Numerical Representation of the Operating Behavior of a Crossflow Friction Turbomachine

Significance 

In an effort to mitigate anthropogenic climate change, many governments have targeted energy savings to reduce greenhouse gas emissions. Efficient building retrofits can be used to save energy and offset requirements for renewable supply. One of these retrofits is the decentralized room ventilation system. This system is simple and is used for the ventilation of interior damp rooms such as bathrooms, kitchens, toilets and utility rooms. Normally, such systems consist of two separate ducts for fresh and exhaust air, with each being supplied by a separate fan as well as a heat exchanger which recovers thermal energy from the exhaust by transferring it to the incoming fresh air. However, with components such as fans fitted, noise insulation is inevitable for quiet rooms as the bladed fans do not operate acoustically neutral. Moreover, the costs for such devices are high compared to the achievable energysavings. Thus, further investigations and redesigning of the decentralized room ventilation system is imperative.

Researchers have been aware of the aforementioned shortfalls for quite some time. Consequently, several preliminary studies on the heat transfer of rotating discs and on the general functional principle of the Tesla turbomachine have been published; however, no studies on the system considered here are known to the authors beyond those done by their own research group. Therefore, to further improve their system, University Erlangen-Nürnberg researchers: Julian Praß (PhD candidate), Jorg Riedel, Andreas Renz, Professor Jorg Franke and Professor Stefan Becker proposed a new type of device in which the two fans and the heat exchanger are bundled in one compact functional element. This friction fan consists of round discs, mounted on a shaft which rotates in between two separate ducts and thus transports air in two directions while exchanging heat from the warm exhaust air to the fresh inlet air. In use, the system can provide a low priced and retrofittable way of providing a healthy indoor climate with minimal loss of heating energy. Their work on the general operating behavior of the friction fan is currently published in the research journal, Chemical Engineering & Technology.

In their work, stationary Reynolds-averaged Navier-Stokes (RANS) simulations were carried out for the investigations. The researchers used the commercial software ANSYS CFX for numerical simulations of the flow processes in the friction fan. Moreover, for them to determine the influence of crossflows, simulations were carried out on a model of a half fan with five discs and associated duct section in addition to simulations of the flow around a single disc. Altogether, simulations of a model with one disc as well as a five-channel model at different grids were performed.

The authors reported that with almost unthrottled operation, secondary flows could be determined at velocity magnitudes of up to 20% of the mean main flow velocity, with secondary currents reaching up to 50% in throttled operation. Besides high dissipation and recirculation, the secondary currencies were found to be capable of reducing the overall efficiency of the system.

In summary, a friction fan, meant to be used as a room ventilation system with heat recovery, was numerically investigated by means of RANS simulations using a kω-SST turbulence model. The simulations were validated with existing measured values as well as numerical characteristics. Overall, the results indicate that the wake of the rotor exhibits recirculation areas as well as strong crossflows and high dissipation rates. In a statement to Advances in Engineering, Mr. Julian Praß, first author highlighted that for one to optimize the flow mechanics of the system, the rotor wake has to be fundamentally redesigned. Thus, topic of further investigations is the potential of increasing efficiency by means of straighteners and geometric adaptions.

Numerical Representation of the Operating Behavior of a Crossflow Friction Turbomachine - Advances in Engineering

About the author

Department of Mechanical Engineering
Working group: Institute for Factory Automation and Production Systems (FAPS)
Phone number: +49 911 5302-96259, Email: [email protected]

Julian Praß is scientific assistant, head of technology field “Resource and Energy efficiency” at the Institute for Factory Automation and Production Systems (FAPS), Department of Mechanical Engineering at Friedrich-Alexander-University Erlangen-Nürnberg (FAU). Furthermore, he is PhD-student at the Institute of Process Machinery and Systems Engineering (iPAT), Department of Chemical and Biological Engineering. He earned his B.Eng. and M.Sc. in mechanical engineering with focus on fluid mechanics from Technische Hochschule Nürnberg and FAU, respectively.

His current research focuses on heat transfer from rotating discs. In this context, the complex flow processes on non-smooth discs are of particular interest. The focus is on the investigation of the mechanisms of heat transfer and flow losses as well as the resulting thermal efficiency.

About the author

Department of Chemical and Biological Engineering
Working group: Institute for Process Machinery and Systems Engineering (iPAT)
Phone number: +49 9131 8529596, Email: [email protected]

Jörg Riedel is a research assistant at the Chair of Process Machinery and Systems Engineering at the Friedrich-Alexander University of Erlangen-Nuremberg (FAU). He received his Bachelor degree (B.Sc.) and Master degree (M.Sc.) in mechanical engineering at the Friedrich-Alexander-University of Erlangen-Nuremberg (FAU). His doctoral thesis deals with the investigation of wind noise generation in hearing aids. In this context numerical methods of fluid mechanics (CFD) as well as experiments in the aeroacoustic wind tunnel are applied. During his master thesis he was engaged in the simulation of a novel fan system consisting of rotating discs.

