Boiling, nucleate

Nucleate Boiling High rates of heat transfer are obtained under nucleate boiling. The mechanism has not yet been clearly established, but, as a result of considerable activity in the field of nucleate boiling, there are available several expressions from which reasonable values of the film coefficients may be obtained. These expressions do not yield exactly the same numerical results even though the correlations were based upon much of the same data. There is thus neither a prominent nor unique equation for nucleate boiling. Either convenience or familiarity usually governs the user s selection of the available equations. 

The following equation has been widely used to predict nucleate boiling heat transfer. The equation includes a term to treat variation of coefficient with pressure. A special Reynolds number is also defined. 

001 for commercial copper and steel surfaces 0.00059 for stainless steel or chromium and nickel alloys 0.0004 for polished surfaces 

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The mathematical formulation of forced convection heat transfer from fuel rods is well described in the Hterature. Notable are the Dittus-Boelter correlation (26,31) for pressurized water reactors (PWRs) and gases, and the Jens-Lottes correlation (32) for boiling water reactors (BWRs) in nucleate boiling. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

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. 

Pressure drop due to hydrostatic head can be calculated from hquid holdup B.]. For nonfoaming dilute aqueous solutions, R] can be estimated from f i = 1/[1 + 2.5(V/E)(pi/pJ ]. Liquid holdup, which represents the ratio of liqmd-only velocity to actual hquid velocity, also appears to be the principal determinant of the convective coefficient in the boiling zone (Dengler, Sc.D. thesis, MIT, 1952). In other words, the convective coefficient is that calciilated from Eq. (5-50) by using the liquid-only velocity divided by in the Reynolds number. Nucleate boiling augments conveclive heat transfer, primarily when AT s are high and the convective coefficient is low [Chen, Ind Eng. Chem. Process Des. Dev., 5, 322 (1966)]. 

Eitch indented into the tube. Tube 48 was a clean copper tube that ad 50 longitudinal flutes pressed into the wall (Gener Electric double-flute profile, Diedrich, U.S. Patent 3,244,601, Apr. 5, 1966). Tubes 47 and 39 had a specially patterned porous sintered-metal deposit on the boihng side to promote nucleate boiling (Minton, U.S. 

Satisfactory performance is obtained with tubes having helical ribs on the inside surface, which generate a swirling flow. The resulting centrifugal action forces the water droplets toward the inner tube surface and prevents the formation of a steam film. The internally rifled tube maintains nucleate boiling at much higher steam temperature and pressure and with much lower mass velocities than those needed in smooth tubes. In modern practice, the most important criterion in drum boilers is the prevention of conditions that lead to DNB. 

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

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

Pool and Nucleate Boiling—General Correlation for Heat Flux and Critical Temperature Difference... 

X = vapor quality of fluid = 0 for pool boiling and is a low fraction, about 0.1 to 0.3, for most nucleate boiling... 

Figure 10-100. Maximum AT for nucleate boiling correlation. (Used by permission Cichelli, M. T, and Bonilla, C. F. Transactions, AlChE, V. 41, No. 6, 1945. American Institute of Chemical Engineers. All rights reserved.)...

A well-recognized and often-used equation for determining a reasonable, even if preliminary, nucleate boiling coefficient is represented by the McNelly equation for boiling outside of tubes ... 

The mechanism of boiling is essentially nucleate pool hoiling. In hoth styles of rehoiler the liquid velocity is relatively low compared to thermosiphon units. Jacobs provides an extensive comparison of advantages and disadvantages of essentially all the reboiler types used in industrial plants. Palen and Taborek conducted extensive studies of available data and proposed nucleate boiling equations to correlate various data from the available 14 equations down to a selected 6 for detailed study. The study was limited to various hydrocarbons and hydrocarbon mixtures. Their conclusions after computer correlations of the results from several equations were as follows. 

Calculate the correction to the nucleate boiling film coefficient for the tube bundle number of tubes in vertical row, hi,. See previous discussion. 

U(j) = single tube overall heat transfer coefficient hj = nucleate boiling coefficient for single tube, outside Btu/hr (ft) (°F)... 

Figure 10-108. Uncorrected nucleate boiling coefficient. (Used by permission Chen, Ning Hsing. Chemical Engineering, V. 66, No. 5, 1959. McGraw-Hill, Inc. All rights reserved.)...

Figure 10-115—1/Xjj values for use in Figure 10-118. Figure 10-118—a for correcting the nucleate boiling coefficient.

Figure 10-118 a from Equation 10-184. d. Nucleate boiling coefficient, h, from Table 10-30 or other source. (An estimate of the film temperature drop is required.)... 

Film boiling should be avoided however, nucleate boiling often can be found at heat flux values greater than the rule-of-thumb values of 10-12,000 Btu/hr-fT. These are often conservative values. See Figure 10-119 from Fair. ... 

Above critical temperature difference for nucleate boiling Vs -in-Vs -in- O.D. (data of correlation)... 

Nucleate Boiling Outside Horizontal or Vertical Tubes... 

Nucleate boiling is boiling at the tube surfece at a temperature difference between outside tube surface temperature and the fluid body, less than the critical temperature difference. At and beyond the critical temperature difference, metastable and film boiling take place. These produce lower transfer coefficients as the temperature difference increases. 

Griffith, P, The Correlating of Nucleate Boiling Burnout Data, ASME Heat Transfer Div. meeting. University Park, PA, Aug. 11 (1957) Paper 57-HT-21. 

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