Kylchap Exhaust Systems
April 30, 2018 at 12:34 pm #4565
I agree with everything you have said, I always thought that the “turbulent entrainment theory” was a bit of a side issue. There is one thing that is puzzling ma about the design of the Kylchap. When two streams of fluid with different velocities mix, there is a loss of kinetic energy. I always thought that if this could occur over a number of stages (i.e. a Kylchap) the overall loss of KE could be reduced, resulting in a lower back pressure. However, for this to occur, the intermediate nozzles would need to be slightly converging, to increase the velocity. But any drawings or photographs I have found seem to indicate that they are parallel. Can you (or anyone else) shed any light on this?
ChrisMay 4, 2018 at 7:15 pm #4612
Hi Guys – pleased to see that this has started a bit of a debate. During my lunchtimes, (when I get them), I trawl the internet for recent research articles. The video was the result of one of these searches.
As we cannot fund research ourselves I look to see what others are doing and whether it has relevance to our interests. Over the weekend I will I post more.May 7, 2018 at 11:59 am #4620
The way I come to that problem is that the mass flow ratio in a locomotive is not in a very good place for efficiency in a jet pump. Obviously, we can’t change the mass flow ratio very much, coal combustion does what it does.
That being the case, one way of overcoming that is to stage the pump, so we have a blast pipe that entrains a small proportion of the flue gas flow, this then feeds to another stage where the flow entrains another load of flue gas, and so on. Each stage is thus working at a more favourable mass flow ratio.
If the flow in each Kylchap petticoat is being used to drive another stage, then we do not want to decelerate the flow, as we need that velocity energy to drive the following stage. Hence we could either use a slightly converging or parallel petticoat.
P.S. Glad to have stirred things up a bit!May 8, 2018 at 4:35 pm #4643
Made a rather hurried reply to Chris’s query yesterday. Should also have said that if each petticoat has significant contraction, then you will need more pressure energy to drive the consequent increase in velocity energy – which is also not a good thing for the stage preceeding. Any interchange of velocity / pressure energy is lossy (pressure to velocity slightly so), hence leave the petticoats parallel.
MartinMay 8, 2018 at 7:25 pm #4644
I agree with you about the jet ejector pump being an inefficient device, but under the environment in which it is operating it’s difficult to see a comparable alternative.
As I understand it the jet has a minimum length to ensure complete mixing, and this is the reason why larger locomotives have multiple jets, however, I struggling to see the purpose of the cowls in a Kylchap if the gas leaves the intermediate nozzle at the same velocity that it entered it. One possible explanation is that the momentum of the smokebox gas hitting the side of the jet will cause the four jets to merge into one.
I agree that a slightly convergent jet will give a limited amount of back pressure, but this may help in preventing the nozzles reaching sonic velocities. I’m thinking of the Kylchap being rather like a steam turbine with the pressure drop divided by a number of stages.
For a conventional single stage jet the gas which merged at the bottom of the jet will help to entrain more gas as it travels to the top, so it will act in the same way that you have described above.
ChrisMay 14, 2018 at 9:02 am #4665
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.
MartinMay 14, 2018 at 10:28 am #4667
Thanks for the analysis, it gives food for thought. I had come to the conclusion that the combined input area of the various bellmouths in a Kylchap system would be greater than for any other common exhaust system, but I am not sure of how relevant this would be.
ChrisMay 15, 2018 at 2:51 pm #4668
I had come to the conclusion that the combined input area of the various bellmouths in a Kylchap system would be greater than for any other common exhaust system
Yes, they have to be Petticoat 2 takes input from Petticoat 1 & some flue gas. Petticoat 3 takes input from Petticoats 1 & 2 plus some flue gas.
MartinMay 16, 2018 at 8:47 am #4669
Your comment upstream about the mass flow being to the left of the effy. peak sounds intriguing. Are there any references (eg text books) where I can read more about this? Regarding the areas of the bellmouth, I was actually thinking of the area where gas enters from the smokebox, i.e the difference between the whole bellmouth area and the area taken by the downstream petticoat or nozzle. Another query, is where you say that the smokebox gas cools down as it passes through the system. This is true, but the steam will also heat up, and will this compensate for the cooling effect.
ChrisMay 19, 2018 at 6:25 pm #4688
In reply to my own question in the post above, I found the attached link.
http://www.thermopedia.com/content/902/May 23, 2018 at 11:50 am #4698
Thanks for that link, Chris. You have partly saved me the trouble of replying. The Thermopedia data follows very closely the approach in the Engineering Science Data Unit Report 85032 – Ejectors and Jet Pumps Design and Performance for Incompressible Flow. There is another report covering compressible flow (sonic choking of blast nozzle) where the maths gets even heavier – Oh joy of joys!
It is report 85032 on which I based my comments. Incidentally, as far as I can untangle the maths, the basic approach is the same as Porta uses.
ESDU 85032 contains the following graph:
ESDU Report Fig 3a
which assumes certain typical loss coefficients for the nozzle, secondary inlet and diffuser, and also assumes that density of the motive fluid (exhaust steam) is equal to the density of the entrained fluid (combustion gas) which is approximately true. It shows the envelope of best basic design against volume ratio and pressure ratio.
The graph is expressed in terms of volume ratio (gas / steam) and pressure ratio (draught / exhaust back pressure) and area ratio (blast pipe / choke). For a typical locomotive M is around 2 (varies with degree of superheat, grate area, working point etc. etc.)
You will see that for M = 2, travel across to the dropping “M” line and you are well to the left of peak effy. You will also see that dropping down to the x axis suggests an area ratio of 0.12. With that set up you can expect an N value around 0.15. You would then need to size a blast pipe to give a back pressure 1/0.15 times the total draught. The choke would be 1/0.12 times the blast pipe area.
The calculation would then be repeated using refined values for individual losses and density ratio etc. to optimise the design.
I have recently waded through the Everett G Young paper which seems to confirm much of what a retired fluid dynamicist knows about diffusers, settling lengths and maximum diffusion angles. There are some very subtle design compromises to be made in terms of choke geometry and diffusion geometry to get the best out of a chimney, much of which will be down to experimental results – not calculation.
MartinMay 26, 2018 at 7:16 pm #4700
So for a Kylchap you would have follow the above procedure for each stage, estimating values of M (entrainment ratio) for each stage taking into account that the propelling fluid is the mixture from the previous stage, and that the pressure ratio will be affected by the back pressure from the upstream stages.May 29, 2018 at 1:59 pm #4703
So for a Kylchap you would have follow the above procedure for each stage, estimating values of M (entrainment ratio) for each stage taking into account that the propelling fluid is the mixture from the previous stage, and that the pressure ratio will be affected by the back pressure from the upstream stages.
Yes. Quite a challenge! But I was only trying to illustrate another way of thinking about the problem.
Maybe better to accept experimental findings that such petticoats will not beat a well designed Le-Maitre or Porta setup.
MartinMay 29, 2018 at 6:41 pm #4705
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.
ChrisJune 7, 2018 at 12:46 pm #4712
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.
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