🚀 go-pugleaf

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 ???

     Heat transfer.

>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.

     I'm willing to explain the details (it's a bit wimpy to allude to 
     details without giving them).

     The classical pool boiling curve looks like (with the limitations of
     this medium)
  
 regime: convective| nucleate | transition | film boiling
          
          |
          |                   * *
          |                 *     *
 heat     |                *       *                     *
 flux     |               *         *                 *
          |              *           *             *
          |             *             *         *
          |            *               *     *
          |           *                   *
          |          *
          |         *
          |        *
          |       *
          |     *
          |   *
          |**____________________________________________________
  
                              T_surf-T_bulk (= delta T)

       Both axes are generally log scales.

       The convective region is nonboiling;  the nucleate region involves
       bubbles boiling from a given surface; the transition region is 
       big patches of film boiling interspersed with nucleate boiling; the 
       film boiling region has the liquid completely detached from the 
       surface by a layer of gas phase.

       As the metal cools, it rides the film boiling part of the curve back 
       to the dip between transition and nucleate.  This is the minimum
       film boiling heat flux.  Now, usually one simply jumps to the 
       nucleate boiling curve at constant heat flux since a) transition
       is unstable on flux-controlled surfaces, b) most surfaces we play
       with (e.g. heaters) are flux-controlled.  However, a coin in a 
       nitrogen bath undergoing steady cooling is not exactly flux-controlled.
       So the flux rides up (usually rather rapidly) the transition part
       of the curve to rather violent nucleate boiling.  The heat transfer
       (and gas phase generation) is correspondingly increased, cooling
       the coin rather quickly to equilibrium.

       For nitrogen plates facing upwards, the peak nucleate flux occurs at 
       a delta T of about 8 K at atmospheric pressure and 77K bulk 
       temperature.  The minimum film boiling flux occurs at a delta T
       of about 20 K.  The ratio of heat fluxes from minimum film to 
       peak nucleate is over 20 for nitrogen under these conditions.

       I know these things because I am the world's largest repository
       of nitrogen, hydrogen and deuterium boiling heat transfer data.
       Write for your own franchise today.

                                dale bass

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