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Heat Chap07-035 - Chapter 7 External Forced Convection Flow...

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Chapter 7 External Forced Convection Flow Across Cylinders And Spheres 7-35C For the laminar flow, the heat transfer coefficient will be the highest at the stagnation point which corresponds to   0 . In turbulent flow, on the other hand, it will be highest when is between 90 and 120 . 13-36C Turbulence moves the fluid separation point further back on the rear of the body, reducing the size of the wake, and thus the magnitude of the pressure drag (which is the dominant mode of drag). As a result, the drag coefficient suddenly drops. In general, turbulence increases the drag coefficient for flat surfaces, but the drag coefficient usually remains constant at high Reynolds numbers when the flow is turbulent. 13-37C Friction drag is due to the shear stress at the surface whereas the pressure drag is due to the pressure differential between the front and back sides of the body when a wake is formed in the rear. 13-38C Flow separation in flow over a cylinder is delayed in turbulent flow because of the extra mixing due to random fluctuations and the transverse motion. 7-39 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-15) 7232 . 0 Pr /s m 10 784 . 1 C W/m. 02724 . 0 2 5 - k Analysis The Reynolds number is 4 2 5 10 228 . 6 /s m 10 784 . 1 m) (0.08 ] s/h) 0 m/km)/(360 1000 ( km/h) (50 [ Re D V The Nusselt number corresponding to this Reynolds number is 1 . 159 000 , 282 10 228 . 6 1 7232 . 0 / 4 . 0 1 ) 7232 . 0 ( ) 10 228 . 6 ( 62 . 0 3 . 0 000 , 282 Re 1 Pr / 4 . 0 1 Pr Re 62 . 0 3 . 0 5 / 4 8 / 5 4 4 / 1 3 / 2 3 / 1 5 . 0 4 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . 0 k hD Nu The heat transfer coefficient and the heat transfer rate become C . W/m 17 . 54 ) 1 . 159 ( m 08 . 0 C W/m. 02724 . 0 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 . 0 ( 2 2 2 W 1130 T T hA Q DL A s s conv s 7-20 Air V = 50 km/h T = 7 C Pipe D = 8 cm T s = 90 C
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Chapter 7 External Forced Convection 7-40 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-15) 7282 . 0 Pr kg/m.s 10 934 . 2 kg/m.s 10 872 . 1 /s m 10 608 . 1 C W/m. 02588 . 0
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