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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 1 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14 Introduction 1.3 their kinetic energy to those in the cool wall. Within solids comparable processes occur as the molecules vibrate within their lattice structure and as the lattice vibrates as a whole. This sort of process also occurs to some extent in the electron gas that moves through the solid. The processes are more efficient in solids than they are in gases. Notice that --dX ĩ k L10 since in steady conduction q is constant Thus solids with generally higher thermal conductivities than gases yield smaller temperature gradients for a given heat flux. In a gas by the way k is proportional to molecular speed and molar specific heat and inversely proportional to the cross-sectional area of molecules. This book deals almost exclusively with S.I. units or Système International d Unites. Since much reference material will continue to be available in English units we should have at hand a conversion factor for thermal conductivity J h ft 1.8 F 0.0009478 Btu 3600 s 0.3048 m K Thus the conversion factor from W m-K to its English equivalent Btu h-ft F is 1 1.731 W m-K 111 1 Btu h-ft- F Consider for example copper the common substance with the highest conductivity at ordinary temperature W m-K kCu at room temp 383 W m-KƯ 1.731 W m o 221 Btu h-ft- F Btu h-It- F The range of thermal conductivities is enormous. As we see from Fig. 1.6 k varies by a factor of about 105 between gases and diamond at room temperature. This variation can be increased to about 107 if we include the effective conductivity of various cryogenic superinsulations. These involve powders fibers or multilayered materials that have been evacuated of all air. The reader should study and remember the order Synthetic insulation Materials occurring in nature Synthetic conductors CT 5 o Ọ c Compound evacuated Evacuated powders s insulation and fibers . Nonmetal lie gases 300 K High temperatures . Unevacuated insulating materials Stone Other nonmetaiiic solids 0.0003 0.001 0.01 0.1 1.0 10 100 1000 10 000 .
