Home – Home – Jornals – Thermophysics and Aeromechanics 2021 number 5

In the present paper, the results of a study of the influence of textile materials on the structure of the aerodynamic wake behind blunt-body models in the form of quasi-two-dimensional and cantilever cylinders are reported. The fabrics used were previously examined in the works by co-authors while determining the drag coefficient of a two-dimensional cylinder. The change in diameter in the latter case proved to be not more than 1%, and it was considered insignificant. The use of fabrics has made it possible to reduce the critical Reynolds number to about 2·10⁵. The two- and three-dimensional flows turned out to be similar in terms of fluctuation level, whereas some narrowing of the wake behind the cantilever cylinder occurred due to the three-dimensional structure of the flow. Based on the results of spectral analysis, complete similarity between three- and two-dimensional flows was demonstrated, both for the flow around a smooth cylinder and for the flows around cylinders fitted with textile materials.

Methodological aspects of stagnation-temperature measurement in high-enthalpy supersonic air flows using a thermoelectric transducer (thermocouple) with shielded measuring junction are analyzed. Specific features of thermo- and gas-dynamic processes under the conditions of interest, leading to a substantial deviation of temperatures measured using traditional measuring methods from the true temperatures, are demonstrated. Refinements to the method of determining the temperature of high-enthalpy supersonic air flows allowing one to reduce the deviation from the true temperature are introduced. A mathematical apparatus developed for analysis of experimental data and for quantitative estimation of temperature-measurement error is described. Approbation of the proposed approaches and validation of the developed mathematical models are performed using the results of experimental studies of high-enthalpy supersonic flows. A satisfactory agreement between the results of indirect temperature measurements and specified temperature values is shown. The results of the study can prove useful in the development of techniques and methods, and in the analysis of physical experiments with high-enthalpy flows.

Nano-tracer Planar Laser Scattering (NPLS) technique and pseudo-color processing technology are employed to investigate the flow field after a thickness plate of non-isobaric initial conditions. The results show that the supersonic mixing layer behind the blunt trailing edge will deflect toward the low pressure side which is consistent with the canonical mixing layer. However, there is no laminar region for any unmatched cases and the turbulent state of the mixing layer seems to be restricted for the case of the low-speed side with a higher pressure.

A physical-mathematical model of a plane supersonic boundary layer of a vibrationally excited dissociating gas in the Prandtl approximation is presented. The simplest case of a diatomic gas parameterized according to the data for nitrogen is considered. A number of simplified formulas for calculating the transport coefficients and the dissociation and recombination rates for a molecular-atomic mixture, which are admissible for typical conditions of supersonic and hypersonic boundary layers, are substantiated. Comparative calculations of numerical and locally self-similar solutions of the boundary layer equations on a flat plate for typical conditions of a high-enthalpy wind tunnel and hypersonic flight in the upper atmosphere are performed. It is shown that the atomic concentration profiles calculated on the basis of these approaches differ significantly. In this regard, in order to adequately take into account the effect of chemical reactions in the boundary layer stability calculations, it is necessary to use the profiles of hydrodynamic parameters calculated numerically on the basis of full boundary layer equations. Locally self-similar solutions can serve as initial conditions for numerical calculations.

Film cooling efficiency averaged over the length of a one-row ring of holes for the case of a flat surface with air blowing through fan-shaped holes was estimated experimentally and normalized by the respective efficiency for cylindrical holes in a wide range of injection parameters m (from 0.25 to 5.5) and blowing angles a (from 30° to 75°). Ranges of regime and geometric parameters of film cooling systems in which fan-shaped holes provide a higher cooling efficiency in comparison with cylindrical holes have been identified. With the case of α = 30° taken as an example, we show that very high injection parameters ( m = 4.5¸5.5) in the entrance region of the mixing zone ( x / d < 10) make the relative efficiency decrease from 1.7 to 1.05 with the increase of x / d . Simultaneously, the relative efficiency increases from 1.05 to 1.6 in the main mixing zone ( x / d > 10). Fan-shaped holes provide a higher film cooling efficiency at optimal and high values of m and α = 45° (under all other conditions being identical) in comparison with cylindrical holes both in the entrance region and in the main mixing zone. At α = 75°, the efficiency is higher than throughout the whole examined range of injection parameters and throughout the whole examined range of the normalized distance from the location of the injection place. The relative efficiency decreases from 4 to 1.5 in streamwise direction in the entrance region of the mixing zone ( x/d < 5), and it ranges between 1.5 and 2.5 in the main mixing zone ( x/d > 5) at all values of injection parameter considered in the paper.

