🚀 go-pugleaf

Albrecht Kadlec  <albrecht@java.vlsivie.tuwien.ac.at> wrote:
>nick@vu-vlsi.ee.vill.edu (Nick Pine) writes:
>> george <jeffe@red.seas.upenn.edu> wrote:
>> >...the thermal conductivity of soap film isn't in my heat transfer text.
>> 
>> I guess it's like a very thin film of water, or anything else, eg plastic
>> film, as far as stopping convection, and good enough to stop some of the
>> IR radiation, ie the bubble wall would ideally be thick enough or opaque
>> enough to slow down IR losses considerably.
>
>I always care about radiation losses last: 
>conduction and convection losses come first.

Sounds like a good idea, until you get up to some temperature. Hmmm. A black
body plate with good insulation on the back and no glazing absorbing 300
Btu/hour/ft^2 of sun in still air on a (average Phila December) 36 F day
with a US R value of 2/3 should have a temp of 236 F. If it had only radiation
loss, ie 0.174x10^-8((T+460)^4-496^4) Btu/hr/ft^2, the temp T might be
234.7 F... It might actually get up to a temperature of 130 F or so.

>So from my point of view, it could be a problem that the thing is NOT SOLID: 

I guess you mean "advantage" when you say "problem"? :-)

>The trick in using bubble plastic is, that the walls are too thin for
>heatloss via conduction.

The thin bubble walls have little conductive thermal resistance for heat
flowing perpendicularly thru the wall, but I guess you are talking about
heat flowing parallel to the walls, lengthwise, say, somehow making it
from one side of a large wall full of bubbles to another, by flowing along
the thin bubble walls over a torturous random path from one side of the
large wall to the other by conduction. If the bubble walls were very thick,
like Swiss cheese, that could happen more easily. 

>And the air in the bubbles is trapped in such small bubbles, that no 
>convection can take place in the bubbles.

Not as much as if they were big bubbles. 

>The only losses left are radiation losses.

Steve Baer discovered that his bubbles had little IR thermal radiation
resistance, because the walls were so thin, he thinks. But he was trying
to solve a much harder problem, because he wanted the sun to shine through
the bubbles and heat up some water in a dish to 130 F, with no glazing (as
I recall.) He wanted his bubbles to have the greenhouse effect, selectively
transmitting shortwave solar radiation and blocking longwave IR re-radiation
in the other direction. A very nice thing, if possible to arrange. In our
case, during the day there would be no bubbles, just 2 layers of glazing,
and there would be no evaporation, and the temp inside the room might be
70 F, not 130 F. These bubbles would only be inside the glazing cavity at
night, trying to stop ALL heat transfer in the longwave IR and shortwave
visible spectra.
 
Bubble walls thinner than light wavelengths look transparent or "black."
As they get thicker, they acquire reflections and colors in the visible
spectrum. We want bubbles that are at least that thick. Reflective bubbles
in the visible spectrum may still be transparent to IR. I suppose these
reflections mainly come from Fresnel losses, ie those resulting from the
impedance mismatch or difference or between the indexes of refraction of
air and water. Each air-water interface might reflect ((n-1)/(n+1))^2 of
the incident normal radiation, assuming the index of air is 1 and the index
of water is n (what is n for water? Less than glass. It's about 1.5 for glass,
which makes the air-glass reflection coefficient rho about 4% at normal
incidence. Reflection coefficients go up for non-perpendicular incidence 
angles. For glass-air with a 60 degree incidence angle, rho becomes 9.3%.
There is a complicated formula on page 218 of Duffie and Beckman's book 
showing how to calculate this, starting with Snell's law.)  

The index and thus reflection coefficient depends on the wavelength, and rho
also depends on the wall thickness somehow, and whether the water has color
or not, eg some sort of dye, in the IR spectrum... Absorbing lots of energy
in the bubble wall may be almost as good as reflecting it, if there are lots
of small bubbles so the heat has to pass from one to the other in a long
chain from indoors to outdoors. Letting the heat pass right thru the bubble
walls is not good.

>With soap bubbles, the water around the bubble can move:

You mean the bubble wall itself... sure. Slowly, one hopes. 

>So a water molecule could absorb energy, and while the bubbles are
>going up, I'd expect warmer bubbles to go up faster.

I'd hope they wouldn't move much at all. Dr. Grosse's didn't move much. Some
of his bubbles lasted more than a year. His principles of bubble health and
long life are: 1. Dust is the enemy of bubbles. 2. Carbon dioxide poisons
bubbles (better bubbles are not blown by humans), and 3. Bubbles love cold.
They also like humidity. _The Unbelievable Bubble Book_ says (in the
"Permanent Bubbles" section)

    As it turns out, bubble death is preventable, or certainly postponable,
    a discovery most definitely made by Dr. Grosse nearly 30 years ago. Among
    Dr. Grosse's many other scientific awards and achievements exists the
    lesser-known fact that he was once the owner of the world's oldest bubble.

