Talking Thermodynamics

[crueby]

Hi MJM just a quick question ........wondering about thermal coal ??!!

Anything like thermal underwear??

[MJM460]

Hi Paul, it has taken a while, but good to be talking about combustion and your questions from a few days ago.  Talking about accelerants is probably not where I would choose to begin, but why not?  I looked up that web site, a secondary science project from Canada, if I have understood correctly.  It's good to see young people working on science projects, they are the scientists and engineers of the future.  Very hard to comment on the project.  I would assume the supervisors main job was to ensure the students did not come to any harm, and not to contribute to the project.    Perhaps help with choice of equipment and so on.  It's not a measure of the supervisors' knowledge.

So let's just look at whether some peroxide will help your locomotive performance.  In principle, adding an oxidiser as an accelerant provides oxygen for combustion without the nitrogen which slows the reaction, basically by getting in the way of the fuel and oxygen getting together. And by absorbing some of the heat produced, it limits the maximum temperature reached in the reaction.  But it does not add to the heat produced, which comes from the reactions between oxygen and the hydrogen and carbon in the fuel. 

The students state that peroxide decomposes into oxygen and water in an exothermic reaction, that is, a reaction that releases heat.  I have not checked that, but have no reason not to believe it.  But the quantity of heat is key, as that is the only source of extra energy.  It would be interesting to know how much heat is released per unit of peroxide decomposed.  I do know that hydrogen peroxide is highly unstable.   The students overcame this issue by using a solution in water.  The concentration available in the local pharmacy is quite low, but adequate for disinfectant and similar household purposes.  I think they used a more concentrated solution, which is probably available from suitable sources.  But it still comes with plenty of water.

The student experiment then involved carefully measuring the temperature in a beaker above the flame as a measure of the heat produced by the flame.   They did this very carefully and they documented it well.

When those students progress in the years ahead and start studying thermodynamics, they will see what was missing from their experiment.  You have been following this thread from the start, so I hope that you can also see the issue.  Otherwise I have some gaps to go back and fill.

So I suggest we go back to the basics, and look at your earlier questions, then return to see if the accelerant would be useful.  Thank you for recognising the time these posts require, but it really is a privilege to share the information with people who are interested.  And it is something I can give in return for all I learn from other members who post on this forum.

Hi Willy, coal is not a pure substance, but is dug out of the ground, where it was laid by the processes of nature in times long past.  It always comes with various earth based impurities, and also a variation in the basic carbon content, density, heating value etc.  It ranges from the soft, wet brown coal we have here, which the metallurgical engineers describe as something made by our creator to sop up water, through to the hard black coals.  The miners separate out what they dig up, and divide it into various quality levels, which generally attract differing prices in the market place. 

Thermal coals are the quality generally selected for burning to produce heat for steam, electricity etc, generally the lower price ones, as opposed to the metallurgical coals which are selected for steel making and other high value processes.  At work, I had the experts down the corridor, but I did not get involved so much in those projects.

Hi Chris, so not thermal underwear.  The best of that comes from other natural sources, particularly merino wool.  There is a very good brand manufactured in New Zealand, and we produce a lot of good merino wool here.  I hope that is not considered advertising, but the farmers can do with some support.  The main other source for thermal underwear is the the fleece of the Wild Orlon, a strange beast of US origin, that is born from parents coming from an ethylene plant.  Also natural, in that the ethylene comes from oil and gas wells, albeit with a little help from chemical engineers as midwives.  Or is it Vets?  But more generally described as 'guaranteed to contain no natural ingredients'.

So Paul, you mentioned flame temperatures that were quite similar over a range of fuels.  I had not seen it stated so clearly before, but it was there when I checked, in plain view in my text book.  Perhaps I never opened that page.  The figure was a little higher than you quoted, but it was clearly stated as for stoichiometric combustion.  That word just means the exact theoretical air fuel ratio.  In practice, excess air is required to ensure that all the fuel atoms actually meet an oxygen atom while the temperature is enough to initiate the reaction.  A bit like you need more guys than gals at a dance to ensure that every girl is able to find a partner.  Of course that means some of the blokes miss out, but some one has to.

