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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 2 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Problems 389 7.23 Check the value of h given in Example 7.3 by using Reynolds s analogy directly to calculate it. Which h do you deem to be in error and by what percent 7.24 A homemade heat exchanger consists of a copper plate 0.5 m square with 201.5 cm I.D. copper tubes soldered to it. The ten tubes on top are evenly spaced across the top and parallel with two sides. The ten on the bottom are also evenly spaced but they run at 90 to the top tubes. The exchanger is used to cool methanol flowing at 0.48 m s in the tubes from an initial temperature of 73 C using water flowing at 0.91 m s and entering at 7 C. What is the temperature of the methanol when it is mixed in a header on the outlet side Make a judgement of the heat exchanger. 7.25 Given that NuD 12.7 at 2 Gz 0.004 evaluate NuD at 2 Gz 0.02 numerically using Fig. 7.4. Compare the result with the value you read from the figure. 7.26 Report the maximum percent scatter of data in Fig. 7.13. What is happening in the fluid flow when the scatter is worst 7.27 Water at 27 C flows at 2.2 m s in a 0.04 m I.D. thin-walled pipe. Air at 227 C flows across it at 7.6 m s. Find the pipe wall temperature. 7.28 Freshly painted aluminum rods 0.02 m in diameter are withdrawn from a drying oven at 150 C and cooled in a 3 m s cross flow of air at 23 C. How long will it take to cool them to 50 C so that they can be handled 7.29 At what speed Ua must 20 C air flow across an insulated tube before the insulation on it will do any good The tube is at 60 C and is 6 mm in diameter. The insulation is 12 mm in diameter with k 0.08 W m-K. Notice that we do not ask for the u for which the insulation will do the most harm. 7.30 Water at 37 C flows at 3 m s across at 6 cm O.D. tube that is held at 97 C. In a second configuration 37 C water flows at an average velocity of 3 m s through a bundle of 6 cm O.D. tubes that are held at 97 C. The bundle is staggered with St Sl 2. Compare the heat transfer coefficients for the two situations. 390 .
