Channel Of Rectangular Cross-section Research Articles

TEST-CASE NO 36: KELVIN-HELMHOLTZ INSTABILITY (PA)

The geometry of the experiment proposed as a benchmark is a glass channel of rectangular cross section that contains two immiscible fluid of different densities. The channel is suddenly tilted at a small angle from the horizontal and a Kelvin-Helmholtz instability develops under cer- tain conditions. It is suggested to compare the numerical simulation with the experimental values of the most unstable wave number, the starting time and the growth rate of the instability, and the wave speed.

Micro-moulded substrates for the analysis of structure-dependent behaviour of nematodes

AbstractMicrostructures, consisting of networks of channels of rectangular cross sections (50 μm high, 40-200 μm wide), were formed in 5 mm square areas on transparent substrates made of silicon rubber. An experimental set-up using the network sealed with a flat glass plate has the potential to function in a way similar to the pore space in soil, and is therefore useful for studies of nematode migration. The set-up allowed the migratory activity of nematodes in water-filled, porous and transparent microstructures to be observed with a microscope. By means of substrates with two different channel dimensions, the structure-dependent behaviour of second-stage juveniles of Meloidogyne incognita was visually demonstrated. Their behaviour was examined on the basis of the migration patterns obtained by superimposing recorded serial images of individual juveniles. In a micro-channel network with 40 μm high channels of 200 μm wide elements, juveniles showed marked activity in migration, forming consistent zigzag patterns spread over the network area. In contrast, in a micro-channel network with 80 μm high channels of 400 μm wide elements, migration showed thick, sparse patterns, restricted around the area where the juveniles were initially deposited. This comparison showed that M. incognita juveniles in a narrow, fine network tended to migrate actively and, in contrast, those in a wide, coarse network were prevented from migrating by the network configuration.

Comparison Between Non-Equilibrium Green's Function and Monte Carlo Simulations for Transport in a Silicon Quantum Wire Structure

We present comparisons of simulations conducted with non-equilibrium Green's functions and Monte Carlo approaches. As prototype, we consider an idealized silicon quantum wire structure, consisting of a conduction channel of rectangular cross-section, terminated by two contacts. The Monte Carlo model treats the particles as semi-classical, but distributed over up to seven subbands and with scattering model similar to the one used for the Green's functions model. Results for drift velocity under various field conditions agree very closely using the two techniques, suggesting that particle simulation may continue to be a useful physical investigation tool at the nanoscale with an appropriate introduction of the most important quantum features of the transport.

Patterns of solidification in channel flows with surface cooling

Understanding the rates of cooling and solidification in laminar flows down sloping channels is central to predicting the advance of lava flows. The mechanisms involved include thermal convection and a competition between shear strain rate and the rate of formation of solid at the chilled surface of the flow. We report experiments in which polyethylene glycol wax flows in a laminar fashion down an inclined, open channel of rectangular cross-section under cold water. Two distinctly different flow regimes are recognized: 'tube' flow in which solidification of the flow surface creates a stationary roof while melt continues to flow through a relatively well-insulated 'tube' beneath, and a 'mobile crust' regime in which a solid surface crust develops only in the centre of the channel. In the latter regime the crust is carried down the channel, separated from the walls by crust-free shear regions in which cooling produces only dispersed fragments of solid owing to the effects of shearing. This flow structure is quasi-invariant over a large distance downstream. We show that thermal convection takes place in organized rolls that have axes aligned with the shear flow, and conclude that transition between the two flow regimes occurs at a critical value of the combined parameter = ψ(Ra/R 0 ) 1/3 , where ψ = U 0 t s /H 0 is the ratio of a surface solidification timescale t, to a shearing timescale H 0 /U 0 , H 0 and U 0 are the flow depth and centreline surface velocity in the absence of solidification, Ra is a Rayleigh number and R 0 is a constant.

