Numerical and Experimental Investigation of the Velocity Field in Friction Ventilators
3.4 Fan Design for Improved Efficiency and Extended Operating Range
In an effort to develop a new and simple concept for a decentralized ventilation system a cross flow friction ventilator was investigated. This friction ventilator consists of multiple circular discs which are driven by a motor and are rotating centrally in between two ducts (inlet and outlet duct of the ventilation system). The drag between the disc surfaces and the fluid induces a countercurrent flow in the two ducts while the discs also act as a heat exchanger between the two air flows. While the concept of the friction ventilator met the basic requirements for volume flow, pressure rise and heat recovery, the hydrodynamic efficiency found from experiments and simulations was lower than expected. In this study, we used Laser Doppler Anemometry to investigate the velocity field at different rotor geometries and operating points. Furthermore, we investigated the secondary currents in the ducts by means of numerical simulations in order to explain the low efficiencies.
The velocity field was measured in two areas, where on area of interest was the section between the discs themselves. The measurements there concentrated on the buildup of the wall boundary layer and its development along the disc. The other area was the velocity field up- and downstream of the friction ventilator.
As a result, we found that the wall boundary layer at the disc surface is thin compared to the distance between the discs and that a large part of the flow is only mildly affected by the rotating surfaces. The LDA measurements showed further that even in optimal operating points the flow field downstream of the friction ventilator was highly turbulent and could be non-uniform from disc duct to disc duct, depending on the rotor design. The numerical investigations showed that in almost unrestricted operation, secondary currents could be determined at velocities of up to 20% of the mean main flow velocity, with secondary currents reaching up to 50% in throttled operation. The investigations contributed to a better understanding of the velocity field of this kind of ventilator, also to the optimization of the rotor geometry and to the flow over rotating discs in general. Although the flow along the discs and the turbulent areas downstream were measured thoroughly, further investigations need to be made to fully understand the energy transfer from the disc to the air flow and the hydrodynamic losses that occur.