Vertical Narrow Channel Research Articles

Oscillatory excitation of Faraday waves on the interface of immiscible fluids in a slotted channel

Recent studies of the oscillatory dynamics of the interface between fluids in Hele–Shaw cells have revealed a new type of instability termed the “oscillatory Saffman instability” in the case of fluids with high-viscosity contrast. The present study is dedicated to the experimental investigation of the dynamics of the interface between low-viscosity fluids of different densities oscillating in a vertical narrow channel. It is discovered that as the amplitude of oscillations increases, a threshold excitation of parametric oscillations of the interface in the form of a standing wave is observed in the plane of the fluid layer. This phenomenon bears a resemblance to Faraday waves, but the dependence of the standing wave wavelength on the oscillation frequency does not align with the classical dispersion relation for low-viscosity fluids. The damping effect of viscous boundary layers near the cell walls and the out-of-plane curvature of the oscillating interface leads to a decrease in the natural frequency of oscillations. The experiments demonstrate a significant role of the dimensionless layer thickness. With its decrease (increase in the dimensionless out-of-plane interface curvature), the threshold oscillation acceleration rises in accordance with a power law. To the best of the authors' knowledge, this type of instability has been discovered and studied for the first time. Another important finding is the excitation of intense time-averaged vortical flows in the channel plane within the supercritical region. The physical mechanism underlying the excitation of the time-averaged vortices is clarified, and the dimensionless parameters that govern their intensity are identified.

Shear‐driven flow of an ionic fluid in a narrow vertical channel under a Hall electric field

AbstractThis paper focuses on demonstrating the shear‐driven convective flow of an ionic optically thin fluid in a narrow channel formed by two vertical parallel plates subject to a Hall electric field. The Hall electric field induces Hall currents, amending the flow dynamics of the ionic fluid. The setup involves a stationary left wall and a right wall that either undergoes impulsive motion (IM) or accelerated motion (AM), which initiates the fluid flow. A unified closed‐form solution for flow‐regulating equations is derived by harnessing the Laplace transform (LT) approach. The upshots of cardinal parameters on the velocity components and temperature distributions, shear stresses, and rate of heat transfer (RHT) are elucidated via graphics for both IM and AM scenarios. The graphs reveal that an intensification in the Hall parameter notably boosts the velocity components in both IM and AM cases. The primary and secondary velocities are consistently higher for IM than AM. The magnitude of shear stresses at the moving wall is always greater for IM than AM. Additionally, the shear stresses at the moving wall are notably greater for IM than AM, and the RHT at the moving wall reduces as the radiation parameter amplifies. The significant findings of this research have potential applications in electromagnetic propulsion systems, like plasma or ion thrusters, commonly employed in propelling spacecraft.

A surface-piercing truncated cylindrical meta-structure operating as a wave energy converter

In this paper, we study the interaction of water waves with a surface-piercing truncated cylindrical meta-structure consisting of two overlapping arrays of closely spaced vertical thin plates. The fluid resonance promoted in the narrow vertical channels formed by the cylindrical meta-structure is exploited by a novel design concept of the wave power converter by covering the surface of the cylinder with an array of small cuboid buoys which float in the gaps between the intersecting plate arrays. Each buoy is attached to its own spring and power takeoff damping mechanism, and the vertical displacement of individual buoys is replaced by a continuous two-dimensional function of position which follows from homogenization of the plate/fluid structure of the cylinder. Effective medium equations and boundary conditions are derived under both full depth-dependent theory and shallow-water theory, allowing semi-analytical methods to be developed to investigate the wave scattering and wave energy absorption properties of this cylindrical meta-structure. Results illustrate that the internal resonance of the cylindrical meta-structure can lead to significant wave power capture across a broad range of frequencies.

