Flow In Horizontal Channel Research Articles

Modeling of void fraction and relative velocity covariance for dispersed bubbly and intermittent flows in horizontal channels

The interfacial drag force governs the momentum exchange between gas and liquid phases and is a key parameter in predicting void fraction using a one-dimensional thermal-analysis code. The interfacial drag force is formulated by the product of the overall drag coefficient and the area-averaged relative velocity between two phases. The area-averaged relative velocity for gas-dispersed two-phase flows has been formulated with the aid of the distribution parameter, which expresses the covariance due to the distributions of void fraction and mixture volumetric flux. It has been pointed out that the term due to void fraction covariance is missing in the formulation of the area-averaged relative velocity adopted in one-dimensional thermal-analysis codes. This study has been conducted to develop void fraction and relative velocity covariance models for gas-dispersed two-phase flows in horizontal pipes. The database of local void fraction distributions for horizontal bubbly flows was established, and an artificial database for intermittent horizontal flows was modeled with assumed distributions of local void fractions. The developed void fraction covariance model for gas-dispersed horizontal two-phase flows could reproduce the data with the mean relative deviation (i.e., bias) of 8.33% and the mean absolute relative deviation (i.e., random uncertainty) of 21.4%. The developed relative velocity covariance model for gas-dispersed horizontal two-phase flows could reproduce the data with the mean relative deviation (i.e., bias) of -0.265% and the mean absolute relative deviation (i.e., random uncertainty) of 7.11%.

The effect of gravity on turbulence modulation in particle-laden horizontal open channel flow

Two-way coupled direct numerical simulation of turbulence in a horizontal open channel with Lagrangian tracking of heavy particles is performed to study the joint influence of particle inertia and wall-normal gravity on turbulence modulation. The Stokes numbers based on the wall units considered in the study are St+= 1 (low inertia), 31 (moderate inertia), and 202 (high inertia) and the gravitational accelerations are g+= 0, 0.0068, 0.068, and 0.2221, varying from case to case. It is found that heavy particles (gravity involved in simulation) with low inertia subtly increase or have little significant influence on turbulence modulation, regardless of gravitational acceleration. Particles with high inertia and large gravitational acceleration significantly reduce the mean streamwise fluid velocity and the scales of turbulence structure near the wall, and vice versa in the outer layer. In this case, turbulence fluctuations are notably suppressed and the turbulence structures tend to be two-dimensional and laminar due to the strong rebound and trajectory-crossing effect. Moderate-inertia particles probably cause anisotropic turbulence modulation due to the competition between particle inertia and gravity settling. In the parameter range studied in this work, particles have indistinctive modification on turbulence when τp+g+ (=St+g+)<0.2 but suppress turbulence when τp+g+>0.2.

On the width and mean value of bubble size distributions under subcooled flow boiling

In this work, a new comprehensive dataset about bubble sizes in flow boiling of water is presented. The experimental setup basically consists of a lower copper heated plate embedded in a horizontal rectangular channel flow. Several experimental conditions, ranging bulk velocities: 0.1–0.9 [m·s−1]; mass fluxes: 96.9–871.8 [kg·m−2·s−1]; subcooling degrees: 16–36 [°C]; heat fluxes: 200–650 [kW·m−2] and pressures: 110–190 [kPa] were set for three cooper plates of different roughness, with Sa: 0.45, 1.23 and 7.43 [µm], respectively. Based on the current samples, the probability function for the normalized bubble diameter has been described by a lognormal pdf for any plate and experimental condition and a dimensionless correlation for the standard deviation of the bubble size distribution is presented. In order to completely describe the diameter distribution for bubbles in subcooled boiling systems, an improved correlation for the mean bubble diameter value is also presented here, after a deep review of previously published datasets. Further work is required to accommodate different channel configurations and surface morphologies.