Influence of channel aspect ratio on heat transfer characteristics in sharp-turn connected two-pass channels with inclined divider wall

Heat transfer characteristics in sharp-turn connected two-pass channels of rectangular cross-section have been investigated using naphthalene sublimation method. Primary interests have been focused on the combined influences of the cross-section aspect ratio of the channel (AR), varying between 1 and 8, and the divider wall's inclination angle with respect to the outer walls ( α), varying between −6° and +6°, on the heat transfer. The results show that AR's variation can bring significant changes to the local Sherwood number distribution characteristics, especially to those of local maximum and minimum areas that are unique to sharp-turn channels. Regionally averaged results reveal that the influence of AR on the Sherwood number distribution becomes more pronounced in the region after the turn of the convergent channel with larger | α|. Heat transfer performance has been evaluated based on the constant pumping power criteria and on the uniformity of the local Sherwood number distribution over the channel wall. It has been found that higher heat transfer performance can be expected with the channels having a relatively low AR and a divider wall that is parallel or slightly inclined toward the converging side with respect to the outer wall.

VELOCITY AND TEMPERATURE FIELDS IN A PLANE POISEUILLE FLOW WITH VOLUME HEATING: MEASUREMENTS UNDER ELECTRIC FIELD

This study concerns the measurement of the velocity and temperature fields in a laminar flow within a vertical channel of rectangular cross section. The fluid is heated in volume by direct electric conduction induced by an alternating electric field. Particular attention is paid to the procedure for temperature measurement under an electric field. The feasibility of such a measurement technique is shown, and specific aspects of the mixed convection regime are analyzed. Flow and temperature fields are also calculated by solving numerically the complete Navier-Stokes and energy equations for comparison with the experimental results. Differences between the experimental and numerical results are discussed, and the underlying physical mechanisms are analyzed.

Mixing and film cooling in supersonic duct flows

A study for gas dynamics and heat transfer characteristics is presented for supersonic turbulent flows inside a channel of rectangular cross-section, with film cooling provided by tangential injection of coolant through the porous slots. A Navier–Stokes-based model with both original and non-equilibrium versions of the k– ε two-equation turbulence model, and an integral method have been assessed with experimental measurements to gain insight into the fluid physics, and to assess the current predictive capability for such flows. Navier–Stokes-based computations show that the interaction between the main and film flows leads to mixing and recirculation, with the overall flow and pressure fields reasonably predicted. However, the computed temperature distribution on the adiabatic wall and pressure at the injection points exhibit noticeable differences compared to the experimental measurements. The integral method is shown to be capable of accounting for the wall temperature profile under the given flow condition and geometry. Further development of the film cooling technologies for supersonic flows can benefit from the present coordinated approach utilizing the detailed Navier–Stokes and integral models along with direct experimental evaluations.

Micro-channel filling flow considering surface tension effect

Understanding filling flow into micro-channels is important in designing micro-injection molding, micro-fluidic devices and an MIMIC (micromolding in capillaries) process. In this paper, we investigated, both experimentally and numerically, ‘transient filling’ flow into micro-channels, which differs from steady-state completely ‘filled’ flow in micro-channels. An experimental flow visualization system was devised to facilitate observation of flow characteristics in filling into micro-channels. Three sets of micro-channels of various widths of different thicknesses (20, 30, and 40 μm) were fabricated using SU-8 on the silicon substrate to find a geometric effect with regard to pressure gradient, viscous force and, in particular, surface tension. A numerical analysis system has also been developed taking into account the surface tension effect with a contact angle concept. Experimental observations indicate that surface tension significantly affects the filling flow to such an extent that even a flow blockage phenomenon was observed at channels of small width and thickness. A numerical analysis system also confirms that the flow blockage phenomenon could take place due to the flow hindrance effect of surface tension, which is consistent with experimental observation. For proper numerical simulations, two correction factors have also been proposed to correct the conventional hydraulic radius for the filling flow in rectangular cross-sectioned channels.

A Reynolds analogy for real component surface roughness

A new Reynolds analogy equation is presented, which is based on flow and thermal behavior in a rectangular cross-section channel with real component surface roughness. This roughness is similar to that which exists on some turbine surfaces under extreme operating conditions, or on surfaces of other industrial devices, with deposit accumulation such as heat exchangers. Skin friction coefficient, Nusselt number, Stanton number, and performance factor experimental results are given over a range of Reynolds numbers for one polished smooth surface, and for two other surfaces with different levels of irregularly shaped and irregularly distributed, three-dimensional surface roughness. The Reynolds analogy deduced from these data, with such roughness, is important because it is different from Reynolds analogy equations for surfaces with uniformly shaped elements arranged in a regular, periodic pattern.

