Boiling, nucleate departure from

FIG. 27-40 Elffect of departure from nucleate boiling (DNB) on tube-metal temperature. 

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

Steam blanketing results in a departure from nucleate boiling (DNB) and typically dryout (localized total evaporation) conditions. It also may result in the formation of secondary (silicate-based) deposits that cannot be rinsed or resolublized, overheating problems, and eventual boiler tube or furnace shell rupture. 

Concentration may also result from waterline evaporation. In higher pressure WT power boilers, it may develop as a result of high-firing cycle operations, which may lead to departure from nucleate boiling (DNB). 

Swenson, H. S., Carver, J. R., and Kakarala, C. R., The influence of axial heat flux distribution on the departure from nucleate boiling in a water-cooled tube, ASME Paper No. 62-WA-297 (1962). 

Cermak, J. O., R. F. Farman, L. S. Tong, J. E. Casterline, S. Kokolis, and B. Matzner, 1970, The Departure from Nucleate Boiling in Rod Bundles during Pressure Blowdown, Trans. ASME, Ser. C, J. Heat Transfer 92(4) 621-627. (4)... 

Swenson, H. S., J. R. Carver, and C. R. Kakarala, 1962a, The Influence of Axial Heat Flux Distribution on the Departure from Nucleate Boiling in a Water Cooled Tube, ASME Paper 62-WA-297, Winter Annual Meeting, ASME, New York. (5)... 

Tong, L. S., 1967a, Prediction of Departure from Nucleate Boiling for an Axially Non-uniform Heat Flux Distribution, J. Nuclear Energy 21. 241-248. (3)... 

Tong, L. S., 1967b, Heat Transfer in Water-Cooled Nuclear Reactors, Nuclear Eng. Design (5 301. (3) Tong, L. S., 1968a, An Evaluation of the Departure from Nucleate Boiling in Bundles of Reactor Fuel Rods, Nuclear Sci. Eng. 33 7-15. (5)... 

Weisman, J., A. H. Wenzel, L. S. Tong, D. Fitzsimmons, W. Thorne, and J. Batch, 1968, Experimental Determination of the Departure from Nucleate Boiling in Large Rod Bundles at High Pressures, AIChE Chem. Eng. Prog. Symp. Ser. 64(82) 114-125. (5)... 

The rate of heat transfer from the tubes to the fluid depends primarily on turbulence and the magnitude of the heat flux itself. Turbulence is a function of mass velocity of the fluid and tube roughness. Turbulence has been achieved by designing for high mass v ocities, which ensure that nucleate boiling takes place at the inside surface of the tube. If sufficient turbulence is not provided, departure from nucleate boiling (DNB) occurs. DNB is the production of a film of steam on the tube surface that impedes heat transfer and results in tube overheating and possible failure. This phenomenon is illustrated in Fig. 27-40. 

L. S. Tong, An Evaluation of the Departure From Nucleate Boiling in Bundles of Reactor Fuel Rods, Nuclear Sci. Eng. (33) 7-15,1968. 

Low heat density of the core, enhanced margins of departure from nucleate boiling, self regulation of coolant flowrate throu the fuel assemblies. 

The main thermal-hydraulic characteristics of the ELENA-NTEP plant are presented in Table in-3 coolant temperatures in all circuits are also shown in Fig. III-l. Under normal operation conditions, the maximum temperature of the fuel is 673 K, that of structural materials (fuel cladding) is 638 K and the departure from nucleate boiling (DNB) is 15 K. 

The departure from nucleate boiling (DNB) Design Basis is that there is at least a 95-percent probability, at a 95-percent confidence level, that DNB does not occur on the limiting fuel rods during normal operation and operational transients and any transient conditions arising from faults of moderate frequency. 

VIPRE-01 is a finite-volume sub-channel analysis code enable of three-dimensional modelling of reactor cores and other similar geometries in steady-state and transient conditions. VIPRE-01 calculates the detailed steady-state and operational transient core flow distributions, coolant conditions, fuel rod temperature and departure from nucleate boiling ratio (DNBR). 

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