Home – Home – Jornals – Thermophysics and Aeromechanics 2009 number 2

In this work we consider an experimentally observed effect of significant increasing of the residual heat in metal targets at their irradiation with femtosecond laser pulses in an ambient gas in respect to the vacuum conditions. Numerical modelling of heating of a platinum target by femtosecond laser pulses in argon under normal conditions has been performed taking into account gas breakdown in the focussing region of the laser beam in front of the target. The applied model is based on a combination of a thermal model describing heating and phase transitions in irradiated samples and a hydrodynamic model to describe motion of the ambient gas perturbed by laser irradiation as a result of multiphoton ionization. The hot ambient gas is shown to heat efficiently the irradiated sample. The hydrodynamic processes in the ambient gas play an important role in heating.

The mathematical model of water vapor condensation was developed for the direct simulation Monte Carlo method. This model describes a chain of reactions providing formation and decay of water clusters with consideration of accompanying energy transfer processes. One-dimensional expansion of water vapor into vacuum from an evaporating spherical surface was studied numerically within the range of parameters, corresponding to the flows transitional by the Knudsen number. The influence of condensation on the gas-dynamic pattern of the flow, including the parameters of the Knudsen layer, is discussed.

Gas-dynamic mechanisms of the shock-wave structure restructuring is investigated in a transonic flow around an airfoil at a pulsed-periodic energy supply. It is shown that it does not reduce only to the effects of the low-density wake forming behind the energy source on the shock wave but represents a combination of several factors, of which the "explosion" character of the disturbance development is determining.

An analysis of the losses of heat into the walls of settling chamber in a hypersonic hot-shot tube has been performed. Tests without diaphragm rupture showed that the fall of settling-chamber pressure during the operating flow regime in the tube was the consequence of the transfer of heat from working body to wall; this has allowed us to evaluate the heat-transfer coefficient α and the inner-surface temperature of the wall T_w. An empirical formula relating the coefficient α with the pressure and working-body temperature in the settling chamber in the range of pressures and temperatures 160 to 540 bar and 700 to 3400 K was obtained. Using the gained dependences of α and T_w on pressure and temperature, we have developed a physical model for calculating the working-body characteristics in the tube with allowance for enthalpy losses. We found that by the hundredth millisecond of the operating regime the disregard, in such calculations, of the wall heat flux in the first settling chamber resulted in overestimation of the stagnation temperature in the test section in comparison with similar calculations made without allowance for the heat losses by 6−18 % in terms of the full-scale temperature for aircraft flight in Mach number range 5 to 8. The developed calculation procedure has been tested in experiments without diaphragm rupture.

An experimental study of spectral characteristics of unstable flow in the mixing layer of supersonic axisymmetric underexpanded jets with Mach number M_a = 1 is reported. The destabilization of the flow is related to the formation, in the mixing layer of the jets, of disturbances in the form of streamwise vortical structures of the Taylor ⎯ Goertler type. As a result, in the mixing layer there forms an azimuthally non-uniform stationary distribution of total pressure. The Fourier transform of azimuthal sweeps of non-uniformity in pressure distributions was used to calculate the amplitude-wave spectra. An analysis of the spectral characteristics has allowed us to evaluate the longitudinal increment of amplitude growth of the disturbances and their dependence on the wavenumber and on the nozzle pressure ratio. The range of wavenumbers in which the streamwise vortical structures grow in amplitude is identified.

The main principles and methods of developing the single-layer monolithic compliant coatings for turbulent drag reduction are presented.

In the present study, the distribution of circumferential velocity components of carrier flow in separation elements of air-centrifugal classifiers of powder materials has been experimentally examined. The study was carried out for variously contoured separation channels, and also for various conditions of a carrying agent (air) input in a separation zone. Optimal conditions for particle separation throughout the whole volume of the separation element can be organized by making the channel contour diverging towards the center of rotation. The experimental data gained in this study supplement the mathematical model for the aerodynamics of turbulent carrier flow in the shaped rotating separation elements of air-centrifugal classifiers and make an analysis of the separation-zone flow possible.

