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The squirrel-cage fan with a newly introduced outward inlet lip is studied experimentally. Previous research has demonstrated the importance of inlet flow control on the general flow field and vortex generation of such fans. The bell-mouth inlet, which is the common industrial and academic practice, has drawbacks that result in flow separation and turbulence enhancement. The adapted experimental approach is a conceptual study. It is a combination of standard characteristic measurements and detailed laser Doppler anemometry. The measured flow pattern inside the volute demonstrates that the separated flow behind the inlet lip, which for an ordinary inward lip occupies a large part of the rotor blades, disappears. This is promising since removing this separated flow diminishes a major loss-making region in this fan and adds to the effective flow area. It also reduces noise and gives uniform blade loading. The results also show an improvement in performance in comparison with that of a fan with an ordinary inward inlet lip. This modification is industrially feasible with no extra manufacturing cost and therefore can represent a substantial advance over the current practice.

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Energy conversion in squirrel-cage fans is sensitive to the inlet geometry. It occurs at the inlet where a separation zone which occupies a major volume in the rotor and the volute starts. In this research, different inlets of inward and outward types were tested on two fans. First, the inlet diameter and position were matched with the rotor, which improved the fan characteristic curves. The results of the experiments were sensitive to the width of the blade retaining ring (shroud). Later the tangential and radial components of the velocity out of the rotor were measured. The resulting velocity profiles across the scroll width showed that outward inlets produce a more uniform velocity angle inside the volute than inward inlets did. This was not because of a more aerodynamic flow through the rotor blades but was due to a better match between the inlet and the volute. The axial energy transfer resulted in tangential velocities larger than the rotor velocity, at axial positions across the volute where there was no flow out of the rotor.