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Unformatted text preview: Chapter 8 Internal Forced Convection Chapter 8 INTERNAL FORCED CONVECTION General Flow Analysis 81C Liquids are usually transported in circular pipes because pipes with a circular crosssection can withstand large pressure differences between the inside and the outside without undergoing any distortion. 82C Reynolds number for flow in a circular tube of diameter D is expressed as D m V = Re where = = = = and 4 ) 4 / ( 2 2 D m D m A m c V Substituting, D m D D m D m 4 ) / ( 4 Re 2 = = = V 83C Engine oil requires a larger pump because of its much larger density. 84C The generally accepted value of the Reynolds number above which the flow in a smooth pipe is turbulent is 4000. 85C For flow through noncircular tubes, the Reynolds number as well as the Nusselt number and the friction factor are based on the hydraulic diameter D h defined as p A D c h 4 = where A c is the cross sectional area of the tube and p is its perimeter. The hydraulic diameter is defined such that it reduces to ordinary diameter D for circular tubes since D D D p A D c h = = = 4 / 4 4 2 . 86C The region from the tube inlet to the point at which the boundary layer merges at the centerline is called the hydrodynamic entry region , and the length of this region is called hydrodynamic entry length . The entry length is much longer in laminar flow than it is in turbulent flow. But at very low Reynolds numbers, L h is very small ( L h = 1.2 D at Re = 20). 87C The friction factor is highest at the tube inlet where the thickness of the boundary layer is zero, and decreases gradually to the fully developed value. The same is true for turbulent flow. 88C In turbulent flow, the tubes with rough surfaces have much higher friction factors than the tubes with smooth surfaces. In the case of laminar flow, the effect of surface roughness on the friction factor is negligible. 89C The friction factor f remains constant along the flow direction in the fully developed region in both laminar and turbulent flow. 810C The fluid viscosity is responsible for the development of the velocity boundary layer. For the idealized inviscid fluids (fluids with zero viscosity), there will be no velocity boundary layer. 811C The number of transfer units NTU is a measure of the heat transfer area and effectiveness of a heat transfer system. A small value of NTU (NTU &lt; 5) indicates more opportunities for heat transfer whereas a large NTU value (NTU &gt;5) indicates that heat transfer will not increase no matter how much we extend the length of the tube. 812C The logarithmic mean temperature difference T ln is an exact representation of the average temperature difference between the fluid and the surface for the entire tube. It truly reflects the exponential decay of the local temperature difference. The error in using the arithmetic mean 81 m, V m T e Chapter 8 Internal Forced Convection temperature increases to undesirable levels when...
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 Spring '08
 BENARD
 Heat Transfer

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