Three-dimensional Regime Research Articles

Experimental study on the characteristics of steam direct contact condensation with non-condensable gas in a T-junction minichannel with flowing subcooled water

In order to investigate the effect of non-condensable gases (NCGs) content on direct contact condensation (DCC) of steam at the mini-scale, a series of experiments on mixed gas condensation were carried out with steam mass flow rate of 0.2–1.0 g/min, subcooled water mass flow rate of 2–20 g/min, water temperature of 40 °C, and NCGs mass flow rate of 0–0.1935 g/min. The results show that the condensation regimes were significantly affected by the content of non-condensable gas and the mass flow rate of the subcooled water. Three typical condensation regimes were observed under different operating conditions, namely chugging, chugging &bubble flow-slug flow-microbubble flow and slug flow-long bubble flow, the last two of which were newly discovered compound condensation regimes. Furthermore, a three-dimensional steam condensation regime diagram was also created. The instantaneous position of the head of the plume was obtained, which has the characteristics of oscillation and significantly by the content of the NCGs content. In addition, the frequency and amplitude of instantaneous pressure oscillations decrease with the increase of non-condensable gas content.

Flow instability in high-Prandtl-number liquid bridges with fully temperature-dependent thermophysical properties

The axisymmetric steady two-phase flow of a differentially heated thermocapillary liquid bridge in air and its linear stability is investigated numerically, taking into account dynamic interfacial deformations in the basic flow. Since most experiments require a high temperature difference to drive the flow into the three-dimensional regime, the temperature dependence of the material properties must be taken into account. Three different models are investigated for a high-Prandtl-number thermocapillary liquid bridge with nominal Prandtl number ${\textit {Pr}}=28.8$ : the Oberbeck–Boussinesq (OB) approximation, a linear temperature dependence of all material properties and a full nonlinear temperature dependence of all material properties. For all models, critical Reynolds numbers are computed as functions of the volume of the liquid bridge, its aspect ratio, its dimensional size and as a function of the strength of a forced axial flow in the ambient air. Under most circumstances the OB approximation overpredicts and the linear model underpredicts the critical Reynolds number, compared with the model based on the full temperature dependence of the material properties. Among the main influence factors are the proper selection of the reference temperature and, at larger temperature differences, the temperature dependence of the viscosity of the liquid.

Experimental investigation on condensation regimes and transition boundary during bubble condensation in narrow rectangular channel

Direct steam contact condensation in subcooled water is widely used in various industries because of high efficiency in heat and mass transfer. A visual experiment is carried out to investigate the condensation process of vapor bubble in narrow rectangular channel. The steam injection mass flux, subcooled water temperature and water mass flow are 0.43–40 kg/m2·s, 50–93 °C, and 0–200 kg/m2·s, respectively. In order to obtain more accurate parameters of vapor bubble in narrow rectangular channel, a new bubble volume model was developed and compared with existed models. A three-dimensional condensation regime map is developed with the coordinates of steam injection flux, water mass flow, and water temperature. Five condensation patterns such as chug, oscillatory bubble, growth bubble, transition region and oscillatory jet are determined based on the dynamic behavior of steam-water interface. The bubble equivalent diameter and aspect ratio during evolution process are calculated, and these two parameters in each pattern exhibit a significant periodicity, except for the oscillatory Jet. The true area of heat transfer interface between bubble and subcooled water is much larger than the actual measured value under high steam injection mass flux. A transition criterion from oscillatory bubble regime to oscillatory jet regime is proposed by the analysis of the bubble critical diameter and the condensation heat transfer of steam-water interface.

The Study of Three-Dimensional Fluid Flow Regimes on the Basis of the Exact Solution to the Convection Equations

We consider the evaporation/condensation process of diffusion type in two-layer fluid flows under the assumption that both viscous heat-conducting incompressible fluids, a liquid and a gas-vapor mixture, have a common interface and occupy an infinite threedimensional rectangular channel. Based on the exact solution to the convection equations, we study the fluid flows arising in the case of a linearly distributed thermal load on the substrate and thermal insulation of the upper and lateral walls of the channel. It is observed that some complex vortex structures are formed under the action of the longitudinal temperature gradient and transversely directed gravity field. For the ethanol–nitrogen system we present the characteristics of hydrodynamic, thermal, and concentration fields, in particular, the profiles of the evaporative mass flow rates and interface tangential stresses, the topology of flows, and the temperature and vapor concentration distributions.