Generally 15 - 20% excess air is required to ensure complete combustion.  This means that the reaction not only has to heat the combustion products to temperature, and the nitrogen that accompanies the oxygen, but also all that extra air has to be heated to the same resulting temperature, and that limits the temperature reached.  It is possible that allowance for excess air explains the difference in the temperature in my text and the figure you have quoted.

So what are the other parameters?  Well temperature is an result of the heat contained in the flue gases, but is not the measure of the heat.  Each component of the flue gas has a property called specific heat, which is a measure of the heat (in Joules) required to raise the temperature of a kilogram of the substance by one degree kelvin.  A difference of one degree Kelvin being the same as a difference of one degree centigrade.  And of course you can use Btu, lbm. and degree F if you prefer.   Immediately, you can see that you need a gas composition, mass of flue gas, (which is equal to the mass of fuel plus mass of air), specific heat of each component, and the temperature rise from atmospheric temperature to that flame temperature.

Now there are two values of specific heat, depending on whether your reaction is at constant volume, so called Cv, or constant pressure, Cp.  Fortunately you only need to know one of these, as the difference between them involves only the universal gas constant and the molecular weight of the substance.  And while it is called a constant, it actually varies with temperature, so you need the appropriate value for the temperature range you are referring to.  However, there is a simple short cut through all this, as the heating value of the fuel is readily available data for common fuels, and the ones you quoted agree with the data I have.  The value for methylated spirits needs a little further explanation, but I will come back to that later.

 The units of that heating value are Joules per Kg, or more conveniently MJ/kg, as none of us like keeping track of too many zeros in our calculations.  Or KJ/g for the quantities in your small locomotives. 

So that flame temperature gives a figure we need for heat transfer calculations, and the mass of fuel burned per second tells us how much heat we have available.

Several more questions to answer before we have dealt with your post (#557, 12 Dec), so I will continue those tomorrow.

Thanks for looking in,

MJM460

[paul gough]

MJM, Thanks for your advice regarding my alcohol addiction thus far. As you say any given amount of fuel has to heat all the gases in the air supporting combustion and this was one of my many 'maybe thoughts' regarding the use of H2O2 as a provider of oxygen, it might be able to reduce primary air volume, but like most of my thinking in this area it is conjectural as I have little knowledge of fuel chemistry or the delicate art of combustion engineering and am only able to apply, mostly forgotten, schoolboy chemistry. I was hoping that I might draw in some model rocketeers or maybe someone who has tinkered with a scale version of the torpedo motors that use H2O2 for the oxygen supply who may have lifted some fog from my thinking and pointed out pitfalls and baseline requirements, e.g. such as whether a stabiliser or catalyst might be required for my uses. I could probably go on and on, but don't want to drag discussion off on too tangential a trajectory, after all this website is primarily dedicated to model engine making and fear too much chemistry might induce a lot of yawning, or worse! turn people off this thread. So I'm happy with a brief indulgence but of course very grateful  if it pleases you to go into some depth. Regards, Paul.

[Steam Haulage]

What part do azeotropic mixtures play?

[MJM460]

Continuing on combustion -

Hi Paul, I hope you mean addiction to alcohol as a locomotive fuel, I will try and go al little further yet, as I am also a great believer in its suitability.  I started the thread after noticing that chemistry and thermodynamics seem to be actively avoided in the usual sources, and hoped that there would be some interest in an introduction to the concepts that help us further our hobby.  I think the number of reads probably speaks for itself, but I hope people will drag me back if I go to far into the esoteric, rather than drift away.

Hi Steam Haulage, I was hoping no one would ask.  An azeotrope is a mixture that boils with a constant composition, and despite a lot of heavy reading on the subject, was unable to be sure if it applied the ethanol water mixtures or not.  It might do, as you cannot purify ethanol beyond 95% by simple distillation.  I am reasonably sure that it does not apply to the mixtures of ethane, propane butane and pentane, the simple paraffin series hydrocarbons, and also the olefins and diolefins.  With those, I am well familiar with separation to high purity ethylene and ethane, or propylene and propane for example.  But even further in left field, you cannot separate normal water from heavy water by simple distillation either.

Quite a few concepts introduced last time.  I hope it was not too confusing, and you were able to keep track of whether 'it' was nitrogen, as intended in one sentence, or oxygen as was intended in the very next sentence.  Comes of writing at the end of a long day.

