Characteristics of liquids lugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity UTP. The product of the liquid slug residence time and the recirculation rate is independent of UTP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug

3D + time blood flow mapping using SPIM-microPIV in the developing zebrafish heart

We present SPIM-μPIV as a flow imaging system, capable of measuring in vivo flow information with 3D micron-scale resolution. Our system was validated using a phantom experiment consisting of a flow of beads in a 50 μm diameter FEP tube. Then, with the help of optical gating techniques, we obtained 3D + time flow fields throughout the full heartbeat in a ∼3 day old zebrafish larva using fluorescent red blood cells as tracer particles. From this we were able to recover 3D flow fields at 31 separate phases in the heartbeat. From our measurements of this specimen, we found the net pumped blood volume through the atrium to be 0.239 nL per beat. SPIM-μPIV enables high quality in vivo measurements of flow fields that will be valuable for studies of heart function and fluid-structure interaction in a range of small-animal models

In Vitro Blood Flow Behaviour in Microchannels with Simple and Complex Geometries

Over the years, various experimental methods have been applied in an effort to understand the blood flow behaviour in microcirculation. The development of optical experimental techniques has contributed to obtain possible explanations on the way the blood flows through microvessels. In recent years, due to advances in computers, optics, and digital image processing techniques, it has become possible to combine a conventional particle image velocimetry (PIV) system with an inverted microscope and consequently improve both spatial and temporal resolution. The present review outlines our most relevant studies on the flow properties of blood at a microscale level by using current micro-PIV and confocal micro-PIV techniques. In this chapter, our recent studies about in vitro blood flow behaviour in microchannels both in straight and with complex geometries are presented. In straight microchannels we present some phenomena such as Fahraeus effect and Fahraeus-Lindqvist effect, the flow of particles and red blood cells (RBCs) in diluted suspensions, the flow of RBCs in concentrated suspensions, the cell-free layer and sedimentations effects. The most recent studies in blood flow through complex geometries such as bifurcations, confluences and stenosis are also reviewed. By using a chromatographic method, the flow of RBC s through a network of microcapillaries is presented.The authors acknowledge the financial support provided by: PTDC/SAUBEB/ 108728/2008, PTDC/SAU-BEB/105650/2008 and PTDC/EME-MFE/099109/2008 from the FCT (Science and Technology Foundation) and COMPETE, Portugal

Confocal micro-PIV/PTV measurements of the blood flow in micro-channels

The development of optical experimental techniques has contributed to obtain explanations on the blood flow behaviour through micro-channels. Although the past results have been valuable, detailed studies on the flow properties of in vitro blood in micro-channels have been limited by several technical factors such as poor spatial resolution and difficulty to obtain quantitative detailed meas-urements at such small scales. In recent years, due to advances in computers, op-tics, and digital image processing techniques, it became possible to combine both particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods with confocal microscopes. As a result, this combination has greatly increased the resolution of the conventional micro-PIV/PTV systems and consequently pro-vided additional detailed description on the blood cells motion not obtainable by traditional methods. In this chapter the most relevant theoretical and technical is-sues related to both conventional and confocal micro-PIV/PTV methods are dis-cussed. In addition, a comparison between them is presented. Furthermore, the most relevant results of in vitro blood flowing in both glass and polydime-thylsiloxane (PDMS) micro-channels are shown

Experimental investigation of the flow in a micro-channelled combustor and its relation to flame behaviour

© 2020 Elsevier Inc. The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled combustor consisting of an array of five planar rectangular channels was found to be influenced by the equivalence ratio and flow-rate of the continuously and steadily injected premixed fuel charge. Three distinct flame stages were observed — planar, chaotic and trident, which were strongly correlated to the flow dynamics. The effect of the flow on the flame behaviour was investigated by characterizing the cold flow in a scaled-up model channel with the same aspect ratio as the combustion micro-channel. Direct flow visualization using flow tracers and quantitative velocity-field data from PIV measurements showed both an increase in the bottom recirculation zone reattachment length (along the floor of the channel) and a decrease in the lateral recirculation zone reattachment length (along the sides of the channel) with increasing flow Reynolds number. Comparison of the flow and flame transition locations downstream of the injection point suggested that the location of trident flame onset coincides with the flow bottom recirculation zone reattachment length. The planar-chaotic flame transition location was observed to be influenced by the homogeneity of the mixture downstream of the injection plane

Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

Radial dispersion of red blood cells in blood flowing through glass capillaries: the role of hematocrit and geometry

The flow properties of blood in the microcirculation depend strongly on the hematocrit (Hct), microvessel geometry, and cell properties. Previous in vitro studies have measured the radial displacement of red blood cells (RBCs) at concentrated suspensions using conventional microscopes. However, to measure the RBCs motion they used transparent suspensions of ghost red cells, which may have different physical properties than normal RBCs. The present study introduces a new approach (confocal micro-PTV) to measure the motion of labeled RBCs flowing in concentrated suspensions of normal RBCs. The ability of confocal systems to obtain thin infocus planes allowed us to measure the radial position of individual RBCs accurately and to consequently measure the interaction between multiple labeled RBCs. All the measurements were performed in the center plane of both 50 mm and 100 mm glass capillaries at Reynolds numbers (Re) from 0.003 to 0.005 using Hcts from 2 to 35%. To quantify the motion and interaction of multiple RBCs, we used the RBC radial dispersion (Dyy). Our results clearly demonstrate that Dyy strongly depends on the Hct. The RBCs exhibited higher Dyy at radial positions between 0.4 R and 0.8 R and lower Dyy at locations adjacent to the wall (0.8 R–1 R) and around the middle of the capillary (0 R–0.2 R). The present work also demonstrates that Dyy tends to decrease with a decrease in the diameter. The information provided by this study not only complements previous investigations on microhemorheology of both dilute and concentrated suspensions of RBCs, but also shows the influence of both Hct and geometry on the radial dispersion of RBCs. This information is important for a better understanding of blood mass transport mechanisms under both physiological and pathological conditions

A simultaneous planar laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry technique for the investigation of thin liquid-film flows

AbstractA simultaneous measurement technique based on planar laser-induced fluorescence imaging (PLIF) and particle image/tracking velocimetry (PIV/PTV) is described for the investigation of the hydrodynamic characteristics of harmonically excited liquid thin-film flows. The technique is applied as part of an extensive experimental campaign that covers four different Kapitza (Ka) number liquids, Reynolds (Re) numbers spanning the range 2.3–320, and inlet-forced/wave frequencies in the range 1–10Hz. Film thicknesses (from PLIF) for flat (viscous and unforced) films are compared to micrometer stage measurements and analytical predictions (Nusselt solution), with a resulting mean deviation being lower than the nominal resolution of the imaging setup (around 20μm). Relative deviations are calculated between PTV-derived interfacial and bulk velocities and analytical results, with mean values amounting to no more than 3.2% for both test cases. In addition, flow rates recovered using LIF/PTV (film thickness and velocity profile) data are compared to direct flowmeter readings. The mean relative deviation is found to be 1.6% for a total of six flat and nine wavy flows. The practice of wave/phase-locked flow-field averaging is also implemented, allowing the generation of highly localized velocity profile, bulk velocity and flow rate data along the wave topology. Based on this data, velocity profiles are extracted from 20 locations along the wave topology and compared to analytically derived ones based on local film thickness measurements and the Nusselt solution. Increasing the waviness by modulating the forcing frequency is found to result in lower absolute deviations between experiments and theoretical predictions ahead of the wave crests, and higher deviations behind the wave crests. At the wave crests, experimentally derived interfacial velocities are overestimated by nearly 100%. Finally, locally non-parabolic velocity profiles are identified ahead of the wave crests; a phenomenon potentially linked to the cross-stream velocity field