From: crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass)
Subject: Re: Metals in liquid Nitrogen !!
Organization: University of Virginia
Date: Fri, 11 Dec 1992 21:29:20 GMT

In article <1992Dec11.200754.6398@magnus.acs.ohio-state.edu> kpearce@magnus.acs.ohio-state.edu (BULLDAWG) writes:
>In article <1992Dec11.192622.6719@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>>In article <1992Dec11.171454.3158@magnus.acs.ohio-state.edu> kpearce@magnus.acs.ohio-state.edu (BULLDAWG) writes:
>>
>>[discussion of boiling in nitrogen deleted]
>>>
>>>But at this point, the metal makes a phase transition from one kind of
>>>metal crystal to another (bcc to alpha).  Whenever a substance under goes
>>>a phase trnsition it gives up heat.  So what was happining was that the
>>>metal was bubbling away as it cooled down giving up heat in a straight-
>>>forward way and then at a centain temperature, the metal says "hey, I'm in the
>>>wrong shape, and I have too much energy" and then gives up that excess energy
>>>_in_addition_ to the heat energy it was giving up to begin with and then
>>>as soon as it makes the phase transition, the copper goes to cooling down as
>>>before (the bubbles slow down).
>>
>>     So all metals have discrete phase transitions near the boiling point of
>>     nitrogen?   And these phase transitions involve increased cooling
>>     /increased heat transfer?
>
>Of course not.

     Check the numbers on such phase transitions.  Further, I think you'll find
     that they are readily taken care of by the relevant specific heat.
     You'll also find that this has precious little to do with the
     phenomenon.

>>
>>     I think you'll find that it is just changes in the heat 
>>     transfer characteristics of the metal/fluid interface down near 
>>     the boiling incipience point that are the real cause.  
>>
>>     See my earlier posting for details.
>>
>
>I saw the earlier post.  The data shown is over a range of temperatures.
>All LN2 that is found in an open dewar will be 77K.  Refering to his chart, 
>the LN2 should be boiling furiously (and is not, I just took a look to 
>make sure).  The data that you show in your chart just doesn't apply to this
>process (since it is all taking place at 77K).

      No, you misread the graph.  The x-axis is surface superheat (T_wall
      - T_bulk, or in this case T_coin-T_bulk), the difference in 
      temperature between the coin and the  bath.  When that goes to zero, 
      the coin goes to 77K.  Then there is no boiling.  And the graph
      applies to *all* saturated pool boiling from a surface in newtonian
      fluids.  You'll note that there are *no* value notations on the axes,
      though I did give representative superheats for nitrogen in a
      specific situation.  Even misreading the graph, how did you decide
      where 77K was?  It could have been in the convective region.
      
      In any case, boiling is driven by delta T in this case.

      As far as the statement that 'all LN2 that is found in an open dewar
      will be 77K', I think you'll find that is a sweeping overgeneralization.
      You could easily put 72K LN2 in an open dewar.  Transients can
      be as small as you wish the dewar to be large.

                             dale bass
-- 
C. R. Bass                                          crb7q@virginia.edu        
Department of Mechanical, 
     Aerospace and Nuclear Engineering
University of Virginia                              (804) 924-7926