Journal of Engineering Thermophysics最新文献

This work studies the stagnation-point flow of a hybrid TiO₂-water nano-fluid over a stretching sheet in the presence of an induced magnetic field. The impacts of heat source and heat transmission processes are investigated by applying induced magnetic field. The similarity transformation is used to transform the modeled equations into nonlinear ordinary differential equations (ODEs). Using fourth order Runge–Kutta (R–K) method, the governing equations obtained from the given mathematical modeling are solved numerically. The significance of the relevant factors on temperature, induced magnetic field and fluid velocity is analyzed using figures and tables. A few key conclusions are that the Grashof number lowers the flow field and the heat source parameter heats it.

This paper presents experimental studies on using CO₂ hydrate to extinguish the flame from burning pine shavings. The area of shavings combustion was exposed to an external air flow. The flame was suppressed by means of various agents common in fire extinguishing: sand, NH₄H₂PO₄, water, and foam. In terms of the minimum mass spent on suppression of burning of shavings, CO₂ hydrate powder demonstrated performance exceeding by far that of the other materials. The pressed cylindrical tablet of CO₂ hydrate showed the maximum time required for extinguishing the flame. Thermal imaging measurements showed that in the presence of forced air motion, the water vapor and CO₂ gas flows resulting from dissociation enter the combustion area unevenly, which leads to decrease in the extinguishing efficiency. Carbon dioxide hydrate powder is the most efficient means for extinguishing flame of wood materials.

The dynamics of HFE-7100 droplet interactions with a heated sapphire surface at superheat from 10 to 100°C for Weber numbers from 25 to 345 are considered. Top- and side-view visualizations are performed using a high-speed camera supplemented by a stereomicroscope. Top view images allow us to follow the transitions to different boiling regimes, as well as to observe bubble formation, bubble density, bubble clusters, bubble absence, and the transparency of the spreading droplet during levitation. While side view images show the rebound and levitation heights and angles. The rebound, the formation of a vapor layer, and the conditions under which the droplet levitates as the substrate temperature rises are studied. The formation of finger-like structures, which can then form secondary droplets, is observed for high We number values.

The paper presents a study of the aerodynamics of flame combustion of coal fuel in a burner device with thermal power of 50 kW. The burner device was a two-stage tangential swirler. In the first stage, air was supplied tangentially with axial pilot supply of propane. In the second stage, the pulverized coal mixture with heated secondary air was supplied. During the experiments, the efficiency of coal fuel combustion was studied with co-swirl and counter-swirl of the second stage relatively to the first one. The temperature was measured on the wall and in the center along the muffle swirl chamber. The gas composition of the combustion products was measured at the outlet of the swirl chamber. The profiles of the mean axial and tangential velocity along the length of the swirl chamber were measured with the two-component laser Doppler anemometer (LDA) system. Particles of unburned coal fuel served as signal tracers. The obtained results have led to a conclusion about the efficiency of the system for pulverized coal fuel combustion in the counter-swirl mode for enhancement of mixing and combustion.

The paper presents experimental results on the use of graphene nanotubes aimed at an increase in thermal conductivity of a phase change material. Graphene nanotubes were dispersed in molten paraffin by ultrasonic treatment in an amount of 0.1–0.5 wt. %. The obtained samples of paraffin with graphene nanotubes were examined using a scanning calorimeter. During heating and cooling, the DSC curves of all samples with different content of nanotubes demonstrated two peaks corresponding to phase transitions of the main hydrocarbons in paraffin composition. The presence of nanotubes in paraffin did not significantly affect the shape of the DSC curves. Experiments with sample heating in a thermostat in terms of a time delay in changing the temperature of samples indicated an increase in thermal conductivity of solid paraffin with addition of graphene nanotubes and its decrease, when graphene nanotubes were added to liquid paraffin. These effects increased with increasing mass content of nanotubes. The method of steady-state heat flux was used to determine the coefficient of thermal conductivity of samples of a phase change material with nanotubes. According to the measurement results, the maximum increase in the thermal conductivity of paraffin in the solid state was 22% at a nanotube concentration of 0.5 wt. %. The thermal conductivity coefficients of various materials with “contrasting” inclusions were compared according to the results of calculations and experiments.