Flame oscillation instability of H2/CO/air at different ignition positions in an open-ended vertical narrow channel

ABSTRACT The propagation instability of a premixed H2/CO/air mixture flame in an open-ended vertical narrow channel was investigated in this paper. The mixture was ignited at five different ignition positions: IP-50, IP-250, IP-500, IP-750, and IP-950. This paper presents a new method for flame image processing. The flame front curve is generally an exponential outflow from the ignition to the top opening at any ignition. The oscillation frequency increases with the increase of flame front curvature. The flame front curvature fluctuates with time when the oscillation propagation occurs, and the flame decelerates and inverts when the curvature is larger. The dominant frequencies are different depending on ignition position, which has a certain effect on frequency. Five different ignition positions of dominant frequency are 78.00 Hz, 125.07 Hz, 170.5 Hz, 125.1 Hz, 170.7 Hz, respectively. Interestingly, IP-500, IP-750 and IP-950 have secondary frequency values of 356.9 Hz, 357.1 Hz and 331.8 Hz, respectively. The secondary frequency is about twice the dominant frequency, and it is caused by parametric instability. Flame oscillation propagation is affected by dimensionless temperature and Lewis value. The interaction between flame and overpressure wave promotes the deformation and oscillation of the flame front. The overpressure, flame front position, flame front curvature, and flame speed coupled oscillation.

Swimming Mode of Two Interacting Squirmers under Gravity in a Narrow Vertical Channel.

The swimming mode of two interacting squirmers under gravity in a narrow vertical channel is simulated numerically using the lattice Boltzmann method (LBM) in the range of self-propelling strength 0.1 ≤ α ≤ 1.1 and swimming type -5 ≤ β ≤ 5. The results showed that there exist five typical swimming patterns for individual squirmers, i.e., steady upward rising (SUR), oscillation across the channel (OAC), oscillation near the wall (ONW), steady upward rising with small-amplitude oscillation (SURO), and vertical motion along the sidewall (VMS). The parametric space (α, β) illustrated the interactions on each pattern. In particular, the range of oscillation angle for ONW is from 19.8° to 32.4° as α varies from 0.3 to 0.7. Moreover, the swimming modes of two interacting squirmers combine the two squirmers' independent swimming patterns. On the other hand, the pullers (β < 0) attract with each other at the initial stage, resulting in a low-pressure region between them and making the two pullers gradually move closer and finally make contact, while the result for the pushers (β > 0) is the opposite. After the squirmers' interaction, the squirmer orientation and pressure distribution determine subsequent squirmer swimming patterns. Two pushers separate quickly, while there will be a more extended interaction period before the two pullers are entirely separated.

Dendritic bubble growth during the sub-atmospheric boiling of water in a narrow vertical channel

The bubbles observed during the boiling of water at subatmospheric pressure (a few mbar) behave in a very atypical way compared to under atmospheric pressure. Moreover, the presence of very close walls significantly affects bubble dynamics and interface disturbances like fingering can be observed during growth. Interface perturbations were reported in a phenomenological analysis. Image processing software was developed to process the obtained high resolution camera recordings. Based on this software, physical quantities characterising the bubble are measured for the purpose of a parametric study of the dentritic growth of the bubbles. It appeared that the bubble interfacial velocity , bubble interface acceleration and nucleation immersion depth, related to inertia and hydrostatic pressure, have a significant influence on the fingering occurrence.

Waves on the surface of a thin liquid film entrained by a turbulent gas flow in a narrow channel

The paper considers the problem of a joint flow in a narrow vertical channel of a turbulent gas flow and a wave film of liquid flowing down its walls. The calculation of tangential stresses on the interface is performed. The components of the Reynolds stress tensor were determined within the framework of the Boussinesq hypothesis. The turbulent viscosity coefficient was taken into account both when calculating the average velocity profile and when considering disturbances caused by wavy interface. The main difference this case and the semi-infinite channel case is observed in the longwave region. Nonlinear waves on a liquid film flowing down under the action of gravity are studied in a known stress field at the interface. For the case of low Reynolds numbers of the liquid (Re ∼1), the problem is reduced to a nonlinear integro-differential equation for the deviation of the layer thickness from the unperturbed level. For this equation, the character of branching from the unperturbed solution of weakly nonlinear steady-state traveling solutions, whose wave numbers lie in the neighborhood of neutral points, is studied. A numerical study of the evolution of periodic perturbations, whose wave numbers noticeably differ from neutral wave numbers, has been carried out. Several characteristic scenarios for the development of such disturbances are presented.

