Vol 55, No 4 (2017)

This paper provides theoretical and numerical investigation carried out on a vertical up-flow model risen to investigate the jet maximum height based on buoyancy frequency parameter σ_i and plume function Γ_i. We study the characteristic solution of the maximum height reached by the fountain through an asymptotic approach. Results show a discontinuity of affinity solution approximation at Γ_i≫1. The fluid became very negligible and ended with a fall back. The entrainment coefficient effect had no significant influence on the topological behavior profiles development of this jet with negative buoyancy force.

Variants of the differential equation of heat conduction in a solid body, which follow from the Fourier and Cattaneo–Vernotte hypotheses and the Lykov equation, are considered. A boundary value problem describing temperature fields in a body (cylinder) upon cyclic heat transfer with cold and hot media is formulated. An analytical solution to the boundary value problem with a hyperbolic differential equation of heat conduction with allowance for thermal relaxation and temperature damping with cyclic boundary conditions of the third kind is given. The thermal transient processes calculated by the classical heat conductance equation and hyperbolic equation of heat conduction on the axis of the cylinder at different values of factors such as the ratio of the thermal damping time to the thermal relaxation time, the duration of cyclic periods, the Fourier relaxation number, and the Biot number are compared. A conclusion is made that the theory of regenerative air heater should be improved by taking into account thermal relaxation and thermal damping in the nozzle and measurements of the thermal relaxation and thermal damping times of the corresponding materials.

A closed method is proposed for recovering heat fluxes to anisotropic bodies under conditions of aero-gasdynamic heating from experimental temperature data at spatial-temporal nodes. The thermal protection of a body is made of anisotropic materials with components of thermal-conductivity tensor, which are dependent of temperature, i.e., are nonlinear. The method is based on approximating a spatial dependence of a heat flux by a linear combination of basis functions with sought coefficients (parameters), which are found by minimization of a quadratic functional of the residual (the discrepancy between experimental and theoretical temperature values) using the implicit method of gradient descent, as well as on constructing and numerically solving problems for the determination of sensitivity coefficients. To increase the degree of correctness of an inverse problem, along with a main functional, the regularizing functionals have been constructed and utilized on the basis of smoothness requirements for spatial functions of heat fluxes to have continuous first and second derivatives, which allowed heat fluxes with the coupled heat transfer to be recovered in the form of arbitrary functions: monotonic, nonmonotonic, having extrema, inflection points, etc. Numerous results of recovering heat fluxes to anisotropic bodies are obtained and discussed, with the regularization parameter being selected for every case.

The formation of an overdriven detonation wave in methane-oxygen mixtures in an axially symmetrical channel with a variable cross section was experimentally investigated. The ignition of gas mixture was carried out using the spark gap, located at the closed end of the channel. To create the overcompressed shock detonation wave, the decay of the stationary detonation wave was performed at the transition to the channel of a larger cross section. The created complex of shock wave and flame front, moving behind it, propagated in a channel with conical narrowing. The formation of the overdriven detonation wave, with parameters exceeding the parameters of Chapman–Jouguet stationary detonation by a few times, was registered at the outlet of conical narrowing. The rates and pressures on the front of the detonation wave were determined, depending on the mixture composition. The sizes of detonation cells, diagrams of compression waves propagation, flame front, and detonation wave in a combustion chamber, depending on the mixture composition, were presented.

An analysis and generalization are performed of experimental data on near-wall turbulence damping under the influence of stable thermal stratification of density in vertical and horizontal tubes with rectangular and circular cross-sections.

Emission spectra of gas discharges resulted from dipole allowed transitions between triplet states of the hydrogen molecule are placed in the near ultraviolet, visible and near infrared wavelength ranges. This makes them attractive for use in spectral diagnostics of gas discharges. An improved collisional-radiative model is used for analysis of applicability of optical emission spectroscopy based on the emission of triplet states of molecular hydrogen for diagnostics of non-equilibrium microwave plasma.

Simple algorithms are developed to proceed the probe characteristics in a number of limiting regimes when the characteristic probe size, r_p, is relatively large (r_p > 10³rD). It relates both to steady plasmas and to movement with the directed velocity, u. We consider the cases of collision-free (the Knudsen number, Kn≫1) and collisional (Kn≪1) plasmas. The majority of the proposed algorithms are tested in practice and confirm their reliability.

Results of calculations are presented for the transition of a charged gas suspension from the initial nonequilibrium state with a spatially nonuniform distribution of the average density to a state with a uniform distribution of the average density and charge in a bounded volume. A numerical solution is obtained for the system of equations describing the motion of a polydisperse multirate and multitemperature gas suspension, the particles of which carry an electric discharge and create a self-consistent electric field.

The heat capacity and thermal diffusivity of a polymer composite based on multiwalled carbon nanotubes (95%), produced by electrospinning in the temperature range of 300 ≤ Т < 450 K, are investigated during heating and cooling. Temperature hystereses are detected characteristic of first-order phase transitions.

The semiempirical expression of the free energy of lithium deuteride in the form of an analytical function of volume and temperature for the pressure range of 0–100 GPa and at 200–2000 K was developed. The thermodynamic description was based on experimental data of thermophysical properties at normal conditions and lithium deuteride Hugoniot. The predictive calculations of a set of thermophysical properties of lithium deuteride at high pressures (including shock conditions) and temperatures were performed. A comparison of isotherms, isobars, heat capacity, and thermal conductivity with literature experimental data was presented. The results made it possible to consider, compare, and coordinate the data on the shock and isothermal compression of lithium deuteride at high pressures and temperatures from a unified point of view.

An equation of state is a fundamental characteristic of a substance. It is necessary in numerous studies and practically important problems of high energy density physics. In this review, we consider the modern requirements to equations of state, theoretical and experimental methods used to study the thermodynamic properties of a substance, different aspects of constructing wide-range equations of state, and examples of application of wide-range equations of state in simulation of high-energy processes.

It is shown that allowance for fluctuations leads to the appearance of additional terms in the Hamiltonian of the system and in the thermodynamic parameters of the substance. As a result, it becomes possible to describe the critical phenomena and phase transitions of liquid–vapor within classical statistical mechanics (without taking into account fluctuations, this is impossible).

A method for experimental determination of the degree of adiabaticity of a sample when measuring the thermal diffusivity by the temperature waves method is described. Analysis of the thermal diffusivity of ARMCO iron at different temperatures is performed as an example. It is experimentally proven that the twodimensional thermal model gives an adequate description of the temperature-wave propagation in a sample of finite sizes.

Results of experimental research on heat transfer in a plane converging channel in a steady regime and at superimposed harmonic pulsations of operating medium (air) are presented. The distribution of the heat transfer coefficient through the length of converging channel has been obtained for all regimes in a range of frequencies from 10 to 190 Hz. Heat transfer enhancement effect has been established for flow pulsations in relation to the steady regime. The integral effect along the converging channel reached 25% compared with heat transfer in the steady flow. The correlation between the increase in heat transfer coefficient and frequency and amplitude of flow velocity pulsations is shown.