Boiling in free flow

Calculation of heat transfer coefficients for boiling in free flow... 

In the following the equations introduced previously for heat transfer in nucleate boiling in free flow are summarised and supplemented by further equations. 

In the calculation of the heat transfer in saturated boiling it is presumed that the heat transfer is predominantly determined by the bubble formation. In addition, convection also has some influence on the heat transfer, although this may only be small. Models and empirical equations start from the heat transfer in boiling in free flow. The influence of forced flow is taken into account with an extra term. This is based on the concept that the growth of the bubbles is only slightly influenced by the flow, as long as the bubbles are smaller than the superheated boundary layer. The heat transfer coefficient is a combination of two parts... 

The part a B emanating from the bubble formation is based on the heat transfer coefficient aB in nucleate boiling in free flow. However because the temperature rise in the boundary layer of a forced flow is steeper than in free flow nucleate boiling, more heat will be released from the wall by conduction and the bubble formation will be partially suppressed in comparison to that in free flow. Chen accounted for this effect with a suppression factor S < 1, which the heat transfer coefficient aB in nucleate boiling in free flow is multiplied by, a B = SaB. 

The factor S approaches one for vanishingly small mass flux, the heat transfer coefficient a B is then the same as that in boiling in free flow, ft approaches zero at large mass flux because then the heat transfer is exclusively determined by the convective part a c in (4.159). 

In the limiting case of S = F = 1, this equation converts into that for saturated boiling, (4.156). The heat transfer coefficients aB for nucleate boiling in free flow were taken by Chen from an equation from Forster and Zuber [4.93]. More recent investigations [4.94], etc. showed however, that this gives somewhat inaccurate results. It therefore seems more sensible to calculate aB using one of the formulae from section 4.2.6. 

Equation (4.89) or (4.91) is only valid in regions of intensive nucleate boiling. However, as Nukijama [4.67] was the first to show there are further regions of vaporization. Fig. 4.37 illustrates the individual regions that appear in boiling water at atmospheric pressure and in free flow. The heat flux q is plotted against... 

The phenomena that are observed in free flow also appear in forced flow, but are more complex. There are two fundamental types of boiling crisis ... 

Natural convection is self-induced and is created by the density differences, which are temperature related the boiling of water in a kettle is an example of free convection. Forced convection is caused by an external force being applied by mechanical means such as a fan or pump the cooling of a warm bottle in cool flowing water is an example of forced convection. 

In the fourth chapter the heat and mass transfer in condensation and boiling with free and forced flows is dealt with. The presentation follows the book, Heat Transfer in Condensation and Boiling (Berlin Springer-Verlag 1992) by K. Stephan. Here, we consider not only pure substances condensation and boiling in mixtures of substances are also explained to an adequate extent. 

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