Determination of heat flux leading to the onset of flow instability in MTR reactors

Abstract The prediction of heat flux leading to the Onset of Flow Instability (OFI) phenomena is an important consideration in the design of Material Testing Reactors (MTR) due to the possibility of flow excursion during postulated accident. From the thermal-hydraulic point of view, OFI is the critical phenomenon limiting MTR reactor power. In a previous work, an empirical correlation is developed to predict the subcooling at OFI in narrow vertical rectangular channels simulating a coolant channel of MTR. In the present work, an innovative model to determine the heat flux leading to OFI in MTR reactors is introduced based on the previous correlation. The developed model gives a very low deviation of only 1.65% from the experimental data of Whittle & Forgan that covers a wide range of MTR operating conditions. The heat flux leading to OFI is also predicted by both Whittle & Forgan and Fabrega correlations for comparison. The present model is then applied on the IAEA 10 MW MTR generic reactor to predict the Best-Estimate (BE) and Best-Estimate-Plus-Uncertainty (BEPU) Onset of Flow Instability Ratio (OFIR) and the power leading to OFI as well as the bubble detachment parameter under different coolant velocities and inlet temperatures. The model is also used to predict both the OFIR and bubble detachment parameter in the reactor under unprotected Loss-of-Flow transient for exponential flow decay with a time constant of 1.0 s (fast LOFA), 10, 15 and 25 s (slow LOFA) from a power level of 10 MW. For BEPU calculation, a combined statistical method with direct propagation of errors is adapted to treat the uncertainty factors for fuel fabrication and measured parameters in the BEPU calculation. The model results is analyzed and discussed.

Behaviors of a settling microswimmer in a narrow vertical channel

An immersed boundary-lattice Boltzmann (IB-LB) method is employed to study the behaviors of a settling microswimmer in a narrow vertical channel (blockage ratio κ = 0.5). We perform the simulations with swimming Reynolds numbers (Res) between 0.1 and 3.0 and the density ratios (γ) between 1.1 and 2.3. A two-dimensional squirmer model is used to mimic a microswimmer, finding four typical locomotive modes, including the vertical mode, the attracted oscillatory mode, the oscillatory mode, and the upward mode. It is found that both the inertia and the density ratio competitively affect the locomotive modes. Increasing Res (0.1 ≤ Res<1.5) enhances the attraction of the sidewall, the vertical mode for a puller (propelled from the front) and the oscillatory mode for a pusher (propelled from the rear) develop to the attracted oscillatory mode. The pushers (−7 ≤ β ≤ −5) swim with the upward mode at Res ≥ 2.0 due to the enhanced swimming intensity generating the greater negative torque-integral when interacting hydrodynamically with the sidewall. On increasing γ, the attracted oscillatory mode of a pusher (puller) develops to the oscillatory mode (vertical mode). When initialized at the centreline of a channel along the direction of gravity, the speeds of the pushers (pullers) are enhanced (hindered) at lower Res (Res<0.59). With the increase of Res (Res>0.59), the speeds of the pushers (pullers) are hindered (enhanced). In addition, a puller swims faster at low γ whereas slower at high γ than a pusher.

Numerical simulation of single-phase heat transfer and nucleate boiling of liquid sodium in narrow annulus channel similar to that of SFR fuel subchannel

The sodium cooled fast reactor (SFR) is a Generation IV nuclear reactor that uses fast neutrons for fission and liquid sodium as the coolant for fission heat removal. Compared to conventional fluids, such as water, liquid sodium shows superior performance in heat transfer because of its high thermal conductivity and low Prandtl number. Understanding of single-phase and two-phase heat transfer characteristics of liquid sodium is essential for the analysis of the normal operation and certain postulated accidental events of SFR. This study is intended towards the development of computational fluid dynamics (CFD) based multiphase models for the safety analysis of SFR using the finite volume method. The numerical simulations of single-phase and two-phase heat transfer processes have been performed for the basic two-dimensional domain that has resemblance with the flow subchannel of SFR fuel subassembly. This study deals with heat transfer considerations under low Peclet number at high temperature, and its boiling inception and further to the nucleate boiling regime in a narrow vertical annulus channel. The Realizable k-ε model with Kays correlation for the turbulent Prandtl number better predicts the single-phase heat transfer characteristics under low Peclet number. A correlation for Nusselt number is fitted from the numerical results for low Peclet number flow conditions in an annulus channel. The flow boiling inception and nucleate boiling are numerically predicted by the Eulerian-Eulerian multiphase model with non-equilibrium wall-boiling for wall heat flux partitioning. The turbulence is modeled using Realizable k-ε model with standard wall function. Under the flow and heat flux values considered the near steady-state boiling conditions are predicted without much pulsating behavior as observed in the reference experiments and during this, the two-phase flow pattern varied from inception to the nucleate boiling regime. The mechanistic model based heat transfer coefficient is evaluated in the boiling zone from the flux partitioning components and compared with the available empirical correlations.

