Home – Home – Jornals – Thermophysics and Aeromechanics 2008 number 4

A thermal conductivity model is proposed for an orthogonally reinforced fiber medium with dispersed hardening of the bonding agent. On its basis, the problem of the design of a composite with a given set of effective thermophysical properties is solved as well as the solutions are derived for some inverse problems of the diagnostics of thermophysical properties of phase materials and the reinforcement structure of a fiber composite from known effective thermophysical characteristics.

Thermal conductivity of ozone-safe liquid refrigerant R507 was studied by the method of high-frequency thermal waves within the temperature range of 297.95 … 332.55 K and pressures from the saturation line up to 3.7 MPa. The estimated errors of temperature, pressure, and thermal conductivity measurements are 0.1 K, 3 kPa, and 1.5 %, correspondingly. Thermal conductivity of liquid R507 was calculated on the saturation line. Approximation dependences for thermal conductivity were derived for the whole range of studied temperatures and pressures and on the saturation line.

The properties of thermal radiation of the axisymmetric systems formed by an absorbing medium are analysed. Analysis of absorption and propagation abilities of the systems with an impermeable boundary on the basis of integral radiation equations is suggested together with absorption, transmission, and reflection abilities of the systems with a boundary permeable for radiation.

The peculiarities of cooling of stationary working hot and cold cathodes are under consideration. Scientific premises of the advisability of the tube electrodes arc-spot scanning are presented. The classification of the scanning methods is given. The assumption has been made that the phenomena of auto-electron emission of electrons in vacuum discharge tubes and in arc chambers of plasmatorches under the pressure equal or above atmospheric are somehow similar. The optimum scanning frequency and achievable values of the end-face tube cathode life-time have been estimated. Owing to the magnetic scanning, the area of the electrode surface, which is swept by the arc spot, increases one order or more, resulting in better cooling and reduced specific erosion. The continuous electrode lifetime becomes several orders longer than it is during the work without scanning.

On the basis of experimental data for a windwheel with large-aspect-ratio (up to 14) cylinders, a method making it possible to determine optimal parameters and main characteristics of a windwheel (power, high-speed) is proposed. Effects due to number of cylinders, their aspect ratio and speed of rotation, stream velocity, and generator load are analysed.

A model is proposed, which describes the work of the viscometer sensor of the physical pendulum type. The model enables the obtaining of data on fluid viscosity directly from the measurement of the settling frequency of sensor oscillations or the amplitude of these oscillations. To describe the sensor operation a numerical computational algorithm is developed. This method enables the solution of a wide class of three-dimensional laminar fluid flow problems involving moving solids of arbitrary geometry. The results of testing the proposed numerical technique are presented.

A new exact solution of a problem for motion equations of a solid and ambient viscous fluid is found.

This paper reviews the works of the 3rd International Workshop and Exhibition on Plasma Assisted Combustion, which dealt with the last achievements in the following scientific fields: fuel conversion and activation, plasma ignition of fuels and flame control, plasma generation and modelling, waste treatment and utilization, and promising industrial technologies.

The compressibility factor was calculated in a wide range of parameters including the critical region using the equations of state obtained previously for real gas and conditions and limitations formulated by the authors for the form (structure) of the equation of state. It was determined that maximal deviations of compressibility factor values calculated by these analytical equations from the experimental (tabulated) values occur mainly within the critical region. A corresponding correction was introduced into the initial equation, and the analytical equation accurately describing experimental data in a wide range of real gas state parameters, including the critical region, was derived. It was shown by the example of analysis of high-accuracy experimental data on thermal properties of helium in the critical region that data description by the proposed analytical equation is at least as good as that obtained by the equation of fluctuation theory (scaling). It is shown that the proposed equation meets the classical conditions at the critical point.

