Author Topic: Talking Thermodynamics  (Read 100091 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1110 on: January 11, 2019, 03:19:04 AM »
Hi MJM,  thanks for the info.......

willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1111 on: January 14, 2019, 08:39:36 AM »
Hi Admiral DK, thanks for joining in once again.  Itís interesting to see the diverse uses for these devices arising once they become available. 

Hi Willy always glad to have a reason to talk about the science and physics that keeps things turning.  There has been a lot of progress since those early experimenters discovered strange effects at the junction of dissimilar metals.

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 #1112 on: February 10, 2019, 07:42:48 PM »
Hi MJM, just a quick digestion question from my hospital  bed. i missed a hot meal so they gave me some frozen sandwiches, They were sealed in plastic bags and i was wondering if they would get up to ambient quicker if they were removed from the bags ??? getting better slowly....

Willy

Offline crueby

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Re: Talking Thermodynamics
« Reply #1113 on: February 10, 2019, 09:56:23 PM »
Hope the date on those was 2019, not 1919....

You must be doing a bit better to be asking thermodynamic questions! Keep that up!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1114 on: February 11, 2019, 12:28:55 AM »
Hi Willy, good to see you thinking about food and thermodynamics again, you must be on the mend.

As usual the question is a bit more complicated than it looks.  The plastic film makes a very thin insulating layer, and adds film coefficients for each side of the plastic.  Then, the air inside the wrap is somewhat restricted for convection.  I assume they are not vacuum packed.

The rough surface of the exposed bread has more surface area than the smooth plastic film, so that would help if unwrapped.  But the generally dry hospital air might make the sandwiches dry out while warming, so taking more time might not be the worst.  If they are like hospital sandwiches here, they donít need drying out!

All in all, I wouldnít want to predict the outcome on theoretical grounds, easier to determine by experiment.  You have the time, so perhaps one of your visitors could bring in your thermocouple probe and meter, and you could conduct the experiment, including the taste testing.  I am sure the nurses would let you use a sterile wipe to clean the probe before using it for a food application.  They would probably prefer that to your having someone bring in some saws, files and material for the next bit of your engine.

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 #1115 on: February 11, 2019, 06:43:16 PM »
thanks MJM, yes there are a few variables as taste and texture come into it....next time i come into hospital that will be first on the list !!!!   except they wright down everything you bring !!!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1116 on: February 19, 2019, 08:25:00 PM »
Hi MJM, I was thinking about a telly programme that i saw 20-25 years ago that was about heating water by using the vibrating water hammer effect.  somebody discovered that the pipes got warm when a water hammer started up. the programme then went on to show in the USA a fire station using electric motor induced water hammers to produce all their hot water, ...Was i dreaming or hallucinating ?and did anything become of it ??   would love to know about the physics behind this !!! and did it come to anything ?

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1117 on: February 20, 2019, 11:37:04 AM »
Hi Willy, it sounds like you are feeling more like your normal self.  Keep it up.  That pneumonia can knock you around quite a bit for such a tiny bug.

Water hammer basically comes from the energy that has to be dissipated when you quickly stop a moving mass of some basically no compressible fluid.  You will remember that a moving mass has momentum, or even zero momentum if it is at rest, and it takes energy to change that momentum.

You may not be so familiar with the basic fact that to change the magnitude of momentum requires a force, and the magnitude of the force is calculated from the change of momentum per unit time.  From this calculation it is easy to see that if you quickly stop a moving body you get a large force.  The force necessary to stop a moving car in a very short time is enough to severely damage the car.  If you are unfortunate enough to be in that car, the seat belts have to apply enough force to stop you before you hit the windscreen, and that force is enough to damage you, but if the seat belt stretches just a little, it can double the time it takes to stop you and literally cushion the blow.  But the windscreen stops you very quickly, with the inevitable result.

Similarly if you have a pipe line of water moving at some velocity, and you suddenly stop the flow with a valve or what ever, the momentum of that column of water has to be changed by the valve plate, and as water is nearly not compressible, the whole column stops very quickly, requiring a large force, and that is the cause of the familiar water hammer description of the noise.   The energy of that moving column has to be absorbed, and is eventually dissipated as heat.

The water hammer that I have come across can be quite damaging, so the usual effort is put into preventing it by accumulators with a gas cushion that slow the rate of change, so reducing the maximum force, and preventing the damage.  The sudden compression of that gas cushion will cause it to get hot.  However, just how you would set up a system to continually produce water hammer and absorb the energy directly into heat, I am not sure.  Nor am I sure if this could be done with great efficiency.  I suspect it must involve gravity, as the efficiency of fuel powered devices to keep accelerating the column would likely make the system uneconomical.  I have not come across any such system, but the energy is there for those ingenious enough to work out how to harness it.

Perhaps in a district water supply system, there is a point where an air cushion accumulator experiences frequent enough water pressure surges due the velocity changes as consumption varies through the day, to produce useful heat. 

Perhaps someone else had heard of it and can add something to the discussion.

But we really want to know how to use that energy in an engine to do useful work before it is eventually dissipated as heat.

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 #1118 on: Today at 12:31:05 AM »
Hi MJM, thanks for that and i'm sure i saw this programme on the TV though...been trying to look it up on the web,...but to no avail....

willy