Forum Replies Created
Jos Koopmans has added this paper to the Nat Pres forum.
That’s good news.
I’m wondering if it would be worth writing up your findings for future reference by other members of the group.
My usual approach with a very inefficient device such as this is to try and identify the losses and then see if there is anything that can be done to reduce them. Obviously it’s necessary to stay within the loading gauge and – these days – limitations imposed by the heritage appearance.
Presumably the kinetic energy of the gases leaving the chimney is included in the losses, but there is not much that can be done about this.
Other losses that spring to mind are
– mixing losses due to steam and smokebox gases at different velocities.
– supersonic shock losses
– internal friction.
– losses due to pulsation effects (as previously discussed, there is not much knowledge about this)
As you can tell from previous discussions I’ve been trying to understand how a Kylchap works. The double Kylchap successfully draughts the boiler but with a much larger orifice area, and lower back pressure than the single chimney it replaced, so there must be something beneficial going on there!
I hope I’ve understood your question correctly, let me know if I’ve missed the point.
My take on this is that it case of “horses for courses”
Conservation of momentum is one of the fundamental laws of physics. To change the momentum of a body it is necessary to apply a force to it. Every force has an equal and opposite reaction, so the body to which the reaction force applies will experience an equal and opposite change in momentum. The total momentum of the system remains constant.
So if you are mixing fluids with different velocities, the overall momentum will remain constant. however the kinetic energy of the mixture will be less that the combined KE of the constituent parts. Therefore it makes sense to use the momentum equation.
If a fluid passes through a nozzle of diffuser, the walls of the nozzle/diffuser will apply a force to the fluid and the momentum of the fluid will change. However, if the nozzle/diffuser is reasonably efficient, the energy of the fluid will remain reasonable constant, based on Bernoulli’s equation, and in this case it makes more sense to use conservation of energy.
Hi Martin, Regarding the “sequential ejector”, for a long time I thought that the various cowls of the Kylchap were slightly convergent, so that the pressure energy in the blastpipe is converted to velocity in a number of small steps. A similar idea to a multi-stage turbine. Looking at drawings and photographs, as far as I can tell, they are parallel, which makes analysis of the deviice a lot easier!
It’s an interesting point about the nozzle peak mach number. According to Wardale’s “Red Devil”, Porta held the view that in some cases it was advantageous to use de laval nozzles. Peak velocity occur at exhaust release which is close to the piston dead centre position, where back pressure doesn’t matter. At mid stroke the nozzles would be operating with subsonic flow conditions imposing a low back pressure on the pistons. This would make best use of the residual exhaust steam pressure.
Further to the above post, I came across some data for a Class 6 locomotive, and at the maximum boiler output this quantity would be of the order of 50kW. This is a significant amount of power and is worthy of further consideration, however, it is calculated on the basis of steady flows. If you take the pulsing effects into account it could easily be double this value.
There is another question that I’ve been mulling over.
Assuming the smokebox gas impinges on the steam jet at an angle, it will be possible the resolve the velocity into two vectors, one parallel with the steam jet, and the other one perpendicular.
Now the Kinetic Energy (KE) of each component can be calculate from the equation KE = ½ mv2
And as the two values of KE are proportional to the square of velocity and the vectors are at right angles to one another, Pythagoras tells us that summing the two together will give the KE of the resultant flow (which is what you would expect).
Now considering the KE of the perpendicular flow in isolation, it would be straight forward to obtain a value of velocity by dividing the volume flow by the surface area of the steam jet.
The question is whether this is a significant quantity. The KE has to be generated by the exhaust acting as an ejector pump, and then the energy is just lost as heat, which will have further detrimental effects.
Basically I’m wondering what the advantage of the Kylchap was, or in other words why Chapelon, Gresley and others went to the trouble of using it.
I’m thinking that the design of the Kylchap, with the various cowls and mixing areas would have maximised the effective surface area of the steam jet, and therefore minimised the “lost” KE of the perpendicular moment of flow. Does this make sense?
The paper mentioned in the post above is “Optimum control of diffuser shapes for non-uniform flow” by G.P.Bentham, I.J.Hewitt, C.P. Please and P.A.D. Bird.
It can be downloaded FOC from the internet.
