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Post by Roger on Feb 14, 2021 22:19:55 GMT
Hi Gary, I think that once the nozzle is choked, IE the gas/steam is at the speed of sound, then the diverging part of the nozzle accelerates the flow and the pressure decreases. Look at the shapes of nozzles on rocket engines. Chris. Surely the basic venturi principle would still apply, no matter what ??......so diverging ie expanding will slow the velocity and increase the pressure ??....Have a look at the final variable ducting shapes used on Concorde's air inlet side to the Rolls-Royce engines... Hi Alan, Sadly not. Venturis assume incompressible flow, and that's not what you have with Steam cones. Concorde has variable geometry to deal with sub-sonic and supersonic flows, hence the sharply sloping inlet ducts. These create two oblique shock waves, one from the first edge of the inlet, and the other where that shock hits the opposing wall at an angle. Two oblique shocks create much less drag than a Normal shock wave, ie one that's perpendicular to the inlet. That's what you would have with a circular inlet which would be very inefficient. You can have circular inlets, such as on the Lightning, but you'll notice that there is a conical point that stands out from the middle of the inlet. This creates a shock wave that intersects the inlet, creating an oblique shock. The SR-71 uses a similar but moving spear to control the shock wave. These are compressible flows, but don't really help us understand what's going on in our nozzle. Chris Vine is absolutely right about the divergent nozzle. It's worth looking up Convergent/Divergent or De-Laval nozzles for a better insight into compressible flow through nozzles. |
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Post by Roger on Feb 14, 2021 22:37:16 GMT
Surely the basic venturi principle would still apply, no matter what ??......so diverging ie expanding will slow the velocity and increase the pressure ??....Have a look at the final variable ducting shapes used on Concorde's air inlet side to the Rolls-Royce engines...[/quote] When an injector is working properly, it is working at below atmospheric pressure until you get to the delivery cone. This is why there is traditionally a check valve (or flap valve) at or on the first half of the combining cone, or the sliding first half of the combining cone that Ed intends to have a go at. Transferring the check valve in effect to the start of the delivery outlet, as Roger had done, could have been done by turning upside down a standard injector and removing the ball from the combining cone check valve, and adding (upside down) a check valve to the outlet to for the overflow from the gap between the end of the combining cone and start of the delivery cone. I would question that Roger's model is easier to make. I am quite happy with press fits and delivery cone and combining cone 2 halves as 3 separate pieces, and I remain unconvinced that 'end regulation' is easier than the annular gap for the steam cone just into the start of the first half of the combining cone.[/quote] Hi Julian, That's an interesting idea for using a standard body for this style of injector with a modified overflow. A ball valve on top would certainly work, in fact there's a picture of one in Bob Bramson's book, albeit with two balls in the housing instead of one. I guess we all have our own thoughts on what's easy or difficult to make. Personally, I can't see how a press fit is easier to achieve than a sliding fit. It's also hard to see how making three parts is easier than one. Drilling the cross holes is easy enough with a dividing head, pr a simple indexing fixture fitted to the tool post, but some people might not agree. I'd agree that setting the regulation gap is a finer adjustment when using End Regulation. However it is a controllable dimension that repeats, which I'd suggest is not the case with Annular Regulation where tiny changes in the Steam Cone nose or the taper angle will upset it. I can swap cones without altering the regulation gap and they still work. I leave it to the reader to make his own judgement on what he thinks he's most capable of achieving. I think the fact that most people won't even consider making their own Injectors speaks volumes. Just reading D.A G Brown's chapter on making the cones would put most people off. The more we can do to rectify that the better. |
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Post by delaplume on Feb 14, 2021 22:45:03 GMT
Thanks Roger--------I'll do just that........Another thing on the "What I've learnt to-day" pensioners guide---LoL !!......... As a RN Marine Engineering Apprentice I'm sure we touched on Laval Nozzles in relation to Gas and Steam turbine design..... but it was during 1965-68 and I've long since lost my reference books... Yes.you're right--I must keep a check on what's going on in my nozzle ( Remember the recent PM ?? )...Lol !! PS}--- We used to have the Alfa Laval Pumps factory at Reading---- but that's a different matter .. OK----here we go then...includes the "choked throat" explanation as well.......Many thanks Chris and Roger for that}---- engineering.fandom.com/wiki/De_laval_nozzle |
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Post by delaplume on Feb 14, 2021 23:02:01 GMT
Just for interest sake the WW2 German jet fighter-- the ME262 --- had engines also with a pronounced extention in the centre but for an additional reason}----- housing a 2-stroke engine which acted as an APU ( Auxilliary Power Unit ) for starting the main turbine if a battery cart was not available....... Have a look at this}---- en.wikipedia.org/wiki/Junkers_Jumo_004 |
