Boiling

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Boiling

A process in which a liquid phase is converted into a vapor phase. The energy for phase change is generally supplied by the surface on which boiling occurs. Boiling differs from evaporation at predetermined vapor/gas-liquid interfaces because it also involves creation of these interfaces at discrete sites on the heated surface. Boiling is an extremely efficient process for heat removal and is utilized in various energy-conversion and heat-exchange systems and in the cooling of high-energy density components. See Heat transfer

Boiling is classified into pool and forced-flow. Pool boiling refers to boiling under natural convection conditions, whereas in forced-flow boiling the liquid flow over the heater surface is imposed by external means. Flow boiling is subdivided into external and internal. In external-flow boiling, liquid flow occurs over heated surfaces, whereas internal-flow boiling refers to flow inside tubes. Heat fluxes of 2 × 108 W/m2, or three times the heat flux at the surface of the Sun, have been obtained in flow boiling. See Convection (heat)

Pool boiling

The illustration, a qualitative pool boiling curve, shows the dependence of the wall heat flux q on the wall superheat ΔT (the difference between the wall temperature and the liquid's saturation temperature). The plotted curve is for a horizontal surface underlying a pool of liquid at its saturation temperature (the boiling point at a given pressure).

Typical boiling curve, showing qualitatively the dependence of the wall heat flux q on the wall superheat Δ T enlarge picture
Typical boiling curve, showing qualitatively the dependence of the wall heat flux q on the wall superheat ΔT

Several heat transfer regimes can be identified on the boiling curve: single-phase natural convection, partial nucleate boiling, fully developed nucleate boiling, transition boiling, and film boiling.

Forced-flow boiling

Forced flow, both external and internal, greatly changes the boiling curve in the illustration. The heat flux is increased by forced convection at temperatures below boiling inception, and after that the nucleate boiling region is extended upward until a flow-enhanced higher maximum flux (corresponding to point C) is achieved. Forced flow boiling in tubes is used in many applications, including steam generators, nuclear reactors, and cooling of electronic components.

Boiling

 

the transition of a liquid into vapor, accompanied by the formation of bubbles of vapor, or vapor cavities, in the fluid. The bubbles result from the evaporation of the liquid within them and float to the surface, where the saturated vapor they contain passes into the vapor phase above the fluid. Boiling begins when the liquid is heated to the point at which the pressure of the saturated vapor above its surface becomes equal to the external pressure. The temperature at which a liquid starts to boil under a constant pressure is called the boiling point (Tb). Strictly speaking, Tb corresponds to the temperature of the saturated vapor (saturation temperature) above the plane surface of the boiling liquid, because the liquid itself is always somewhat superheated relative to Tb. For a steady-state boiling condition there is no change in the temperature of the boiling fluid. If the pressure is increased, Tb, rises. The maximum boiling temperature is called the critical temperature of a substance. The boiling point under atmospheric pressure is usually cited as one of the basic physicochemical properties of a chemically pure substance.

To keep a liquid boiling, the heat that is expended in the formation of bubbles and in the work performed by the vapor against external pressure during the increase in volume of the vapor phase must be supplied. Thus, boiling is inseparably associated with heat transfer, which results in the transmission of heat from the heating surface to the liquid. The heat exchange during boiling is one of the forms of convective heat transfer.

A definite temperature distribution (Figure 1) is established in a boiling liquid: near the heating surfaces (the walls of the container, pipes, and so on) the liquid is appreciably superheated (T > Tb). The degree of superheating depends on a number of physicochemical properties of both the liquid and the boundaries of the solid surfaces. Carefully purified liquids that are free of dissolved gases (air) can be superheated by dozens of degrees without starting to boil when special measures are taken. However, when such a superheated liquid finally begins to boil, the process occurs very violently, resembling an explosion. The onset of boiling is accompanied by spattering of the liquid, hydraulic shocks, and sometimes even the destruction of the container. The heat of superheating is expended in the formation of vapor, so that the liquid is rapidly cooled to the temperature of the saturated vapor, with which it is in equilibrium. The substantial superheating of a pure liquid without boiling is explained by the difficulty of formation of initial small bubbles (nuclei); their formation is hindered by the considerable mutual attraction of the molecules of the liquid. The situation is different when the liquid contains dissolved gases and various extremely fine suspended particles. In this case even very little superheating (by tenths of a degree) produces steady, smooth boiling because the initial nuclei of the vapor phase are provided by gas bubbles and solid particles. The principal centers of vapor formation are at the points of the hot surface where there are very small pores holding adsorbed gas, as well as various irregularities, inclusions, and deposits that reduce the molecular adhesion between the liquid and the surface.

Figure 1. Temperature distribution in a layer of boiling liquid 6 cm thick at atmospheric pressure

After a bubble has been formed, it grows only if the vapor pressure in it is slightly greater than the sum of the external pressure, the pressure of the overlying layer of liquid, and the capillary pressure, which is due to the surface curvature of the bubble. To create the necessary vapor pressure within a bubble, the surrounding liquid, which is in thermal equilibrium with the vapor, must have a temperature higher than Tb. This type of boiling, called nucleate boiling, is observed in everyday practice (for example, when water is boiling in a teakettle). It takes place when the temperature T of the heating surface is slightly higher than the boiling temperature—that is, when the thermal head ΔT = TTb is very small. As the temperature of the heating surface rises, the number of vaporization centers increases sharply and an increasing number of freed bubbles comes to the surface of the liquid, thus causing intense agitation. This results in a substantial increase in the flow of heat from the heating surface to the boiling liquid (a higher heat transfer coefficient α = qT, where q is the heat flow density at the heating surface; see Figure 2). The amount of vapor being created increases correspondingly.