I suspect that it is possible to calculate that flame temperature from the energy balance, though it requires data for the specific heat of the reactants over a wide temperature range, while I only have data for  one relatively low temperature.  It is not so surprising that propane and butane have very similar heating values, as they all and made up of carbon and hydrogen, with very similar carbon to hydrogen ratios. 

Ethanol has one oxygen in the chemical makeup of the molecule, which can be written symbolically as CH3-CH2-OH and the lower heating value is possibly explained partly by the lack of that extra hydrogen, but it is also because one of the hydrogens has already combined with oxygen in that hydroxyl (-OH) group attached to one of the carbons, so we get no heat from that reaction either.   The oxygen which has sort of taken the place of a hydrogen atom does not add to the energy released during combustion, but actually reduces the heat released compared with the release if had not been there, as in say ethane which also has 2 carbon atoms and 6 hydrogens.  That is certainly an oversimplified explanation, but adequate for our purposes.  Clearly the effect of this oxygen is anything but an accelerant, so does not tell us much about that peroxide experiment.

There is another factor which, in practice, further reduces the heating value of ethanol in the normal form of methylated spirits.  If you look carefully at the container, you will see somewhere the composition.  It is normally 95% ethanol and 5% water.  There is a perfectly valid reason for this, in that ethanol is normally obtained from fermentation processes (I don't know why I am telling you this!) which produce a solution of ethanol in water.  The ethanol is purified by a distillation process, but it is only possible to achieve 95% purification by this process.  A result of those vapour pressures and liquid concentrations when a two phase mixture of substances is boiled, that we spoke about earlier.  There is also another compound added to methylated spirits to make it undrinkable, well to most of us anyway.  When we then take that 95% ethanol mixture, and burn it, we end up having to evaporate that water, which means we have to supply that latent heat, and which goes along for the temperature ride up to the flame temperature.  It does not add to the energy produced, but simply dilutes the flue gases and reduces the maximum temperature reached from the reaction.  It does not reduce the energy produced either, except perhaps by getting in the way, similar to nitrogen, so may result in a bit less complete combustion.  You can see the result of this in the sticky black soot deposited on your boiler if the flames are allowed to lick the boiler.

So at this stage, I hope I have drawn a clear enough distinction between temperature and heat release, and introduced some of the concepts that make methylated spirits a different fuel to burn.  We can come back to that later, but first let's look at how that distinction between heat and temperature helps us understand the process of boiling water in our boilers.

The heat transfer area in the boiler has to enable the transfer of energy from the combustion products to the water in order to generate steam.  The rate at which the heat can be transferred is determined by the temperature difference between the gases and the water at steam pressure and temperature, and in addition by the heat transfer film coefficient.  Then, as heat is transferred from the gases, the gas temperature is reduced, so the temperature difference is reduced and the rate of energy transfer per unit of area is reduced as the gases travel over the heating surface.  This is where that log mean temperature difference comes in.  The simple one to understand intuitively is the marine centre flue boiler, a description I would also apply to Chris's Lombard model.  The hot gas from the burner starts at one end and travels along the flue, loosing heat to the water, and consequently falling in temperature as it travels along to the smoke box end.  Clearly the biggest temperature difference is at the burner end.  The first, say one third of the flue length transfers the biggest part of the heat due to the larger temperature difference.  The next third starts with a much lower temperature, so looses heat at a much lower rate, and similarly for the final third, where the heat transfer rate is lower so the temperature change is not so much.  Clearly there is diminishing returns on adding length to the boiler in the hope of cooling the flue gases further to raise more steam.

There is also limited usefulness in raising that flame temperature, say by using an accelerant, or perhaps using some oxygen enriched air.  Yes it would increase that initial heat transfer coefficient, but at very high temperatures, the water side boiling coefficient is affected, I believe it is something along the lines of the whole area becoming vapour on the metal surface, instead of boiling liquid, and this dramatically reduces the heat transfer rate.  Just where on the temperature scale, I am not sure, quite possibly a little benefit before you get to that point, but just not linear, ad infinitum.  On the other hand, burning more fuel so there is more heat and a greater volume of flue gas does two things.  It increases the velocity of the flue gases, which tends to increase the heat transfer rate a little, but the greater heat content of the flue gas means that the temperature at the end of that first third of the length is higher, hence the temperature difference at the start of the next third would be higher, and so more heat transferred there, and so on to the end of the boiler.  At the smoke box, with the same length of boiler, the flue gas is still at a higher temperature than it would have been with less fuel burned, so more heat is lost up the stack.

