Coanda-effect mixing

Recycle-flow Coanda-effect Mixing Based on Taylor Dispersion Most Relevant Citations... 

Smokeless Steam Inspirating Air Coanda effect nozzles are used to inspirate air into the mixing chamber. Claim high efficiency of steam use. Claim low steam noise. Complex nozzles are a high-cost component. Brochure shows much R D back-up of basic design, in marked contrast to those of other vendors. Coanda effect is well understood and widely used, therefore working principle is well established. 

The use of the Coanda effect is based on the desire to have a second passive momentum to speed up mixing in addition to diffusion [55, 163], The second momentum is based on so-called transverse dispersion produced by passive structures, which is in analogy with the Taylor convective radial dispersion ( Taylor dispersion ) (see Figure 1.180 and [163] for further details). It was further desired to have a flat ( in-plane ) structure and not a 3-D structure, since only the first type can be easily integrated into a pTAS system, typically also being flat A further design criterion was to have a micro mixer with improved dispersion and velocity profiles. 

Another interesting planar structure able to induce chaotic advection has been reported by Hong et al. [126] (Fig. lid). This micromixer comprised a modified tesla structure that redirected the streams, by exploiting the Coanda effect. The authors demonstrated an efficient mixing at relative low Re number Re < 10). 

One of the promising designs is the modified Tesla structures [46]. It uses the Coanda effect to split part of the fluid stream and direct it so that it recombines with the opposing flow of the other part of the stream. Coanda effect micromixer relies on the redirection of a flow by a special guiding structure that creates new interfaces within the flow [47]. This special passive structure provides good mixing at low flow rates. In this way the Coanda mixer can also be seen as a special realization of the SAR approach using recycle flows [32]. 

In this section, data will be given on the mean velocity and turbulence components of water flow in and near an air-water bubbling jet subjected to the Coanda effect, measured with a two-channel laser Doppler velocimeter. These quantities are closely associated with mixing in metallurgical reactors and the erosion of the side wall of the reactors [22], Particular attention is paid to whether or not the horizontal distributions of the liquid flow characteristics near the side wall are similar in the vertical region above the attachment position. 

The other swirl motion of the second kind occurs for a bath depth larger than about twice the bath diameter, D. The radial displacement of the jet becomes large and the jet approaches the side wall of the vessel. The swirl period therefore becomes much longer than the period of the first kind of swirl motion. The liquid at a radial location opposite to the jet falls as it swirls. The bath would be highly mixed by such a large-scale swirl motion, and hence, the mixing time would become shorter. In fact, Murthy et al. [25] reported that the mixing time has a minimum at Hi/D 2. This motion is caused by the Coanda effect [26-28] and called the second kind of swirl motion. The Coanda effect has been fully discussed in Chap. 3. 

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