Home – Home – Jornals – Thermophysics and Aeromechanics 2013 number 4

Results of experimental investigation of heat transfer and wall friction in the upward bubble flow in a flat inclined channel are presented. Measurements were carried out in the range of superfi-cial liquid velocities of 0.3–1.1 m/s and different values of volumetric void fraction. The hydrodynamic structure was measured by means of the electrochemical method with the use of miniature friction sensors. The values of average friction and heat transfer at different channel orientation were determined. It is shown that in the bubble gas-liquid flow we can observe a significant dependence of friction and heat transfer on the angle of channel inclination.

Мethods of measuring lifetime and continuous pressure decrease were used to study kinetics of spontaneous boiling-up of superheated ethane-methane solutions. The attainable superheats of solutions at two pressure values (1.0 and 1.6 MPa) and two concentrations of methane (2.1 and 6.0 mole %) were determined experimentally in the range of nucleation rate J = 1∙10⁴–3∙10⁸ s^–1m^–3. At temperatures 266.5, 270.0, and 273.15 K, the attainable stretching of the studied solutions was measured. The experimental results were compared with the theory of homogeneous nucleation. At nucleation rates J ≥ 2.5∙10⁶ s^–1m^–3, there is a fair agreement of the theory and experiment. The discrepancy in attainable superheat temperatures Tn does not exceed 0.8 K. It is shown that significant underheating of solution to theoretical values Tn at J < 2.5∙10⁶ s^–1m^–3 cannot be bound only with heterogeneous nucleation but is conditioned by other factors as well.

The method of penetrating gamma-radiation was used to measure the density of copper-aluminum melts containing from 0 to 100 % aluminum in the range of temperatures from liquidus to 1300–1400 ºС; the concentration dependences of their molar volume were calculated as well. High accuracy of measurements (error not exceeding 0.2 %) allows using the values as reference data. Substantial non-monotonicity of the molar volume isotherms that proves the complexity of the specified melts especially in the area of compositions rich in aluminum was found. The parameters of polynomials describing concentration dependencies of the molar volume with the error not exceeding 1 % are presented.

A numerical analysis of the unsteady regimes of the natural convection and thermal radiation in a square enclosure with heat-conducting walls has been carried out in the presence of a heat source of finite sizes located in the base zone under the conditions of convective exchange with the ambient medium. The mathematical model formulated in the dimensionless variables “stream function — vorticity — temperature” has been implemented numerically by a finite difference method. The influence of the reduced emissivity of internal surfaces of bounding walls on the local characteristics (the streamlines and temperature fields) and on the integral complex (the mean Nusselt number on typical boundaries) has been analyzed in detail for 0 ≤ ε < 1, the location of the heat source 0.1 ≤ l/L ≤ 0.4, and its length 0.2 ≤ l_hs/L ≤ 0.6 for Ra = 10⁶, Pr = 0.7. The approximation relations have been derived for the mean convective and radiation Nusselt numbers depending on the reduced emissivity of the internal surfaces of bounding walls and the energy source location relative to vertical walls.

The analysis of energy balance equation for viscous laminar flow of fluid or gas in the cylindrical channel in the area (zone) of warm-up bounded along the longitudinal coordinate is made. It was found that at laminar flow of fluid or gas in a round pipe, in each warm-up area bounded along the longitudinal coordinate there are the areas of direct and reverse flows separated by a plane that is a locus of points where temperature is maximal for each fixed value of radial coordinate r.

It is shown that with an increase in elasticity modulus of the coating material from 0.01 MPa to 100 MPa the pulsation velocity of coating surface changes not more than by 30 % and equals 0.17–0.24 of the value of dynamic flow velocity, and this can change significantly Reynolds stresses in the near-wall area. According to performed analysis, it was found out that the deformation value of the compliant coatings within the velocity range, optimal for their interaction with the turbulent flow, is only several units of the viscous scale. Moreover, these deformations are very gentle: the ratio of amplitude of deformation to wavelength is less than 10^–3. It is assumed that while modeling the interaction between the compliant coating and turbulent flow it is not necessary to transfer the boundary conditions to the moving coating surface. Perhaps, it will be sufficient to determine the velocity of wall motion over the undisturbed coating surface.

