John H. Lienhard IV, John H. Lienhard V. A Heat Transfer Textbook (776116), страница 66
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Wärme- und Stoffübertragung, 7:1–13, 1974.[8.41] P. J. Marto. Condensation. In W. M. Rohsenow, J. P. Hartnett, andY. I. Cho, editors, Handbook of Heat Transfer, chapter 14. McGrawHill, New York, 3rd edition, 1998.[8.42] J. Rose, H. Uehara, S. Koyama, and T. Fujii. Film condensation. InS.
G. Kandlikar, M. Shoji, and V. K. Dhir, editors, Handbook of PhaseChange: Boiling and Condensation, chapter 19. Taylor & Francis,Philadelphia, 1999.[8.43] E. M. Sparrow and S. H. Lin. Condensation in the presence of anon-condensible gas. J. Heat Transfer, Trans. ASME, Ser. C, 86:430, 1963.9.Heat transfer in boiling andother phase-changeconfigurationsFor a charm of powerful trouble,like a Hell-broth boil and bubble.. . .. . .Cool it with a baboon’s blood,then the charm is firm and good.Macbeth, Wm. Shakespeare“A watched pot never boils”—the water in a teakettle takes a long timeto get hot enough to boil because natural convection initially warms itrather slowly.
Once boiling begins, the water is heated the rest of the wayto the saturation point very quickly. Boiling is of interest to us becauseit is remarkably effective in carrying heat from a heater into a liquid. Theheater in question might be a red-hot horseshoe quenched in a bucket orthe core of a nuclear reactor with coolant flowing through it. Our aim is tolearn enough about the boiling process to design systems that use boilingfor cooling.
We begin by considering pool boiling—the boiling that occurswhen a stationary heater transfers heat to an otherwise stationary liquid.9.1Nukiyama’s experiment and the pool boiling curveHysteresis in the q vs. ∆T relation for pool boilingIn 1934, Nukiyama [9.1] did the experiment described in Fig. 9.1. Heboiled saturated water on a horizontal wire that functioned both as anelectric resistance heater and as a resistance thermometer. By calibratingthe resistance of a Nichrome wire as a function of temperature before the457458Heat transfer in boiling and other phase-change configurations§9.1Figure 9.1 Nukiyama’s boiling hysteresis loop.experiment, he was able to obtain both the heat flux and the temperatureusing the observed current and voltage.
He found that, as he increasedthe power input to the wire, the heat flux rose sharply but the temperature of the wire increased relatively little. Suddenly, at a particular highvalue of the heat flux, the wire abruptly melted. Nukiyama then obtaineda platinum wire and tried again. This time the wire reached the samelimiting heat flux, but then it turned almost white-hot without melting.§9.1Nukiyama’s experiment and the pool boiling curveAs he reduced the power input to the white-hot wire, the temperaturedropped in a continuous way, as shown in Fig.
9.1, until the heat flux wasfar below the value where the first temperature jump occurred. Thenthe temperature dropped abruptly to the original q vs. ∆T = (Twire −Tsat ) curve, as shown. Nukiyama suspected that the hysteresis would notoccur if ∆T could be specified as the independent controlled variable. Heconjectured that such an experiment would result in the connecting lineshown between the points where the temperatures jumped.In 1937, Drew and Mueller [9.2] succeeded in making ∆T the independent variable by boiling organic liquids outside a tube. Steam wasallowed to condense inside the tube at an elevated pressure. The steamsaturation temperature—and hence the tube-wall temperature—was varied by controlling the steam pressure.
This permitted them to obtain afew scattered data that seemed to bear out Nukiyama’s conjecture. Measurements of this kind are inherently hard to make accurately. For thenext forty years, the relatively few nucleate boiling data that people obtained were usually—and sometimes imaginatively—interpreted as verifying Nukiyama’s suggestion that this part of the boiling curve is continuous.Figure 9.2 is a completed boiling curve for saturated water at atmospheric pressure on a particular flat horizontal heater.