Speed of sound in the gaseous freon R-236ea with the purity of 99.68 mol. % has been measured by the method of ultrasonic interferometer in the range from 263 to 423 K and at pressures from 17 kPA to 4.2 MPa. Errors of temperature, pressure, and speed of sound measurement were estimated to be within +/- 20 mK, ± 1.5 kPa, and ±(0.1+0.2) % respectively. Temperature dependence of ideal-gas heat capacity of R-236ea has been calculated on the basis of the obtained data.

Experimental techniques for investigation of heat transfer in liquid mixtures of different types complimenting each other have been presented. The main attention is focused on the measurements under conditions of pulse heating. The paper opens the set of papers dealing with the elucidation of features characteristic of the heat transfer in pulse superheated mixtures, including metastable states with respect to the liquid-vapor and liquid-liquid equilibriums.

The gas motion through porous objects in the gravity force field with a non-uniform distribution of heat sources, which may arise as a result of natural or man-caused catastrophes (as the damaged power unit of the Chernobyl NPP), is investigated. The influence of different parameters of the heat-releasing zone on the process of cooling of such objects is analyzed with the aid of computational experiment. It is shown that the porous element heating is affected not only by the height of the heat-releasing zone and the heat-release intensity therein but also by the distance of the heat-releasing zone from the element inlet as well as by the width of the heat-releasing zone. The phenomenon of a reduction of the porous element heating with increasing distance of the heat-releasing zone from the porous element inlet is revealed. An ambiguous dependence of the porous object heating on the width of the heat-release zone is identified: at a growth of the heat-releasing zone width, the heating of the element may both increase and decrease depending on the distance of the heat-release zone from the element inlet.

Three-dimensional wave processes in vertically falling films of viscous liquid are considered. The 3D localized perturbations, which are studied insufficiently, are of a particular interest. The numerical method for watching evolution of initial perturbations was developed. The final stage of this evolution is formation of the 3D localized structures: solitons. The boundaries of 3D soliton stability were determined.

A Particle Image Velocimetry (PIV) system was used to perform an investigation into the effect of low concentration of dispersed phase on time-average and pulsating characteristics of the flow over the self-similar part of a two-phase jet emanating into ambient space filled with the same fluid. A phase discrimination procedure based on reflected intensity was introduced into routine practice. Distributions of mean and pulsating velocities in the carrier and dispersed phases of the gas-droplet jet were obtained. In spite of low concentration of the dispersed phase, large droplets present in the flow were found to reduce the intensity of velocity pulsations in the gas phase.

The PIV/LIF method was used to experimentally examine turbulent characteristics of a submerged gas-saturated axisymmetric impact jet. A novel method to analyze the dynamics of vortex formations is proposed, making it possible to obtain spectral characteristics of turbulent motion at low sampling frequency. A comparison of data obtained with theoretical models is reported.

Under conditions of a model experiment in nonswept-wing boundary layer, forerunner wave packets were obtained in the flow regions preceding the fronts of streaky structures. A model to describe the formation and development of the fronts is proposed. An external-flow pressure gradient is shown to be a necessary condition for forerunner growth. Besides, the spatial geometry of the forerunners and the evolution of this geometry in the course of downstream development of the wave packet were studied.

Results of parametric calculations of the total aeropropulsive characteristics and characteristics of acceleration of a small-scale high-velocity flying vehicle with an air-breathing engine are presented. Integral parameters of acceleration from the flight Mach number М_∞ = 4 to М_∞ = 7 are determined, namely, the time, required fuel stock, and range. A schematic configuration of the vehicle is considered, which allows studying the basic parameters, such as the forebody shape, the angles of surfaces of compression of the stream captured by the inlet, angles of external aerodynamic surfaces of the airframe, relative planform area of the wing panels, and relative area of the nozzle cross section. A comparative estimate of the effect of these parameters shows that it is possible to improve the characteristics of acceleration of vehicles of the type considered.

Aerodynamic characteristics of a sharp cone with the semi-apex angle of 15° are investigated by the methods of ballistic modelling in the Mach number range from 0.5 to 3.7, which are computed under the assumption of their almost linear behavior at the angles of attack up to 10°. Using the direct shadow pictures the geometric characteristics of near wake are measured and analyzed within the considered transonic and supersonic range. They provide the basis for semi-empirical computation of integral parameters of the base cavity: the pressure, density, and temperature.