This study intends to give a detailed research of surface angles of impingement, and jet-plate distances to enhance heat transfer and improve the channel’s flow structure features for twin jet impingement. For this aim, numerical analysis of twin jet impingement was carried out for distinct impingement surface angles ((alpha)) of 30°, 45°, and 60° and impinging jet-plate distances (H) of 3, 4.5, and 6. The numerical investigation was performed as a steady, two-dimensional, employing the (k)-(varepsilon) turbulence model and the Ansys–Fluent program in the twin jet rectangular channel. While the isothermal flat plate impingement surface has 310 K (T(_{{rm H}})), the impinging air jet fluid temperature is 300 K (T). The obtained outcomes were compared with the numerical and experimental outcomes of the work in the literature and it was noticed that they are determined to be compatible. The results were presented as the local Nu number (Nu(_{L})) variations for (alpha=30^{circ}), 45° and 60° impingement surface angles along the flat plate impingement surface, and the variations of mean Nu numbers (Nu(_{m})) for different (alpha) and H values with the Re number ranges of 20000–50000. Besides, streamlines and temperature isotherms were evaluated for different (alpha), H and Re number values by visualising them in detail. The results displayed that while the Nu(_{m}) values are higher than the case without angle ((alpha=0^{circ})) for all angles and H values, after the 30° angle, the Nu(_{m}) values decrease slightly at 45° and 60°. Accordingly, for H = 6, the Nu(_{m}) value of 30° impingement surface angle is 3.09% higher than that in 60°.

A method has been developed for constructing correction factors to compensate for distortions introduced by 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 spectral densities of noise signals obtained during illumination of a photodetector with a reference white light source, constructing the inverse function to the resulting averaged spectral density with truncation in the low- and high-frequency bands, and applying digital low-pass filtering to it. In the first part, a model numerical experiment was carried out. The gas flow velocity at the Vitoshinsky nozzle exit was measured with a laser Doppler anemometer in high-velocity modes. It has been shown that in high-velocity modes (200 m/s), the application of correction factors to the spectral density of Doppler signals allows estimating the average flow velocity with an error of no more than 5%, while calculating velocities without preliminary correction of the spectral density leads to statistical measurement errors of more than 100%.

In the search for a flow pattern optimal for culture growth in vortex bioreactors, our experiments discovered unusual flow structures that existing theoretical models cannot explain. Three immiscible liquids fill a vertical open cylindrical container of radius R = 47 mm whose sidewall is still while the bottom disk rotates driving a fluid motion. The centrifugal force pushes the lower liquid (L, aqueous glycerol) from the axis to the periphery near the bottom, creating its toroid circulation that in turn drives toroid circulations of the middle liquid (M, sunflower oil) and the upper liquid (U, alcoholic glycerine). With increasing rotation, counterflows develop near interfaces LM and MU: the liquids move towards (away from) the axis below (above) the interface that seems paradoxical. At a small thickness of M, a stagnation zone arises where the axial and radial velocities vanish and the M liquid only rotates. As M thickness decreases, this stagnation zone expands and occupies the entire volume of the middle liquid. These counterintuitive results await their theoretical explanation and formulation of new contact conditions at the LM and MU interfaces.

The paper presents a numerical modeling of boundary layer equations supplemented with the (k)-(omega)-(gamma) turbulence model, that describe an accelerated xenon flow at the helium injection through the wall. Authors set values of the acceleration parameter (K) of (4cdot10^{-7}) and the injection parameter (overline{j_{w}}) in the range from (10^{-4}) to (10^{-3}). The problem was solved by the finite difference approach with the implicit scheme. The study showed that the helium as a light gas injection into the accelerated xenon flow may results to the occurrence of the local flow laminarization near the wall together with turbulent flow in the outer part of the boundary layer. It was shown that the increase of the injected gas temperature leads to the increase of the skin-friction in two times under considered conditions. At that, the thermal and mass Stanton numbers increase by no more than 20% relatively the flow with the quasi-isothermal light gas injection.