Flow pattern visualization of liquid film conduction pumping using flush mounted electrodes

Conduction pumping is one convenient method for pumping of dielectric liquid films. Flow visualization can be useful for understanding the hydrodynamic behavior of this kind of flow. In this study, the flow pattern of the liquid film conduction pumping using flush mounted electrodes has been visualized for various electrode arrangements. Symmetric and asymmetric configurations of electrodes have been examined for pumping of silicon oil. Two major factors affect the conduction pumps; ion mobility difference and electrodes configuration. For known ion mobility difference two cases have been considered: Case 1 in which electrode configuration strengthens the ion mobility difference pumping effect and Case2 in which electrode configuration has the opposite effect. The results show that, for one pair of electrodes acting as unit pump, a primary vortex is generated on the electrodes which arises from rotational part of the Coulomb force. For more than one pair of electrodes, secondary vortices appeared which are generated by the primary vortices. For Case 1, the vorticity of primary and secondary vortices increases with electrode width ratio because of the strengthening effect of the two factors but it decreases for Case 2 because of weakening effect of them.     

On the refracted patterns produced by liquid vortices

A theoretical analysis of the refracted shadows produced by steady and time-decaying liquid vortices under uniform illumination from above is given in this article. An expression for the induced shadow intensity is derived and found to be a function of the vortex’s free surface profile, i.e., function of the static pressure distribution. The patterns for different focusing depth are given and compared with previous visualization results from the literature. The phenomenon is examined and illustrated as a bench mark case by using both steady and time-decaying algebraic vortex models. However, this study can be extended to check the feasibility of recovering the main flow properties by analyzing the luminous image intensity of the refracted patterns. The present analysis is valid only when the swirl velocity is order of magnitude higher than the meridional flow components and the vorticity is concentrated within the core region and of intense conditions.     

Experimental study of the gas flow in an air-blasted liquid sheet

An experimental study has been performed to improve the understanding of the initial air–liquid interaction in the near field of an air-blasted breaking water sheet. For the first time, planar laser-induced fluorescence (PLIF) has been used to visualize the air-flow field, seeding the air streams with acetone vapor. Mie scattering from the liquid sheet, together with the acetone fluorescence signal has enabled simultaneous determination of the instantaneous water sheet location and the air-flow structures. The two-phase flow visualization has revealed detachment of the air boundary layer over the air–water interface behind the zones of strong curvature. The pressure field induced by these vortices has been identified as a cause of the enhanced sheet flapping and the instability growth. Received: 30 October 2000/Accepted: 29 March 2001     

Decay of vortices in a visco-elastic liquid

Summary Decay of vortices in the flow of a certain class of visco-elastic liquids (incompressible second-order fluids) is investigated. It is found that in the flow of double array of vortices, the vortices decay much faster in such a liquid than in ordinary viscous liquids. For the flow in the wake of such a liquid past a two-dimensional grid, a pair of bound eddies occur behind the single elements of the grid as in the viscous case but their scale diminishes with increase in the elastic parameter.

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Sommario Si studia l'attenuazione dei vortici nel flusso di una certa classe di liquidi viscosi (fluidi incompressibili di secondo ordine).Si trova che nel flusso di una schiera di vortici, i vortici si attenuano molto più rapidamente in tali liquidi che nei liquidi viscosi ordinari. Per il flusso nella scia di un reticolo bidimensionale si formano una coppia di vortici limitati dietro i singoli elementi del reticolo come nel caso viscoso, ma la loro scala diminuisce con l'aumentare del parametro elastico.

     

Rivulet flow of a viscoelastic liquid

A theoretical investigation is made of the rivulet flow of a viscoelastic liquid down an inclined plane. It is shown that elasticity causes a significant change in the shape of the rivulet, with height rise at the center. There is also a change in the relationship between the flow rate and the geometry of the rivulet. Elasticity is found to cause a flow in the cross-sectional plane in the form of counter-rotating vortices. In the Newtonian case the flow is purely axial.     

