🚀 go-pugleaf

In article <1992Dec11.194258.6013@magnus.acs.ohio-state.edu> kpearce@magnus.acs.ohio-state.edu (BULLDAWG) writes:
>In article <1992Dec11.023731.17721@lynx.dac.northeastern.edu> mkagalen@lynx.dac.northeastern.edu (michael kagalenko) writes:
>>In article <10DEC199219275429@trentu.ca> lchardon@trentu.ca (PETIT PAPA NOEL.) writes:
>>>
>>>	I'm sorry if this has been discussed here before, but I'm quite 
>>>puzzled by the following:
>>>
>>>	In my thermodynamics class, we were all happily playing with liquid 
>>>nitrogen, freezing squash balls, making erasers explode (BTW, why do 
>>>plastic erasers explode when in liquid nitrogen?) and all that kind of 
>>>things that you do when you have 10 litres of nitrogen to waste, when a 
>>>prof dropped by and told us to try to immerge some piece of metal in the 
>>>nitrogen. Sure enough, somebody pulled out a penny and dropped it in a 
>>>full styrofoam cup. What happens is the following:
>>>The nitrogen will boil for some time, like it boils when you drop anything 
>>>in it, but then suddenly it will boil really hard, about 3 times more 
>>>violently than it did before, for about half a second, and then it will 
>>>just gently bubble away (When the metal is in thermal equilibrium with it, 
>>>i guess...).
>>>
>>>	WHY ???????????????
>>>	WHY JUST METALS ???
>>>	
>>This is an interesting question. You can do the same observation with
>>the iron ;
>>1. Heat it 
>>2. Turn it off
>>3. drop small amounts of water on the hot surface(make sure it's horizontal) 
>>You'll see 2 different situations :
>> - When iron is hot enough, water doesn't touch metal ! Steam production 
>>allows drop of water to levitate. If you can fix an iron strictly 
>>horizontally, even small drops can survive more than 1 minute.
>> - When iron become cooler, the rate of steam production is too small 
>>to isolate the drop from the hot surface; in that case water touchs the
>>metal & evaporates in several seconds. It seems a bit paradoxical,
>>but I'm unwilling to explain the details (surface tension of drop of
>>water does matter). Liquid nitrogen + coin works the same way.
>>
>
>These are two different processes!
>
>The situation is not the same.  The drop is levitating away from the heat
>source on a cushion of steam.  And since steam doesn't conduct heat very
>well, the steam acts as an isulator, allowing the drop to survive.
>If you were to invert the iron and go from the
>bottom (iron on top) you would not get this two step effect, as the steam would
>escape immediatly, and the water would remain in contact (to a point, for
>if the steam had enough pressure to overcome the weight of the iron, the iron
>would levitate as well).  But my point is is that they are two entirely
>situations.

     Well, gravity is different in the two situations but that is     
     about all.  Film boiling is film boiling.  If you get above
     about 20K delta T surface superheat then you get film boiling, top, 
     bottom or side.

>The situation in the LN2 is that the LN2 is touching the object over all
>parts and there is no isolation of the LN2 from the object, for as soon as the
>LN2 turns into N2 vapor, the bubble breaks off and heads for the surface of
>the dewar.  So there is no "levitation" or isolation of the LN2 from the
>object.

     Not exactly true, a bubble may more easily break of the surface in 
     a given situation, but it still may be in 'isolation' or film
     boiling.  In nitrogen, the required superheat for film boiling on 
     a downward-facing surface is several degrees lower than for
     upward-facing surfaces for this very reason.  However, it is
     still film boiling.

                           dale bass


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