Flow In Horizontal Channel Research Articles

Measurement of Steady-State CHF in Horizontal Channels for Low-Pressure, Low-Flow Conditions

Experimental investigations on critical heat flux (CHF) are mostly on vertical channels involving high mass fluxes and high system pressures. Reported studies on CHF in horizontal flow channels under low-pressure, low-flow (LPLF) conditions are limited. Understanding CHF is essential in the design and operation of heat exchangers and heat-generating devices including fuel channels of nuclear reactors. The present work investigates CHF in horizontal tubes for low steady flow at atmospheric pressure conditions. Appearance of a “red hot” spot on the test section is considered to be the occurrence of critical heat flux condition in this study. Present data could not be predicted using the reported method of applying a correction factor for the vertical lookup table data. A correlation using the experimental data is developed incorporating the fluid-to-fluid modeling parameters for the prediction of CHF in horizontal channels under LPLF conditions. Numerical study using thermal hydraulic system code RELAP5 suggests liquid film dryout as the mechanism of CHF occurrence in the present investigations.

Two-phase flow in a horizontal rectangular microchannel

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.

Northward channel flow in northern Tibet revealed from 3D magnetotelluric modelling

The Kunlun fault defines one of the major northern tectonic boundaries of the Tibetan plateau. Previous geophysical studies have detected a major change in rheology across this boundary, but it is not clear how this is related to models that have invoked crustal flow. The lithospheric resistivity structure of the Kunlun fault has been investigated by both the INDEPTH III and IV magnetotelluric (MT) transects. All the MT data were processed using modern statistically-robust methods, and have been analysed for directionality and dimensionality. In order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the INDEPTH III MT data, a variant approach on 3D inversion of 2D profiles was investigated to explore and improve lateral resolution. In addition to the apparent surficial deformation associated with the sinistral strike-slip Kunlun fault, the 3D modelling of the INDEPTH MT data reveals that complex deformation processes are occurring at mid-crustal depths in northern Tibet. The 3D MT inversion results, supported by synthetic modelling, particularly confirm and highlight the presence of separate north–south intrusions of conductive material crossing the Kunlun fault into the more resistive Kunlun–Qaidam block. These north–south intrusions are interpreted to be associated with the horizontal channel flow of partially molten Songpan–Ganzi crust into two (or more) separated channels moving northwards and crossing the surficial trace of the Kunlun fault at mid-to-lower crustal depths.

Role of fluids in the exhumation of the Southern Marginal Zone of the Limpopo Complex, South Africa

The Limpopo Complex (LC) of southern Africa displays structural, metamorphic, and geochronological evidence for a complex exhumation-controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions (P>9kbar, T>900°C) in the root zone (Central Zone, CZ) of the orogen before ∼2.72Ga. The Southern Marginal Zone (SMZ) of the LC, in addition, provides evidence for thrust-controlled emplacement of a hot granulite facies nappe onto the adjacent low-grade granite greenstone terrane of the northern Kaapvaal Craton (NKVC) in the interval ∼2.69–2.62Ga. A tectono-metamorphic scenario in which initial near vertical channeling (exhumation) at ∼2.69Ga was transferred into horizontal channeling (thrusting) at ∼2.67–2.64Ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen (CZ). This resulted in formation of melt-weakened crust (migmatized gneisses) that with continuous shortening led to vertical transfer of lower crustal material, accompanied by fluid-fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material. This material detached to form buoyant diapirs that accompanied the emplacement at ∼2.65Ga of mega sheath folds at the middle crustal level throughout the entire CZ of the LC. This early stage of near-vertical exhumation is reflected (i) by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the SW throughout the entire CZ, and (ii) by decompression-cooling (DC) segments of P–T paths constructed for metapelitic rocks. DC paths in the far northern part of the SMZ that are associated with the steep north-dipping portion of the Hout River Shear Zone (HRSZ) traversed from P∼8.5kbar, T∼850°C to P∼6.5kbar, T=720°C. Early near-vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe (SMZ). Horizontal channeling (thrusting) is recorded in metapelitic granulite by near-isobaric cooling (P∼6kbar, T∼700–550°C) segments of earlier DC P–T paths that are associated with the shallow north-dipping portion of the HRSZ. Devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe. Immiscible CO2-rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at P>7.5kbar, T>750°C. Similar fluids established a regional retrograde isograd and associated zone of 4500km2 retrograde hydrated granulite at P∼6kbar, T=600–630°C located in the hanging wall of the shallow north-dipping HRSZ that bounds the SMZ in the south. Shear zone-hosted metasomatic alteration including formation of lode-gold deposits occurred at T=600–630°C, P∼6kbar in the zone of rehydrated granulite, and at higher P–T conditions in the granulite zone. The granulite facies SMZ of the LC thus preserves clear evidence for a direct link between persistent fluid flux and fluid-rock interaction as an integral component of a conveyor-like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen (CZ). This mechanism explains the important link between the thrust-controlled emplacement of the SMZ onto the NKVC and deep crustal fluid flow.

