Flow visualization methods

Using this assumption, one ean apply this flow visualization method to any working medium. 

In order to control the movements of contaminants it is useful to be able to see how both the contaminant and the induced airflows move. A number of flow visualization methods have been developed some are more suitable for laboratory research applications whereas others are quite widely used in industrial situations. We are primarily interested in this latter category. The methods involve releasing a tracer (for example gas, aerosol, or heat) and making visible its path. While in most cases the methods are subjective, their use is invaluable. Ideally the tracer should be nontoxic, nonirritating, inexpensive, and highly visible at low concentrations. The system should be easily portable, self-contained, easy to use, and be controllable. 

P 63] A reactive-type flow visualization method was used for quantification of mixing [47] (citing a protocol described in detail in [20] and given in [P 40]). Colorless solutions of iron(III) nitrate and sodium rhodanide form a colored compound,... 

The appearance of flow visualization methods [61, 62, 63, 64] has made possible the study of two-phase flows in flow field channels. These methods should be perfected considering the potential measurement artifacts introduced by the transparent element (change in thermal and current distribution, and flow field channel surface properties). Mathematical representations of the pressure drop in presence of two-phase flow will be needed to modify existing stack reactant flow distribution models [65]. 

To be able to design devices based on microfluidics and nanofluidics, it is crucial to quantitatively visualize the flow of fluids in the microfluidic and nanofluidic channels. There have been many flow visualization methods being developed for macroscale fluid flow (e.g., hot-wire anemometry), but most of them are not suitable for micro- and nanoscale measurements because they are too intrusive for micro- and nanoscale fluid flows [3]. Fluorescence measurements are very suitable for quantitatively visualizing flow in micro- and nanoscales, because it is nonintrusive and it allows for measurements with a high spatial resolution. 

Flow visualization has a long history. A well-known early flow visualization experiment is that of Reynolds in 1883, who studied the transition to turbulence in a pipe flow by observing the transport of an injected dye. Although many flow visualization methods have evolved since, all... 

To be able to design devices based on microfluidics and nanofluidics, it is crucial to quantitatively visualize the flow of fluids in the microfluidic and nanofluidic channels. There have been many flow visualization methods being... 

The discussed microscale flow visualization methods discussed here were divided into particle- and scalar-based approaches. However, as hybrid techniques are developed... 

Zeta Potential Measurement, Figure 6 Plot of the electro-osmotic velocity as a function of the applied electric field for two buffers (a) 1xTAE and b) 1xTBE. Results for both the direct flow visualization method and the current-monitoring method are shown [6]... 

An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method. How-evec, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow. 

There are of course a number of methods that can be classific d a- methods for the visualization of airflow and contaminant dispersion. This i.hapter describes some of these that are useful for designers of industrial vcntilatiou. Methods that not are presented in more detail here are, for example, to fill small soap bubbles or ordinary balloons with helium in order to stuiiy the airflow field in large rooms. A large number of textbooks focus on flow- visualization. The research in this area can also be followed in The lournal of Floif Visualization and Image Processing. -... 

Analysis of numerous experimental data obtained by various methods of flow visualization led the investigators to the conclusion that in all cases it is possible to separate three main regions with essentially different flow patterns. Fig. 4.52 shows these regions for a rectangular channel. 

Winkelmann et al. (54) have studied air-water flows in a corrugated heat exchanger. Flow visualization and two-phase pressure drop measurements have been performed. The flow visualizations have shown that the flow pattern is complex and that a wavy or a film flow occurs in most cases (Figure 29). The two-phase pressure drop depends on the total flow rate and vapor quality, and Chisholm-type correlation is proposed. More work is required to characterize the flow structure in compact heat exchangers and to develop predictive methods for the frictional pressure drop and the mean void fraction. 

For experimental characterization, flow visualization by colored or fluorescent streams is the most facile method. Dilution-type experiments contact dyed and pure water streams (passive mixing) or standing volume portions (active mixing) in a type of photometric experiment. This is usually monitored with the aid of microscopic, photo, video or high-speed camera techniques (see e.g. [20]). 

D visualization methods in, 117—118 for close-to-focus images, 112—113 CTF/B factor corrections in, 101—4 data processing flow chart of, 106... 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

There is also a variety of wall-slip techniques used in rheological analysis (Gupta, 2000). The methods described here are flow visualization, capillary flow and torsional flow. 

Quantitative information on the flow pattern can also be determined by direct flow visualization, although this method is limited to optically transparent liquids, sufficient light, suitable flow followers, and adequate observation points. Direct observation provides information about recirculation (back-mixing), short circuits, and dead zones. 

The radioactive isotope rubidium 82—another photon emitter—is also finding a useful niche in diagnostic medicine. As the tracer in PET (positron emission tomography) scans, rubidium is cheaper than the more commonly used ammonia, which must be produced in a particle accelerator. Combined with a CTscan that helps visualize blood flow, this method is currently the most accurate when imaging blood flow restrictions that may lead to cardiac arrest. 

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