Home – Home – Jornals – Thermophysics and Aeromechanics 2018 number 5

Here, we present the results of computational studies of gas parameters in the high rarefaction zone formed behind a molecular shield mounted on the OKA-T spacecraft. The method of accounting for the effect of solar cell rotation on the rarefaction zone is described. The influence of jets of on-site engines and gas emission of the apparatus on the rarefaction zone has been estimated.

An experimental study of the influence of unit Reynolds number on the position of laminar-turbulent transition in swept-wing boundary layer at supersonic flow velocities was carried out. In the experiments, a swept-wing model with 3-% circular arc airfoil and 45-deg gliding angle of wing edges was used. The position of the transition was identified using a hot-wire anemometer. It was found that at М = 2 and 2.5, an increase in the unit Reynolds number (Re₁) leads to a transition delay. It was shown that an increase in freestream Mach number and in the level of flow pulsations in the wind-tunnel test section leads to a less pronounced influence of Re₁ on the transition position. At a high noise level due to the growth of Mach number or due to the introduction of vortical disturbances, no effect due to unit Reynolds number on the transition position was observed.

The influence of the low-frequency modulation of flow behind a rectangular backward-facing step on the amplitude characteristics of disturbances in the separated laminar boundary layer has been studied. The experimental data were obtained by the method of hot-wire anemometry in a wind tunnel at a low subsonic velocity. Response of the separated flow to the long-wave oscillations generated by a local source of disturbances on the surface of the experimental model was clarified. The low-frequency nonstationarity of the separation region leads to a growth of velocity fluctuations in the separated boundary layer, which dominate the laminar-turbulent transition and the state of the flow in the near-wall region.

The promising method of drag reduction with the use of micro-blowing through the streamlined surface has been proposed for its use to the external surface of high-speed train. The advantages of high-speed train as an object of micro-blowing application are introduced. The corresponding RANS-based mathematical model is elaborated, and the computations of the external flow around a long train body are performed. Predictions of the turbulent boundary layer over penetrable surface with different modes of micro-blowing have been presented and analyzed. The developed modifications of mathematical model of turbulence have been used to take into account the micro-blowing influence in the inner region of turbulent boundary layer. The obtained results of parametric analysis of drag reduction depending on the area of permeable sections, intensity of micro-blowing, and high-speed train length have been analyzed. In particular, the dependence between drag reduction effect and length of train body with realized micro-blowing as well as its intensity is established. Realization of micro-blowing with blowing velocity just 0.25 % of train speed (V = 100 m/s) on the 70 % of the streamlined surface area for just one train carriage (L = 25 m) allows one to reduce the aerodynamic drag (including the most actual friction and head-tail pressure components) of the whole train (L = 200 m) by about 5.25 %, so in case of micro-blowing realization on all its 8 carriages, the train’s aerodynamic drag can be reduced approximately by 42 %.

Under the experimental studies carried out to reduce the sonic boom intensity created by the aircraft by means of active action, the problem of measuring the shock wave parameters in nonuniform flow is considered. Comparison of the pressure profiles behind the shock wave obtained by means of a drained measuring plate and a comb of brake pressure probes is given. It is shown that the technique using brake pressure probes provides an acceptable reliability of pressure distributions measurements in the near zone of the model. The use of the drained plate leads to significant errors in measuring under nonuniform flow conditions. Analysis of the causes of signal distortion is carried out. The procedure for experimental data processing is presented.

The article presents the results of experimental studies of the influence of different designs of mixing spacer grids on the coolant flow in FA-KVADRAT PWR. The investigations were carried out by simulating the coolant flow in the core on the experimental air stand being an aerodynamic open loop through which air is pumped. To measure the local hydrodynamic characteristics of the coolant flow, special pneumometric sensors were used for measuring the total velocity vector and the flow rate value. During the investigations of the local hydrodynamics of the coolant, transverse flow velocity, as well as the coolant flow rates in the cells of the experimental model FA-KVADRAT, was measured. The analysis of spatial distribution of projections of absolute flow velocity allowed studying and detailing the coolant flow pattern behind the mixing spacer grids with different designs of deflectors as well as selecting the op-timal design of the deflector. The accumulated database on the coolant flow in FA-KVADRAT has formed the basis for engineering assessment of active zones’ structures of PWR. The results of experimental studies are used to verify CFD codes (in both foreign and domestic development) as well as programs for detailed cell-by-cell calculation of active zones in order to reduce conservatism when assessing heat engineering reliability.

The heat transfer at superposition of high-frequency oscillations on a laminar flow of a liquid in flat and rectangular channels at a distance from an inlet of a heated site is investigated under boundary conditions on channel walls of the first and second kind. For the flat channel, the obtained analytical expressions for the amplitude and phase profiles of the longitudinal velocity oscillations are used as a function of the dimensionless oscillation frequency in the form of functions of a real variable. It is shown that the mean value taken for the perimeter of the channel and also the period of oscillations, the Nusselt number for large amplitudes of mean velocity oscillations over the cross section can significantly exceed its stationary value. The limiting value of the ratio of Nusselt numbers for a pulsating and steady flow in the region of high pulsation frequencies is found.