Three-dimensional convection regimes in a two-layer system with evaporation for different types of thermal loads on a substrate

Three-dimensional convection regimes in a two-layer system with evaporation for different types of thermal loads on a substrate

Two dimensional LiMgAs: A topological quantum catalyst for hydrogen evolution reaction

Quantum materials, such as topological insulators (TIs), are promising due to diverse applications of their robust surface/edge states in the bulk three-dimensional (3D) and two-dimensional (2D) regimes. Such conducting surface states in 3D systems host “electron baths,” which are known to facilitate catalysis. However, the analogous effects in 2D scenarios wherein conducting helical edge states leading to Fermionic accumulation have been scarcely addressed. Using first-principles calculations, we demonstrate that the conducting edge states in 2D TIs, such as LiMgAs, can be exploited to facilitate excellent catalytic response toward hydrogen evolution reactions. The Gibbs free energy in such cases was found to be as low as −0.02 eV, which is quite superior compared to other materials reported in the literature. The concept presented herein can be extended to other well-known 2D TIs and used to realize unconventional topological quantum catalysts for ultra-high performance and efficient catalytic applications.

Flow Over a Square Cylinder at Intermediate Reynolds Numbers

Abstract This study experimentally investigates the flow over a stationary square cylinder in the test section of a subsonic wind tunnel. Hotwire anemometry (CTA type), particle image velocimetry (2C-PIV), and flow visualization techniques are used in the study to characterize the flow properties. The study is carried out at four intermediate Reynolds numbers (blocking width 'B' as the characteristic length) 600, 800, 1000, and 2000. The flow over the circular cylinder and square cylinder differs, particularly at intermediate Reynolds numbers, but the literature lacks detailed flow behavior over a square cylinder beyond the wake transition regime. Re = 600 is the critical Reynolds number as the flow changes from a disordered fine-scale three-dimensional regime to a shear layer transition regime. The effects of the regime alterations on the flow properties, vortex shedding frequency, and mean drag coefficient are reported in the study for all Reynolds numbers. The flow visualization supplements the CTA and PIV measurements. The turbulent kinetic energy budget has also been compared for all Reynolds numbers.

Numerical study on the effect of solar radiation intensity on the fresh water productivity of solar still equipped with Thermoelectric Cooling System (TEC) for hot and dry areas of Semnan

The fresh water crisis is one of the most important issues in the world and as a solution one of the most important methods of freshwater production is the evaporation of saline water by using of solar stills. Semnan province is one of the largest provinces in Iran. The province covers an area of 97,491 km 2 , which is 5.9% of the total area of the country. This province is the seventh largest province in Iran and is located almost in the central desert region of Iran. Due to its location on the solar belt, the desert areas of Semnan province are prone to the installation of solar cells, and according to the statistics of the General Meteorological Department of Semnan, this province has 325 sunny days in 365 days of the year. Therefore, this area is a great location for using clean solar energy. In this research, using the numerical method, the effect of the presence of the Thermoelectric Cooling System (TEC) on the performance of solar still, intensity and direction of solar radiation has been investigated to check their effect on the rate of freshwater production. The simulations were performed for Semnan city condition with the method of humid air in a three-dimensional, steady and laminar regime by solving the equations of continuity, Navier-Stokes, and water vapor concentration. The results show that there is a direct relationship between radiation intensity and water production rate and increasing radiation intensity increases the amount of produced freshwater. It was also observed that applying a TEC on a part of the glass can increase water production from 15% to 62%. The reason for this increase can be the increase in free convection due to the increase in the flow temperature gradient in designed solar still.

