Author Topic: Talking Thermodynamics  (Read 81538 times)

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1020 on: August 10, 2018, 03:04:14 AM »
Hi M.J.M., Here are the charts I mentioned previously, see attachments. I hope they are of some interest.
 Loco specs; 4-4-0 express type; cylinders 19x26 inch, valve travel 3 5/8", 1/8" lead, 1" lap at full gear; Driving wheels 7'1"; boiler 4'4" Dia, 11'4" between tube plates, 240 x 1 3/4" O.D. tubes, copper firebox 5'7" x 3'2" at grate, grate area 18.14 sq. ft., firebox heating surface 112.45 sq. ft., tubes 1246.2 sq. ft., total 1358.65 sq. ft., total area through tubes 2.47 sq.ft.; Boiler pressure 175 p.s.i. Regards, Paul Gough

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1021 on: August 12, 2018, 12:52:52 AM »
Hi MJM, Just a few observations ... today i had a really cold smoothie and i was thinking about warming it up slightly so i was blowing into it. I noticed that when doing this cold air was being returned into my face ?!!! so was this coldness being driven from the smoothie? When you have hot food and drink you blow on it to cool it down .....so does the same thing happen with ice cold drinks in a reciprocative way, to get back to ambient temperature ??  Just wondering.............Paul , those charts will need some figuring out to draw some conclusions......I use to travel on that line years ago when they had steam engines It would have given something to do to while away the time !!!I was stationed at Arborfield cross when i was an electronics apprentice !!



Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1022 on: August 12, 2018, 08:00:47 AM »
Hi Willy, The charts can provide various levels of analysis. You don't necessarily have to jump into applying mathematical methods to get something from them. Firstly, they show the kinds of things that were important to know and what was measured under operating conditions and the results/performance then considered against the existing dimensions of the loco. Some things are obvious once you know them, but often it is not until you see figures for various things and start making comparisons and connections that the relationship between them becomes apparent. Have a look at the boiler and engine efficiencies. Why such a difference?

 Some sense of what is happening and what the driver is doing can be got by comparing the vacuum profiles against the speed and line profiles while noting the indicated horsepower trace. Noting  the normal range for the vacuum in the smokebox, what difference between it and the fire hole door, asking are the changes co-incident at each location, does smokebox temperature and vacuum always parallel each other. Just simple comparisons, maybe obvious, but until the numbers are checked, it is only speculation. The numbers help verify ideas, which might then be improved upon.

The book from which these charts are taken is from; 'A Manual of Locomotive Engineering', by W. F. Pettigrew, Charles Griffin & Co., London, 1899. There are a number of later editions, and in the U.K. it should not be too hard to get a copy. I have always considered this book the best I have come across because it explains things comprehensively but easily. Perhaps there one in a library you can check out. 

 In the end, pondering all these things along with some of awareness of the design reasoning/decisions provides insights into what might or might not be  good in model design, always with the caveat, that not all full size ideas or results can be replicated when scaled down. Hope all this is not boring people. I trust M.J.M. can fathom something from the information that is useful. Regards, Paul Gough

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1023 on: August 13, 2018, 01:24:13 AM »
hi MJM saw this in the newspaper ...i have not seen this before

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1024 on: August 13, 2018, 12:56:47 PM »
Hi everyone, back in civilisation after a few days under the starry skies, with magnificent views of stars, planets and the Milky Way.  Also wonderful surf beach, only one, but 60 km long, magnificent bird life and a four ft. long goanna which was obviously well versed in where there might be food left stored or accidentally dropped.  Got to love the new car (to us), confidently towed the van through deep sand up steep slopes.  Only need a tow out once, but did engage the diff lock a couple of times which got us through.  But my wife says I have to learn to drive more gently, the box of tissues fell off the table, along with two place mats while we came out this morning!

Thanks everyone for keeping up the conversation while I have been thinking of other things.

Hi Derek, not entirely driverless I believe.  I think there are drivers in the control room in Perth, just not on the train.  I wonder if they have changed the electric locomotive that takes over at the end of the line.  It actually had a power cord connected into the mains when I last saw it. 

Hi Paul, those are interesting memories of those times, which do not really seem so long ago to most of us.  We should all  be writing down or recording some of those stories for future generations, we all have them.  And thank you for that interesting information on the locomotive trial runs.

Hi Willy, back to smoothie time again?  When you use a straw to blow through the smoothie, the close contact between the drink and the air enables good heat transfer from the warm breath to the cold drink, leaving the air close to drink temperature when it emerges from the surface.  Just simple heat transfer from a hot material (your breath), to a cooler material (the drink).  When you blow on a hot drink, the cooling is mostly by evaporation at the liquid surface, which is accelerated by the breath carrying away the vapour, which is composed of the higher energy molecules in the liquid, so results in cooling of the liquid.  Also the breath is basically cooler than the hot drink so any heat transfer is from hot to cold, but not very good contact, so only a minor contribution to cooling.  If you blow over a cold drink, there is still cooling by evaporation, but there is less evaporation at the lower temperature, so the cooling is not so effective.  Also the breath is actually warmer than the drink, so the small amount of convection heat transfer that occurs is in the direction of heating the drink.  So in each case, heat travels from the hot material to the cooler material.  In your examples your breath temperature is somewhere between the temperatures of the hot and cold drinks, so the heat flow is the opposite direction in the two cases.