A computational study is carried out to assess the suitability of various RANS based turbulence models for slot jet impingement on flat and ribbed surfaces with various values of Reynolds number and jet to plate spacing. The computed results are compared with the reported experimental data. It was observed that none of the turbulence models considered predicted the heat transfer data accurately. However, some models predicted the experimental data with good trends, e.g., secondary peak and several spikes in Nusselt number for ribbed surface, with a precise computation of the stagnation point Nusselt number. Further, the effects of slot width, rib pitch and jet to ribbed surface spacing were investigated for jet impingement on a ribbed surface. It was observed that the local Nusselt number increased with slot width and rib to plate spacing. It was also observed that increasing Reynolds number had a positive effect on the local heat transfer. With increasing rib pitch the local Nusselt number increased near the stagnation zone but decreased downstream. The observed flow pattern was different for jet impingement on a ribbed surface than that on a flat surface.

The thermal conductivity and the thermal diffusivity coefficients of samarium have been measured by the laser flash method in the temperature interval of 293 – 1773 K in solid and liquid states including the regions of phase transitions. The measurement errors of the heat transfer coefficients were ±(3–6)%. The approximation equations and the tables of reference data for the temperature dependence of properties have been obtained. The obtained results have been compared with the available literature data

We propose an improvement of an analytical approach presented previously for determining the surface shape formed during laser cladding process at the goods manufacturing in additive technologies. The approach is based on the balance of pressures on the liquid metal surface, which occurs under the gravity and surface tension. A method generalization is proposed for the case of a curvilinear shape of a substrate, which allows determining the surface geometry at arbitrary contact angles for single beads, vertical walls, and coatings formed by overlapping beads. The verification of the considered approach was carried out for laser cladding problems with the use of experimental data obtained by other authors.

The process of hollow jet formation during steam outflow through a thin nozzle was studied; the water steam was initially at high pressure and in the supercritical state. The numerical solution for this problem was obtained with the sonicFoam solver library from the open-source CFD software package OpenFOAM in 2D axisymmetric formulation. The reliability of results is estimated by comparing two approaches for simulation of the dynamics of unloading wave propagating in the high-pressure nozzle using the OpenFOAM package with Peng-Robinson equation of state and numerical solution of a similar problem by method of through computation in the case of 1D planar approximation for perfect gas equation of state.

Experimental study was performed on the influence of nanosized refractory nanopowders on the structure and electrochemical characteristics of aluminum alloy AlZn4 used as protective anode. It was found that modification of alloy AlZn4 with nanopowder of aluminum nitride or diamond induces refining of alloy structure by the factor of 1.5, increases the anode strength and its protective negative potential. Those factors have practical value for higher efficiency of protection from corrosion in water-contacting vessels.

Numerical simulation of thermal state was performed for semitransparent medium; the thermal state is formed under impact of incident radiation and convective heat transfer to the ambient medium. The model takes into account the heat transfer to the semi-infinite opaque substrate below the layer of semitransparent material. Computation was performed for governing parameters typical of snow and ice coating during winter period. Solving the radiative part of problem uses the modified average flux method. This method takes into account the dependence of optical properties on the wavelength of incident radiation, scattering, and reflective properties of the layer boundaries.

The results of theoretical studies of the processes of ignition of water-coal fuel droplets based on brown coal, semi-anthracite, anthracite, long-flame and fat coal under the conditions corresponding to the combustion spaces of typical modern boilers are presented. The influence of the degree of metamorphism (structural-molecular transformation of organic matter of coal) and concentration of the organic component of the base fuel (coal) on the conditions of ignition of water-coal fuel particles is analyzed. It is determined that the type and grade of coal have a significant impact on the dynamics of fuel ignition. It was shown that in the case of ignition of coal-water fuel made of mineral coal, the ignition of particles based on semi-anthracite and anthracite is the fastest (by 20%), and ignition of coal-water fuels of fat coal is the slowest. The latter is explained by the lower heat capacity and thermal effect of pyrolysis of this fuel, as well as the relatively high heat conductivity of anthracite coal as compared to fat coal. It has been determined that drops of coal-water fuel made of brown coal ignite substantially (2 times) faster than drops prepared from coal of coal-water particles. This is due to the high content of volatiles in the composition of brown coal.

Comparative analysis of the main characteristics of the process: ignition delay times (t_ign ) obtained by mathematical modeling and experiments showed a satisfactory agreement between the theoretical and experimental values of t_ign .

The velocity fields obtained by PIV (Particle Image Velocimetry) in supersonic flows are not sufficient to determine the integral characteristics of the flow. Additional data, for example, on pressure can be obtained from the solution of the Navier-Stokes equations. For incompressible flows, the solution of these equations is not too complicated. However, for supersonic flows, the need to take into account the flow density and the increasing number of experimental errors make it more difficult.  This paper proposes a new method for calculating density and pressure from PIV data on the basis of the continuity equation. This method is robust and easy to implement for compressible flows.