Heat Transfer Handbook part 44. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 422 FORCED CONVECTION INTERNAL FLOWS Surface condition ks mm Riveted steel Concrete Wood stave Cast iron Galvanized iron Asphalted cast iron Commercial steel or Wrought iron Drawn tubing Figure also documents the effect of wall roughness. It is found experimentally that the performance of commercial surfaces that do not feel rough to the touch departs from the performance of well-polished surfaces. This effect is due to the very small thickness acquired by the laminar sublyyer in many applications . because UyVSL v is of order 102 Bejan 1995 where yVSL is the thickness of the viscous sublayer . In water flow through a pipe with U 10m s and v s yVSL is approximately mm. Consequently even slight imperfections of the surface may interfere with the natural formation of the laminar shear flow contact spots. If the surface irregularities are taller than yVSL they alone rule the friction process. Nikuradse 1933 measured the effect of surface roughness on the friction factor Uy coating the inside surface of pipes with sand of a measured grain size glued as tightly as possiUle to the wall. If ks is the grain size in Nikuradse s sand roughness the friction factor fully rough limit is the constant TURBULENT DUCT FLOW 423 f f In ks The fully rough limit is that regime where the roughness size exceeds the order of magnitude of what would have been the laminar sublayer hi time-avaraged turbulent flow over a smooth surfcee k k- To p 1 2 10 s V The roughness effect described by Nikuradse is illustrated by the upper curves in Fig. . Heat Transfer in Fully Developed Flow There are several empirical relationships for calculating the time-averaged coefficient for t t tfansVeu between tee diKh urnface and the fulld devoloped flow h q T0 -Tm . The analytical form of these relationships is based on exploiting the analogy between momentum and heat transfer by eddy rotation. One of the .
