High Temperature最新文献

Abstract

A method is proposed for solving the problem of a pulsating quasistationary flow in a channel, based on the use of calculation results for stationary flow. This approach is applicable at low relative oscillation frequencies (for Womersley numbers less than one). The solution to the system of stationary equations of motion and continuity in the initial section of a flat channel was carried out by the finite difference method using an iterative implicit unconditionally stable scheme. The hydrodynamic characteristics of a developing pulsating laminar flow in a flat channel have been studied. The results of calculating the longitudinal velocity component and the Poiseuille and Euler numbers are presented as a function of the relative amplitude of the oscillation of the cross-sectional average velocity A and dimensionless length of the channel. It was found that for A values exceeding unity, the period-averaged coefficients of hydraulic resistance and friction resistance near the inlet to the channel are significantly higher than these values for a stationary flow. It has been shown that in order to achieve a pulsating flow with large amplitudes of oscillations, it is necessary to create a time-average pressure drop approximately three times higher (at A = 5) than for a stationary flow.

Abstract

The influence of porosity and pressure on the parameters of the equation of state of a porous material is studied. The model takes into account the presence of gas in the pores. The equations of state of the mixture and its components, both solid and gaseous, are presented uniformly (in the form of the Mie–Grüneisen equation with the Grüneisen coefficient depending on density). To describe the Grüneisen coefficient, a logarithmic dependence on density is used. Numerical calculations of shock-wave loading and isentropic unloading of copper samples of various porosities are carried out. It is shown that the parameters of the equation of state obtained under normal conditions adequately describe the behavior of the substance both under loading and unloading.

Abstract

The paper studies the thermal conductivity and specific heat capacity of carbon fiber reinforced plastic (CFRP) with various reinforcements using the methods of a stationary heat flow (SHF) and differential scanning calorimetry with temperature modulation. The values of the thermal conductivity and heat capacity, as well as their dependence on temperature, are established in the temperature range from –20 to 100°C. The changes in the thermal conductivity range from 0.400 to 0.515 W/(m K); and the specific heat capacity coefficient, from 923 to 984 J/(kg K). The results obtained can be used to calculate and design systems and installations using PCMs as structural materials and to calculate the parameters of the technological process for the production of these materials.

Abstract

In the thermodynamic theory of phase equilibria in disperse systems, the size dependences of jumps in entropy and the heat of fusion of particles (including those in the nanometer range) are obtained with strict and consistent consideration of surface phenomena in the approach with separating surfaces. The consideration is carried out taking into account the dimensional dependences of the molar volume, melting temperature, and interfacial tension. Using the obtained relations, calculations were performed for spherical sodium and tin nanoparticles. This implies a decrease in the entropy and heat of fusion with decreasing nanoparticle size. The results are in close agreement with the experimental and calculated data available in the literature.

Abstract

The separation of water–oil emulsions differing in the content and ratio of asphalt-resinous substances, as well as dielectric properties when exposed to a high-frequency (HF) electromagnetic field is investigated. The results of experimental studies are presented, showing the correlation between the degree of phase separation from dielectric parameters and the ratio of asphaltenes and resins in oils. It is shown that when water–oil emulsions are exposed to an HF electromagnetic field, the degree of heating and the efficiency of phase separation of the emulsion depend on the total content of asphaltenes and resins, their ratio, and whether the operating frequency of the generator falls within the region of resonant frequencies of the emulsions.

Abstract

The relations of the model of a two-phase local equilibrium region are used to calculate the temperature dependences of the heat capacities and coefficients of thermal linear expansion of the palladium triad (Ru, Rh, Pd) and platinum triad (Os, Ir, Pt) in the presence (absence) of an aggregate transition in the studied temperature range. In contrast to the approximation functions used in the scientific literature in individual temperature intervals (using, in particular, the Einstein function), the proposed formulas are simple, universal, and adequately describe the experimental data in the temperature range from 0 K to high temperatures. They can be used to create computer programs for calculating the specified characteristics of various solids, for example, when developing technologies for the rational use of noble metals.

Abstract

In this study the applicability of the binary mixture model utilizing the first-order gas-kinetic Chapman–Enskog theory is substantiated for describing diffusion processes at different degrees of ionization of a single-temperature simple gas plasma consisting of three components: atoms, ions and electrons. On the same bases, the obtained expressions for a trinary mixture are applicable to a plasma with a fourth component that is difficult to ionize. The thermal diffusion relations of a trinary mixture are derived, whose peculiarity is the electronic component, which does not affect the diffusion flows of atoms and ions. It is shown that in a highly ionized He arc plasma with developed diffusion and ionization nonequilibrium, thermal diffusion is insignificant. It is noted that when thermodynamic equilibrium is violated, the diffusion coefficients may not change at all or decrease by half at most.

Abstract

Using experimental data available in the literature on the contact angles of some refractory metals with oxide melts, calculations of their interfacial values at certain temperatures were performed. Because the surface energies of solid metals are measured at temperatures different from their melting points, these values are reduced to the temperatures of contact angle measurements so that Young’s equations can be applied.

Abstract—This paper studies the development of approaches to expand the method for calculating the properties of liquid sodium in the region of overheating and at high temperatures. The expansion is carried out using the hypothesis of the linearity of isochores and interpolation of the properties of sodium on the saturation line using the theory of critical exponents. Using modified techniques, the position of the spinodal is calculated and an expression is given to estimate its position. A comparison is made with the available published data. Satisfactory agreement is obtained.

Abstract

Using direct numerical simulation, a problem of the interaction of a Mach wave with a boundary layer on a flat plate, streamlined by a supersonic perfect gas flow at the Mach number M = 2.5 is considered. The influence of the intensity of the incident Mach wave on the laminar-turbulent transition (LTT) is studied. It is shown that the incidence of a Mach wave with an amplitude of 5%, simulating the relative thickness of the roughness on the side wall of the wind tunnel, on the boundary layer leads to the formation of a turbulent wedge in the boundary layer on a flat plate.