


Vol 28, No 1 (2019)
- Year: 2019
- Articles: 11
- URL: https://journal-vniispk.ru/1810-2328/issue/view/13162
Article
Interaction of Water Droplets in Air Flow at Different Degrees of Flow Turbulence
Abstract
Presented are results of experiments on high-speed video recording of collisions of water droplets in a gas medium as part of aerosol. Parameters of the gas (air) flow and aerosol cloud were monitored using cross-correlation complexes and optical methods of Particle Image Velocimetry, Particle Tracking Velocimetry, Interferometric Particle Imagine, and Shadow Photography. Conditions at different degrees of gas flow turbulence were considered. The characteristic Reynolds numbers ranged from 1100 to 2800. The relative probabilities of coagulation, scattering, and fragmentation of water droplets upon their collisions were calculated. Experimental dependences of probabilistic criteria on parameters of droplets and flow have been obtained for subsequent mathematical modeling. It has been shown that fragmentation and complete breakup of droplets can enable several-fold increase in the relative area of the liquid. The effect of the degree of gas flow turbulence on parameters of recorded processes of interaction of droplets has been established. The results of the experiments were subjected to criterion processing, theWeber and Reynolds numbers taken into account.



Numerical Modeling of Convection in the Zone of Spreading and Subduction



Constructal Design of Conductive Asymmetric Tri-Forked Pathways
Abstract
This work relies on the constructal design method associated with exhaustive search and genetic algorithms to perform geometric optimization of an asymmetric tri-forked pathway of highly conductive materials (inserts) that remove a constant heat flux from a body and deliver it to three isothermal heat sinks. It is shown numerically that the global thermal resistance, represented by the maximum excess of temperature, can be minimized by means of geometric evaluation subject to two constraints, the total rectangular area where the forked pathway is circumscribed and the tri-forked pathway area, and seven degrees of freedom. A parametric study is performed to show the influence of the degrees of freedom over the global thermal resistance. The optimal geometry was obtained for a 40% area fraction, leading to a maximum excess temperature seven-times minimized with a thermal performance 627% better than a once optimized architecture, showing the importance of the design for thermal performance. For higher values of aspect ratio, height/length, the optimal configuration is highly asymmetrical, while for lower ratios the bifurcated branches has low influence over the thermal performance of the system. The optimal tri-forked pathway presented a 307% lower thermal global resistance compared to a V-shaped pathway on the same conditions.



Experimental Study of Regularities in Suppression of Flame Combustion and Thermal Decomposition of Forest Combustible Materials Using Aerosols of Different Dispersiveness
Abstract
This paper presents results of experimental studies on the heat and mass transfer and phase transformations in suppression of thermal decomposition and flame combustion of a group of typical forest combustible materials (leaves and needles used as examples) due application of water aerosol and a group of droplets of a fixed total volume. The main attention is paid to the experimentally found effect of aerosol dispersiveness (via variation in the concentration and size of droplets) on the integral characteristics of the heat and mass transfer processes and phase transformations. The effective irrigation density, the minimum (necessary and sufficient) volume, and the respective specific water consumption for complete termination of destruction of forest combustible materials are calculated. Three most typical schemes of sprayed liquid interaction with the surface of forest combustible material under intensive pyrolysis are considered with the aim of establishing conditions for inhibition, localization, and complete cessation of the process of destruction of forest combustible materials. Dependencies illustrating how the properties and structure of layers of forest combustible materials affect the characteristics of these processes are established.



Effect of Fluid Viscoplasticity on the Constructal Design of Elliptic Tubes Subjected to Forced Convection
Abstract
The main purpose of this work is to study the effects of flow on the Herschel–Bulkley number (HB), a parameter that should account for fluid viscoplasticity, in the optimal designs of elliptical section tubes subjected to forced convection of viscoplastic fluids. The constructal design was associated with exhaustive search to obtain the ellipse aspect ratios that maximize the Nusselt number and minimize dimensionless pressure drop, for fixed ellipse area and distance between tubes.We employed a modified Herschel–Bulkleymodel to predict viscoplastic behavior, and a fixed flow index equal to 0.4. To solve the system of differential equations for flow and heat transfer, we used a finite volume method code. We investigated the HB ranging from 1 to 100. The maximum dimensionless heat transfer increased with the increase of the HB. Still, the HB did not affect the ellipse aspect ratio that maximized dimensionless heat transfer. For all values of HB, the optimal aspect ratio was equal to 0.96. The Herschel–Bulkley number strongly affected dimensionless pressure drop. The minimum pressure drop increased with the increase of HB, and the optimal geometry was slender as HB increased.



