Making your own Bellmouths

Recently I had the need for a bellmouth – one of those flared shaped pieces of tube that goes at the beginning of a pipe to aid flow. I could have bought one, but in the end it was cheaper and quicker to make one. Here’s how I did it.

Here’s the starting point - a short length of 50mm diameter, 1.6mm wall thickness aluminium tube. (Obviously, pick the diameter of the tube to match the intake pipe with which you’re working.) I don’t know what grade of aluminium it is – it was just a piece of tube I had lying around. Even if you are making only one bellmouth, you will need sufficient tube to make three or four – it’s very hard to turn out a perfect one first go.

I then grabbed this cone-shaped mandrel. It’s actually a store-bought plumb-bob that just happens to have the right shape. You could also turn one up in a lathe.

Using a hydraulic press, I then forced the mandrel down into the tube. There are a few things to note. Use a lubricant between the mandrel and the tube. Also, see how the base of the tube has distorted? This is because the two supports on which the tube is sitting are forced apart by the mandrel as it descends. To overcome the problem, push the mandrel into the tube in a series of steps, rotating the tube between each step.

Here’s where it gets interesting. The flared tube was then placed in a lathe and a round bar, braced against the tool post, was used to form the flare further outwards. This is effectively ‘metal spinning’, but without the complex former that would normally be used. You need to place a lot of lubricant on the aluminium and steel bar, and take things easily until you see how (a) the metal changes shape, and (b) how much force is required.

To allow the aluminium tube to be held in the chuck without distorting the tube, I first placed a round plug inside the tube. I just used whatever I had lying around – and found this plug that had formed when I cut out a 2-inch hole in a thick piece of nylon with a holesaw. Note in this pic how the steel bar has burnished the inner lip of the bellmouth as it was formed. (Perhaps a piece of hardwood could replace the steel bar, and not mark the aluminium?)

The inner part of the lip was then sanded. Be careful that there aren’t any sharp bits of aluminium to go into your fingers!

The finished bellmouth prior to being trimmed to length and cleaned.

The bellmouth was made for the exit duct in an intercooler end tank. Here it is pictured in place before the tank was completed.

The difference? Do bellmouths make a difference to flow? A few years ago I got a company to put theory to the test. The test was performed on a 3-inch pipe, but just the same idea applies to smaller diameter tubes. Three intake designs were tested on the flow-bench. First up was a straight piece of 3-inch tube, cut off square at the end. The actual ID of the tube was 73mm, giving a cross-sectional area of 41.8 square cm. At a pressure differential of 3 inches of water, the 215mm-long tube flowed 255 cfm. Next up was a 3-inch pipe running a straight flare. Again the tube was 215mm long, with the length of the 11-degree flare being 75mm. The mouth opening was 115mm in diameter, giving an intake area of about 104 square cm. The airflow with this design was noticeably improved, with a measured flow of 318 cfm recorded. This represents a 25 per cent increase over the straight pipe! Finally, a custom-made bellmouth was trialled. Again 215mm long, the bellmouthed intake used the same flare angle as the previous pipe. This time, though, a 22mm tube was rolled around the top, being brazed to the edge of the flare. Plastic body filler was used to fill any imperfections, giving a very smooth radius entry into the tube. And the result? 322 cfm was the measured, up over the straight flare by 1.2 per cent.

What shape? The sharpness of the radius of the lip that surrounds a bellmouth is important. If it is too tight, the airflow ‘unsticks’ on the transition around the corner and the intake flow is reduced. In Axial Flow Fans and Ducts (R. Alan Wallis, 1983, John Wiley and Sons, ISBN 0-471-87086-2) the minimum radius of the bellmouth lip is specified as being best between 0.25 and 0.3 times the diameter of the tube. So for example, a 7.5cm tube should have a bellmouth that has a radius of curvature that is 1.9 – 2.3cm. (Where did these figures come from? 7.5 x 0.25 = 1.9, and 7.5 x 0.3 = 2.3). To turn it into data that can be more readily used, the diameter of a disc that can be nestled inside the outer face of the bellmouth lip should be 0.5 – 0.6 times the diameter of the tube.

Conclusion

If you’ve got a lathe and a hydraulic press, have a go at making your own bellmouths. You can get whatever shape you want and they cost literally only cents to make.