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re: a bubblewall house
8 jun 1997
eric swanson  wrote:

>nick@ufo.ee.vill.edu says...

>>consider a 12' cube with a south bubblewall sandwich containing some drums
>>full of water, as in the report "liquid foam--greenhouse insulation..." by
>>john f. groh & t. l. thompson, u arizona, ca 1974, the subject of us patent
>>no. 3,672,184, now expired, i guess, based on an earlier swedish technique
>>for insulating shop windows at night...

>hey nick,

hey eric :-)

>do you ever build any of the wild devices you propose?

sure. you can soon see pictures on my web page of our 18x24' solar cabin,
with lots of instrumentation, a data logger and modem, etc. 

>anyone thinking of building one of these should do a little small scale
>experimentation first.  try good old bubble bath mix from k-mart.

starting small seems like a good idea. the first thing i did was make bubbles
on top of the water in an 10 gallon aquarium. the second was to make bubbles in
a 2'x2'x4" thick box with polycarbonate walls attached to a recycled plastic
1x3 picture frame with aluminum angle and silicone caulk, drilled and tapped
and plumbed with garden hose, using a squirt of green dawn in about a quart
of water. the water usually foamed up fine, with a fine airstone and small
aquarium pump, but after 24 hours of pumping air into the water, the bubbles
sometimes disappeared. i don't understand that yet, but it doesn't seem like
rocket science... 

>i built a bubble generator and was able to get the bubbles to last
>quite a while, but i realized that a bubble wall would be a serious mess. 

a mop is a useful development tool. i came in one morning to find lots of
water and foam all over the physics lab floor. people have made bubbles that
lasted over a year, in bell jars, eg the 19th century physicist plateau...

>for example, maintaining the bubbles overnight requires a continual
>supply of new bubbles, which means a compressor running all the time.

"compressor" sounds like overkill. a 100 watt 100 cfm 1" h2o blower is plenty
big for a 3000 ft^2 greenhouse, and the bubble machine only has to run a small
fraction of the time.  

>there must be a top vent in the bubble wall for the excess to flow out,
>once the wall is full.

we might stop making bubbles when the wall is full, using a photocell to
sense when the cavity is full or to sense bubbles in the return line, or
we might keep slowly producing and recycling bubbles... 

>what does one do with the excess bubbles?

ideally, they might be separated into air and water and recycled. if the
air pump can handle moisture, i guess they don't need to be separated.

>if they are to last a while, they don't break up very well either.

some of the things that destroy bubbles are sharp points, dust, co2,
heat and dry air.

>then, what does one do when the sun rises the next day?  some sort of
>bubble busting system must be built, such as a spray of clean water?

i used a 5 micron sediment water filter in line with the bubble outlet
at the top of the polycarbonate chamber, to break up the bubbles and
separate them into air and water. this required a fairly high backpressure.
it now seems better to just pump the bubbles around, eg with a wet/dry
shop vac, vs. an air pump, or maybe pop them with a small hot nichrome wire
in an inline chamber. incidentally, here's groh's formula for the foam's
back pressure as it moves through a polyethylene film glazing cavity:

    dp = taupl/a,                                              (1)

         where dp is the pressure differential in lb/ft^2,
               l is the length in feet,
               p is the wetted perimeter in feet,
               a is the cross sectional area between films in ft^2, and
               tau is the shear stress in lb/ft^2.

the report says "for foam velocities of 0.94 to 3.3 ft/min, tau is constant at
0.013 lb/ft^2 with a standard deviation of 0.001 lb/ft^2. for drained foam
around 30 minutes old tau appears to increase by around 33%. this data is based
on a greenhouse roof approximately 16' wide x 47' long with a = 20.7 ft^2 and
p = 35.1 ft. equation (1) gives only the pressure differential required to
drive the foam down the greenhouse, and does not include the pressure drop
across the foam generator." one lb/ft^2 is equivalent to 0.1875" h20.

>how much of your bubble solution will be lost each day?

very little, i'd think. it seems like a good idea to make this a closed system,
to avoid loss of water and buildup of dust.

>are you going to use an antifreeze system, and how much antifreeze
>will be lost each day?  

the bubbles probably wouldn't freeze in the system i suggested.
groh and wells mention this needs more work, in general. 

>sorry, i haven't looked at the patent yet.  maybe those problems have
>been solved.

there's the patent, which i haven't seen yet, and a couple of papers by
john groh and otho wells. prof wells said the system basically worked,
and the reasons they didn't pursue them were more economic than technical,
eg the problem of economically finding and fixing leaks in a structure
that costs 50 cents per square foot. to me, this really does seem like
fertile ground for a serious tinkerer.

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html
web site: http://www.ece.vill.edu/~nick 




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