TAILIEUCHUNG - Heat Transfer Handbook part 107

Heat Transfer Handbook part 107. 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. | ROUGH SURFACES 1057 on the basis of equal pumping power it was shown that CHFs for the rough tubes were up to 80 higher than those for smooth tubes. Murphy and Truesdale 1972 have shown further that subcooled CHF tends to decrease by as much as 15 to 30 with increasing roughness height. In another study Gomelauri and Magrakvelidze 1978 with two-dimensional structured roughness CHF is found to be dependent on the degree of subcooling. Swenson et al. 1962 and Watson et al. 1974 employed integral multistart helical ribs and found that at subcritical pressures film boiling was inhibited and much higher heat 11 rixcs could be sustaind wtüi lower mass duxes in comparison with a smooth tube. Kitto and Wiener 1982 have reported as much as a threefold increase in CHF under nonuniform circumferential heating ribbed tubes. Three more recent studies Celata et al. 1994 Kabata et al. 1996 Lan et al. 1997 have eiperimented with wire-coil inserts and found considerable enhancement in CHF. Celata et al. 1994 have reported up to 50 higher subcooled boiling CHF in comparison with that in a smooth tube for the same conditions. The subcooled waterboiling data of Kabata et al. 1996 show as much as 90 CHF enhancement. Similarly Lan et al. 1997 found two- to threefold increases in CHF in a R-113 system they have also proposed a tentative correlation for predicting CHF under subcooled conditions. The use of helical ribbed tubes in high-pressure power boilers have long been considered in the literature Bergles 1998 and they have been shown to increase the heat transfer coefficient and CHF in once-through boiling of water Swenson et al. 1962 . Ribbed tubes also tend to suppress pseudo-film boiling Ackerman 1970 and increase the heat transfer coefficient in the post-CHF dispersed-flow film boiling regime Bergles 1998 . Commercial structured rough surfaces in the form of corrugated or fluted or convoluted tubing have come to be employed eitensively in refrigerant evaporators. Withers and .

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