Local Heat Transfer and Flow Characteristics in Two-Pass Channels with an Inclined Divider Wall.

Experimental study has been conducted on local heat/mass transfer and flow characteristics in rectangular cross-sectioned two-pass channels with a sharp turn having an inclined divider wall. Detailed distributions of local Sherwood numbers are presented that are obtained on all the walls of the channels by the naphthalene sublimation method, and the influence of the inclination angle of the divider wall on the local heat/mass transfer are examined. Then the flow characteristics in the channels, such as flow separation and reattachment around the turn section, secondary flow velocities in and after the turn, are described based on the time-mean flow vectors obtained by using the PIV. The correlation between the local heat/mass transfer and flow structure is discussed.

On the flows produced by sudden application of a constant pressure gradient or by impulsive motion of a boundary

Some properties of the time-dependent Navier–Stokes equations are discussed for flows impulsively started from rest by sudden application of a constant pressure gradient or by the impulsive motion of a boundary. Five illustrative examples are given. They are: unsteady flow in a circular cylinder moving parallel to its length, starting flow in a circular pipe, unsteady flow in a rotating cylinder, starting flow in a rectangular channel moving parallel to its length and unsteady flow in a channel of rectangular cross-section. It is found that the expressions of the quantities such as velocity, flux and skin friction are in series forms which may be rapidly convergent for large values of the time but slowly convergent for small values of the time or vice versa. It is shown that if their expressions can be found for one of large values of the time or small values of the time, these expressions can be used for the other.

Heat (mass) transfer in rectangular cross-sectioned two-pass channels with an inclined divider wall

Heat transfer characteristics in rectangular cross-sectioned two-pass channels with an inclined divider (inner) wall have been examined experimentally. Local heat (mass) transfer rates were measured by the naphthalene sublimation method; seven kinds of divider inclination angles were tested for three turn clearances under the Reynolds numbers of (2.0–5.0)×10 4. The influence of the inclined divider wall on the local heat transfer characteristics is discussed in detail. Then, the optimum combination of the inclination angle and the turn clearance is examined based on the trade-off between the heat transfer enhancement and pressure loss penalty, and on the improvement of uniformity in the distribution of local heat transfer rates.

Three-Dimensional Steady Vapor Bubbles in Rectangular Microchannels

We develop a mathematical model of a long steady vapor bubble in a channel of rectangular cross section with given temperature distributions on the walls of the channel. Evaporation near the heated bottom of the channel is balanced by condensation in colder areas of the vapor–liquid interface near the top. An asymptotic method is developed in the limit when the shape of the bubble is dominated by capillary forces everywhere except near the walls. A lubrication-type analysis is used to find local vapor–liquid interface shapes and mass fluxes near the walls. Contact lines are present at the bottom and side walls and are modeled through a disjoining pressure. The total length of the bubble is determined from the integral mass balance after all local solutions are found and characterized. The length increases as the average heater temperature becomes higher as expected. The length of the bubble is also studied as a function of the material properties for fixed temperature distribution at the heated bottom. It is shown that longer bubbles can be obtained when kinetic effects at the vapor–liquid interface are less significant or when the side-walls conduct heat better.

Field Applications of a Variable-Parameter Muskingum Method

This study demonstrates field applications of a physically based variable-parameter Muskingum method for routing floods using daily and two hourly flood data available for six reaches of three Australian rivers and using hourly flood data available for a specified stream-network segment of the Tyne River in the United Kingdom. Some of the flood events studied for Australian rivers inundated the floodplain. The study illustrates how to estimate the routing parameters at every routing time interval using limited channel cross section data, and the wave speed-discharge relationship developed for the routing reach; this can be derived from past observed flood hydrographs or the rating curves available at the inlet and outlet of the study reach. Overbank floods are routed through a two-stage rectangular compound cross section channel, and the method used to determine the floodplain width on the basis of the applicability criterion of the method is described. The major advantage of the routing approach followed...

