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Unformatted text preview: the primary quantity of energy cen58933_ch10.qxd 9/4/2002 12:38 PM Page 517 517
CHAPTER 10 transferred during boiling heat transfer. The hfg values of water at various temperatures are given in Table A-9.
Bubbles owe their existence to the surface-tension at the liquid–vapor interface due to the attraction force on molecules at the interface toward the liquid phase. The surface tension decreases with increasing temperature and
becomes zero at the critical temperature. This explains why no bubbles are
formed during boiling at supercritical pressures and temperatures. Surface
tension has the unit N/m.
The boiling processes in practice do not occur under equilibrium conditions,
and normally the bubbles are not in thermodynamic equilibrium with the surrounding liquid. That is, the temperature and pressure of the vapor in a bubble
are usually different than those of the liquid. The pressure difference between
the liquid and the vapor is balanced by the surface tension at the interface. The
temperature difference between the vapor in a bubble and the surrounding liquid is the driving force for heat transfer between the two phases. When the liquid is at a lower temperature than the bubble, heat will be transferred from the
bubble into the liquid, causing some of the vapor inside the bubble to condense and the bubble to collapse eventually. When the liquid is at a higher
temperature than the bubble, heat will be transferred from the liquid to the
bubble, causing the bubble to grow and rise to the top under the influence of
Boiling is classified as pool boiling or flow boiling, depending on the presence of bulk fluid motion (Fig. 10–3). Boiling is called pool boiling in the absence of bulk fluid flow and flow boiling (or forced convection boiling) in the
presence of it. In pool boiling, the fluid is stationary, and any motion of the
fluid is due to natural convection currents and the motion of the bubbles under the influence of buoyancy. The boiling of water in a pan on...
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