Thin Horizontal Layer Research Articles

Effect of shape and thermal conductivity of microstructured capillary-porous coatings on heat transfer during evaporation and boiling in thin horizontal liquid layers

Investigation results on heat transfer during evaporation and boiling in thin horizontal liquid layers on 2D modulated capillary-porous coatings with a sinusoidal profile in a wide range of relative pressures and layer heights are presented in this paper. Coatings were made on a 3D laser printer using the technology of additive selective laser sintering (SLS). One coating was made of bronze with a modulation wavelength equal to the Laplace constant of the working fluid and two coatings were made of stainless steel and bronze with a doubled wavelength. The experimental data obtained on the coatings were compared with each other and with the values obtained by evaporation and boiling of n-dodecane on a smooth surface under the same conditions. It is shown that in the region of low relative pressures in liquid layers with a height less than the Laplace constant, heat transfer intensification achieved on a sample made of a material with low thermal conductivity (stainless steel) was five times higher as compared to a smooth surface. In the range of relative pressures, when nucleate boiling was observed in the layers, the temperature difference reached on the bronze coatings was approximately 3–4 times less than on a smooth surface. The temperature difference for the stainless steel coating changed little with an increase in the heat flux. In pre-crisis conditions, the temperature difference during boiling on the stainless steel coating was less than on the bronze coatings in the entire range of relative pressures and layer heights. The highest critical heat fluxes were obtained on a bronze coating with a modulation wavelength equal to the Laplace constant.

Electromagnetically driven flow in unsupported electrolyte layers: lubrication theory and linear stability of annular flow

We consider a thin horizontal layer of a non-magnetic electrolyte containing a bulk solution of salt and carrying an electric current. The layer is bounded by two deformable free surfaces loaded with an insoluble surfactant and is placed in a vertical magnetic field. The arising Lorentz force drives the electrolyte in the plane of the layer. We employ the long-wave approximation to derive general two-dimensional hydrodynamic equations describing symmetric pinching-type deformations of the free surfaces. These equations are used to study the azimuthal flow in an annular film spanning the gap between two coaxial cylindrical electrodes. In weakly deformed films, the base azimuthal flow and its linear stability with respect to azimuthally invariant perturbations are studied analytically. For relatively thick layers and weak magnetic fields, the leading mode with the smallest decay rate is found to correspond to a monotonic azimuthal velocity perturbation. The Marangoni effect leads to further stabilisation of the flow while perturbations of the solute concentration in the bulk of the fluid have no influence on the flow stability. In strongly deformed films in the diffusion-dominated regime, the azimuthal flow becomes linearly unstable with respect to an oscillatory mixed mode characterised by the combination of radial and azimuthal velocity perturbations when the voltage applied between electrodes exceeds the critical value.

Integrated CAD/CAM Approach for Experimental Stands in Hydrogen Explosion Analysis

Hydrogen is considered a promising clean energy carrier gas for the future and is used in fuel cell systems to generate electricity without producing greenhouse gas emissions. However, the safe storage and handling of hydrogen are essential for the widespread adoption of hydrogen as an alternative energy source. Research into hydrogen explosions is aimed at developing safe storage methods, safety guidelines, and accident prevention measures - crucial objectives for the sustainable growth of the hydrogen economy. Fundamental research into fast hydrogen combustion requires specially designed and built experimental stands to ensure both safe operation and recording of explosion parameters. More than a simple combustion chamber, often the geometry of the stands must contain elements to increase the complexity of the explosion process. In this respect, this paper deals with the possibilities of designing and 3D printing obstacles that can be placed in the path of hydrogen explosions on research stands to modify the cross-section, direct the pressure waves, turbulence the fluid movements, or even suppress the flame front. By combining the computer-aided modeling and design facilities offered by the Autodesk Fusion 360 software, digitized models are converted into 3D objects by the process of dividing them into thin horizontal layers (slices) and transmitting the information thus generated to the 3D printer, using Ultimaker Cura. Here, through the process of material deposition and fusion (FDM), the thermoplastic material is extruded, deposited layer by layer, rapidly cooling, and solidifying to form the final object.

