Author Topic: Talking Thermodynamics  (Read 108035 times)

Offline AVTUR

  • Full Member
  • ****
  • Posts: 7
Re: Talking Thermodynamics
« Reply #1155 on: April 25, 2019, 05:49:46 PM »
Good afternoon MJM

My reply was to Willy's first question concerning the amount of energy required to heat something up and then released on cooling down [I will try using quotes]

Hi MJM thanks very informative,,,Another question  does it take the same amount of energy to cool things down ,as it does to heat them up ?? conservation says yes but ....... also we can heat things up very quickly but can we cool them down very quickly as well ?? we can use microwaves to heat things up but what are the alternatives with cooling ??

Sorry my questions are so short  and takes a fraction of your time for your answers !!!  Also, Does all those collisions of the molecules on the walls of the containers have a detrimental effect on the containers ??

Willy

My reply was basically a "knee jerk" reaction by a retired aerothermal engineer.

I should add that I have seen the detrimental effects of high energy molecular collisions on containers far too many times. As MJM says, the container gets hot and if there is enough energy transported by conduction or convection (by moving gas molecules) or radiation it weakens, melts, burns and becomes useless.

I am now going to go away and read all the pages of this thread!

AVTUR

There is no such thing as a stupid question.

Offline MJM460

  • Full Member
  • ****
  • Posts: 817
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #1156 on: April 26, 2019, 09:31:44 AM »
Hi Avtur,

Great to have you on board and I am delighted that you will be reading all those pages,  I hope many of them anyway.

I am sensing that you will not find too much that is new to you.  If you see any posts that need correction or clarification please let me know.  There is so much high quality information on this forum and I dont want the theory side to let down the community.  It will be great to have another contributor to the discussion.  It can be quite difficult to cover all the aspects of a complex question in a manageable post.

As for the effect of very high energy molecules on metals, I suspect that many of the forum members would be most interested in the effect of those molecules in the energy range you have been dealing with, and the special alloys used in that field to extend the operating range and power output.  Definitely way beyond what most have to deal with on the engines we are more familiar with here, which is what I was thinking of in my reply to Willy’s question.  Perhaps you could add a little to my response.

Thanks for joining in.

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

Offline steam guy willy

  • Full Member
  • ****
  • Posts: 2082
Re: Talking Thermodynamics
« Reply #1157 on: May 08, 2019, 01:29:45 AM »
Hi MJM, I have acquired this old 1/16th HP ic engine and it is fitted with a water jacket and a cooling tower . I was wondering exactly how this works. The water level has to be higher than the top pipe connection . As heat rises why dose'nt the hot water just stay at the top of the tower ? as this does work which way does the water travel ? Does the water actually move ,as there is no pump, or is it gust the temperature traveling through the water some how...? or is this another silly prognoses !!!!

Offline 10KPete

  • Full Member
  • ****
  • Posts: 1558
  • Nordland, WA, USA
Re: Talking Thermodynamics
« Reply #1158 on: May 08, 2019, 03:48:01 AM »
Hot water rises pulling cooler water in from the tank. Just like a Ford Model T.

Pete
Craftsman, Tinkerer, Curious Person.
Retired, finally!
SB 10K lathe, Benchmaster mill. And stuff.

Offline MJM460

  • Full Member
  • ****
  • Posts: 817
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #1159 on: May 08, 2019, 11:33:40 AM »
Hi Willy, no silly prognosis, but the common perception that “heat rises” causes more confusion than clarity.  Heat travels from higher temperature to lower temperature, it does not matter whether that is up or down, gravity has no influence on heat.

We all get taught when quite young that heat is transferred by conduction, convection and radiation.  Conduction and radiation are easily understood as independent of gravity. Convection is broadly divided into forced convection and natural convection.  In forced convection, heat transfer is in the direction of the flow whether up or down.  All the confusion comes from our observation of natural convection.  Basically when a fluid is heated, whether it is gas or liquid, it usually expands, and when a given mass expands, the density becomes less.  Then the low density fluid is displaced upwards by higher density cooler fluid which sinks downwards.  So the fluid flows under the influence of that density gradient.

There is a very interesting anomaly in the general observation that in fact we have all seen.  When water cools from 4 degrees C, instead of contracting and becoming more dense (as it does when cooling but still above 4 deg C), it expands, so get less dense.  So water cooling from 4 deg C rises and the surface water is cooler than the water deep below, and when it finally freezes at O degrees C, the ice is also less dense than the water and so floats.  Ice is not a fluid an might be expected to have a different density than the surrounding water, but the water freezes first where it is coolest, which is at the surface at the surface as we all have seen.

So in your engine, there is one place we know the pressure in the cooling system, that is at the water surface.  If you calculate the increase in pressure as you go down to the bottom of the water tower, you get a somewhat higher pressure than at the surface, the increase being proportional to the density and the vertical height.  Now you now do the calculation in the other side of the circuit, the water tower above the side connection, the copper tube, the cylinder jacket and through the bottom connection to the bottom of the water tower.  The same procedure as the first path, but this time, part of the column is heated in that cylinder jacket and so is lower density than the cooler water in the tower.  The result of that lower density is a lower pressure when you get back to the connection at the bottom of the tower.  That pressure difference drives circulation of the water, up through the cylinder jacket, through the copper tube (so increasing the temperature in the tube and reducing the density of more of that side of the circuit) and back into the top of the tower.  Then it flows down in the tower as it looses heat to the surrounding air.

