Author Topic: Talking Thermodynamics  (Read 194390 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #765 on: March 11, 2018, 03:38:55 PM »
Hi Admiral, a lot of the Stuart Turner cylinders have cast in steam passages of rectangular section. However they do need a thorough clean out before use. I think the reason most built up cylinders have drilled holes is because it is quick and easy to do so !!
Willy.

Offline Dan Rowe

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Re: Talking Thermodynamics
« Reply #766 on: March 11, 2018, 04:07:18 PM »
Admiral, K.N. Harris in his book "Model Stationary and Marine Steam Engines" has nothing good to say about simple drilled steam passages except for what Willy said it is easy.

He gives two methods to make proper passages, one is to have a false port face so the passages can be milled, and the other is to chain drill then fill the holes with brass silver soldered in and chain drill the webs and clean up with a file.

Dan
ShaylocoDan

Offline Gas_mantle

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Re: Talking Thermodynamics
« Reply #767 on: March 11, 2018, 06:14:35 PM »
On the subject of steam passages / ports etc, this model loco has had larger cylinders fitted and an unusual port shape to enable things to fit on the existing engine.

The relevant part starts at 2min 30.

<a href="https://www.youtube.com/watch?v=eJu9IzNpJd4" target="_blank">http://www.youtube.com/watch?v=eJu9IzNpJd4</a>

Online paul gough

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Re: Talking Thermodynamics
« Reply #768 on: March 11, 2018, 08:39:57 PM »
Hi Zephyrin & MJM, Here is a photo, (hope its clear enough), of an old sketch of ideas for a 'wet' firebox. Never drew it up carefully so dimensions and proportions are only tentative and for memory jogging. I think the angled water tubes where the flame passes through them to get to the fire tubes is potentially a better idea than the vertical straight tubes as there is not much radiant heat with meths wicks, but only building a few boilers would sort out which was the most effective. Obviously the lower water leg(s) is shown without a connecting tube from the barrel, hope this does not cause confusion. Regards, Paul Gough.
« Last Edit: March 11, 2018, 08:44:25 PM by paul gough »

Offline Zephyrin

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Re: Talking Thermodynamics
« Reply #769 on: March 11, 2018, 10:52:41 PM »
HI Admiral_dk
Yes, steam passages have to be enlarged, streamlined and polished, removing elbows and smoothing curves, to facilitate high speed steam flow, in line with the Chapelon's studies.
But on a small model, streamlining the tiny steam passages is not that easy, grilling them the largest we can should do it !

I post a plan of one of my Gauge 1 loco boiler with 3 flue tubes through the barrel and water tubes in the furnace. Only the sides of the alcohol burner are surrounded with water. This boiler is not difficult to do; in 2 brazing sessions, plus a 3thd one for ferrules and fittings.
pictures in this google album:
https://goo.gl/photos/tn1b8kkCZrqG1Z2V9

Z.
« Last Edit: March 11, 2018, 10:59:08 PM by Zephyrin »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #770 on: March 12, 2018, 10:38:02 AM »
Hi Admiral dk, good to have you joining in again.  You are quite right about the multiple small passages.  As others have said, it is mostly about simplicity.  For small engines which are most likely to spend their lives running quietly and unloaded, it is probably a reasonable simplification for a beginner.  After all, if you are going to run with the throttle partly open, it does not really matter if some of the throttling happens in the ports and passages.  However, for an engine expected to work hard, such as passenger hauling locomotives, all these losses are clearly undesirable.

The basic theory is that the friction drag that resists flow comes from the viscous forces which lead to a shear force per unit area at the passage wall which is proportional to the velocity gradient near the wall.  The total drag force is then proportional to velocity gradient and the passage wall area, which in turn is proportional to the diameter for a round passage.  The heat transfer is a little more complex, but as I see it, there is nothing wrong with your theory.  The small engines we build still follow the normal rules.  I don't know what happens if we go much smaller again, to nano scales.  Always dubious to extrapolate too far.

The flow through the passage is increased by differential pressure over the length, and flow is resisted by the drag force of that shear force at the wall.  The total flow is then determined by flow  velocity and the flow area.  The flow area in turn is proportional to the diameter squared.  So, small diameter passages have more wall surface area per unit of mass flow, and hence present more resistance to flow.  Rectangular or oval passages have more flow area compared with the wall area especially in comparison with multiple small diameter holes.