Seeing the invisible: convection cells revealed with thermal imaging

Fluid instabilities are ubiquitous phenomena of great theoretical and applied importance. In particular, an intriguing example is the thermocapillary or Bénard-Marangoni instability which occurs when a thin horizontal fluid layer, whose top surface is free, is heated from below. In this phenomenon, after passing a certain temperature difference threshold, the fluid develops a regular pattern, usually hexagonal, of convection cells known as Bénard convection. In general, this pattern is not visible to the naked eye unless specific tracers are incorporated into the fluid. The use of thermal imaging is a simple alternative not only for directly observing this phenomenon but also for obtaining valuable quantitative information, such as the relationship between the critical wavelength and the thickness of the fluid layer. Here, we propose an experiment specially suited for laboratory courses in fluid mechanics or nonlinear physics that involves the use of thermal cameras, or appropriate smartphone accessories, to study Bénard convection.

Heat transfer during boiling in a thin horizontal layer of the dielectric liquid

Heat transfer during boiling in a thin horizontal layer of the dielectric liquid

Axially symmetric continuum mechanical model for the oscillon

The oscillon is a highly localized dynamical phenomenon occurring in a thin horizontal layer of granular material, which rests on a rigid plate oscillating in the vertical direction. The geometry is axially symmetric and physically resembles a splash of liquid due to a falling drop, except that it continually perpetuates itself and does not generate a spreading wave, as is the case for a liquid splash. The oscillon moves from “peak” to “crater” and “crater” to “peak” such that the time from “peak” to “peak” or “crater” to “crater” is twice the period of the oscillating plate. The physics of granular phenomena is not properly understood, and there is no continuum mechanical theory of granular materials which is widely accepted as accurately describing their behaviour. Here for a free-flowing (cohesion-less) granular material, under axially symmetric conditions, we present a partial continuum mechanical analysis assuming the Coulomb–Mohr yield function and non-dilatant double-shearing theory. We examine small perturbations superimposed upon a purely vertical vibration, and make the assumption that throughout the motion, the lower surface of the layer remains in contact with the rigid metal plate. We show how the temporal dependence, which decouples from the spatial structure, is governed by Mathieu’s equation for the physically relevant case of the rigid plate oscillating sinusoidally, and therefore stability is determined by certain key parameters. We explore a variety of possible forms for spatial dependence. The present axially symmetric analysis complements that presented by the authors for plane strain conditions, and we find, quite remarkably, that apart from constants, both flows are governed by similar fourth-order systems of ordinary differential equations. This means that for both plane strain and axially symmetry, analogous pattern forming conditions can operate.

Onset of double-diffusive convection in water and ester based magnetic nanofluids

The present paper studies the onset of double diffusive convection in water based magentic nanofluids (WMNF) and ester based magnetic nanofluids (EMNF) for a thin horizontal layer. The fluid is heated and salted from below. The effect of three important slip mechanisms viz., Brownian diffusion, thermophoresis and magnetophoresis has been incorporated. The Eigen value problem is solved by the numerical method. The results are presented for rigid-rigid boundaries in the gravity environment. Critical Rayleigh number is determined in terms of the governing non-dimensional parameters like solutal Rayleigh number, solutal Lewis number and nanoparticle concentration Rayleigh number.

Coarsening Dynamics of 2D Subaqueous Dunes

AbstractFluid flow over an initially flat granular bed leads to the formation of a surface‐wave instability. The sediment bed profile coarsens and increases in amplitude and wavelength as disturbances develop from ripples into dunes. We perform experiments and numerical simulations to quantify both the temporal evolution of bed properties and the relationship between the initial growth rate and the friction velocity u∗. Experimentally, we study underwater bedforms originating from a thin horizontal particle layer in a narrow and counter‐rotating annular flume. We investigate the role of flow speed, flow depth and initial bed thickness on dune evolution. Bedforms evolve from small, irregular disturbances on the bed surface to rapidly growing connected terraces (2D equivalent of transverse dunes) before splitting into discrete dunes. Throughout much of this process, growth is controlled by dune collisions which are observed to result in either coalescence or ejection (mass exchange). We quantify the coarsening process by tracking the temporal evolution of the bed amplitude and wavelength. Additionally, we perform Large Eddy Simulations (LES) of the fluid flow inside the flume to relate the experimental conditions to u∗. By combining the experimental observations with the LES results, we find that the initial dune growth rate scales approximately as . These results can motivate models of finite‐amplitude dune growth from thin sediment layers that are important in both natural and industrial settings.

