Thread: Bell mouth hood entry for ventilation impellers; some performance info

** WARNING ** Nerdy technical data ahead.

Still bored with this broken ankle and browsing for "stuff" on the internet and I stumbled across this.
I've seen some performance info on Bell Mouth Hoods for regular air intakes but this is the first time I have seen data specifically for impellers even if they are really are only ventilation type impellers.

The article is
Journal of Mechanical Science and Technology 25 (9) (2011) 2267~2276
Effects of bell mouth geometries on the flow rate of centrifugal blowers
by Pham Ngoc Son, Jaewon Kim and E. Y. Ahn

The setup investigated was quite different to a standard WW DC impeller but the results could be useful to anyone setting up blowers without ducting for venting.

This is what the tested impellers look like.
Screen Shot 2020-02-28 at 4.34.38 pm.jpgScreen Shot 2020-02-28 at 4.33.27 pm.jpg

The impeller was a squirrel cage (300mm) centrifugal impeller with 48 blades - CF a conventional WW type impeller which typically has around 6-8 blades.
The impeller blades were only about 25 mm wide
The rotational speed of the impeller was 500rpm only and flow rate was around 350 cfm.
The opening into the impeller was relatively wide - nominally 250 mm or 83% of the impeller diameter. For WW DCs its 50%.
The impeller was not connected to ducting so for optimum effect had the BMH on the outside of the impeller.
The Static Pressure was very low - only 69Pa or 0.25"WC, WW impellers are usually around 8-10"WC

CONCLUSIONS:
For this set up the
1) Bell mouth radius has a major effect* on the flow rate of the centrifugal blower.
2) Optimum inlet radius was 86% of the impeller radius
3) Too small an impeller opening radius will result in a noticeable loss* in the flow rate because of the appearance of a vortex under the bell mouth region.
4) Optimum bell mouth radius ratio was approximately 9% of the blower radius (9% is small - it's only a 13.5mm high blip/bump in the impeller metal casing, CF WW and loudspeakers inlet use a 50% ratio)
5) Larger bell mouth radii will produce further small increases in flow but are not always physically possible and also cost more to make.
6) Noise loudness and frequency were not altered significantly by the impeller inlet radius or the bell mouth radius




* (author's words) my understanding was the "major" effect and "loss of flow" were around 6% between best and worst scenarios/tests.

Bear in mind this is quite different setup to wood work impellers which operate at greater flows and pressures, but it does appear to show that using bell mouth hoods even at low pressures and flows may be worth considering.