FTFS Chap19 P032 - Chapter 19 Forced Convection Flow Across...

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Unformatted text preview: Chapter 19 Forced Convection Flow Across Cylinders And Spheres 19-32C The local heat transfer coefficient is highest at the stagnation point ( θ = 0 ° ), and decreases with increasing angle θ measured from the horizontal, reaching a minimum at the top point of the cylinder ( θ = 90 ° ). 19-33C At Reynolds numbers greater than about 10 5 , the local heat transfer coefficient during flow across a cylinder reaches a maximum at an angle of about θ = 110 ° measured from the stagnation point. The physical phenomenon that is responsible for this increase is flow separation (the break-up of the boundary layer) at this angle in turbulent flow, and the associated intense mixing. 19-34C For the laminar flow, the heat transfer coefficient will be the highest at the stagnation point which corresponds to θ ≈ ° 0 . In turbulent flow, on the other hand, it will be highest when θ is between 90 ° ° and 120 . 19-35 A steam pipe is exposed to windy air. The rate of heat loss from the steam is to be determined. √ Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas with constant properties. Properties The properties of air at 1 atm and the film temperature of (T s + T ∞ )/2 = (90+7)/2 = 48.5 ° C are (Table A-22) 7232 . Pr /s m 10 784 . 1 C W/m. 02724 . 2 5- = × = ° = υ k Analysis The Reynolds number is 4 2 5 10 228 . 6 /s m 10 784 . 1 m) (0.08 ] s/h) m/km)/(360 1000 ( km/h) (50 [ Re × = × = υ =- ∞ D V The Nusselt number corresponding to this Reynolds number is ( 29 [ ] ( 29 [ ] 1 . 159 000 , 282 10 228 . 6 1 7232 . / 4 . 1 ) 7232 . ( ) 10 228 . 6 ( 62 . 3 . 000 , 282 Re 1 Pr / 4 . 1 Pr Re 62 . 3 . 5 / 4 8 / 5 4 4 / 1 3 / 2 3 / 1 5 . 4 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . = × + + × + = + + + = = k hD Nu The heat transfer coefficient and the heat transfer rate become C . W/m 17 . 54 ) 1 . 159 ( m 08 . C W/m. 02724 . 2 ° = ° = = Nu D k h length) m (per = C 7)- )(90 m C)(0.2513 . W/m 17 . 54 ( ) ( m 0.2513 = m) m)(1 08 . ( 2 2 2 W 1130 ° ° =- = = = ∞ T T hA Q DL A s s conv s π π 19-26 Air V ∞ = 50 km/h T ∞ = 7 ° C Pipe D = 8 cm T s = 90 ° C Chapter 19 Forced Convection 19-36 A hot stainless steel ball is cooled by forced air. The average convection heat transfer coefficient and the cooling time are to be determined. Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas with constant properties. 4 The outer surface temperature of the ball is uniform at all times. Properties The average surface temperature is (350+250)/2 = 300 ° C, and the properties of air at 1 atm pressure and the free stream temperature of 30 ° C are (Table A-22) 7282 . Pr kg/m.s 10 934 . 2 kg/m.s 10 872 . 1 /s m 10 608 . 1 C W/m. 02588 ....
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FTFS Chap19 P032 - Chapter 19 Forced Convection Flow Across...

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