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A new statistical parameter for identifying of the main transition velocities in bubble columns
Nedeltchev, S. N.; Rabha, S.; Hampel, U.; Schubert, M.;
The identification of the main flow regime boundaries in bubble columns is essential since the degrees of mixing, mass and heat transfer vary with the flow regime. Most of the available methods in the literature for flow regime identification provide controversial results about the transition velocities and they are applicable mainly to pressure fluctuations. That is why, in this work a new statistical parameter was extracted from gas holdup time series (60,000 points) of the cross-sectional averaged gas holdup. The measurements were performed in a narrow (0.15 m in ID) and a large (0.4 m in ID) bubble column by means of conductivity wire-mesh sensors at very high sampling frequency (2000 Hz). The wire-mesh sensors in both columns were always installed at a height of 1.3 m above the gas distributor. Both columns were equipped with a perforated plate distributor, each with an open area of 1 %. The columns operated with an air-deionized water system at ambient conditions.
As a regime indicator, a new dimensionless statistical parameter called ‘relative maximum number of visits in a region’ Nrmax was introduced. This new parameter is a function of the difference between the maximum number of visits in a region arising from two different division schemes of the signal’s range. The identified two transition velocities were found to be independent of the different division schemes studied in this work. In both small and large bubble columns, the Nrmax profiles exhibited two well-defined local minima, which identified two transition velocities Utrans, indicating the end of the gas maldistribution regime and the onset of the churn-turbulent regime. It was found that the column diameter affects only the second transition velocity.
Keywords: flow regime identification, bubble column, maximum number of visits in a region, gas maldistribution, gas holdup fluctuations, conductivity wire-mesh sensor

Publ.-Id: 21147 - Permalink