Simulation of multilayer particle resuspension in an obstructed channel flow

The present work deals with the multilayer resuspension of solid aerosol particles off a multilayer deposit exposed to a sudden gas flow increase. The heavy detachment of particles spans a wide range of industrial and non-industrial applications. It is used extensively in applications dealing with the resuspension of dust by wind, the resuspension of particles in ventilation ducts and the resuspension of radioactive graphite particles in high temperature reactors. A new numerical approach is suggested to simulate the particle resuspension off a multilayer deposit initially at rest in the cavity of horizontal obstructed turbulent channel flow. The present resuspension model is based on alternating iterations of a Large Eddy Simulation (LES) for the gas flow and of a Discrete Element Method (DEM) for the particle detachment. The combination of LES and DEM simulates the effect of a sudden increase in the turbulent gas flow on the topology of the granular interface, i.e. the surface separating the multilayer deposit from the turbulent gas phase. After tuning two parameters of a simple cluster re-entrainment criterion, results show good agreements with experimental data performed on-site. Both the shape and the wall roughness of the granular interface are predicted with a good level of accuracy. Findings from this study also confirm that the friction velocity is a major resuspension agent.

An investigation of two-phase flow pressure drop in helical rectangular channel

Numerical simulations have been carried out to evaluate the two-phase frictional pressure drop for air-water two-phase flow in horizontal helical rectangular channels by varying configurations, inlet velocity and inlet sectional liquid holdup. The investigations performed using eight coils, five different inlet velocity and four different inlet sectional liquid holdups. The effects of curvature, torsion, fluid velocity and inlet sectional liquid holdup on two-phase frictional pressure drop have been illustrated. It is found that the two-phase frictional pressure drop relates strongly to the superficial velocities of air or water, and that the curvature and torsion have some effect on the pressure drop for higher Reynolds number flows in large-scale helical rectangular channel; the inlet sectional liquid holdup only increases the magnitude of pressure drop in helical channel and has no effect on the development of pressure drop. The correlation developed predicts the two-phase frictional pressure drop in helical rectangular channel with acceptable statistical accuracy.

Horizontal turbulent channel flow interacted by a single large bubble

Flow field modified by a single large bubble in a horizontal wall turbulent boundary layer is measured by particle tracking velocimetry. We focus on intermediate bubble size being comparable to the thickness of boundary layer to find out the events altering the original turbulent shear stress field. The results are all presented by quantities relative to single phase flow on Lagrangian grid system that moves with the bubble. The measurement results reveal bubble-produced secondary flow around itself, which involves twin roll vortices, separation on the bubble surface, and strong sweeping flow. For a small and nearly spherical bubble, the sweeping flow is provided strongly to enhance the turbulent momentum exchange while negative exchange is detected for a large flat bubble. This effective length approximately corresponds to the size of the bubble.

Particle Tracking in Large Eddy Simulated Turbulent Channel Flow Using Generalized Lattice Boltzmann Method

The deposition of spherical solid particles with 0.00192 ≤ τ+ ≤ 160.56 in three-dimensional turbulent vertical and horizontal channel flows with friction Reynolds number was studied. A generalized lattice Boltzmann equation (GLBE) was used for computation of instantaneous turbulent flow field for which large eddy simulation (LES) was employed. The subgrid scale (SGS) turbulence effects were simulated through a shear-improved Smagorinsky model (SISM), which can predict turbulent near wall region without any wall function. The Lagrangian particle tracking approach was used, and qualitative distribution of particles in a vertical channel center plane and in a near wall region was presented. It was shown that, in horizontal channels, the rate of deposition is higher than in vertical channels since gravitational sedimentation is the dominant mechanism of deposition on the lower wall. Combination of SISM in GLBE with particle tracking analysis in turbulent channel flow is the novelty of the present work. Since the accuracy of the flow field is essential to correctly predict particles' behavior, a reliable method should be used to simulate the flow field. In the proposed method, the flow field equations are solved locally. The collaboration of this method and particle tracking can be considered a good candidate for simulation of particulate flow.