The dependence of the aerodynamic characteristics of typical multi-beam structures of bridge spans on the angle of attack, the incident flow turbulence, the number of beams, and the distance between them has been studied. For the model of three beams bridge span, the characteristic vortex structures were obtained, the thermo-anemometer probing of the near wake was performed, and the diagrams of the distribution of the averaged pressure were plotted.

The work presents the results of mathematical modeling of large-scale flows in areas with a complex topographic relief. The Reynolds-averaged Navier—Stokes equations constitute the basis of the three-dimensional unsteady modelling. The well-known Volume of Fluid method implemented in the solver interFoam of the open package OpenFOAM 1.7.1 is used to track the free-boundary location. The mathematical model adequacy is checked by comparing with experimental data. The efficiency of the applied technology is illustrated by the example of modeling the breakthrough of the dams of the Andijan (Uzbekistan) and Papan (near the Osh town, Kyrgyzstan) reservoirs.

The statements and numerical solution of the problem of igniting the wood wall as a result of the fire seat effect based on the mathematical model of a porous reacting medium are proposed. The original reagent ignition is found to be determined by the processes of drying, pyrolysis (decomposition and synthesis reactions) of dry wood, reaction of the carbon oxide oxidation as well as by the wood thermophysical properties.

Thermal conductivity of refrigerant R-415A in the vapor phase has been studied by the steady-state method of coaxial cylinders in the range of temperatures 308–415 K and pressures 0.12–1.68 MPa. The approximation dependence of thermal conductivity on pressure and temperature was determined. Thermal conductivity on the dew line and in the ideal gas state was calculated.

Tests were performed at Mach number M = 6. Observations of flow pulsations were performed with the help of high-frequency measurements of pressure and heat fluxes and with the help of hot-wire anemometry. Measurements were carried out in a broad range of Re1 numbers at various orientations and locations of sensors with respect to the nozzle exit plane. It is shown that the Transit-M noise varies weakly over the longitudinal coordinate, whereas on moving along the radial coordinate, some increase of the fluctuation level is observed due to approaching the mixing layer. Also, the distribution of axial asymmetry of the noise level was obtained.

The problem of pressure increase at a thermal effect through a non-deformable wall on a liquid filling an enclosure is considered. The solutions are obtained for the cases of the plane, radially symmetric, and spherically symmetric problem statements, which describe the pressure variation due to thermal expansion and phase transitions. The influence of the initial liquid state and the enclosure shape on the pressure variation pace are analyzed.

Both theoretically and experimentally, the suitability of a technique for studying the finely dispersed spraying of liquids based on an analysis of salt residue particles formed as a result of evaporation of test solutions of NaCl in water was evaluated. Data gained in studying the dispersion of droplets in the droplet clouds produced by pulsed and ultrasonic atomizers are reported. During pulsed atomization, salt residue particles of various morphologies were found to form, this finding pointing to realization of unsteady conditions for particle evolution.

A new method is proposed for computing the thermal conductivity of a gaseous mixture with regard for chemical reactions occurring therein, which enables the estimation of the thermal conductivity from each separate reaction to the chemical component of thermal conductivity. The method for determining the paired collision integrals, which is used in the work, is also briefly presented. The results computed by the proposed method are compared with experimental data and the data obtained by the existing computational methods in the temperature range from 500 to 2500 K under atmospheric pressure. The comparison was done both for pure gases and for the gaseous mixtures, including the combustion products of solid fuels. The obtained results may be applied for the development and design of systems related to heat exchange, gas dynamics, processing of solid and liquid fuels, for example, boiler aggregates, aviation engines, and in other applications.