Measured data for the temperature of a porous spherical surface to which an evaporating binary liquid mixture was supplied are reported. In the experiments, solutions of ethyl and methyl alcohols in water, and also solutions of acetone in water, were used. The concentration of mixture components was varied throughout the widest possible range of X_L = 0−1, and the temperature of dry air flow past the sphere was in the range t₀ = 15−300 °C. In the present study, a strong influence of the composition of the mixtures on their adiabatic evaporation temperature was established. In the heat- and mass-transfer process, the air temperature is also of paramount importance. An experimental correlation is obtained which generalizes data on adiabatic evaporation temperature in a broad range of component concentrations and temperatures for the experimentally examined binary liquid mixtures.

The investigation of the influence of additional water injection in a classifier on the particle separation process characteristics has been performed on the basis of numerical modelling. It has been shown that the increase in water injection velocity leads to the increase of both the cut size and the minimal value of separation curve. The change of the injector opening size influences only the minimum value of the separation function not altering the cut size at the specified water injection velocity.

Wave processes in chemically active multicomponent media: liquid ⎯ gas bubbles ⎯ liquid drops have been studied experimentally. Existence of detonation waves in multicomponent (bubble-drop) media has been proved. Structure of detonation waves in bubble-drop and bubble media is qualitatively identical: detonation waves are solitary waves with pulsation profile the pressure behind which is close in value to the one in unperturbed medium. Propagation velocity of detonation waves in bubble and bubble-drop media drops with the increase in medium gas phase concentration and with the decrease in carrier liquid viscosity. Presence of liquid drops decreases detonation wave velocity compared with bubble medium that does not contain liquid drops. Detonation wave propagation in multicomponent media causes gas bubbles fragmentation as well as fragmentation of individual liquid drops.

Thermal conductivity and thermal diffusivity of "ozone-safe" refrigerant R134а in liquid state within the range of temperatures 295.9−354.9 K and pressures from the liquid ⎯ vapor equilibrium line up to 4.08 MPa have been studied by high-frequency thermal-wave method. The experimental uncertainties of temperature, pressure, thermal conductivity and thermal diffusivity measurement errors were estimated to be 0.1 K, 3 kPa, 1.5 and 2.5 %, respectively. Values of thermal conductivity and thermal diffusivity of liquid R134а on saturated line have been calculated. Approximation dependences for thermal conductivity and thermal diffusivity within the whole studied range of temperatures and pressures as well as on the saturated line have been obtained.

The critical regimes of heat exchange at a laminar steady flow of a pseudo-plastic liquid in the initial interval of a coaxial duct are investigated with regard for both dissipative and chemical sources in the Arrhenius representation [5] under the conditions of an insignificant variation of the concentrations of reacting substances.

Here we consider one-dimensional heating of a layer of gray semitransparent medium by an outer source of radiation and convection. The sample boundaries reflect, absorb (radiate), and transmit radiation. It is shown that heating dynamics and character of temperature fields depend significantly on optic parameters of the boundaries.

The work of a pulsed aerosol system for fire fighting is modelled, which is designed for fire fighting at oil storages and at the spills of oil products, whose vapors were modelled by gaseous methane. The system represents a device for separate installation, which consists of a charge of solid propellant (the gas generator) and a container with fine-dispersed powder of the flame-damper substance. The methane combustion was described by a one-stage gross-reaction, the influence of the concentration of vapors of the flame-damper substance on the combustion process was taken into account by reducing the pre-exponent factor in the Arrhenius law and was described by an empirical dependence.
The computational experiment showed that the application of the pulsed aerosol system for fire fighting ensures an efficient transport of fine-dispersed aerosol particles of the flame-damping substance and its forming vapors to the combustion zone; the concentration of particles ensures the damping of the heat source.

An exergy analysis of possible regimes of energy supply to the air flow in the ramjet duct is carried out. A condition for the supply of a given heat amount to supersonic flow and a condition of the passage across the sound velocity are obtained for a duct with variable cross-sectional area. An analysis of the flow in a model ramjet duct at a pulsed-periodic energy supply is carried out. For a clear demonstration of possible schemes of heat supply in such a duct, a diagram in the temperature-exergy coordinates is proposed. A duct configuration in which the heat supply to supersonic flow is realized with regard for the limitation of the gas static temperature is proposed.

Fundamental principles of complex exergy analysis of new boiler technologies within power-generating units of thermoelectric power stations are set out. Results of analysis are stated.