Radiation shield

Radiative Properties of material adn radiation shield

1. Module -7 Radiation Heat Transfer By Faculty: Mr. LALAN KUMAR Assistant Professor Department of Mechanical Engineering Katihar Engineering College Katihar

2. 01 Version: 1, KEC Katihar Radiative Properties When radiation strikes a surface, a portion of it is reflected, and the rest enters the surface. Of the portion that enters the surface, some are absorbed by the material, and the remaining radiation is transmitted through. The ratio of reflected energy to the incident energy is called reflectivity , ρ . Transmissivity ( τ ) is defined as the fraction of the incident energy that is transmitted through the object. A bsorptivity ( α ) is defined as the fraction of the incident energy that is absorbed by the object. The three radiative properties all have values between zero and 1. Furthermore, since the reflected, transmitted, and absorbed radiation must add up to equal the incident energy, the following can be said about the three properties: a + t +r = 1

3. Emissivity 02 Version: 1, KEC Katihar A black body is an ideal emitter. The energy emitted by any real surface is less than the energy emitted by a black body at the same temperature. At a defined temperature, a black body has the highest monochromatic emissive power at all wavelengths. The ratio of the monochromatic emissive power E l to the monochromatic blackbody emissive power E bl at the same temperature is the spectral hemispherical emissivity of the surface.

4. 03 Version: 1, KEC Katihar The total (hemispherical emissive power is, then, given by Define total (hemispherical) emissivity, at a defined temperature Here, e can be interpreted as either the emissivity of a body, which is wavelength independent, i.e., e l is constant, or as the average emissivity of a surface at that temperature. A surface whose properties are independent of the wavelength is known as a gray surface. The emissive power of a real surface is given by

5. 04 Version: 1, KEC Katihar Absorptivity a , Reflectivity r, and Transmissivity t Consider a semi-transparent sheet that receives incident radiant energy flux, also known as irradiation, G . Let dG represent the irradiation in the waveband l to l + d l . Part of it may be absorbed, part of it reflected at the surface, and the rest transmitted through the sheet. We define monochromatic properties, Monochromatic Absorptivity : Total Absorptivity :

6. 05 Version: 1, KEC Katihar Monochromatic reflectivity : Total reflectivity : Monochromatic Transmissivity : Total Transmissivity :

7. Radiation Shields And The Radiation Effect 06 Version: 1, KEC Katihar Radiation heat transfer between two surfaces can be reduced greatly by inserting a thin, high-reflectivity (low-emissivity) sheet of material between the two surfaces. Such highly reflective thin plates or shells are called radiation shields. The role of the radiation shield is to reduce the rate of radiation heat transfer by placing additional resistances in the path of radiation heat flow. The lower the emissivity of the shield, the higher the resistance .

8. 07 Version: 1, KEC Katihar The radiation shield placed between two parallel plates and the radiation network associated with it.

9. 08 Version: 1, KEC Katihar The resistances are connected in series, and thus the rate of radiation heat transfer is F 13 = F 23 = 1 and A 1 = A 2 = A 3 = A for infinite parallel plates Note :

10. 09 Version: 1, KEC Katihar Then the radiation heat transfer through large parallel plates separated by N radiation shields becomes If the emissivities of all surfaces are equal

11. 10 Version: 1, KEC Katihar A thin aluminum sheet with an emissivity of 0.1 on both sides is placed between two very large parallel plates that are maintained at uniform temperatures T 1 = 800 K and T 2 = 500 K and have emissivities Ɛ 1 = 0.2 and Ɛ 2 = 0.7, respectively. Determine the net rate of radiation heat transfer between the two plates per unit surface area of the plates and compare the result to that without the shield.