Properties of thermal radiation

This chapter aims to systematically develop the background information needed to formulate and evaluate thermal radiation properties of gases for engineering applications, and to review the literature of present works and approaches for future research in this area. The scope of the chapter is limited by the assumption that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect. The chapter introduces the general concepts concerning gaseous radiation and presents a review of the physics of atomic and molecular spectra. The radiation resulting from transitions of electronic, atomic, or molecular states has been discussed; they are line radiation, band radiation, and continuum radiation. The evaluation of total (engineering) emissivity and its applications to radiation from homogeneous gas bodies of complex geometry have been discussed. Consideration has been given to the appropriate absorption coefficients for use in the radiative transport calculations. • Previous chapter in volume • Next chapter in volume

• All objects give off thermal radiation • The hotter an object is, the more thermal radiation it emits • Thermal radiation is the part of the electromagnetic spectrum called infrared • Thermal radiation is the only way in which heat can travel through a vacuum • It is the way in which heat reaches us from the Sun through the vacuum of space • The colour of an object affects how good it is at emitting and absorbing thermal radiation: EXTENDED Thermal Equilibrium • As an object absorbs thermal radiation it will become hotter • As it gets hotter it will also emit more thermal radiation • The temperature of a body increases when the body absorbs radiation faster than it emits radiation • Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation • At this point, the object will be in thermal equilibrium • If the rate at which an object receives energy is less than the rate at which it transfers energy away then the object will cool down • If the rate at which an object transfers energy away is less than the rate at which it receives energy then the object will heat up • The process will always move towards thermal equilibrium • The amount of thermal radiation emitted by an object depends on a number of factors: • The surface colour of the object (black = more radiation) • The texture of the surface (shiny surfaces = more radiation) • The surface area of the object (greater surface area = more a...

Thermal radiation is one of the three basic forms of heat transfer (conduction, convection, and radiation), being a basic property of matter depending on its temperature. In microcosmic point of view, thermal radiation is the result of random particle motion (due to the material temperature) resulting in charge acceleration or dipole oscillation of the electrons and protons. In contrast to conduction and convection, thermal radiation has two unique properties: (1) it does not require a medium to be transferred but being electromagnetic energy can interact with intervening materials; and (2) during the emission/absorption of radiation, there is transfer between electromagnetic energy and kinetic energy of atoms, namely, thermal energy. These two properties result... • Bergman TL, Incropera FP, DeWitt DP, Lavine AS (2011) Fundamentals of heat and mass transfer. Hoboken, New Jersey, Wiley • Cohen JD, Butler BW (1998) Modeling potential structure ignitions from flame radiation exposure with implications for wildland/urban interface fire management. In: Proceedings of the 13th fire and forest meteorology conference, International Association of Wildland Fire. pp 81–86 • Finney MA, Cohen JD, McAllister SS, Jolly WM (2013) On the need for a theory of wildland fire spread. Int J Wildland Fire 22(1):25–36 • Frankman DJ (2009) Radiation and convection heat transfer in wildland fire environments[D], PhD Thesis, Brigham Young University • Hankinson G, Lowesmith BJ (2012) A considerati...

Thermal Radiation – Radiant Heat Thermal radiation is electromagnetic radiation in the infra-red region of the electromagnetic spectrum although some of it is in the visible region. The term thermal radiation is frequently used to distinguish this form of electromagnetic radiation from other forms, such as radio waves, x-rays, or Thermal radiation does not require any medium for energy transfer. In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. In contrast to heat transfer by Stefan–Boltzmann Law Radiation heat transfer rate, q [W/m 2], from a body (e.g. a black body) to its surroundings is proportional to the fourth power of the q = εσT 4 where σ is a fundamental physical constant called the Stefan–Boltzmann constant, which is equal to 5.6697×10 -8 W/m 2 K 4. The Stefan–Boltzmann constant is named after Josef Stefan (who discovered the at very high temperatures and in a vacuum. The emissivity, ε, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0. By definition, a blackbody in thermal equilibrium has an emissivity of ε = 1.0. Real objects do not radiate as much heat as a perfect black body. They radiate less heat than a black body and therefore are called gray bodies. To take into account the fact that real objects are gray bodies, the Stefan-Boltzmann law must include emissivity. Quantitatively, emissivity is the ratio of the thermal rad...