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Abstract

Centrifugal fans are common in many fields of application. A centrifugal fan consists of a drive, an impeller, and a volute. The volute casing is designed to turn the dynamic pressure at the outlet of the fan impeller into static pressure and to guide the flow towards a connected geometry, e.g. a flange. However, centrifugal fans without a volute are being used more and more often. A common application is the installation in an air conditioning unit, where typically the dynamic pressure at the outlet is dissipated. In this way, both manufacturing costs and installation space can be saved. On the other hand it must be noted that the air at the outlet of the fan is strongly swirled, which may lead to inadequate acoustic and aerodynamic behaviour of the flow in subsequent devices. These two disadvantages of a fan without casing led to the consideration of a contra-rotating fan.
The opportunities of a contra-rotating fans are well known and examined for axial fans, but rarely investigated for centrifugal fans. In this approach we present the design and the measurement of a contra-rotating radial fan comprising two concentrically rotating impellers and two drives. The fan is designed for a volume flow of 150 m³ h-1 and 125 Pa (static pressure). The diameter is 150 mm and the rotational speed of the inner impeller (impeller A) and the outer impeller (impeller B) is 3000 min-1 and 1500 min-1, respectively.
The impeller were designed by means of CFturbo latest approach for the calculation of contra-rotating fans. This design’s prototype was created by means of 3d printing and measured on a test-rig as well as examined by means of CFD.
The results indicate that the simulations are in good agreement with the experiment and provide more detailed insights into the internal flow structure of the impellers. Based on these findings, investigations were made at the leakage point between the two impellers. The results of the test rig measurements and the laser optical measurements demonstrate the influence of different sealing geometries on the leakage.
It has been shown that, compared to a centrifugal fan without volute, a contra-rotating fan can significantly reduce the power loss due to swirl at the outlet and increases the power transfer to the fluid compared to a diffuser with guide vanes at the same time.