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A HEAT TRANSFER TEXTBOOK - THIRD EDITION Episode 1 Part 10

Tham khảo tài liệu 'a heat transfer textbook - third edition episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 214 Transient and multidimensional heat conduction 5.4 Solution. After 1 hr or 3600 s at k _ _ . 3600 s o rO pc 20OC 0.05 m 2 _ 0.603 J m-s-K 3600 s _ 0 208 997.6 kg m3 4180 J kg-K 0.0025 m2 . Furthermore Bi 1 _ hro k 1 _ 6 0.05 0.603 1 _ 2.01. Therefore we read from Fig. 5.9 in the upper left-hand corner 0 _ 0.85 After 1 hr Tcenter _ 0.85 30 - 5 C 5 C _ 26.3 C To find the time required to bring the center to 10 C we first calculate 0 - 10-1 - 0 2 and Bi 1 is still 2.01. Then from Fig. 5.9 we read Fo _ 1.29 _ at ro2 so 1.29 997.6 4180 0.0025 t _----------- -------------_ 22 300 s _ 6 hr 12 min 0.603 Finally we lookup at Bi _ 1 2.01 and Fo _ 1.29 in Fig. 5.10 for spheres t Qdt _ 0.80 _ T4 V pc f nr3 Ti - Tx so p Qdt _ 997.6 4180 4n 0.05 25 0.80 _ 43 668 J apple 0 3 Therefore for the 12 apples total energy removal _ 12 43.67 _ 524 kJ 5.4 Temperature-response charts 215 The temperature-response charts inFig. 5.7 throughFig. 5.10are without doubt among the most useful available since they can be adapted to a host of physical situations. Nevertheless hundreds of such charts have been formed for other situations a number of which have been cataloged by Schneider 5.5 . Analytical solutions are available for hundreds more problems and any reader who is faced with a complex heat conduction calculation should consult the literature before trying to solve it. An excellent place to begin is Carslaw and Jaeger s comprehensive treatise on heat conduction 5.6 . Example 5.3 A 1 mm diameter Nichrome 20 Ni 80 Cr wire is simultaneously being used as an electric resistance heater and as a resistance thermometer in a liquid flow. The laboratory workers who operate it are attempting to measure the boiling heat transfer coefficient h by supplying an alternating current and measuring the difference between the average temperature of the heater Tav and the liquid temperature TM. They get h 30 000 W m2K at a wire temperature of100 C and are delighted with such a high value. Then a colleague