Chris Corney
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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.
In reply to my own question in the post above, I found the attached link.
http://www.thermopedia.com/content/902/Martin,
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.
ChrisMartin,
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.
ChrisThe above link (to the book) describes various vortex generators, but it doesn’t say much about how much back pressure these generate. I w3as also thinking about the visualisation video. There is some evidence that a convergent mixing chamber is better that a parallel one, and in quantitative terms its interesting to mull this over in conjunction with turbulent entrainment theory.
Thanks for posting that John. It gives useful information for the sizing and proportions of a blastpipe/exhaust system, but I still agree with Martin that the fundamentals are Bernoulli and the conservation of momentum.
Martin,
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.
Best Regards
ChrisPulsations are not necessarily detrimental to a locomotive exhaust, unless they become excessive. It has been noted that a locomotive with an off-beat exhaust can steam better than one with the valves set correctly, although the effect of back pressure is likely to be greater.
Hi Chris,
I’ve been reading about Porta’s water treatment, and to adds more additives to the boiler water to arrive at a mixture which is benign from a corrosion point of view. However it occurred to me that the boiling point of this mixture would be higher than for pure water, even though the steam produced would be pure and the saturation temperature equivalent to that produced by boiling pure water. I was wondering if there was any evidence that this increased boiling point reduced boiler efficiency, heat transfer or output.
Hi John,
It’s difficult to imagine what is being described, just from the abstract, but it might help locomotives with excessive smokebox pressure pulsations. It also reminds me of the drawing of Porta’s Lemprex, with the auxiliary chambers to smooth out the flow. My feeling is that it would be less effective in terms of basic draughting, and there are other options we could consider.
I agree entirely with David’s analysis. I am of the opinion that it is necessary to have some form of control system to regulate the quantity of steam being supplied to the grate.
It’s also interesting to consider this analysis with the discussion of various coal types currently being held on the Nat. Pres. forum.
It would be difficult to engineer a durable probe that could monitor the grate temperature directly, but as a half way house, my suggestion would be to provide a valve which shuts down the undergrate steam if the boiler pressure drops below, say 85%, of its nominal value. I’ve been advised that suitable valves are commercially available to achieve this. Admittedly, there are other reasons why the boiler pressure might fall, but I don’t think that shutting off the undergrate steam would be too much of a disadvantage.
The other point that needs considering is whether there is sufficient secondary air to burn the gases produced by the system (i.e. hydrogen and carbon monoxide). We seem to be assuming that sufficient secondary air passes through the firebox door, but is this actually the case? If I remember correctly, for “Red Devil” additional openings were created in the side of the firebox. Gases passing unburned through the chimney obviously represent a loss of usable energy. Perhaps it may be possible to monitor the carbon monoxide content in the smokebox gas.
Rather than cutting holes in the side of the firebox, and thus penetrating the pressure vessel, my preference would be to have secondary air pipes passing up alongside the grate from below the locomotive. This opens up the possibility of considering pre-heating the secondary air, using exhaust steam. (Preheating primary air could lead to further clinkering problems).
I remember many years ago hearing that pulverised fuel was once trialled on the Great Central, but obviously never became widespread. In power stations the coal is crushed using specialised mills, then entrained in a stream of compressed air before being fed into the boiler using purpose designed burners. It sounds a bit complicated for a locomotive.
I think that the “heat balance” (for want of a better description) between the various chemical reactions in a GPCS grate needs more consideration. My suspicion is that some GPCS grates are being supplied with too much undergrate steam.
Am I correct in thinking that CFD can only model steady flows? There are a number of components in an exhaust system whereby pulsating flows can be exploited, for example Kordinas and de Laval nozzles. Also I think the inertia of the gas in the long chimney of a Lempor will have an effect. If the velocity of the gas is rising and falling all the time, what happens to the energy when it is reducing? Atmospheric pressure isn’t going to change, but the “peristaltic or piston” effect could cause some additional vacuum in the smokebox.
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