Vertical 3D gallium nitride field-effect transistors based on fin structures with inverted p-doped channel

This paper demonstrates the first vertical field-effect transistor based on gallium nitride (GaN) fin structures with an inverted p-doped channel layer. A top-down hybrid etching approach combining inductively coupled plasma reactive ion etching and KOH-based wet etching was applied to fabricate regular fields of GaN fins with smooth a-plane sidewalls. The obtained morphologies are explained using a cavity step-flow model. A 3D processing scheme has been developed and evaluated via focussed ion beam cross-sections. The top-down approach allows the introduction of arbitrary doping profiles along the channel without regrowth, enabling the modulation of the channel properties and thus increasing the flexibility of the device concept. Here, a vertical npn-doping profile was used to achieve normally-off operation with an increased threshold voltage as high as 2.65 V. The p-doped region and the 3D gate wrapped around the sidewalls create a very narrow vertical electron channel close to the interface between dielectric and semiconductor, resulting in good electrostatic gate control, low leakage currents through the inner fin core and high sensitivity to the interface between GaN and gate oxide. Hydrodynamic transport simulations were carried out and show good agreement with the performed current–voltage and capacitance–voltage measurements. The simulation indicates a reduced channel mobility which we attribute to interface scattering being particularly relevant in narrow channels. We also demonstrate the existence of oxide and interface traps with an estimated sheet density of 3.2 × 1012 cm−2 related to the Al2O3 gate dielectric causing an increased subthreshold swing. Thus, improving the interface quality is essential to reach the full potential of the presented vertical 3D transistor concept.

Thermoacoustic analysis of lean premixed hydrogen flames in narrow vertical channels

Thermoacoustic instabilities arise for lean hydrogen-air flames propagating in narrow channels. We provide here a detailed experimental analysis of such phenomena in a semi-confined vessel, analyzing the effect of the mixture composition, geometry and gravity on the onset of acoustic-driven flame vibrations.Downward-propagating flames leaner than a critical value vibrate smoothly and transit to the secondary oscillating instability, which develops strong variations of pressure that couple with the propagation dynamics. The transition threshold changes during the propagation along very narrow channels, where heat losses are no longer negligible. The parametric region of equivalence ratio for the secondary thermoacoustic instability diminishes, showing an additional transition for very lean flames. There, the front breaks into several structures and the flame-wave feedback becomes weaker.The influence of gravity is studied by comparing upward and downward propagating flames, where the Rayleigh–Taylor instability arises for sufficiently small values of the Froude number in slow-propagating lean flames. For a constant mixture, buoyancy-driven upward-propagating flames develop less wrinkled fronts than those propagating downwards, and remain unresponsive to acoustic-front interaction. We show here a direct relation between front shape and thermoacoustics.In agreement with previous studies [1–3], curvature and strain effects on conduction and diffusion characterize the response of the flame to pressure perturbations, with the Markstein number controlling the aforementioned transition. Nevertheless, the theoretical analyses found in the literature can only be used on nearly equidiffusional mixtures, and are not accurate enough to describe the highly diffusive fuel mixtures (i.e. lean hydrogen-air flames) considered in our experiments.

