Thermodynamics HW Solutions 572

# Thermodynamics HW - 7 1 10 778 1 7336 10 045 2 06 10 045 2 4 2 Pr Re 06 Re 4 2 4 1 5 5 4 3 2 4 5 5 4 1 4 3 2 5 = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ×

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Chapter 7 External Forced Convection 7-44 The average surface temperature of the head of a person when it is not covered and is subjected to winds is to be determined. Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas with constant properties. 4 One-quarter of the heat the person generates is lost from the head. 5 The head can be approximated as a 30-cm-diameter sphere. 6 The local atmospheric pressure is 1 atm. Properties The properties of air at 1 atm pressure and the free stream temperature of 10 ° C are (Table A-15) 7336 . 0 Pr kg/m.s 10 802 . 1 kg/m.s 10 778 . 1 /s m 10 426 . 1 C W/m. 02439 . 0 5 C 15 @ , 5 2 5 - = × = × = × = ° = ° s k μ υ Air V = 35 km/h T = 10 ° C Head Q = 21 W D = 0.3 m Analysis The Reynolds number is [ ] 5 2 5 10 045 . 2 /s m 10 426 . 1 m) (0.3 m/s 1000/3600) (35 Re × = × × = υ = D V The proper relation for Nusselt number corresponding to this Reynolds number is
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Unformatted text preview: [ ] [ ] 7 . 344 10 802 . 1 10 778 . 1 ) 7336 . ( ) 10 045 . 2 ( 06 . ) 10 045 . 2 ( 4 . 2 Pr Re 06 . Re 4 . 2 4 / 1 5 5 4 . 3 / 2 4 5 . 5 4 / 1 4 . 3 / 2 5 . = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ × × × + × + = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ μ μ + + = = − − ∞ s k hD Nu The heat transfer coefficient is C . W/m 28.02 ) 7 . 344 ( m 3 . C W/m. 02439 . 2 ° = ° = = Nu D k h Then the surface temperature of the head is determined to be C 12.7 ° = ° ° = + = ⎯→ ⎯ − = = = ∞ ∞ ) m C)(0.2827 . W/m 02 . 28 ( W (84/4) + C 10 ) ( m 0.2827 = m) 3 . ( 2 2 2 2 2 s s s s s hA Q T T T T hA Q D A & & ππ 7-27...
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## This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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