Front End situation

Hi,
Since outside temperatures are some 34C/93F at the moment I am retired to the basement and could do some calculations.
Firstly, based on the calorific value of diesel, carbon and hydrogen, I concluded that the combustion products plus 20% extra air plus 5% burner steam were almost identical in weight to full coal burning. So it does not matter that much for the calculations.
Secondly, based on averages used in the past, I used 65 kg/m2 steam generated for a total of 3900 kg/hr. Please supply some figures for #17.
Thirdly, with a load of assumptions I concluded that the present layout would be sufficient for a vacuum of about 4 inches of water in the smokebox which then
fourthly, lead to some calculation possibilities for a multiple with diffuser stack.
I arrived at 4 orifices of 2,25 in ea. exhausting in a chimney starting with a 12 in diam mixing chamber. Anything larger just does not work.
It appears now that Nigel Day and Michael Guy have 12 in. and 10 in. resp. so we will have some fun.
Kind regards
Jos

Jos- see separate sub-forum/thread for #17 here:

https://cliffside110.forumotion.com/t43-exhaust-modifications-for-17

Hugh,
I just checked, the #17 data show a steam generation at a rate of 68.35 kg/m2/hr.
I used 65, fits nicely.
Btw what do you folks want, metric or imperial calculations/measurements.
Kind regards
Jos

Jos,

I plugged your 3900kg/hr (1.08 Kg/s) into the sheet and set the mixing chamber at 12" leaving everything else the same as my #2 yesterday. With vacuum set at 4" water the sheet says the nozzles should be 2.2" diameter. Very close results *but* the results graph at the bottom of the sheet shows a curve rising to the right. If I adjust the numbers to get the convex curve indicating "best results" the nozzles come down to 1.9" dia and the vacuum rises to 8.5" water.

Michael.

Michael,
For the time being I don't know what the really needed vacuum should be. Hopefully they will use a U-tube shortly on #17 to supply an answer for oil firing.
Kind regards
Jos

Plans are in the works for manometer and back pressure readings in July. I'll be among the last to know if it gets carried out, but I'll pass it along as soon as I do.

17 doesn't draft as well as we'd like, so it is probably going to be a low point baseline measurement.

Dave

Dave,
They could use the blower as an extra to define the vacuum needed.
Kind regards
Jos

OK, gentlemen, here's my drawing of Jos' exhaust arrangement explained a few posts back:



Jos, please let me know if there are any glaring errors.  I made the nozzle stand as low as I thought it could reasonably be fabricated.  I still had to shorten the straight portion of the stack slightly (about 6 inches below the preferred L/D = 2) in order to fit everything in the smokebox. I worked the exhaust nozzle angle out to be 6 degrees based on the diagram on Michael's page (drawing of that not included here); please advise if I missed something on that as well.

Hi Hugh,
Sorry for the work, but yesterday at 2:47 I concluded that anything over about 12 in. mixing did not give satisfying results in the calculations. I ended with 4 orifices of 2,23 in. mixing chamber 12.49 in and exit 19.82 in. It even fits better since the 15 in can be reduced to 13.32 and as a consequence the mixing chamber length is 25.2 in where 25 would suffice. Perfect!
Again, my apologies,
kind regards
Jos

Jos,

No problem; I will re-draw when I get a chance (proabably this weekend) and post. 

Hugh

Michael supplied a new drawing for Jos' revised exhaust:



I added a bellmouth to the stack inlet.  The optimum shape for a bellmouth probably deserves it's own thread.  Personally, I'm a fan of the type Wardale uses (not shown here), where the radius of the bellmouth is "wrapped" around the stack inlet and up onto the sides.  This provides the gas flowing from above the stack inlet a smooth surface to flow across rather than meeting the sharp edge of a bellmouth, which it seems would have to cause unnecessary turbulence and thereby increase the gas flow restriction into the stack.

Hugh, thanks for posting. Sorry about the coloured dimensions, that works a lot better on my normal black drawing background than it does on white. I will change that on any future sketches.

>Personally, I'm a fan of the type Wardale uses (not shown here),
>where the radius of the bellmouth is "wrapped" around the stack inlet
>and up onto the sides. This provides the gas flowing from above the stack
>inlet a smooth surface to flow across rather than meeting the sharp edge
>of a bellmouth, which it seems would have to cause unnecessary turbulence
>and thereby increase the gas flow restriction into the stack.

This is the sort of situation (and fractured gasdynamics) that comes of not using the Master Mechanics style plating to define the path that the combustion gas follows to get into the front end.