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Gary L
Elder Statesman
Posts: 1,208
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Post by Gary L on Feb 14, 2021 23:54:04 GMT
Hi Gary, I think that once the nozzle is choked, IE the gas/steam is at the speed of sound, then the diverging part of the nozzle accelerates the flow and the pressure decreases. Look at the shapes of nozzles on rocket engines. Chris. Surely the basic venturi principle would still apply, no matter what ??......so diverging ie expanding will slow the velocity and increase the pressure ??....Have a look at the final variable ducting shapes used on Concorde's air inlet side to the Rolls-Royce engines... Yes, that’s my understanding too Alan, though I have no claim to any expertise on the subject. It is counter-intuitive, so not surprising that it is often misunderstood. I’m not sure if it adds anything to Roger’s experimentation of oourse, because if something works well empirically, theory isn’t going to make a lot of difference! EDIT. Particularly if it is not the relevant theory! I think I now see the difference between a normal venturi (like in a blast pipe) and these supersonic ones. Who’d have thought it?  Gary |
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Post by David on Feb 15, 2021 0:37:17 GMT
Amazing stuff! Other than making the reamers (which traditional injectors also require) these look to be within the grasp of many of us without all the press fit and spacing issues, and the single cone looks like it has more metal left on so also probably easier to machine.
The performance you've videoed is as good as anything I've seen in our club and the easier to make design is a breakthrough.
We are lucky to have a very good injector supplier down here and as long as everything else in the chain is good (including the inside of your boiler as I've found out) they work really well. But it's nice to think the rest of us might now have half a chance at making a working injector.
Have you measured the delivery rate on yours? It looks really fast. I was surprised at how quickly the water was going up in the glass, and then looked at the plastic bottle and you could also see the level going down. How would you throttle this if you wanted to? Just with the water valve? If you wanted to make one with a different delivery rate, what would change?
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Post by Roger on Feb 15, 2021 8:37:24 GMT
Amazing stuff! Other than making the reamers (which traditional injectors also require) these look to be within the grasp of many of us without all the press fit and spacing issues, and the single cone looks like it has more metal left on so also probably easier to machine. The performance you've videoed is as good as anything I've seen in our club and the easier to make design is a breakthrough. We are lucky to have a very good injector supplier down here and as long as everything else in the chain is good (including the inside of your boiler as I've found out) they work really well. But it's nice to think the rest of us might now have half a chance at making a working injector. Have you measured the delivery rate on yours? It looks really fast. I was surprised at how quickly the water was going up in the glass, and then looked at the plastic bottle and you could also see the level going down. How would you throttle this if you wanted to? Just with the water valve? If you wanted to make one with a different delivery rate, what would change? Hi David, I agree, there's no reason why anyone can't make these in my opinion with a little care. As long as someone in your club (maybe you) can make acceptable reamers, the rest is pretty easy. Every club could have reamers so members can make Injectors. I'll probably put together a guide in a magazine article when I'm happy with it all. That's a good point about the delivery rate, I need to measure it. That ought to be easy with the balanced check valve, but it tends to overflow when I'm using that, so if won't be accurate if I do it that way. I'll find out one way or another. I've used the geometric ratios from Bob Bramson's book in my 3D model, so it ought to be possible to quickly home on on something that works in a smaller size. This is why I'm more interested in his approach than D.A.G Brown's where he serves up finished designs without any indication as to how they were arrived at. Most people don't care, and that's not the purpose of his book. In short, everything changes when you alter the delivery volume. You have to go back to the tables in the book to find suitable throat sizes and then work it all out again. In reality, it's pretty much a scaled down version of the same thing, but you can't just do that and expect it to work. There's not much else you can do to reduce the delivery rate, because you're having to overcome the boiler pressure. Reducing the Steam pressure would probably cause the Injector to drop out, but I've never tried it. Something else to play with! |
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oldnorton
Statesman
5" gauge LMS enthusiast
Posts: 728
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Post by oldnorton on Feb 15, 2021 9:59:01 GMT
Well done Roger for your persistence and skill. I think you have achieved something that a lot of us admire but might have preferred not to go near personally!