Interesting picture of that "firenado", I also have not seen anything like that.  Obviously some sort of effect of an intense heat source in a cold atmosphere, but it must involve something more, that might be explainable to someone who knows more about conventional tornados.  A coincidental occurrence of such a fire and the atmospheric conditions that cause little "Willy Willies" or "dust devils" I suppose.

Hi Paul, I have certainly been interested to see that real locomotive performance data, and to have your drivers view interpretation of them.  They certainly contain a wealth of information. 

I have been fascinated to see the breakdown of where the heat goes on a real locomotive.  There is so much to learn from these figures, and interesting to consider what we can use to help us understand our models. 

As you say, it is always good to go beyond the simple comparison, and try and understand which is cause and which is effect, and the why of those variations in the various parameters.  Understanding some of the issues will require more understanding of some of the systems, but theory can certainly add some understanding of the why.  But your drivers eye looks at what response is required, when the variation is ok and when something has to be done in response.  Even more importantly, what is to be done.  I will definitely leave that part to you.

With regard to your first question, why are the boiler and engine efficiencies so different, perhaps that is a good one for me to start.

We all know that heat is work and work is heat, and that the first law of thermodynamics allows us to calculate the equivalence.  But transferring between them is not so simple or symmetrical.

It is easy to convert 100% of work to heat, but when we try and change heat to work, we find that we always loose some as heat.  This is stated more emphatically in the second law of thermodynamics.

When we look at how the energy in the random motion of molecules is turned into work, we find it is the linear displacement motion component of that random motion that exerts a force on the piston, and produces work.  But the energy of the molecules is not all linear displacement.  There is energy in spin around each axis of the molecule, as Jo mentioned very early in this thread. The spin results in angular momentum, and that angular momentum has minimal effect on the piston.  Not zero, but not very effective at producing work.  That spin energy tends to stay with the molecule through conservation of angular momentum.  We can see the proportions in the steam tables.  The enthalpy column figure is always slightly higher than the internal energy column.  The difference is approximately the amount of energy that can be made to produce work of we expand the fluid to low enough pressure.

When we apply these concepts to the boiler, we burn fuel to release chemical energy.  The energy released increases the temperature of the combustion gases, and the heat involved is all eventually transferred to the surroundings.  With control of excess air to minimise stack losses, generous heat transfer area to help transfer to the water, and insulation to minimise external losses from the boiler, we can get quite good boiler efficiency, measured as the proportion of heat arriving in the boiler water.  I have seen figures in the low 70% for full size high pressure boilers, in your locomotives and even come close in my model boilers. 

When we look at the engine, the most obvious problem is that of the energy delivered to the engine  in the steam, most of it goes out in the exhaust steam, and only a relatively small proportion is turned to work.  There is still energy that can potentially produce work until the steam is expanded to zero absolute pressure.  However, at this pressure, the volume is so large that expansion to this pressure is quite impractical, so not only is it only possible to turn a part of the incoming energy to work, we can't even practically produce work from all of that small proportion.  The difference in enthalpy values of the inlet and exhaust gives us an idea of the maximum possible efficiency of the engine, but in practice there are further losses due to friction of the moving parts, steam leakage, and heat losses from the repeated heating and cooling of part of the cylinder.  The second law of thermodynamics, in saying that when we transfer heat to work we will loose some, really understates the issue.

I recently saw a report that the newest combined cycle gas turbine power plants from GE had exceeded the previous benchmark a little over fifty percent, but most industrial size plants don't make anywhere near this.  Our little steam plants are nowhere near this level, and the approximately 10% quoted in your tables is representative of real world locomotive performance.   I believe more normal power generation plants achieve around thirty percent.  The efficiency competition results I have seen seem to reach a maximum of around 5% for the best five inch gauge locomotives, and considerably less for smaller gauges.

My own testing so far has all been without load, so quite low pressure.  As there is no work produced above what is consumed by friction, the efficiency is zero.  The adiabatic efficiencies for the steam conditions are also quite low, but should improve when I get to a load test, when higher pressure will be possible with reasonable speed and some work output.

I hope that offers some insight into why the boiler efficiency is so much higher than engine efficiency.  In summary, the boiler is only dealing with heat transfer which can be quite efficient, while the engine is converting heat to work, a process which must always involve losses, which in practice are quite large.

Thanks everyone for following along,

MJM460




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