Nucleate boiling curve

Suspended inert gas bubbles, too small to be seen, seem to be present in some liquids in an as-received condition. These motes can also come into being if a liquid containing dissolved gases is heated. The evidence is clear that such motes have a strong effect on the nucleate-boiling curve. Pike and co-workers (PI) boiled tap water, deaerated water, and water which had been brought to equilibrium with bubbling air at various temperatures. It was found that an increase in air content causes an increase in h. Similar results are reported by McAdams (M4). 

Figure 5 Nucleate boiling curves. F + (PF-5060), F- (PF-5050), LL+ (high liquid level), LL- (low liquid level), P+ (high pressure), and P- (low pressure)...

The use of structured surfaces to enhance thin-film evaporation has also been considered recently. Here, in contrast to the flooded-pool experiments noted above, the liquid to be vaporized is sprayed or dripped onto heated horizontal tubes to form a thin film. If the available temperature difference is modest, structured surfaces can be used to promote boiling in the film, thus improving the overall heat transfer coefficient. Chyu et al. [43] found that sintered surfaces yielded nucleate boiling curves similar to those obtained in pool boiling. T-shaped fins did not exhibit low AT boiling however, a threefold convective enhancement was obtained as a result of the increased surface area. 

The results presented in this section are a summary of our previously published work on the nucleate boiling curves of drag-reducing polymer solutions [5]. 

To provide a common basis for comparisons, all of the boiling curve data for the polymer solutions have been run at constant relative viscosities. Figures 1(a) and 1(1 ) show the nucleate boiling curves for water and the six aqueous polymer solutions... 

Fig.l. The nucleate boiling curve data of water and the six polymers of interest at concentrations corresponding to relative viscosities of (a) 1.08 and (b) 1.32. 

The observed temperature shifts in the nucleate boiling curves for polymer solutions are reasonably well predicted based only on how the polymers affect the solution viscosity. As the solution viscosity increases, the temperature difference of the boiling curve increases in direct proportion to the relative temperature shift predicted by the Rohsenow pool boiling correlation. However, the polymer boiling curve temperature shifts from the pure solvent line, determined by using heated platinum wires, are in the opposite direction to those reported by other investigators for heated flat plates Cl-3]. While the hydrodynamics of bubble growth and departure on wires are different from those on flat surfaces, it is not clear why the results should be opposite. 

Answer by Author It is our understanding that the Forster—Zuber correlation applies also to the entire nucleate-boiling curve. No correlation was attempted with the peak flux data since our peak flux data are slightly lower than a true peak, and we were not primarily concerned with locating this point. 

The lower Emit of applicability of the nucleate-boiling equations is from 0.1 to 0.2 of the maximum limit and depends upon the magnitude of natural-convection heat transfer for the liquid. The best method of determining the lower limit is to plot two curves one of h versus At for natural convection, the other ofh versus At for nucleate boiling. The intersection of these two cui ves may be considered the lower limit of apphcability of the equations. 

If point F in Fig. 2 is reached without physical burn-out occurring, then, as shown by Nukiyama, a further increase in heat flux will raise the surface temperature in the direction of E until physical burn-out does occur. If, however, the heat flux at point F is decreased, the surface temperature does not revert to the value at C, but moves along the curve towards D. On reaching D, it was observed by Nukiyama that the surface temperature undergoes another jump discontinuity along the dotted line DG, and stabilizes at G in the nucleate-boiling region. Both the transition lines CF and DG can be passed only in the direction shown by the arrows in Fig. 2. 

Figure 4.3 Boiling curve for partial nucleate boiling. 

Figure 4.4 Procedure for construction of curve for partial nucleate boiling. (From Bergles and Roh-senow, 1964. Copyright 1964 by American Society of Mechanical Engineers, New York. Reprinted with permission.)... 

The relationship between the wall temperature and the coolant temperature can be seen in Figure 4.5. The wall temperature starts to bend at the incipience of subcooled boiling, where the coolant temperature is defined as Tm. The wall temperature follows a curve of partial boiling and then reaches an approximately constant value at a fully developed nucleate boiling where the coolant temperature... 

Consider a tube heated uniformly at a heat flux q/A fed with saturated water at the base at a velocity Fo. For this velocity and heat flux, nucleate boiling will take place, and a temperature difference aTo will be established. At some distance up the tube vaporization will occur and increase the volumetric flow of material and hence the velocity to, say, Fi. The line for forced convective heat transfer meets the boiling curve below the heat flux of q/A and so nucleate boiling will still be the mode of heat transfer and the temperature difference AT, and hence the heat transfer... 

Fig. 30. Effect of surface texture on nucleate boiling. Ether was boiled on a nickel plate polished with different grades of emery paper. The pressure was atmospheric. Typical data are given for one curve only (C4).

As would be expected, agitation increases the heat transfer rate for nucleate boiling. Schweppe and Foust (S2) pumped saturated water through a heated tube to obtain the boiling curves in Fig. 34. Note the... 

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