Double Boundary Layer Research Articles

PROCESSING OF OLD SLUDGE DEPOSITS INTO COMPOSITE FUEL

An urgent problem in Ukraine is overcrowded sludge sites with outdated sludge deposits. Due to the food problem, the world needs to maximize the release of land areas that have been under technical use and reclaim them for agricultural land. Therefore, the processing of obsolete sludge deposits makes it possible to reuse land for activated sludge or for reclamation for other needs of the country. The paper sets the task of studying obsolete sludge deposits to create a technology for their processing. Due to the high ash content of old silt deposits, it was proposed to combine them with peat and biomass. This will reduce ash content by 1.5–2.0 times. Since the created composite granules and the sludge deposits themselves have a high moisture of 61–68 %, the drying kinetics was studied on a convective experimental stand to reduce it. The research results showed that composite granules have a drying time of 28–39 minutes to a moisture content of 6–7 %, which is 2.3–3.1 times faster than peat. The equilibrium moisture content of composite granules was determined to be 6–7 % and not exceeding the standard moisture content for fuel granules. The heat of combustion of obsolete sludge deposits, which is 11.8 MJ/kg, and the resulting composite granules based on them, was determined, which is quite high and, depending on the composition, is in the range of 13.4–15.5 MJ/kg. During the study of the combustion of a separate fraction of aged sludge deposits, it was found that the combustion process occurs in the upward convective flow of the oxidant behind the double boundary layer. It was determined that the combustion rate is independent of its size and is the same, but the mass affects the time of ignition. Further studies on the combustion of composite granules showed that the combustion rate of the granules is significantly lower than the combustion rate of the original biomass, but the combustion rate of a separate fraction of silt deposits prevails. Bibl. 32, Fig. 8, Tab. 1.

Condensation characteristics of air–water vapor mixture on the surface of vertical flat plate

The condensation heat transfer of air–water mixed vapor is an important heat transfer phenomenon in production life. An experimental system for condensation heat transfer of the air–water vapor mixed vapor is built in this paper. The condensation characteristics of the mixed vapor and the double boundary layer structure characteristics were investigated. The study mainly discussed the influence of flow velocity on the condensation performance of mixed vapor and summarized the experimental correlation equations considering flow velocity. Results indicate that the condensation performance decreases significantly as the air concentration increases. The mixed vapor flow velocity was increased from 1 to 3 m/s, and the condensation heat transfer coefficient was increased by 34.9–121.0%. The enhanced effect of flow velocity was also observed at high air concentration and low subcooling. The ratio of convective and condensation heat transfer coefficients increases with the rise in air concentration, reaching a maximum of 0.12. The proportion of convective heat transfer can be reduced by increasing the mixed vapor flow velocity. The gas film thickness is substantially higher than the liquid film thickness in mixed vapor condensation, and the gas/liquid film thermal resistance ratio can be more than 30 times. And increasing the flow velocity can significantly reduce the gas film/liquid film thermal resistance ratio. A new experimental correlation equation is fitted based on the experimental results, and this equation can be used as a reference for the design of related heat exchangers.

Effect of flow rate on condensation of CO2-water vapor mixture on a vertical flat plate

The presence of non-condensable gas significantly deteriorates the heat transfer of water vapor condensation, and the condensation process of a CO2-water vapor mixture is a key process for CO2 separation and purification. In this study, we fabricated an experimental system to investigate the heat transfer characteristics of the mixed vapor condensation using a CO2 mass fraction of 32–85 % at a flow rate of 0.4–1.2 m/s and surface subcooling of 3–70 K. Liquid film thickness, gas-phase diffusion layer thickness, and interface temperature were calculated based on the double boundary layer model. An increase in the flow rate (0.4–1.2 m/s) improved the mixed vapor heat transfer, especially at high CO2 concentrations, and increased the heat transfer coefficient by 177 % at a surface subcooling of 40 K. The lower the surface subcooling, the greater the effect of the flow rate. As the CO2 concentration increased, the liquid film thickness decreased, whereas the gas-phase diffusion layer thickness increased to 1.7 mm, which is tens and hundreds of times the liquid film thickness. Increasing the flow rate slightly changed the liquid film but significantly reduced the gas-phase diffusion layer thickness and afforded a better heat transfer performance. A new heat transfer correlation equation was fitted, and the error of the predicted value was within ± 30 %. This study provides fundamental insights for the design of related heat exchangers.

