3 cm2 8590 kgm3004663 m203 m 244 cm 1252 kg therefore

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Unformatted text preview: l conductivity, L is the length, and T is the temperature difference across the copper bar. Solving for the cross-sectional area A and substituting the specified values gives L· Q kT A 0.3 m (180 W) (386 W/m · °C)(3°C) 0.04663 m2 466.3 cm2 Then the diameter and the mass of the copper rod become A 1 D2 4 m V → AL D 4 A/ 4(466.3 cm2)/ (8590 kg/m3)(0.04663 m2)(0.3 m) 24.4 cm 125.2 kg Therefore, the diameter of the copper rod needs to be almost 25 times that of the heat pipe to transfer heat at the same rate. Also, the rod would have a mass of 125.2 kg, which is impossible for an average person to lift. SUMMARY Boiling occurs when a liquid is in contact with a surface maintained at a temperature Ts sufficiently above the saturation temperature Tsat of the liquid. Boiling is classified as pool boiling or flow boiling depending on the presence of bulk fluid motion. Boiling is called pool boiling in the absence of bulk fluid flow and flow boiling (or forced convection boiling) in its presence. Pool and flow boiling are further classified as subcooled boiling and saturated boiling depending on the bulk liquid temperature. Boiling is said to be subcooled (or local) when the temperature of the main body of the liquid is below the saturation temperature Tsat and saturated (or bulk) when the temperature of the liquid is equal to Tsat. Boiling exhibits different regimes depending on the value of the excess temperature Texcess. Four different boiling regimes are observed: natural convection boiling, nucleate boiling, transition boiling, and film boiling. These regimes are illustrated on the boiling curve. The rate of evaporation and the rate of heat transfer in nucleate boiling increase with increasing Texcess and reach a maximum at some point. The heat flux at this point is called the critical (or maximum) heat flux, q·max. The rate of heat transfer in nucleate pool boiling is determined from · q nucleate l hfg g( l ) 1/2 Cpl (Ts Tsat) Csf hfg Prln 3 The maximum (or critical) heat flux in nucleate pool boiling is determined from q·max Ccr hfg[ g 2 ( l )]1/4 where the value of the constant Ccr is about 0.15. The minimum heat flux is given by cen58933_ch10.qxd 9/4/2002 12:38 PM Page 552 552 HEAT TRANSFER · q min...
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