Due to the gap between the impeller and the casing of a radial fan a gap flow rate occurs, leading to a higher flow rate through the impeller than through the fan. The gap flow rate has an impact on the performance of the radial fan and must be considered in the design process. There is only little known from the literature about the gap flow rate. Furthermore, as to the knowledge of the authors, no information on the effect of scaling on the gap flow rate is reported in literature.

In this work a systematic study of the effect of scaling on the gap flow rate has been performed. For this first a baseline radial fan has been designed. This model and its full characteristics have been computed with the commercial Navier-Stokes solver Ansys CFX. Then the impeller width, the outer diameter, the inner diameter, the gap width, and the rotational speed were varied. These designs have been simulated with the commercial Navier-Stokes solver Ansys CFX. The effect of these parameters on the performance characteristics pressure and efficiency and its impact on the gap flow rate were evaluated.

Based on these results, a dimensional analysis model for the gap flow rate have been developed and evaluated with the full characteristics and the main dimensions of all simulated models. It has been shown that the gap flow rate can be estimated based on data that is typically available, such as pressure, volume flow rate, speed, outlet diameter, outlet width, gap width, fluid density and dynamic viscosity. The developed dimensional analysis model was validated with a test case. These results are presented and discussed in detail.

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