9.89. Boiling is of importance in nuclear reactor systems both as a means of achieving high heat-transfer rates from fuel to coolant and for generating steam in a heat exchanger. The mechanisms involved are complex and depend upon many factors, including surface conditions. For the prelim­inary treatment of boiling heat transfer from a solid fuel rod, it is convenient to consider a heated surface at temperature ts, immersed in a pool of liquid. Suppose the temperature difference between the heated surface and the liquid saturation temperature, i. e., ts — tSSLt, is steadily increased;[6] the corresponding variation in the heat flux, i. e., q/A, across the surface is then as shown in Fig. 9.13, in which both scales are logarithmic. Although the data in this figure are representative of natural convection boiling from a heated surface in a pool of water at atmospheric pressure and a liquid temperature of 100°C, some of the same general characteristics apply to forced convection boiling and to other pressure and temperature conditions.

9.90. The curve for pool boiling can be divided into a number of regions, in each of which the mechanism of heat transfer is somewhat different from that in the others. Until the heated surface exceeds the saturation temperature by a small amount, heat is transferred by single-phase con­vection; this occurs in region I. The system is heated by slightly superheated liquid rising to the liquid pool surface where evaporation occurs. In region II, vapor bubbles form in crevices on the heated surface; this is the nucleate boiling range in which formation of bubbles occurs upon nuclei, such as solid particles or gas adsorbed on the surface, or gas dissolved in the liquid. Nucleate boiling is a common phenomenon, since it is encountered in standard power-plant steam generators.

9.91. The steep slope of the heat flux curve in region II is a result of the mixing of the liquid caused by the motion of the vapor bubbles. A maximum flux is attained when the bubbles become so dense that they coalesce and form a vapor film over the heated surface. The heat must then pass through the vapor film by a combined mechanism of conduction and radiation, neither of which is particularly effective in this temperature range. Consequently, beyond the maximum, the heat flux decreases ap­preciably despite an increase in temperature. The maximum flux, which is a design limitation, is referred to as the DNB (departure from nucleate boiling) value (§9.98). In region III, the film is unstable, it spreads over a part of the heated surface and then break down. Under these conditions, some areas of the surface exhibit violent nucleate boiling, while film boiling, due to heat transfer, occurs in other areas.

9.92. For sufficiently high values of ts — £sat, as in region IV, the film becomes stable, and the entire heated surface is covered by a thin layer of vapor; boiling is then exclusively of the film type. If attempts are made to attain large heat fluxes with film boiling, as high as those possible with nucleate boiling, for example, the temperature of the heated surface may become so high as to result in damage to the material being heated. This is called a burnout and is, of course, to be avoided. It may also be noted from Fig. 9.13 that if a system undergoing nucleate boiling is operating at conditions near the maximum of the curve, a slight increase in the heat flux will cause a sudden change to film boiling, which could result in burnout.

9.93. Subcooled {or local) boiling occurs when the bulk temperature of the liquid is below saturation but that of the heated surface is above sat-

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uration. Vapor bubbles form at this surface but condense in the cold liquid, so that no net generation of vapor is realized. Very high heat fluxes can be obtained under these conditions; values as large as 4 x 107 W/m2 have been reported in forced-convection heating of water, but 6 x 106 W/m2 appears more realistic if the surface temperatures are to be kept low enough to avoid burnout. When the bulk temperature of the liquid reaches the saturation point, the vapor bubbles no longer collapse and then bulk boiling occurs.