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Experimental research on two-phase mixing between two vertical channels connected through a gap

Tazi Hnyine, Z. (2017) Experimental research on two-phase mixing between two vertical channels connected through a gap.

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Abstract:Inter-channel mixing through gaps is a process of mass transfer which naturally occurs in applications ranging from small household-size heating systems, to nuclear reactors, or any large industrial facility that requires heat exchange. Mixing between different flow channels may occur due to natural or forced pressure differences between flow regions. Also, fluid from one channel may be entrained by turbulent flowing regions surrounding this fluid, introducing inter-channel turbulent mixing. Large structures from vortex shedding due to flow separation around blunt objects may be another cause means of this mixing, given a geometry allowing inter-channel vortex motion. Injection of nominally monodispersed air bubbles of 5 to 15 mm through needle arrays into two adjacent, vertical flow channels connected by a narrow gap has been shown to drastically decrease the liquid mixing and mass transfer between the channels compared to the single-phase case. The amount by which mixing is inhibited (because of bubble injection) depends on the combination of injected gas flux (we studied the 0 to 150 slpm range), the Reynolds number (ranging from 4×104 to 1×105) , and the gap height (20 and 50 mm). The main mechanism of reducing the mixing appears to be the inhibition of the formation of large coherent structures explicitly observed at the single-phase cases. Indications of preferred mixing conditions have been found using the measurements performed. Mixing values were shown to collapse across a range of Reynolds numbers with respect to the volumetric quality, or ‘the ratio of gas and liquid in the system’, as well as with the two different gap heights studied. The inhibition number seemed to collapse with the volumetric quality as well, which was a way of collapsing the data without using a length scale. Visual observations of bubbles being convected between the channels along with the water due to their entrainment in the vortices were reviewed, and will be explored further to aid in the understanding of the reduced mixing for multi-phase flows as compared to the single-phase flows. Additionally, the researchers are further exploring the parameter space, including varying gap heights and flow rates, to further develop a physical understanding of the multi-phase mixing phenomenon. We suggest higher-resolution measurements gap heights in particular, so that we may find more information on stacking mixing values with the gap height. Acquisition of additional data, analysis or the results accompanying CFD effort and more detailed comparison between the experimental and CFD data are ongoing. Furthermore, in trying to solve the mixing disparities—mass transfer from one channel to the other appeared to be stronger than the other way around—found in some data sets, future research will start using only one type of wire mesh sensor (WMSs) for all channel positions, as we suspect different WMSs to be causing the disparity. On top of that, bringing the WMSs closer together will also help in obtaining higher cross-correlating values between two subsequent sensors which are needed for velocity measurements. Integrating optical probe measurement into the system will help in obtaining more accurate velocity measurements as well. Finally, as a few data sets have been found to contain fluorescein dye data that contained inaccuracies, it is further suggested to perform some re-measurements of the corresponding data sets.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Programme:Mechanical Engineering MSc (60439)
Keywords:Inter-channel mixing through gaps, mass transfer, flow channels, numerical simulations
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