|
|
re: ot: what's the physics?
11 feb 2006
papapeng wrote:
>i have a $10 electric kettle that has a whistle as part of the spout.
>it is as simple as it gets. in the 5 minute run up to a full boil
>where the steam will blow hard on the whistle there is a puzzling
>sequence of noises. the kettle will sound like it is building a head
>of stream, a kind of muted roar consistent with air and steam forming,
>then go quiet. this happens two or three times...
this may be unstable film boiling: bubbles form so rapidly that they blanket
the heated surface, preventing water from moving in to take their place. the
increased resistance of this film reduces the heat flux, and the heat transfer
decreases with increasing temp diff (a negative resistance region of the
boiling curve between nucleate boiling with bubble columns and stable film
boiling.) the film intermittently collapses and reappears.
the 1998 schaum's outline on heat transfer by d. pitts and l. sissom says
using the hydrodynamic instability of the liquid-vapor boundary,
n. zuber and m. tribus [ucla eng. report 85-5 (1985)] found the
equation for min heat flux in film boiling:
q/a = 0.09rvfhfb[g(rl-rvf)/(rl+rvf)]^(1/2)x
[gcsigma/(g(rl-rvf))]^(1/4) ~ 13 kw/m^2.
based upon zuber-tribus analysis, p. berensen [trans. asme, j. heat
transfer 83: 351 (1961)] found the excess temp dt = tsurface - tsat
at the point of min heat flux:
dt = 0.127rvfhfg/kvf[g(rl-rv)/(rl+rv)]^(2/3)x
[gcsigma/(g(rl-rv)]^(1/2)[muf/(g0(rl-rv)]^(1/3) ~ 150 c.
my 240 v 3 kw uk stainless steel swan teakettle (5 pounds at a jumble sale :-)
has an exposed underwater heating coil about a foot long and 1/4" diameter, so
it operates in the stable/radiation film boiling regime, with increasing noise
until it senses full steam and lights the handle switch and shuts itself off
in 56 seconds, starting with 20 oz of 112.5 f tap water. the steam thermostat
resets itself after another 2 minutes. if there's too little water, the spring-
loaded overtemp thermostat bangs the cord connector out of the handle sooner.
nick
|
|
