Effect Of Channel Confinement Research Articles

Dimeric colloidal inclusion in a chiral active bath: Effective interactions and chirality-induced torque.

Colloidal inclusions suspended in a bath of smaller particles experience an effective bath-mediated attraction at small intersurface separations, which is known as the depletion interaction. In an active bath of nonchiral self-propelled particles, the effective force changes from attraction to repulsion, an effect that is suppressed when the active bath particles are chiral. Using Brownian dynamics simulations, we study the effects of channel confinement and bath chirality on the effective forces and torques that are mediated between two inclusions that may be fixed within the channel or may be allowed to rotate freely as a rigid dimer around its center of mass. We show that the confinement has a strong effect on the effective interactions, depending on the orientation of the dimer relative to the channel walls. The active particle chirality leads to a force imbalance and, hence, a net torque on the inclusion dimer, which we investigate as a function of the bath chirality strength and the channel height.

Effect of channel confinement on wake dynamics and forced convective heat transfer past a blunt headed cylinder

In this paper, a two-dimensional numerical simulation is carried out to understand the effect of confinement (blockage ratio β) on fluid flow and forced convective heat transfer characteristics past a blunt headed cylinder. Utilizing air as an operating fluid, flow simulations are carried out for wide ranges of blockage ratios (110≤β≤13) and Reynolds numbers (60≤Re≤200). The flow characteristics and heat transport are analysed critically for different β. The functional dependence of CD (Drag Coefficient) and CL (Lift Coefficient) on blockage ratio is examined. It has been found that CD reduces with increasing Re, while the Strouhal number and the average Nusselt number show an increasing trend when the blockage ratio is increased. The average Nusselt number also increases with increasing Re.

Effect of Channel Confinement on Mixed Convective Flow Past an Equilateral Triangular Cylinder

The present work investigates the mixed convective flow and heat transfer characteristics past a triangular cylinder placed symmetrically in a vertical channel. At a representative Reynolds number, Re = 100, simulations are carried out for the blockage ratios β=1/3,1/4, and 1/6. Effect of aiding and opposing buoyancy is brought about by varying the Richardson number in the range -1.0≤Ri≤1.0. At a blockage ratio of 1/3, suppression of vortex shedding is found at Ri = 1, whereas von Kármán vortex street is seen both at β=1/4 and 1/6, respectively. This is the first time that such behavior of blockage ratio past a triangular cylinder in the present flow configuration is reported. Drag coefficient increases progressively with increasing Ri and a slightly higher value is noticed at β=1/3. For all β, heat transfer increases with increasing Ri. Flattening of Nuavg –Ri curve beyond Ri>0.75 is observed at β=1/3.

Effect of Channel Confinement on the Coarsening Kinetics of Nanoparticles Deposited on Semicrystalline Polymer Templates.

Dodecanethiol-capped gold nanoparticles (NPs) deposited onto a poly(ethylene glycol) substrate quickly coarsen over a timescale of days when stored under ambient conditions. The NP coarsening was studied at different surface coverages and temperatures. The coarsening of NPs depended on their location on the film; NPs located inside the channels coarsened in a different fashion than NPs deposited on the plateaus. Size distributions studied by image analysis and comparison to theoretical distribution functions shed light on the coarsening mechanism in each case, revealing the role of dimensionality.

Study of sediment transport in granular avalanches

AbstractUnderstanding the characteristics and mechanics of granular avalanches along sloping channels is fundamental and vital for the study of different geophysical flows in the field. By using the discrete element method (DEM), three‐dimensional (3D) granular (mass) flows along a sloping channel are numerically modelled to study the contact behaviour between solid particles. The evolution of 3D unsteady granular flows from a quasi‐static state to an inertial flow is systematically investigated through numerical simulations. Captured velocity profiles along the flow height show variation in the flow regimes, while utilisation of the definition of the Savage number allows study of the vibration of solids inside a granular body along the sloping channels. These numerical results are compared with field measurements to observe the effect of channel confinement on the solids discharge per unit width of granular flows. This particulate study illustrates that, for unsteady granular flows, solid particles in the front head are more collision dominated, with shear rates and Savage numbers greatly enhanced. Also, the solids discharge inside a granular body is not uniformly distributed. Finally, channel width acts as a confinement to the granular debris flow, which can significantly affect sediment transport.

