Nucleate boiling heat transfer

Correlations for Boiling Heat Transfer Nucleate boiling is a complex phenomenon. Several mechanisms have been proposed to explain the boiling process, such as latent heat transport, microconvection, vapor-liquid exchange, wake flow, enhanced convection, and microlayer evaporation, details of which can be found in Ginoux (1978). 

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. 4.29 Heat transfer in boiling water at 100 °C on a horizontal heated surface, according to Jakob etal. [4.43], [4.45]. Curve a stagnant boiling region, curve b nucleate boiling region...

P. Sadasivan, C. Unal, and R. Nelson, Nonlinear Aspects of High Heat Flux Nucleate Boiling Heat Transfer, / Heat Transfer (117) 981-989,1995. 

Experimental investigations of heat transfer at boiling of polymeric liquids cover highly diluted (c = 15 to 500 ppm), low-concentrated (c 1%), and concentrated solutions (c>10%). The data represent diversity of physical mechanisms that reveal themselves in boiling processes. The relative contribution of different physical factors can vary significantly with changes in concentration, temperature, external conditions, etc., even for polymers of the same type and approximately equal molecular mass. For dilute solutions this is clearly demonstrated by the experimentally detected both intensification of heat transfer at nucleate boiling and the opposite effect, viz. a decrease in the heat removal rate in comparison with a pure solvent. 

All the limitations discussed earlier depend upon the axial heat transfer. The boiling limit, however, depends upon the evaporator heat flux (radial). Boiling limit occurs when the radial heat flux into the heat pipe causes the liquid in the wick to boil and evaporate causing dryout. It also occurs when the nucleate boiling in the evaporator creates vapor bubbles that partially block the return of fluid. The presence of vapor bubbles requires both (1) the formation of bubbles and also (2) the subsequent growth of these bubbles. Let us imagine a spherical vapor bubble that is very close to the heat pipe surface. At equilibrium, we have... 

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. 

The boiling mechanism can conveniently be divided into macroscopic and microscopic mechanisms. The macroscopic mechanism is associated with the heat transfer affected by the bulk movement of the vapor and Hquid. The microscopic mechanism is that involved in the nucleation, growth, and departure of gas bubbles from the vaporization site. Both of these mechanistic steps are affected by mass transfer. 

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)]. 

Highest heat-transfer coefficients are obtained in FC evaporators when the liquid is aUowed to boil in the tubes, as in the type shown in Fig. 11-122 7. The heating element projects into the vapor head, and the hquid level is maintained near and usuaUy slightly below the top tube sheet. This type of FC evaporator is not well suited to salting solutions because boiling in the tubes increases the chances of salt deposit on the waUs and the sudden flashing at the tube exits promotes excessive nucleation and production of fine ciystals. Consequently, this type of evaporator is seldom used except when there are headroom hmitations or when the hquid forms neither salt nor scale. 

The following nucleate or alternate designs procedure, suggested by Kem, is for vaporization (nucleate or pool boiling) only. No sensible heat transfer is added to the boiling fluid. 

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

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

Magrini, V. and E. Mannei, On the Influence of the Thickness and Thermal Properties of Heating Walls on the Heat Transfer Coefficients in Nucleate Pool Boiling, Trans. ASMEfoumal of Heat Transfer, May (1974) p. 173. 

Raben, 1. A., R. T. Beaubouef, and G. Commerford, A Study of Nucleate Pool Boiling of Water at Low Pressure, 6 Nat l. Heat Transfer Conference, Boston, Aug. (1963), AlChE Preprint No. 28. 

Rohsenow, W. M., Nucleation with Boiling Heat Transfer, Heat Trans. Div. ASME, Conference, Detroit, Ml, May (1970), Paper No. 70-HT-18. 

Heat transfer rates in modern boilers are relatively high, and when the first stage of boiling (incipient boiling point) is quickly reached, small bubbles of steam begin to form on the heated, waterside metal surface (steam bubble nucleation) but initially collapse when cooled by contact with the bulk water. 

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

High heat-transfer rates at boiler surfaces promote rapid nucleate boiling and other forms of convective boiling, which in turn may cause steam blanketing. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

H7. Hines, W. S., Forced convection and peak nucleate boiling heat transfer characteristics for hydrazine flowing turbulently in a round tube at pressures to 1000 psia, Rept. No. 2059, Rocketdyne, Canoga Park, California (1959). 

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