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Unformatted text preview: ct like “nests” for the bubbles to form and thus increase the
rate of bubble formation. Berensen has shown that heat flux in the nucleate
boiling regime can be increased by a factor of 10 by roughening the heating
surface. However, these high heat transfer rates cannot be sustained for long
since the effect of surface roughness is observed to decay with time, and the
heat flux to drop eventually to values encountered on smooth surfaces. The effect of surface roughness is negligible on the critical heat flux and the heat
flux in film boiling.
Surfaces that provide enhanced heat transfer in nucleate boiling permanently are being manufactured and are available in the market. Enhancement
in nucleation and thus heat transfer in such special surfaces is achieved either
by coating the surface with a thin layer (much less than 1 mm) of very porous
material or by forming cavities on the surface mechanically to facilitate continuous vapor formation. Such surfaces are reported to enhance heat transfer
in the nucleate boiling regime by a factor of up to 10, and the critical heat flux
by a factor of 3. The enhancement provided by one such material prepared by
machine roughening, the thermoexcel-E, is shown in Figure 10–14. The use
of finned surfaces is also known to enhance nucleate boiling heat transfer and
the critical heat flux.
Boiling heat transfer can also be enhanced by other techniques such as mechanical agitation and surface vibration. These techniques are not practical,
however, because of the complications involved. Water
108°C Heating FIGURE 10–15
Schematic for Example 10–1. EXAMPLE 10–1 Nucleate Boiling of Water in a Pan Water is to be boiled at atmospheric pressure in a mechanically polished stainless steel pan placed on top of a heating unit, as shown in Figure 10–15. The
inner surface of the bottom of the pan is maintained at 108°C. If the diameter
of the bottom of the pan is 30 cm, determine (a) the rate of heat transfer to the
water and (b) the rate of evaporation of water. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 527 527
Tsat = 0°C
105 0.5 be n tu F...
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