A moment's reflection will tell you that the gas path does NOT follow 'around' some curved lip past the sharp edge, nor do you want flow to be induced from 'above' the edge of the vane. (If you do -- follow the Kylchap et al. approach and put stacked diffusers in series).

Turbulence is NOT the enemy here (entrainment in the stack, for example, depends on it in large part). You are much more likely, however, to get flow inatabilitiy (eddies and so forth) around the edge of some airfoil profile purporting to get a few lbm or kg of gas mass per second into the bell mouth.

That edge is sharp for a reason, and ought to continue to be. Consider it as a kind of 360-degree 'splitter' (in the automobile-aerodynamics sense). I sympathize with the general logic of inducing flow pattern down along the stack into the lower-pressure region... but it better not be INTO the zone producing vacuum flow induction. And the general reverse-bell of the 'outside' probably accomplishes this better than anything purporting to have 'aerodynamic continuity' around the edge...

My vote on the shape of the bellmouth curve is to shape it so that the velocity and direction of the inrushing combustion gas are both optimized and 'matched' to the average conditions in the jet; you are essentially shaping a gas plume by inducing flow, and then imparting momentum to it once it's imoving substantially in axial alignment.

Understandably, if gas from the superheaters is going to be accommodated straight into the front end, you may see advantages from flow shaping the induced flow from up high. But look at what is actually being 'induced' by the airfoil; it's two separate flow patterns, and they do NOT meet either smoothly or stably across ... well,some zone between the downrush along the outside of the stack and the point where stable flow induction into the bellmouth is occurring. If you want to optimize flow from 'above' the petticoat area, it might be better to put helical vanes on the outside of the stack, to get shaping of the downcoming flow into (fairly) discrete regions of lower pressure. But absent competent CFD, I think that this is another example of Sinclair's comment about fooling the steam (or here, combustion gas) into following the clever little arrows that show where the designer wants it to go.

Robert,

Very interesting thoughts there.  I suppose I'm thinking more as if there's an exhaust fan on top of the stack and the most important thing is to minimize the flow restriction across the stack inlet. You're concentrating on avoiding disruptions to the steam/gas mixing process to ensure highest "pumping" efficiency. 

I'd definitely like to see CFD models of alternative arrangements.  Jos included some CFD in his thesis and book, I wonder if he looked at this area. Alternatively, a clear plastic scale model could be used with smoke- that might be a fun project.

Hugh

Michael supplied an updated drawing for better legibility for Jos' revised exhaust arrangement for #110.  I converted it to a negative which seems to be more readable:

Nigel forwarded me this sketch to post; he promises he will post some explainations to follow.

Dave

This is getting good!

As a note -- would not soot settle preferentially in the 'curl' of the volute at the bell mouth and perhaps cause problems?

The diagram above is baied mainly on the work of Porta andWrdale. The relivent information can be all found in the'Red Devil'.It's all done and tested so it gives the info that I have seen here being debated thus simplifying the who design process. It goes to the point now where a lempor is ' bolt in and play' no messing garantteed good result. The reason of writing this is to educate, nothing more. To add relivence to this the coming weekend marks the start of the twentieth anniversary of the fitting of the first lempor to the Welshpool and Llanfair loco fleet all built to the basic principles shown here. All have worked with no problem since then. To me this loco is just another job, I understand the desire to experiment but to re invent the wheel?

The sketch shows a section of a lempor in a horizontal plane rather than vertical just because I happen to draw them out this way. To the left is the blast pipe nozzle which i make out of machined bar. Easy to produce. The important factor here is that the throat to the exit has a six degree taper on it relating to supersonic seeds. Thus the nozzle can work efectivly at any pressure . The ratio between the throat and exit areas is 1 to1.1.
The bell mouths are fabricated so as to give a taper in the lower set into avoid boundary separation. The degree of eternal rounding is dependent uon the hight of the mouth relative to the tube bundle. Remember the vacuum will pull the gasses round into the mixing chamber so some areas are dead space.

The proportions of the mixing chamber are self explanatory so I am not saying much on that.

Right for what is probably the most critical item of understanding on front ends is the way the steam is impinged on the wall of the mixing chamber. Any nozzle like the flame on a welding torch has a core (the red lines) and also the expanding products of energy conversion around it. With a welding torch you hold it at an angle for the greatest heat transfer from the entre core. The same logic applies with steam to achive the greatest energy conversion between the steam and mixing chamber wall. In principe this is a compromise due to the pulsation flow of a steam locos exhaust. What is critical is holding the ore of the steam from the nozzle parallel to the mixing chamber wall and this was found to be 7.25 degrees confirmed by tests in Datong. That it period no messing no change.

