Bachelor's thesis Tobias Czarnecki

 

Design and measurement of a counter-rotating axial compact fan

The use of two serially connected contra-rotating axial fans (CRF) allows on the one hand the reduction
of the rotational momentum at the outlet of the second impeller and on the other hand, a
higher power density compared to a conventional configuration of impeller and discharge guide vanes.
In practice this is already being used to expand the application range of axial fans up in higher
differential pressures. In conventional drive concepts both impellers are driven by their own motors
or via a gear. At ILK Dresden, a gearless one-engine concept for a compact CRF was developed
(nominal point 650m3/h, 2100 U/min, fan diameter 200 mm).
First, the experimental setup of the demonstrator is modified. An optimization of the mounting
concept requires a redesign of the ball bearings. An improvement of the electric power supply of
the demonstrator is to be achieved through the commissioning of a contactless inductive power
transfer. The first successful installation of the prototype in the mode of continuous operation was
achieved. Later, the prototype was integrated into a test pipe and measurements of all the relevant
fluid mechanical characteristics were performed. A significant increase of both the efficiency and
performance of the CRF are detected by fan characteristics measurements compared to the configurations
of the first impeller alone and in combination with two dischare guide vanes.
The performed measurements of the velocity field by means of stereo particle image velocimetry
(stereo PIV) could successfully visualize the improved operating characteristics. In addition, sound
power spectra were recorded from all configurations in several operating points. This allows a statement
about acoustic advantages and disadvantages of each configuration.
Significant improvement potentials were detected by the Stereo PIV-measurements in an adjustment
of the blade geometries to achieve a more uniform axial flow between the hub and the outer
tube. In addition, further efficiency improvements are expected when the rough surfaces of the impeller
caused by the manufacturing process are replaced by impellers made out of aluminum.