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A HEAT TRANSFER TEXTBOOK - THIRD EDITION Episode 1 Part 4

Tham khảo tài liệu 'a heat transfer textbook - third edition episode 1 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 64 Heat conduction thermal resistance and the overall heat transfer coefficient 2.3 Figure 2.9 The one-dimensional flow of current. proportional to the gradient of its mass concentration mi. Thus Ji -pDi2 mi 2.19 where the constant D12 is the binary diffusion coefficient. Example 2.3 Air fills a thin tube 1 m in length. There is a small water leak at one end where the water vapor concentration builds to a mass fraction of 0.01. A desiccator maintains the concentration at zero on the other side. What is the steady flux of water from one side to the other if D12 is 2.84 X 10-5 m2 s and p 1.18 kg m3 Solution. I twater vapor I 1.18 2.84 X 10 01 TO ttrn 3.35 X 10 7 kg m2 -s Contact resistance One place in which the usefulness of the electrical resistance analogy becomes immediately apparent is at the interface of two conducting media. No two solid surfaces will ever form perfect thermal contact when they are pressed together. Since some roughness is always present a typical plane of contact will always include tiny air gaps as shown in Fig. 2.10 2.3 Thermal resistance and the electrical analogy 65 Interstitial heat transfer Interstitial heat transfer Temperature gradient Figure 2.10 Heat transfer through the contact plane between two solid surfaces. which is drawn with a highly exaggerated vertical scale . Heat transfer follows two paths through such an interface. Conduction through points of solid-to-solid contact is very effective but conduction through the gas-filled interstices which have low thermal conductivity can be very poor. Thermal radiation across the gaps is also inefficient. We treat the contact surface by placing an interfacial conductance hc in series with the conducting materials on either side. The coefficient hc is similar to a heat transfer coefficient and has the same units W m2K. If AT is the temperature difference across an interface of area A then Q Ahc AT. It follows that Q AT Rt for a contact resistance Rt 1 hcA in K W. The interfacial .