Sufficient Condition for the Existence of an Additional Zone in Singularly Perturbated Second-Order Boundary Problem

Studies the Dirichlet, Neman and Robin boundary value problems for a singularly perturbed linear inhomogeneous second order ordinary differential equation. The considered boundary value problems have three features: the singular presence of a small parameter; the solution of the corresponding unperturbed equation has a k order pole and an additional boundary layer. The singular presence of a small parameter generates the classical boundary layer, and the singular point of the corresponding unperturbed equation generates the second boundary layer. As a result, we get a double boundary layer. A sufficient condition for the existence of an additional boundary layer is found.

Study on the synergistic heat transfer of double boundary layers in the jacketed vacuum membrane distillation process

During the process of membrane distillation (MD) heat recovery, due to the adverse effects of the heat exchange boundary layer and membrane surface boundary layer, the latent heat is hindered in its transfer and ultimately leading to the loss of heat recovery and mass transfer capacity of the system. In this study, a novel type of jacketed hollow fiber vacuum membrane distillation module (J-HF-VMD-M) has been successfully developed. It has the dual functions of “feed evaporation” and “vapor condensation”, which is helpful in reducing the volume and complexity of the system. Based on the structural characteristics of the jacketed module, a “Synergistic heat transfer model of double boundary layers” (SHT-DBL Model) is proposed for the first time. Compared with the previous “independent heat transfer” method between boundary layers, the “synergistic heat transfer” can overcome the above challenges more effectively. Compared with the non-heat recovery process in experimental range, the equivalent membrane distillation flux of the jacketed-type heat recovery process increased by 246.1 %, especially the gain output ratio increased by 145.1 %, and the corresponding increment is more significant than that of the traditional heat recovery process.

Clearance flow patterns and pressure distribution of a pump-turbine: Measurement and simulation of a rotating disk flow

The clearance flow patterns and pressure distribution determine the clearance axial hydraulic thrust of a pump turbine, which has a substantial impact on the unit axial imbalance. However, due to the tiny size and complex shape of the clearance flow channel, there is no detailed description of the flow patterns and pressure characteristics. In this study, we conducted a model test with particle image velocimetry (PIV) measurements and CFD simulation of a rotating disk flow that was a simplified model of the pump-turbine clearance flow. It is shown that a typical Batchelor flow is formed in the clearance region, demonstrating a “core region + double-boundary layers” distribution for the circumferential velocity along the clearance height direction; the core region rotates at a speed of only around 41–42% of the rotating disk speed and is independent of the clearance inlet pressure and clearance height. Driven by centrifugal force, the flow is radially outward around the rotating disk, while inward around the stationary disk in the meridian section, showing secondary flow vortices. The pressure in the clearance region has a circumferentially symmetric and radially quadratic distribution. Based on the liquid differential equilibrium equation and core region circumferential velocity, the pressure and clearance axial hydraulic thrust in the clearance region can be expressed as a function of the clearance inlet pressure and the square of the runner rotating speed. These findings can be used to investigate axial force imbalance issues of the pump-turbine unit.

Speed sensorless control of a bearingless induction motor based on sliding mode observer and phase-locked loop

To realize the low-cost and high-performance of a bearingless induction motor (BIM), a speed sensorless control strategy combined with the improved sliding mode observer (SMO) and phase locked-loop (PLL) is proposed. Based on analyzing the principle of the traditional SMO, the improved SMO adopts a switching function that has a double boundary layer structure to reduce the high-frequency jitter. Firstly, the observation equation of the rotor flux is established where the stator current, as well as the rotor flux are two state variables, and then the system stability is discussed with the Lyapunov theory. Secondly, to reduce the rotor chatter and enhance the speed tracking performance a flux electrical angle detecting module is constructed based on the PLL structure that extracts the rotor flux electrical angle and speed from the observed flux component. Finally, the BIM speed sensorless vector control system is established and the simulation results and experimental results are respectively presented by the MATLAB/Simulink simulation platform and a laboratory prototype. Both results prove that this strategy can not only enhance the speed self-detection capability and tracking performance, but can guarantee the excellent self-suspension function as well.

