Journal of Engineering Thermophysics最新文献

This paper studies the validity of prediction tools for two-phase flow pressure drops in wide range of reduced pressures based on the comparison between new experimental results and theoretical results predicted with the commonly used methods. The original dataset was obtained in a vertical uniformly heated minichannel 1.1 mm inner diameter with R125 and RC318 as working fluids. Uniform heating was carried out by electric current, simulating real flow conditions in heat exchangers, which is a distinctive feature of this work from most similar studies. The mass velocity varied in the range from 200 to 1400 kg/(m²s), the reduced pressure varied from 0.132 to 0.70, the heat flux density range was from 4 to 322 kW/m², the inlet vapor quality was set from −0.2 to −0.06 and outlet vapor quality reached 1 at minimum flow rates. The database is composed of 115 data points of two-phase flow boiling and was compared against well-known two-phase pressure drop prediction methods. The effect of the reduced pressure on the ability of the methods to predict the pressure drop was pointed out.

Analytical modeling of heat transfer during condensation onto a horizontal round tube placed in a hydrophobic granular layer has been performed. According to generalized experimental results of the authors, the area under study was divided into three regimes of the condensate film flow: Re (< 5), (5 < {rm Re} < 10), and Re (> 10). For the first two regimes, in the absence of effect of capillary forces, theoretical solutions were found based on the representation of a near-wall pore channel in the form of a flat annular slot with hydrophobic side surfaces; the solutions are in good agreement with the experimental data. At Re (> 10), a self-similar regime of the hydrodynamics of the condensate film settles, independent of the Re number, with a constant mean film thickness over the tube perimeter and the condensate part not involved in the heat transfer in the tube draining into the pore space of the layer. In all analyzed cases, there is heat transfer deterioration by two to three times on a horizontal tube in a hydrophobic layer in comparison with a smooth hydrophilic tube, because of the peculiarities of the hydrodynamics of the condensate flow in the wall pore channels. For all modes, formulas were obtained for calculation of the Nusselt numbers of heat transfer in dependence on the Re number.

The dynamic development and enhancement of the efficiency and safety of power engineering in the Arctic and remote Siberian regions of Russia are promising and relevant. The aim of this work is search for scientifically based approaches and methods against icing, which is one of the main problems hindering efficient use of wind turbines for autonomous power supply to remote settlements in the Far North. The necessity in the research and its relevance are confirmed by the growing interest in the development of the Arctic region by the leading world countries. The presented research strives for an optimal strategy against icing of wind turbine blades in climatic conditions typical of the Arctic coast of Russia. The efficiency of combined anti-icing methods relying on the use of aerodynamically transparent substrates, hierarchical superhydrophobic (HSH) coatings, and materials based on polytetrafluoroethylene fiber for wind turbine blades in Arctic climatic conditions was experimentally studied. It is a fundamentally new approach, which has no world analogues. For verification of the efficiency of the de-icing systems and identification of the most efficient protection methods or combination of them, an experimental comparison was made for the efficiency of superhydrophobic coatings when used separately and together with various traditional de-icing methods based on heaters and ultrasonic and vibration devices. It has been shown that the integral use of the proposed methods and approaches successfully solves the problem of developing a general anti-icing strategy.

Currently, we investigate the collision process of water droplets on cold cylindrical copper surfaces by means of a video camera and a cooling testbed. The solidification of water vapor on cold metal surfaces increases the friction of contacting liquids is an unavoidable factor, so we experimented with a uniform atmospheric pressure and relative humidity environment. The paramount purpose of this experiment was to avail oneself of the change in viscosity due to temperature change and the change in radius of copper cylinder to understand its effect on droplet impact conducting heat and freezing. The results show that the substrate viscosity (frost layer) has marginal effect on the time for a droplet to reach maximum diffusion in the two main droplet movement directions. In addition, droplet diffusion on cold cylindrical copper surfaces consists of three processes: spreading stage, transitional stage and steady stage. Among these three phases, power function fitting works best in the spreading stage. Besides, we have used the composite spreading coefficient (gamma) to describe the speed of spreading. For any radius cylinder, the cooler the temperature, the bigger the average value of the composite spreading coefficient (gamma) below 0°C than above 0°C. The larger the composite spreading coefficient (gamma) is, the more slowly the droplet dimensionless spreading arc length changes with dimensionless time. Moreover, droplets between 0°C and −5°C sometimes show post-collision supercooling, which is related to surface viscosity instability and the contribution of surface shape to droplet retraction.

The paper presents an experimental study of the process of methane hydrate formation from stabilized water foam. In all cases, the hydrate formation front started from the region inside the foam. Upon reaching the foam-solution boundary, it initiated the formation of polycrystalline conical conglomerates at this boundary - hydrate needles oriented deep into the solution. A mechanism for their formation and subsequent spontaneous shortening of some of them is proposed.

A numerical analysis of the flow structure and thermal efficiency of a gas-droplet jet injected through a radial annular slot into a single-phase air cross-flow has been performed. The calculations were carried out via the axisymmetric RANS approach for the following range of the main parameters of a two-phase flow: the initial size of water droplets (d_{1}=0)–20 (mu)m and their mass concentration (M_{L1}= 0{–}0.1). Gas turbulence was described in a model of transfer of the Reynolds stress components for a two-phase flow. Because of the presence of evaporating liquid droplets, even at relatively small mass concentrations not exceeding 5% of the mass of the secondary flow, the thermal efficiency during transverse injection could more than double compared to the injection of a single-phase radial jet.

The results of experimental investigation of thermal pulsations in nucleate and film modes of water boiling under Joule heating of a wire heater and a porous cylindrical rod are presented. Pulsation power spectra have been determined from experimental data. It has been shown that in the transition modes of boiling from nucleate to film boiling the frequency dependence of the power spectra acquires a characteristic (1/f) form. Such frequency dependence of the spectra indicates the possibility of large-scale low-frequency emissions. The pulsation power spectra found from experimental data can be used to diagnose the transition to the crisis mode of heat transfer.

The hybrid nanofluid is extensively used in manufacturing for industrial uses because of its exceptional property of enhancing the heat transfer process. The purpose of the present work is to find novel explanations for the behavior of thermal radiation and inclined magnetohydrodynamics effects on the convective viscoelastic flow of water Al₂O₃–Cu hybrid nanofluids over an accelerating permeable surface with mass transpiration. The viscoelastic liquid concept is postulated with the benefit of hybrid nanofluids employing conventional flow patterns that are impacted by the magnetic field. Thermophysical properties of Al₂O₃–Cu and water are employed. Nonlinear PDE for momentum, temperature, and concentration are converted into non-dimensional ODE by employing the proper similarity transformations. The current study is reported to be in very good accordance with earlier research. The velocity field and energy distributions were depicted graphically to show the influence and typical behaviors of physical factors such as the viscoelastic parameter, the Richardson number, the radiation number, etc. In industrial applications, the temperature distribution influenced by radiation is quite important, specifically in accelerated plates where cooling the liquid is necessary to achieve the desired outcome.