About the author

Department of Chemical and Biological Engineering
Working group: Institute of Process Machinery and Systems Engineering (IPAT)
Phone number: +49 9131 85-29464,  Email: [email protected]

Andreas Renz is scientific assistant at the Institute of Process Machinery and Systems Engineering (iPAT), Department of Chemical and Biological Engineering at Friedrich-Alexander-University Erlangen-Nürnberg (FAU). He earned his B.Eng. and M.Sc. in mechanical engineering with focus on fluid mechanics and technical thermodynamics from Baden-Wuerttemberg Cooperative State University and FAU, respectively. He has also worked for four years as an engineer at /H&B/ Electronic GmbH & Co. KG developing handheld medical devices.

His current research focuses on the aerodynamics and aeroacoustics of friction ventilators. The main objective of the research is to investigate the momentum transport from the rotating ventilator’s discs to the fluid, the occurring losses and the aeroacoustic sources of such a device.

About the author

Head of Chair
Department of Mechanical Engineering
Working group: Institute for Factory Automation and Production Systems (FAPS)
Phone number: +49 9131 85-27569,  Email: [email protected]

Jörg Franke studied production technology at FAU before receiving the title Dr.-Ing. in 1995. He then became consultant, senior consultant and project leader at McKinsey & Company, Inc. before he took the responsibility for the Japan-strategy of Robert Bosch GmbH. In 2000 he became member of the upper management circle at ZF Lenksysteme (ZFLS) GmbH. In 2005 he changed to INA Schaeffler KG as head of all national CEOs before becoming CEO and CFO at ABM Greiffenberger Antriebstechnik GmbH. Since 2009 he has been Head of Institute for Factory Automation and Production Systems (FAPS) with over 80 scientific assistants distributed over seven research sectors. His scientific focus lies on assembly automation and electronics production as well as production of electric drives and Computer-aided development and simulation of production processes.

Among many other voluntary activity, he is speaker of the DFG Research Group Optical Packaging and Interconnection Technology, member of the Supervisory Board at SHW AG, Aalen, member of the Board EnergieRegion Nordbayern e.V. Research Association for Energy, spokesman of the Department of Mechanical Engineering at FAU, member of the International Academy for Production Engineering (CIRP), and member of the Executive Board of Bayerischer Cluster Mechatronik und Automation (CMA) e.V.

Furthermore he is consultant for various research promotion agencies, e.g. AiF, BMBF, DFG, IEEE, Landesstiftung Baden Württemberg, Austrian Accreditation Council, Austrian Research Promotion Agency (FFG), Italian Ministry of Education, University and Research (MIUR), International Journal of Production Economics (Elsevier), IEEE projects in automation science and technology, scientific working group in plastics technology, etc. He offers 17 lectures, one seminar and seven internships during winter semesters as well as 18 lectures, one seminar and nine internships during summer semesters at FAU. Besides those, he offers five virtual courses that can be taken throughout the year.

About the author

Department of Chemical and Biological Engineering
Working group: Institute of Process Machinery and Systems Engineering (IPAT)
Phone number: +49 9131 85-29455, Email: [email protected]pat.fau.de

Stefan Becker received his diploma in fluid mechanics and thermodynamics from Technical University Dresden. He concluded his PhD about flow around three-dimensional obstacles in wall boundary layers in 1996. Within his habilitation, he investigated local measuring methods and their application for determining the flow and turbulence characteristics of wall-bound flows. Besides fluid mechanics, he teaches machine acoustics, flow acoustics, turbomachines and turbocompressors. He is member of the German society for Acoustics, the European Research Community on Flow, Turbulence and Combustion, German Society for Aviation and Aerospace and the German University Network.

As head of research sector “Fluid System Dynamics and Flow Acoustics” he currently focusses his research on efficient low-noise turbomachines, automatic optimization of flow systems, and the development of modern design methods for axial and radial impellers as well as flow-acoustic calculation methods and experimental investigation procedures.

Reference

Julian Praß, Jorg Riedel, Andreas Renz, Jorg Franke, Stefan Becker. Numerical Representation of the Operating Behavior of a Crossflow Friction Turbomachine. Chemical Engineering & Technology 2019, volume 42, No. 9, page 1853–1860.

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