Figure 2. Change in the heat flow density q and the heat transfer coefficient α for water boiling at atmospheric pressure as a function of the thermal head ΔT T T,: (A) region of weak bubble formation, (B) nucleate boiling, (C) transitional boiling, (D) stable film boiling

When the maximum (critical) value of the heat flow (qmax) is reached, a second, transitional boiling mode begins. In this mode most of the heating surface is covered with dry spots because of the progressive coalescence of the vapor bubbles. The heat transfer and the rate of vapor formation are sharply reduced because the vapor has a lower heat conduction than the fluid, and so the values of q and α are markedly lower. The boiling approaches a critical point. When the entire heating surface is covered with a thin film of vapor, the third, or film, boiling mode develops. In this mode the heat is transferred by heat conduction and radiation from an incandescent surface to the liquid through a film of vapor. The nature of the change in q upon transition from one boiling mode to another is shown in Figure 2. When the liquid does not wet the wall (as, for example, mercury and alloyed steel), boiling occurs only in the film mode. All three boiling modes can be observed in the reverse order when a massive metal body is immersed in water for quenching: the water begins to boil and the body is cooled slowly at first (film boiling), then the cooling rate begins to increase rapidly (transitional boiling) and achieves the highest values in the final stage of cooling (nucleate boiling). The heat transfer in the nucleate boiling mode is one of the most efficient means of cooling; it is used in atomic reactors and for cooling jet engines. Boiling processes are also used extensively in chemical engineering, in the food industry, in the production and fractionation of liquefied gases, and for cooling units in electronic apparatus. The mode of nucleate boiling of water is used most widely in modern steam boilers at power plants to produce steam at high temperatures and pressures. Film boiling in steam boilers cannot be tolerated, since it may result in over-heating of the tube walls and in boiler explosions.

Boiling is possible not only when a liquid is heated under conditions of constant pressure. A decrease in external pressure at constant temperature can also cause a liquid to become super-heated and boil (because of a decrease in the saturation temperature). This explains, among other things, the phenomenon of cavitation—the formation of vapor cavities at low-pressure spots in a liquid (for example, in the turbulent area behind a steamship propeller). Boiling under reduced pressure is used in refrigeration engineering and in physics experiments.

REFERENCE

Kikoin, I. K., and A. K. Kikoin. Molekuliarnaia fizika. Moscow, 1963.
Radchenko, I. V. Molekuliarnaia fizika. Moscow, 1965.
Mikheev, M. A. Osnovy teploperedachi, 3rd ed. Moscow-Leningrad, 1956. Chapter 5.

D. A. LABUNTSOV

boiling

[′bȯil·iŋ]
(astronomy)
The telescopic appearance of the limbs of the sun and planets when the earth's atmosphere is turbulent, characterized by a constant rippling motion and lack of a clearly defined edge.
(physical chemistry)
The transition of a substance from the liquid to the gaseous phase, taking place at a single temperature in pure substances and over a range of temperatures in mixtures.

Boiling

A process in which a liquid phase is converted into a vapor phase. The energy for phase change is generally supplied by the surface on which boiling occurs. Boiling differs from evaporation at predetermined vapor/gas-liquid interfaces because it also involves creation of these interfaces at discrete sites on the heated surface. Boiling is an extremely efficient process for heat removal and is utilized in various energy-conversion and heat-exchange systems and in the cooling of high-energy density components. See Boiler, Heat exchanger, Heat transfer

Boiling is classified into pool and forced-flow. Pool boiling refers to boiling under natural convection conditions, whereas in forced-flow boiling the liquid flow over the heater surface is imposed by external means. Flow boiling is subdivided into external and internal. In external-flow boiling, liquid flow occurs over heated surfaces, whereas internal-flow boiling refers to flow inside tubes. Heat fluxes of 2 × 108 W/m2, or three times the heat flux at the surface of the Sun, have been obtained in flow boiling. See Convection (heat)

Pool boiling

The illustration, a qualitative pool boiling curve, shows the dependence of the wall heat flux q on the wall superheat ΔT (the difference between the wall temperature and the liquid's saturation temperature). The plotted curve is for a horizontal surface underlying a pool of liquid at its saturation temperature (the boiling point at a given pressure).

Several heat transfer regimes can be identified on the boiling curve: single-phase natural convection, partial nucleate boiling, fully developed nucleate boiling, transition boiling, and film boiling.

Forced-flow boiling

Forced flow, both external and internal, greatly changes the boiling curve in the illustration. The heat flux is increased by forced convection at temperatures below boiling inception, and after that the nucleate boiling region is extended upward until a flow-enhanced higher maximum flux (corresponding to point C) is achieved. Forced flow boiling in tubes is used in many applications, including steam generators, nuclear reactors, and cooling of electronic components. See Steam-generating unit

blowing

1. See popping.
2. The upward movement of soil material at the base of an excavation or cofferdam as a result of groundwater pressure.