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Unformatted text preview: 183 0.000110T Propane, 90 to 10°C: 0.0092 0.000087T *Multiply by 0.06852 to convert to lbf/ft or by 2.2046 to convert to lbm/s2. Csf n 0.0130 0.0068 0.0130 0.0060 0.0058 0.0130 0.0060 0.0060 0.0130 0.0154 0.0150 0.1010 0.0027 0.0130 0.0025 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.7 1.7 1.7 1.7 1.7 1.7 Minimum Heat Flux Minimum heat flux, which occurs at the Leidenfrost point, is of practical interest since it represents the lower limit for the heat flux in the film boiling regime. Using the stability theory, Zuber derived the following expression for the minimum heat flux for a large horizontal plate, q·min 0.09 hfg g( (l ) l 1/4 (10-4) 2 ) where the constant 0.09 was determined by Berenson in 1961. He replaced the theoretically determined value of 24 by 0.09 to match the experimental data better. Still, the relation above can be in error by 50 percent or more. TABLE 10–4 Values of the coefficient Ccr for use in Eq. 10–3 for maximum heat flux (dimensionless parameter L* L[g( l )/ ]1/2) Heater Geometry Large horizontal flat heater Small horizontal flat heater1 Large horizontal cylinder Small horizontal cylinder Large sphere Small sphere 1 K1 /[g( l v)Aheater] Ccr Charac. Dimension of Heater, L Range of L* 0.149 Width or diameter L* 27 18.9K1 Width or diameter 9 L* 20 0.12 Radius L* 1.2 0.12L* 0.25 Radius 0.15 L* 1.2 0.11 Radius L* 4.26 0.227L* 0.5 Radius 0.15 L* 4.26 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 525 525 CHAPTER 10 · q Film Boiling Using an analysis similar to Nusselt’s theory on filmwise condensation presented in the next section, Bromley developed a theory for the prediction of heat flux for stable film boiling on the outside of a horizontal cylinder. The heat flux for film boiling on a horizontal cylinder or sphere of diameter D is given by · q film Cfilm gk3 ( )[hfg 0.4Cp (Ts D(Ts Tsat) l Tsat) (Ts Tsat) (10-5) 0.62 for horizontal cylinders 0.67 for spheres (Ts4 4 Tsat) (10-6) where is the emissivity of the heating surface and 5.67 10 8 W/m2 · K...
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