TAILIEUCHUNG - 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 Solution. After 1 hr or 3600 s at k _ _ . 3600 s o rO pc 20OC m 2 _ J m-s-K 3600 s _ 0 208 kg m3 4180 J kg-K m2 . Furthermore Bi 1 _ hro k 1 _ 6 1 _ . Therefore we read from Fig. in the upper left-hand corner 0 _ After 1 hr Tcenter _ 30 - 5 C 5 C _ 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 . Then from Fig. we read Fo _ _ at ro2 so 4180 t _----------- -------------_ 22 300 s _ 6 hr 12 min Finally we lookup at Bi _ 1 and Fo _ in Fig. for spheres t Qdt _ _ T4 V pc f nr3 Ti - Tx so p Qdt _ 4180 4n 25 _ 43 668 J apple 0 3 Therefore for the 12 apples total energy removal _ 12 _ 524 kJ Temperature-response charts 215 The temperature-response charts inFig. throughFig. 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 . 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 . Example 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

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