Two-phase flow characteristics in gas–liquid microreactors

Multiphase chemical microreactors require a detailed knowledge of the flow conditions inside the reaction system. This paper reports flow visualization measurements of the two-phase gas–liquid flow pattern and the liquid velocity distribution inside liquid plugs of an intermittent flow. Rectangular cross-section silicon microchannels with hydraulic diameters between 187.5 and 218 μm are fabricated. Laser Induced Fluorescence (LIF) is used to determine the flow pattern. To analyze the influence of the liquid properties and the channel diameter on the two-phase flow pattern, we present flow regime maps using different channel geometries and fluids. A universal flow pattern map based on dimensional analysis is presented. In contrast to microchannel flows, a great number of correlations for flow characteristics for multiphase flow in (round) pipes with diameters >1 mm exist. We compare our experimental results from optical flow visualizations in microreactors with common flow correlations and regime maps for macro- and microchannels. The recirculation motion in the liquid segments of an intermittent gas–liquid flow is analyzed using micron-resolution particle image velocimetry (μPIV). The velocity distribution influences the mixing and the mass transport towards the reactive phase interface dealing with two-phase chemical reactions. For straight microchannels hardly any mass transport over the center line is quantified. For enhanced mixing geometrical adaptations are suggested.     

PTV investigation of phase interaction in dispersed liquid–liquid two-phase turbulent swirling flow

An investigation of dispersed liquid–liquid two-phase turbulent swirling flow in a horizontal pipe is conducted using a particle tracking velocimetry (PTV) technique and a shadow image technique (SIT). Silicone oil with a low specific gravity is used as immiscible droplets. A swirling motion is given to the main flow by an impeller installed in the pipe. Fluorescent tracer particles are applied to flow visualization. Red/green/blue components extracted from color images taken with a digital color CCD camera are used to simultaneously estimate the liquid and droplet velocity vectors. Under a relatively low swirl motion, a large number of droplets with low specific gravity tend to accumulate in the central region of the pipe. With increasing droplet volume fraction, the liquid turbulence intensity in the axial direction increases while that in the wall-normal direction decreases in the central region of the pipe. In addition, the turbulence modification in the present flow is strongly dependent on the droplet Reynolds number; however, the interaction of droplet-induced turbulences is significant due to vortex shedding, particularly at high droplet Reynolds numbers and higher droplet volume fraction.     

Flow patterns of an array of electromagnetically-driven cellular vortices

 A visualization study has been made of the patterns of a quasi-two-dimensional flow consisting of a linear array of four main cellular vortices. The flow is driven electromagnetically by the Lorentz force due to the interaction between a constant magnetic field arranged with alternating polarities and an electrolyzing current through a horizontal thin layer of an electrolytic aqueous solution. A peculiar flow consisting of a middle vortex pair reduced in size and two outer grown-up vortices is found to form when the reduced Lorentz force exceeds about 100. Received: 3 July 1996/Accepted: 15 December 1996     

The Dean instability in power-law and Bingham fluids in a curved rectangular duct

The laminar flow of power-law and yield-stress fluids in 180° curved channels of rectangular cross section was studied experimentally and numerically in order to understand the effect of rheological fluid behavior on the Dean instability that appears beyond a critical condition in the flow. This leads to the apparition of Dean vortices that differ from the two corner vortices created by the channel wall curvature.Flow visualizations showed that the Dean vortices develop first in the near-wall zone on the concave (outer) wall, where the shear rate is higher and the viscosity weaker; then they penetrate into the centre of the channel cross section where power-law fluids have high viscosity and Bingham fluids are unyielded in laminar flow. Based on the complete formation on the concave wall of the new pairs of counter-rotating vortices (Dean vortices), the critical value of the Dean number decreases as the power-law index increases for the power-law fluids, and the Bingham number decreases for the Bingham fluids. For power-law fluids, a diagram of critical Dean numbers, based on the number of Dean vortices formed, was established for different axial positions. For the same flow conditions, the critical Dean number obtained using the axial velocity gradient criterion was smaller then that obtained with the visualization technique.     