Experimental study of graphite particle deposition upstream of a forward-facing step

Multilayer graphite particle deposition was experimentally investigated upstream of a forward-facing step (FFS) in stationary flows in order to better understand graphite dust contamination in HTR primary circuits. Stationary flows of ambient air with homogeneously dispersed graphite particles were created in a small-scale low-speed horizontal flow channel (SSHFC). Flow field measurements were carried out by means of Particle Image Velocimetry (PIV). The FFS-Reynolds numbers ReS determined for the flow scenarios under consideration were in the range of 610–6000. The thickness of particulate deposits created in front of the FFS was measured spatially resolved in regular time intervals by means of laser triangulation. Gravitational settling and inertial impaction accounted for the majority of particle deposition right upstream of the step. Both volume and shape of particulate deposits were observed to depend on local flow characteristics as well as on the size distribution of the used graphite particles. The bulk density of particulate deposits increased with increasing ReS and with increasing mean graphite particle size. The amount of particles adhering to the front side of the FFS due to inertial impaction decreased with increasing Stokes number.

0215 水平チャネル内気泡流における単分散気泡のクラスター形成

Bubble cluster is naturally formed during the bubbles' advection in horizontal turbulent channel flows. The phenomenon cannot be mathematically modeled by the linear sum of bubble-liquid interaction, but it should be assessed with multi-scale modeling in densely dispersed bubbly flow. A series of original geometrical definitions have been proposed to characterize the bubble cluster behavior, such as statistical uniformity of spatial bubble arrangement and bubbles' radial distribution function. From the data analysis, we have found that bubble clustering remains potentially in any flow conditions including the case of large bubble-size deviation

Friction Drag Characteristics in Horizontal Air-Silicone Oil Bubbly Channel Flow

Friction Drag Characteristics in Horizontal Air-Silicone Oil Bubbly Channel Flow

Friction Drag Characteristics in Horizontal Air-Silicone Oil Bubbly Channel Flow

Friction Drag Characteristics in Horizontal Air-Silicone Oil Bubbly Channel Flow

Two-phase turbulence models for simulating dense gas–particle flows

The two-fluid model is widely adopted in simulations of dense gas–particle flows in engineering facilities. Present two-phase turbulence models for two-fluid modeling are isotropic. However, turbulence in actual gas–particle flows is not isotropic. Moreover, in these models the two-phase velocity correlation is closed using dimensional analysis, leading to discrepancies between the numerical results, theoretical analysis and experiments. To rectify this problem, some two-phase turbulence models were proposed by the authors and are applied to simulate dense gas–particle flows in downers, risers, and horizontal channels; Experimental results validate the simulation results. Among these models the USM-Θ and the two-scale USM models are shown to give a better account of both anisotropic particle turbulence and particle–particle collision using the transport equation model for the two-phase velocity correlation.

Large Eddy Simulations of turbulent particle-laden channel flow

This paper scrutinises the Large Eddy Simulation (LES) approach to simulate the behaviour of inter-acting particles in a turbulent channel flow. A series of simulations that are fully (four-way), two-way and one-way coupled are performed in order to investigate the importance of the individual physical phenomena occurring in particle-laden flows. Moreover, the soft sphere and hard sphere models, which describe the interaction between colliding particles, are compared with each other and the drawbacks and advantages of each algorithm are discussed. Different models to describe the sub-grid scale stresses with LES are compared. Finally, simulations accounting for the rough walls of the channel are compared to simulations with smooth walls. The results of the simulations are discussed with the aid of the experimental data of Kussin J. and Sommerfeld M., 2002, Experimental studies on particle behaviour and turbulence modification in horizontal channel flow with different wall roughness, Exp. in Fluids, 33, pp. 143–159 of Reynolds number 42,000 based on the full channel height. The simulations are carried out in a three-dimensional domain of 0.175m×0.035m ×0.035m where the direction of gravity is perpendicular to the flow. The simulation results demonstrate that rough walls and inter-particle collisions have an important effect in redistributing the particles across the channel, even for very dilute flows. A new roughness model is proposed which takes into account the fact that a collision in the soft sphere model is fully resolved and it is shown that the new model is in very good agreement with the available experimental data.

