Reply To: Porta's Derivation of Lempor Theory

[Martin Johnson]

Hi chris,
Shape of Mixing Chamber – I am currently reading a paper on air ejectors by LJ Kastner & JR Spooner on air ejector design. They make the point that you can design (and calculate) on the assumption of constant pressure mixing (which gives a reducing / expanding chamber) or constant area (parallel chamber) or indeed any combination of the two. They make no conclusions about which might give better efficiency. The paper can be found here:

https://journals.sagepub.com/doi/abs/10.1243/PIME_PROC_1950_162_021_02

It is interesting in other respects:
Convergent – divergent driving nozzles were not found to be best for transonic flow in the driving nozzle.
The theory advanced takes account of increasing entropy in the driving and mixing sections (which Koopman & Porta do not)
It confirms that a diffuser taper of 5 to 10 degrees included is the right range – even with the highly disturbed entry flows.

Chimney Machining – What size chimney are we talking about? If full size, I would suggest casting would be the way to go for complex shapes. That being the case, the problem resolves into making the corresponding male shape which is easy. For model work, boring bars are best if kept to a length / diameter ratio of 5 to 6 max. Even in my wee shop, my biggest boring bar is 25 diameter, giving about 6″ of reach, but remember that reach goes up with the size of hole being tackled.

Martin