Thermodynamics HW Solutions 714

Thermodynamics HW Solutions 714 - + + = Nu [ ] 2 2 m 5 . 4...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 9 Natural Convection 9-24 Some cans move slowly in a hot water container made of sheet metal. The rate of heat loss from the four side surfaces of the container and the annual cost of those heat losses are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 3 Heat loss from the top surface is disregarded. Properties The properties of air at 1 atm and the film temperature of ( T s + T )/2 = (55+20)/2 = 37.5 ° C are (Table A-15) Water bath 55 ° C Aerosol can 1 - 2 5 K 003221 . 0 K ) 273 5 . 37 ( 1 1 7261 . 0 Pr /s m 10 678 . 1 C W/m. 02644 . 0 = + = = = × = ° = f T k β υ Analysis The characteristic length in this case is the height of the bath, Then, m. 5 . 0 = = L L c 8 2 2 5 3 -1 2 2 3 10 565 . 3 ) 7261 . 0 ( ) /s m 10 678 . 1 ( ) m 5 . 0 )( K 20 55 )( K 003221 . 0 )( m/s 81 . 9 ( Pr ) ( × = × = = L T T g Ra s 84 . 89 7261 . 0 492 . 0 1 ) 10 565 . 3 ( 387 . 0 825 . 0 Pr 492 . 0 1 Ra 387 . 0 825 . 0 2 27 / 8 16 / 9 6 / 1 8 2 27 / 8 16 / 9 6 / 1 = + × + =
Background image of page 1
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: + + = Nu [ ] 2 2 m 5 . 4 ) m 5 . 3 )( m 5 . ( ) m 1 )( m 5 . ( 2 C . W/m 75 . 4 ) 84 . 89 ( m 5 . C W/m. 02644 . = + = = = = s A Nu L k h and W 1 . 748 C ) 20 55 )( m 5 . 4 )( C . W/m 75 . 4 ( ) ( 2 2 = = = T T hA Q s s & The radiation heat loss is [ ] W 9 . 750 ) K 273 20 ( ) K 273 55 ( ) .K W/m 10 67 . 5 )( m 5 . 4 )( 7 . ( ) ( 4 4 4 2 8 2 4 4 = + + = = surr s s rad T T A Q & Then the total rate of heat loss becomes W 1499 = + = + = 9 . 750 1 . 748 rad convection natural total Q Q Q & & & The amount and cost of the heat loss during one year is kWh 131 , 13 h) 8760 )( kW 499 . 1 ( = = = t Q Q total total & $1116 = = ) kWh / 085 . )($ kWh 131 , 13 ( Cost 9-15...
View Full Document

This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

Ask a homework question - tutors are online