Step response of turbulent structure inside a horizontal bubbly channel flow due to sudden change in void fraction

Step response of turbulent structure inside a horizontal bubbly channel flow due to sudden change in void fraction

Experimental investigation on flow regimes and transitions of steam-water two-phase flow in narrow rectangular horizontal channels

Experimental investigations on atmospheric steam-water two-phase flow regimes and transitions in two transparent narrow rectangular channels with cross-section areas of 1 mm × 20 mm and 2 mm × 20 mm were carried out under conditions of 0.007 ≤ Jf ≤ 0.27 m s−1, 0.05 ≤ Jg ≤ 65.2 m s−1, 0 < q ≤ 18 kW m−2. Four flow patterns were observed in narrow rectangular horizontal channels under heated condition, i.e., bubbly flow, slug flow, churn flow and annular flow. New dryout phenomenon of annular flow was observed under high wall heat flux conditions. The liquid phase occupies half of the middle of narrow channel above the bottom rectangular plate. Below the top plate, liquid phase is concentrated to a slender streamline in the middle of upper half narrow channel by high speed steam flow. For the flow regime transitions, the critical values of void fraction for transitions from bubbly flow to slug flow, slug flow to churn flow, and churn flow to annular flow in narrow rectangular horizontal channels under heated condition are 0.7, 0.8 and 0.9, respectively. The heat flux decays flow regime transitions because the generated bubbles from heated wall surfaces promote the separation of bubbles in the upstream and downstream. Flow regime transitions under larger surface tension condition need greater vapor phase velocities to disturb the mainflow. In a general, new flow regime transition criteria were developed and can present excellent predictions for the present data of flow regimes and transitions in narrow rectangular horizontal channels under heated condition.

Spatial distribution of particles in turbulent channel flow of dilute suspensions

We experimentally investigated the kinematics and suspension of particles in the near-wall region of a horizontal turbulent channel flow of water. The particles were glass beads with a diameter of 0.014H, where H is the full channel height. The experiments involved dilute particle suspensions at bulk volumetric concentrations (ϕ) of 0.05, 0.12, and 0.27%, and at Reynolds numbers (Re) of 20,200, 40,400, and 60,500. Measurement of Lagrangian position and velocity of the particles was carried out using 3D particle tracking velocimetry. At the lowest Re, the results showed a large near-wall accumulation of particles due to gravitational settling at all three bulk concentrations. The analysis of turbulence statistics suggested negligible effect due to inter-particle collisions even when the local volumetric particle concentration reached 2%. With increasing Re to 40,400 and 60,500, different concentration profiles were observed for ϕ = 0.05% with respect to the profiles of ϕ = 0.12 and 0.27%. A bi-model particle concentration distribution, with an inner and outer peak, was present for ϕ = 0.05% at Re = 40,400 and 60,500. In contrast, the particles at ϕ = 0.12 and 0.27% had a wall-peaking profile. The autocorrelation function of the streamwise Lagrangian velocity and wall-normal dispersion of the particles showed greater stability of particle motions in the near-wall region for ϕ = 0.12 and 0.27% with respect to ϕ = 0.05% at the two higher Re. The observations are associated with a stronger particle-wall lubrication at the higher volumetric concentrations of ϕ = 0.12 and 0.27%.

Wall shear stress modified by bubbles in a horizontal channel flow of silicone oil in the transition region

We performed laboratory experiments on bubbly channel flows using silicone oil, which has a low surface tension and clean interface to bubbles, as a test fluid to evaluate the wall shear stress modification for different regimes of bubble migration status. The channel Reynolds numbers of the flow ranged from 1000 to 5000, covering laminar, transition and turbulent flow regimes. The bubble deformation and swarms were classified as packing, film, foam, dispersed, and stretched states based on visualization of bubbles as a bulk void fraction changed. In the dispersed and film states, the wall shear stress reduced by 9% from that in the single-phase condition; by contrast, the wall shear stress increased in the stretched, packing, and foam states. We carried out statistical analysis of the time-series of the wall shear stress in the transition and turbulent-flow regimes. Variations of the PDF of the shear stress and the higher order moments in the statistic indicated that the injection of bubbles generated pseudo-turbulence in the transition regime and suppressed drag-inducing events in the turbulent regime. Bubble images and measurements of shear stress revealed a correlated wave with a time lag, for which we discuss associated to the bubble dynamics and effective viscosity of the bubble mixture in wall proximity.

