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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 364 Forced convection in a variety of configurations 7.3 The heat transfer coefficient on a rough wall can be several times that for a smooth wall at the same Reynolds number. The friction factor and thus the pressure drop and pumping power will also be higher. Nevertheless designers sometimes deliberately roughen tube walls so as to raise h and reduce the surface area needed for heat transfer. Several manufacturers offer tubing that has had some pattern of roughness impressed upon its interior surface. Periodic ribs are one common configuration. Specialized correlations have been developed for a number of such configurations 7.16 7.17 . Example 7.4 Repeat Example 7.3 now assuming the pipe to be cast iron with a wall roughness of E 260 ụm. Solution. The Reynolds number and physical properties are unchanged. From eqn. 7.50 f 1.8 logio 6.9 260 X 10-6 0.12 1.11i 573 700 3.7 -2 0.02424 The roughness Reynolds number is then Re 5-3 700 260x 10-6 T 0 68.4 0.12 8 This corresponds to fully rough flow. With eqn. 7.49 we have 0.02424 8 5.74 X 105 2.47 up 1 V0.02424 8 4.5 68.4 0 2 2.47 0 5 - 8.48 2 985 so h 2985 0.661 16.4 kW m2K . In this case wall roughness causes a factor of 1.8 increase in h and a factor of 2.0 increase in f and the pumping power. We have omitted the variable properties corrections here because they were developed for smooth-walled pipes. 7.3 Turbulent pipe flow 365 Figure 7.7 Velocity and temperature profiles during fully developed turbulent flow in a pipe. Heat transfer to fully developed liquid-metal flows in tubes A dimensional analysis of the forced convection flow of a liquid metal over a flat surface recall eqn. 6.60 et seq. showed that Nu fn Pe 7.51 because viscous influences were confined to a region very close to the wall. Thus the thermal b.l. which extends far beyond Ỗ is hardly influenced by the dynamic b.l. or by viscosity. During heat transfer to liquid metals in pipes the same thing occurs as is illustrated in Fig. 7.7. The region of .
