Journal of Engineering Thermophysics最新文献

The first part of the work sets out the problem for a method of constructing correction coefficients to compensate for distortions introduced by the non-uniform frequency characteristics of the electronic path of a laser Doppler velocimeter into the spectral density of Doppler signals. The method relies on averaging a statistical ensemble of the spectral densities of noise signals obtained during illumination of a photodetector by a white light source, constructing the inverse function to the averaged spectral density acquired with truncation in the low- and high- frequency bands, and applying digital low-pass filtering to it. A model numerical experiment has been carried out. The results of the experiment are presented, and they confirm increase in the velocity measurement error as the flow rate grows.

This study focused on the numerical investigation of heat transfer from various shaped copper plate surfaces exposed to a constant heat flux of 1000 W/m². The analysis employed a combination of cross-flow and impinging jet flows. The numerical simulations were carried out by solving the energy and Navier–Stokes equations using the Ansys–Fluent computer program with the (k)-(varepsilon) turbulence model. To guide the combined jet flow towards the heated surfaces in the channel, horizontal fins with 30° and 60° angles were placed in the channel. The channel height ((H)) was set at 4 times the diameter ((D)), and the fin distance from the jet inlet ((N)) was equal to (D). Different fin lengths of (D), (1.166D), and (1.333D) (K) were utilized in the channels. The study considered three types of fluids: water, 2% CuO-Water, and 0.02% GO-Water nanofluids. The upper and lower surfaces of the channel and the fin were assumed to be adiabatic, and the Reynolds number of the flow ranged from 5000 to 15000. The results of this work were compared with experimental studies from the literature, and good agreement was found between them. Unlike previous studies, this research explored the effects of fin lengths and fin angles on heat transfer from differently shaped surfaces and the flow structures created by the fins in the channels, using GO-Water and CuO-Water nanofluids. The results were presented in terms of the mean Nusselt number variations for each model surface. Additionally, velocity and temperature contour distributions of the combined jet flow along the channel for CuO-Water nanofluid were analyzed. Performance Evaluation Coefficient (PEC) values, along with average Nusselt number (Nu(_{m})) and surface temperature ((T_{m})) values, were assessed at different Reynolds numbers for all three patterned surfaces in the channels. At Re = 5000 and (K=D), there were significant increases in Nu(_{m}) values for cube, roofed, and square hollow model surfaces when using a 60° fin and GO-Water nanofluid compared to channels with water flow and no fins.

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.