Channel Of Uniform Cross Section Research Articles

Simulation of Rarefied Gas Flows in Microchannels

In this work the rarefied gas flows in microchannels is investigated. With low pressure owing to low gas density the gas slippage affetcting the channel walls is observed. The task of sliding flow on initial segments of flat (two-dimensional) channel of uniform cross section is solved. Intercomparison of two calculation methods is carried out. The analytical method is based on approximate approach developed by S. M. Targ. The small channel extension does not always allow application of the formulas of calculation of hydraulic resistance, received for long (theoretically infinitely long) channels. The physics of process are also answered, more by development of flow on the so-called initial segment, when the flow characteristics change not only on a cross section of the channel, but also along its length. As a result of the reasonable simplification of the initial tasks it was possible to receive the simple formulas for practical calculations of the main hydrodynamic characteristics and the value of velocity and pressure of flow on channel length. The numerical solution of the task is carried out with the control-volume method in an ANSYS CFX system with use of structured computational mesh, constructed in ANSYS ICEM CFD. The task is solved in a flat statement: cells are located in one layer and on the sidewalls of layer the symmetry conditions are set. The closed system of the Navier-Stokes equations is solved. The laminar flow mode taking into account a slipping condition on wall of “Finite Slip Wall” is considered. Calculations are carried out for Knudsen numbers from 0 to 0,1 with step in 0,02. The intercomparison of the modeling results obtained by analytical and numerical methods showed good agreement.

Detection of Abrupt Flow Discontinuities in Open-Surface Channels by Transient Analysis

AbstractFrom a linearized form of the equations of unsteady (transient) motion in an open channel of uniform cross section using the well-known technique of the characteristic lines, it is shown that under a set of not-too-restrictive conditions, the sudden occurrence of abrupt flow discontinuities, such as a sudden leak, burst, or whatever flow perturbation, can be detected and localized by monitoring, and the pressure variations in a purposefully designed array of transducers located along the channel bottom can be suitably processed in real time. This study, which belongs to the general theme of transient analysis, extends the techniques of burst detection, localization, and quantification to the case of open-surface flows, which are tested for the first time by the authors in the case of single pressurized pipes. The proposed methodology can easily be applied to instantaneously detect and assess unexpected events that can occur in the more sensible and vulnerable channel stretches.

Porous Gravity Currents of Non-Newtonian Fluids within Confining Boundaries

Motion of non-Newtonian gravity currents in horizontal impermeable channels filled with a porous material is investigated theoretically and experimentally. A constant or time-variable volume of fluid, characterized rheologically by the Ostwald-de Waele constitutive equation, is released from a point source into a channel of uniform cross-section, whose boundary height is described by a monomial relationship. The mathematical problem is formulated and solved at the Darcy scale coupling the local mass balance equation with a modified Darcy's law, taking into account the nonlinearity of the rheological equation. The resulting non-linear ODE is integrated numerically in the general case; for the release of a constant volume, a closed-form analytical solution is derived. Earlier results for Newtonian currents inside confining boundaries and power-law currents in two-dimensional geometry are generalized. The experiments were conducted in a transparent channel of semi-circular cross-section filled with uniform size glass ballotini. The position of the current front, recorded by a photo camera, was generally in a good agreement with the theory. The propagation of the current is described by L∼ tF2 where F2 is a scalar depending on (i) the time exponent of the volume of fluid in the current, α, (ii) the geometry of the channel, parameterized by β and (iii) the exponent n of the rheological equation. It is found that for a critical value αc = n/(n + 1), F2 is independent on the shape of the channel; for α < αc, F2 is a decreasing function of β; the reverse is true for α > αc. Upon comparing results with free-surface viscous flow in open channels, it is found that: (i) the same expression for αc holds; (ii) the exponent F2 increases or decreases monotonically with β, while for the triangular section (β = 1) in open channels, a maximum or minimum value of F2 is attained for α < αc and α > αc, respectively.

Negative capillary-pressure-induced cavitation probability in nanochannels

The capillarity-induced negative pressure of water flow has been investigated innanochannels of a rectangular cross section by using computational fluid dynamics (CFD)simulation. As a consequent effect of negative pressure, the cavitation probability has beenanalyzed numerically. The numerical relation between the critical radius of cavitation (Rc) and geometrical characteristics of channels shows that cavitationdoes not occur in channels of uniform cross section (UCS), since2Rc is larger than the smallest dimension of the channel. However, it may occur in channels ofnon-uniform cross section (NUCS), except for planar or high aspect ratio channels. Theinequality in height and width is favorable for the absence of cavitation. The findingscan also be applied to channels of elliptical or circular cross section. The resultsshow the influence of geometrical characteristics of channels on the cavitationprobability, which is important to engineer the channel geometrical structure inorder to avoid undesirable filling problem ‘cavitation’ during the flow process.

