Figure 1011 different relations are used to determine

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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 400°C Vapor · qfilm boiling · qrad 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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