Home – Home – Jornals – Thermophysics and Aeromechanics 2021 number 6

The three-dimensional structure of the flow in the near field of a wake behind a cylinder in a slot channel was experimentally investigated. Based on direct measurements of three-component volumetric velocity distributions, the ave-raged flow structure in a wake behind the cylinder is presented for the cylinder height-to-diameter ratio of 0.4 and Reynolds number Re _D = 3500. It is shown that the horseshoe vortices formed in front of the cylinder affect the flow structure significantly. It is found that two quadrupole distributions of mean longitudinal vorticity are formed in the near wake.

Various studies have shown that a baffle installation can enhance heat transfer in the backward facing step flows. It increases the pressure drop in the channel. In this study, the main focus is to find the best installation location and orientation for a given baffle to maximize its thermal performance. A steady incompressible laminar flow is considered in a channel with an expansion ratio of 2. The bottom wall of the channel is partially heated with a constant heat flux. For numerical modeling, the Navier-Stokes equations were solved using the finite element method. Two new concepts entitled the maximum temperature constraint and the performance evaluation parameter (PEP) were defined to characterize the problem. Grid independence study was performed, and the numerical simulation was validated successfully with the published results. As the main result, a small zone close to the step was identified for the baffle installation which gives higher values of the PEP and constrained PEP. It was shown that under the present circumstances, the case ( X _b, Y _b, a ) = (0.3, 0.9, -15°) gives the highest heat transfer enhancement (75 %) and the case ( X _b, Y _b, a ) = (0.3, 0.9, 30°) is the most optimum case from the thermal performance point of view with constrained PEP which is of 1.257.

The article presents the results of studies of a coolant flow behind the mixing intensifier grids of TVS Kvadrat fuel assemblies of the PWR. The purpose of the work is to evaluate the efficiency of mixing the coolant behind the intensifier grids of various designs and to choose their optimal design. To achieve this goal, a number of experiments are carried out on the aerodynamic research stand with scale models of fuel rod bundle fragments of fuel assemblies with mixing intensifier grids, equipped with turbulizing deflectors of various profile shapes. The cells located near the guide channel and regular cells are selected as the research area. The choice of the research area is due not only to the need to obtain a hydrodynamic picture of the coolant flow in characteristic cells and the choice of the optimal shape of the deflector, but also due to the necessary assessment of the influence of transverse coolant flows from the area of the guide channel on the flow motion in adjacent cells. The coolant flow pattern is represented by vector fields of transverse velocities, cartograms of the distribution of transverse and axial velocities, as well as graphical dependences of the distribution of the flow velocity components. Analysis of the spatial distribution of transverse and axial flow velocities allows studying and detailing the flow pattern of the coolant. Evaluation of the efficiency of the coolant mixing behind the grids and determination of the optimal shape of the deflector profile are carried out on the basis of a comprehensive analysis of the hydrodynamic pattern of the coolant flow and the parameters of intracellular vortex formation and intercellular mixing. The experimental results can be used in the engineering justification of structural solutions for the design of active zones of PWR reactors with TVS-Kvadrat. The accumulated database of experimental data is used for verification of CFD programs (both foreign and domestic development), as well as programs for thermal-hydraulic cell-by-cell calculation of active zones.

Two wind tunnels were employed for study of a thick teardrop airfoil with the relative thickness of 40 %. Experiments were conducted for Reynolds number in the range from 2 10⁴ to 12 10⁴ and the angle of attack from -10° to +10°. The weight-in-flow measurements and PIV-method visualization were performed. Experiments revealed a strong impact of the free-stream pulsations on the aerodynamic force coefficients. The following phenomena were demonstrated: drag crisis, hysteresis for the lift force vs. angle of attack, and reversing for the lift force direction. Data analysis investigates the roots of these effects.

The influence of the Pitot probe on total pressure measurements in the near-wall flow in the zone of reattachment of a supersonic separated flow past a compression corner is considered. If the total pressure is measured near the model wall, a local maximum is observed in the region downstream of the reattachment line. This maximum can be either a physical structural element of the separated flow (high-pressure layer in which the total pressure reaches 0.8-0.95 of the free-stream total pressure) or a possible measurement error. The present study reveals the existence of a measured total pressure peak in the boundary layer on a horizontal flat plate and flat wedges. This peak is not associated with the high-pressure flow, but is rather a result of probe interaction with the model wall. The amplitude of this peak is found to depend on the ratio of the probe size and the boundary layer thickness. It is shown that the degree of the probe influence leading to distortion of the measurement results is smaller approximately by an order of magnitude than the maximum value of the measured total pressure in the high-pressure layer in the reattachment zone.

Results of numerical simulations of a supersonic (М_¥ = 7) flow around a cylinder with a gas-permeable frontal insert made of a high-porosity cellular material are reported. A toroidal skeleton model of a high-porosity medium is developed and implemented to describe air filtration in the gas-permeable insert. The aerodynamic coefficients of the cylinder model for various angles of attack ( a = 0¸15°) are obtained. They are found to agree well with available experimental data, which confirms that the proposed skeleton model adequately describes the real properties of high-porosity materials.