Experimental Study of the Onset of Nucleate Boiling in Vertical Narrow Channels

Heat transfer mechanism in flow boiling through vertical narrow channels differs from that in larger circular tubes with diameters more than 10mm due to the increased surface forces. Basing on the experimental data, the main factors which have influence on heat flux of the onset of nucleate boiling (ONB), such as mass flux, system pressure and dimension of the narrow channel, are analyzed in this paper. The empirical correlation which used to calculate heat flux of the onset of nucleate boiling in vertical narrow annular channels in similar conditions is given here. The calculation results of this correlation are compared with the experimental data and agree well with it while the deviation is less than ±15%. And it’s concluded that narrow channels could enhance flow boiling heat transfer.

Algorithms for determination of the vector velocity field in a two-phase gas-liquid flow

Energy efficiency is a key issue of sustainable development. During the design of industrial devices, it strives to achieve the highest possible energy efficiency. In the industrial systems, two-phase flow is a difficult task, especially the prediction, and maintenance of the two-phase flow regime. That is why this research proposes the evaluation and choice of an algorithm that will give a hint of the device design for which the hydrodynamic conditions of the two-phase mixture flow may be evalu-ated. The tests were carried out in a rectangular vertical narrow channel, as this type of device is in common use. The work aimed to show which algorithm is better for such evaluation. Parameters such as pressure drop, heat, mass, and momentum transfer are influenced by the phase velocity field. Still, various models are used for the determination of the velocity field. Therefore, there is a problem of choosing a model that will give the results closest to the real conditions. Flow visualization gives the non-invasive determination of the actual velocity field. An analysis of the velocity field was performed, which showed that for different two-phase flow regimes there are differences for given algorithms. The following algorithms were used to determine the velocity vector field: adaptive correlation method and adaptive particle image velocity method were used which are the parts of the general digital particle image velocimetry. The determination of the velocity fields in the quantitative and qualitative assessment of a given two-phase flow re-gime was obtained. The result of the research is the evaluation of algorithms for characterization two-phase gas-liquid flow.

Effect of porosity on the settling behavior of a 2D elliptic particle in a narrow vessel: A lattice-Boltzmann simulation

Dynamics of a single porous, rigid, two-dimensional (2D) elliptic particle settling in a narrow vertical channel filled with a Newtonian fluid is numerically studied using the lattice-Boltzmann method. The main objective of the work is to investigate the role played by the particle’s permeability on its trajectory, orientation, and terminal velocity when released from the rest state with prescribed initial conditions. Assuming that the flow induced in the fluid surrounding the particle is laminar, incompressible, isothermal, and two-dimensional, numerical results could be obtained over a wide range of parameter settings suggesting that permeability can strongly affect the modes of sedimentation reported in the literature for impermeable elliptic particles provided that the particle’s permeability is larger than a threshold. Above this threshold, permeability is predicted to increase the terminal velocity of the particle with its severity depending on the blockage ratio. It is also predicted that a permeable particle is less sensitive to initial orientation and position as compared with an impermeable particle.

Numerical study of laminar flow and friction characteristics in narrow channels under rolling conditions using MPS method

Abstract Modern small modular nuclear reactors can be built on a barge in ocean, therefore, their flow characteristics depend upon the ocean motions. In the present research, effect of rolling motion on flow and friction characteristics of laminar flow through vertical and horizontal narrow channels has been studied. A computer code has been developed using MPS method for two-dimensional Navier-Stokes equations with rolling motion force incorporated. Numerical results have been validated with the literature and have been found in good agreement. It has been found that the impact of rolling motions on flow characteristics weakens with increase in flow rate and fluid viscosity. For vertical narrow channels, the time averaged friction coefficient for vertical channels differed from steady friction coefficient. Furthermore, increasing the horizontal distance from rolling pivot enhanced the flow fluctuations but these stayed relatively unaffected by change in vertical distance of channel from the rolling axis. For horizontal narrow channels, the flow fluctuations were found to be sinusoidal in nature and their magnitude was found to be dependent mainly upon gravity fluctuations caused by rolling.

