Showing papers in "Journal of Engineering Physics in 2016"

Evaporation of Water Droplets in a High-Temperature Gaseous Medium

Abstract: A numerical solution of the problem of heat and mass transfer in evaporation of a droplet of water moving in a stream of high-temperature (up to 1200 K) gases is done on the basis of a system of nonlinear nonstationary partial differential equations describing conductive and radiative heat transfer in the droplet, as well as composite heat transfer at the ″liquid–gas″ interface. The values of the water evaporation rate have been determined. It is shown that the dependence of the evaporation rate on the droplet surface temperature has a nonlinear character. Characteristic relationships between the convective and radiative heat fluxes on the droplet surface (the radiative flux substantially exceeds the convective one; on decrease in the difference between the gas and droplet surface temperatures the difference between the radiative and convective heat fluxes decreases), the lifetimes (total evaporation) of droplets, as well as of the temperature and concentration of steam and gases in the vicinity of droplets have been determined. The calculated characteristics of the water droplet evaporation under conditions of high temperatures of the gas medium differ considerably from those obtained within the framework of the "diffusional" model of evaporation. A comparison of the results of numerical simulation with the experimental data obtained with the use of high-velocity panoramic optical methods of visualization by ″tracing particles″ is carried out.

Regimes of Spreading of a Water Droplet Over Substrates with Varying Wettability

Abstract: The authors have presented the results of experimental investigations into the physical mechanisms of change in the dynamic angle of contact of a droplet of distilled water on a substrate under the conditions of its wetting at high velocities of motion of the three-phase contact line. The investigations were carried out on three copper substrates and one superhydrophobic substrate. The authors have singled out three regimes of spreading of the droplet on the copper substrates with different surface roughnesses: formation of a droplet, spreading of the droplet, and formation of an equilibrium angle of contact. A distinctive feature of the droplet spreading over the surface with a superhydrophobic coating has been revealed, which lies in the monotonic increase in the advancing dynamic contact angle. The influence of the volume rate of formation of a droplet on the value of the dynamic contact angle has been established. On the copper surfaces with roughnesses of 5.190 and 6.210 μm, increase in the rate tended to increase the maximum value of the contact inflow wetting angle. However, on the copper surface with a roughness of 0.591 μm, the dynamic contact angle was independent of the droplet′s volume in the range of volume rates of droplet growth 0.040–0.160 ml/s.

Experimental Study of the Effects of Collision of Water Droplets in a Flow of High-Temperature Gases

Abstract: Using high-speed video recording and cross-correlation "tracer" visualization, the authors have investigated the regularities of the processes of collision of water droplets (characteristic parameters: radii 0.025–0.25 mm, velocities of motion 0.5–12 m/s, and relative concentration 0.001–0.0012 m3 of liquid droplets in 1 m3 of the gas) in their motion in a flow of high-temperature (about 1100 K) gases. The characteristic effects of collision of two droplets, at which combined droplets are formed (coagulation occurs) and conditions for spreading or fragmentation of the latter are implemented, have been singled out. The values of the Weber and Reynolds numbers for droplets before and after the collisions have been established. The influences of the velocities of motion, the dimensions, and the angles of intersection of mechanical trajectories of droplets on the effects of collisions have been determined.

Analysis of Convective Heat Transfer Enhancement by Nanofluids: Single-Phase and Two-Phase Treatments

Abstract: Nanofluids have been investigated regarding their advantages and potentialities for the purpose of increasing convective heat transfer rates inside thermal systems where they are used as working fluids. Researchers in thermophysics have investigated these fluids experimentally and numerically. This review provides extensive theoretical information concerning nanofluids in the single-phase and two-phase treatments. Important published works on nanofluid properties and correlations are summarized and reviewed in detail. Heat transfer enhancement by nanofluids is a challenging problem due to the difficulties inherent in the model of the physical mechanism of interaction between the paricles. Here the interaction between the phases is modeled by several two-phase models, and the results are given in graphical and tabular forms. Despite the advantages of the mixture model, such as imlementation of physical properties and less computational power requirements, some studies showed that the results of the single-phase and two-phase models are very similar. The main difference consists in the effect of the drift velocities of the phases relative to each other.

Numerical Modeling of Heat Transfer Through a Triple-Pane Window

Abstract: The regularities of heat transfer through a triple-pane glass window with air and argon fillings have been investigated by the method of numerical modeling. Distinctive features of this process have been elucidated which affect the increase in the thermal resistance of triple-pane windows compared to double-pane ones. The thermal resistances of the triple-pane window as functions of the gas-interlayer thickness and of temperature on the window’s exterior surface have been found.