Experimental study of heat transfer during boiling in a thin layer of liquid on surfaces with structured porous coatings

The paper presents the results of the study of evaporation and boiling in a thin horizontal layer of liquid on microstructured surfaces in a wide range of changes in pressure. It is found that the thermal conductivity of materials of microstructured surfaces significantly affects the mechanism of steam removal from the pores and circulation of liquid along the heat transfer surface. It is determined that the pressure change leads to three regimes of heat transfer: evaporation, transition regime, and bubble boiling. The lowest values of the heat transfer coefficients and CHF were obtained in the transition regime; the highest ones were obtained in the bubble regime on both surfaces. Due to the higher thermal conductivity, the higher heat transfer coefficients and CHF were obtained on the bronze coating than on stainless steel over the entire pressure range.

A map of regimes of evaporation and boiling in the horizontal liquid layer on the modified surface

The paper presents the results of the study of the evaporation and boiling regimes in thin horizontal layers of liquid on a modified surface in a wide range of changes in the pressure and height of the liquid layer. Depending on the heat flux, pressure, and height of the liquid layer, the formation of various structures was observed. In this paper, maps of the evaporation and boiling regimes are obtained, which show the heat fluxes from the natural convection regime up to the boiling crisis, depending on the height of the liquid layer. The results are compared with the calculation dependencies.

Influence of a Thin Horizontal Weak Layer on the Mechanical Behaviour of Shallow Foundations Resting on Sand

The presence of minor details of the ground, including soil or rock masses, occurs more frequently than what is normally believed. Thin weak layers, shear bands, and slickensided surfaces can substantially affect the behaviour of foundations, as well as that of other geostructures. In fact, they can affect the failure mechanisms, the ultimate bearing capacity of footings, and the safety factor of the geotechnical system. In this research, numerically conducted through Finite Element Code Plaxis 2D, the influence of a horizontal thin weak layer on the mechanical behaviour of shallow footings was evaluated. The obtained results prove that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than two to four times the footing width. In fact, under these circumstances, the failure mechanisms are always mixtilinear in shape because the shear strains largely develop on the weak layer. However, the reduction in the ultimate bearing capacity is a function of the difference between the shear strength of the foundation soil and the layer. The presence of a thin weak layer decreases the ultimate bearing capacity up to 90%. In conclusion, this research suggests that particular attention must be paid during detailed ground investigations to find thin weak layers. Based on the obtained results, it is convenient to increase the soil volume investigation to a depth equal to four times the width of the foundation.

Electrically inferred subsurface fractures in the crystalline hard rocks of an Experimental Hydrogeological Park, Southern India

We have investigated a network of fractures forming the flow paths within the crystalline granitic rocks of an Experimental Hydrogeological Park (EHP) with the help of electrical resistivity surveys. The experimental study located at the managed aquifer recharge (MAR) site of EHP has measured a distinct variation in the apparent resistivity for deeper electrical signals that localize the presence of interconnected water-saturated fractures. Usually, profiles close to the MAR tank depict low apparent resistivity values from deep signals across in situ fractures and resistivity amplitude increases away from the tank. We modeled and simulated the presence of water-saturated fractures by a simple three-layered model having embedded shallow heterogeneities in the saprolite layer, vertically interconnected multiple thin conductive horizontal layers in the fissured zone, and an underlain unweathered crystalline granitic basement. These fractures produce a distinct variation in the resistivity for modeling and inversion exercises. The decadal time-lapse electrical resistivity surveys, after the establishment of the MAR tank, mark similar repetitive main features with a distinct drop in resistivity depicting the presence of water-saturated fractures. An overview of the 3D resistivity model characterizes the subsurface heterogeneities and the presence of possible flow paths for shallow depths <30 m, and it is indicative of possible flows in the interconnected deep fractures for depths >30 m.

Maps of hydrodynamic regimes of evaporation and boiling in the thin horizontal liquid layer on the modified surface

The paper reports the results of the study of evaporation and boiling regimes of a thin horizontal liquid layer on a modified surface. The formation of various structures was observed at the different heat fluxes, pressures, and heights of the liquid layer. The paper presents maps of the hydrodynamic regimes of evaporation and boiling of thin liquid layers on a modified surface. The regions of existence of various structures observed in the layer are indicated depending on the pressure and heat flux. The results are compared with the calculation dependencies.