Transient velocity profiles and drag reduction due to air-filled superhydrophobic grooves

This paper presents an experimental study of horizontal channel flow with air-filled superhydrophobic grooves. Air–water interfaces in the grooves are visualized in a range of the channel Reynolds number, Re, (2000 ≤ Re ≤ 5000) while flow characteristics are evaluated using particle tracking velocimetry measurements at Re = 3000 and 4000. Near the air–water interface in the superhydrophobic groove, turbulent eddies and hence the Reynolds shear stress appreciably attenuate owing to a lack of energy supply through the interface, and it takes a notable distance for the Reynolds shear stress to recover downstream of the groove. Additionally, a secondary cross-flow from the solid surface region between two grooves towards the air–water interface appears and sweeps eddies between the grooves towards the interface. The decay of turbulent eddies and the sweeping of eddies from the solid surface to the air–water interface reduce friction drag both in and immediately downstream of the grooved region.

Enhancement of flow boiling heat transfer using heterogeneous wettability patterned surfaces with varying inter-spacing

This study experimentally investigated the influence of heterogeneous wettability-patterned surfaces with varying inter-spacing on flow boiling heat transfer characteristics. The test surfaces consisted of three fluorooctyltrichlorosilane hydrophobic-patterned array structures having a triangle, inverted triangle, and circular shape on a SiO2- hydrophilic substrate, with an inter-spacing of 0.75 or 1 mm. The working fluid was deionized water, and the Reynolds number was 6,000 at atmospheric pressure. Among the test surfaces, varying the inter-spacing between neighboring hydrophobic patterns slightly enhanced the heat transfer coefficient (HTC) due to changing the bubble characteristics. In terms of the shape effect of hydrophobic patterns, the heterogeneous wettability-patterned surfaces dominated the overall flow boiling heat transfer performance showing a significant increase in critical heat flux (CHF) compared to the Si surface, by 40–43%. In addition, all of the wettability test surfaces showed a markedly higher heat transfer coefficient than the Si surface, by 35–163%. This experiment is explained by analyzing the relationship between bubble lift forces and the various hydrophobic-patterned shapes in a horizontal flow channel, in an attempt to better understand flow boiling heat transfer and optimize the pattern design.

Multi-fluid modelling of bubbly channel flows with an adaptive multi-group population balance method

Mass, momentum, and energy transfer in bubbly flows strongly depends on the bubble’s size distribution, which determines the contact area between the interacting phases. Characterization of bubble sizes in polydisperse flows requires empirical modelling of sub-grid physical mechanisms such as break-up and coalescence. In the present work an adaptive multiple size-group (A-MuSiG) method is incorporated into the Eulerian multiphase solver available in Simcenter STAR-CCM+ in order to model polydisperse bubbly flows in horizontal and vertical channels. The disperse phasespace is discretized into multiple size-groups each represented by its own size, number-density, and velocity field. The diameter of the bubbles in each of the size-groups varies in time and space, dynamically adapting to the local flow conditions. The interphase momentum transfer between the continuous phase and polydisperse bubbles is modelled through drag, virtual mass, turbulent dispersion, and lift forces. For modelling sub-grid bubble break-up and coalescence processes, different phenomenological kernels are evaluated. The empirical parameters of the adopted kernels are calibrated in two steps. The initial stage of the analysis considers experimental channel flows at low Reynolds number and zero-gravity conditions, under which the bubble size distribution is solely dependent on coalescence. As part of the second phase of the evaluation, additional parametric simulations in turbulent channel flows are performed in order to calibrate the break-up models, assuming the coalescence scaling constants derived in the previous step. The obtained results demonstrate that in flows with high turbulent mixing the ensuing bubble dynamics are strongly coupled to the internal properties of the population, which in turn influence the developing multiphase interactions in a transient manner.

Structural and sedimentary discontinuities control the generation of karst dissolution cavities in a carbonate sequence, Potiguar Basin, Brazil

Epigenetic karstic systems in carbonate rocks commonly result from progressive dissolution by acidic meteoric waters over thousands to millions of years. The generation of secondary porosity and permeability improvement due to dissolution in carbonate reservoirs of geofluids (e.g., groundwater, hydrocarbons, and CO2) can profoundly impact reservoir storage capacity and subsurface fluid flow. This study investigates the control of structural discontinuities such as stylolites, fractures, and primary sedimentary discontinuities on the generation of multiscale karst dissolution cavities by epigenetic fluid percolation in a Late Cretaceous carbonate sequence (Jandaíra Formation) in the Potiguar Basin, Northeastern Brazil. The study relies on micro- and macroscale analyses such as stratigraphic logs, field structural investigations, rock strength data collected in the field (Schmidt hammer), microtomographic and drone images, thin section analyses, porosity and permeability laboratory measurements. The results show that bed-perpendicular stratabound and non-stratabound stylolites and fractures can be enlarged due to meteoric water percolation until they merge and form a single channel system that crosscuts all sedimentary multilayers. Bed-parallel stylolites are ubiquitous in carbonate sequences overprinting bed interfaces and layers. Where not dissolved, bed-parallel stylolites have low porosity and permeability and thus can act as barriers to vertical fluid flow. Where dissolved, such stylolites can contribute to horizontal fluid flow and form channel porosity. The results of this study led to a formulation of a conceptual model of rock dissolution along structural and sedimentary discontinuities that affects carbonate rock successions in the subsurface.

