Cen58933ch10qxd 942002 1238 pm page 522 522 heat

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Unformatted text preview: ng (from J. W. Westwater and J. G. Santangelo, University of Illinois at Champaign-Urbana). cen58933_ch10.qxd 9/4/2002 12:38 PM Page 522 522 HEAT TRANSFER · q W —2 – m · · qmax qmax = constant C E Sudden jump in temperature Ts, °C Tmelting FIGURE 10–9 An attempt to increase the boiling heat flux beyond the critical value often causes the temperature of the heating element to jump suddenly to a value that is above the melting point, resulting in burnout. temperature reaches a point at which it no longer rises and the heat supplied can be transferred to the fluid steadily. This is point E on the boiling curve, which corresponds to very high surface temperatures. Therefore, any attempt · to increase the heat flux beyond qmax will cause the operation point on the boiling curve to jump suddenly from point C to point E. However, surface temperature that corresponds to point E is beyond the melting point of most heater materials, and burnout occurs. Therefore, point C on the boiling curve is also called the burnout point, and the heat flux at this point the burnout heat flux (Fig. 10–9). · Most boiling heat transfer equipment in practice operate slightly below qmax to avoid any disastrous burnout. However, in cryogenic applications involving fluids with very low boiling points such as oxygen and nitrogen, point E usually falls below the melting point of the heater materials, and steady film boiling can be used in those cases without any danger of burnout. Heat Transfer Correlations in Pool Boiling Boiling regimes discussed above differ considerably in their character, and thus different heat transfer relations need to be used for different boiling regimes. In the natural convection boiling regime, boiling is governed by natural convection currents, and heat transfer rates in this case can be determined accurately using natural convection relations presented in Chapter 9. Nucleate Boiling In the nucleate boiling regime, the rate of heat transfer strongly depends on the nature of nucleatio...
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