Methane Gas Hydrate Decomposition in a Porous Medium Upon Injection of a Warm Carbon Dioxide Gas

Abstract: The characteristic features of methane gas hydrate decomposition upon injection of a warm carbon dioxide gas into a porous medium saturated with methane and its hydrate are investigated. A mathematical model is presented for heat and mass transfer in a porous medium accompanied by substitution of methane for carbon dioxide gas in the original gas hydrate. Self-similar solutions of a one-dimensional problem that describe the distribution of basic parameters in a stratum have been constructed. It is shown that there are solutions according to which methane gas hydrate may decompose either with the formation of carbon dioxide gas hydrate alone, or with the formation of both carbon dioxide gas hydrate and a mixture of methane with water. Critical diagrams of the existence of each type of solutions have been drawn.

Analytical Investigation of Heat Transfer in an Anisotropic Band with Heat Fluxes Assigned at the Boundaries

Abstract: An analytical solution of a nonstationary problem of the theory of heat conduction in an anisotropic band under heat transfer conditions at the boundaries has been obtained for the first time by applying the Fourier transformation with respect to the longitudinal variable and the Laplace transformation for time. The problem is formulated and solved in regions with anisotropy of general form when the principal axes of the heat conduction tensor are set at an angle that orients these axes relative to the Cartesian coordinate system.

Boiling of a Liquid on Microstructured Surfaces Under Free-Convection Conditions

Abstract: The authors have shown the possibilities of replacing complex and expensive technologies of manufacture of nanorough, microrough, and porous materials for boiling surfaces by a simple and resource-saving technique of mechanical treatment of surfaces: by the strain-cutting method. It has been established that the maximum levels of heat-transfer intensification (as high as four to six times) during the boiling of distilled water and increase (of six times) in the critical heat fluxes are inherent in surfaces obtained by the strain-cutting method with three-dimensional microfinning with spacings of width 120–180 μm at a height of fins of 340–570 μm and their longitudinal spacing of 240–400 μm.

Mathematical Models of Heat Conduction with a Two-Phase Lag

Abstract: A mathematical theory has been developed for boundary-value problems of nonstationary heat conduction with a dual-phase lag. Features of analytical solutions of such heat problems are described. On the basis of the integral transformations proposed, an exact analytical solution of the practically important third boundary-value problem on the indicated heat conduction in the general formulation (plate, cylinder, sphere) has been obtained in the form of new functional constructions.

Acoustoconvection Drying of Meat

Abstract: The dynamics of moisture extraction from meat samples by the acoustoconvection and thermoconvection methods has been investigated. To describe the dynamics of moisture extraction from meat, we propose a simple relaxation model with a relaxation time of 8–10 min in satisfactorily describing experimental data on acoustoconvection drying of meat. For thermoconvection drying the relaxation time is thereby 30 and 45 min for the longitudinal and transverse positions of fibers, respectively.

Plane Waves in a Rotating Monoclinic Magnetothermoelastic Medium

Abstract: The governing equations for a rotating monoclinic magnetothermoelastic medium are formulated in the context of the Lord–Shulman theory and are solved to yield the velocity equation that points to the existence of three quasiplane waves. Some particular cases are obtained, i.e., waves in the absence of anisotropy, rotation, and thermal and magnetic fields. A procedure for computing the angles of reflection is carried out. A numerical example is considered to show the dependence of the speeds of various plane waves on the angle of incidence, angle of reflection, rotation rate, and magnetic field strength.

Influence of Pressure on Stable Film Boiling of Subcooled Liquid

Abstract: Film boiling of subcooled liquids is an integral part of the hardening process. Understanding of the mechanisms underlying film boiling is important for modeling processes in atomic power engineering and cryogenic technology. Stationary processes of film boiling of subcooled liquids under conditions of their free motion near cylindrical heaters, just as subcooled liquid turbulent flow past high-temperature surfaces, represent quite a different type of process. In cooling metal spheres heated to a high temperature by a subcooled water, a special regime of film boiling is observed (microbubble boiling) distinguished by high intensity of heat transfer. Such a regime has not been revealed up to now for nonaqueous liquids. The paper presents new experimental data on heat transfer regimes in cooling nickel spheres in subcooled isopropanol and perfluorohexane at pressures of up to 1 MPa. It has been established that stable film boiling is the main regime of heat transfer that accounts for the larger part of the total time of cooling. The regimes of highly intensive film boiling heat transfer were not observed in the entire range of operational parameters even in the case of extreme subcoolings of liquid below their saturation temperature (to 170 K). The intensity of heat transfer in stable film boiling increases noticeably with subcooling of a chilling liquid.

