Announcement

Collapse
No announcement yet.

liquid insulator

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    As I understand it, you want to prevent heat flow across a 2" gap and gas or fluid are both possibilities. So, I would conclude that the volume is closed, but may change shape and possibly size.

    I think gas would be the better solution just from the possibility of fluid leaks that may be problematic. And air is probably the cheapest gas you can use so without knowing of any reason not to, I would suggest sticking with air.

    But you probably need to decrease the thermal transfer through the air. The primary mechanism for this transfer in a space like this is going to be convection currents. This simply means there will be a bit of wind inside there. Air on the hot side expands and rises and on the cold side it will contract and fall. This creates a circular air current which transferes the heat from the hot side to the cold side. So, what you need to do is add something to prevent this circulation. Depending on the types and amount of changes the volume can undergo and on the temperature range it may experience and on other substances that may come in contact with anything in the volume, this may be something like foam rubber, or a loose fiber type of insulation, or fixed baffels, or many other possibilities. The insulating material they sell in hobby and crafts stores for things like quilting is very resiliant. It can be crushed very easily and yet will easily recover to it's former size when allowed to. Or fixed baffels could be constructed from an insulating material like card stock or plastic. They would be roughly parallel to the hot and cold surfaces of the volume. Air would circulate in each of the chambers created by them, but each such chamber would be a further hinderance to heat flow. The more such chambers, the better. You could get ten or even more in a space of two inches. The baffels could have a small hole in the center to allow for changes in volume.

    Air is still the actual insulating material. The added material or baffels is just to prevent thermal currents. Either of my suggestions would also work with a fluid, including water. The primary transfer mechanism in a fluid would also be circulation and they would cut down on the circulation there also. A very viscous fluid would also be a possibility. High viscousity equals less flow. There are some oils that are almost solid. Or perhaps grease. I would bet the manufacturers could provide thermal transfer properties. Small insulating beeds could even be mixed in with the grease. Or in an oil if they had the same density so they wouldn't float or sink.

    This analysis has been made working with the information you have supplied above. I hope it helps. But, as elements may be missing, please take it with a grain of salt. Designing with incomplete information is difficult.
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there IS a penalty for trying!

    Comment


    • #17
      Thanks everyone for your inputs. We're actually thinking now that a much smaller gap would be far better all around. I wonder what kind of surface finish would help to impede air convection currents in a gap this small? Hopefully a very smooth surface, unlike a golf ball. At any rate, it's more of a constructional issue to have the gap that small, nothing that can't be overcome. It seems that controlling the water level will be easier as well because less will have to be displaced, even though the pressure required to displace it will be the same. Thanks again for the ideas.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

      Comment


      • #18
        I don't know if the surface finish would have any effect on the magnitude of the air current. It might help slow down a fluid current. But a smooth surface would have less area than a rough one and therefore the heat transfer would be less.
        Paul A.
        SE Texas

        Make it fit.
        You can't win and there IS a penalty for trying!

        Comment


        • #19
          The golf ball effect isn't applicable to what you are contemplating. A dimpled surface has more drag than a smooth surface. What it does is cause some of the energy of motion to be imparted to the air. That energy is expressed as turbulence which causes the air to slow since the motion is more randomized. That in turn allows it to remain attached to the boundary layer longer which in turn reduces the overall induced drag of the ball to a greater extent than the energy cost of making the air turbulent.

          Drag has two forms, induced drag which is the drag caused by the creation of lift forces and parasitic drag which is the flat plate drag of an object. In your application all you have is the parasitic drag of the boundary layers so maximizing that will slow flow just as it does water in a hose. The rougher the surface the slower the movement of the fluid.

          Keep in mind that air is a fluid and is subject to exactly the same principles as liquids.
          Free software for calculating bolt circles and similar: Click Here

          Comment


          • #20
            Originally posted by Evan
            .....

            Keep in mind that air is a fluid and is subject to exactly the same principles as liquids.
            But air has a lot lower viscosity. Same principles, yes. Different result, definitely. A rough surface would act like a lot of cooling fins, increasing the contact area and thereby add to or increase the heat transfer. Within reason, it would have little effect on the flow with air as the fluid and not a lot with a viscous, liquid fluid. I think with a rougher surface the change in the flow rate will be very small and the net effect would be an increase in heat transfer but it would take actual experiments to confirm this.

            Using a viscous fluid would definitely slow the heat flow down. I believe I suggested that already.
            Paul A.
            SE Texas

            Make it fit.
            You can't win and there IS a penalty for trying!

            Comment


            • #21
              I think with a rougher surface the change in the flow rate will be very small and the net effect would be an increase in heat transfer but it would take actual experiments to confirm this.
              Ah, but the rougher surface will tend to trap the boundary layer next to it because of the very pronounced drag near a rough surface. This will prevent convection currents from reaching the surface of the container leaving an insulating layer of fluid that has already adjusted to the ambient temperature of the walls. This effect will also operate very well with air and is how even a small amount of hair on your arms helps to keep you warm.
              Free software for calculating bolt circles and similar: Click Here

              Comment


              • #22
                Originally posted by J Tiers
                Well, presumably they know what they are doing..... better than the Soviet experiment of saving power by running at lower voltages........ motors didn't like it very much.
                I thought that was the US with their 110 volt motors ?

                .
                .

                Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                Comment


                • #23
                  Originally posted by Evan
                  ..... This effect will also operate very well with air and is how even a small amount of hair on your arms helps to keep you warm.
                  You must have better hair than I do.
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there IS a penalty for trying!

                  Comment


                  • #24
                    To summarize what I think has been implied: You have an air gap (perhaps 0.5") between two panels. The air gap will be periodically flooded with water. During flooding you want heat transfer between the two panels, otherwise not.

                    If you are concerned about convection during the air phase, this may not be an issue if the mechanism is natural convection, as distinct from forced convection. Natural convection in an enclosed space occurs due to the buoyancy of heated air rising. If the panels are horizontal, there will be essentially no natural convection, and heat transfer will be governed by the thermal conduction across the air gap. If the panels are vertical, natural convection may still not be very significant if the distance from bottom to top is only a few feet.
                    Last edited by aostling; 01-08-2008, 01:33 AM.
                    Allan Ostling

                    Phoenix, Arizona

                    Comment


                    • #25
                      You must have better hair than I do.
                      I wouldn't know. What do you think?

                      Free software for calculating bolt circles and similar: Click Here

                      Comment

                      Working...
                      X