which is difficult to predict the type and the

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Unformatted text preview: n (the number of active nucleation sites on the surface, the rate of bubble formation at each site, etc.), which is difficult to predict. The type and the condition of the heated surface also affect the heat transfer. These complications made it difficult to develop theoretical relations for heat transfer in the nucleate boiling regime, and people had to rely on relations based on experimental data. The most widely used correlation for the rate of heat transfer in the nucleate boiling regime was proposed in 1952 by Rohsenow, and expressed as q·nucleate l hfg g( l v) 1/2 Cp(Ts Tsat) Csf hfg Prn l 3 where · q nucleate l hfg g l v Cpl Ts Tsat Csf Prl n nucleate boiling heat flux, W/m2 viscosity of the liquid, kg/m · s enthalpy of vaporization, J/kg gravitational acceleration, m/s2 density of the liquid, kg/m3 density of the vapor, kg/m3 surface tension of liquid–vapor interface, N/m specific heat of the liquid, J/kg · °C surface temperature of the heater, °C saturation temperature of the fluid, °C experimental constant that depends on surface–fluid combination Prandtl number of the liquid experimental constant that depends on the fluid (10-2) cen58933_ch10.qxd 9/4/2002 12:38 PM Page 523 523 CHAPTER 10 It can be shown easily that using property values in the specified units in the Rohsenow equation produces the desired unit W/m2 for the boiling heat flux, thus saving one from having to go through tedious unit manipulations (Fig. 10–10). The surface tension at the vapor–liquid interface is given in Table 10–1 for water, and Table 10–2 for some other fluids. Experimentally determined values of the constant Csf are given in Table 10–3 for various fluid–surface combinations. These values can be used for any geometry since it is found that the rate of heat transfer during nucleate boiling is essentially independent of the geometry and orientation of the heated surface. The fluid properties in Eq. 10–2 are to be evaluated at the saturation temperature Tsat. The condition of the heater surfa...
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