Dimensionality-dependent type-II Weyl semimetal state in Mo0.25W0.75Te2

Weyl nodes and Fermi arcs in type-II Weyl semimetals (WSMs) have led to lots of exotic transport phenomena. Recently, Mo$_{0.25}$W$_{0.75}$Te$_{2}$ has been established as a type-II WSM with Weyl points located near Fermi level, which offers an opportunity to study its intriguing band structure by electrical transport measurements. Here, by selecting a special sample with the thickness gradient across two- (2D) and three-dimensional (3D) regime, we show strong evidences that Mo$_{0.25}$W$_{0.75}$Te$_{2}$ is a type-II Weyl semimetal by observing the following two dimensionality-dependent transport features: 1) A chiral-anomaly-induced anisotropic magneto-conductivity enhancement, proportional to the square of in-plane magnetic field (B$_{in}$$^{2}$); 2) An additional quantum oscillation with thickness-dependent phase shift. Our theoretical calculations show that the observed quantum oscillation originates from a Weyl-orbit-like scenario due to the unique band structure of Mo$_{0.25}$W$_{0.75}$Te$_{2}$. The in situ dimensionality-tuned transport experiment offers a new strategy to search for type-II WSMs.

Anisotropic transport properties of graphene-based conductor materials

We analyzed nanographite-based materials in a combined study including experimental analysis via 4-point probe STM and simulation to provide a complete picture of microscopic and macroscopic properties of the material. The two- and three-dimensional transport regimes were determined and evaluated regarding the anisotropy of the conductivity. The experimental results yield the full macroscopic conductivity tensor. Microstructural simulations are used to map those macroscopic properties to the microscopic building blocks of the sample. By combining those two, we present a coherent and comprehensive description of the electrical material parameters across several length scales.

Three-Dimensional Mixed Convection in a Rectangular Enclosure Containing a Circular Cylinder

This study carried out a three-dimensional, unsteady, numerical simulation to investigate the effects of the cylinder radius, Reynolds number, three-dimensionality and unsteadiness on the mixed convection in a rectangular enclosure containing a cold circular cylinder. The variations in the cylinder radius in the range of 0.1 to 0.3 times the length of the enclosure and the Reynolds number in the range of 100 to 1000 were considered. The flow and thermal structures reached three different regimes depending on the radius of cold cylinder and Reynolds number. The patterns in the flow and thermal fields were categorized into three regimes: steady two-dimensional convection, steady three-dimensional convection, and unsteady three-dimensional convection. The temperature distribution, vortical structure, and Nusselt number for a heated bottom wall were extensively analyzed. The transition of flow regime, flow instability and the heat transfer performances were influenced by the presence of cylinder. In addition, the three-dimensional flow characteristics were quantified using orthogonal enstrophy, and the unsteady characteristics were investigated by carrying out the frequency analysis of Nusselt number. As the three-dimensionality in flow and thermal fields increases and the flow regime changes to the three-dimensional unsteady convection regime, the difference in the results of two and three-dimensional simulations also increases.

Three-dimensional wake transition behind an elliptic cylinder near a moving wall

Three-dimensional flow past an elliptic cylinder with an aspect ratio of 0.5 near a moving bottom wall is investigated numerically for gap ratios of G/D=0.1,0.2,0.3, and 0.4 (where G denotes the gap between the cylinder bottom and the moving wall, and D is the major-axis length of the cylinder) with Reynolds numbers (Re) ranging from 100 to 200 (based on a constant inlet velocity and the major-axis length of the cylinder); the transition between two- and three-dimensional flow regimes is described in detail. For G/D=0.4, the flow is first two-dimensional with a Kármán vortex street followed by a two-layered wake, then it evolves into a three-dimensional flow regime with near-wake and far-wake elliptic instabilities of vortex pairs; for Re≥180, the near-wake elliptic instability disappears (i.e., the near wake becomes two-dimensional) while the far-wake elliptic instability persists. For G/D=0.3, the flow is first two-dimensional without the development of the two-layered wake, then it evolves into a three-dimensional flow regime with streamwise vorticity pairs propagating periodically in the spanwise direction; this propagation becomes irregular for Re≥160. For G/D=0.2, the flow is first two-dimensional as for G/D=0.3, then it becomes three-dimensional, exhibiting a behavior of modified mode C instability; for Re≥140, this flow exhibits a chaotic behavior. For G/D=0.1, the flow is first three-dimensional and steady without vortex shedding, and then develops into an unsteady flow with a dominating upper shear layer in the near-wake and a chaotic wake structure farther downstream.