Hi Willy, thanks for coming in.  I find I can remove metal when necessary with a large file, but when dimensions compel me to use a small file, I find it very difficult to remove much metal.  I don't know if there are better files than the needle files I use, but your comment makes total sense in my limited experience.  I can't match the beautiful job you are achieving with your freelance build, it is opening my eyes to the possibilities of bench work with files, but I suspect that I don't have your skilled artists eye.  You clearly use the skills from your previous life to great effect.

Hi Dan, thank you.  Glad you are still following along.  We are clearly all in agreement that the only justification is easy.

Hi Gas Mantle, good to have you coming in again.  I hope you got your boiler working as you wanted and settled that feedwater question.  I don't have enough data at the moment for videos, but I am sure that many will appreciate your post.

Hi Paul.  The sloped tubes are probably longer so present more heat transfer area, but I don't think they count as water cooled walls.  The outer walls are still solid sheet, but the tubes certainly increase the heating area, so if they are easier to fabricate, they are worth trying.  The vertical tubes are rather like the water tube walls of a full size boiler, and probably still need insulation on the outside.  You have to be careful with the dimensions of the ligaments between the tubes where they join into the header boxes.  May not be in the end any easier to fabricate than Zephyrin's design.

Hi Zephyrin, can you tell us more about Chapelon, or where his or her work can be found?  In the mean time I certainly follow your suggestion to drill the largest hole practical in my small engines.  I mostly aim for the full inside diameter of the steam pipe as a minimum.

Thanks for posting the drawing of that boiler.  It is a very interesting design, with quite a few interesting details.  It certainly shows what can be achieved with a small scale boiler.  Is the design publicly available or available for purchase anywhere?

Travelling again tomorrow.  Will see how I feel in the evening, but will definitely check in.  It is great to see so many replies on these topics.  Thank you all for following and especially for contributing.

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #771 on: March 13, 2018, 11:15:59 AM »
Been a long day today, including the 300 km drive.  Started some reading to help me sort out how to tackle radiation heat transfer.  It's a long time since I did that stuff on any detail, but it starts to look familiar with a little reading as a refresher.  We will see how it goes tomorrow.

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #772 on: March 13, 2018, 07:34:04 PM »
Hi MJM, there is a swiss firm called Grobert and Valorbe that specialise in bizarre small files   quite expensive but most of mine came from secondhand stalls and car boot sales  so a file for every eventuality !! Having a good handle is quite important...especially the ergonomic non vegan type !!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #773 on: March 14, 2018, 12:06:07 PM »
Hi Willy, that is quite a collection of files.  As others have mentioned in previous threads, those non-vegan ones are quite impressive, even apart from the imaginative use of materials at hand, rather than the expensive commercial product.  Do you find that old files from a car boot sale still cut well enough to be useful?  I have always understood that they should not be just thrown in a drawer, and you don't know how the previous owner looked after them.  A blunt file is only metal that needs heat treatment to be useable for engine parts.  (Obviously you don't throw the files in the drawer, but rather, place them carefully?)

Before moving on to radiation calculations, I thought I would have a quick look at the little boiler design that Zephyrin posted along side the Australian Model Boiler codes.  I don't know what design pressure these little locomotives use, but if it is less than 520 kPa it would even fit within the Sub-miniature Boiler Code.  I don't consider this a design check, but just a first step to see if it would be likely to be acceptable for club running.  It seems to fit within the acceptable types (as type F if you have the code) and I think while some minor adjustments to the design would be required, it would be worth talking to the club boiler inspector about.  I suspect the firebox plate thicknesses might have to be increased a little, but I think they would still be workable.  The firebox width might also have to be increased to comply with the ligament requirements on the flue tubes.  It is always interesting to see the differences in detail that occur when a different code is applied.  They are not all equal except for their design intent of providing a safe design and being based on the same basic formulae.

I am wondering if it would be a practical compact design to use in a model boat, so steam could be used in smaller models than most of those I have seen.  Perhaps I have just not seen so many.  But when I sit my boilers, particularly the commercially made centre flue one beside the plans I have a dream of building, the boats all seem too small.  Not sure I could launch them if I scale them up too much.  Of course, more space than you would imagine is always required for the piping and such.  Just as in a full size plant!