Experimental research on bubble size distribution and vapor quality at the outlet of vertical narrow channel

The presence of bubbles exerts a strong influence on pressure drop, heat transfer, flow pattern, and many other flow characteristics. Due to the complexity of two-phase flow boiling, it is not easy to carry out experimental research. An experimental setup based on ultrasonic detection method is built up in this paper. The present study investigates bubble size distribution and vapor quality in liquid-gas two-phase flow in a vertical narrow channel with the cross section of 3×20 mm. Bubble size distribution is heavily affected by the heat power and mass flux, which means that different flow patterns show different bubble size distributions. Vapor quality is also obtained by the ultrasonic attenuation method, which is compared to the theoretical calculation. The ultrasonic detection model is mainly applied in the bubble-coalesced flow. As the vapor quality is small, the detection value is close to the theoretical value, and this detection model is suitable for nucleate boiling. As the vapor quality is increased, the deviation is larger. By comparison with the theoretical calculations, it is necessary to modify the ultrasonic detection model to fit different flow patterns, which is helpful to study the liquid entrainment mechanism in the micro-channel (especially when the inner diameter is less than 5 mm) in the future.

Study of natural convection development in narrow vertical channels

The process of heat transfer due to natural convection in narrow vertical water-filled pipes is considered. Experimental and simulation data are given. The mechanism of natural convection development is analysed.

Heat transfer in counterflow fluidized bed of oxide particles for thermal energy storage

The potential for inert oxide particles as a heat transfer and thermal energy storage (TES) media in concentrating solar power (CSP) depends in part on particle receiver designs that provide high wall-to-particle heat transfer rates. This paper presents a novel continuous-flow approach to achieve high heat transfer coefficients hw for particle receivers by fluidizing net-downward-flowing particles in a narrow vertical channel bounded by an external irradiated/heated wall and a parallel interior wall with a metal mesh opening that allows the upward-flowing fluidizing gas to exit at the top of the channel. To demonstrate the high hw of this flow configuration, a fluidized bed in a 10 cm × 10 cm × 0.64 cm deep channel was heated through an external aluminosilicate wall with mid-IR quartz lamps that provided external wall heat fluxes up to 20 W cm−2. Extensive heat transfer measurements with fluidized Carbo Accucast ID50 particles (diameters between 150 and 350 μm) in steady-state continuous downward flow and in transient batch mode assessed total hw as functions of particle bed temperatures Tb, bed solids volume fractions αb, and superficial gas velocities Ug. Results showed that the narrow-channel fluidized bed can achieve overall hw as high as 1000 W m−2 K−1. The highest hw were measured at upward Ug between 2 and 4 times the minimum bed fluidization velocities, Umf, which decreased to0.12 m s−1 for the mean particle diameter at Tb=600°C. Increasing Ug further above Umf decreased hw due to an associated decrease in αb. hw increased strongly with Tb in part, because gas-phase conductivity and the radiative heat transfer contribution increased with Tb. The extensive measurements were fit to a modified version of the Nusselt number correlation by Molerus (1992). For αb⩾0.1, the Molerus correlation with adjusted dependence on excess fluidization velocity (Ug-Umf) provided an excellent fit to the measured convective fraction of hw (with <10% error). Adding the radiation component with the Molerus correlation provides an effective tool for calculating hw for this counterflow fluidized bed configuration. A simple analysis explored the impact of such high hw for an indirect receiver design with angled external walls to spread solar aperture fluxes. Results from the analysis indicated that total hw=1000 W m−2 K−1 can enable solar collection efficiencies approaching 90% with external wall temperatures Tw,ext≈1020°C. This potential performance motivates further exploration of this fluidized bed configuration for particle receivers for CSP applications.

Numerical study of two disks settling in an Oldroyd-B fluid: From periodic interaction to chaining.

In this article, we present a numerical study of the dynamics of two disks sedimenting in a narrow vertical channel filled with an Oldroyd-B fluid. Two kinds of particle dynamics are observed: (i) a periodic interaction between the two disks, and (ii) the formation of a two-disk chain. For the periodic interaction of the two disks, two different motions are observed: (a) the two disks stay far apart and interact periodically, and (b) the two disks interact closely and then far apart in a periodic way, like the drafting, kissing, and tumbling of two disks sedimenting in a Newtonian fluid, due to a weak elastic force. Concerning the formation of a two-disk chain occurring at higher values of the elasticity number, either a tilted chain or a vertical chain is observed. Our simulations show that, as expected, the values of the elasticity and Mach numbers are the determining factors concerning the particle chain formation and its orientation.