Re: H2O Microwave Criticality

["Elizabeth M. Brackett" <brackett@bright.net>]

My husband is a nuclear engineer whose expertise is thermal fluids and
fluid flow.  His opinion/explanation was as follows:
>>
This is moderately believable.

The physics are correct - you can superheat water without boiling it.
Temperatures as high as 241C have been achieved in water without boiling
given the right conditions.  I would think you'd have to heat it in a
microwave an inordinate length of time to achieve anything like this.

Superheating is easier with microwaves than with heating elements because
the hottest part of the fluid is more diffusely within the bulk fluid rather
than right on the wall.  Nucleation sites will allow a fluid to boil with
lower superheat, while no nucleation sites allow boiling to be delayed.
Because the hottest fluid may not be on the wall, where the most nucleation
sites are, the story becomes more likely.  This would be most likely if the
water were filtered well, something many people do these days.

The main phenomenon that would limit superheating is the natural convection
circulation of the water.  The cooler water in the center (self-shielding of
the microwaves) would sink while the hotter water on the periphery would
rise.  This mixing limits local superheat and promotes boiling.

I would think boiling violent enough to eject the water would be unlikely
but possible.  Certainly very unlucky!
<<

Liz Brackett


******************************
  Elizabeth M. Brackett, CHP 
     Sr. Health Physicist     
     MJW Corporation, Inc.       
       (330) 644-3757        
  mailto:brackett@bright.net 
******************************
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html