The results are presented for numerical modelling of two-dimensional flows with large pressure gradients in a wide range of freestream parameters (М = 2−4, Re₁ = 5−30⋅10⁶ 1/m) and the intensities of perturbing factors. Computations were performed with the use of averaged unsteady Navier - Stokes equations of a viscous heat-conducting gas. The structure of a turbulent boundary layer at its passage through a single shock and a system of shocks of different strengths, which lie at a fixed distance from one another, was investigated numerically. In the case of the boundary layer passage through a system of shocks, the influence of the first interaction on the structure and separation properties of the boundary layer behind the second shock was investigated. The presence of a preliminary shock was shown to improve the boundary layer capability to withstand separation ahead of the secondary interaction region.

Based on the solution of unsteady Reynolds equations (URANS) closed with the aid of differential equations for shear stresses transport (MSST), the evolution of the vortex structure in a square cavity with a moving lid and an unsteady turbulent heat transfer in the air medium are computed while maintaining constant temperatures of the hot moving and cold motionless walls (Re = 5⋅10⁴). The phases of the development of dynamic and thermal processes are analyzed.

Specific features of laminar diffusion boundary layer on permeable surface with isothermal helium blowing into airflow are experimentally examined. Measured profiles of helium, nitrogen, and oxygen concentrations show that the proportion between the volume molar fractions of N₂ and О₂ varies across the boundary layer. At high intensities of helium blowing into air a difference from binary diffusion obeying the Fick law is observed. Our estimates show that, in boundary layer with foreign blowing, manifestation of effects due to multi-component diffusion is possible.

A physical and mathematical model is proposed and numerical investigations are carried out for processes of the deformation and non-equilibrium crystallization of a liquid metal drop with modifying refractory solid nano-inclusions at its collision with a solid substrate under the typical conditions of the gas-thermal and also plasma-sprayed coating. An analysis of the formation of splats is carried out depending on the substrate temperature.

A mathematical model is constructed for the bubble dynamics, in which the interphase surface variation is presented in the form of a series in spherical harmonics, and the equations are written with the accuracy up to the squared amplitude of the distortion of the spherical shape of the bubble. In the oscillation regimes close to periodic sonoluminescence of a single bubble in a standing acoustic wave, the character of air bubble oscillations in water was studied depending on the bubble initial radius and the amplitude of the liquid pressure variation. It was found that non-spherical oscillations of bounded amplitude can take place outside the region of linearly stable spherical oscillations. Both the oscillations with a period equal to one or several periods of the liquid pressure variation and aperiodic oscillations are observed. It is shown that neglecting the distortions in the form of spherical harmonics with large numbers (i > 3) may lead to a change of oscillation regimes. The influence of distortions on the bubble surface shape for the harmonics with i > 8 is insignificant.

The influence of acoustic resonators on the acoustic and propulsion performance characteristics of a ramjet ejector chamber under conditions with vibration hydrogen combustion was experimentally examined. In the study, resonators having identical throats and different cavity diameters were used. For fixed-volume resonators the best propulsion performance characteristics were achieved in the case in which the cavity diameter differed little from the resonator throat diameter.

The flow of reacting mixture of methanol and steam in a 2D microslot was studied numerically at activation of the reactions on the channel wall. This modelling was carried out in the framework of Navier - Stokes equations for a laminar flow of multicomponent compressible gas. Correlations between thermal, diffusion, and physical-chemical processes were studied under the conditions of intense endothermic reaction and external heat supply distributed along the channel. It is shown that not only the amount of heat supplied to the reaction zone is essential, but also the mode of heat supply along the channel length is important, which allows optimization of the compact reactor for hydrogen production.