Numerical Simulation of Fluid Flow Past an Oscillating Triangular Cylinder in a Channel

The structure of laminar incompressible flow in a two-dimensional horizontal channel past a triangular cylinder undergoing vertical oscillating motion is simulated using the finite element method. The oscillating motion is carried out for a fixed Reynolds number equal to 100 and dimensionless oscillating amplitudes of 0.125, 0.25, 0.5, and 1, while the dimensionless forced oscillation frequencies are chosen from St/4 to 4 St, where St is the natural Strouhal number of a stationary triangular cylinder. The arbitrary Lagrangian–Eulerian kinematics is utilized to simulate the oscillating motion. The present problem is first solved for a stationary triangular cylinder to obtain the natural Strouhal number. Then, the fluid dynamics for an oscillating triangular cylinder are solved in terms of instantaneous drag and lift, mean of drag, and root mean square (RMS) of lift coefficients. Detailed illustrations of flow streamlines and vortices contours are presented. The results indicate that when the oscillating frequency is equal to the natural Strouhal number of the stationary cylinder, the RMS of lift coefficient reaches its maximum and the oscillating amplitude has no effect at high oscillating frequency.

Nonlinear resonant interactions of interfacial waves in horizontal stratified channel flows

AbstractWe consider the problem of nonlinear resonant interactions of interfacial waves with the presence of a linear interfacial instability in an inviscid two-fluid stratified flow through a horizontal channel. The resonant triad consists of a (linearly) unstable wave and two stable waves, one of which has a wavelength that can be much longer than that of the unstable component. Of special interest is the development of the long wave by energy transfer from the base flow due to the coupled effect of nonlinear resonance and interfacial instability. By use of the method of multiple scales, we derive the interaction equations which govern the time evolution of the amplitudes of the interacting waves including the effect of interfacial instability. The solution of the evolution equations shows that depending on the flow conditions, the (stable) long wave can achieve a bi-exponential growth rate through the resonant interaction with the unstable wave. Moreover, the unstable wave can grow unboundedly even when the nonlinear self-interaction effect is included, as do the stable waves in the associated resonant triad. For the verification of the theoretical analysis and the practical application involving a broadbanded spectrum of waves, we develop an effective direct simulation method, based on a high-order pseudo-spectral approach, which accounts for nonlinear interactions of interfacial waves up to an arbitrary high order. The direct numerical simulations compare well with the theoretical analysis for all of the characteristic flows considered, and agree qualitatively with the experimental observation of slug development near the entrance of two-phase flow into a pipe.

Direct numerical simulation of horizontal open channel flow with finite-size, heavy particles at low solid volume fraction

We have performed direct numerical simulation of turbulent open channel flow over a smooth horizontal wall in the presence of finite-size, heavy particles. The spherical particles have a diameter of approximately 7 wall units, a density of 1.7 times the fluid density and a solid volume fraction of 5 × 10−4. The value of the Galileo number is set to 16.5, while the Shields parameter measures approximately 0.2. Under these conditions, the particles are predominantly located in the vicinity of the bottom wall, where they exhibit strong preferential concentration which we quantify by means of Voronoi analysis and by computing the particle-conditioned concentration field. As observed in previous studies with similar parameter values, the mean streamwise particle velocity is smaller than that of the fluid. We propose a new definition of the fluid velocity ‘seen’ by finite-size particles based on an average over a spherical surface segment, from which we deduce in the present case that the particles are instantaneously lagging the fluid only by a small amount. The particle-conditioned fluid velocity field shows that the particles preferentially reside in the low-speed streaks, leading to the observed apparent lag. Finally, a vortex eduction study reveals that spanwise particle motion is significantly correlated with the presence of vortices with the corresponding sense of rotation which are located in the immediate vicinity of the near-wall particles.