Microfluidic liquid-liquid flows exhibit a wide range of different flow patterns. The most important point in practical applications is the transition from the segmented to the continuous flow patterns, as well as the prediction of this transition for an arbitrary combination of fluids. This paper presents a detailed analysis of the existing experimental data on flow patterns of immiscible liquids and provides data generalization through dimensional analysis. It is shown that the previously proposed criterion (We^0.4Oh^0.6) composed from Weber and Ohnesorge numbers provides a prediction on transition continuous-to-segmented flow with good accuracy if viscosity ratio λ = m _d/ m _c is less than unity. For the viscosity ratio λ > 1 this criterion ceases to work, and additional experimental data are required to construct a generalizing parameter for microfluidic flow in such systems.

The present study aims at obtaining the optimum contoured crater for a cylindrical-based film cooling hole with maximum area-averaged cooling effectiveness via computational fluid dynamics and optimization method. The influences of 5 geometrical parameters, which depict completely the dimensions of the contoured crater, were discussed through performing the orthogonal experiment design and range analysis. The optimum designs at blowing ratios of 0.5 and 1.5 were obtained by using the range analysis and the genetic algorithm combined with back propagation neural network respectively. From the analysis of the results, it can be found that the latter optimization method outperformed with higher area-averaged cooling effectiveness at both blowing ratios. The area-averaged cooling effectiveness of the optimized cratered holes were improved by 17.21 % at blowing ratio of 0.5 and 101.96 % at blowing ratio of 1.5, respectively, compared to those of the reference geometry. The improvements on the film-cooling performance were explained in terms of the flow filed and the vortex structures.

The wetting of a surface with a combined texture is investigated. Contact angles are measured for various surface textures. The method for hydrophobic texture formation using a combination of mechanical punching and patterning the surface made on the basis of polymers and aluminum oxide nanoparticles is proposed and analyzed.

The regularities of liquid film formation in the vicinity of capillary holes of a die when starting the droplet generator, as well as the regularities of film rupture and the formation of jets on its surface are experimentally and theoretically investigated. It is shown that the transient processes of film formation and rupture last several seconds, and the defining mechanism of film rupture is the development of bending instability of the jet nucleus.

The paper presents a problem of one-dimensional flow of water and steam separated with an interface; the flow is arranged in an inclined porous layer and subjected to lateral heat flux. The analytical stationary solution was found and the properties analyzed. Instability of flow for a certain region of parametric space was found and the nature of instability was described.

In this paper, energy and exergy analysis has been done for irreversible Brayton cycle with regenerator, reheater, and intercooler. In this work, the influence of the different parameters such as the efficiency of cycle's components surveyed based on the first and the second laws of thermodynamics. The lost exergy in the different components and the total lost exergy of the irreversible Brayton cycle are calculated under several conditions. Also, the optimum pressure of the intercooler and the reheater are obtained under different conditions. To obtain the optimum pressure, irreversible Brayton cycle with regenerator, reheater, and intercooler is simulated in engineering equation solver software and optimum pressure is obtained based on the first and the second laws of thermodynamics in each simulation. The obtained optimum pressures are compared with the geometric mean of the low and the high pressure of the cycle in each simulation.

Results of a numerical study of ignition of a cold supersonic (М_jet = 1.46) hydrogen jet surrounded by an annular supersonic (M_air = 1.86) jet of hot vitiated air expanding into a submerged space are reported. The simulations are performed under the experimental conditions of Cohen and Guile (1969) based on the Reynolds-averaged Navier-Stokes equations supplemented with the k-w SST turbulence model, a detailed kinetic mechanism of hydrogen combustion in air, and various models for taking into account the turbulence-chemistry interaction. The calculations are performed in the ANSYS Fluent 2020 R1 software in a transient two-dimensional axisymmetric approach by using a pressure-based solver. The instantaneous, mean, and RMS components of the main aerodynamic parameters and species mass fractions are obtained. A detailed comparison of the calculated profiles of the Mach number, total temperature, and species mass fractions along the jet axis and in several jet cross sections for non-reacting and reacting flows with experimental data is performed, revealing reasonable agreement in all parameters. It is demonstrated that the use of the transient approach combined with a detailed kinetic scheme makes it possible to reproduce the vortex structures developing at the combustion layer boundary, which make a significant contribution to hydrogen-air mixing, and, thus, affect the hydrogen combustion process.

The article presents experimentally obtained temperature dependences of the degree of blackness of metals of group VIII of the periodic table: cobalt, nickel, palladium and platinum.

Using the methods of mathematical modeling in a single-phase formulation of the Stefan problem, the problem of melting of an ice layer, which scatters radiation, was formulated and solved. To solve the radiation part of the problem, a modified method of mean fluxes was used. Anisotropic scattering was taken into account using the method of expansion of the scattering indicatrix in terms of Legendre polynomials. The influence of the scattering direction on the rate of ice layer melting is shown. Satisfactory agreement of the calculation results with the experimental data and with the data of calculation by the transport method has been obtained.