Thermodynamics HW Solutions 380

Thermodynamics HW Solutions 380 - kJ 1278 . kJ/kg) kg)(2490...

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Chapter 4 Transient Heat Conduction 4-114 The time it will take for the diameter of a raindrop to reduce to a certain value as it falls through ambient air is to be determined. Assumptions 1 The water temperature remains constant. 2 The thermal properties of the water are constant. Properties The density and heat of vaporization of the water are ρ = 1000 kg/m 3 and h fg = 2490 kJ/kg (Table A-9). Analysis The initial and final masses of the raindrop are kg 0000141 . 0 m) 0015 . 0 ( 3 4 ) kg/m 1000 ( 3 4 kg 0000654 . 0 m) 0025 . 0 ( 3 4 ) kg/m 1000 ( 3 4 3 3 3 3 3 3 = = = = = = = = π πρ ρ f f f i i i r V m r V m whose difference is kg 0000513 . 0 0000141 . 0 0000654 . 0 = = = f i m m m The amount of heat transfer required to cause this much evaporation is
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Unformatted text preview: kJ 1278 . kJ/kg) kg)(2490 0000513 . ( = = Q The average heat transfer surface area and the rate of heat transfer are J/s 0.2777 = C ) 5 18 ( ) m 10 341 . 5 ( C) . W/m 400 ( ) ( m 10 341 . 5 2 m) (0.0015 + m) 0025 . [( 4 2 ) ( 4 2 5 2 2 5 2 2 2 2 = = = = + = T T hA Q r r A i s f i s & Air T = 18 C Raindrop 5 C Then the time required for the raindrop to experience this reduction in size becomes min 7.7 = = = = = s 460 J/s 0.2777 J 8 . 127 Q Q t t Q Q & & 4-91...
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This note was uploaded on 01/19/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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