The heat transfer rate increases with increasing

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Unformatted text preview: d the liquid is responsible for the low heat transfer rates in the film boiling region. The heat transfer rate increases with increasing excess temperature as a result of heat transfer from the heated surface to the liquid through the vapor film by radiation, which becomes significant at high temperatures. A typical boiling process will not follow the boiling curve beyond point C, as Nukiyama has observed during his experiments. Nukiyama noticed, with surprise, that when the power applied to the nichrome wire immersed in wa· ter exceeded qmax even slightly, the wire temperature increased suddenly to the melting point of the wire and burnout occurred beyond his control. When he repeated the experiments with platinum wire, which has a much higher melting point, he was able to avoid burnout and maintain heat fluxes higher than · qmax. When he gradually reduced power, he obtained the cooling curve shown · in Figure 10–8 with a sudden drop in excess temperature when qmin is reached. Note that the boiling process cannot follow the transition boiling part of the boiling curve past point C unless the power applied is reduced suddenly. The burnout phenomenon in boiling can be explained as follows: In order to · move beyond point C where qmax occurs, we must increase the heater surface temperature Ts. To increase Ts, however, we must increase the heat flux. But cen58933_ch10.qxd 9/4/2002 12:38 PM Page 521 521 CHAPTER 10 (a) (b) (c) the fluid cannot receive this increased energy at an excess temperature just beyond point C. Therefore, the heater surface ends up absorbing the increased energy, causing the heater surface temperature Ts to rise. But the fluid can receive even less energy at this increased excess temperature, causing the heater surface temperature Ts to rise even further. This continues until the surface FIGURE 10–7 Various boiling regimes during boiling of methanol on a horizontal 1-cm-diameter steam-heated copper tube: (a) nucleate boiling, (b) transition boiling, and (c) film boili...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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