Droplet size measurements inside a Venturi

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.

Droplet size measurements inside a Venturi - Advances in Engineering

About the author

David Breitenmoser is a PhD student working in the department of Radiation Safety and Security at the Paul Scherrer Institute in Switzerland. He graduated with a master’s degree in Nuclear Engineering at the Swiss Federal Institute of Technology in Zurich and Lausanne and joined the Paul Scherrer Institute in 2020 for his PhD. His main research interest lies in the combination of advanced modern computational and experimental techniques in the broad field of nuclear engineering. During his master’s, he worked on the experimental investigation of droplet size distributions in Venturi scrubbers using high-speed imaging and digital imaging analysis to improve our understanding in the performance of Filtered Containment Venting Systems (FCVS) and thereby in nuclear power plant safety. For his master’s thesis, he visited the Experimental and Computational Multiphase Flow Laboratory (ECMF) at the University of Michigan to perform high-resolution high-speed void fraction measurements in helical coiled tubes using X-ray radiography. He combined advanced unsupervised and supervised machine learning techniques to characterize the flow regime dynamics in these coils and thereby contributed to the development of advanced heat exchangers for small modular reactors. Currently, he is working on the improvement in the calibration and evaluation of airborne gamma-ray spectrometry (AGRS) systems for his PhD project combining experimental radiation measurements, numerical radiation transport simulations and machine learning techniques.

Aside his academic career, he contributes to the sustainable development and application of nuclear energy systems as well as to the strengthening of the radiation protection for our society. During his master’s, he worked at the National Cooperative for the Disposal of Radioactive Waste (NAGRA) on the development of deep geological repositories. In addition, he shares his expertise in several organizations and expert groups including the Swiss Federal Expert Group in AGRS or the Swiss Study Foundation. He is recipient of the ETH Medal for outstanding master’s theses.

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About the author

Horst-​Michael Prasser has been full professor of Nuclear Energy Systems at ETH Zurich from April 2006 to January 2021. From 2007 to 2017, he was also head of the Laboratory for Thermal-​Hydraulics at the Paul Scherrer Institute. From 2008 to 2011, he was elected as a member of the supervisory body of the Swiss Federal Nuclear Safety Inspectorate (ENSI Council).

Professor Prasser was born in Görlitz, Germany in 1955. He studied at the Moscow Energy Institute from 1974 to 1980 and received his doctorate in 1984 from the Zittau University of Applied Sciences on the topic of flow studies in nuclear reactors. In early 1987, he started working for the Central Institute for Nuclear Research Rossendorf near Dresden. During the German reunification phase, he also served as a personal advisor to the scientific director during the founding of the Rossendorf Research Center. From 1994, there Prof. Prasser headed the Accident Analysis and Experimental Thermal Fluid Dynamics departments at the Institute for Safety Research. From earlier work at the Rossendorf Research Center (today Helmholtz Zentrum Dresden-​Rossendorf) came results related to boron mixing in pressurized water reactors which made direct contributions to reactor safety. This was in addition to development of high-​resolution measurement techniques used for liquid-​gas flows, including wire mesh sensors and time-​resolved gamma and X-​ray tomography. Large scale thermal-​hydraulic test facilities were constructed where these techniques were applied.

At ETH Zurich, Prof. Prasser continued working primarily in the field of thermal fluid dynamics in the context of nuclear facilities. Further innovative measurement methods for various applications emerged, such as high speed measurement of liquid film thickness distributions, using either conductivity probes or the attenuation of infrared radiation. Under his leadership, the Paul Scherrer Institute developed a deuterium-​deuterium plasma neutron source that is suitable for fast neutron imaging. The laboratory of Prof. Prasser provided contributions to the development of CFD codes used to describe single and two-​phase flow phenomena in fuel elements of nuclear reactors, while also providing experimental results for the validation of such models. One focus was on the detailed description of liquid films in modern fuel assemblies of boiling water reactors. Other fields of activity were in the area of containment thermal-​hydraulics, e.g. work on the distribution of hydrogen in severe accident scenarios and filtered containment pressure relief.

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About the author

Dr. Terttaliisa Lind is a Senior expert in the area of severe accident research at Paul Scherrer Institute (PSI), Switzerland. She graduated from the Helsinki University of Technology in Finland with a master of science and a PhD in nuclear engineering and physics. She worked at the Finnish nuclear regulator STUK, at VTT Technical Research Centre of Finland, and made a post-doc at Sandia National Laboratories in Livermoore, California. Her research areas are aerosol formation and transport in various energy and environmental applications as well as filtering of particulate matter.

From 2006, she works at PSI in the area of nuclear reactor severe accident research. After 2011, she has supported Swiss nuclear power plants in their post-Fukushima action plans, and collaborated in many international projects in support of accident analysis and de-commissioning of Fukushima Daiichi nuclear power station. Her work has also included fundamental investigations of aerosol transport and mass transfer in multi-phase flows and gas cleaning from both solid and gas phase contaminants. She is a lecturer at the Swiss Federal Institute of Technology in Zürich and is a member of various international working groups.

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Petros Papadopoulos is an instructor at the Nukleartechnikerschule where he contributes to the education of Swiss nuclear operators. He graduated with a master’s degree in Nuclear Engineering at the Swiss Federal Institute of Technology in Zürich and Lausanne and joined the Paul Scherrer Institut in 2015 for his PhD. His research topic focused on the relevance of the two-phase flow structure on the retention of soluble gases relevant to nuclear pool scrubbers. The studies included the advancing of established two-phase flow measuring methodologies, the analysis of the hydrodynamics of commercial wet scrubber nozzles and the introduction of a new measuring technique for two-phase flow mass transfer.

Aside his academic work, he was actively involved from 2015 to 2020 in the European Nuclear Society. He helped to increase the networking possibilities of young professionals across the European borders. Thanks to his academic background, he was also participating in the discussion on the attraction of nuclear professionals in Europe which eventually lead to his current working position as an instructor.

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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 injectionInternational Journal of Multiphase Flow, 142, 103694.

Go To International Journal of Multiphase Flow

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