Still working on those radiation calculations.  The section in the book starts with saying radiation involves understanding of quantum mechanics and related sciences.  Fortunately it quickly moves on to a simpler approach.  Reading is slow, as we are back to reassembling our home after that carpet laying.  It had to wait while we went to see that classic boat festival.  Which reminds me, I promised some photos.  I have the computer reconnected, so with some luck I will load up a few tomorrow if people are interested.  There were a few steam driven ones, even one model, but most were 1:1 scale wood and sail.

Oh, by the way, many guests to dinner tomorrow, so next post will probably be Friday.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #774 on: March 17, 2018, 11:30:26 AM »
Radiation Heat Transfer-

As you know, I have been slogging through the radiation chapters in the heat transfer text book, it's been a long time since I looked at it in quite so much detail.  But it is interesting to review it all again with a view to calculation of a simple cooling problem that is relevant to our modelling.  At last I feel that I am in a position to write a brief summary that might help with a basic understanding.

The book I use starts in a quite intimidating manner, talking about electromagnetic radiation, statistical mechanics, quantum mechanics as well as thermodynamics.  Quite a mouthful.  So why do these terms from modern physics belong in an introductory text on heat transfer?

Electromagnetic radiation because radiant heat is an electromagnetic radiation just like light, radio waves, microwaves, X-Ray's and so on.  It is interesting that some problems in electromagnetic radiation can also be solved by treating it as a stream of particles.  It is not dependant on having any material in between objects to be part of the process.  Think of heat from the sun radiated to earth through the deep vacuum of space.

Quantum physics is the science used to analyse and understand atomic and molecular activities associated with the energy transfer.  The energy is evident in the vibration and rotation of molecules and movement of electrons between shells.

Statistical mechanics is involved because the energy is transported over a range of wavelengths, there is no single wavelength associated with a temperature or material or energy level.

Thermodynamics is the science that deals with the properties of bulk materials, rather than single atoms or molecules.

But in the end, knowing all that won't have any direct application to our attempts to calculate a heat transfer coefficient.  There is no need to be intimidated by all the big words.

Heat is transmitted in a range of the electromagnetic spectrum that includes all the infra red, all the visible light and a small part of ultra violet.  Higher energy is associated with higher energy and shorter wavelength.

Just like in engine cycles, there is the concept of an ideal adiabatic engine, in heat transfer, there is the concept of an ideal "black body".  This concept describes a body that emits or absorbs the maximum amount of energy possible for any temperature or wavelength.  Like the ideal engine, no real object behaves exactly like a black body, and most objects are approximated as a grey body, defined as a body that emits or absorbs some percentage of the maximum possible energy, always less than 100%, but still the same fraction of the black body emissivity for any wavelength.  Unfortunately no real surface even behaves like that definition of a grey body, and real surfaces have emissivity and absorptivity that vary with wavelength.  When this factor is included in analysis of transparent materials such as glass, we have the basis of the greenhouse effect.  And in addition, the intensity of the radiated heat is dependant on the direction you are looking from, it is not equal in strength in all directions from the radiating surface.

You might think your intuitive knowledge of the behaviour of light would help you understanding of radiative heat transfer.  Unfortunately again our intuition let's us down again.  Both black paint and white paint are quite close to black body behaviour in heat transfer.

Early workers in the area worked out from theory that the total heat energy radiated is proportional to the absolute temperature raised to the fourth power.  The constant of proportionality cannot be deduced from theory, but is determined experimentally.  The constant is called the Stefan-Boltzmann constant, and it's value is 5.675 x 10^-8.  We will use it shortly to calculate the heat transfer from Paul's locomotive firebox.  It is obviously a very small number, but then normal temperatures as absolute temperature are quite large (20 deg C is 293 K) so T^4 is a very large number, and it all works out.  The number can be found in tables of physical constants.  You might even have it on your calculator.

Every surface radiates this radiant heat in accordance with that Stefan-Boltzmann's law, so radiative heat transfer occurs when the surface radiates more or less energy than it is receiving from everything else around it.  So the heat transfer equation is q is proportional to T1^4-T2^4.

Now the proportionality constant for heat transfer is not just the Stefan-Boltzmann constant, that number must be multiplied by the area of the surface, the emissivity and a view factor.