Second-Law Analysis of the Peristaltic Flow of an Incompressible Viscous Fluid in a Curved Channel

Abstract: The present investigation extends a consideration of peristaltic flow in curved channels through the second-law analysis. The lubrication approximation is employed to linearize the momentum, energy, and entropy generation rate equations. The stream function and temperature distribution are used to calculate the entropy generation number and the Bejan number. It is shown that the entropy generation rate in a peristaltic pump increases with the occlusion parameter. The entropy generation increases at the upper wall and decreases near the lower wall of the peristaltic channel as the curvature parameter increases. A curved surface acts as a strong source of entropy generation.

Simulation of the Thermal Process of Butt Welding of Polyethylene Pipes at Low Temperatures

Abstract: A theoretical study has been made of the thermal process of welding polyethylene pipes for gas pipelines at low ambient air temperatures. The mathematical model used takes into account the heat of the phase transition in the temperature range, as well as the thermal effect of the fin formed by the slip. Computing experiments have shown that it is possible to control the temperature regime in welding at low ambient air temperatures and provide, in the thermal influence zone, the same change in the temperature field as at permissible air temperatures.

General Boundary-Value Problems for the Heat Conduction Equation with Piecewise-Continuous Coefficients

Abstract: A constructive scheme for the construction of a solution of a mixed problem for the heat conduction equation with piecewise-continuous coefficients coordinate-dependent in the final interval is suggested and validated in the present work. The boundary conditions are assumed to be most general. The scheme is based on: the reduction method, the concept of quasi-derivatives, the currently accepted theory of the systems of linear differential equations, the Fourier method, and the modified method of eigenfunctions. The method based on this scheme should be related to direct exact methods of solving mixed problems that do not employ the procedures of constructing Green′s functions or integral transformations. Here the theorem of eigenfunction expansion is adapted for the case of coefficients that have discontinuity points of the 1st kind. The results obtained can be used, for example, in investigating the process of heat transfer in a multilayer slab under conditions of ideal thermal contact between the layers. A particular case of piecewise-continuous coefficients is considered. A numerical example of calculation of a temperature field in a real four-layer building slab under boundary conditions of the 3rd kind (conditions of convective heat transfer) that model the phenomenon of fire near one of the external surfaces is given.

Comparison of the Results of Modeling the Flame Propagation in a Hybrid Gas Suspension with Experimental Data

Abstract: This article considers the laminary combustion of a hybrid gas suspension consisting of a gaseous combustible, an oxidizer, and an inert gas, as well as particles capable of interacting with the gas-phase oxidizer. The mathematical model takes into account the thermal expansion of the gas and its related relative motion of particles. The problem has been solved numerically in dimensionless variables. The dependences of the steady velocity of the flame front on the initial concentration of the combustible in the gas phase at various values of the parameters of the dispersed phase are presented. A comparison has been made between experimental and calculated data for a gas suspension consisting of a gaseous oxidizer, an inert gas, and reactive particles.

Mathematical Modeling of Chemical Vapor Deposition of Material on a Curvilinear Surface

Abstract: In this work, a mathematical model has been constructed that describes the process of chemical vapor deposition of material on a curvilinear plate. On the boundary where the deposition occurs, account is taken of convective heat transfer, heat transfer by radiation, and heat and mass transfer during the attachment of the substance to the surface. A numerical algorithm is proposed for finding the temperature profile at any instant of time; results and an analysis of numerical calculation are given for different materials.

Permeability of Hollow Microspherical Membranes to Helium

Abstract: This work is devoted to the study of the sorption characteristics of various hollow microspherical membranes to reveal particles most suitable for application in the membrane-sorption technologies of helium extraction from a natural gas. The permeability of the investigated sorbents to helium and their impermeability to air and methane are shown experimentally. The sorption–desorption dependences of the studied sorbents have been obtained, from which the parameters of their specific permeability to helium are calculated. It has been established that the physicochemical modification of the original particles exerts a great influence on the coefficient of the permeability of a sorbent to helium. Specially treated cenospheres have displayed high efficiency as membranes for selective extraction of helium.