Oscillations of a cantilevered rod - fuel element simulator - were experimentally studied in an annular channel with a longitudinal ascending fluid flow using an original sensor built on the basis of the electrical impedance method. The data on the trajectories of the rod axis movement and changes in amplitude and frequency of rod vibrations depending on the velocity of the axial fluid flow are given. The data on rod vibration obtained using the electrical impedance method and high-speed video recording are compared.

A model of a rivulet formed on the coils of a tubular heat exchanger has been developed. The cross-sectional shape of the rivulet is determined taking into account the forces of gravity of liquid, the forces of surface tension, and centrifugal forces. The equations for the coordinates of the free boundary points of the rivulet are presented in the integral form based on the analytical solution of the problem. The cross-sectional areas of rivulet and liquid bridge are compared, as well as the areas of their contacts with the working tubes.

The results of experimental studies on the properties of a capillary liquid bridge in the gap between two glass spheres of an equal diameter are presented. It is shown that for the case when the diameter of the spheres is much larger than the capillary scale of liquid, the shape of the capillary liquid bridge can be described as a figure formed by two “drops”, touching the spheres, and the central catenoid. The contact angle between the “drop” and the sphere depends on the effective mass of the “drop”, and the relative position of the catenoid and the “drops” is set by the condition that the contact angle between them is equal to zero. In the field of gravity, the position of the minimum cross section of the catenoid does not coincide with the middle of the gap between the spheres and is determined by the magnitude of the surface energy and the way how the mass of liquid in the bridge is distributed over the “drops” and the catenoid.

The paper presents the experimental study on the influence of parameters of a multi-jet impingement spray on the cooling efficiency for a large flat surface. The study is based on common principes for engineering systems with high-rate heat and mass transfer using the impinging jets; this enables the draining of hogh heat loads with a low flow rate of the coolant. These results, along with using the Nusselt and Reynolds criteria, give the approach for estimating the aggregated efficiency of heat transfer coefficient while cooling with a multi-jet impingement spray.

In industry, there is a whole class of heat exchangers with low values of specific heat fluxes: evaporators of refrigerating machines, steam generators of binary freon stations, steam generators of heat pumps, etc. Such heat exchange modes are realized when boiling in the film is just beginning, and the intensity of heat exchange during evaporation is commensurate with the heat exchange during boiling. The article presents the results of an experimental study of heat transfer during boiling on copper finned pipes, the entire surface of which is subjected to special treatment in an air stream containing corundum particles with the size below 500 microns. The result of such treatment is a significant decrease in the temperature head, at which boiling and a noticeable intensification of heat transfer, compared to a finned pipe without treatment with corundum particles, begin.

The paper presents an experimental study of the processes of ignition and combustion for a stoichiometrical propane-oxygen mixture in a closed volume (a single cylinder of internal combustion engine). The object of study was the dynamics of gas ignition and combustion in a cylinder of diameter D = 72 mm and height h = 4 mm as a function of location of N spark points for ignition ( N = 1÷9) under the condition of simultaneous ignitions. The big number of ignition points reduces the interval for generating the force impact on the engine’s piston and increases the maximum amplitude of the force.

A Raman spectrometer with the full use of energy of the probing laser radiation, repeatedly passing through the object of study to increase the scattering, is described. In a flame of a propane and air mixture, the local values of temperature and composition of gases were measured simultaneously with broadband registration of Raman spectra. The results of their processing using the original software are presented.

The paper presents the study of oxidation of benzene, pyridine, and pyrrole in a dense steam at uniform heating of the reactor. The time dependencies for temperature and pressure of reaction mixtures suggest that the fuel oxidation is enhanced by catalytic action of the Pt-Rh/Pt thermocouple inserted into the center of the reaction volume. It was shown that the fuel ignition temperature T _in^* increases within the fuel molecular series C₆H₆ < C₅H₅N < C₄H₄NH. The higher fuel equivalence ratio and steam density create a lower T _in^*. The benzene oxidation is a dual-stage process and it is described by low carbon burning. For a lean fuel mixture, combustion of pyridine occurs with detonation due to shock waves generation in the vicinity of the Pt-Rh/Pt thermocouple. The higher density of steam prevents the complete burnout of fuel. These features of fuel combustion are explained by differences in molecular structure and reactants adsorption on the platinum surface.