Effect of Channel-Confinement and Rotation on the Two-Dimensional Laminar Flow and Heat Transfer across a Cylinder

Effect of channel-confinement and rotation on the flow and heat-transfer across a cylinder is studied for various blockage ratio (β = 0–50%), nondimensional rotational velocity (α = 0–2), and Reynolds number (Re = 35–170). The cylinder is maintained at a constant wall temperature with air as the working fluid. Criss-cross motion of the shed-vortices is noticed for the channel-confined flow across a rotating cylinder at intermediate blockage ratio. The effect of channel-confinement (rotation) is an enhancement (reduction) in the drag force and heat transfer. A downward lift force is generated under the influence of counterclockwise rotation, which increases with increasing blockage ratio. Rotation and channel-confinement have a stabilizing effect and can be used for flow control.

Macro-to-microchannel transition in two-phase flow: Part 2 – Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51–3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 ( Co ≈ 0.99) and 1.03 ( Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].

Macro-to-microchannel transition in two-phase flow: Part 1 – Two-phase flow patterns and film thickness measurements

The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. The main objective of this study focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of three different sizes with three different refrigerants over a range of saturation conditions to investigate the effects of channel confinement on two-phase flow patterns and liquid film stratification in a single circular horizontal channel (Part 2 covers the flow boiling heat transfer and critical heat flux). This paper presents the experimental two-phase flow pattern transition data together with a top/bottom liquid film thickness comparison for refrigerants R134a, R236fa and R245fa during flow boiling in small channels of 1.03, 2.20 and 3.04 mm diameter. Based on this work, an improved flow pattern map has been proposed by determining the flow patterns transitions existing under different conditions including the transition to macroscale slug/plug flow at a confinement number of Co ≈ 0.3–0.4. From the top/bottom liquid film thickness comparison results, it was observed that the gravity forces are fully suppressed and overcome by the surface tension and shear forces when the confinement number approaches 1, Co ≈ 1. Thus, as a new approximate rule, the lower threshold of macroscale flow is Co = 0.3–0.4 while the upper threshold of symmetric microscale flow is Co ≈ 1 with a transition (or mesoscale) region in-between.

Bubble confinement in flow boiling of FC-72 in a “rectangular” microchannel of high aspect ratio

Boiling in microchannels remains elusive due to the lack of full understanding of the mechanisms involved. A powerful tool in achieving better comprehension of the mechanisms is detailed imaging and analysis of the two-phase flow at a fundamental level. Boiling is induced in a single microchannel geometry (hydraulic diameter 727 μm), using a refrigerant FC-72, to investigate the effect of channel confinement on bubble growth. A transparent, metallic, conductive deposit has been developed on the exterior of the rectangular microchannel, allowing simultaneous uniform heating and visualisation to be achieved. The data presented in this paper is for a particular case with a uniform heat flux applied to the microchannel and inlet liquid mass flowrate held constant. In conjunction with obtaining high-speed images and videos, sensitive pressure sensors are used to record the pressure drop across the microchannel over time. Bubble nucleation and growth, as well as periodic slug flow, are observed in the microchannel test section. The periodic pressure fluctuations evidenced across the microchannel are caused by the bubble dynamics and instances of vapour blockage during confined bubble growth in the channel. The variation of the aspect ratio and the interface velocities of the growing vapour slug over time, are all observed and analysed. We follow visually the nucleation and subsequent both ‘free’ and ‘confined’ growth of a vapour bubble during flow boiling of FC-72 in a microchannel, from analysis of our results, images and video sequences with the corresponding pressure data obtained.

