Chris Corney
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Hi Martin,
Good points,we should also consider how much steam is used by auxiliaries (injectors etc.) and doesn’t pass through the blastpipe, and then there is leakage!
Hi Martin,
I thought that JK’s book was very good as a historic record, and useful as piece of reference material, but if you are looking for a textbook on fluid mechanics, there are better options available.Regarding nozzles etc., in my mind the jury is still out until we can get some decent test results from a main line sized locomotive such as an S160. There are some people putting these sort of devices on locomotives with slide valves and saturated boilers, which is probably a waste of effort.
On the other hand, some people would say that messing about with steam engines is a waste of effort!Regards
ChrisOn page 140 of “The fire burns much better…” at the end of section 7.6.3, Koopmans says “It is conceivable that a properly designed petticoat could act as a sequential ejector. However, this approach does not appear to have been the subject of any investigation.” Does anyone have any more information on sequential ejectors?
Martin,
Looking at Equation A28.1 on page 446 of Jos Koopmans book “The Fire burns much better…”, Porta considers kinetic energy at each stage, however he also includes terms 2 and 3 which cover the shock losses included in the mixing chamber. Am I correct in thinking that these terms would allow for the difference between the results obtained by conservation of momentum and conservation of energy?
Chris
Hi Martin,
I was looking at my copy of David Wardale’s book “Red Devil and other tales from the age of steam” last night.
It’s now a few years since I read it. There are some references to comparative trials between de Laval and plain nozzles, although he describes the various projects he worked on in chronological order, and the nozzle trials are mixed in with all the other aspects of design he worked on.Going back to my post on 29 Jan, the description of operation of the nozzles is on page 97, not 92.
Regards
ChrisHi Martin,
If we were talking about a turbine engine, I would agree entirely with you.
For a simple expansion engine (and probably a compound) the engine is unable to fully expand the steam, and there is considerable residual energy in the steam when the exhaust valve opens. Porta’s argument (as described in “Red Devil”) is that the de Laval nozzle utilises this pressure close the dead centre position of the piston (when back pressure doesn’t matter) and as a trade off, the back pressure can be lower during the piston mid stroke. Obviously with a two cylinder engine, both pistons will be subject to the same back pressure, and Porta introduced the “Kordina” which was a crude ejector arranged so that the draught from one cylinder reduced the back pressure in the other cylinder.
(Strictly speaking, a Kordina is a concentric device. The more common arrangement where the two exhausts merge as two semi circlular pipes is known as a “Goss wall”)
My suggestion is that the de Laval “chokes” should be located in the Kordina, to reduce the back pressure even further, but I don’t think anyone has designed such a system.
Regarding the argument of “off beat” engines, you obviously have to consider the overall efficiency of the locomotive, and I’m sure the engine with correctly set valves would be better in this respect.
It is true that de Laval nozzles were rare. Apart from the Lempor, the only other example I am aware of is the 4-8-4 Niagara on the New York Central.
Regards
ChrisHi Martin,
I’m not sure that storage would be a good idea. It’s been noted that an “off-beat” engine steams better than one with the valves set correctly, and the three cylinder Jubilee required a smaller diameter blastpipe than the two cylinder Black Five to steam, despite the two having similar boilers. I think the peak value of steam flow through the blastpipe is important, so from this point of view storage would be detrimental.Regards
ChrisHi 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
My background is electrical rather than fluids, but I’m just wondering if the de Laval nozzles in the Lempor amplifying the effect of peak flows is relevant. The is described on P92 (if I remember correctly) of “Red Devil”. I’ve come across the effect of pulsing flows on non-linear devices in electrical applications before. The effect does exist, but is very difficult to analyse mathematically. From previous comments on the “Nat Pres” forum, Jos Koopmans denies that this makes any difference, but I think it could explain any differences between calculated and measured results. I hope this is relevant to the question!
Assuming the locomotive driving wheels are supported on rollers, I would think it’s relatively straight forward to design them for multiple gauges. A similar priciple to dual gauge or multigauge track. I think one of the locomotive manufacturers (I can’t remember which – possibly Hunslet) had a multiple gauge test track on which it could run locomotives of most gauges.
Yes, I like the idea, especially with modern instrumentation. The Lynton and Barnstaple is a the same gauge as the Festiniog, so it would be good for their new-builds.
And now for my next question!
Given the advantage of the Kylchap on the “efficiency curve” principle described above, and if I’ve understood the explanation in Wardale’s book “Red Devil” correctly, the basis of the Lempor exhaust is the de Laval nozzle. These oscillate between supersonic flow/high back pressure during the “chuffs” and subsonic flow/low back pressure between the chuffs. Since the high back pressure more or less co-incides with the dead-centre position of the piston, it gives an improvement in efficiency.
I’m just mulling over what would happen if you combined de Laval nozzles with a Kylchap chimney.Chris
Martin,
I don’t know if your familiar with Jos Koopmans’ book “The Fire burns much better…” which is included in the Books for Sale section of this website. It’s basically the thesis of his Ph.D. where he made a detailed study of the history of the design of locomotive exhausts. It’s a bit heavy-going for the average enthusiast, but with your knowledge, I’m sure you would find it interesting. The reason I mentioned it is because Jos was quite critical of the Bulleid-Lemaitre exhaust. Jos identified some key proportions which he had distilled from test results of locomotives with successful draughting, and I’m thinking that if you combine this with your approach using the efficiency curve, it should be the basis for a good design.
Chris
Hi Martin,
There are some effects on the chart records from the tests that we don’t fully understand. My opinion is that it’s a resonance in the chimney of the Lempor, but other people have other opinions. I think there would be too much damping to get a resonance between the smokebox and firebox, but I could be wrong.
Chris
I think I’m correct in saying that the smokebox pressure returned to “atmospheric” between each “chuff”.
A while ago we had a discussion about pulsating smokebox pressure. My background is as an electrical engineer, and we are working with pulsating quantities all the time (i.e. ac). Adapting a method from this area, it occurred to me that there might be some advantage in having a double chimney with one being a Kylchap and the other being a Lempor. For each “chuff”, due to the difference in steam operating velocities, it would take slightly longer for the Lempor to reach “terminal velocity”, by probably a few tens of milliseconds, than the Kylchap and therefore the pulses of suction from the two chimneys would be slightly out of step. The overall magnitude of the oscillation would be reduced, compared with an equivalent double Kylchap or double Lempor. Following the discussion on the “Kylchap” thread I’m now thinking that under these circumstances the Lempor could be acting as a more efficient pump, if pulling against the vacuum which has just been created by the Kylchap.Obviously this would require a lot of work to optimize!
Does it make any sense?
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