Studies of nonstationary conductive heat transfer on a cylindrical surface are presented in the approximation when the thickness of the thermal boundary layer is less than the surface diameter. The temperature profiles at the cylindrical surface and the specific heat flux on this surface are obtained. Using the experimentally established law for the stationary mode of thermal conduction of a cylindrical surface (Nu = 0.5), the value of the Fourier number at which the stationary mode occurs (Fo = 1.27) is determined. For mantle plumes having conduit diameters d = (7.4 _¸ 85)×10³ m, the time of plume ascent to the surface t _a and the time of establishment of the stationary conductive heat transfer t _s are estimated depending on the dimensionless value Ka = N / N_H , where N is the thermal power at the base of the plume, and N_H is the thermal power transferred from the plume conduit to the surrounding mantle in stationary mode. For Ka = 0.97 _¸ 1.58 ( t _a / t _s = ¥ _¸ 1) and Ka = 1.58 _¸ 129 ( t _a / t _s = 1 _¸ 0.005), the effect of unsteady heat transfer to the sur-rounding mantle on the plume ascent to the surface is shown and it is established that for Ka > 4 ( t _a / t _s < 0.19, d > 15 km), the effect of unsteadiness during the plume rise may be neglected.

A mathematical model of dissolution of a titanium carbide particle in a titanium melt containing nano-sized refractory particles is developed. The model allows one to study the influence of the parameters of the medium with nanoparticles on the stability of dissolution of refractory compounds under the conditions of laser cladding of a nano-modified coating with a hardening phase (titanium carbide). The time needed for the carbon-containing chemical compound to dissolve is determined as a function of the concentration of the nano-modifying additive, inclusion size, and melt temperature. It is found that the presence of nanoparticles in the fluid reduces the mass transfer intensity, resulting in a longer time of solid inclusion dissolution.

The results of studies of solid tungsten rod-type cathodes in different gases are presented. The thermal states of the electrodes under the action of cathode plasma, radiation and joule heat release are determined. The thermal nature of the rate of destruction (erosion) of the thermocathode is shown depending on the main determining parameters of the arc discharge and the design features of the cathode node. The obtained experimental data indicate the possibility of a long service life of W-cathodes in technological plasma torches.

The paper presents the results of simulation and experimental study on the efficiency of selecting a pair of excitation lines of OH hydroxyl radical for the (1-0) transition for the A²Σ⁺-X²Π system for local temperature measurement in a hydrocarbon flame. The LASKIN software was used for the numerical simulation. The temperature field for a laminar methane-air premixed flame (with equivalence ratio equal 1.1) at the atmopsheric pressure was measured. Different combinations of literature-recommended pairs of excitation lines were considered. The results of numerical simulation agree with a theoretical dependency for the temperature range of 1200 - 2100 K for the coupled excitation lines Q₁(5):Q₁(14) and Q₁(5):Q₂(11). However, a moderate discrepancy is observed for the pairs R₂(2):R₂(13) and R₂(2):R₂(10). It is concluded that the coupled excitation of Q₁(5):Q₁(14) and R₂(2):R₂(13) lines provide a higher sensitivity to the temperature variation. The benefit of the latter pair is that these transitions correspond to close values of the excitation wavelengths in the vicinity of 282 nm. Therefore, this can be convenient for the arrangement of experiments.

Studies of hydrodynamics and heat exchange of an alcohol-water mixture during its circulation through a heated cylindrical channel are carried out. Non-azeotropic and azeotropic mixtures with a mass concentration of a volatile component of 22 and 95 %, respectively, are used as the working fluid. The experiments are realized at a pressure of 0.018-0.02 MPa. The working section is made of stainless-steel pipe with an outer diameter of 8 mm and a length of 4 m. For visual observation of the composition of the two-phase flow, an optical section installed behind the working section is used. Experiments show that the formation of a vapor phase during the circulation of a non-azeotropic mixture through a heated channel begins at a wall temperature below the equilibrium temperature of the mixture. The governing condition for vaporization is the excess of the wall temperature over the equilibrium temperature of the volatile component.

The study deals with unsteady heat transfer between a spherical encapsulated nanoparticle with a phase transition material as a core and the ambient medium. The calculations take into account the Kapitsa’s contact thermal resistance at the nanoparticle interface. It was demonstrated that this phenomenon can be critical for heat transfer in nanofluids.