accounts for thermophysical properties of the metal nanoparticle suspension and n-decane. The data are presented for heating the nanosuspension and the model duct walls for the cases of different content of metal nanoparticles in nanosupension. The range where the use of nanosuspension gives advantages in terms of heat removal relative to the n-decane has been shown.

Heat conductivity for a Novec 7100 fluid sample was measured with a method of coaxial cylinders. Experiments were performed in the temperature range 350 - 385 K and pressure range 0.12 - 0.21 MPa. The error for experimental data on heat conductivity is about 1.5 - 2.5 %, The error in measuring temperature and pressure was less than 0.05 K and 4 kPa, correspondingly. The general equation for calculating the heat conductivity as a function of pressure and temperature was formulated. The heat conductivity was defined for the ideal gas state. A previously developed approach was tested in application for a single-measurement prognosis of heat conductivity.

Numerical simulation of unsteady processes proceeding during the laser welding of plates made of porous and monolithic (non-porous) metals was carried out. The influence of the welding speed on the quality of the resultant joints and on the seam morphology was studied. The calculated characteristics of connections between porous and monolithic stainless steel plates are in qualitative agreement with the results of physical experiments.

Physicochemical and thermal processes occurring during methane conversion into synthesis gas under non-isothermal conditions in microstructural heat exchanger-reactors based on microchannels are considered in this paper. A method for synthesizing a rhodium-based composite thin-layer catalyst for steam reforming of methane and carbon monoxide is proposed, and the results of experimental and numerical studies of the features of steam reforming under controlled thermal conditions of a microchannel reactor are presented. The determining influence of thermal processes on the rate and sequence of multi-stage heterogeneous reactions was obtained; the methods for controlling the steam reforming process have been developed to achieve high completeness of chemical transformations.

The paper presents results of experimental study on the solubility (VLE-properties) for crystalline tricosane in supercritical carbon dioxide (in pure and modified form) in the temperature range 308.15 - 315.15 K and the pressure range 8.00 - 20.32 MPa using the dynamic method. Modification of carbon dioxide fluid was performed by adding several organic solvents, such as dimethylsulfoxide, ethanol, acetone, and chloroform. It was found that using the fluid co-solvents at the concentration of 5 wt. % improves the tricosane solubility by factor of two. The measurement results can be described by the Penga-Robinson equation of state.

The volumetric properties of liquid indium-lead alloys containing 20 and 33 at. % Pb have been measured using gamma-ray attenuation technique at temperatures from the liquidus line to 880 K. The density changes of these alloys during solid-liquid phase transition have been calculated. The obtained experimental values of the molar volume and the volume thermal expansion coefficient of melts and the results of calculations according to the laws for an ideal solution and data of other authors have been compared.