Deposition of micron liquid droplets on wall in impinging turbulent air jet

The fluid mechanics of the deposition of micron liquid (olive oil) droplets on a glass wall in an impinging turbulent air jet is studied experimentally. The spatial patterns of droplets deposited on a wall are measured by using luminescent oil visualization technique, and the statistical data of deposited droplets are obtained through microscopic imagery. Two distinct rings of droplets deposited on a wall are found, and the mechanisms of the formation of the inner and outer rings are investigated based on global diagnostics of velocity and skin friction fields. In particular, the intriguing effects of turbulence, including large-scale coherent vortices and small-scale random turbulence, on micron droplet deposition on a wall and coalescence in the air are explored.     

Visualization of flow boiling of liquid nitrogen in a vertical mini-tube

The flow boiling patterns of liquid nitrogen in a vertical mini-tube with an inner diameter of 1.931 mm are visualized with a high-speed digital camera. The superficial gas and liquid velocities are in the ranges of 0.01–26.5 m/s and 0.01–1.2 m/s, respectively. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are observed. Some interesting scenes about the entrainment and liquid droplet deposition in the churn and annular flow, and the flow reversal with the indication of negative pressure drop, are also presented. Based on the visualization, the two-phase flow regime maps are obtained. Compared with the flow regime maps for gas–water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and that of churn to annular flow moves to lower superficial gas velocity. Moreover, time-averaged void fraction is calculated by quantitative image-digitizing technique and compared with various prediction models. Finally, three kinds of oscillations with long-period and large-amplitude are found, possible explanation for the oscillations is given by comparing the instantaneous flow images with the data of pressure, mass flux and temperature recorded synchronously.     

Vortex structure in laminar boundary layers on a concave wall

Naturally occurring Görtler vortices have been investigated, using laser anemometry, in the laminar concave-surface boundary layers of water channels with 90° bends. Distributions of streamwise and spanwise velocities are presented, supported by flow visualization. Amplification was found to cease at Görtler numbers in the region of 9, followed by increasing distortion and spanwise wandering of the vortices. Development of a vortex system from an imposed disturbance has also been predicted by numerical computations.     

The turbulent flow field around a circular cylinder

The flow field around a circular cylinder mounted vertically on a flat bottom has been investigated experimentally. This type of flow occurs in several technical applications, e.g. local scouring around bridge piers. Hydrogen bubble flow visualization was carried out for Reynolds numbers ranging from 6,600 to 65,000. The main flow characteristic upstream of the cylinder is a system of horse-shoe vortices which are shed quasi-periodically. The number of vortices depends on Reynolds number. The vortex system was found to be independent of the vortices that are shed in the wake of the cylinder. The topology of the separated flow contains several separation and attachment lines which are Reynolds number dependent. In the wake region different flow patterns exist for each constant Reynolds number.     

Penetration height correlations for non-aerated and aerated transverse liquid jets in supersonic cross flow

Experimental results on the mixing of non-aerated and aerated transverse liquid jet in supersonic cross flow (M = 1.5) are presented in this paper. The goal of this study is to investigate the effect of the gas/liquid mass ratio on the penetration and atomization of an aerated liquid jet in high speed cross flow and to develop correlations for the penetration heights. High speed imaging system was used in this study for the visualization of the injection of aerated liquid jet. The results show the effect of jet/cross flow momentum flux ratio, the gas/liquid mass ratio and the Ohnesorge number on the penetration of aerated liquid jet in supersonic cross-flow. New correlations of the spray penetration height for the non-aerated liquid jet (GLR = 0) and the net gain in spray penetration height for the aerated liquid jet (GLR > 0) are presented.     

Steady slow rotation of a sphere in a visco-elastic liquid

Summary In this paper, we obtain the flow due to slow steady rotation of a sphere in a visco-elastic liquid characterized by the constitutive relation given by Rivlin. The non-Newtonian effects are strongly dependent on a non-dimensional parameter K independent of the angular velocity of the sphere. If 1<K3, we notice four vortices symmetrically placed around the sphere. When K lies outside this range, the direction of the flow pattern is the same as that in the Newtonian case but displaced towards the sphere as K decreases. Also the expression for the couple on the sphere has been obtained which depends on K.     