Design of Multichannel Optical Splitter without Bends

The theory of a planar geometry splitter involving only parallel waveguide channels of rectangular cross-section is presented. The strictly parallel geometry allows for a semi-analytical analysis that makes possible the design of a splitter, which can divide an optical signal into uncoupled outputs carrying designated fractions of the input power. The theoretical intrinsic losses in the proposed splitter is shown to be minimal, and device design parameters are provided.

Design of Multichannel Optical Splitter without Bends

The theory of a planar geometry splitter involving only parallel waveguide channels of rectangular cross-section is presented. The strictly parallel geometry allows for a semi-analytical analysis that makes possible the design of a splitter, which can divide an optical signal into uncoupled outputs carrying designated fractions of the input power. The theoretical intrinsic losses in the proposed splitter is shown to be minimal, and device design parameters are provided.

Saturated Flow Boiling of Water in a Narrow Channel: Experimental Investigation of Local Phenomena

This paper describes some observations of flow boiling of water at near-atmospheric pressure in a single vertical channel of rectangular cross-section 2 mm × l mm, heated on three sides over a length of 248 mm, with a window on the fourth side. Measurements have been made of the fluctuating wall temperatures and pressures at several stations along the channel, with simultaneous high-speed video recordings of the flow patterns at a mass flux of 134 kg m −2 s −1 and heat fluxes of 50–100 kW m −2 . The frequency spectra of the fluctuations depend on the compressibility of the inlet flow to the channel. It is shown that the processes of initiation of boiling and of heat transfer differ from flow boiling in large channels. The acceleration of liquid slugs by confined bubbles causes pressure pulses to propagate the full length of the channel. Heat transfer is distributed in time between convection and nucleate boiling. The experimental limitations are discussed.

Capillary waves on a Eulerian jet emerging from a channel

This paper considers the flow of a Eulerian fluid jet emerging out of a slightly compressed circular or rectangular cross-section channel. The emerging fluid is subject to surface tension, which provides a restoring force and produces steady waves along the free surface. In this paper, the full nonlinear problem for the jet is formulated and a dimensionless surface tension parameter is identified. The linearized problem is then solved by the Wiener–Hopf technique. The free surface shape consists of a steady sinusoidal oscillation and a component which decays exponentially with distance from the channel opening. Asymptotic methods are used to find the surface profile near the channel exit.

Evaluation of the Green function for 3-D wave-body interactions in a channel

This study deals with a 3-D boundary-value problem that arises when free-surface waves interact with a stationary body or body system in a channel or wave tank of rectangular cross-section. A consistent asymptotic analysis and an efficient numerical solution is presented of the Green function that satisfies the linear free- surface condition and the non-penetration condition on the channel bottom and the sidewalls. The formulation is based on the open-sea Green function and the complete series of images is evaluated accurately based on the asymptotic analysis. It is demonstrated that the Green function has a square-root singular behavior due to the sidewalls when the wave frequency approaches one of the resonant frequencies. The numerical results for the Green function presented in this paper are believed to have an absolute accuracy of 10−5.

Heat Transfer Characteristics in Sharply Turned Serpentine Channels with Inclined partition Wall.

Experimental study was conducted on heat transfer characteristics in rectangular cross-sectioned two-pass serpentine channels with an inclined partition wall. The cross section of the channel at the entrance was 50 mm×25 mm, and the inclination angle of the partition (inner) wall was changed from -6° (convergent channel) to +6° (divergent channel) for three turn clearances of 30 mm, 50 mm, and 70 mm. Distributions of local heat (mass) transfer rates on the long-side wall were measured by the naphthalene sublimation method under a single Reynolds number of 3.5×104. In this paper, influences of the inclination angle of the partition wall and of the turn clearance on heat transfer characteristics in the channels are discussed in detail. Then, the optimum combination of the inclination angle and the turn clearance is examined from viewpoints of the trade-off between the mean heat transfer rate and pressure loss, and of the improvement of uniformity in the distribution of local heat transfer rates.