Under consideration is the physical nature of the auto-scanning in plasmatorches with tube electrodes; the exclusive simplicity of this process comparing to the aeromagnetic and gas methods of scanning has been demonstrated.

Various forms of icing of flying vehicles are considered. Dimensionless parameters of similarity are chosen and justified. The necessity of creating a climatic wind tunnel for modelling the icing processes is grounded. A possible structural scheme of the climatic wind tunnel is given. It seems reasonable to develop a small-scale test bench for testing methods aimed at generating a uniform field of concentrations of supercooled droplets of a given size and for identifying the efficiency of various anti-icing systems and coatings.

Fuel economy at boost trajectory of the aerospace plane was estimated during energy supply to the free stream. Initial and final flight velocities were specified. The model of a gliding flight above cold air in an infinite isobaric thermal wake was used. The fuel consumption rates were compared at optimal trajectory. The calculations were carried out using a combined power plant consisting of ramjet and liquid-propellant engine. An exergy model was built in the first part of the paper to estimate the ramjet thrust and specific impulse. A quadratic dependence on aerodynamic lift was used to estimate the aerodynamic drag of aircraft. The energy for flow heating was obtained at the expense of an equivalent reduction of the exergy of combustion products. The dependences were obtained for increasing the range coefficient of cruise flight for different Mach numbers. The second part of the paper presents a mathematical model for the boost interval of the aircraft flight trajectory and the computational results for the reduction of fuel consumption at the boost trajectory for a given value of the energy supplied in front of the aircraft.

Results of an experimental study of turbulent breakdown in gradient boundary layer at high freestream turbulence are reported. For the first time it is shown that, like the flat-plate boundary layer, the wing boundary layer at high freestream turbulence is modulated with streaky structures. One of possible mechanisms underlying the generation of turbulence spots in wing boundary layer is modelled assuming the interaction of streaky structures with high-frequency waves. Qualitative and quantitative data concerning the evolution of streaky structures in swqpt-wing boundary layer and in swept-wing boundary layer are presented. Certain differences between the evolution of streaky structures in wing boundary layer and in flat-plate boundary layer are revealed.

The harmonics of Tollmien-Schlichting waves in a compressible boundary layer of a plate are computed with the aid of nonlinear parabolized stability equations. At the (downstream) growth of the second harmonic amplitude up to the values of the order of the basic harmonic amplitude, the amplification rate of the latter is shown to increase abruptly. A similar rapid deviation from the results of the linear theory characterizes the onset of the boundary-layer transition to turbulent state. Computations are carried out for the Mach numbers М = 0.01 and 2.

Results of experimental investigation of temperature distribution over the surface of a complex heat exchanger (the Frenkel packing type) are presented. Measurements were carried out in the air flow between two sheets with triangular corrugations directed at 90° to each other. Measurements were carried out by the microthermocouples glued on the heated outer surface. The effect of Reynolds numbers, a gap between corrugated sheets, and substitution of one corrugated sheet by the smooth one on temperature distribution over the heat exchanger surface in the turbulent air flow is analysed. According to the performed experiments, there is a significant effect of a gap and applied perturbations on the type of temperature distribution over the perimeter of a heated cell.

Visualization data and results of combined measurements of flow quantities in flow with separation past a rib at nominally laminar regime of channel flow are reported. In the separation region, the flow is found to be essentially three-dimensional and unsteady, exhibiting a distinct cellular structure and flow zones with transverse motion. It is shown that the rib-induced flow separation gives rise to low-frequency fluctuations of flow velocity and initiates the turbulence transition in the channel flow. The critical Reynolds number at which flow instability starts developing in the channel is estimated. It is shown that at Reynolds numbers higher than the critical Reynolds number the linear integral scale of flow velocity fluctuations in the channel is defined by the duct size.

Effects of the main flow pulsations on the unsteady adiabatic film cooling efficiency were investigated. The possibility of using the critical value of the modified Strouhal number for the single-row perforation to identify the quasi-steady flow in the double-row perforation was proved. The penetration of disturbances into the perforation channels due to “plunger” effect was observed. The influence of the imposed pulsations on the adiabatic film cooling efficiency was shown to be weaker for the double-row perforation as compared to the single-row perforation.

