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 k–ω-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.
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.