# [ExI] Skylon heat exchangers

spike spike66 at att.net
Sat Dec 20 21:13:21 UTC 2014

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> On Behalf Of Rafal Smigrodzki
Subject: Re: [ExI] Skylon heat exchangers

On Fri, Dec 19, 2014 at 11:32 PM, spike <spike66 at att.net> wrote:

Here's why I haven't completely lost faith in the whole notion, an
experiment you can do at home.  Go get a garden hose and find a spider web.
Check out how much flow you can spray at that web and still it stands.
Every notion I have about flow and strength of materials would tell me there
is no way that web could hold up to a garden hose, even with a nozzle on it,
but it does.  Now don't take my word for it; go out there and start
spraying.  I have no equations that would explain why the heck that happens.

### The spider web is a miracle of impact energy dissipation, in part by an ingenious use of surface tension-induced coiling of net strands. I would imagine that by comparison, the brazed tubes in the heat exchanger are very primitive mechanically.

Rafał

Ja, well there is a good example for the engineering students, a lesson that all the old familiar formulas don’t necessarily scale all the way down.

Experiment: go to the local university, round up all the really smart engineering students, the ones who have taken fluid flow, strength of materials, all those hard junior and senior mechanical engineering classes, get those who understood it and know all their formulas.  Show them a spider web and have them come up with an expression for how fast the water flow would need to be to tear up that web.  Let them get a microscope and measure the diameter, let them look up a tensile strength or maybe that could be measured in the lab, let them use any equations they want, all of which are applicable in our world, let them scale right on down to the regime where surface tension, negligible in our world, dominates everything.

Let those guys use all their favorite tricks in their mathematical toolbag, everything they thought they knew about impact energy dissipation, drag on objects in hydraulic flow, all the tricky fun stuff, and have them predict what will happen if they spray a spider web with a water hose.

If they use only the equations in the book, every one of them will get the wrong answer, and miss it by an order of magnitude.  They will go away humble and smarter: equations don’t always scale all the way down.

When I did my calcs on a supersonic jet intercooler, I used the smallest tubing I knew all the numbers for: 1/64 inch ID Alnico tubing, great for heat exchangers.  But it wasn’t nearly good enough, if all the equations are linear.  From Keith’s description, the Brits are using even smaller diameter tubing.

spike

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