{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Thermodynamics HW Solutions 650

Thermodynamics HW Solutions 650 - m 20 m 02 10 10 = = ≈...

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

Chapter 8 Internal Forced Convection 8-40 Water is to be heated in a tube equipped with an electric resistance heater on its surface. The power rating of the heater and the inner surface temperature are to be determined. Assumptions 1 Steady flow conditions exist. 2 The surface heat flux is uniform. 3 The inner surfaces of the tube are smooth. Properties The properties of water at the average temperature of (80+10) / 2 = 45 ° C are (Table A-9) Water 10 ° C 3 m/s 80 ° C (Resistance heater) L D = 2 cm 91 . 3 Pr C J/kg. 4180 /s m 10 602 . 0 / C W/m. 637 . 0 kg/m 1 . 990 2 6 - 3 = ° = × = ρ μ = υ ° = = ρ p C k Analysis The power rating of the resistance heater is kg/s 132 . 0 kg/min 921 . 7 ) /min m 008 . 0 )( kg/m 1 . 990 ( 3 3 = = = ρ = V m W 38,627 = ° ° = = C ) 10 80 )( C J/kg. 4180 )( kg/s 132 . 0 ( ) ( i e p T T C m Q The velocity of water and the Reynolds number are V m c V A == × = (8 / ) (. / . 10 60 002 4 0 4244 3 m/ s m) m/s 3 2 π 101 , 14 /s m 10 602 . 0 m) m/s)(0.02 (0.4244 Re 2 6 = × = υ = h m D V which is greater than 10,000. Therefore, the flow is turbulent and the entry lengths in this case are roughly
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: m 20 . m) 02 . ( 10 10 = = ≈ ≈ D L L t h which is much shorter than the total length of the duct. Therefore, we can assume fully developed turbulent flow in the entire duct, and determine the Nusselt number from 79 . 82 ) 91 . 3 ( ) 101 , 14 ( 023 . Pr Re 023 . 4 . 8 . 4 . 8 . = = = = k hD Nu h Heat transfer coefficient is C . W/m 2637 ) 79 . 82 ( m 02 . C W/m. 637 . 2 ° = ° = = Nu D k h h Then the inner surface temperature of the pipe at the exit becomes C 113.3 ° = ° − ° = − = e s s e e s s T T T T hA Q , 2 , C ) 80 )]( m 7 )( m 02 . ( )[ C . W/m 2637 ( W 627 , 38 ) ( & 8-12...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online