MHD mixed convection on an inclined stretching plate in Darcy porous medium with Soret effect and variable surface conditions

AbstractThis work is concerned with a steady 2D laminar MHD mixed convective flow of an electrically conducting Newtonian fluid with low electrical conductivity along with heat and mass transfer on an isothermal stretching semi-infinite inclined plate embedded in a Darcy porous medium. Along with a strong uniform transverse external magnetic field, the Soret effect is considered. The temperature and concentration at the wall are varying with distance from the edge along the plate, but it is uniform at far away from the plate. The governing equations with necessary flow conditions are formulated under boundary layer approximations. Then a continuous group of symmetry transformations are employed to the governing equations and boundary conditions which determine a set of self-similar equations with necessary scaling laws. These equations are solved numerically and similar velocity, concentration, and temperature for various values of involved parameters are obtained and presented through graphs. The momentum boundary layer thickness becomes larger with increasing thermal and concentration buoyancy forces. The flow boundary layer thickness decreases with the angle of inclination of the stretching plate. The concentration increases considerably for larger values of the Soret number and it decreases with Lewis number. The skin friction coefficient increases for increasing angle of inclination of the plate, magnetic and porosity parameters, however it decreases for rise of thermal and solutal buoyancy parameters. In this double diffusive boundary layer flow, Nusselt and Sherweed numbers increase for rise of thermal and solutal buoyancy parameters, Prandtl number, but they behave opposite nature in case of angle of inclination of the plate, magnetic and porosity parameters. The Sherwood number increases for increasing Lewis number but it decreases for increasing Soret number.

Effect of Soret and Dufour numbers on double diffusive mixed convection boundary layer flow induced by a shrinking sheet

The numerical study of double diffusive mixed convection boundary layer flow over a shrinking sheet are presented in the presence of Soret and Dufour effects. Shrinking velocity, wall temperature and wall concentration are assumed to have exponential function forms. Non-similarity method is applied for the governing basic equations (flow, momentum, energy and concentration equations) before they are solved numerically using bvp4c program in Matlab software. The numerical results of velocity, temperature and concentration are reported graphically for various values of suction parameter, dimensionless coordinate along the plate parameter, shrinking parameter, mixed convection parameter, buoyancy ratio parameter, Soret parameter and Dufour parameter. Since dual solutions is obtained, stability analysis is performed to select the stable solution.

Concomitant Double Ion and Electron Populations in the Earth's Magnetopause Boundary Layers From Double Reconnection With Lobe and Closed Field Lines

AbstractWhile double ion populations, with a cold population originating from the solar wind and a hotter one from the magnetosphere, are frequently observed in the low‐latitude boundary layers at the Earth's magnetopause, similar double electron populations are observed less often. A preliminary study of magnetopause crossings characteristics was used to determine the typical locations and energy spectra of ion and electron populations near the magnetopause. Then, we set up an automated detection algorithm for identifying regions with concomitant double populations in both ion and electron energy spectra. The statistical study was carried out on 7 years of Time History of Events and Macroscale Interactions during Substorms particle data, to determine the interplanetary magnetic field conditions in the upstream solar wind for these double populations. The results suggest that such concomitant ion and electron double population boundary layers form preferentially in the subsolar region and under northward interplanetary magnetic field but with a significant BY component. We interpret this finding as the result of reconnection of the same magnetosheath field line in both hemispheres with at least one end reconnecting in its hemisphere at lower latitude with a closed magnetospheric field line that already contains a hot electron source.

On a double boundary layer in a nonlinear boundary value problem

A nonlinear second-order differential equation with a small parameter at the derivatives is considered in the case where the limit algebraic equation has a multiple root. The matching method is applied to construct an asymptotic expansion of the solution of the boundary value problem. Two boundary layer variables with different scales are used to describe the asymptotic solution near the boundary.

