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Significance
Preventing the release of gaseous pollutants and harmful aerosols is a major challenge for most industries. Venturi scrubbers have been proved to be an attractive solution for cleaning gas streams from hazardous gases and aerosols owing to their low installation and maintenance costs as well as superior collection efficiency compared to other scrubbers. They have been successfully applied in a broad variety of industries including waste incineration plants, gasification plants, coal and nuclear power plants, shipping and mining. Generally, Venturi scrubbers can be categorized based on the liquid injection mechanism (the film, spray and jet injection designs) and the operating mode (forced feed and self-priming modes). Venturi scrubbers are often part of critical safety installations. For those applications, self-priming Venturi scrubbers with liquid film injection are predominantly adopted due to their passive and self-regulating characteristics.
For the design and operation of Venturi scrubbers, the collection efficiency is of great importance and has therefore been studied extensively over the past seven decades. These studies revealed a strong relationship between the collection efficiency and the droplet size distribution within the throat of the Venturi scrubber. The droplet size distribution itself showed high sensitivity on the liquid injection mechanism, operating mode and geometrical size of the scrubber. Although they are a critical part of safety installations in various nuclear power plants around the globe, currently there are no experimental studies available investigating droplet size distributions in industrial self-priming Venturi scrubbers with liquid film injection.
To address this gap, PhD Candidate David Breitenmoser, Dr. Petros Papadopoulos and Professor Horst-Michael Prasser from the Swiss Federal Institute of Technology together with Dr. Terttaliisa Lind from the Paul Scherrer Institute used high-speed imaging analysis to investigate the droplet size distributions in an industrial self-priming Venturi scrubber with liquid film injection. Multiphase measurements were carried out at different gas flow rates and submergence levels. The work is currently published in the International Journal of Multiphase Flow.
The authors showed that the cumulative droplet size distribution function shifts consistently to smaller droplet diameters when the gas flow rate is increased. In contrast, no significant difference was observed for changes in the submergence level. In general, the droplet size distribution showed a unimodal shape and could be satisfactorily represented by Nukiyama-Tanasawa distribution functions. In addition, the authors compared the experimental results with two standard droplet size correlations, which are used in most of the thermohydraulic codes today to compute the collection efficiency of Venturi scrubbers. This comparison revealed a significant droplet size overprediction (>250%) of those correlations, especially at low gas flow rates.
The authors conjectured that this overprediction could be explained by the fact that the adopted correlations were developed for liquid spray injection and forced feed operating mode. This hypothesis is motivated by the fundamentally different droplet generation processes for the liquid spray and liquid film injection designs, i. e. pneumatic atomization and entrainment, respectively. To test their hypothesis, the authors compared their experimental results with annular flow correlations, which are based on the same droplet generation process as for Venturi scrubbers with liquid film injection. The annular flow correlations show a superior consensus in magnitude with the experimental data thereby supporting the authors’ hypothesis. Differences in the pronounced decreasing trend with increasing gas flow rate could be explained by the self-priming characteristics and geometry effects.
The obtained experimental results play a critical role in improving our understanding of the complex droplet dynamics in Venturi scrubbers. Furthermore, this study has direct relevance for the design and operation of self-priming Venturi scrubbers with liquid film injection. The current droplet size correlations used in thermohydraulic codes to compute the collection efficiency for these Venturi scrubbers should be revised. As a temporary solution, annular flow correlations could be applied. For a comprehensive and accurate droplet size correlation framework, further investigations are required.
Reference
Breitenmoser, D., Papadopoulos, P., Lind, T., & Prasser, H. (2021). Droplet size distribution in a full-scale rectangular self-priming Venturi scrubber with liquid film injection. International Journal of Multiphase Flow, 142, 103694.
