Jos Koopman Conundrum
January 29, 2019 at 8:23 am #5055
In the course of surfing about, I found the following:
Take a look at the sidebar at bottom left which gives details of a paper delivered at York (ASTT perhaps?). In the “Answers to Mr. Wardale’s Comments”, Page 12 under “Orifice to chimney distance” purports to show how a blast pipe mounted below the chimney throat gives improved draught. However, if you look at the “Lempor CFD” button, you find (second and fourth figures):
Blast pipe in throat – pressure change -570 (Pascals?) to 5.13 x 10^3 at chimney top (delta p = 5600)
Blast pipe below throat – pressure change -856 to 2.65 x 10^3 at chimney top (varies a bit, but give it the benefit of doubt) hence delta p = 3506
In both cases the blast pipe is running at just over 100 m/s and the terminal velocities are 38 and 34 m/s respectively, suggesting steam flow and gas flow are the same in both cases.
Does this not show the precise opposite to Koopman’s assertion? Am I missing something?
MartinJanuary 29, 2019 at 9:37 am #5056
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!January 30, 2019 at 2:48 pm #5057
Currently, I am reading his book but not completed it. I think JK’s initial comment applies to a ‘coventional exhaust’ ie single blastpipe and chimney which may or may not be taper (diffuser).
The CFD is for a Lempor Exhaust, which uses 4 convergent/divergent nozzles which discharge into a mixing chamber and then into a diffuser.
In a conventional exhaust if the blastpipe is too close to the chimney, the exhaust can be ineffective as the jet from the blastpipe does not fill the chimney and you get a backflow into the smokebox. I am currently dealing with a locomotive which has been modified by others and I have been told that this happens (I have not seen it as I have not had a chance to ride the engine) but I have seen evidence in the chimney that it is happening.
We are doing some more drawings to check whether the jet of steam from the blastcap actually fills the chimney as it expands and entrains the flue gasses as they flow from the smokebox up into the chimney.
If the steam and the flue gasses do not fill the chimney then you get back flow into the smokebox.
In the 1930’s rules of thumbs were established for the spreads of the steam jets as they emerged from the blast cap and in the chimney. The spread of the jet in the chimney is less than the spread of a jet coming out of the blast pipe. I am over-checking these rules of thumb against current research in other fields.
For a Lempor, Porta’s recommendation was that the nozzle exhaust is at the bottom of the mixing chamber. We followed this recommendation on the Lempor we fitted to S160 No 5820. The nozzles are angled so that steam jets impinge the mixing chamber walls so that there is thorough mixing before the entrance to the diffuser, so that there is a ‘flat velocity’ profile and turbulent flow which diffusers need to work effectively. If the exit from the nozzles is too far away from the mixing chamber then the jets may not even enter the mixing chamber. This is the opposite of a conventional exhaust.
Regarding the CFD examples I cannot really comment on the results as I do not know enough about the configuration and the flows he was testing.
I do not believe you compare how you configure a Lempor with a conventional exhaust.
Over the next couple of weeks I will be tabulating steam and gas flows for typical locomotive exhausts (using Rugby Test Data) in the hope that we can persuade someone to do some free CFD studies for us, then we will be able to evaluate the CFD against actual test result and have a useful tool.
JK has done a service by compiling the history of exhaust system development and offering some ideas of his own guided by his tutors, however, he is handicapped by having to communicate with the rest of the world in his second language. Heseems to have overlooked research in other fields that could help us to understand one of the great unsolved mysteries in Fluid Dynamics – what is the optimum steam locomotive exhaust!!!February 2, 2019 at 11:40 am #5058
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?
MartinFebruary 3, 2019 at 9:03 am #5059
I have just realised that in my second to last paragraph I give 16 inlet diameters to achieve something around 75% effy on a diffuser, that should be 8. The design chart I work from (Miller, D.S. Fig.11.3) gives the length in radii, not diameters.
We are all fallible (me included as just demonstrated), However, if there are glaring contradictions in publicly quoted material, and considerable exaggeration in work by Porta it needs to be aired.
Porta gives comparisons of performance between various locomotives his Fig. 3 (His Rio Turbio 2-10-2 being top of the league, a Geisl 9F being about mid way) – does anybody know if the curves quoted are all TESTS or is he comparing test with calculation?
MartinFebruary 3, 2019 at 2:00 pm #5060
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.
ChrisFebruary 3, 2019 at 4:12 pm #5061
I begin to see your point, the nozzle may well flip sonic to sub-sonic and it would produce some “interesting” effects. Like you, I am eager to see just how much of an issue it is from the S160 tests.
With regard to designing with DeLaval nozzles, I tend to come at the problem from a designer’s point of view (since that is what I was). Ipso, if you can avoid supersonic flow, then do so as it eliminates the issues noted above. I also note that many locomotives have minimal storage volume in the exhaust system, so there is no storage to even out the flow reaching the blast nozzle. I realise there is not much room at the front end of a loco to put such storage. However, I have been making an effort over the last few years to look at steam engine design from fundamentals and if theory dictates that there ought to be storage, then space would need to be designed in. That is also how I come to be picking holes in what others have said before – as the song goes “It ain’t necessarily so”
MartinFebruary 3, 2019 at 4:58 pm #5062
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.
ChrisFebruary 5, 2019 at 11:38 am #5068
Well yes, that might be so. But an offbeat engine means the valve events are not as they should be, so more energy is going up the stack and less into turning the wheels. Therefore, more energy is being expended in the blast pipe, meaning more energy to draw the fire, to make more steam to feed that hungry off-beat engine.
As to the relative merits of a Jube Vs. a Black 5, I do not really know enough detail of the two designs to comment.
I remain un-convinced that peak flow through the blast is a good thing. The more “peaky” the flow, the more likely you are to be running part of the cycle in the sonic regime. However, if you can smooth things out, you should be able to run the whole cycle sub-sonic. The attraction of sub-sonic is that a basic contracting nozzle will work well across the whole flow range. Once you start putting DeLaval nozzles on, it will be great if you get sonic velocity at the throat, the expanding section will INCREASE exit velocity. However, if you do not get sonic velocity, the expanding section simply re-expands the steam and you DECREASE exit velocity. So at anything below design point, you are making things worse, not better – unless somebody can explain how they have matched a DeLaval nozzle to the draught requirement ACROSS THE WHOLE OPERATING ENVELOPE then I maintain it is not the way to go. Porta certainly does not touch on this subject, nor E.G. Young. Any suggested reading for me?
MartinFebruary 5, 2019 at 12:59 pm #5077
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.
ChrisFebruary 5, 2019 at 4:24 pm #5078
That starts to make a bit of sense – provided there is a sharp enough exhaust to get sonic velocity at some stage. Those S160 results are going to be very interesting in that respect.
MartinFebruary 6, 2019 at 8:27 am #5079
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.
ChrisFebruary 10, 2019 at 1:11 pm #5096
Hi Martin – in earlier you post you wrote:-
‘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 can’t tell anything about the positioning of the nozzles from the images or am I missing something blindingly obvious.
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