Dynamic Stability of a Cylindrical Shell Reinforced by Longitudinal Ribs and a Hollow Cylinder Under the Action of Axial Forces

Abstract: The dynamic stability of a cylindrical orthotropic shell reinforced by longitudinal ribs and a hollow cylinder under the action of axial forces changing harmonically with time was investigated with regard for the axial contact interaction of the shell with the ribs. A solution of the differential equations defining this process has been obtained in the form of trigonometric series in the angular and time coordinates. A two-term approximation of the Mathieu–Hill equations of motion was used for construction of the main region of instability of the shell. As a result, the problem was reduced to a system of algebraic equations for components of displacements of the shell at the locations of the ribs. The problem for uniformly spaced ribs was solved in the explicit form. A numerical example of this solution is presented.

Calculation of the Rate of Combustion of a Metallized Composite Solid Propellant with Allowance for the Size Distribution of Agglomerates

Abstract: A physicomathematical model of combustion of a metallized composite solid propellant based on ammonium perchlorate has been presented The model takes account of the thermal effect of decomposition of a condensed phase (c phase), convection, diffusion, the exothermal chemical reaction in a gas phase, the heating and combustion of aluminum particles in the gas flow, and the velocity lag of the particles behind the gas The influence of the granulometric composition of aluminum particles escaping from the combustion surface on the linear rate of combustion has been investigated It has been shown that information not only on the kinetics of chemical reactions in the gas phase, but also on the granulometric composition of aluminum particles escaping from the surface of the c phase into the gas, is of importance for determination of the linear rate of combustion

Experimental Investigation of the Supersonic Flow over an Axisymmetric Ring Cavity

Abstract: This paper presents the results of the experimental investigation of the supersonic flow over a ring cavity of rectangular cross-section on a cylindrical body with a conical tip. The evolution of the flow over a cavity with its continuously changing extent has been investigated. The transition zone boundaries within which both an open and a closed schemes of flow are possible have been determined by the parameter of the relative extent of the cavity. It has been shown that the flow conditions in the transition zone depend on the prehistory of the flow. The main stages of cavity flow restricuting at the transition zone boundaries have been described.

Pyrolysis of Rubber in a Screw Reactor

Abstract: On the basis of an analysis of thermal methods described in the literature and from the results of experimental investigations of steam conversion, the authors have developed and created a facility for thermal processing of rubber waste. Rubber crumb was used as the raw material; the temperature in the reactor was 500°C; nitrogen, steam, and a mixture of light hydrocarbons (noncondensable part of pyrolysis products) represented the working medium. The pyrolysis yielded 36–38% of a solid fraction, 54–56% of a liquid hydrocarbon fraction, and 6–9% of noncondensable gases. Changes in the composition of the gas mixture have been determined at different stages of processing. Gas chromatography of pyrolysis gases has shown that the basic gases produced by pyrolysis are H2 and hydrocarbons C2H4, C3H6, C3H8, C4H8, C2H6, C3H6O2, and C4H10, and a small amount of H2S, CO, and CO2. Noncondensable gases will be used as a fuel to heat the reactor and to implement the process.

The Godunov Method for Calculating Multidimensional Flows of a One-Velocity Multicomponent Mixture

Abstract: Computational formulas of the Godunov method are given for the equations of a generalized-equilibrium model of a heterogeneous medium on a curvilinear grid; with the use of this method, the problems of interaction of air shock waves with bubbles of various gases are investigated. Flow of a gas–liquid mixture in a nozzle mouthpiece is considered. The Prandtl–Meyer flow of a water–air mixture is calculated and compared with a self-similar solution.

Estimation of the Drop Size in Dispersed Flow

Abstract: The formulas for calculating the characteristic drop size for the mean Sauter diameter have been compared. The question on various forms of the size distribution of drops has been considered. To substantiate the applicability of the compared formulas for calculating the thermohydrodynamics in the circuits of nuclear power plants, experimental data on the wall temperature in a dispersed flow have been used. It has been shown that the Sauter diameter values calculated using the wall temperature in the supercritical region are in good agreement with sparse direct measurements of the drop size in steam–water flows. The drop sizes calculated using the tested formulas obtained for two-component gas–liquid flows or for single-component flows of coolants (various kinds of freons) and liquefied nitrogen turned out to be much lower. It has been shown that it is necessary to recalculate the numerical coefficients in the considered formulas in using them for steam–water flows.

