TAILIEUCHUNG - Heat Transfer Handbook part 66

Heat Transfer Handbook part 66. 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. | 644 BOILING TABLE Advancing Contact Angles Liquid Surface ß deg Water Copper 86 Brass 84 Benzene Copper 25 Brass 23 Ethanol Copper 14-19 Brass 4 8 Methanol Copper 25 Brass 22 n-Propanol Copper 3 Brass 8 Source Shakir and Thome 1986 . Size Range of Active Nucleation Sites Above a single uniform temperature was assumed for the wall and liquid. A more practical case is when there is a temperature gradient in the form of a thermal boundary layer in the liquid adjacent to the wall such as illustrated in Fig. for a conical nucleation site where a vapor nucleus of radius rnuc sits at the cavity mouth. The bulk liquid temperature is T the wall temperature is Tw where Tw T and a linear temperature gradient is assumed in the thermal boundary layer of thickness 8. If anc is the natural convection heat transfer coefficient and cL is the thermal conductivity of the liquid the boundary layer thickness is approximately 8 anc Hsu 1962 postulated that a nucleus sitting in such a temperature gradient activates if the superheat at the top of the vapor nucleus is greater than that required for its equilibrium . eq. including the distortion of the temperature profile by the bubble nucleus itself. Nucleation occurs if the local liquid temperature profile intersects the equilibrium nucleation curve. The first site to activate is at the tangency between the nucleation superheat curve and the liquid temperature profile line. Hsu assumed that distortion put the location of this temperature at a distance 2rnuc from the surface while Han and Griffith 1965 put the distance at based on potential flow theory. If the liquid pool is at the saturation temperature . if Tm Tsat and is assumed for displacement of the isotherms the aavity size satisfy gg the condition of tangency is rnuc 8 2 which is approximately 50 pm for watea at hs normal boiling point. Much larger superheats or heat fluxes are typically necessary however to initiate boiling on a heated

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