The science is that the steam exerts presure on the gas layers on the mixing chamber wall. The viscosity of these gas layers is reduced making a greater gasflow at a higher velocity as the friction is over come. The whole shaping of the nozzles and these boundary layer conditions is similar to a skate on ice. The velocity of the exhausted steam is thus higher and the pressure lower rating a greater vacuum.

The diffuser is the secondry consideration to the mixing chamber. You design the mixing chamber first. The ideal angle can be comprised but it dose affect the overal eficiency.

Thank you Nigel. A very straightforward approach, with simple calculations based on the proportons of parts relative to the mixing chamber as I understand it. The rest is just stock degrees of incidence applied by rote.

I'm not sure I understand what the criteria you use to come to the value for D might be........since everything else depends on that dimension.

And even more thanks for the specific information about the divergent angles for the nozzles. Does anybody else use anything different?

Dave

Dave your welcome. Although the ultimate science is important I believe it's important to explain all of this in terms that are simple to both understand and apply.

I have not explained how to find D, I know there are many ways to find this and it was not part of the intention in that post to explain that. There are also many if's and buts in calculating D so the subject is not so cut and dry as what is explained above. Please also note that when you will have a different back pressure to vacuum graph for oil than coal firing. This is mainly due to the lack of fire bed resistance and the presence of atomising steam.

The nozzle angles and proportions are still a Porta and Wardale standard. The lower section on to the flange to Porta standards are parallel, but mine have a taper down to the throat due to the geometry of my method and retro fitting.
Robert, the Abt three dose not suffer the condition you describe with soot and cylinders in the back of the bell mouth. They are all simply thrown out of the chimney. It's not a problem.

On of the joys of my work in the 1985 to 2005 period was reading through the vast amount of railway literature. It really was a treasure trove of ingenious engineering described by past generations. In that treasure trove one item is sadly missed, proper information on Lempor experience. Even Porta’s earliest contribution to the subject, his 1957 text for the Pan American Railway Congress can not be retrieved, I have never been able to get a photocopy.
Were I a believer in conspiracy theory, I would be inclined to think that it not even existed! With “proper information” I mean experience with tests in which dimensions were varied to arrive at the “best solution”. The only written material we have is the two pages (474-476) on the Datong tests by Wardale in his book, I cannot find the orifice inclination of 7.25 degrees  mentioned in there.  In this I sympathise very much with Prof. Dr. A. Giesl-Gieslingen who wrote in his 1986(!) book “Lokomotiv-Athleten”  p 154: “As long as the Lempor is not investigated as precisely as my Flatejector in 1959 in Rugby its supposed superiority is in doubt…”
As for the explanation of the functionality of a diffuser chimney, why not regard it as a bundle of very long slender ejectors? As such the orifices are inclined to lie on the axis of such a slender ejector with a proper distance between orifice and chimney entrance based on 200 years of experience with working front-ends and tests by Troske, Goss (Purdue, MM), Young (Illinois) and Ell (BR).
Please let us experiment and falsify or verify some of the possible explanations!
Kind regards
Jos

Red Devil pages 152 show inclination of nozzles on a drawing and page 305 shows various test results of nozzle angle. It also shows comparative draughting systems including Lempors and Giesels. There is no full lempor design paper, most of that info was done by private communications and hence some details in the above are new to common knowledge.

If you don't want to understand or believe what I have been saying for many years that's fine by me, my results are in hardware not words and developments still under way so its not time to write the book yet. If you want to do it by your method again carry on, I will tell you when you have caught up with what I know.

I think it's a matter of wanting to work through the process to completely understand how the results and design specs were arrived at rather than to challenge belief systems. It's fine to know that if you make part A fit at X degrees relative to part B it will work....but to know why X degrees is the best alignment between parts A and B.....that's real understanding of the theory in practice. Since not all of us are well experienced in practice or grounded in academic study of theory, knowing the process involved is important in acquiring understanding.

Dave

RD pages 152 and 305 both show results of 12 and/or 18 degree angles. I was after the 7.25 degree angle.
As for your second comment, it would take an extremely well educated engineer to design a really lousy Lempor, I admire and envy your capabilities, but why are you so scared of submitting proof of your findings?
Kind regards
Jos

The angle of one nozzle is 7.25. The spread of two noles is 7.25x2 = 14.5 which is between 12 and 18 degrees.