Hello all----------- whilst trawling around for inspiration re}---The Bear's front end I came upon this from the Bure Valley people............Makes interesting reading with some innovative approaches and revealing photographs..ROGER, some ideas for you here as well >>>>>>>>>>>>>>>>>>>>>>> www.steam-loco-design.co.uk/zb_article_2.html
While researching exhaust systems I found this two years old thread an interesting read, so I wanted to bring it to date. I also found this www.trainweb.org/tusp/ex_dwgs.html Any practical experiences with the lempor or similar exaust systems in miniature locomotives?
Looking for information about blast nozzle dimensions and the right setup in the smoke box I found the sketch from julian in the web. I wanted to know what the right height and diameter for nozzle is and in my simple thinking I thought changing the diameter of the nozzle is all I need to do to improve the steaming😂. After reading this thread I know now better that it’s easier to order a beer at the pub. My dilemma with my existing setup is that I can’t get everything right how it should be. If I choose a nozzle diameter and distance to the choke with the 1:6 ratio I don’t get the1:3 right. To get the 1:3 ratio right I would need to raise the nozzle, but then the 1:6 cone would be far off. What to do? Thank you for help in advance.
You may have to modify the length and/or the diameter of the chimney/petticoat arrangement to get both cones to coincide - as well as adjusting the diameter of the blast nozzle. It is about juggling all the different variables.
But that may be more drastic than you want, so the next best would be to aim for a reasonable compromise and accept "good enough".
2.5"g narrow gauge modeller - Bagnall 0-4-2T, L&B 2-4-2T "Lyn" and just completed Burma Mines Rly 0-6-0 - all to 1.25": 1'. Now getting started on the next 2.5"g model - ex-Mecklenburg Pommersche Schmalspurbahn No.99 3462, currently on the Waldeisenbahn Muskau....
Just remember that of the variables, changes to the nozzle and throat diameters are the most significant effect as the effect on pressure is a square law. The linear effects of changes in length between the nozzle and throat are much less. I would tend to leave the diameters as they are and adjust the distances to fit it in if possible, probably concentrating on getting the 1:3 ratio as close as possible. A 5 degree change in cone angle particularly on the 1:6 will not be measurable without test equipment. Dave
Building a Don Young 5" gauge Aspinall, Driving truck finished. Member of Fareham and District Society of Model Engineers
Greenly's 1:3 and 1:6 rules will only work within a few parameters; common sense and a bit of maths shows they are unobtainable on lots of locos with large diameter smokeboxes and short chimneys, unless you severely throttle the exhaust at the blast nozzle - increasing back pressure - that is most undesirable IMHO.
Jos Koopmans has provided the formulae for all this. Dave ('Kipford'), on various threads over the years, has a most interesting perspective on all this with his expert knowledge, and I would welcome further discussion on all of this.
We tend to forget that the blast pipe stack/chimney combination is just an ejector. Consequently we need to approach design and assessment from this angle rather than get lost in all the peripheral stuff. From some of the old design papers it is hard to actually establish what is the dimensional relationship between the stack length and diameter and the blast nozzle (See the Master Mechanics design which was the "bible" for USA locomotives).
The 1:3 and 1:6 are not something dreamed up by Greenly but rather his take on the know properties of jets, both steam and other fluids. On leaving and orifice jets have a Gaussian distribution of velocity across their diameter. Further, the steam jet in the smoke box en-trains smoke changing the velocity profile. The 1:3 approximates the diameter of the edge of the jet and en-trained smoke whilst the 1:6 approximates this jet combination at the highest velocity point. If the 1;6 cone just engages the chimney top maximum entraiment of smoke occurs.
Where a chimney is forced to be shorter than the 1:6 "rule" allows then an alternative must be found. Multiple blast orifices or multiple chimneys are necessary. Multiple blast orifices with one stack mean that the beginning of the 1:6 cone is lower. Bullied explored this with his choice of the Lemaitre exhaust system for the Merchant Navy locos. It didn't give optimum results because he gave the locos an unnecessarily short stack with no proper petticoat. Porter modified the Lemaitre approach by modifying the stack to give a defined mixing length then a conical expanding section above. Arguably the Lempore approach is the most effective practical solution to front end design, it basically ticked all the boxes.
Should the 1:3/1:6 rule not work then my suggestion is to put in a multi port exhaust nozzle or a radially ported nozzle; the former will be easier to make. The truncate the 1:3/1:6 cone such that the bottom diameter is the same as the outer diameter of a circle just enclosing all the exhaust nozzles. This truncated cone then sits on top of the blast pipe to set the stack/chimney dimensions.