The tower looses heat to the air, keeping its average temperature lower, while the heat from the cylinder heats the other side of the circuit so maintaining a flow.  This mechanism is called a thermosyphon.  It requires no pumps, just a continuous fluid circuit.  As you mention, the water level has to be above the side connection to the tower near the top. 

If the tower has extended surface area, such as fins or a complete radiator configuration, the tower will stay closer to the outside air temperature, and faster circulation.  The tower on your engine appears to have a good thick layer of paint.  That tends to insulate it just a little, so a very thin layer, just sufficient to prevent corrosion would be preferred.  So a good rub back before freshening the paint, not just another coat on top.

The water does move, it flows around the circuit, carrying and redistributing the heat as it goes. The average temperature around the circuit is something above the temperature of the surrounding air, and the temperature it was at before the engine started.  So the average density is less due to the temperature rise, and the level of the surface rises compared with the level when all is cold.  But the difference in density through the tower compared with the density down through the cylinder jacket is what drives the flow.  And of course, the heat lost by the tower increases the temperature and so lowers the density of the surrounding air.  So this warmed air tends to rise and be replaced by cooler air from the larger surroundings.  Again the rising air carries the heat, which is interpreted as heat rising.  I think that brings us back to where we started!

I hope that clears the fog a little.

Hi Pete, thanks for joining in.  It’s interesting that the radiators in cars have not increased in size as much as the engine power since the model T.  Instead of increasing radiator size and heat transfer area to dissipate the extra heat of our larger engines, the heat transfer coefficient has been increased by moving to forced convection by introducing a water pump.  And of course the air side heat transfer is increased by the higher speed of modern cars which gives more air flow over the radiator.  Its not all proportional of course.  Engine efficiency has improved, radiator design has improved and so on.  But the design of Willy’s engine does have many parallels in other fields.

Thanks everyone for looking in,

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

Offline steam guy willy

  • Full Member
  • ****
  • Posts: 2082
Re: Talking Thermodynamics
« Reply #1160 on: May 08, 2019, 03:13:54 PM »
Hi MJM, thanks for your detailed explanation about  Rising Heat...  I really like the way that modern misconceptions and urban myths abound !! The reason i asked about if the water was  is moving was that in a solid medium  (metal) the actual molecules do not move !! So thanks for your time spent replying in such detail
 :praise2:

Willy

Offline AVTUR

  • Full Member
  • ****
  • Posts: 7
Re: Talking Thermodynamics
« Reply #1161 on: May 08, 2019, 06:22:58 PM »
Just a little add on. With thermosyphoning the top of the radiator was usually the highest point under a car bonnet. This led to quite powerful cars in the 1930s, such as the SS (forerunner of the Jaguar), having high bonnet lines. Things changed when a water pump and thermostat were used. The engine could run hotter and the radiator did not need the height. This led to controlled engine temperatures which gave other advantages, in addition to efficiency, such as better lubrication.

MJM460 - I still mean to reply to you but at present I am long way from home and on the edge of the known world.
There is no such thing as a stupid question.

Offline MJM460

  • Full Member
  • ****
  • Posts: 817
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #1162 on: May 10, 2019, 12:58:05 PM »
Hi Willy, it is always a pleasure to be able to answer your questions.  You have a talent for seeing things happen around you and wondering why, when most people just take things for granted (or perhaps assume it is magic).  The main thing is so long as I am giving clear explanations.  If not, I need to have another go.

Yes the water actually flows around the circuit.  But the metal molecules also move.  Not travelling like the molecules in a fluid, but jiggling around in the limited space available at their location in the lattice arrangement of the molecules in the structure. 

When heat is applied to a surface of the metal, the amplitude of the jiggling of atoms near that surface  increases, and the collisions with the surrounding atoms increases their jiggling and so on, to transfer the heat through the solid metal.  So long as there is a temperature difference, heat continues to be transferred.  But the atoms do essentially stay in their space in the lattice.

Hi Avtur, I know what you mean about travelling, I am four hours from home at the moment, and in a relatively remote area by the standards of my home state, but the real travel to the edge of the earth for me starts later in the year when we head up to Darwin.  I think it involves dropping off the edge and appearing the other side, or something like that.  Over four thousand km by the shorter route.  That gets boring after the fourth trip, though only a little, and it never looses its appeal.  But the alternative routes are always longer, and we enjoy them too.

Interesting also that you mention that SS model Jag.  Only a month or so back, one of my friends was telling me he had one of those which he had fully restored.  He had started a conversation with one of his business clients, who said it would not be one of those as there were so few around.  The client was somewhat of an expert on them and had the “official” list of all known examples in the world.  Quite a short list.  My friends car was not on it, but was soon proven to be genuine.  I believe it has now been transported from here to US for a show, where it was sold to a collector in The Netherlands, so it is almost home.  A well travelled vehicle.

Thanks for the additional information on the cooling systems.  It is always hard to know when to stop, so always room for additional comments to add to the discussion.

I think we will all be glad to hear your comments when you have the opportunity to read the earlier posts, but it is quite a long thread.  Please let me know if you notice any errors, and I will correct them.  And enjoy your travels.

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