Reply To: Porta's Derivation of Lempor Theory

[Martin Johnson]

I have been doing some data mining on E.G. Young’s paper of 1930. First step – what is a realistic Cd for a steam nozzle? I have accounted for velocity pressure in the pipe where (I have inferred) Young took the pressures and come up with a scatter around 0.85 for all the nozzles which underwent extensive testing. The “Pepperbox” averages 0.78.

Now the proportion of energy used is Cd ^2, so about 70 to 65% of the incoming energy is being usefully turned into velocity energy. That still leaves at least 30 % which goes to increase the temperature, and hence the specific volume, and hence the velocity of the remaining gases, leading to even more energy going up the chimney to waste. So a properly designed blast nozzle is essential. It is quite possible to design nozzles with Cd values up around 0.95 but they tend to need axial length – which is what a steam loco seldom has available.

That leads on to another observation – neither Porta or ESDU 85032 calculations account for density and temperature changes due to increasing entropy of steam and gas. I believe the latter to be insignificant, but the former would give around 8 degrees increase in steam temperature. Not large, but it can be accounted for.

I have also continued attempting to resolve Porta and ESDU 82032 – without success. I cannot see where Porta accounts for momentum.

Martin