Taking the chimney top as the starting point (as I can't change this), it's 1 11/16" diameter, so I take six times that and mark a point 10 1/8" down, on the centre line - and draw a cone (red). Then, as the standard blast nozzle is 1/4", I mark a point six times that, so 1 1/2" up. That's where the top of my nozzle should go, the red one-sixth cone intersecting both the top of this nozzle and the chimney top. And it's 0.4375" above the as-drawn position, so I need to raise my blast pipe. Then I mark a point 3/4" down from my new nozzle and construct a 1:3 cone (green) up from this, to see where the petticoat pipe/choke should go. It's almost right as drawn, but could be extended down a little bit.
Then I do it all again for a slightly larger nozzle - say 0.3". This intersects the 1:6 cone at a higher point still, 0.7375" above standard. But the 1:3 cone (blue) intersects the standard choke OK.
Rather than the red cone starting at the very top of the chimney, I could come down a little bit, and bring the nozzles and choke down a little too.
Does that look OK?
PS, Jessie's bores are 1 1/2", for what it's worth.
What I get from interesting observations at tracks and the BLAME Banter afterwards with their Locos slowly fading into almost useless. Then they come off the track!!! I usually go for an early Lunch then get to play with something struggling while the owner goes on RELIEF and Lunch. We have a woodland section which is away from prying eyes and full of useful sticks with which to DE CLOG THE CHIMNEY!!! An over oiling lubricator and lots of dust bunged into the fire makes for a small chimney with the build up. Not bad but in ripples just goes and slows down the Escaping Gasses.
I have gone for 1 5/16" at the top of the Petticoat and 1 1/2" at the top of mine. Yes Lily is using it for a dried flower Vase or Varse as mentioned in Model Engineer so I just went and Measured it. Quite a lump when you build it up from solid bar.
David and Lily.
Builder of far too many projects, but I love them all. The more Locomotives you have building, the more solutions to other Projects Problems get Solved. Reworking Simplex into a Fowler Complex. Making a Jinty more to scale. And going up a size in the shape of a Jessie.
I paid more attention to the contributions of Dr Koopmans, following Julian's thoughts that this man knows what he's talking about.
It seems to me that in Jessie's design the choke is too large. At 1 1/2" it's six times the blast nozzle size of 1/4". And the distance between the blast nozzle and the choke is too much - 11.6 times the nozzle diameter - more than Dr Koopmans preferred ratio of 7.
In my first sketches, I was raising the blast nozzle, but it might be better instead to extend the petticoat downwards, when I can reduce the choke by incorporating a taper. So here's another sketch:
This shows three different choke sizes, but in all cases the blast nozzle to choke distance is 2.1", being seven times the nozzle diameter of 0.3" (which is larger than the designer's 0.25"). Also I've brought the base of the petticoat down to just above the superheater flues, so they're still accessible. Jessie is blessed with a decent chimney, so the 1:6 cone intersects 1/2" below the top.
The smallest choke is 1.125". This is 3.75 times the blast nozzle diameter. This results in a petticoat pipe taper of 8°, which is perhaps too much. The 1:3 cone intersects 0.638" above the choke.
The medium choke is the result of choosing a 6° taper (which is the most Dr Koopmans recommends). The choke ends up at 1.206" and the 1:3 cone intersects 0.800 above.
The largest choke size is 1.265". This is what results if the chimney taper of 4.34° is continued downwards. The 1:3 cone intersects 1.029" above the choke.
All of this is confined to the smokebox alone - since fluid dynamics is not my specialist subject, I've not extended the analysis into the grate area, or tubes, cylinders, passages etc. Most of these I can't change, whereas I can put a modified petticoat and blast nozzle in easily, and revert to standard if it all goes wrong.
For what it's worth, the cylinders are 1 1/2" bore, the grate area is 20.2 sq in, and the total tube area is 3.78 sq in (making no deduction for the area of flues occupied by superheaters).
Please tell me if this is all rubbish.
PS - I could make things easier with a bigger blast nozzle. The petticoat would not need to be so long, and the choke could be larger - but I'm already quite a bit bigger than the designer, and I'm not confident enough to stray too far away from conventional wisdom.
So my worries are if I use the right point for the top of the 1:3 cone. I used the smallest dia in the chimney. Cheers Michael
Those videos look OK, but stick to the tried-and-tested 1:3, 1:6 and 1:7 formulas (I think - I've not watched right through) - these will work.
You'll see from my earlier post (not the most recent one) that I had to raise the blast nozzle to get the 1:3 cone to touch the sides, at or just above the choke point (the top of the flared petticoat skirt). With the blast nozzle in it's original position the 1:3 cone missed completely. I'm lucky - the chimney is long enough for the 1:6 cone to also touch. With a shorter chimney it would miss, and there would be nothing I could do about that. It sounds like you're in that position - you can get the 1:3 cone to touch - so you should be OK.