Flow velocimetry

Fig. 12.20 Flow velocimetry as a function of the radial position of the polyethylene inside the capillary die for a series of flow rates. Measurements were taken at x = —0.2 mm (no PPA). [Reprinted by permission from K. B. Migler, Extensional Deformation, Cohesive Failure, and Boundary Conditions during Sharkskin Melt Fracture, J. Rheol., 46, 383-400 (2002).]...

Piorek B, Mechler A, Lai R, Freudenthal P, Meinhart C, Banerjee S (2006) Nanoscale resolution microchannel flow velocimetry by atomic force microscopy. Appl Phys Lett 89(15) 153123... 

Several variations of microflow visualization have been developed for microfluidic applications such as particle-based flow velocimetry and scalar-based flow velocimetry [5]. In terms of the zeta potential measurement, these visualization techniques such as micro-PlV are used to measure the velocity profile and flow rate under electroosmotic flow. Once the velocity is known, the zeta potential can be calculated from Eq. 6. The main advantage of using a flow visualization technique is that the electroosmotic velocity can be measured directly and in real time. In general, the small amount of particles or dye used has a negligible effect on the electroosmotic flow being measured. The oifly significant disadvantage of this technique is that the extent and cost of the hardware may be prohibitive. 

Scalar-based flow velocimetry, where the motion of the bulk fluid is inferred from the observed velocity of a conserved scalar, has been widely applied in fluid mechanics from the work of Reynolds (1883) to the present for macro- and microscale applications [32]. The basis is the use of light emitting molecules (fluorescent or phosphorescent). In this section the laser-induced fluorescence (LIF) measurement technique will be discussed in detail. This technique tags one of the liquids by a fluorescent dye and records the fluorescence intensity originating from that dye, which is linked to the concentration of the respective liquid. In contrast to the application of fluorescence intensity techniques to macroscopic flows (LIF), the present application to microflows does not allow one to span up a light sheet and instead illuminates the complete microchannel volume. 

This procedure offers the possibiUty of remote noncontact velocity measurement, where no probes disturb the flow. It is thus compatible for use with hot or corrosive gases. Commercial laser velocimeters have become weU-developed measurement tools. Examples of laser velocimetry include remote measurement of wind velocity, measurement of vortex air flow near the wing tips of large aircraft, and in vivo measurement of the velocity of blood flow. 

Maynes and Webb (2002) presented pressure drop, velocity and rms profile data for water flowing in a tube 0.705 mm in diameter, in the range of Re = 500-5,000. The velocity distribution in the cross-section of the tube was obtained using the molecular tagging velocimetry technique. The profiles for Re = 550,700,1,240, and 1,600 showed excellent agreement with laminar flow theory, as presented in Fig. 3.2. The profiles showed transitional behavior at Re > 2,100. In the range Re = 550-2,100 the Poiseuille number was Po = 64. 

Hao et al. (2007) investigated the water flow in a glass tube with diameter of 230 Lim using micro particle velocimetry. The streamwise and mean velocity profile and turbulence intensities were measured at Reynolds number ranging from 1,540 to 2,960. Experimental results indicate that the transition from laminar to turbulent flow occurs at Re = 1,700—1,900 and the turbulence becomes fully developed at Re > 2,500. 

For most existing measuring methods, the actual motion of individual nano-particles in two-phase flow cannot be observed easily. Conventional particle image velocimetry (PIV) apparatus can measure the particles in micro scale... 

Kim, M. J., Beskok, A., and Kihm, K. D., "Electro-Osmosis-Driven Micro Channel Flows A Comparative Study of Microscopic Particle Image Velocimetry Measurements and Numerical Simulations, Exp. Fluids, Vol. 33, No. 1, 2002, pp. 170-180. 

On comparing the two flames, it is evident that the flow structure of the lean limit methane flame fundamentally differs from that of the limit propane one. In the flame coordinate system, the velocity field shows a stagnation zone in the central region of the methane flame bubble, just behind the flame front. In this region, the combustion products move upward with the flame and are not replaced by the new ones produced in the reaction zone. For methane, at the lean limit an accumulation of particle image velocimetry (PIV) seeding particles can be seen within the stagnation core, in... 

This system produces a steady laminar flow with a flat velocity profile at the burner exit for mean flow velocities up to 5m/s. Velocity fluctuations at the burner outlet are reduced to low levels as v /v< 0.01 on the central axis for free jet injection conditions. The burner is fed with a mixture of methane and air. Experiments-described in what follows are carried out at fixed equivalence ratios. Flow perturbations are produced by the loudspeaker driven by an amplifier, which is fed by a sinusoidal signal s)mthesizer. Velocity perturbations measured by laser doppler velocimetry (LDV) on the burner symmetry axis above the nozzle exit plane are also purely sinusoidal and their spectral... 

Figure 7.2.5 provides a visualization of a localized extinction event in a turbulent jet flame, using a temporal sequence of OH planar LIF measurements. The OH-LIF measurements, combined with particle image velocimetry (PIV) reveal that a distinct vortex within the turbulent flow distorts and consequently breaks the OH front. These localized extinction events occur intermittently as the strength of the coupling between the turbulent flow and the flame chemistry fluctuates. The characteristics of the turbulent flame can be significantly altered as the frequency of these events increases. 

Fajardo, C.M. and V. Sick, Flow field assessment in a fired spray-guided spark-ignition direct-injection engine based on UV particle image velocimetry with sub crank angle resolution. Proceedings of the Combustion Institute, 31(2) 3023-3031, 2007. 

Ultrasonic Doppler velocimetry is a nonintrusive technique that has been developed into a very useful technique for opaque liquid flows [3]. This technique provides good measurement of velocity new high-frequency techniques give a space resolution on the millimeter level, and even the large turbulent scales can be resolved. 

Hammoudi, M. et al. (2008) Flow analysis by pulsed ultrasonic velocimetry technique in SulzerSMX static mixer. Chem. Eng.]., 139 (3), 562-574. 

Virdung, T. and Rasmuson, A. (2008) Solid-liquid flow at dilute concentrations in an axially stirred vessel investigated using particle image velocimetry. Chem. Eng. Commun., 195 (1), 18-34. 

One characteristic of shear banded flow is the presence of fluctuations in the flow field. Such fluctuations also occur in some glassy colloidal materials at colloid volume fractions close to the glass transition. One such system is the soft gel formed by crowded monodisperse multiarm (122) star 1,4-polybutadienes in decane. Using NMR velocimetry Holmes et al. [23] found evidence for fluctuations in the flow behavior across the gap of a wide gap concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The timescale of these fluctuations appeared to be rapid (with respect to the measurement time per shear rate in the flow curve), in the order of tens to hundreds of milliseconds. As a result, the velocity distributions, measured at different points across the cell, exhibited bimodal behavior, as apparent in Figure 2.8.13. These workers interpreted their data... 

In summary, we have commented briefly on the microscopic applications of NMR velocity imaging in complex polymer flows in complex geometries, where these applications have been termed Rheo-NMR [23]. As some of these complex geometries can be easily established in small scales, NMR velocimetry and visc-ometry at microscopic resolution can provide an effective means to image the entire Eulerian velocity field experimentally and to measure extensional properties in elastic liquids non-invasively. 

J. Bouillard, B. Alban, P. Jacques, C. Xuereb 2001, (Liquid flow velocity measurements in stirred tanks by ultrasound doppler velocimetry), Chem. Eng. Sci. 56, 747. 

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