Investigation of low-temperature plasma generator with divergent channel of the output electrode and some applications of this generator

A review is made of experimental and theoretical investigations of processes occurring in low-temperature plasma generators (LTPG) with divergent channel of the output electrode, and the possibilities of utilizing these generators in new plasma technologies are analyzed. Comparison is made of the characteristics of discharge (including the current-voltage characteristic) in a divergent channel and in a cylindrical channel of uniform cross section. The effect of divergent channel of the output electrode and of its expansion ratio on the pattern of physical processes in LTPGs of different designs is studied. Investigations are performed of the distribution of electric current and heat flux density along a channel with a segmented output electrode. The voltaic equivalents of heat fluxes to cathode and anode are determined. The process of “shunting” of discharge is investigated, which causes fluctuations of electric arc-burning voltage. The investigations involving an LTPG with divergent channel reveal that the voltage amplitude in the case of shunting decreases with increasing current strength and, at high currents of argon arc, does not exceed 1–2 V. Results are given of spectral and visual investigations of LTPG. It is demonstrated that, in an LTPG with divergent channel, the plasma temperature in the region of energy input at currents of 300 A and higher exceeds 30 000 K. The significant part is found which is played by vacuum ultraviolet radiation in the process of closing the arc to anode. The mechanisms of erosion of the tungsten cathode tip are investigated, which play an important part in increasing the cathode service life by way of recirculation of tungsten atoms because of their ionization in the discharge gap. Results are given of using an LTPG with divergent channel of the output electrode in plasma technologies of surface hardening, cutting, and hard-facing of metals. The technology of plasma hardening of wheel pairs, adopted by the RZhD (Russian Railroads) Joint-Stock Company, provides for increasing the service life of railroad wheels by a factor of 1.5–2.

Calculation of the Dynamics of Ignition of an Air-Hydrogen Mixture by Nonequilibrium Discharge in a High-Velocity Flow

The paper deals with the problem of initiation of ignition of an air-hydrogen mixture by means of a nonequilibrium electric discharge under conditions of high-velocity flow. A model is constructed of calculation of the parameters of electric discharge in a chemically reacting gas in view of the heating and motion of gas in a channel of uniform cross section. It is demonstrated that a minor energy contribution may be accompanied by a strong reduction of the time of ignition induction, which cannot be attained in the case of thermal heating of gas corresponding to this energy contribution.

Thermal dispersion effect on MHD flow of dusty gas and dust particles through hexagonal channel

In the field of dynamics for a dusty fluid, the volume of the dust particles and flow behaviour of particles in different conditions is very important in engineering problems such as atmospheric fallout, nuclear reactor, powder technology, performance of solid fuel rocket nozzles, air craft icing and so many others. An analysis is presented in this paper to study the effects of thermal dispersion and Viscous dissipation on unsteady flow of a viscous incompressible dusty gas through a hexagonal channel of uniform cross section under the influence of magnetic field and time dependent pressure gradient. The results show the change in velocity profile of gas and particles in the presence of magnetic field with time, thermal dispersion and volume fraction φ.

Dispersion by Pressure-Driven Flow in Serpentine Microfluidic Channels

Microfluidic hydrodynamic chromatography performed in serpentine microchannels etched on chips is analyzed in the limiting case of chips containing a large number of periodically arrayed turns. Comparison is made between these results and those for a straight channel of the same length as the curvilinear channel, all other things being equal. Explicitly, generalized Taylor−Aris dispersion (macrotransport) theory for spatially periodic systems is adapted to compute the chip-scale solute velocity Ū* and dispersivity D* for effectively point-size, physicochemically inert Brownian particles entrained in a low Reynolds number, pressure-driven solvent flow occurring within the curvilinear interstices of such serpentine devices. Attention is focused upon relatively thin channels of uniform cross section, enabling the various transport fields pertinent to the problem to be expressed as regular perturbation expansions with respect to a small dimensionless parameter e, representing the ratio of channel half-width ...

Visualisation studies of the transition regime flow in a channel of varying cross section under the influence of Coriolis force

This paper is a flow visualisation study of the effect of Coriolis force on the flow in the transition regime in a channel with a mild change in cross section. Transition in this flow is found to share a salient gross feature with its counterpart in a rotating channel of uniform cross section, viz. that it takes place at a Reynolds number around two orders of magnitude lower than the critical Reynolds number in a non-rotating system and it is then to a state of flow with highly ordered steady longitudinal vortices. The change in channel cross section has the effect that the longitudinal vortices may arise or be annihilated in neighbouring sub-domains within the flow region. While in a channel of decreasing cross section the flow may undergo transition as it proceeds downstream, it may also revert from the state with vortices to one without in a channel of increasing cross section. Viewed in terms of a stability diagram with local flow parameters alone, the cross-over points from one state to another do not coincide for the two cases, with the reversal of transition exhibiting a kind of “hysteresis”.

