Nucleate, Transition, and Film Boiling

Subsequent evidence proved that Nukiyama was right. At least three types of boiling exist. In Fig. 3, which shows some recent data (W3) for methanol boiling on a horizontal copper tube, the portion of the curve 

The region BCD is the transition region of boiling. No active centers exist. The heat flux from the hot solid to the boiling liquid decreases continuously as the temperature-difference driving force is increased. 

This type of boiling has been called metastdble by at least one writer. The word transition is preferred inasmuch as the word is well established for the in-between region encountered in fluid-flow studies. 

From about E, in Fig. 3, on beyond indefinitely to increasing temperature differences, film boiling exists. This terminology is good, because a real film of vapor coats the hot solid during this type of boiling. 

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Nucleate boiling is the most desirable boiling regime in practice because high heat transfer rates can be achieved in this regime with relatively small values of A7 c, ss typically under 30"C for water. The photographs in Fig. 10-7 show the nature of bubble formation and bubble motion associated with nucleate, transition, and film boiling. 

The enhanced convection provided by stirring dramatically improves pool boiling at low superheat. However, once nucleate boiling is fully established, the influence of the improved circulation is small. Pramuk and Westwater [255] found that the boiling curve for methanol was favorably altered for nucleate, transition, and film boiling, with the improvement increasing as agitator speed increased. 

Heat transfer coefficients (HTCs) between melt and water fields are provided via a boiling curve, which describes nucleate, transition, and film boiling. (In the present problem, only the film boiling regime occurs.) At high vapor volume fractions, a transition is made between film boiling heat transfer to water and convective heat transfer to vapor from the melt. 

In convective vaporization, the same boiling regimes are encountered, but modified by the net motion of the two-phase fluid past the surface. At low velocities or high heat fluxes, the convection effect is small, and nucleate boiling dominates. At higher velocities, the heat-transfer rate is dominated by the two-phase mixture sweeping across the surface. It is still important to avoid transition and film boiling, but the onset of these phenomena is complicated by many factors. (See [1, 34].)... 

There are upper and lower limits of applicability of the equation above. The lower limit results because natural-convection heat transfer governs at low temperature differences between the surface and the fluid. The upper limit results because a transition to film boiling occurs at high temperature differences. In film boiling, a layer of vapor blankets the heat-transfer surface and no liquid reaches the surface. Heat transfer occurs as a result of conduction across the vapor film as well as by radiation. Film-boiling heat-transfer coefficients are much less than those for nucleate boiling. For further discussion of boiling heat transfer, see Refs. 5 and 6. 

It is obvious that much more experimental work is needed. No theoretical treatment of transition boiling has ever been given. A convincing explanation of why there should be a smooth decrease in h with an increase in AT has never appeared. It is possible to explain the effect in terms of film thickness, but this is a superficial explanation based on the false assumption of a stable film. It is also possible to explain that transition boiling is a mixture of nucleate and film boiling, but this is contrary to photographic evidence. 

After an initial excursion into film boiling, as soon as the test element cooled below the critical AT, power was re-established at a level slightly below the indicated critical value and then was slowly increased until the transition to film boiling recurred. Three or more such determinations of peak nucleate boiling flux were made at each pressure. 

Their results showed the following. Surface 1 gave direct transition from liquid-phase natural-convection heat transfer to film boiling with CHF values of 160,000 Btu/hr ft2 (503 kW/m2), independent of the pressure. Surface 2 gave stable nucleate boiling with CHF values much greater than those obtained with surface 1, and... 

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