Guys, I realize that water has some disadvantages as a coolant for any
length of time. However, would it be a good idea to replace one's normal
glycol mix with distilled water prior to one's initial test flight(s) to
give a little extra margin on the cooling system? If adequate cooling

is the

great unknown for first flights, then an extra bit of margin would be great
to have during that first hour cruising in one's new experimental. Replace
it with the original glycol on the ground. I have a hard time believing

that

using it for an hour or two could lead to instant problems and it certainly
would be an easy little safety margin during first flight.

David Doshay wrote:

It depends upon what extra margin you think you really need. The water
would be better at soaking up heat from the walls up to the point it starts
to boil, and it soaks up heat really well at the moment it is boiling (the
heat required for the phase transition), but once it has boiled you can say
good bye to cooling system efficiency. I'll stick with water-glycol mix and
keep the headroom of higher temperature before boiling. You may decide
otherwise.

David

Take another look at that chart I uploaded David. You will not normally
notice nucleate boiling. I think the point you are referring to is
where it transitions to "unstable film boiling". I think that is the 
classic radiator boiling over point we have all experienced. 

I don't think anybody is advocating allowing the coolant to do that. 
I think Jeff is referring to the amount of heat transferred during 
the nucleate boiling stage. Note; that nucleate starts at only a 
10 degree C difference in coolant temp to wall temp. As the temp 
difference reaches 50 degrees the rate of heat transferred 
is 100 times higher and we have still not reached unstable film boiling.

IMHO It will pay to optimize things within that range and that 
could mean using pure water.

BTW with both a water temp sensor and a combustion wall temp sensor
we can get a more precise handle on this situation.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org