Experimental study of the effect of surfactants on the characteristics of waves in three-dimensional wave regimes

Effect of a surfactant Triton X-100 on three-dimensional wave regimes of the falling liquid film flow was experimentally studied. The shadow method and laser induced fluorescence measurements revealed that for surfactant concentrations at which complete suppression of wave motion is not observed, the flow characteristics at the three-dimensional wave regime differ from the characteristics of a pure water flow. Most likely, these differences are associated with differences in the structure of flows under the three-dimensional wave, which were revealed by simultaneous measurements of the thickness and volumetric measurements of the velocity in the wave at the initial section of the flow. For films with the surfactant concentration of 2000 mg/l, the capillary ripples in front of the three-dimensional wave are weakly expressed, there are no backflows, and transverse flows in the area of the capillary precursors are much weaker than in the case of pure water.

Excess Conductivity Analysis of Polycrystalline FeSe Samples with the Addition of Ag.

Bulk FeSe superconductors of the iron-based (IBS) “11” family containing various additions of silver were thoroughly investigated concerning the microstructure using optical microscopy and electron microscopy (TEM and SEM). The measurements of electrical resistivity were performed through the four-point technique in the temperature interval 2–150 K. The Aslamazov–Larkin model was employed to analyze the fluctuation-induced conductivity (FIC) in all acquired measurements. In all studied products, we found that the FIC curves consist of five different regimes of fluctuation, viz. critical region (CR), three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and shortwave fluctuation (SWF) regimes. The critical current density (), the lower and upper critical magnetic fields ( and ), the coherence length along the c-axis at zero-temperature ((0)), and further parameters were assessed with regards to the silver amount within the products. The analyses discloses a diminution in the resistivity and a great reduction in (0) with Ag addition. The optimal silver doping amount is achieved for 7 wt.%, which yields the best superconducting transition and the greatest value.

Influence of a soluble surfactant on the wave characteristics of a liquid film flow

In this paper we present results of experimental investigation of the influence of soluble surfactant (Triton X-100) on wave characteristics of vertically falling liquid film in the range of film flow Reynolds number 25 < Re < 95. Shadowgraph technique is used to analyze wave patterns on the whole length of the test section (140 cm). Film thickness fields obtained with the help of high-speed Laser Induced Fluorescence (LIF) technique in two areas in the bottom part of test section allow analyzing statistical characteristics of the film flow. Expectedly, adding surfactant suppresses wave motion. For some surfactant concentrations the absence of waves is observed in the whole test section. For other concentrations the wave repetition rate decreases with distance from the liquid inlet. Analysis of spectral characteristics reveals that for water with some concentrations of surfactant, as for pure liquids, the steady-state three-dimensional wave regimes are observed. The characteristics in such flow regimes differ from the characteristics of pure water flow.

Application of the PTV with the use of a light-field camera to study three-dimensional wave regimes of liquid film flow

We present the results of studying the influence of surfactants on three-dimensional waves, in particular, on the flow structures in the wave. Volumetric fluid velocity measurements in waves, as well as the determination of their shape, were carried out using a light field camera. An aqueous solution of the nonionic surfactant Triton-X was used as a working fluid. The presence of surfactant leads to a significant change in the structure of the transverse flow regions under the main hump of the wave. Besides, capillary precursor and backflow in it are suppressed.

Microstructure and Fluctuation-Induced Conductivity Analysis of Bi2Sr2CaCu2O8+δ (Bi-2212) Nanowire Fabrics

Resistance measurements were performed on Bi2Sr2CaCu2O8+δ (Bi-2212) fabric-like nanowire networks or nanofiber mats in the temperature interval 3 K ≤T≤ 300 K. The nanowire fabrics were prepared by means of electrospinning, and consist of long (up to 100 μm) individual nanowires with a mean diameter of 250 nm. The microstructure of the nanowire network fiber mats and of the individual nanowires was thoroughly characterized by electron microscopy showing that the nanowires can be as thin as a single Bi-2212 grain. The polycrystalline nanowires are found to have a texture in the direction of the original polymer nanowire. The overall structure of the nanofiber mats is characterized by numerous interconnects among the nanowires, which enable current flow across the whole sample. The fluctuation-induced conductivity (excess conductivity) above the superconducting transition temperature, Tc, was analyzed using the Aslamzov-Larkin model. Four distinct fluctuation regimes (short-wave, two-dimensional, three-dimensional and critical fluctuation regimes) could be identified in the Bi-2212 nanowire fabric samples. These regimes in such nanowire network samples are discussed in detail for the first time. Based on this analysis, we determine several superconducting parameters from the resistance data.

