Home – Home – Jornals – Thermophysics and Aeromechanics 2012 number 3

In the present paper, we discuss various approaches to measurement of velocity fields and velocity fluctuations in incompressible turbulent boundary layer by digital particle flow visualization methods. In particular, performing measurements in the vicinity of streamlined surface is considered. A description of the optical measuring system and methods to seed the flow with tracer particles is presented. Measured data for flow velocity fields and turbulent velocity pulsations in boundary layer on a flat impermeable plate are analyzed. Comparative data illustrating the efficiency of various algorithms for treating PTV images are presented.

A two-dimensional inlet of external compression with the increased flow rate factor at high supersonic velocities is constructed by the method of gasdynamic design. Its feature is that a flow with the initial oblique shock wave and the subsequent centered isentropic compression wave is formed over the external compression ramp of the inlet. These waves interact with one another so that a resulting stronger oblique shock wave and a velocity discontinuity arise in front of the entrance to the inlet internal duct. An example of an inlet configuration with the design flow regime corresponding to the Mach number M_d = 7 is considered. The characteristics of this inlet were obtained in the range of the free-stream Mach numbers M = 4−7 with the use of a Navier-Stokes code for turbulent flow. They are compared with characteristics of an equivalent conventional shocked inlet. As computations have shown, the inlet with the isentropic compression wave has much higher values of flow rate factor ϕ at Mach numbers М < M_d. So, for example, at М = 4 the value ϕ ≈ 0.72 for it is by 33 % higher in comparison with ϕ ≈ 0.54 for the equivalent shocked inlet.

When the stagnation temperature of the combustion chamber or ambient air increases, the specific heats and their ratio do not remain constant any more, and start to vary with this temperature. The gas remains perfect, except, it will be calorically imperfect and thermally perfect. A new generalized form of the Prandtl⎯Meyer function is developed, by adding the effect of variation of this temperature, lower than the threshold of dissociation. The new relation is presented in the form of integral of a complex analytical function having an infinite derivative at the critical temperature. A robust numerical integration quadrature is presented in this context. The classical form of the Prandtl-Meyer function of a perfect gas becomes a particular case of the developed form. The comparison is made with the perfect gas model for aim to present a limit of its application. The application is for air.

In the present work, the regimes of the flow and mixing of fluids in a T-shaped micromixer in the range of the Reynolds numbers from 1 to 1000 are investigated systematically with the aid of numerical modeling. The flow and mixing regimes are shown to alter substantially with increasing Reynolds numbers. Five different flow regimes have been identified in the total. The dependencies of the friction coefficient and mixing efficiency on the Reynolds number are obtained. A sharp increase in the mixing efficiency at a flow transition from the symmetric to asymmetric steady regime is shown. On the other hand, the mixing efficiency slightly drops in the laminar-turbulent transition region. A substantial influence of the slip presence on walls on flow structure in the channel and mixing efficiency has been revealed.

This paper presents physical modeling of thermal tornado under lab conditions. For the tested range of 0÷300 Hz, selective frequencies were discovered which facilitate the tornado decay. Data analysis was comp-lemented by velocity profile measurement using LDV system LD-05M. The results on velocity pulsation at selected points were recalculated into coefficient of correlation between velocities and function cos(2π f Δt_i) describing the acoustic oscillations. In the theoretical part of this paper, we present solution of dispersive equation of Euler's model and resulting boundary of stability for tornado existence. Satisfactory agreement between experiment and calculation has been observed.

The authors have formulated the problem of joint optimization of pressure and temperature of combustion products before gas turbine, profiles of nozzle and rotor blades of gas turbine, and cooling air flow rates through nozzle and rotor blades. The article offers an original approach to optimization of profiles of gas turbine blades where the optimized profiles are presented as linear combinations of preliminarily formed basic profiles. The given examples relate to optimization of the gas turbine unit on the criterion of power efficiency at preliminary heat removal from air flows supplied for the air-gas channel cooling and without such removal.

Numerical technique was developed for simulation of cavitating flows through the flow passage of a hydraulic turbine. The technique is based on solution of steady 3D Navier-Stokes equations with a liquid phase transfer equation. The approch for setting boundary conditions meeting the requirements of cavitation testing standard was suggested. Four different models of evaporation and condensation were compared. Numerical simulations for turbines of different specific speed were compared with experiment.

It was confirmed in experiments that during contact between cool and hot liquids, the lower subcooling of the cool liquid below the saturation temperature changes the characteristics of a vapor layer covering the fragments of hot liquid. This factor also decreases the probability of spontaneous direct contact between two kinds of liquid, explosive incipience of the cool liquid, and pressure pulse generation (the latter triggers fine fragmentation of hot coolant and vapor explosion). The mechanism that describes this trend in vapor layer behavior has been described.