Computational modeling of microbubble coalescence and breakup using large eddy simulation and Lagrangian tracking

AbstractThe flow of dispersed microbubbles was studied with an Eulerian–Lagrangian technique using large eddy simulation to predict the continuous liquid flow and Lagrangian tracking to compute bubble trajectories. The model fully accounts for bubble coalescence and breakup and was applied to horizontal and vertical channel flows. With low levels of turbulence, gravity in horizontal, and lift in vertical, channel flows govern the bubble spatial and collision distribution. When turbulence is sufficiently high to, at least partially, oppose bubble preferential concentration, more uniform collision and coalescence distributions are found, although these remain peaked near the wall in both configurations. Almost 100% coalescence efficiency was always found, due to bubbles colliding along similar trajectories, with breakup only recorded in a flow of low surface tension refrigerant R134a. Models like this can provide the required quantitative understanding of the microbubbles complex behavior, as well as supporting the development of more macroscopic modeling closures.

Characteristic of Flow and Heat Transfer in Horizontal Channel with mounted Rectangular Crosss section Bars. (Dept. M.)

Characteristic of Flow and Heat Transfer in Horizontal Channel with mounted Rectangular Crosss section Bars. (Dept. M.)

Development of a Momentum Transfer Coefficient in Particle Horizontal Channel Flow

Abstract A developed momentum transfer coefficient based on Wen’s 1966 model was proposed, in which the two threshold values were used to keep the continuous particle porosity function. A second-order moment gas–particle two-fluid turbulent model that coupled this coefficient, was established to model and simulate the particle horizontal channel flows. An improved Reynolds stresses transport to close fluctuation velocities correlation between gas and particle phases was set to reveal the fully anisotropic characteristics of particle dispersions. The hydrodynamics of horizontal gas–particle turbulent flow was numerically simulated and validated by experimental measurement with satisfied agreements. Distributions of particle Reynolds stresses are significantly affected by two-phase interactions, turbulent diffusions and particle collisions. Much wider size and flatter profiles distributions were obtained in terms of the horizontal and vertical root-means-square of particle velocities in comparison with Wen’s 1966 model.

Turbulence statistics and flow structure in fluid flow using particle image velocimetry technique: A review

AbstractParticle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag‐reducing fluid flows. Such structures include large‐scale‐events that constitute an important portion of the Reynolds stress tensor; shear layers of drag‐reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag‐reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single‐ and two‐phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas‐liquid flows such as S‐shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag‐reducing agents (DRAs)‐imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag‐reducing flows showed that DRAs act to dampen wall‐normal‐, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall‐normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall‐normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the reduced strength and population of vortices close to the wall, increased spacing between low‐speed streaks, reduced frequency of large‐scale events and reduced concentration of small eddies. Effect of DRAs on flow structures depends not only on the level of DR but also on the concentration and molecular weight. The effect of concentration is linked to the elastic properties of the DRA. The influence of DRA on oil‐water flows in similar to its effect on single phase water flow with regards to changes in near wall structures and turbulence statistics. A few research gaps and limitations of the PIV techniques were also highlighted.

