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Unformatted text preview: ion 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 heat transfer in this regime. FIGURE 10–11
Different relations are
used to determine the heat
flux in different boiling regimes.
P = 1 atm 100°C
qfilm boiling ·
Heating FIGURE 10–12
At high heater surface temperatures,
radiation heat transfer becomes
significant during film boiling. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 526 526
HEAT TRANSFER Note that the gravitational acceleration g, whose value is approximately
9.81 m/s2 at sea level, appears in all of the relations above for boiling heat
transfer. The effects of low and high gravity (as encountered in aerospace
applications and turbomachinery) are studied experimentally. The studies
confirm that the critical heat flux and heat flux in film boiling are proportional
to g1/4. However, they indicate that heat flux in nucleate boiling is practically
independent of gravity g, instead of being proportional to g1/2, as dictated by
Eq. 10–2. Enhancement of Heat Transfer in Pool Boiling Liquid
Vapor Nucleation sites for vapor FIGURE 10–13
The cavities on a rough surface act as
nucleation sites and enhance
boiling heat transfer. P = 1 atm 100°C The pool boiling heat transfer relations given above apply to smooth surfaces.
Below we will discuss some methods to enhance heat transfer in pool boiling.
We pointed out earlier that the rate of heat transfer in the nucleate boiling
regime strongly depends on the number of active nucleation sites on the surface, and the rate of bubble formation at each site. Therefore, any modification that will enhance nucleation on the heating surface will also enhance heat
transfer in nucleate boiling. It is observed that irregularities on the heating
surface, including roughness and dirt, serve as additional nucleation sites during boiling, as shown in Figure 10–13. For example, the first bubbles in a pan
filled with water are most likely to form at the scratches at the bottom surface.
These scratches a...
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