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Investigation of the Radial Effect On the Transition Velocities in a Bubble Column Based On the Modified Shannon Entropy

Nedeltchev, S.; Hampel, U.; Schubert, M.

A new method for flow regime identification in a bubble column (0.15 m in ID) based on a modification of the Shannon entropy (SE) algorithm was developed. The bubble column was equipped with a perforated plate distributor (14 holes, Ø 4×10-3 m) and operated with an air-deionized water system at ambient conditions. The newly introduced dimensionless ratio of minimum SE to maximum SE was capable of identifying the main transition velocities at three different dimensionless radial positions (r/R): 0.0 (core), 0.63 (inversion point) and 0.88 (annulus).
In the core of the column the new parameter identified successfully three transition velocities Utrans at 0.034, 0.089 and 0.134 m/s. They mark the end of the gas maldistribution regime, the onset and the end of the churn-turbulent flow regime, respectively. Three Utrans values (at 0.045, 0.089 and 0.124 m/s) were also identified in the annulus of the column. However, the second transition velocity identified the boundary between the first and second transition sub-regimes. The third transition velocity distinguished the onset of the churn-turbulent flow regime. It was found that in the core of the column both the transition and churn-turbulent flow regimes start earlier, which is due to the earlier onset of the bubble coalescence caused by higher gas fraction in the column center.
At the inversion point of the axial liquid velocity the end of the gas maldistribution regime is shifted to a somewhat higher Utrans value (0.067 m/s). The second transition sub-regime begins at 0.101 m/s, whereas the onset of the churn-turbulent regime is identified at 0.124 m/s.
The SE algorithm was also applied to both the first and second half of the time series and the ratio of both SEs was successfully used as a flow regime identifier.

Keywords: Bubble column; Gas holdup fluctuations; Wire-mesh sensor; Transition velocities; Modified Shannon entropy

Publ.-Id: 23515