Thermal counterflow in a diverging channel: a study of radial heat transfer in He II

Heat transfer in superfluid helium is studied in a channel which imposes a radial 1/ r divergence in the heat flux density, q( r). A very sensitive AuFe thermocouple/SQUID system is employed to measure the temperature difference between various radial positions along the channel. Both laminar and turbulent dissipation are observed. The Gorter-Mellink mutual friction model, used to describe the turbulent thermal dissipation in channels of uniform cross-section, is applied locally to this diverging flow. Comparison of the turbulent dissipation measured over two sections of the channel shows that the Gorter-Mellink model alone is inadequate to describe the thermal resistance of He II in this non-uniform flow. Effects such as a suppression of the turbulence at the channel entrance or a transition to turbulence at some position within the channel may contribute to the discrepancy. Until these effects are carefully understood, use of the Gorter-Mellink model alone to characterize radial heat transfer in He II is highly suspect, even at high heat flux densities.

Remarks on some nonconservation problems

A horizontal channel of uniform cross section has an impervious channel bed to the left of x = 0 x = 0 and allows infiltration at a constant rate to the right of x = 0 x = 0 . Initially there is water at constant depth and zero velocity. There are left and right moving interfaces and, between them, water with positive velocity. At a certain time there will be a water edge to the right of which there is no water in the channel. The time history of this water edge is a free boundary. The solution of this problem, which is nonconservation because mass and momentum are carried away by infiltration, is discussed below. A single equation, which is also nonconservation, has an explicit solution. The characteristics of this single equation have a geometry similar to that the C 2 {C_2} characteristics of the channel problem.

A statistical model of the longitudinal dispersion process in turbulent flow in a channel

The important practical problem of the dispersion of a passive contaminant in a fluid flowing through a pipe or channel of uniform cross-section is usually analysed in terms of the distribution of concentration. In this paper however a different though approximate approach is adopted which both illustrates the essential statistical nature of the process and may be quicker to employ when approximate answers are acceptable in a practical problem. A simple statistical model is proposed for the motion of a single molecule of contaminant and leads to an expression for the covariance of the velocity of the molecule in terms of the fluid velocity, and hence to a value of Taylor's longitudinal diffusivity. The model is applied to two simple flows in a channel, one of which illustrates the effect of the viscous sub-layer. Despite the number of simplifying assumptions made in constructing the model it gives results which are close to those obtained by conventional means. Ways in which the model could be adapted to give even better results are discussed.

Hydrodynamics in channels with porous walls

An experimental study has been made concerning the hydrodynamics of turbulent air flow through porous channels of uniform cross section with drain or feed along the path. Formulas are proposed for calculating the variations in pressure and flow rate along such a channel.

Modelling Tidal Flow from Harbors

Problems concerned with the tidal flow of water into and out of harbors are difficult to solve mathematically. For the case of flow in a channel the Froude number may be considered to be the most important parameter, and it is therefore necessary to determine an expression for the velocity of flow in the channel connecting the harbor to the sea. For the case considered herein, it is assumed that there is a large harbor connected to the sea by a wide channel of uniform cross section and length L .

Optimum power generation from a moving plasma

The possibility exists of directly using the plasma, resulting from a controlled fusion reaction, to generate electricity by electromagnetic induction. Two special cases of a more general problem are considered here: (1)the extraction of optimum power from the steady one-dimensional flow of an incompressible inviscid plasma across a uniform transverse magnetic field in an externally loaded channel of arbitrarily varying cross-section, and (2) the extraction of optimum power from the steady one-dimensional flow of a compressible inviscid plasma across a uniform transverse magnetic field in a channel of uniform cross-section. In each case, the magnitude of the required external loading at optimum power operation is determined as a function of the parameters which characterize the hydromagnetic interaction. Also determined are the magnitudes of the terminal voltate, power, fluid mechanical to electrical conversion efficiency, and the variation of the fluid dynamical variables along the channel at optimum power.

Two-Dimensional Flow Through a Diffuser With an Exit Length

Abstract A diffuser with an “exit length” is one with a channel of uniform cross section following the diffuser. Tests were made of different diffusers with and without exit lengths. The data were for steady, incompressible, two-dimensional flow. The performance of each diffuser was improved by the presence of an exit length. As compared to flow without an exit length, flow with an exit length is characterized by a reduced boundary-layer growth, by a small decrease in the pressure rise across the diverging section, and by a decrease in the static-pressure gradient at the diffuser inlet.