TAILIEUCHUNG - Computational Fluid Mechanics and Heat Transfer Third Edition_14

Tham khảo tài liệu 'computational fluid mechanics and heat transfer third edition_14', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 496 Heat transfer in boiling and other phase-change configurations Lienhard obtained pf . 017Pf pgựDf i00 A 9-40 where if we call pflpg r A Inr - lnr 2 lnr 3 This correlation represents all the existing data within 20 over the full range of the data. The influence of fluid flow on film boiling. Bromley et al. showed that the film boiling heat flux during forced flow normal to a cylinder should take the form X 1 2 kg Pgh fg ATu q constant1 ------1 D for Ú2Ol gD 4 with h fg from eqn. . Their data fixed the constant at . Witte obtained the same relationship for flow over a sphere and recommended a value of for the constant. Additional work in the literature deals with forced film boiling on plane surfaces and combined forced and subcooled film boiling in a variety of geometries . Although these studies are beyond our present scope it is worth noting that one may attain very high cooling rates using film boiling with both forced convection and subcooling. Forced convection boiling in tubes Flowing fluids undergo boiling or condensation in many of the cases in which we transfer heat to fluids moving through tubes. For example such phase change occurs in all vapor-compression power cycles and refrigerators. When we use the terms boiler condenser steam generator or evaporator we usually refer to equipment that involves heat transfer within tubes. The prediction of heat transfer coefficients in these systems is often essential to determining U and sizing the equipment. So let us consider the problem of predicting boiling heat transfer to liquids flowing through tubes. Wall and fluid temperature variation Flow patterns Heat transfer regions Singlephase vapor Convective heat transfer to vapor Dryout point Bubbly iling Singlephase liquid Convective heat transfer to liquid Forced convective heat transfer through liquid film Annular flo w with entrainment id deficient region Saturated nucleate

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