🚀 go-pugleaf

In article <1992Dec11.172416.3281@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.
>
>This is not the same thing.  The water touches the metal because the force
>of evaperating stem is not enough to over come the force of gravity and the
>drop makes contact with the metal.  Drop the iron in a bucket of water
>and you will _not_ see the same effect.  In fact, make a small puddle on a
>table, put the iron on _top_ of the water and again you will not see this
>effect.  In this case the bubble is riding on a film of evaporated water.
>

     It is exactly the same thing.  Drop an iron into a pool of water
     (* not recommended, I do not recommend this, I really do not recommend
     this, and I have a really good attorney *), and look at the surface.  
     If the iron is hot enough, you'll see a film of water vapor form on 
     the surface.  It's called film boiling.  It also happens on vertical
     and downward-facing surfaces.  And it would happen in free-fall
     under appropriate circumstances.

                              dale bass


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