Mixed convection flow of Jeffrey fluid along an inclined stretching cylinder with double stratification effect

This paper addresses double stratified mixed convection boundary layer flow of Jeffrey fluid due to an impermeable inclined stretching cylinder. Heat transfer analysis is carried out with heat generation/absorption. Variable temperature and concentration are assumed at the surface of cylinder and ambient fluid. Non-linear partial differential equations are reduced into the non-linear ordinary differential equations after using the suitable transformations. Convergent series solutions are computed. Effects of various pertinent parameters on the velocity, temperature, and concentration distributions are analyzed graphically. Numerical values of skin friction coefficient, Nusselt, and Sherwood numbers are also computed and discussed.

Scalings for unsteady natural convection boundary layers on a vertical plate at time-dependent temperature

In this paper, a scaling analysis using a simple three-region structure was used to develop scalings for the unsteady natural convection boundary layer (NCBL) of a homogeneous Newtonian fluid with Pr>1 adjacent to a finite vertical plate evenly heated with a time-varying sinusoidal temperature, where Pr is the Prandtl number. Three distinct sub-stages are identified in the heating stage, i.e., a start-up stage, a transitional stage, and a quasi-steady stage. After the heating stage, there is a very short cooling stage, in which a double thermal boundary layer structure is developed, which leads to a new, thinner thermal boundary layer adjacent to the plate co-existing with the original thermal boundary layer. A series of scalings are developed for the thermal and viscous boundary thicknesses, the maximum vertical velocity within the boundary layer, and the Nusselt number which are the major parameters representing the flow behavior, in terms of the governing parameters of the flow, i.e., the Rayleigh number Ra, Pr, and the dimensionless natural frequency of the time-varying sinusoidal temperature at various flow development stages. These scalings are tested by full numerical solutions of the governing equations and are shown to, in general, provide an accurate description of the flow.

Magnetoplasmon Excitations at Graphene Vortex Hall Fluid Edge

We investigate magnetoplasmon dynamics localized on the edges of graphene vortex Hall fluid. The vortex matter captures an anomalous term that causes vortex localization near fluid boundary and creates a double boundary layer, Δ0∝(β-1)lB with β being filling factor. The term also has qualitative effect on resonant excitations of edge magnetoplasmons. We found that, for sharp edges under experimental conditions, graphene edge magnetoplasmon (EMP) resonances have similar behavior as in recent experiments. Gradual distinctions arise for smooth edges in the presence of the anomalous term, where a weak EMP peak appears. The second peak becomes well noticed as the smoothness is increased. We identified the resonant mode as an Inter-EMP. It originates from the oscillations of charges in the inner boundary of the double layer. The present observation brings to light the direct cause of Inter-EMP which remained to be detected in graphene experiments.

The asymptotic equivalence of fixed heat flux and fixed temperature thermal boundary conditions for rapidly rotating convection

The influence of fixed temperature and fixed heat flux thermal boundary conditions on rapidly rotating convection in the plane layer geometry is investigated for the case of stress-free mechanical boundary conditions. It is shown that whereas the leading-order system satisfies fixed temperature boundary conditions implicitly, a double boundary layer structure is necessary to satisfy the fixed heat flux thermal boundary conditions. The boundary layers consist of a classical Ekman layer adjacent to the solid boundaries that adjust viscous stresses to zero, and a layer in thermal wind balance just outside the Ekman layers that adjusts the normal derivative of the temperature fluctuation to zero. The influence of these boundary layers on the interior geostrophically balanced convection is shown to be asymptotically weak, however. Upon defining a simple rescaling of the thermal variables, the leading-order reduced system of governing equations is therefore equivalent for both boundary conditions. These results imply that any horizontal thermal variation along the boundaries that varies on the scale of the convection has no leading-order influence on the interior convection, thus providing insight into geophysical and astrophysical flows where stress-free mechanical boundary conditions are often assumed.