0312 周期的気泡注入により持続されるボイド率変動(OS3-3 流れの抵抗低減,OS3 流れの抵抗低減)

In the frictional drag reduction using bubbles, we proposed a periodic bubble injection technique to improve the performance and the new technique improved the performance 66% in Re 2200 at a horizontal channel flow. In this paper, the void fraction fluctuation in higher Reynolds number condition is investigated by visualization. The uncontrolled fluctuation of the void fraction fluctuation doesn't have any specific frequency, however the artificial fluctuation of void fraction maintain its frequency in streamwise. We indicate that it is possible to control density and shape of a swarm of bubbles by controlling the injection frequency and gas volume.

Turbulent Flow Phenomena and Ce2O3 Behavior during a Steel Teeming Process

Non-metallic inclusions in molten steel have received worldwide attention due to their serious influence on both the steel product quality and the steel production process. These inclusions may come from the de-oxidation process, the re-oxidation by air and/or slag due to an entrainment during steel transfer, and so on. The presence of some inclusion types can cause a termination of a casting process by clogging a nozzle. Thus, a good knowledge of the inclusion behavior and deposition rate in steel flows is really important to understand phenomena such as nozzle clogging. In this thesis, inclusion behaviors and deposition rates in steel flows were investigated by using mathematical simulations and validation by experiments.A ladle teeming process was simulated and Ce2O3 inclusion behavior during a teeming stage was studied. A Lagrangian method was used to track the inclusions in a steel flow and to compare the behaviors of inclusions of different sizes. In addition, a statistical analysis was conducted by the use of a stochastic turbulence model to investigate the behaviors of different-sized inclusions in different nozzle regions. The results show that inclusions with a diameter smaller than 20 μm were found to have similar trajectories and velocity distributions in the nozzle. The inertia force and buoyancy force were found to play an important role for the behavior of large-size inclusions or clusters. The statistical analysis results indicate that the region close to the connection region of the straight pipe and the expanding part of the nozzle seems to be very sensitive for an inclusion deposition.In order to know the deposition rate of non-metallic inclusions, an improved Eulerian particle deposition model was developed and subsequently used to predict the deposition rate of inclusions. It accounts for the differences in properties between air and liquid metals and considers Brownian and turbulent diffusion, turbophoresis and thermophoresis as transport mechanisms. A CFD model was firstly built up to obtain the friction velocity caused by a fluid flow. Then, the friction velocity was put into the deposition model to calculate the deposition rate.For the case of inclusion/particle deposition in vertical steel flows, effects on the deposition rate of parameters such as steel flow rate, particle diameter, particle density, wall roughness and temperature gradient near a wall were investigated. The results show that the steel flow rate/friction velocity has a very important influence on the rate of the deposition of large particles, for which turbophoresis is the main deposition mechanism. For small particles, both the wall roughness and thermophoresis have a significant influence on the particle deposition rate. The extended Eulerian model was thereafter used to predict the inclusion deposition rate in a submerged entry nozzle (SEN). Deposition rates of different-size inclusions in the SEN were obtained. The result shows that the steel flow is non-uniform in the SEN of the tundish. This leads to an uneven distribution of the inclusion deposition rates at different locations of the inner wall of the SEN. A large deposition rate was found to occur at the regions near the SEN inlet, the SEN bottom and the upper region of two SEN ports.For the case of an inclusion/particle deposition in horizontal straight channel flows, the deposition rates of particles at different locations of a horizontal straight pipe cross- section were found to be different due to the influence of gravity and buoyancy. For small particles with a small particle relaxation time, the gravity separation is important for their deposition behaviors at high and low parts of the horizontal pipe compared to the turbophoresis. For large particles with a large particle relaxation time, turbophoresis is the dominating deposition mechanism.

0214 超音波流速分布計を用いた水平チャネル内気液二相乱流のモニタリング(OS2-3 混相流の多次元可視化計測,OS2 混相流の多次元可視化計測)

In the frictional drag reduction by using bubbles, fluctuations of void fraction take an important role to enhance the effect. We have investigated the mechanisms on formation of bubble clusters that provide the large fluctuation. The objective of this study is establishing a new measurement system of passing interval of bubbles with ultrasonic wave in order to find out the property of void fluctuation. We conducted synchronous measurement of ultrasonic pulse emission and video camera recording of bubble images in a horizontal bubbly channel flow. The present technique provided reasonable results on the passing interval in comparison with the estimates from the images. Probability distributions of the passing bubbles obtained at three different points in the downstream represent influence of bubble coalescence.