Boiling regimes and the boiling curve boiling is

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Unformatted text preview: ling FIGURE 10–5 Different boiling regimes in pool boiling. Boiling Regimes and the Boiling Curve Boiling is probably the most familiar form of heat transfer, yet it remains to be the least understood form. After hundreds of papers written on the subject, we still do not fully understand the process of bubble formation and we must still rely on empirical or semi-empirical relations to predict the rate of boiling heat transfer. The pioneering work on boiling was done in 1934 by S. Nukiyama, who used electrically heated nichrome and platinum wires immersed in liquids in his experiments. Nukiyama noticed that boiling takes different forms, depending on the value of the excess temperature Texcess. Four different boiling regimes are observed: natural convection boiling, nucleate boiling, transition boiling, and film boiling (Fig. 10–5). These regimes are illustrated on the boiling curve in Figure 10–6, which is a plot of boiling heat flux versus the excess temperature. Although the boiling curve given in this figure is for water, the general shape of the boiling curve remains the same for different fluids. The specific shape of the curve depends on the fluid–heating surface material combination and the fluid pressure, but it is practically independent of the geometry of the heating surface. We will describe each boiling regime in detail. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 519 519 CHAPTER 10 Natural convection boiling Bubbles collapse in the liquid 106 · qboiling, W/m2 Nucleate boiling Transition boiling C Film boiling Maximum (critical) · heat flux, qmax E 105 B 104 103 Bubbles rise to the free surface A 1 ~5 10 D · Leidenfrost point, qmin ~30 100 ~120 ∆Texcess = Ts – Tsat , °C 1000 Natural Convection Boiling (to Point A on the Boiling Curve) We learned in thermodynamics that a pure substance at a specified pressure starts boiling when it reaches the saturation temperature at that pressure. But in practice we do not see any bubbles forming on the heating surface until the liquid is heated a few degrees ab...
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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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