Lower overall boiler efficiency, but more steam generated in the same boiler heat transfer area.  That is the point we were discussing a few days ago about waving the hand over the smoke stack to see if the boiler could handle more heat.  The actual flue gas in the smoke box cannot be cooler than the steam in the boiler, so is quite hot enough to burn your hand.  But as the flue gas exits the stack, it mixes with air, and is cooled in a relatively short distance.  So start with your hand high, and cautiously bring it down, and get out of there before it feels to uncomfortable.  Experience with several different boilers will soon show you the difference between boilers by how close to the stack is comfortable.

So if the flame temperature is about the same for many fuels and if we don't really want to increase it too much anyway, is there anything we can do to increase the steam production from our model?  And how do we decide on the best fuel for our model?

This has been pretty heavy going, I might need to answer a few questions on the topic, otherwise I will continue on those last two questions tomorrow.

Thanks for looking in,

MJM460

[steam guy willy]

Hi MJM ,when i made violins i used industrial meths for the spirit varnish as it had
 no colour . The meths you buy in the tool shops has a blue dye in it to make it not so attractive to the people that drink it!!! is there a difference between the two ?Willy......

[Steamer5]

Hi Willy,
 No difference except the blue stuff added & the stuff to make you sick!

Now having a dad in his 9 decade has some advantages....I know how to take the blue stuff out of meths, I hasten to say NOT TO DRINK.....but best not to put how here.........

Cheers Kerrin

[paul gough]

Hi MJM, My alcoholic tendencies pertain only to its use as a fuel in burners. I am following your words carefully and will refrain from further questions until you have covered those already put. You might be interested in a slightly more advanced discussion of ethanol and H2O2 in a paper from the 21st Brazilian Congress of Mechanical Engineering, 2011, at the following site; <http://www.abcm.org.br/anais/cobem/2011/PDF/197101.pdf>. Regards, Paul.

[MJM460]

Hi Willy, here they put blue dye in kerosene and Meths is normally clear.  I doubt that it makes much difference to to the properties as a fuel.  Possibly responsible for the smell some people complain about.  We use it as our main cooking fuel on a small boat, much as hikers use those Trangia outfits, and don't notice any particular smell.

Hi Kerrin, thanks for joining in.  Taking the blue stuff out might overcome the reported odour problems.

Hi Steam Haulage, are you able to add anything about azeotropes.  I thought I understood it, but I usually check these things before including them in the post unless I am really confident.  When it came to it, I could not find a satisfactory definition that made it clear whether that water alcohol mixture is an azeotrope.  That is why I did not mention them earlier.  Can you confirm the term applies to that mixture of water and ethanol?

Paul, I hope I have made some progress on your questions in this post.  I will look up that site later, we are obviously all writing at the same real time.  You can see that the heat necessary to get any amount of steam your boiler can produce, can generally be obtained from almost any fuel so long as you have a burner that will burn it at the required rate.  You are quite right, to observe that you have to burn proportionately more ethanol than butane to produce the same amount of steam.  It is important to be careful with the units.  Each of the fuels has a different density, ethanol density of 790 kg/m^3, compared with butane about 570 (depending on the ratio of normal to ISO butane), helps partly overcome that difference.  It is probably better to stay with mass measurement for comparative purposes.

Of course, sometimes you will see heating values based on vapour volume.  The reason is that one mole of any substance occupies the same volume.  So volume measurements are proportional to the number of moles of the substances.  But only if you are using vapour volumes.  You should not mix vapour and liquid volumes when calculating air fuel ratios and so on.

So the choice of fuel is based on convenience, safety, availability and the practicality of the burner in the particular boiler setting.  For your small locomotives, I suspect there are practical difficulties associated with coal, that some will see as a challenge and persist for the sake of following prototype, but probably not the simplest choice until the models are larger.

Gas requires a pressure vessel for containment and has safety issues in terms of the consequence of leakage.  However it burns cleanly, and suitable burners are commercially available in a range of  sizes.  Whether to use propane or butane will depend on availability in your area, which in turn probably depends to some extent on climate.