A mathematical model was developed to simulate two-phase gas-dispersed flow moving through a pipe with axisymmetric sudden expansion. In the model, the two-fluid Euler approach was used. The model is based on solving Reynolds-averaged Navier - Stokes equations for a two-phase stream. In calculating the fluctuating characteristics of the dispersed phase, equations borrowed from the models by Simonin (1991), Zaichik et al. (1994), and Derevich (2002) were used. Results of a comparative analysis with previously reported experimental and numerical data on two-phase flows with separation past sudden expansion in a plane channel and in a pipe are given.

Oscillations of nonuniform fluidized bed in the slugging regime are considered. A nonlinear one-dimensional model of bed oscillations is developed and investigated, including the equations of the bed surface motion and the oscillations of the pressure drop in the bed for the phase of the surface ascent and for the phase of its fall. A quasi-discrete process of the gas escape from the layer with large bubble and the relaxation (discontinuous) type of oscillations of the bed surface and of the pressure, which are related to the above process, are shown based on the model analysis, the obtained numerical solutions, and the comparison with experiment.

The effect of artificial perturbations on structure formation in the water film flow over a vertical plate with a heater was studied experimentally. To measure the film thickness an eight-channel capacitance probe was used. It is shown that artificial perturbations on the liquid film surface can change the distance between rivulets from the values corresponding to the thermocapillary-wave regime of rivulet formation to the values related to the thermocapillary regime. The distance between the rivulets can be changed at Reynolds numbers higher than those corresponding to the thermocapillary regime. Artificial perturbations do not change significantly the relative amplitude of large waves, but they affect the character of amplitude dependence on the heat flux density.

It is difficult to eliminate the effect of vessel walls and impurities in practical studies of boiling-up of strongly superheated liquids. Therefore, some doubts are always cast upon validity of experiments and their agreement with the classical theory of homogeneous stationary nucleation, impairing the verification of theoretical considerations. This paper reports main results of the theory of homogeneous nucleation and presents formulas for calculating the work of formation of a critical nucleus by an incline of the accessible superheating boundary on isobars and isotherms. Matching of experimental and theoretical values of the work of formation was considered as the homogeneous boiling-up criterion. Calculations by isobars and isotherms were made for some liquids. The theoretical values of the work of formation of a critical nucleus were 1.23-4.5 times higher than the corresponding empirical values for all the liquids, i.e., they were much higher than the calculation error. Thus, boiling-up of the studied liquids was not homogeneous and, hence, it did not correspond to the classical theory of nucleation.

A numerical and analytical model is proposed for the impact of a hollow melt droplet onto the surface of a solid polished substrate. The model is based on integral laws of the mass and energy conservation of the colliding droplet, it accounts for capillary and adhesion properties of the melt. The main parameters of the high-velocity deformation of a hollow particle have been computed: the variation of its height, shell thickness, and the contact spot diameter up to the moment of the spreading droplet solidification as well as the pressure variation inside the droplet until the moment of the shell rupture. The critical pressure value at which the rupture occurs is estimated by a formula characterizing the spherical shell strength. Quite a fair agreement of the computed values of the final diameter of the splat of a spread and solidified droplet with the data of physical experiment  is shown.

The problem of drying a porous permeable material is considered. Self-similar solutions are derived for the process of the moisture diffusion transport. The dependence of drying intensity on the porous medium initial state as well as on the parameters of external effect is investigated.

Based on a mathematical model of a porous reacting medium the statement and mathematical solution are presented for a problem of the peat smoldering rise as a result of the effect of a lower seat of fire. It is found that at moderate temperatures T ≤ 750 K the firing and smoldering of the initial reagent are determined by the intensity of the external burning seat as well as by the processes of the peat drying, pyrolysis, and the oxidation reaction of the coked fixed bed.