Film boiling convection

Figure 1.1 Pool boiling regimes A-B, natural convection B-C, nucleate boiling C-D, partial film boiling D-E, stable film boiling.

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... 

Convective heat transfer correlations for film boiling... 

The combined effect of turbulent convection from liquid with high subcooling and radiation for film boiling on a flat surface was analyzed by Hamill and Baumeister (1967), resulting in the expression... 

The primary mode of heat transfer at the wall is forced convection of the vapor phase. As the liquid does not wet the heating surface during film boiling, heat transfer due to drop-wall collisions is relatively small, resulting in low wall-drop heat transfer (only a few percent of the total heat input). Most of the droplet evaporation occurs because of vapor-drop heat transfer. Just after dryout, the... 

Hynek, S. J., 1969, Forced Convection Dispersed Flow Film Boiling, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge, MA. (4)... 

The problem of burn-out prediction is a difficult one, and one on which a great deal of experimental work is being carried out, particularly in connection with nuclear-reactor development. Much of the earlier literature is rather confused, due to the fact that the mechanics of the burn-out were not carefully defined. Silvestri (S8) has discussed the definitions applicable to burn-out heat flux. It appears possible to define two distinctly different kinds of burn-out, one due to a transition from nucleate to film boiling, and one occurring at the liquid deficient point of the forced-convection region. The present discussion treats only the latter type of burn-out fluxes. The burn-out point in this instance is usually determined by the sudden rise in wall temperature and the corresponding drop in heat flux and heat-transfer coefficient which occur at high qualities. 

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. 

You may be tempted to simply add the convection and radiation heat transfers to determine the total heat transfer during film boiling. However, these two mechariisms of heat transfer adversely affect each other, causing the total heal transfer to be less than their sum. For example, the radiation heat transfer from the surface to the liquid enhances the rale of evaporation, and thus the thickness of the vapor film, which impedes convection heat transfer. For flfiimi Broitdey dclcnuincd that die relation... 

S. S. Kuiateladze. On the Transition to Film Boiling under Natural Convection. Kotloturbostroenie 3 (1948), p. 48. 

Convective boiling-heat transfer coefficient Local effective cooling-condensing heat transfer coefficient, partial condenser Fouling coefficient based on fin area Heat transfer coefficient based on fin area Film boiling heat transfer coefficient Forced-convection coefficient in equation 12.67 Local sensible-heat transfer coefficient, partial condenser... 

Fig. 21. Film boiling with forced convection. The liquid flow was normal to a horizontal tube at one atmosphere. Velocity = 0 to 14 ft./sec. Diameter = 0.387 to 0.637 inch (B5).

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].)... 

Enhancement techniques can be classified as passive methods, which require no direct application of external power, or as active schemes, which require external power. The effectiveness of both types depends strongly on the mode of heat transfer, which might range from single-phase free convection to dispersed-flow film boiling. Brief descriptions of passive techniques follow. 

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. 

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