Methylated spirits, the common name for ethanol based fuel, has a huge safety advantage for model uses.  You can put out Meths fires with water.  Alcohol is soluble in water in all proportions, the alcohol is absorbed into the water and cooled below its vapour pressure, so the flame is cooled and snuffed out.  It is the vapour that burns, not the liquid.  It is also relatively cheap and readily available.  But as you know, it can can be a bit tricky to burn.  Ok, a wick will burn a certain amount, and a bigger wick or more wicks will burn more Meths for more heat.  Those Trangia burners start with a pool of liquid which you light at the surface, and the arrangement starts vapourising some of the fuel, while the pool surface fire is extinguished. 

I have seen comments about the air requirement, the theoretical air for ethyl alcohol s 14.3 to one by volume, compared with 23.8 for propane and 31.0 for butane for stoichiometric combustion.  And then there is the heat absorbed to evaporate the water content.  But you have done a lot of work in this area and I would be very interested to learn more about it if you would write it up, either here or in a separate thread to help us all get a bit further on understanding this fuel.

The stove I mentioned that we use on our boat, is very effective.  It does totally evaporate the fuel into a burner much like a gas burner in a domestic stove, but with the straight liquid tube over the burner to evaporate the fuel.  The tank is slightly elevated, so it has about 75 mm of liquid as pressure to drive the flow through the needle valve regulator.  I have attempted to copy this without much success, but need to get back to that experiment.  However that is all getting off the track of your carefully thought out questions.

You mentioned the difference between different burner configurations, such as the poker type, ceramic, or wicks.  When heat transfer is discussed we nearly always jump straight to convection heat transfer.  However, for our boilers, particularly at the burner end, I expect that radiation is also a significant factor. 

Radiation is a little different from conduction and convection.  It travels in straight lines, not dependent on the air or gas in the way, or at least only minimally so.  But it requires line of sight.  A large part of the maths related to radiation is about calculation of this view factor.  Radiation heat transfer is proportional to the fourth power of the absolute temperature, so that flame temperature is quite important in this case.  And the radiation intensity is dependent on the frequency of the radiation and most heat transfer is in the visible to infra red region of the spectrum.  If you can't see it, or feel the heat on your hand outside the hot gas stream, then radiation is not very important.  The frequency is dependent on the temperature.  For the conventional locomotive boiler, that firebox has a very effective view factor for the coal fire on the grate.  Now the heat transfer area also radiates back, but because of its lower temperature the net heat transfer proceeds from the hot coal to the cooler plate.  Now that is a very brief summary of the key points for radiative heat transfer, but it might be enough to guide some useful experiments.  Once the combustion products leave that firebox and enter the flue tubes, radiation is a much less important factor, as gas tends to be a poor radiator.  In the flue tubes, convection is the predominant heat transfer mechanism, and this is where gas velocities have an influence on the gas side film coefficient.

A coal fire is a very good radiator, providing it can see the receiving surface, and it provides plenty of hot gas for convection transfer in the tubes.  Gas fires however, with a much less visible and predominantly blue flame, are not such good radiators.  This is where that ceramic grate comes in.  The ceramic is heated to red hot which radiates to the heat transfer area.  Some people advocate some stainless steel mesh placed so it is heated to glowing by the flame, and then radiates to the boiler tube.  The idea is to increase the total heat transfer by harnessing some radiation transfer.  I have no experience to say how effective this is.  I hope others who have tried this scheme can tell us how well it works.

Remember the importance of that line of sight.  When ethanol is burned on a wick, located like that picture you provided a few posts back, it appears to be outside the boiler, with only a shroud to guide the flue gases into the tubes.  This shroud will get hot, but if I understand the arrangement correctly, it has limited ability to transfer that heat to the boiler water, so it appears that the radiant heat is largely wasted.  Not easy to analyse in detail, as the shroud is able to transfer some heat to  the flue gases by convection, so it is not all lost.  But possibly worth thinking about boiler arrangements that have some sort of water wall around and over the wicks.  My little boilers have superheater coils around the firebox.  Nothing fancy, about two turns around the inside of the casing.  They are not in the flame like the main pot boiler but receive heat by radiation.  They superheat the steam from about 110 C in the boiler to 138 C so clearly absorb significant heat.  I had not thought of it quite this way before, but it possibly points to the way to arrange the water space around the flame to take advantage of the radiated heat.

