HEAT TRANSFER THEORY (cont'd)
TRANSFER TO BOILING LIQUIDS
When the presence of a heated surface causes a liquid near it to boil,
the intense agitation gives rise to high local coefficients of heat
transfer. A considerable amount of experimental work has been carried
out on this, but generalized correlations are still not very adequate.
It has been found that the apparent coefficient varies considerably
with the temperature difference between the heating surface and the
liquid. For temperature differences greater than about 20°C,
values of h decrease, apparently because of blanketing of
the heating surface by vapours. Over the range of temperature differences
from 1°C to
20°C, values of h for boiling water increase from 1200
to about 60,000 J
m-2 s-1 °C-1.
For boiling water under atmospheric pressure,
the following equation is approximately true:
h = 50(DT)2.5 (5.36)
where DT is
the difference between the surface temperature and the temperature
of the boiling liquid and it lies between 2°C and 20°C.