The view factor accounts for detailed problems like heat transfer in an oven for example where an object is being heated from some surfaces while other surfaces are passive insulation which looses heat.  Calculation of these view factors becomes quite complex, and takes up a disproportionate amount of the introductory chapter of most heat transfer text books.  For cooling of a small object in a large relatively constant temperature surrounding, the whole lot can be considerably simplified. 

So let's move on from the heavy theory, and look at how it is used to solve the simple cooling problem we started out with.

The most simple calculation method assumes the surface being cooled is a black body.  Let's use the example of Paul's firebox, 35 mm x 25 mm in area at about 200 deg C.  If we assume the surroundings with a direct view of that plate are very large, approximately infinite compared with the plate, and at a uniform 20 deg C, then the view factor becomes 1, and the rate of cooling by radiation is given by the following formula.  (Remember, length is measured in metres.)

Q = 0.035 x 0.025 x 5.675 x 10^-8 x ((200+273)^4- (20+273)^4)  = 1.3 Watts.

If we assume the emissivity of the firebox plate is 0.8, that is, not black, but a bit oxidised rather than shiny polished, and allow for the surroundings to be more of a grey body than the ideal black, say emissivity of 0.92,  the calculation is a bit more complex, but the answer turns out close enough to 0.8 x 1.3 = 1.04.

If you remember back, the forced convection cooling rate for 200 deg C and the locomotive moving at 1 m/s was 3.2 watts, or for 2 m/s 4.6 watts.  So the radiation cooling, which occurs in addition to convection, is about an additional 30%, a significant enough figure to suggest it should not be ignored.  Unlike convection, radiation heat transfer is not affected by the locomotive speed.

I hope that was not too hard, and helps with a basic understanding of radiation heat transfer.  I think you can see why it is often mentioned then forgotten as people move on to conduction and sometimes even convection.

I happy to discuss it further if someone would like some more information.  Otherwise I suspect it is time for a new topic. 

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Online paul gough

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Re: Talking Thermodynamics
« Reply #775 on: March 17, 2018, 01:13:32 PM »
Hi MJM, Thank you indeed for the very well communicated summary of radiative losses, and in particular for the worked example pertaining to one side of my little firebox. Having quantifications for losses goes a long way in allowing one to get a feel for what is happening and also allows one to make a considered choice as to whether some action should be taken to reduce it. I am very grateful for all the time you put in chasing the convective and radiative losses as well as the broader guide through the 'thorn thicket' of thermodynamics. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #776 on: March 18, 2018, 11:27:07 AM »
Hi Paul, thank you for your encouragement, it is much appreciated.  I never cease to be amazed when you take fourth power of absolute temperatures, quite large numbers, and multiply it by a standard physics constant, and a very small area in square meters, that the answer comes out quite reasonable, even when the dimensions are only a few millimetres.

However, now that we have some estimates of the heat loss from the firebox sides, we really need to fire up that boiler and measure the fuel consumption, so we can see the size of the losses in proportion to the burner output.  This will tell us how much effort to put into reducing those losses.

It is also important to look at where the frames are, to see what shielding is already in place.

I should have mentioned that the radiated heat travels at the speed of light, as do all other electromagnetic waves, but not aeroplanes, as was reported in a recent issue of New Scientist magazine, which reported the effect on a plane doing 1.2 times the speed of light.  I think they meant the speed of sound in their little news item.  A plane travelling at more than the speed of light would be a really big news item, surely worth much more than a short paragraph!

Insulation for radiant heat is interesting.  The sun on a tin roof heats that roof which in turn radiates the heat down into the shed, or house underneath unless there is a good lining.  The air under the iron gets hot and much of that heat is transferred inside.  To insulate the roof from the radiant heat of the sun, a second roof above the first, just separated by an air space that is well ventilated, is very effective.  The outer roof receives the heat from the sun, blocking the direct path to the building roof.  It of course gets hot, but radiates to the inner roof based on its temperature, which is much lower than the sun's temperature, so much less heat transferred.  The air that is heated between the layers flows out the ventilation spaces and is replaced by cooler air.  You can carry this further and put a third layer over the top for even more reduction of the heat transmitted to the building.  Probably diminishing returns on any more than three layers.  This is the same principal as setting a tent fly over the tent, though it is less effective unless the outer surface is highly reflective (as some are).  Part of the issue with a tent is that the material is not totally opaque, so some of the Suns rays are transmitted through the cloth to the inner layer, but it is definitely cooler inside a tent with a fly than in one without. 

