CHAPTER 5
HEAT TRANSFER THEORY (cont'd)

HEAT 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.