tailieunhanh - A HEAT TRANSFER TEXTBOOK - THIRD EDITION Episode 3 Part 4

Tham khảo tài liệu 'a heat transfer textbook - third edition episode 3 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 564 Radiative heat transfer even emit additional photons. The result is that fluids can play a role in the thermal radiation to the the surfaces that surround them. We have ignored this effect so far because it is generally very small especially in air and if the distance between the surfaces is on the order of meters or less. When other gases are involved especially at high temperatures as in furnaces or when long distances are involved as in the atmosphere gas radiation can become an important part of the heat exchange process. How gases interact with photons The photons of radiant energy passing through a gaseous region can be impeded in two ways. Some can be scattered or deflected in various directions and some can be absorbed into the molecules. Scattering is a fairly minor influence in most gases unless they contain foreign particles such as dust or fog. In cloudless air for example we are aware of the scattering of sunlight only when it passes through many miles of the atmosphere. Then the shorter wavelengths of sunlight are scattered short wavelengths as it happens are far more susceptible to scattering by gas molecules than longer wavelengths through a process known as Rayleigh scattering . That scattered light gives the sky its blue hues. At sunset sunlight passes through the atmosphere at a shallow angle for hundreds of miles. Radiation in the blue wavelengths has all been scattered out before it can be seen. Thus we see only the unscattered red hues just before dark. When particles suspended in a gas have diameters near the wavelength of light a more complex type of scattering can occur known as Mie scattering. Such scattering occurs from the water droplets in clouds often making them a brilliant white color . It also occurs in gases that contain soot or in pulverized coal combustion. Mie scattering has a strong angular variation that changes with wavelength and particle size . The absorption or emission of radiation by molecules rather than .