Of course, if you are setting up the tent in the snow that seems to be falling in many northern parts at the moment, you have to reverse the thinking a bit.  Remember, the net heat transfer is from the higher temperature to the lower temperature.  And the higher temperature, if you can manage it, is inside the tent.  So you want the shiny material facing in to keep the heat in.  Of course convection is probably the bigger issue.  I have camped in the snow in around -16 from memory (in Ontario) when the scouts needed a few more nights camping experience to qualify to go to the World Jamboree, but I don't think we thought much about heat radiation on that occasion.  We probably put the shiny sides of the fly facing out as you would in summer.  The principal also applies to your picnic Esky (or drink cooler), if you are putting a shiny blanket over it to slow the drinks warming.  Heat is entering from the outside, so you need the shiny side of the blanket facing out.  However, for the emergency heat blanket in your first aid kit, you want to conserve the patient's heat on the inside of the blanket, so shiny side in.  It all depends on which way the heat is travelling.

For your firebox, which is the source of the heat, a second layer perhaps provided by the frame is very close to the heat source.  It heats up with the radiated heat, and radiates back more heat as its temperature rises.  This in effect reduces the temperature difference between the firebox and the surroundings in its view, so reduces the net heat loss.  So either an air gap, or a layer of insulation (opaque to radiated heat) would reduce the heat loss from the firebox.  However, as more heat is lost through convection, the insulation would probably be the best approach.

Roofs, tents, drink coolers, first aid emergency blankets, despite its bad press, it is worth a little effort to understand radiant heat as it has so many applications.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #777 on: March 18, 2018, 02:19:10 PM »
Hi MJM, More good stuff here .....I always wondered about the reason for flysheets ..??so nothing to do with fly's then !! Also found out about calculations for model boat 'ratings'  Quite a long formulae.....his day job might have been a thermodynamics engineer !!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #778 on: March 19, 2018, 01:09:28 AM »
Hi MJM just a few technical questions........... on my electric boiler the output pipe is 5/32" ,and the inlet pipe to my new engine will be larger , about 1/4"  if i want to connect the two together should i have the connection as close to the boiler as possible or to the engine ?.... when the two pipes are joined is there a significant pressure drop  into the larger pipe ? Also is it possible to work out with all the info on my boiler how large an engine can be run assuming the boiler is kept fed with cold water ? The new engine i am making is double acting with slide valve , running at say 150 RPM. with 1. 1/2" stroke by 3/4' bore......Also could you insulate a boiler using chicken 'Bacofoil' with the shiny side inside and could you use several layers at a suitable distance apart ?
Willy.

Offline Zephyrin

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Re: Talking Thermodynamics
« Reply #779 on: March 19, 2018, 08:21:41 AM »
Hi MJM460
The boiler I have shown is for a plain Gauge 1 loco, 1/32 scale. Its a small loco even for its gauge.
The pressure is usually between 1.5 to 4 bar depending on the load, and the water contain is 0.1 litre, an axial pump try to keep this value constant.
I did another boiler on the same design for a larger loco, in this album.
https://goo.gl/photos/dY6fYHntSb5wJUUP9
the boiler is surrounded with a layer of 3mm of cork, and a foil of tinned steel.
After the run the loco is too hot to handle with bare hand, not a surprise, as is the case for all these tiny machines...
And yes, the frame also is hot, but heated mostly by the cylinders between the frame more than by the firebox, owing to a too powerful draught ?

I don't known the code you are referring to...I should say that in our club we don't have a boiler inspector...Yes some incidents may occur, loco catching fire, train derailments and falling on the floor etc.

all my boilers are properly tested and safe, I did 17 locos in this gauge or smaller...
A popular loco boiler drawing is present in "the Project" loco booklet, edited by the Gauge 1 Association, UK, which certainly has the approval of boiler's inspectors...
in this plan, as in a large majority of plan, a single larger flue tube is traversed with inaccessible cross tubes.

For live steamed boat, maybe all the boiler are now butane fired, a much easier solution, owing to the risk of alcohol spilling during the launching of the boat.

 

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