Reply To: Jos Koopman Conundrum

[Chris Corney]

Hi Martin,
It’s great to have your contribution to the discussion.
If I can comment on a few points, regarding “chuffing”, in the electrical engineering world we have the oxymoron of “steady state alternating current” which is a very valuable concept, and very usable mathematics has been developed to carry out calculations. All this is of course based on sinusoidal waveforms and components with linear characteristics, which is unlikely to be the case with steam locomotive exhausts.
I see the de Laval nozzle as operating a little like a “switch-mode” power supply in electronics, with the nozzle changing rapidly between subsonic and supersonic operation. The ASTT are carrying out tests, when the opportunity arises, using a data logger with a 1kHz sampling rate. It would be interesting to what proportion of the blast pipe pressure is above the critical value for supersonic flow, when the locomotive is working hard. Of course the nozzle size would also influence this. I think as well that there would be a shock travelling up and down the nozzle with each chuff. I don’t know what the effect of this would be, but it might be possible to adjust the nozzle design to mitigate its effect.
You mention that de Laval nozzles are possibly not relevant to heritage steam, but steam does operate on the “big railway” over hauling loads over Shap etc. often in excess of those in BR days, and sharing the track with Pendolinos etc.
Finally you mentioned the Giesl. In his analysis, Jos Koopmans concluded that the design of the Giesl was flawed. If I remember correctly he was unhappy with the ratio of the orifice area to the area of the choke.

Chris