Heat transfer in laminar viscous flow in a channel with one porous wall

Similarity solutions to the two-dimensional steady Navier–Stokes and energy equations for viscous flow in a semi-infinite horizontal channel with upper porous and lower impermeable walls are presented. The finite end of the channel is closed. Both fluid suction and injection through the porous boundary are considered. Two cases of thermal conditions at the walls are studied. In one case the temperature of both channel boundaries decreases linearly along the channel. In the second one, the upper wall has the same decreasing temperature, while the lower boundary is thermally insulated. By means of similarity transformations the dynamic and heat transfer equations are reduced to three ordinary differential equations for the velocity, pressure and temperature functions. The velocity equation is solved numerically for different values of the suction/injection Reynolds number. Analytical solution of the dynamic problem in a series of small Reynolds number is also found. The thermal problem is solved numerically for arbitrary Peclet number and two values of the Prandtl number. Analytical solution for small Peclet and Reynolds numbers is also obtained. The dependence of the Nusselt number on the Peclet number at both walls is studied. The stream lines and isotherms are presented graphically for some values of the Reynolds number at the Prandtl number equal one. It is shown the strong influence of the flow behavior on the heat transfer inside the channel.

INVESTIGATION OF COUNTER-CURRENT FLOW LIMITATION FOR AIR-WATER IN A PWR HOT LEG EXPERIMENTAL LOOP FOR DIFFERENT GEOMETRY

Gas/liquid two-phase stratified flows in horizontal channels are frequently encountered in nuclear reactors, oil and gas pipelines, steam generators, refrigeration equipment, reflux condensers, packed columns, and heat pipes. The phenomenon known as countercurrent flow limitation, or flooding, is the limiting condition where the flow rates of neither the gas nor the liquid can be further increased without changing the flow pattern. This is the condition where the maximum air mass flow rate at which the down-flowing water mass flow rate is equal to the inlet water mass flow rate. This limiting condition, also known as onset of flooding, can occur in vertical or horizontal geometry. This work is a review of recent experimental investigations of countercurrent flow limitation (CCFL) for various hot-leg geometries of pressurized water reactors (PWRs). We compare results with those obtained from the Nuclear Technology Development Centre (CDTN) in 2005. Recent experimental results in the literature are in good agreement with the 2005 findings.

Geometrical Evaluation of a Channel with Alternated Mounted Blocks under Mixed Convection Laminar Flows Using Constructal Design

This work presents a numerical study of incompressible, laminar, two-dimensional, mixed convection flows in horizontal channels with two rectangular alternated blocks mounted in the surfaces. The influence of the geometry and positioning of the blocks on the heat transfer rate is evaluated using Constructal Design. The problem is subjected to two geometric constraints associated with the areas of the channel and the two blocks and three degrees of freedom: $$H_{1}/L_{1}$$ (the ratio of the height to the width of the upstream block), $$H_{2}/L_{2}$$ (the ratio of the height to the width of the downstream block), and $$L_{3}$$ (the horizontal distance between the centers of the blocks). The purpose is to maximize the heat transfer rate per length unit $$q'$$ considering three different Reynolds ($$Re_{H}= 10$$, 100, and 200) and Grashof numbers ($$Gr_{H} = 650$$, 6,500, and 65,000). The conservation equations of mass, momentum, and energy are solved using the Finite Volume Method, and the buoyance forces are modeled with the Boussinesq approximation. In general, the best performance is reached for the largest intrusion of the blocks. However, changes in $$Gr_{H}$$ and $$Re_{H}$$ affected the optimal distance ($$L_{3})_{o}$$, and for $$Gr_{H}=65,000$$ non-symmetrical blocks led to the best performance, showing the importance of design investigation for different flow conditions.

Numerical investigation of a single intermediate-sized bubble in horizontal turbulent channel flow

Numerical investigation of a single intermediate-sized bubble in horizontal turbulent channel flow

Mixed convection Ag-MgO/water hybrid nanofluid flow in a porous horizontal channel

Heat transfer and fluid flow of hybrid nanofluid into horizontal rectangular porous channel is numerically investigated. The Darcy-Brinkman-Forchheimer model is used and the finite volume method is employed to solve the governing equations. The effect of nanoparticle volume fractions (φ), permeability (Darcy number Da) and porosity (e) of porous medium and Richardson number (Ri) on the flow field and heat transfer rate are analyzed and commented. The results, presented by streamlines, isotherms, temperature, velocity, and, local, average and normalized Nusselt numbers, reveal the periodic character of the flow in both of space and the time. The flow is characterized by thermoconvective cells which are influenced by the variation of the permeability and the addition of hybrid nanoparticles into the base fluid. The heat transfer rate increases by increasing the permeability and the porosity of porous medium. It enhances for certain nanoparticle volume fractions depending on the Darcy number. The effect of nanoparticles on the enhancement rate of heat transfer mount by increasing Richardson and Darcy numbers.