Gap Cavitation in the End Clearance of a Guide Vane of a Hydroturbine: Numerical and Experimental Investigation
Abstract
The article presents results of numerical and experimental investigation of cavitation flow around two subscale models of guide vanes of hydroturbines, one of which has a simplified geometry, while the other has an approximately real geometry. The emphasis was on studying the fluid flow through the gap between the end surface of the hydrofoil and the channel wall, and also on the onset and development of gap cavitation. In the study we used high-speed imaging to analyze the spatial structure and dynamics of cavities. The two-dimensional flow velocity distributions above the hydrofoil surface were measured by the PIV method. In the numerical simulation of cavitation flows around the vanes we used computational fluid dynamics methods based on the solution of Reynolds equations for turbulent flows by means of the finite volume method on a three-dimensional grid of hexahedral cells. The vapor phase was accounted for by solving the equation of transport of vapor fraction. Turbulence was described by the DES method based on a two-parameter model, k-ω SST. As a result, we compared the calculated profiles of flow velocity in the boundary layer on the low-pressure side of the vane with the measurement results; they were in good agreement. In the experiment and in the numerical simulation cavitating vortex structures were observed along with gap cavitation as a vapor film while the fluid was flowing through a narrow gap. At a small angle of attack the vapor film in the gap forms in the vicinity of the trailing edge of the hydrofoil, and immediately behind its leading edge at large angles of attack. In unsteady flow regimes the gap cavitation dynamics substantially depends of the phase of development of the main cavity on the low-pressure side of the vane decompression.



Effects of Slip on MHD Flow of a Dusty Fluid over a Stretching Sheet through Porous Space
Abstract
In this paper the effect of slip on the MHD flow of a dusty fluid over a porous stretching sheet in a porous medium is studied. The equations are modeled using Darcy’s law and simplified by using boundary layer approximations. The coupled system of nonlinear ordinary differential equations is obtained by employing similarity transformations. These coupled equations are then solved analytically by perturbation method and numerically by Runge–Kutta method of order four. The results are presented graphically and influence of various parameters of interest on the velocity of both fluid and dust phases and skin friction coefficient is analyzed. The comparison of analytical and numerical solutions is given and found in an excellent agreement.



Reversible Heat Exchange in the Nozzle with Water–Ice Phase Transition in Filtration of Air
Abstract
Results of experimental study of reversible heat exchange with water–ice phase transition in filtering the airflow through a tube heat exchanger are presented. The heat exchanger is made of thin-walled plastic tubes (3 and 5 mm in diameter) filled with water and sealed at both ends. The heat exchanger was tested in a laboratory setup with a Ranque tube applied to create a flow of cold air. The heat exchanger operation was investigated in steady-state and various reversible modes. Themeasurements showed the possibility of substantially increasing the time between switching the direction of airflow with maintaining a high coefficient of heat regeneration. The results can be used in the development of regenerative heat and mass transfer devices, including VENTIREG systems of ventilation emission heat regeneration.



Ice Melting with Allowance for Selective Absorption in the Medium
Abstract
Melting of an ice layer on a vertical substrate heated by long-wave radiation is numerically simulated in a one-phase formulation of the Stefan problem. A simple model is proposed to take into account the presence of a thin water film on the irradiated surface. Ice is considered as a selectively absorbing material with two absorption bands. The calculated results are in reasonable agreement with the experiment.



Investigation of the Capability of Carbon Nanotube Membranes in Separating the Heavy Metal Ions from Aqueous Solutions by Molecular Dynamics Simulation
Abstract
In this study, the ability of carbon nanotube membranes in separating three heavy metal ions from aqueous solutions, namely Zn2+, Ni2+, and Cd2+, under reverse osmosis process is investigated by using molecular dynamics simulation. Also, the impact of the type of heavy metal ion on the number of water molecules moving through membranes is evaluated. The simulation results ascertained that the highest number of water molecules is separated in the presence of nickel ion. Finally, the carbon nanotube membrane with chirality of (8, 8) under 200 MPa is introduced as the most appropriate choice for eliminating the heavy ions from water.



Magnetohydrodynamic Three-Dimensional Couette Flow of a Second-Grade Fluid with Sinusoidal Injection/Suction
Abstract
A mathematical model for magnetohydrodynamic (MHD) three-dimensional Couette flow of an incompressible second-grade fluid is developed and analyzed theoretically. The application of normal sinusoidal injection at the lower fixed plate and its equivalent deduction by suction through the uniformlymoving upper plate leads to three-dimensional flow.Approximate solutions for components of velocity, pressure and skin friction are obtained. The effects of flow parameters such as Hartmann number, Reynolds number, suction/injection parameter, non-Newtonian parameter on velocity components, skin friction factors along main flow direction and transverse direction, and pressure through parallel porous plates are discussed graphically. It is noted that Hartmann number provides a mechanism to stable the skin friction component along the main flow direction.


