Reply To: Kylchap Exhaust Systems
My comment about efficiency was more in relation to where the peak efficiency of a jet pump tends to lay relative to where it is used in a locomotive. As a general rule, for the mass flow ratio (flue gas/steam) found in a coal fired loco you are well to the left of the effy. peak. One way to get round that is to take the flue gas in stages. So the first petticoat is working at a more favourable mass flow ratio (hence greater efficiency), as is the second etc.
The net result should be more draught flow for less steam back pressure – always the goal. As an added bonus you get a better flow distribution across the tubeplate!
I haven’t worked the numbers, but surely sonic limits would only be an issue at the blast pipe orifice – everything else is working at much lower velocities. Another point I only realised this morning is that the flue gas is cooling as it flows through a chimney system, due to mixing with the exhaust steam. Therefore, the flow is de-celerating even in a paralell pipe, because the density is increasing and hence volume is decreasing for a given mass flow.
Perhaps another way to look at things, which addresses some of the other points you raise, is that surfaces usually help to control diffusion of flow and uncontrolled diffusion is the enemy of efficiency. I am afraid that enigmatic statement has taken a career in fluid dynamics to really sink in.