Now that poker burner.  I have not seen one in action close up.  But it does look like it would provide significant radiant heat over its length.  So making the flue tube large enough to accommodate the poker burner is probably a worthwhile idea, as the tube is submerged in the boiler.  The combustion chamber end of the tube has to be large enough to allow the burner to function properly.  The combustion gases then continue through the rest of the tube, where cross tubes or in small sizes possibly even solid cross rods can collect extra heat by convection.

 Unfortunately my theoretical knowledge is not enough to quantify the different arrangements or to determine the optimum layout.  We are back to being reliant on experiment.  And of course the experiment is only really useful if we can arrive at a suitable method to organise and analyse the data.  But I hope those few thoughts on the subject will help with understanding, and help guide productive experiment.

Thanks for looking in,

MJM460

[Noitoen]

Hi Willy,
 No difference except the blue stuff added & the stuff to make you sick!

Now having a dad in his 9 decade has some advantages....I know how to take the blue stuff out of meths, I hasten to say NOT TO DRINK.....but best not to put how here.........

Cheers Kerrin

In South Africa, they used a loaf of dried bread to filter out the blue stuff for drinking purposes

[paul gough]

Hi MJM, Your discussion and insights into the goings on concerning fuel, combustion and associated boiler issues has provided much to ponder and look into. One of the useful things also about all these exchanges is that many, if not all, the parameters that one has to consider are brought together in a couple of pages of text rather than randomly scattered throughout myriad publications etc. This should make it easier and quicker for anyone researching to get a thorough grip on things.

Coal firing in Gauge One is not unheard of for locos of middling to larger sizes, a number of the locos from Aster had a coal firing option as do a few from Accucraft, quite a number of youtube videos show methods of firing etc. Quite a number of self builds also exist and a few with 'wet' fireboxes but the ones I have seen are just conversions from meths dry fireboxes. With regard to my very small model, the wick flame does impinge on the back end of the underside of the boiler barrel and the dry firebox or shroud has ceramic sheet insulation around the inside so does not, if at all, contribute to heat transfer to the boiler. It is only 1mm thick sheet but I intend doubling this if not tripling its thickness, more changes, more experiments, more time. This one little loco and all the things I think of to try out both with boiler/burner/fuel  and the mechanism will probably more than occupy me up to the end of my tenure on the planet. With the wicks, it is important to establish a good blue flame, which involves establishing the best wick length, the number or how tightly packed the strands are in the wick tube, the height the wick sits above the end of the tube and ensuring proper balanced fuel delivery to each wick, how high the wicks sit inside the firebox, the size of the opening at the base of the firebox too big and you suck in too much cold air, also the cross section area of any narrowing of the flame/gas pathway from wicks to fire tubes, not too small but not too big, also the flame length should be determined by setting up burners in the open or outside the fire box in still conditions and measuring it, there should be enough length of flame getting to the tubes in my experience. Some restriction to air flow in experiments can be had by placing metal fly screen or other fine mesh at the bottom of the firebox or in the flame/gas path to observe results. Some have experimented with small holes drilled around wick tubes near their ends, but the only person I know who did this found no significant changes, but as far as I am aware it was a one off try and no extensive tests were performed. As to what can be done on such small model locos, it is generally restricted by the dimensional limitations inherent with any particular example being modelled as well as the general smallness of gauge one models, obviously I have made a rod for my back trying to advance performance with one of the smallest of models, bigger U.S. types would make things considerably easier, however I just happen to like pre 1850 locos. Flame behaviour is observed using a small inspection mirror from an auto parts supplier, I prefer the more rectangular ones rather than circular types. At some point I intend to make a glass walled firebox, a glass 'spy hole' to observe the flame entering the fire tubes and a glass fronted smokebox which will allow direct observation, even if only for a short time due to possible 'sooting'. An easy experiment is to try various wick materials, eg. stainless scouring pad, steel wool or tightly rolled fine mesh etc., etc. As too superheating, I agree it is useful, especially in attempting to eliminate condensation and maintaining higher cylinder body temps, but lubrication needs to be assured when going to much above wet steam temps especially for the slide valves, obviously metal on metal, whereas the pistons are frequently supported by Viton or Teflon rings so not quite so critical. For the moment thats about all I can bring to mind on general principles, no doubt I haven't covered everything, if I remember anything important I'll put it up. If I ever get to setting up a stationary boiler test bed for small engines I think I would go for  a 'colonial', nice and simple but allows various number or sizes of wicks for metho, other fuels, and burner types to be easily installed under the barrel for comparative tests. Once again, thank you for sharing what is in your brain and the results of 'reading up', knowledge is a precious thing and to get it as freely as we do from this thread and website shows considerable generosity. Best wishes for the Christmas, Paul.