Effects of channel confinement on pioneer woody vegetation structure, composition and diversity along the River Drôme (SE France)

AbstractRiver embankments are one of the main causes of a loss in active channel movement. The knock‐on effects of river channel stabilization include important shifts in the ecohydrological conditions in the protected zone behind embankments, which has already been well exemplified. However, few authors have attempted to investigate changes to pioneer units of vegetation colonizing the active channel following the construction of dikes. As a consequence we studied the structure, composition and diversity of pioneer vegetation units dominated by willow and poplar along the River Drôme in two adjacent, but contrasting, river reaches: an embanked reach and an unconstrained reach, each subject to the same flow regime, but offering very different hydrogeomorphological contexts for vegetation colonization. Our results clearly demonstrate that river embankments affect channel geometry. A reduction in channel width (from between 200 and 500 m to less than 100 m) leads to steeper elevational gradients with an overall coarser grain size and consequently drier overall conditions on colonizable landforms in the channelized reach. Pioneer vegetation responded by colonizing landforms at higher elevations above the low water mark and by occurring in smaller units within the channelized study reach. There was also a reduction in species diversity, a homogenization in vegetation unit structure and a predominance of more drought‐tolerant ligneous species (P. nigra) in pioneer vegetation units in the channelized study reach. Copyright © 2007 John Wiley & Sons, Ltd.

Effect of channel-confinement and aiding/opposing buoyancy on the two-dimensional laminar flow and heat transfer across a square cylinder

The effect of channel-confinement of various degree (blockage ratio of 10%, 30% and 50%) on the upward flow and heat transfer characteristics around a heated/cooled square cylinder is studied by considering the effect of aiding/opposing buoyancy at −1 ⩽ Ri ⩽ 1, for Re = 100 and Pr = 0.7. With increasing blockage ratio, the minimum heating (critical Ri) required for the suppression of vortex shedding decreases up to a certain blockage ratio (=30%), but thereafter increases. The influence of buoyancy and channel-confinement on the recirculation length, drag and lift coefficient, pumping power, Strouhal number and heat transfer from the cylinder, is also investigated.

A conceptual model for block-and-ash flow basal avalanche transport and deposition, based on deposit architecture of 1998 and 1994 Merapi flows

The 1998 block-and-ash flow (BAF) deposits on the western flanks of Merapi were described both immediately following emplacement, and after one and two wet seasons of erosion. This afforded a unique combination of data on deposit morphology and internal structure from proximal to distal reaches. The deposits of the basal avalanches from individual BAF events were classified into three major and two minor facies in channelised reaches, primarily resulting from effects of channel confinement and slope. Termination reaches were classified into three facies, related to preevent topography. The deposit facies architecture was interpreted to understand particle transport and deposition within the framework of granular flow systematics. The transport model we propose for these basal avalanches involves unsteady flow conditions, generated by pulses of collapse as the source, and/or by development of kinetic waves during the granular flow. The coarsest clasts along with low-density material migrated to the wave/pulse fronts within these flows. Whereas, in bulk flow, grain dispersive pressures and kinetic sieving result in coarsest clasts being buoyed to the flow top, with this tendency being greatest on higher slopes (=greater kinetic energy) and with greater flow confinement (maximising particle collisions). Deposition begins as the flow reaches near its full extent. An initial pulse or flow wave freezes rapidly from its base upward and its rear forward as it looses kinetic energy and dispersive pressure, such that an upper higher energy part of the flow is still able to move downstream over the frozen base. Later waves or pulses then accrete progressively horizontally upslope banking up onto or overtopping earlier deposited sediment. Hence, the overall flow freezes from its front to its tail. At termination reaches in unconfined areas, the overflowing upper part of flow gradually deposits from its base upward, resulting in a slightly tapering sheet formed deposit, marked by a front made up of wood fragments and large boulders. On sudden slope breaks in termination areas, piling up of sediment occurs in flat reaches, producing whaleback type forms as the flows rapidly lost competence. One implication of this model is that each kinetic wave or flow-pulse crest developed within the basal avalanche represents a portion of greater flow depth (or stage height). Hence, development of these crests control the ability of parts of the basal avalanche to escape from confined channels and generate unexpected hazards.

Resource Division in an Understory Herb Community: Responses to Temporal and Microtopographic Gradients

Resource Division in an Understory Herb Community: Responses to Temporal and Microtopographic Gradients