Stall margin improvement is a crucial issue during fan design, selection and installation. In fact, several industrial fans are operated in extreme applications and requested to be highly flexible in order to be able to withstand abrupt changes in operating conditions. This is the case of fans operated under distorted inlet conditions or interacting with other fans or used for reversible operations in tunnel and metro applications.

This paper reports on a systematic experimental study on the effects of the use of casing treatments on the performance of a single stage, reversible, axial fan. Tests focused on the performance of up-stream single treatment and up- and downstream double treatment, respectively designed to prevent the fan to run into stall in forward-only operations and forward- and reverse- operations. The study reports on the assessment of the change in performance, in normal and stalled operations, with an emphasis on the relative axial position between casing treatments and the impeller blades. The analysis demonstrates the achievement of significant improvements in performance in the unstable region for all the tested configurations with marginal losses in the stable counter-part.

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