Boiling regimes

Heat transfer characteristics for saturated boiling were considered by Yen et al. (2003). From this study of convective boiling of HCFC123 andFC72 in micro-tubes with inner diameter 190, 300 and 510 pm one can see that in the saturated boiling regime, the heat transfer coefficient monotonically decreased with increasing vapor quality, but independent of mass flux. 

Hetsroni et al. (2003b, 2003, 2005a,b) observed the explosive saturated boiling regime in micro-channels, which exists before the annular flow regime. The test... 

There are several boiling regimes in pool boiling as well as in flow boiling. The only difference lies in the influence of flow effect. The buoyancy effect is significant... 

Figure 1.1 Pool boiling regimes A-B, natural convection B-C, nucleate boiling C-D, partial film boiling D-E, stable film boiling.

The single tube values for hb have been correlated by equation 14.2, which applies to the true nucleate boiling regime and takes no account of the factors which eventually lead to the maximum heat flux being approached. As discussed in Volume 1, Chapter 9, equations for maximum flux, often a limiting factor in evaporation processes, have been tested by Palen and Taborek , though the simplified equation of Zuber is recommended. This... 

FIGURE 10-5 Different boiling regimes in pool boiling. 

Various boiling regimes during boiling of methanol on a horizontal l-cm-diametet Steam-healed copper tube (n) nucleate boiling, (fc) transition boiling, and (c) film boiling. 

Nucleate boiling is the most desirable boiling regime in practice because high heat transfer rates can be achieved in this regime with relatively small values of A7 c, ss typically under 30"C for water. The photographs in Fig. 10-7 show the nature of bubble formation and bubble motion associated with nucleate, transition, and film boiling. 

Different relations are used to determine the heat flux in different boiling regimes. 

Minimum heat flux, which occurs at the Lcidenfrost point, is of practical interest since it represents the lower limit for the heat flux in the film boiling regime. Using the stability theory, /uber derived the following expression for the minimum heat flux for a large horiiontalplate,... 

Operation in the transition boiling regime is normally avoided in the design of heat transfer equipment, and thus no major attempt has been made to develop general correlations for boiling heal transfer in this regime. 

Water in a tank is to be boiled at sea level by a 1-cm-diameter nickel plated steel heating element equipped with electrical resistance wires inside, as shown in Fig. 10-16. Determine the maximum heat flux that can be attained in the nucleate boiling regime and the surface temperature of the heater in that case. 

Discussion Note that the 5-mm-diameter copper heating element consumes about 1 kW of electric power per unit length in steady operation in the film boiling regime. This energy is transferred lo the water through the vapor film that forms around the wire. 

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