Numerical Study of Magnetic Field Influence on Three-Dimensional Flow Regime and Combined-Convection Heat Exchange Within Concentric and Eccentric Rotating Cylinders

Abstract The forced and natural flows of fluid within an annulus caused by the rotation of cylinders and temperature differences of the inner and outer walls are observed in various engineering applications. In this research, the laminar flow regime and mixed convection inside a ring-shaped horizontal concentric and eccentric space for an incompressible fluid are studied in the existence of an axial magnetic field. The present work is the first effort to investigate the influence of a magnetic field on flow and combined-convection heat exchange characteristics within an annulus with a cold outer cylinder and an inner hot cylinder. Here, the properties of the flow and heat transfer characteristics are studied using the finite volume method. Numerical procedures are mainly investigated for recognizing the influence of Hartmann number (in the range of 0 ≤ Ha ≤ 100), as the representative of the magnetic force, on velocity components, Nusselt number, streamlines, and isothermal lines. One of the notable effects is that when Ha number increases, it will reduce the vorticity of the fluid and buoyancy forces. As a result, streamlines and isothermal lines can be seen more constant as regular concentric circles. A rise in Ha number decreases the range of local Nu number variation for both cylinders. The average Nu number for the outer and inner cylinders has different trends when Ha number increases. Taking concentric cylinders as an example, this parameter for the inner and the outer cylinders increases and decreases by about 1.2 and 1.6, respectively.

Modelling double emulsion formation in planar flow-focusing microchannels

Double emulsion formation in a hierarchical flow-focusing channel is systematically investigated using a free energy ternary lattice Boltzmann model. A three dimensional formation regime diagram is constructed based on the capillary numbers of the inner ($Ca_i$), middle ($Ca_m$) and outer ($Ca_o$) phase fluids. The results show that the formation diagram can be classified into periodic two-step region, periodic one-step region, and non-periodic region. By varying $Ca_i$ and $Ca_m$ in the two-step formation region, different morphologies are obtained, including the regular double emulsions, decussate regimes with one or two alternate empty droplets, and structures with multiple inner droplets contained in the continuous middle phase thread. Bidisperse behaviors are also frequently encountered in the two-step formation region. In the periodic one-step formation region, scaling laws are proposed for the double emulsion size and for the size ratio between the inner droplet and the overall double emulsion. Furthermore, we show that the interfacial tension ratio can greatly change the morphologies of the obtained emulsion droplets, and the channel geometry plays an important role in changing the formation regimes and the double emulsion sizes. In particular, narrowing the side inlets or the distance between the two side inlets promotes the conversion from the two-step formation regime to the one-step formation regime.

Dimensionality and superconducting parameters of YBa2Cu3O7−d/(WO3 NPs)x composites deduced from excess conductivity analysis

Superconducting composite products of type YBa2Cu3O7−d (noted Y-123) added with different contents (x wt.%) of tungsten oxide nanoparticles (WO3 NPs), x = 0.0–1.0 wt%, were produced via the standard solid-state reaction technique. Samples were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrical resistivity measurements. XRD examination showed that the Y-123 orthorhombic structure is not altered by WO3 NPs additions. SEM observations indicated a reduction in the grains size with the increase of nanoparticles content. The investigation of the excess conductivity of different prepared composites was analyzed using Aslamazov-Larkin (AL) model. The investigated samples comprised of five distinct fluctuation regimes, namely short-wave (SWF), one-dimensional (1D), two-dimensional (2D), three-dimensional (3D), and critical (CR) fluctuations regimes. The coherence length along c-axis at zero-temperature (ξc(0)), lower and upper critical magnetic fields (Bc1 and Bc2), critical current density (Jc) and numerous other superconducting parameters were estimated with respect to WO3 NPs content. Compared to undoped Y-123 sample, it was found that an amount of x = 0.05 wt% of WO3 NPs is appropriate to improve the physical performances of Y-123, whereas with further increasing x, these performances were weakened.