Consequently the heat flux increases at a lower rate

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Unformatted text preview: icult for the liquid to reach the heater surface and wet it. Consequently, the heat flux increases at a lower rate with increasing Texcess, and reaches a maximum at point C. The heat flux at this point is · called the critical (or maximum) heat flux, qmax. For water, the critical heat 2 flux exceeds 1 MW/m . Nucleate boiling is the most desirable boiling regime in practice because high heat transfer rates can be achieved in this regime with relatively small values of Texcess, typically under 30°C for water. The photographs in Figure 10–7 show the nature of bubble formation and bubble motion associated with nucleate, transition, and film boiling. Transition Boiling (between Points C and D on the Boiling Curve) As the heater temperature and thus the Texcess is increased past point C, the heat flux decreases, as shown in Figure 10–6. This is because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation due to the low thermal conductivity of the vapor relative to that of the liquid. In the transition boiling regime, both nucleate and film boiling partially occur. Nucleate boiling at point C is completely replaced by film boiling at point D. Operation in the transition boiling regime, which is also called the unstable film boiling regime, is avoided in practice. For water, transition boiling occurs over the excess temperature range from about 30°C to about 120°C. Film Boiling (beyond Point D ) · q W —2 – m · qmax Sudden jump in temperature Bypassed part of the boiling curve 106 Sudden drop in temperature 1 · qmin 1000 10 100 ∆Texcess = Ts – Tsat, °C FIGURE 10–8 The actual boiling curve obtained with heated platinum wire in water as the heat flux is increased and then decreased. In this region the heater surface is completely covered by a continuous stable vapor film. Point D, where the heat flux reaches a minimum, is called the Leidenfrost point, in honor of J. C. Leidenfrost, who observed in 1756 that liquid droplets on a very hot surface jump around and slowly boil away. The presence of a vapor film between the heater surface an...
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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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