Instability of vertically stratified horizontal plane Poiseuille flow

Abstract

The effect of pressure on heat transfer at evaporation/boiling in horizontal liquid layers of various heights on a microstructured surface produced by 3D laser printing

Heat transfer was experimentally studied under evaporation/boiling conditions in thin horizontal layers of n-dodecane at low relative pressures. A capillary-porous coating from a stainless steel powder with a sinusoidal 2D modulated profile was applied using a 3D laser printer. The selective laser sintering (SLS) additive technique was applied in the study. The dependence of a heat transfer coefficient on the heat flux under different pressures was studied for liquid layers various heights. The resulting experimental data were compared with the values obtained during evaporation/boiling on a smooth surface under the same conditions. A rise in the relative pressure results in an increase of the heat transfer coefficient at bubble boiling. The study demonstrates that the use of a capillary-porous coating leads to a 3.5-fold increase of the heat transfer coefficient in the regimes of bubble boiling. The heat transfer coefficient on the capillary-porous coating was lower compared to a smooth surface in heat transfer regimes with evaporation at low relative pressures. The critical heat fluxes (CHF) on the capillary-porous coating increase with the increasing height of the liquid layer.

Crisis of Nucleate Boiling in a Finite-Height Horizontal Layer of Liquid

This paper presents the results of measurement and calculation within an upgraded Zuber model of critical heat flux (CHF) during nucleate boiling in thin horizontal layers of liquid. The layer height was increased in regimes of developed nucleate boiling. The two-phase layer was foam with vapor columns ascending at the corners of a rectangular square lattice. In the calculations, the foam was considered as an isotropic homogeneous system. The model assumes that the diameter of the vapor columns and the critical Helmholtz instability wavelength are independent of the foam density and are determined by the length of the most dangerous Rayleigh--Taylor instability wavelength. The proposed model fits the CHF experimental data for boiling liquid and can explain the CHF decrease in solutions with addition of surfactants that increase foaming.

Heat transfer at evaporation/boiling in the thin horizontal liquid layer on microstructured surfaces under low pressures

The results of the experimental study of heat transfer during boiling of n-dodecane on a capillary-porous surface are presented. The surface is coated using a 3D laser printer. The dependence of the heat transfer coefficient on the heat flux density under different pressures and liquid layer heights is studied. The obtained experimental data were compared with the values obtained during evaporation/boiling on a plain surface under the same conditions.

Simulating nonlinear wavy flow modes developing in a thin horizontal layer of heavy liquid entrained by a gas flow

The paper considers nonlinear waves generated on a surface of a horizontal liquid layer put into an assigned stress field at the gas-liquid interface. The nature of branching for wavy modes from the undisturbed flow was studied. To accomplish this, the solution of a model nonlinear equation written for the deviation of the layer thickness from the undisturbed layer is found. Analytical solutions were constructed for nonlinear steady state-travelling solutions of this equation with the wavenumbers belonging to the vicinity of neutral wavenumbers. Steady state-travelling periodic solutions for the first family were simulated for the case of wavenumbers beyond this vicinity.

Perspectives and problematic issues in the development of heat transfer enhancement methods at boiling and evaporation

The paper presents an analysis of modern methods of heat transfer intensification and critical heat flux increase during boiling and evaporation using micro-nanostructuring of surface. Special attention is paid to the analysis of such promising methods of modifying heat-transfer surfaces as the application of combined microstructured surfaces/coatings by SLM/SLS (3-D printing), the organization of structures with contrast wettability on the micro-scale. The conclusions formulated on the basis of generalization of results of complex research on studying of heat transfer, transitional and crisis phenomena at boiling in the conditions of free convection, at evaporation and boiling in flowing down films and in thin horizontal layers of liquids at use of various types of microstructuring of heat- transfer surfaces are presented.

Heat transfer at evaporation and boiling in thin horizontal liquid layers on smooth and micro-structured surfaces under low pressures

The analysis of the heat transfer investigation results has been carried out at boiling and evaporation in thin horizontal layers of liquid on smooth and structured surfaces. Intensive evaporation of thermal plumes at reduced pressure is considered to be the basic mechanism of heat transfer. The thermal plumes could make a significant contribution in heat transfer under conditions of bubble boiling on micro-structured and ribbed surfaces in the areas where vapour bubbles don’t emerge.