Reply To: Jos Koopman Conundrum

[Martin Johnson]

Thank you both. But I am not sure you really answer the conundrum.

Chris – I am not convinced the DeLaval nozzle has an influence here. However, the time dependent regime (chuffs) is something I think is not well understood, and something I want to put a few words about on here.

John – If you look at the sidebar item “Lempor CFD?” on the page I gava shortcut to, you wil see that both CFD models are of a Lempor style arrangement, the only difference being the first example (2 plots on of pressure, one of velocity) has the blast nozzle level with the intake bellmouth, while the second (2 plots again) has it below the intake bellmouth. I cannot see how the statement by JK squares with the results the CFD plots are showing.

I think your other comments are not really relevant to the present case. But in the interest of stimulating discussion, I notice that Porta claims a typical inlet bellmouth loss of 0.04 – whereas ESDU 85032 (and my own experience of 40 years in fluid flow) suggests at least 0.05 and probably nearer 0.1.

Similarly Porta claims a nozzle discharge coefficient of 0.99 (inferred from his 1.01 factor tacked onto Equation 9). A well designed slow taper nozzle might give a Cd of 0.94 or so. What a DeLaval nozzle would give when working off design point is anybody’s guess – and DeLaval nozzles have quite a tight optimum performance band, which is hardly likely to suit heritage rail applications. Back in the day applications London to Edinburgh flat out, for example might be a different matter. What I think has happened is that Porta’s number “taken from tests at Rugby” does not account for the velocity head of steam – but I have no proof of that. I am currently struggling to reconcile EG Youngs test results which lack clarity as to where blast pipe pressure was measured and whether it includes significant velocity head.

Once again, Porta claims a diffuser efficiency of 0.8 to 0.85 with an area ratio of 4. Data in D.S. Miller – Internal Flow Systems 2 suggests you would need a diffuser at least 16 inlet diameters high to achieve 0.75 efficiency. I’d like to see that go through a tunnel!

So it is quite easy to convince everybody that your system is best if you bury some assumptions to that effect deeply enough. Am I missing something? Or does everybody else wear rose tinted spectacles?

Martin