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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Problems 39 1.11 A hot water heater contains 100 kg of water at 75 C in a 20 C room. Its surface area is 1.3 m2. Select an insulating material and specify its thickness to keep the water from cooling more than 3 C h. Notice that this problem will be greatly simplified if the temperature drop in the steel casing and the temperature drop in the convective boundary layers are negligible. Can you make such assumptions Explain. TM 1OO C Vacuum T 20 C Figure 1.17 Configuration for Problem 1.12 1.12 What is the temperature at the left-hand wall shown in Fig. 1.17. Both walls are thin very large in extent highly conducting and thermally black. Tright 42.5 C. 1.13 Develop S.I. to English conversion factors for The thermal diffusivity a The heat flux q The density p The Stefan-Boltzmann constant Ơ The view factor F1-2 The molar entropy The specific heat per unit mass c In each case begin with basic dimension J m kg s C and check your answers against Appendix B if possible. 1.14 Three infinite parallel black opaque plates transfer heat by radiation as shown in Fig. 1.18. Find T2. 1.15 Four infinite parallel black opaque plates transfer heat by radiation as shown in Fig. 1.19. Find T2 and T3. T2 75.53 C. 1.16 Two large black horizontal plates are spaced a distance L from one another. The top one is warm at a controllable tem- 40 Chapter 1 Introduction Figure 1.18 Configuration for Problem 1.14 Figure 1.19 Configuration for Problem 1.15 perature Th and the bottom one is cool at a specified temperature Tc. A gas separates them. The gas is stationary because it is warm on the top and cold on the bottom. Write the equation qrad qcond fn N 0 Th Tc where N is a dimensionless group containing a k L and Tc. Plot N as a function of 0 for qrad qcond 1 0.8 and 1.2 and for other values if you wish . Now suppose that you have a system in which L 10 cm Tc 100 K and the gas is hydrogen with an average k of 0.1 W m - K. Further suppose that you wish to operate in such a way that the .
