Heat Chap07-035 - Chapter 7 External Forced Convection Flow Across Cylinders And Spheres 7-35C For the laminar flow the heat transfer coefficient

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
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 θ ° 0 . In turbulent flow, on the other hand, it will be highest when θ 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 - = × = ° = υ 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 × = × = υ = - D V The Nusselt number corresponding to this Reynolds number is ( 29 [ ] ( 29 [ ] 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 π 7-20 Air V = 50 km/h T = 7 ° C Pipe D = 8 cm T s = 90 ° C
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This homework help was uploaded on 03/19/2008 for the course ME 410 taught by Professor Benard during the Spring '08 term at Michigan State University.

Page1 / 16

Heat Chap07-035 - Chapter 7 External Forced Convection Flow Across Cylinders And Spheres 7-35C For the laminar flow the heat transfer coefficient

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online