Numerical Simulation of the Intensification of the Heat Exchange in a Plane-Parallel Channel with a Cylindrical Shallow Dimple on the Heated Wall

Abstract: A comparison of the results of numerical calculations of the convective heat exchange in narrow plane-parallel channels with shallow cylindrical and spherical dimples on their wall heated by a constant heat flow under the conditions of low-velocity turbulent water flow in a channel at Re = 20,000 has been performed. Hydrodynamic features of the vortex intensification of the heat exchange in the indicated channels as a result of the interaction of the swirling flows in them with the side walls of a dimple were determined. The thermal and thermohydraulic efficiencies of a shallow cylindrical dimple were estimated. It is shown that this dimple offers substantial advantages over a spherical dimple of the same depth.

Optimal Thermal Design of a Multishield Thermal Protection System of Reusable Space Vehicles

Abstract: We have solved the problem of the optimal thermal design of a multishield thermal protection system of reusable space vehicles due to the choice of the optimal position and materials of radiation shields.

Mathematical Model of the Processes of Heat and Mass Transfer and Diffusion of the Magnetic Field in an Induction Furnace

Abstract: We propose a mathematical model describing the motion of a metal melt in a variable inhomogeneous magnetic field of a short solenoid. In formulating the problem, we made estimates and showed the possibility of splitting the complete magnetohydrodynamical problem into two subproblems: a magnetic field diffusion problem where the distributions of the external and induced magnetic fields and currents are determined, and a heat and mass transfer problem with known distributions of volume sources of heat and forces. The dimensionless form of the heat and mass transfer equation was obtained with the use of averaging and multiscale methods, which permitted writing and solving separately the equations for averaged flows and temperature fields and their oscillations. For the heat and mass transfer problem, the boundary conditions for a real technological facility are discussed. The dimensionless form of the magnetic field diffusion equation is presented, and the experimental computational procedure and results of the numerical simulation of the magnetic field structure in the melt for various magnetic Reynolds numbers are described. The extreme dependence of heat release on the magnetic Reynolds number has been interpreted.

Calculation of the Melting Process of a Quartz Particle Under Low-Temperature Plasma Conditions

Abstract: The melting process of a quartz particle under low-temperature plasma conditions has been considered. The melting stages of the quartz particle in an experimental electroplasma plant have been modelled mathematically, and the value of the limiting melting radius of particles has been established.

Use of the Piv Method for Investigation of Motion Near a Cylinder in Transverse Flow

Abstract: Turbulent flow past a cylinder in a wind tunnel has been investigated experimentally. Averaged velocity fields near the cylinder have been obtained with the optical PIV method and comparative characteristics have been given for noncavitation and cavitation regimes. From the vector patterns of the averaged velocity fields, the author has determined the angles of separation of the boundary layer from the cylinder surface in the considered regimes of flow. It has been shown that cavitation causes the vortex zone behind the cylinder to increase, the separation angles to displace upstream, and the hydraulic resistance to grow. A comparative calculation of the separation angles and the coefficients of hydraulic resistance of cylinders manufactured from different materials has been given. It has been shown that the vortex zone of a Teflon cylinder in flow having a hydrophobic surface differs from the vortex zone of a steel cylinder, particularly for the cavitation regime in which the angles of separation, especially from the upper part, decrease appreciably and the resistance grows.

Verification of the Shear-Stress Transfer Model and its Modifications in the Calculation of a Turbulent Flow Around a Semicircular Airfoil with a Zero Angle of Attack

Abstract: A comparative analysis of different versions of the Menter shear-stress transfer model, including with correction for the curvature of streamlines, has been performed as applied to a periodic flow around a semicircular airfoil with a zero angle of attack at Re = 45,000. A comparison of the calculated aerodynamic coefficients Cx and Cy and the surface distribution of the pressure coefficient, averaged over the period of oscillations of the lift coefficient Cy, with the corresponding experimental data has shown that the Menter shear-stress transfer model modified within the framework of the Rody–Leshtsiner–Isaev approach with the use of the inverse function of the Richardson number correcting the eddy viscosity of a flow with a semiempirical Isaev–Kharchenko–Usachov constant equal to 0.02 is preferred compared to the other versions of this model. The errors of the calculations performed with the use of the multiblock computational technologies based on intersecting structured meshes of different scales were analyzed. The interrelation between the evolution of the structure of the periodic flow around the semicircular airfoil and the distribution of its integral force characteristics in the period of Cy oscillations is discussed.