    Prof. Grosse's office was home to a huge collection of aging bubbles
    in various states of decay. Under Prof Grosse's meticulous care,
    bubbles did not burst. They flattened. But it was a process that often
    took many months, in some cases, years. They were created out of an
    esoteric bubble solution, and protected fromm deadly dust particles 
    by glass bell jars. 
 
    Prof. Grosse has described his peculiar collection in this way:
    "My first bubbles all died in infancy. They were blown with much
    the same kind of soap that Isaac Newton used in his classic bubble
    experiments. I turned to the literature and found that a much better
    soap was described more than a century ago by the blind Belgian physicist
    Joseph A. F. Plateau, who laid the foundations of our present knowledge
    of soap bubbles. Bubbles blown with Plateau's solution will last for
    several minutes in an ordinary room and for several hours with
    proper protection."

The book mentions that Plateau also created bubbles lasting more than a year.

>So on sloping or curved construction they tend to reach the outside pretty
>soon, losing their heat energy via conduction to the PE film / outside.

Steve said that as he recalled, this heat transfer by moving mass wasn't a
problem. Yes, if you pump air and especially liquid at a high rate thru an
empty glazing cavity, that would reduce the cavity resistance, but he said
that didn't seem to be a problem. 
 
>The bubbles would be great if they were stationary. (not moving)

That's the idea. Bubbles that are fairly stationary, with a long lifetime.
Humidity is one thing that makes bubbles burst. Dust is another. Neither
would be much of a problem inside a glazing cavity. But recall that Dr.
Grosse made bubble foams that he hung on his office walls, with no glazing...
Sprinkled with sequins, even.

>I can't believe they're of great value, when they're moving up,
>especially, if the air is exhausted to the outside.

I agree this should be a closed system, at least to avoid the buildup of dust.
Loss of water and heatloss by massflow out of the cavity would be minor, it
seems to me, even if the air were exhausted to the outside. 
 
>I'd expect it to be equivalent to have a 50 Watt blower blowing air
>between the two PE sheets which is exhausted through various gaps to
>the outside.

That doesn't sound like a great way to insulate a window :-)

>Maybe even worse, since the air would absorb some moisture thus also
>absorbing heat from the surroundings. (evaporative cooling!!!!)
>just imagine the HUGE water surface the bubbles create.

It sure is mind-boggling. But these would be clumped bubbles, not free
floating bubbles. A foam.

>Evaporative cooling would be eliminated, if the PE sheets were leak
>free and the water AND air would be cyclead over & over (sucking air
>from the top to create the new bubbles).

Good idea. I wonder how to pop the bubbles and separate the water and
air at the top, so the air can go thru an aquarium air pump. Something
like this? 
                        ---------------------------- 
		       |  ----------------------    |
       sand filter?--> | |             <--     |    | <--bubbles
                       | |                     |    |
                       | |   ----------        |    |
                       | |  |  -  air  | -->   |    |
                       |  --  | | pump  ----   |    |
		       |  ----  ----------  |  |    | <--water
                       |w|                | |  |wwww| <--water levle
		       | |                |  --     |
		       | |                 ----     |
		       | | water return->      |    |
                       |  ---------------------     |
                        ----------------------------

Or maybe the right kind of air pump wouldn't mind pumping a few bubbles,
reliability-wise, anymore than it would mind pumping air with 100% RH.

>So what is left is the cycling of the water & air from the inside
>sheet to the outside sheet:

Very slowly, one hopes.

>There would be a significant amount of water running down the inside
>sheet (it's a curved/sloped construction, and gravity does it's job),
>absorbing pretty much heat from the inside.

The flow rate seems like an experimental question, and a matter of
picking the right bubble solution... And as you say, geometry. The
flow rate would be different for a vertical window or a horizontal
bubble ceiling or the sloped underside of a bubble cathedral ceiling.
It would be nice if these were all minimal water flow rates. Larger
air flow rates are (1000 times) more tolerable. 

                   . 
                       .
                       .     .                               
                     . .             . <-- plywood or OSB under EPDM rubber
       <-- south    .  .                       .           
                   .20'. poly film bubble ceiling        .   ...........
                  .    .                           .         .         .
                 .     .                                     . chicken .
    refl. pond  .      .<-              38'     .          ->.   coop  .
     ............................................p ~~~~~~~~~ p................
                          thermal storage pond->  ppppppppppp
   
In the above case it might be nice to try to make the bubbles flow up
from the chicken coop along the underside of the sloped roof and over
the peak and down the south glazed wall at night. During the day, it
would be nice to have bubbles along the sloped ceiling, but not in the
south glazed wall. 

>Some of this water is then contacting the outside sheet as it's
>re-bubbled, losing its energy to the outside.
 
Little, we hope.

>What we would need to gain any profit over the blower-alone approach
>would be an air-tight construction of the two sheets and a liquid
>that'd produce very "sturdy" bubbles, so that the air-movement rate
>could be lowered.

Right.

>just my thoughts....

Thanks Albrecht. Time for less gedanken experiments?

Nick