Design and Investigation of a Multistage Axial Contra-Rotating Fan
3.2 Application of Analytical, Computational and Experimental Methods
Axial blowing fans are common in many fields of application. By using an axial fan it should be noted there is a swirl occurring at the trailing edge of the blades due to the working principle. The swirl is not needed in most cases and may have unfavourable influence on subsequent devices, e.g. higher pressure drop or lower heat transfer coefficient. As the static pressure rise is an evaluation criterion for the fan efficiency there are different possibilities for converting the dynamic pressure of the swirl into a static pressure rise. The most common application for rising the efficiency is the installation of discharge guide vanes.
Contra-rotating axial fans (CRF) are well known as one opportunity to increase the efficiency of a fan, too. The efficiency increase is driven by the conversion of the dynamic pressure of the swirl at the trailing edge of the first axial fan blades into a static pressure at the end of the whole stage. In comparison with fans using discharge guide vanes, a higher power density can be obtained.
The development of a multistage axial fan using contra-rotating wheels at each stage is described here. The outer diameter of the fan is 120 mm and the rotational speed for the first and the second fan is are 133.3 s-1 and 83.3 s-1, respectively. The design of the complete fan based upon the design approach for contra-rotating axial fans of CFturbo. The first impeller was designed for a performance of 2/3 of the overall power. Hence, the balancing of momentum was not fulfilled although that is an available design option in CFturbo amongst others.
The contra-rotating axial fan is examined with different methods. Numerical calculations as well as performance measurements and measurements on the velocity profile with optical methods (PIV) are provided to understand the working principle of every stage. The high efficiency and the axial outlet flow without tangential velocity components are demonstrated with these measurements.