Reply To: Jos Koopman Conundrum

[John Hind]

Currently, I am reading his book but not completed it. I think JK’s initial comment applies to a ‘coventional exhaust’ ie single blastpipe and chimney which may or may not be taper (diffuser).

The CFD is for a Lempor Exhaust, which uses 4 convergent/divergent nozzles which discharge into a mixing chamber and then into a diffuser.

In a conventional exhaust if the blastpipe is too close to the chimney, the exhaust can be ineffective as the jet from the blastpipe does not fill the chimney and you get a backflow into the smokebox. I am currently dealing with a locomotive which has been modified by others and I have been told that this happens (I have not seen it as I have not had a chance to ride the engine) but I have seen evidence in the chimney that it is happening.

We are doing some more drawings to check whether the jet of steam from the blastcap actually fills the chimney as it expands and entrains the flue gasses as they flow from the smokebox up into the chimney.

If the steam and the flue gasses do not fill the chimney then you get back flow into the smokebox.

In the 1930’s rules of thumbs were established for the spreads of the steam jets as they emerged from the blast cap and in the chimney. The spread of the jet in the chimney is less than the spread of a jet coming out of the blast pipe. I am over-checking these rules of thumb against current research in other fields.

For a Lempor, Porta’s recommendation was that the nozzle exhaust is at the bottom of the mixing chamber. We followed this recommendation on the Lempor we fitted to S160 No 5820. The nozzles are angled so that steam jets impinge the mixing chamber walls so that there is thorough mixing before the entrance to the diffuser, so that there is a ‘flat velocity’ profile and turbulent flow which diffusers need to work effectively. If the exit from the nozzles is too far away from the mixing chamber then the jets may not even enter the mixing chamber. This is the opposite of a conventional exhaust.

Regarding the CFD examples I cannot really comment on the results as I do not know enough about the configuration and the flows he was testing.

I do not believe you compare how you configure a Lempor with a conventional exhaust.

Over the next couple of weeks I will be tabulating steam and gas flows for typical locomotive exhausts (using Rugby Test Data) in the hope that we can persuade someone to do some free CFD studies for us, then we will be able to evaluate the CFD against actual test result and have a useful tool.

JK has done a service by compiling the history of exhaust system development and offering some ideas of his own guided by his tutors, however, he is handicapped by having to communicate with the rest of the world in his second language. Heseems to have overlooked research in other fields that could help us to understand one of the great unsolved mysteries in Fluid Dynamics – what is the optimum steam locomotive exhaust!!!