Nucleate boiling transition

Natural convective boiling Nucleate boiling Transition boiling... 

Most steam generating plants operate below the critical pressure of water, and the boiling process therefore involves two-phase, nucleate boiling within the boiler water. At its critical pressure of 3,208.2 pounds per square inch absolute (psia), however, the boiling point of water is 374.15 C (705.47 °F), the latent heat of vaporization declines to zero, and steam bubble formation stops (despite the continued application of heat), to be replaced by a smooth transition of water directly to single-phase gaseous steam. 

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. 

Equation (2-113) means that any cylindrical cavity for any liquid-solid combination under a given pressure has a minimum heat flux below which boiling will not be stable, and a transition between natural convection and stable nucleate boiling (bumping) is always observed. 

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... 

Dhir, V. K., 1990, Nucleate and Transition Boiling Heat Transfer under Pool and External Flow Conditions, Proc. 9th Int. Heat Transfer Conf, vol. 1, pp. 129 155 see also Int. J. Heat Fluid Flow 12(4) 290. (2)... 

Dhir, V. K., and S. P. Liaw, 1989, Framework for a Unified Model for Nucleate and Transition Pool Boiling, Trans. ASME J. Heat Transfer 777 739-746. (2)... 

Moissis, R., and P. J. Berenson, 1962, On the Hydrodynamic Transitions in Nucleate Boiling, ASME 62-HT-8, Natl. Heat Transfer Conf., Houston, TX. (2)... 

The reproducibility of the RPT [(100)(number of spills that ended in an RPT)/(total number of spills)] varied from 100 to 12%. At temperatures less than those necessary for an RPT, there was generally nucleate boiling and sometimes the substrate froze. The temperature of the transition from nucleate boiling to an RPT was quite sharp. At temperatures above the RPT region, film boiling took place. However, the temperature of this transition was not definite. The transition would often take the form of decreased reproducibility and RPT intensity at higher temperatures. 

The transition from nucleate boiling to forced-convection heat transfer is gradual. As fluid velocity increases, nucleation becomes more difficult, and therefore this boiling mechanism makes up a decreasing part of the total heat transfer. The transition will occur at higher velocities as pressure increases, and because the velocity at a constant quality decreases with pressure, a higher quality will also be required to suppress nucleation. 

Thonon B, Feldman A, Margat L, Marvillet C. Transition from nucleate boiling to convective boiling in compact heat exchangers. Int J Refrigeration 1997 20(8) 592-597. 

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