The work presents calculation of Ag₂ dimers emission from the substrate into the vapor medium in case of joint deposition of silver and water vapors on ideal substrate formally modeling the crystal of water ice in terms of energy properties. It is assumed that the dimers are formed on the condensation surface as a result of random collisions of atoms at their surface migration, and the dimers emission is conditioned by thermal fluctuations of crystal lattice of water ice. The calculations based on the modified Langmuir adsorption model allowed concluding that emission of silver and water dimers takes place in the entire range of the studied water vapor pressure, binding energy of silver-water, and crystal temperature. Dynamics of emission from the beginning of deposition and dependence of dimers emission on micro-roughness of the condensate surface have been investigated. Statistical processing of results has shown that the probability of dimers emission from the condensate surface is determined not only by the value of the binding energy between the dimer and condensate but by configuration of the nearest dimer environment on the condensation surface. It has been found that there is a certain value of micro-roughness of condensation surface providing the maximal intensity of dimer emission. Dimers emission from the surfaces bordering on the flows of vapor mixtures contaminates the flows with unsuspected admixtures. The latter one makes investigation of this phenomenon important for aeromechanics of vapor and gas mixtures.

Results on conjugate convective heat transfer in a system geometrically similar to the system crystal-gas-growing vessel shell of a growing vessel obtained by the Czochralski method are presented. Equations of thermal gravitation convection in Boussinesq approximation are solved by the finite element method in the following variables: temperature, stream function, and vorticity in cylindrical coordinates. Heat transfer from the crystal was studied within the range of Grashof number 100 ≤ Gr ≤ 10⁴ for the fixed configuration of calculation domain. Dependence of the temperature field in the silicon crystal on convection intensity in gas (argon) was examined.

The article presents the results of studying the thermal expansion of MPG-6 and AXF-5Q graphites and graphite composite based on ¹³С isotope in the temperature range 293−1650 K. Measurements were performed with dilatometric method in DIL-402C unit manufactured by NETZSCH (Germany), with the accuracy of (1÷4)×10⁻⁷ K⁻¹. Approximation dependences of average integral coefficient of linear expansion on temperature have been obtained and reference tables calculated. It is shown that the coefficient of thermal expansion of ¹³С composite is much lesser than for MPG-6 and AXF-5Q. The explanation of this phenomenon is provided in the paper.

Results of computer simulation of reflective properties of the oxide film-metal system in the process of oxidation in the air environment are presented. The complex refractive indices for oxide film and metal were used as the initial data. Thin films (the thickness is comparable with the wavelength of incident radiation) and thick films (thickness is much larger than the wavelength of incident radiation) are considered. The parameter characterizing the cyclic character of system reflectivity during the growth of film thickness was derived for the thin film. It is shown that the cyclic parameter does not depend on optical properties of a metal substrate. In the air environment, this parameter is determined by a complex refractive index of the film, its thickness, and direction of incident radiation. Relationships for the estimate of system reflectivity in the process of oxide film growth are presented for the thick film.

The radiation network method has been applied to calculate the net radiation heat transfer between two concentric hemispheres separated by two hemispherical radiation shields with temperature-dependent surface emissivities. Three different materials are chosen for radiation shields: aluminum oxide, silicon carbide, and tungsten. The reduction in heat transfer with shields depends not only on the surface characteristics of the two shields, but also on the locations of the shields. Three illustrative examples are presented to illustrate the effects of temperature dependent emissivities and shield locations on the percentage heat transfer reduction. The analysis can be used to study other cases as warranted.

In the present analysis, a numerical study is performed to examine the heat transfer characteristics of a convective flow over a vertical plate under the combined effects of magnetic field and thermal radiation in the presence of heat source/sink. The surface of the plate is subjected to a variable surface temperature. The boundary layer equations governing the flow are reduced to non-dimensional equations valid in the free convection regime using the suitable non-dimensional parameters. The dimensionless governing equations are solved by an implicit finite difference method of Crank-Nicolson type which is fast convergent, more accurate and unconditionally stable. Numerical results are obtained and presented for velocity, temperature, local and average wall shear stress, local and average Nusselt number in air. The present results are compared with the results available in the literature and are found to be in a excellent agreement.

The furnace processes of the combustion of poly-fraction high-ashes Ekibastuz coal in the furnace chamber of the boiler aggregate PK-39 and of the combustion of highly humid brown Berezov's coal in the furnace of the BKZ-210-140 boiler are investigated by mathematical modeling using the package of applied programs FIRE 3D [1-3]. Results of the numerical modeling of the processes of aerodynamics, heat exchange, and combustion in the furnace volume and their comparison with the results of nature tests are presented.