If frogs had wings they could glide and do a nice little flare before
touching softly on the ground, too.
But they don't.

----- Original Message -----
From: "Daniel L. Katz" <katzd54@YAHOO.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, March 05, 2004 9:46 PM
Subject: Re: Purging Cooling System of Air / Where Would We Be Without This
Topic?

> it is easy to fill a straight verticle pipe, closed at the bottom, open on
> top, with water. the pipe can be filled from the top by just pouring the
> water in, or from the bottom by pumping through an inlet. either way, all
> the air gets buoyed up and out.
>
> so why is there so much black magic about refilling and purging the air
> from vanagon cooling systems?
>
> a clue might be that when a cup of coffee is turned upside down the coffee
> spills out. let's say the area of the free liquid surface is 30 sq in.
> then the force on the surface exerted by the atmosphere is
> 15 psi X 30 sq in = 450 lb, which is much greater than the weight of the
> coffee. therefore, the atmosphere should easily hold the coffee in the cup
> when it is turned upside down. essentially, the atmosphere is pushing from
> both directions, squeezing the cup and coffee together. but the coffee
> really does spill out, despite all the squeezing. the reason is that the
> lighter fluid (air) cannot support the heavier fluid (coffee) because of a
> fast growing instability. similarly, it is not possible for the free
> surface of coolant in a pipe to push an air bubble down - the coolant will
> just fall through the air bubble, which will then end up on top. whence,
> if there is a local maximum in height, a place for an air  bubble to
> reside, it is non-trivial getting all the air out. therefore, it is
> necessary to run the engine at a high rpm, or use some other pumping
> method, to circulate air bubbles up to an elevated bleeder.
>
> if the van's cooling system was like a straight pipe, all that would be
> necessary would be to fill the system from its highest point, and all the
> air would be buoyed up and out.
>
> dlk