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26 - Boiling Chapter 10 Sections 10.1 through 10.5 Pool...

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11/2/2010 1 Boiling Chapter 10 Sections 10.1 through 10.5 Pool Boiling Correlations Minimum Heat flux ( ) ( ) 4 / 1 2 " min 09 . 0 + = l v v l v fg g h q ρ ρ ρ ρ σ ρ Why is the transition boiling regime of little practical interest? a) You never want to achieve a minimum heat flux b) This is a theoretical rather than an actual regime c) You can only get there by controlling the surface heater temperature d) The correlation has over 100% error Correlations for the film pool boiling regime a) Tend to have liquid properties b) Tend to have vapor properties c) Have an almost even mixture of liquid and vapor properties d) It depends on the geometry
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11/2/2010 2 Film Boiling ( ) ( ) 4 / 1 3 ' = = sat s v v fg v l v conv D T T k D h g C k D h Nu υ ρ ρ C = 0.62 for cylinders; 0.67 for spheres Film Boiling The cumulative (and coupled effects) of convection and radiation across the vapor layer 3 / 1 3 / 4 3 / 4 h h h h rad conv + = Appropriate for T s 300 o C A silicon chip of thickness L = 2.5 mm and thermal conductivity k s = 135 W/m K is cooled by boiling a saturated fluorocarbon liquid (T sat = 57 o C) on its surface. The electronic circuits on the bottom of the chip produce a uniform heat flux of q” o = 5 X 10 4 W/m2 while the sides of the chip are perfectly insulated. Find the temperature at the bottom of the chip at the given q” o and at 90% of the critical heat flux . Plan: a) Governing equation (what type of boiling is it?) b) Properties (given) c) What do we have? What do we need?
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