I look forward to seeing the magazine write up. It will then be interesting to see how many people have a go at making one and what the immediate success rate is.
What do you think the flow delivery of your 8X version is in the old fashioned oz/min calibration, or numbers; have you made a number 3?
Norm
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Post by jon38r80 on Feb 15, 2021 13:43:34 GMT
Would timing the increase in boiler weight with the injector running be the simplest, just put a scale under the boiler?
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Post by Roger on Feb 15, 2021 14:42:18 GMT
Would timing the increase in boiler weight with the injector running be the simplest, just put a scale under the boiler? Now there's a cunning plan, that wouldn't have occurred to me. I've got the arrangement I made for the axle weighbridge, so I could use that. The only problem is that I can't put enough heat into the boiler using the element to maintain the pressure, but it would give a minimum figure. |
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Post by andyhigham on Feb 15, 2021 15:07:38 GMT
Would timing the increase in boiler weight with the injector running be the simplest, just put a scale under the boiler? Or measure the amount of water in the supply tank before and after a timed injection |
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,993
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Post by JonL on Feb 15, 2021 17:31:22 GMT
I must measure the temperature of the water in my side tanks during running. I was suprised that even with the gap between my tanks and boiler/firebox the water still rapidly gets to the temperature of a warm bath.
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Post by delaplume on Feb 15, 2021 18:08:14 GMT
Would timing the increase in boiler weight with the injector running be the simplest, just put a scale under the boiler? Or measure the amount of water in the supply tank before and after a timed injection Don't forget to allow for any lost through the overfow.. |
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Post by Roger on Feb 15, 2021 19:01:33 GMT
I must measure the temperature of the water in my side tanks during running. I was suprised that even with the gap between my tanks and boiler/firebox the water still rapidly gets to the temperature of a warm bath. I'd certainly be interested to know what that is. An air gap is pretty hopeless for insulation if you have a trapped body of air circulating in the gap. You also are providing a direct line of sight between the surfaces. I've noticed that the reservoir on my test setup can go from 15C to 25C if I leave it for a while with the boiler up to temperature. I've bought some insulation to put behind the tanks on the locomotive because I think it's a problem and it's so simple to address. |
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Post by 92220 on Feb 15, 2021 19:26:30 GMT
Hi Roger.
I think you are using it, but if not, ceramic fibre is great for insulation. As a test, I picked up a saucepan of boiling water, off the stove, with a sheet of 3mm ceramic fibre under each hand. It just felt warm, and I held it up for a good minute. I always remember a TV program about new inventions. They showed a man with a piece of 1/2" ceramic fibre blanket on his hand, and someone was playing a plumbers butane gas torch directly onto the ceramic blanket from just 6" away!
Bob.