On swirl development in a square cross-sectioned, S-shaped duct

The flow in a uniform square cross-sectioned, S-shaped duct was investigated experimentally, at Reynolds number (Re) = 4.73 × 10⁴ and 1.47 × 10⁵, using three S-ducts of different curvature and turning angle. The hydraulic diameter (D) for each S-duct is 150 mm. Besides studying the square cross-sectioned S-duct flow at moderately higher Re than current literature, the S-ducts’ geometry used in this study also have larger curvatures and higher turning angles than those reported in the literature. With surface pressure measurement and smoke wire flow visualization, flow separation at the inside wall of the first bend was detected. Using surface oil flow visualization on the bottom wall of the S-duct and cross-wires measurement at the duct exit, it is shown here that the swirl developed in the first bend was partly attenuated in the second bend due to the formation of swirl of opposite direction. The swirl of an opposite sign results in the formation of a clear dividing or separation line on the bottom wall (and top wall) of the duct. Additional flow features include the formation of streamwise vortices on the outer-wall of the second bend. These streamwise vortices can either be a pair of counter-rotating vortices or a single vortex. The formation mechanism of these streamwise vortices is explained using the Squire and Winter [J Aeronaut Sci 18(4):271–277, 1951] formula and it is shown that the said mechanism is applicable to both Re in the present study.     

Coherent motion of turbulent thermal plume in stably stratified fluid

The purpose of this study is to clarify the existence of an ordered and large scale coherent motion in a turbulent plane thermal plume in a thermally-stable stratified fluid inside a comparatively large enclosure. First, the upper part of the thermal plume was carefully observed by a flow visualization. Secondly, a wave form of plume temperature variation was measured. Thirdly, a spectrum analysis was carried out on time series data of the thermal plume. Finally, physical characteristics were investigated on vortices in the thermal plume based on results of the wave form and the spectrum analysis of the plume temperature. As a result, the main conclusions are obtained as follows. (1) An existence of vortices near the upper part of the thermal plume was firstly found by careful flow visualization. (2) From the wave form of temperature variation and the spectrum analysis of the thermal plume, it was clarified that the vortices are generated in the transition state and are transported to the turbulent state. (3) The vortices are ordered and they behave as a large scale coherent motion in the turbulent thermal plume.     

Mass and momentum transport in longitudinal vortical structures in liquid flow Example of Görtler vortices

The difference between mass and momentum transport in longitudinal vortical structures is shown in this paper. The patterns observed by visualization using laser induced fluorescence are the signature of the mass transport, while the flow structures revealed by anemometry show the momentum transport. Experiments and numerical simulations are performed in order to compare the velocity field of the Görtler instability with the mass distribution of a passive scalar. The typical scales involved in this problem are discussed with relation to the Schmidt number, in order to compare the size of the observed “mushrooms” with the size of the longitudinal Görtler vortices. It is found that the nonlinearities which strongly influence the velocity perturbation do not modify the shape and the size of the scalar structures.     

Surface oil flow technique and liquid crystal thermography for flow visualization in impulse,wind tunnels

This paper describes flow visualization techniques employing surface oil flow and liquid crystal thermography suitable for use in impulse wind tunnels. High spatial resolution photographs of oil flow patterns and liquid crystal thermograms have been obtained within test times ranging from 7 to 500 ms and have been shown to be very useful for revealing the detailed features of 3-D separated flow. The results from oil flow patterns, liquid crystal thermograms, schlieren photographs and heat flux measurements are shown to be in good agreement.     

等速上仰翼型分离流动结构的研究

本文通过在翼型上游和翼表面边界层内放置产生氢气泡的铂丝的方法,清楚地显示了上仰翼型分离剪切层的结构。揭示了在不同的翼型转动角速度范围内,存在三种分离流结构。研究了失速涡,剪切涡及起动涡随时间的演变,它们之间的相互作用和转动角速度等参数的影响,分离剪切层的流动显示结果,结合翼型上气动力与流场中涡量矩的关系的理论,定性地解释了上仰翼型产生非定常高升力的原因。