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.
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.
Velocity pro les outside the rotor of four squirrel cage fans are measured in order to calculate their local slip factors. They show that the uid exit angle from the rotor and the blade outlet angle of such fans are very different. Inlet con guration and volute spread angle both affected the direction of the ow out of the rotor and hence the slip factor. The general understanding in centrifugal turbomachines is that more energy transfer per unit mass is equivalent to a larger tangential component of velocity and therefore a larger slip factor. In squirrel cage fans a small slip factor results from a large radial velocity component out of the rotor. This gives a larger volumetric owrate with no sensible head loss. The advantages of a large incidence angle and a large deviation mean that ow adherence to the blades is not always a prime design criterion in such fans.
Squirrel cage fans with non-cylindrical rotors, shaped as frustums of right circular cones, are compared with a similar fan but with a cylindrical rotor. Performance measurements for cone angles ranging from 2108 to þ108 showed that for positive angles, which means that a larger diameter at the rotor inlet decreases towards the back plate, it was possible to have higher fan efficiencies for a similar head. Alternatively, at negative cone angles, the head coefficient could be superior for a similar efficiency. The optimum angles were different for each case. These statements are true for the working part of the performance curves, which is located to the right of the maximum head coefficient. The velocity profiles at selected circumferential positions at the rotor exit showed that the flow did not spread evenly round the rotor when fan geometry was changed or the fan was throttled. If air flowed out of the rotor at a position closer to the fan exit, this flow then travelled a shorter distance inside the volute. A smaller loss and, therefore, a higher efficiency ensued. This pattern was true for both cylindrical and conical rotors and, therefore, the better efficiency for the positive half-cone rotor was a result of the way the rotor and volute combination guided the flow. The negative cone rotor had a higher head as the rotor diameter was larger where the flow passed through the blades.
This article presents an improvement for the blade configuration of a squirrel cage fan. Numerical simulations have shown that a −2 • lean angle applied to a half-cone rotor of +10 • produces a higher efficiency. This is expected to be a result of blade alignment with the inlet stream. Performance curves in this article compare this new rotor with both a simple half cone of the same cone angle and a corresponding cylindrical rotor. The half-cone rotor with leaning blades had improved head coefficient and efficiency in the normal operating range, which is to the right of maximum head or efficiency points. The simple half-cone rotor had almost constant efficiency for all flow ranges, which is a very demanding feature for any fan. A study of velocity profiles inside the volute showed that maximum flow exited the rotor close to the cut-off. A leaning blade had negligible effect on radial and tangential components at this location but it changed the axial component. This was due to a movement of the vortex behind the inlet and a larger active region of the rotor blades.
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