In reply to your points above:-
There was a paper recently which Jos Koopmans mentioned on the National Preservation forum (I’ll try to find it later). From this I drew the conclusions that a convergent mixing chamber could be shorter than a parallel type, with similar performance. This would be useful for railway applications.
I’m just trying to interpret your second point with respect to the principle of de Laval nozzles, which have been around for eons. I’m thinking that if transonic flow occurred in a de Laval nozzle it would probably just create shocks.
For transonic flow in diffusers, Prof. Eames at Nottingham University has done some work on CRMC theory (constant rate of momentum change), with one or two papers availableon the internet; you may find these interesting.
While various arrangements of ejectors using conical and parallel sections are proposed, I’m wondering if a smooth curve would be better. Hence my suggestion of a hyperboloid. CRMC theory also proposes a curve, although it is a different curve and potentially more difficult to manufacture than a hyperboloid.
For chimney machining we are thinking of miniature up to standard gauges, although I’m thinking that a de Laval nozzle with a CRMC divergent section could theoretically have some advantages with pulsating flows.
We discussed castings with complex curves in the group a while ago, and someone asked how you would go about improving the surface finish of a casting.
There are a number of miniature locomotives running with saturated boilers and exhausts with de Laval nozzles, and I can’t help wondering how these would operate with wet steam. If there is a sharp drop in pressure in the nozzle, does this just cause an appreciable amount of condensation?
Further to the post above, I was thinking that the convergent-divergent shape of a hyperboloid might be of advantage when designing an ejector.
A chimney would have to be machined internally, and I have been advised that the typical reach of a profile follower attachment is about 125mm.
The exhaust on the S160 was designed before I joined the group, and I understand that the nozzle angle was carefully considered. The exhaust certainly performs succesfully.
No, a helix is the shape of a coil spring. When steam leaves the nozzle it will travel in a straight line.
An example of a hyperboloid is the traditional concrete power station cooling tower, where the steel reinforcing bars travel in a straight line from the base to the top of the tower, inclined at the appropriate angle. Other examples are in the pictures in the Wikipedia page above.
Martin, I’m probably not the best person to discuss this with, because I am agreeing with everything you say!. I guess the angle comes from interpretation (or otherwise!) of Porta’s Lempor paper.
My background is as an electrical engineer, but I’m happy to pick up any snippets of wisdom that I can on this subject. I have my doubts about the turbulent entrainment theory, which seems to suggest the more turbulence you produce the better the entrainment will be.
Regarding the geometry I’ve been toying with idea of the nozzles angled to produce a hyperboloid steam jet, possibly feeding into a hyperboloid chimney.
How would you make one? It’s possible to produce a casting from a polystyrene pattern, and you can cut polystyrene using a hot wire, set to the appropriate angle to give a hyperboloid.
If you then wanted to improve the surface finish you could electrochemically machine the surface using an electrode made from wires arranged in a hyperboloid.
I once found a patent from the 1950’s describing a hyperboloid ejector, but the internet link has now disappeared. If anyone is interested in making one I’d be happy to help with the mathematics.
In your experience have you come across anything like this?
Sorry for the delay in replying.
I think some people hold the view that the jet should “bounce” off the sides of the mixing chamber.
My thoughts on the operation of the ejector are based on Bernoulli’s theorem and the conservation of momentum. Others seem to support the “turbulent entrainment” theory. I suppose that there needs to be sufficient mixing to obtain a reasonable velocity profile in the diffuser to prevent recirculation, and this will inevitably produce some turbulence, but otherwise my view is that turbulence should be kept to a minimum.
I would be pleased to hear your views on this.
There is plenty of evidence of spark-throwing by locomotives, in other words particles of burning coal going up the chimney. I don’t know what the effect the heat produced by this combustion would have on draughting.
Yes, it’s all very complex. I think it would be an acheivement if calculated values could approach those achieved on the road.
I forgot to mention the Gas Producer Combustion System (GPCS), if used.
A proportion of the exhaust steam is added to the combustion air. This reacts with hot coal to produce Hydrogen and Carbon Monoxide, which then burn back to carbon dioxode and steam in the combustion chamber.
This obviously reduces the blastpipe steam, but adds to the mass of the combustion gas flow.