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Post by Roger on Feb 15, 2021 19:54:31 GMT
Hi Roger. I think you are using it, but if not, ceramic fibre is great for insulation. As a test, I picked up a saucepan of boiling water, off the stove, with a sheet of 3mm ceramic fibre under each hand. It just felt warm, and I held it up for a good minute. I always remember a TV program about new inventions. They showed a man with a piece of 1/2" ceramic fibre blanket on his hand, and someone was playing a plumbers butane gas torch directly onto the ceramic blanket from just 6" away! Bob. Hi Bob, I've got that for direct contact with the boiler, ie under the cladding. However, I've bought some paper backed expanded polystyrene type of insulation which ought to be more effective between the cladidng and the tanks. I've got room for about 5mm thickness, so it ought to be very effective. |
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Post by steamer5 on Feb 15, 2021 19:56:46 GMT
Hi Rodger,
Isn’t your test rig designed so you can put the injector delivery to a container to measure the output? I seem to remember when reading the ME article some years ago that was part of the object of the exercise....also to stop filling the boiler!
I just got to hope now that ME will start delivering to this side of the plant again!
Cheers Kerrin
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Post by Roger on Feb 15, 2021 20:21:29 GMT
Hi Rodger, Isn’t your test rig designed so you can put the injector delivery to a container to measure the output? I seem to remember when reading the ME article some years ago that was part of the object of the exercise....also to stop filling the boiler! I just got to hope now that ME will start delivering to this side of the plant again! Cheers Kerrin Hi Kerrin, You are quite correct, however the Balanced Check valve I made won't cope with the pressure fluctuations in the output. The valve works, but the ball chatters on the seat and it unsettles the flow even more, to the point where it causes the Injector to overflow. The one in D.A.G Brown's book uses a much heavier plunger arrangement and an O-ring that presumably dampens out this effect. To be honest, it's not that useful a device because what you need to keep testing is the ability to work at a wide range of pressures. It's just as easy to run water into the boiler and fill it up to quickly find out the whole working range in one quick test. I use the blowdown valve to drain the water, bubbling the output through cold water to condense the steam. The main use is for doing this measurement as far as I can see. If it was really going to be that useful, I would have designed another one, but I didn't see the point. I've tested 54 cones by filling the boiler, and didn't find it inconvenient. |
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Post by Roger on Feb 15, 2021 20:54:49 GMT
Just a quick update on further tests.
I've just run the injector for one minute with the heater on, starting at 95psi and it quickly dropping to about 75psi which it managed to sustain. In that time it delivered 350ml so that's 11.8 fl oz/min. The rating of the injector is 16 fl oz/min, so that seems about right. Obviously it delivers water much faster when the pressure is at 95psi.
I've also performed a test with the water at 30 and 35C. Hot water necessitates a larger regulation gap, so it's a compromise as to how far you go to accommodate hot water at the expense of low pressure cold water performance.
Setting the regulation gap so it's happy and dry with water at 35C, it drops out at about 50psi on cold water. It picks up straight away as soon as the water is restricted as you would expect, and then goes down below 30psi as before.
I guess you could adjust them slightly differently, with one optimised for lower temperatures and one for higher. In the low temperature setup it wouldn't want to pick up cleanly above 30C and would give up at 35C, but it would run dry without regulating the water below 40psi. In the high temperature one, it would be happy picking up at 35C down to 50psi, but would need regulating the water to go lower.
Anyway, with adjustment, you can change the characteristics at the track if it's a scorching hot day and the water is really hot. You may even get it to pick up approaching 40C. However, at that point, I'll be heading for the shade and buying and ice cream, so it probably doesn't matter.
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Post by delaplume on Feb 15, 2021 21:21:28 GMT
Hi,
For a "drinking man" pint/min seem more appropiate (hic)...... so at 20 fluid ozs/pint your 16 fluid ozs equates to 0.8 which is a tad over a No.3 injector at 0.75 pints per min...
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