[MJM460]

Some pictures at last!

Hi Noitoen, simple technology the best perhaps?  Unfortunately I suspect the bread is only acting as a filter and removing the dye particles.  Remember how they used to demonstrate Brownian motion in junior science?  But a filter does not remove solutions of other compounds from the mixture   So I would assume the poison they add to discourage drinking is still there.  It is also added to our Meths, which like Willy's industrial brew, is colourless, so the poison does not in itself add the colour.  Of course, if the dye is responsible for the odour when burnt, or perhaps some residue deposits on the burner or boiler, it may still be worth removing the dye, even for burning as a fuel.

Hi Paul, thank you for writing up such a great description of your experiments.  So many things to try before you even think of starting on gas burners.  Some of those experiments I guess contribute quite small differences.  How are you assessing the results?  Keeping a good record of what you have done must be a challenge.

While confining your experiments to a small engine will have its limitations, it's much more enjoyable to keep to a prototype you like, and by confining yourself to one platform, you must be getting a really good idea of how things all work together.

My reference to my superheater may have been confusing.  I was only intending to point out that there is useful radiant heat available at the sides of the flame, and presumably over the flame, as demonstrated by the heat collected by my superheater, and due to the T^4 effect, possibly worth harnessing instead of relying solely on convection.  By collecting some radiant heat and the remainder by convection, there may be an overall increase in efficiency.  It is quite hard to predict how much however, because radiant heat that is not collected almost certainly at least partly leaves more available heat in the flue gas for convection.  But radiant heat reaching the dry firebox wall probably also adds to the heat loss from the boiler.  Obviously more experiment required. 

So far I have not appeared to have lubrication issues with the superheat I am achieving, and using just a displacement lubricator.  But those viton and teflon rings will be getting near their temperature limits anyway, so may not tolerate much superheat.  However, with graphited packing?  Oh, and slide valves.  Not necessarily metal on metal.  Almost certainly an oily film of steam or condensate between the surfaces of even the most smooth surfaces unless they are tightly bolted together.  The principle of working out the balance force on the valve usually assumes an approximately linear gradient between the pressure at the edges where the pressure is definitely known.  For example in the exhaust cavity and around the outside of the valve.  That is what I was working on when commenting about the asymmetry of the valve possibly tilting the valve in Chris' Marion Shovel engines.

The glass smokebox idea sounds interesting.  Yes, it will probably need cleaning for each run, but you may not need to use it often to learn what it can tell you.  If you can just make it interchangeable with the normal metal one.

Thanks for your kind words on the thread.  I am delighted that you are finding it interesting.  But please remember that the comments and questions by you and Willy and so many others are giving me wonderful pointers as to just where the knowledge that I gained in my very different work environment can be useful in helping us understand our models.  Together we have gone way further than I imagined when I started this thread.  The sharing is definitely a two way street.

I have attached a photo of the burner from the metho stove I mentioned last time.  The mirror underneath was not as helpful as I had hoped.  The liquid from the fuel tank comes in at the left, and the needle valve is in the tube on the right, with the seat near the left hand end of that hex bar.  The length of the tube through the burner is all that it takes to evaporate the fuel, the little plate on the left hand side increases the heat transfer to the tube, perhaps most importantly while the stove is being preheated for lighting.  The jet is in the side of the little vertical pin on the bottom of the hex bar.  When the needle valve is opened, the vapour stream induces air as it crosses the gap to the opening in the side of the burner.  To light it, about a teaspoon of liquid is allowed to dribble out of the jet into the bottom tray, then the needle shut.  You light the liquid surface with a match or empty spark lighter, and open the needle valve again just before the flame goes out, and you now get vapour to the burner.  The last of the flame lights the main burner.   Boils a kettle quite well, perhaps a bit slower than a propane gas stove, more fierce than a wick flame and works well for us.

