TAILIEUCHUNG - Heat Transfer Handbook part 72

Heat Transfer Handbook part 72. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 704 BOILING local flow boiling heat transfer coefficient which is most pronounced at high vapor qualities and at high local oil mass fractions. ENHANCED BOILING In the foregoing sections we have addressed boiling when it occurs on plain surfaces either outside or inside tubes. Boiling on specially formed microsurfaces to enhance nucleate boiling or convective boiling or both is referred to as enhanced boiling and these surfaces as enhanced boiling surfaces. Typically enhanced surfaces on the outside of tubes are for the purpose of enhancing nucleate boiling whereas those on the inside are for enhancing forced-convective boiling. The exception is the porous layered surfaces that enhance nucleate boiling whether applied inside or outside a tube. In this section first enhancement of nucleate boiling is discussed and then enhancement of forced-convective boiling. Both types of enhanced surfaces are used widely in industry particularly in the refrigeration and air-conditioning industries. Refer to Bergles 1996 Thome 1990 and Webb 1994 for comprehensive treatments of this subject. Enhancement of Nucleate Pool Boiling Boiling on plain smooth surfaces is a weak function of the roughness of the surface which increases nucleate pool boiling heat transfer coefficient with increasing roughness. This is only marginal on the order of up to 30 and may also be temporary if the surface becomes fouled. For substantial and sustainable enhancement numerous geometries have been proposed and patented over the years. The earliest commercially successful enhancement was the integral low finned tube . a continuous helical fin that is formed on the outside of an otherwise plain tube which is still used for appropriate applications. Applying a porous metallic coating on the surface of a tube is another important historical development giving up to 15 times the heat transfer coefficient of a plain tube. In recent years attention has focused on mechanically deformed low finned .

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