Experimental laser-Doppler anemometry

In view of the different requirements as to computer power, it is very worthwhile to compare the outcome of RANS and LES simulations mutually and/ or with quantitative experimental data. Several authors have done this, e.g., Derksen and Van den Akker (1998, 1999), Derksen (2001), Lu et al. (2002), Ranade (2002), Derksen (2002b), and Yeoh et al. (2004a,b). In most cases, the experimental data have been obtained by means of Laser-Doppler Anemometry (LDA, or LDV) see, e.g., Yianneskis et al. (1987), Wu and Patterson (1989), Schafer et al. (1997, 1998), and Derksen et al. (1999). In this review, we will mainly refer to the validation study due to Hartmann et al. (2004a). 

There have been very few studies of the effects of non-Newtonian properties on flow patterns in hydrocyclones, although Dyakowski et al.,AU have carried out numerical simulations for power-law fluids, and these have been validated by experimental measurements in which velocity profiles were obtained by laser-doppler anemometry. 

There have been several studies in which the flow patterns within the body of the cyclone separator have been modelled using a Computational Fluid Dynamics (CFD) technique. A recent example is that of Slack et a/. 54 in which the computed three-dimensional flow fields have been plotted and compared with the results of experimental studies in which a backscatter laser Doppler anemometry system was used to measure flowfields. Agreement between the computed and experimental results was very good. When using very fine grid meshes, the existence of time-dependent vortices was identified. These had the potentiality of adversely affecting the separation efficiency, as well as leading to increased erosion at the walls. 

Figure 1. Experimental set up for measuring the mixing length by a redox reaction and the flow field by Laser Doppler Anemometry ( LDA) in the 16.4 mm tube...

Since 1995 the experimental work has been concentrated on preparation of the second phase of investigations in water (WAMIX II) and providing a comparable testing arrangement in sodium (NAMIX experiment). For measurements of local velocity (see Fig. 1) and temperature as well as their fluctuations in the WAMIX test-section, laser Doppler anemometry and resistance thermometry are applied in a modified test-section. To make these measuring techniques applicable, some modifications on the water loop were also made. 

Levins and Glastonbury (1972a, b, c), Kuboi et al. (1974a), and Lee (1981,1984) refined Harriott s approach subsequently. The importance of turbulence parameters, such as intensity/scale of turbulence, in deciding the mass transfer coefficient was expanded in these studies. Evidently, application of this approach required more elaborate information on the relevant turbulence parameters. Some of the experimental investigations, particularly in stirred vessels, are discussed in the following. The early investigations used equipment with limited capabilities and accuracy. Contemporary techniques such as laser Doppler anemometry/velocimetry have far more sophistication and accuracy. Nonetheless, the initial studies deserve mention because many of them included simultaneous measurements of mass transfer. 

Figure 18.18 (left) exhibits the calculated gas flow field from an individual gas jet at atomization pressure po = 0.5 MPa (polPa = 5). The diameter at the nozzle exit is 3 mm. The simulation is conducted based on the 2D axisymmetric geometry (see Fig. 18.15). Five cells with shocks can be found after the nozzle exit. Figure 18.18 (right) exhibits the velocity distribution at the centre line of the jet. The experimental data were obtained by laser Doppler anemometry (LDA) [26]. A good agreement is achieved between experimental data and numerical simulation results, for example in the location and number of shock cells, the calculated length of the supersonic core of the jet and the decay rate of the gas velocity in the subsonic region. Only the amplitudes of the velocity fluctuation differ between experiment and simulation the peak in velocity values behind the shock is more intense than those measured in the experiment. The experimental deviation may be caused by the behaviour of the tracer particles used for LDA measurements. These small but still inertial tracer particles cannot follow the steep velocity gradients across a shock exactly. The k-co SST model indicates a better performance than the standard k-e model.

During the last four-five decades, the laser-Doppler anemometry (LDA) has become a commonly used experimental technique to measure the instantaneous velocity of seeded single phase flows, and dispersed two-phase flows of very low concentration. A major reason is that LDA is a non-invasive optical technique and does not disturb the flow. Moreover, the LDA system has a high spatial resolution with a fast dynamic response and range. 

Our understanding of the hydrodynamics of multiphase flows has progressed substantially in the recent three decades, thanks to the development of advanced experimental techniques, particularly laser Doppler anemometry (LDA), particle image velocimetry (PIV), computer-automated radioactive particle tracking (CARPT), and optical bubble probes. In addition, computational fluid dynamics (CFD) simulations allow for inner views in two-phase process equipment. 

This procedure has been used to determine droplet size in sprays. Oseillations in the curve relating x and D can be smoothed out by the use of an incident laser beam having a broad speetral bandwidth [83]. An accumulation of independent scattering intensities from multiple scatterers ean be used to measure the mean droplet size of a group [84]. This procedure has been applied to water sprays and the experimental data confirmed by phase Doppler anemometry [85]. The applicability of the polarization ratio technique is strongly influenced by the complex refractive index of the dispersed media and is limited to media having a relative refractive index below about 1.44 [86]. 

For experimental determination the flow pattern produced by the stirrer was initially visualized using different photographic methods (e.g. [574, 497]), but hydraulic probes were also used to determine the pressure distribution (e.g. [135]) and velocity distribution (e.g. [437]). Also, convection probes (spherical probes) and pressure probes (Prandtl s Pitot tube) were used. Later constant temperature hotwire/hot-film anemometry was used. Currently contactless laser doppler velocim-ctry (LDV)/anemometry (LDA) is exclusively utilized. 

Lance and Bataille [78] studied grid-generated turbulent bubbly flow in a 45 (cm) X 45 (cm) rectangular pipe. Laser-Doppler and hotfilm anemometry were used for the experimental investigation. The high-frequency of spectra of the velocity components were determined by the power —8/3. A decade later Mudde et al [102] reported LDA-measurements characterizing 15 (cm) and... 

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