I also promised a few days ago to attach the diagram of butane properties.  Attached is the pressure enthalpy diagram for iso-butane.  Normal butane is very similar, just the pressure at any temperature is a bit lower.  I have attached the same diagram for water so you can see the form is very similar, just the numbers differ for two quite different substances.  The diagram I was previously using for steam, when discussing engines and work output, was a temperature entropy diagram.  Again a similar form with that two phase area very obvious, but a slightly different shape.  So I have also included the T-S diagram for water for comparison.  Unfortunately I do not have a T-S diagram for butane.  These are copied from the Gas Processors Association handbook.  I hope it is acceptable to use that extract for illustrative  purposes, in a discussion such as this.  Unfortunately I had to fold the i-butane diagram to fit my scanner.  The sloped lines that lost their label are the specific volume lines.  You can see the diagrams are more use to help understanding how the other properties change with a change in any selected property, than they are for picking off accurate values over a small range.  Always better to use tables if you have them.  But the point of including them is to show that very different substances behave in a similar manner in the two phase region.  If you understand the temperature-pressure relationship for boiling water in your boiler, then you also understand the behaviour of the liquified gas in your fuel tank.  You don't have to start learning from the beginning for gas.  Or for the refrigerant in your refrigerator or your air conditioner.

A little shorter post this evening to make up for a few recent longer ones.

Thanks to everyone following.  If you are packing up to travel to friends and family for Christmas, safe travels and best wishes,

MJM460

[MJM460]

Hi Paul, I looked up that Brazillian site on peroxide in rocket fuel.  Quite interesting.  They were looking at very high temperatures and pressures as you might expect in rocket propulsion.  You clearly would not want those pressures and temperatures in the firebox of your locomotives.  I also looked up some other properties of hydrogen peroxide, and can see why the supervisors of those students were pretty careful to use low concentration solutions.  At these levels, it appears that the heat from decomposition of peroxide is less that the heat required to evaporate the associated water.  This would seem pretty safe, but a bit pointless if you need more heat to evaporate the water than you gain from the decomposition.  The decomposed peroxide, consisting of water and oxygen reduces the amount of air required for combustion of the ethanol, so perhaps gives a higher peak temperature, but at the cost of less available heat due to the energy contained in that water vapour.  I say perhaps gives a higher temperature because it is possible that the water contained in the flue gas absorbs nearly as much heat as the nitrogen that comes if you simply use air.  Remember that adding more oxygen in this way does not in itself produce extra heat from burning the fuel.  So while there is a contribution of heat from the peroxide decomposition, a safe dilution absorbs even more heat, simply to evaporate the water that comes with it.

It appears that at much higher temperatures, the reaction is highly unstable.  Read 'verging on explosive', if not actually explosive.  As far as I can see, it would be very hard to control the reaction safely, and I don't have the necessary knowledge to find a safe way through the various parameters, so don't have much to offer in this area.

I don't know if you have found other accelerants that might be more controllable, but unless there is actually more heat output from the accelerant, it looks unlikely to be useful while staying with safe parameters.  It might be more productive to work on burner design to burn Meths at a greater rate in air for more heat, and experiment with the heat transfer area arrangement to make most steam from this.

I wanted to take a photo of the burner I have been using for my model, that might be promising in this direction, but time ran away today, and I could not get it done.  I will keep that on the list, for later.

I think you implied that you had more questions and I suspect we had better turn our attention to those.

If you do further work on those accelerants, I think we would all be pleased to hear how it went.  I am sure that I don't have the last words on that one.

A short post tonight.  I have spent too much time trying to find out about peroxide, and on some other calculations I want eventually to get back to.

I hope the Christmas preparations are going smoothly for everyone, and wish you safe travels if you are travelling to relatives or friends for your celebrations.

Thanks for looking in,

MJM460

[steam guy willy]

hi MJM  I was wondering what the graph would look like if the squares were linear rather than logarithmic ?? is there a reason for this on the middle graph ??  Lots of really good info to take on here as usual  ....Thanks and a merry winterval and a preposterous new year !!!........

[steam guy willy]

Sorry ,that should be prosperous !!!!