RADIATIVE TRANSFER IN COMBUSTION SYSTEMS
Leading to: Combustion phenomena affected by radiation; Radiative transfer in laminar flames; Radiative transfer in turbulent flames; Radiative transfer in combustion chambers; Radiative transfer in two-phase combustion; Thermal radiation in unwanted fires
Fossil fuel utilization, primarily in the form of combustion transformations, has been the backbone of worldwide development for about two centuries. The reliance on fossil fuel is not likely to change in the foreseeable future because gas and remaining supplies of coal, oil, shale oil, and tar sands appear to be adequate for decades. Some of these sources are to be gradually replaced by renewable fuels, and alternate technologies such as direct conversion of solar radiation to electricity, wind power, etc., are expected to replace fossil fuels. In spite of these trends and the anticipated continuing cost advantage of fossil fuel–based combustion technologies, there are and will continue to be research, development, and design requirements for advanced as well as sustainable combustion technologies in the decades to come.
Combustion is the oldest technology of mankind and has furnished man with a major source of energy for more than one million years, and at present about 90% of our worldwide energy needs (e.g., in electrical power generation, transportation, heating) is provided by combustion of hydrocarbon fuels. More recently, nuclear energy has provided a significant fraction of energy for electric power. However, many years will elapse before combustion loses its predominance, and for the foreseeable future it will continue to be an energy source for power, industrial processes, human comfort, etc.
Combustion is a rapid oxidation generating heat or both heat and radiation. This definition emphasizes the intrinsic importance of chemical reactions to combustion and why combustion is so useful. Combustion transforms energy stored in chemical bonds of a fuel to heat that can be utilized in a variety of ways.
The purpose of combustion is to retrieve energy from the burning of fuels in the most efficient way possible. Combustion can occur in either a flame or a nonflame mode. A flame is considered to be a combustion reaction that can propagate subsonically through space. It is usually accompanied by the emission of radiation (ultraviolet, visible, and infrared). The property of spatial propagation is the important one that distinguishes flames from other combustion reactions. The spatial propagation of flames is a result of strong coupling between chemical reaction kinetics, the transport processes of mass and heat diffusion, and fluid flow. Heat transfer, thermal radiation, and active species can all accelerate a chemical reaction. Qualitatively, this can be considered as a positive feedback. If the feedback exceeds some threshold, the system will be self-sustaining. The existence of flame motion implies that the reaction is confined to a small zone. This reaction zone is called the flame front, combustion wave, or combustion zone.
In view of the preceding discussion, it is certain that in the future scientists and engineers engaged in development of combustion technologies will be confronted with complex phenomena that depend on interrelated processes of thermodynamics, chemical kinetics, fluid flow, heat and mass transfer, turbulence, and radiative transfer. Thermal radiation in combustion systems at high temperatures is an important energy transport process that needs to be considered for both fundamental understanding of the process and for its implementation in practical combustion systems. In this contribution to THERMOPEDIA, we apply the fundamental concepts and methodologies of radiative transfer theory to combustion situations arising in ignition of solids, laminar and turbulent flames, combustion chambers, furnaces for materials processing, and unwanted fires.
This article focuses on recent interest of incorporating radiative transfer in fundamental combustion analysis and practical combustion systems. Radiation does not directly affect the physicochemical reaction processes and major species, but the transfer of radiation indirectly alters the flame temperature distribution and, consequently, the local rate of elementary chemical reactions and minor species through, for instance, chemoluminesence and other de-excitation processes. Combustion textbooks (Williams, 1985; Kuo, 1986; Turns, 2000) have decoupled combustion and radiation. In the past, flame radiation was considered a posteriori based on the adiabatic temperature, which is determined from combustion analysis without considering the effects of radiative transfer. More recent works that have accounted for radiative transfer in fundamental combustion studies have revealed that radiation can significantly affect the flame temperature, minor species, the NOx emissions, soot formation, flame extinction, and other phenomena (Chan, 2005; Viskanta, 2005). In the articles to follow, knowledge of radiative transfer is used to account for radiation in combustion phenomena and in combustion systems. Reference is made to textbooks on radiation in participating media (Brewster, 1992; Siegel and Howell, 2002; Modest, 2003) for fundamentals of the theory and for practical results that can be applied to combustion.
Brewster, M. Q., Thermal Radiative Transfer and Properties, Wiley, Hoboken, NJ, 1992.
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