The dynamics of the Taylor vapor bubble rise in a circular (16 mm cross-sectional diameter) and annular (25 and 16 mm cross-sectional diameters) channel at saturation pressure was experimentally studied. The bubble formed during boiling of superheated degassed liquid. It was found that the movement velocities of its front and rear surfaces are variable and depend on the rise regime, its position in the channel, and the previous rise dynamics. It is shown that this behavior is determined by the spatiotemporal distribution of liquid temperature at the bubble boundary. Based on the obtained results, a map of rise regimes was plotted, linking the nature of movement with the magnitude of liquid superheating near the vapor bubble surface.
The paper presents the results of measurements and calculations for key parameters of a thermoelectric cooling system. Measurements on temperature and electrophysical parameters at different temperature conditions and operation modes defined the values of heat exchanger’s thermal resistance, cooling capacity, and cooling coefficient of performance and temperature drops in the constructive parts. Heat transfer processes in a model are described through integral relations with the account for measured working characteristics of a thermoelectric unit and thermal resistances of heat supply/removal devices. Results of analysis suggest recommendations for a range of current for a thermoelectric unit that provide high values for cooling capacity, cooling coefficient of performance and optimal economic aspects of the cooling system.
The influence of heat flux density on the pressure drop of two-phase gas-liquid flows in a slit microchannel with local heating was experimentally studied at a fixed liquid mass velocity and varying mass velocities of gas. It was shown that at low gas mass velocities, the pressure drop increases due to increased interfacial interaction; at high gas mass velocities, it decreases due to intense evaporation, wall drying, and reduced liquid-wall friction. At moderate gas mass velocities, it remains virtually unchanged due to the mutual compensation of two opposing effects: increased interfacial interaction and reduced liquid-wall friction.
This paper presents a comprehensive study on rheological and viscoelastic properties of aquatic solutions of polyacrylamide (PAM) which is modified by single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWVNTs). Research was conducted with a modular rheometer at the temperature of 25 °C. The tested compositions with the base of PAM (0.05 wt.
Comparative modeling of plasma jets generated by a direct-current plasma torch using straight and angled nozzles was carried out. The geometry of the angled nozzle developed by the authors was numerically analyzed; this nozzle was created for air-plasma spraying of powder coatings onto the inner surfaces of small-sized parts. The gas dynamics of the mixing of a high-temperature plasma gas jet with flows of cold carrier and focusing gases in the nozzle was studied. The proposed aerodynamic model and calculation algorithm were validated by comparison of the coordinates of shock diamonds along the jet length in the off-design mode of plasma torch operation, which showed acceptable agreement with full-scale experiments. The effect of wall cooling on the jet enthalpy was calculated, and its significant role was demonstrated in the case of using an angled nozzle. The distances and optimal gas flow rates at which the velocity and temperature profiles along the jet cross-section acquire an axisymmetric and uniform appearance were determined.