It displays thebehavior shown in Fig. 9.1, but it has been rotated to place the independent variable, ∆T , on the abscissa. (We represent Nukiyama’s connectingregion as two unconnected extensions of the neighboring regions for reasons that we explain subsequently.)Modes of pool boilingThe boiling curve in Fig. 9.2 has been divided into five regimes of behavior. These regimes, and the transitions that divide them, are discussednext.Natural convection. Water that is not in contact with its own vapor doesnot boil at the so-called normal boiling point,1 Tsat . Instead, it continuesto rise in temperature until bubbles finally to begin to form.
On conventional machined metal surfaces, this occurs when the surface is a fewdegrees above Tsat . Below the bubble inception point, heat is removedby natural convection, and it can be predicted by the methods laid out inChapter 8.1This notion might be new to some readers. It is explained in Section 9.2.459460Heat transfer in boiling and other phase-change configurations§9.1Figure 9.2 Typical boiling curve andregimes of boiling for an unspecifiedheater surface.Nucleate boiling. The nucleate boiling regime embraces the two distinctregimes that lie between bubble inception and Nukiyama’s first transitionpoint:1. The region of isolated bubbles.
In this range, bubbles rise from isolated nucleation sites, more or less as they are sketched in Fig. 9.1.As q and ∆T increase, more and more sites are activated. Figure 9.3a is a photograph of this regime as it appears on a horizontalplate.2. The region of slugs and columns. When the active sites becomevery numerous, the bubbles start to merge into one another, and anentirely different kind of vapor escape path comes into play.
Vaporformed at the surface merges immediately into jets that feed intolarge overhead bubbles or “slugs” of vapor. This process is shownas it occurs on a horizontal cylinder in Fig. 9.3b.461d. Film boiling of acetone on a 22 gage wire atearth-normal gravity. The true width of thisimage is 3.48 cm.b. Two views of transitional boiling in acetone on a 0.32 cmdiam.
tube.Figure 9.3 Typical photographs of boiling in the four regimes identified in Fig. 9.2.c. Two views of the regime of slugs and columns.3.75 cm length of 0.164 cm diam. wire in benzeneat earth-normal gravity. q=0.35×106 W/m23.45 cm length of 0.0322 cm diam. wire in methanolat 10 earth-normal gravities. q=1.04×106 W/m2a. Isolated bubble regime—water.462Heat transfer in boiling and other phase-change configurations§9.1Peak heat flux. Clearly, it is very desirable to be able to operate heatexchange equipment at the upper end of the region of slugs and columns.Here the temperature difference is low while the heat flux is very high.Heat transfer coefficients in this range are enormous. However, it is verydangerous to run equipment near qmax in systems for which q is theindependent variable (as in nuclear reactors).
If q is raised beyond theupper limit of the nucleate boiling regime, such a system will suffer asudden and damaging increase of temperature. This transition2 is knownby a variety of names: the burnout point (although a complete burningup or melting away does not always accompany it); the peak heat flux (amodest descriptive term); the boiling crisis (a Russian term); the DNB, ordeparture from nucleate boiling, and the CHF, or critical heat flux (termsmore often used in flow boiling); and the first boiling transition (whichterm ignores previous transitions). We designate the peak heat flux asqmax .Transitional boiling regime.
It is a curious fact that the heat flux actually diminishes with ∆T after qmax is reached. In this regime the effectiveness of the vapor escape process becomes worse and worse. Furthermore, the hot surface becomes completely blanketed in vapor and qreaches a minimum heat flux which we call qmin . Figure 9.3c shows twotypical instances of transitional boiling just beyond the peak heat flux.Film boiling.
Once a stable vapor blanket is established, q again increases with increasing ∆T . The mechanics of the heat removal processduring film boiling, and the regular removal of bubbles, has a great dealin common with film condensation, but the heat transfer coefficients aremuch lower because heat must be conducted through a vapor film insteadof through a liquid film. We see an instance of film boiling in Fig. 9.3d.Experiment 9.1Set an open pan of cold tap water on your stove to boil. Observe thefollowing stages as you watch:• At first nothing appears to happen; then you notice that numeroussmall, stationary bubbles have formed over the bottom of the pan.2We defer a proper physical explanation of the transition to Section 9.3.Nukiyama’s experiment and the pool boiling curve§9.1These bubbles have nothing to do with boiling—they contain airthat was driven out of solution as the temperature rose.• Suddenly the pan will begin to “sing.” There will be a somewhathigh-pitched buzzing-humming sound as the first vapor bubblesare triggered.
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