Hydrodynamic structures generated by a rotating magnetic field in a cylindrical vessel∗∗This article was accepted for publication in Fluid Dynamics Research 2014 Vol 46, No 4; which was a special issue consisting of papers from the 5th International Symposium on Bifurcations in Fluid Dynamics. Due to an unfortunate error on the part of the journal, this article was not published

The hydrodynamic structures arising in a cylinder under the influence of a rotating magnetic field were considered, and the stability of a primary stationary flow in an infinitely long cylinder was investigated by linear approximation. The curves of neutral stability were obtained for a wide range of flow parameters and the calculations generated a single-vortex (in the radial direction) structure of Taylor’s vortices. The flow stability in the infinitely long cylinder was evaluated based on energy balance. The problem of three-dimensional stationary flow of a viscous incompressible conducting liquid induced by a rotating magnetic field in a cylindrical vessel of limited length was solved using an iteration method. The values of the parameters were found for which the iterative process still converges. Numerical experiment made it possible to investigate the arising spatial flow patterns and to track their evolution with changes in the flow parameters. Results of modelling showed the appearance of a three-dimensional structure of Taylor-type vortices in the middle portion of a sufficiently long vessel. The appearance of a double laminar boundary layer was demonstrated under certain conditions of azimuthal velocity distribution along the vessel height at the location of the end-wave vortex.

Effect of Tube Geometry and Curvature on Film Condensation in the Presence of a Noncondensable Gas

Based on the double boundary layer theory, a generalized mathematical model was developed to study the distributions of gas film, liquid film, and heat transfer coefficient along the tube surface with different geometries and curvatures for film condensation in the presence of a noncondensable gas. The results show that: (i) for tubes with the same geometry, gas film thickness, and liquid film thickness near the top of the tube decrease with the increasing of curvature and the heat transfer rate increases with it. (ii) For tubes with different geometries, one need to take into account all factors to compare their overall heat transfer rate including gas film thickness, liquid film thickness and the separating area. Besides, the mechanism of the drainage and separation of gas film and liquid film was analyzed in detail. One can make a conclusion that for free convection, gas film never separate since parameter A is always positive, whereas liquid film can separate if parameter B becomes negative. The separating angle of liquid film decreases with the increasing of curvature.

Probing membrane and interface properties in concentrated electrolyte solutions

This study deals with the membrane and interface electrical properties investigation by electrochemical impedance spectroscopy (EIS). The EIS is a powerful technique for characterizing electrical behavior of systems in which coupled electrical processes occur at different rates. A systematic study on the effect of solution concentration, temperature and velocity, on the electrical resistance of anion- and cation-exchange membranes (AEMs and CEMs) and their interfaces (electrical double layer and diffusion boundary layer), was carried out. At the best of our knowledge, for the first time electrolyte concentrations up to 4M were used for the study of membranes and interface by EIS.Moreover, Pulsed Gradient Spin Echo Nuclear Magnetic Resonance (PGSE-NMR) technique was used to measure the water self-diffusion coefficients in swelled membrane as a function of the solution concentration and temperature. These measurements gave additional important insights about the effect of the electrolyte solution and fixed charges concentration in membrane, on membrane microstructure and its transport and electrical properties.

Soret and Dufour Effects on Transient Double Diffusive Free Convection of Couple-stress Fluid past a Vertical Cylinder

This study examines the influence of Soret and Dufour effects on double diffusive transient free convective boundary layer flow of a couple-stress fluid flowing over a semi-infinite vertical cylinder. A set of non-dimensional governing equations namely, the continuity, momentum, energy and concentration equations is derived and these equations are unsteady non-linear and coupled. As there is no analytical or direct numerical method available to solve these equations, they are solved by using the CFD techniques. An unconditionally stable Crank-Nicolson type of implicit finite difference scheme is employed to obtain the discretized forms of governing equations. These equations are solved using the Thomas and pentadiagonal algorithms. The numerical results are compared and found to be in good agreement with previously published results as special cases of the present investigation. Transient velocity, temperature and concentration profiles, average skin-friction, Nusselt number and Sherwood number are shown graphically for different values of Soret (So) and Dufour (Du) numbers. In all these profiles it is observed that, as the values of So decreases or the Du increases, the time required to reach the temporal maximum and the steady-state increases. It is also observed that the average values of skin-friction and heat transfer rate increases with the increasing values of So or decreasing values of Du. Whereas the reverse trend is observed for the average mass transfer rate.

On the double laminar boundary layer at MHD rotation of the conducting liquid in the cylinder

On the double laminar boundary layer at MHD rotation of the conducting liquid in the cylinder