The present research focuses on the efficiency improvement at part-load of a centrifugal fan for a 30 kW fuel cell combined heat and power (CHP) unit. For this purpose, the fan stage is equipped with a partially vaned diffuser with a variable cross-sectional area using a moving backplate.

The design and the performance of the partially vaned diffuser with a variable cross-sectional area are described in the first part of this paper. The performance results are compared to measurements of the same centrifugal fan with a vaneless diffuser carried out for the previous investigation. For the second part, the influence of the variable cross-sectional area on the diffuser flow field is investigated using optical PIV (Particle Image Velocimetry) measurements and CFD (Computational Fluid Dynamics) simulations.

The combination of a variable cross-section, partially vaned diffuser was able to achieve a 10 percent increase in pressure ratio, a 5 percentage points increase in part-load efficiency while maintaining the whole operating range of the vaneless, constant cross section reference design.

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