Thermal radiation exchange with gases

In some cases it is not possible to consider the modes separately. For example, if a gas, such as water vapor or carbon dioxide, which absorbs and generates thermal radiation, flows over a surface at a higher temperature, heat is transferred from the surface to the gas by both convection and radiation. In this case, the radiant heat exchange influences the temperature distribution in the fluid. Therefore, because the convective heat transfer rate depends on this temperature distribution in the fluid, the radiant and convective modes interact with each other and cannot be considered separately. However, even in cases such as this, the calculation procedures developed for convection by itself form the basis of the calculation of the convective part of the overall heat transfer rate. 

Figure 5 Thermal history of a chondrule (sohd fines) passing through a 7.5 km s shock wave, over timescales of (a) hours and (b) minutes. Chondrules are heated for hours before the shock by radiation. At the shock front, chondrules must slow to the gas velocity in 1 min it takes to do so, gas drag heating causes a spike in temperature. Afterwards, chondrules are heated for hours by radiation and thermal exchange with the hot gas (dashed line) (Desch and Connolly, 2002) (reproduced by permission of Sheridan Press and Meteoritical Society from Meteorit.

If two bodies of different temperature are separated by a gas, heat is exchanged in general by radiation, convection, and conduction. If the gas does not absorb in the frequency range of the thermal radiation (which is normally the case with air but not with CO2), the transport mechanisms in the gas and the radiation can be regarded as independent of one another. The total heat exchanged represents the sum of heat transported through the medium (via conduction and convection) and by radiation. In other words, the overall thermal conductance (i.e., the reciprocal thermal resistance) is obtained as the sum of the inchvidual thermal conductances of the respective transport mechanism. 

Much of the radiation with which we are familiar in everyday life is of thermal origin, arising by definition from matter in thermal equilibrium. In an ideal atomic gas in thermal equilibrium, for example, the upward versus downward transitions of bound electrons between energy levels in individual atoms are in close balance due to the exchange of energy between particles via collisions and the absorption and emission of radiation. The velocities of particles in an ideal thermal gas follow the well-known Maxwellian distribution, and the collective continuous spectrum of the radiating particles is described by the familiar Planck black-body radiation curve with its characteristic temperature-dependent profile and maximum. 

Method Explicit Matrix Relations for Total Exchange Areas, Int.J. Heat Mass Transfer, 18, 261-269 (1975). Rhine, J. M., and R. J. Tucker, Modeling of Gas-Fired Furnaces and Boilers, British Gas Association with McGraw-Hill, 1991. Siegel, Robert, and John R. Howell, Thermal Radiative Heat Transfer, 4th ed., Taylor Francis, New York, 2001. Sparrow, E. M., and R. D. Cess, Radiation Heat Transfer, 3d ed., Taylor Francis, New York, 1988. Stultz, S. C., and J. B. Kitto, Steam Its Generation and Use, 40th ed., Babcock and Wilcox, Barkerton, Ohio, 1992. 

A numerical model is presented to describe the thermal conversion of solid fuels in a packed bed. For wood particles it can be shown, that a discretization of the particle dimensions is necessary to resolve the influence of heat and mass transfer on the conversion of the solid. Therefore, the packed bed is described as a finite number of particles interacting with the surrounding gas phase by heat and mass transfer. Thus, the entire process of a packed bed is view as the sum of single particle processes in conjunction with the interaction of the gas flow in the void space of a packed bed. Within the present model, neighbour particles exchange heat due to conduction and radiation with each other. 

Additional parameters specified in the numerical model include the electrode exchange current densities and several gap electrical contact resistances. These quantities were determined empirically by comparing FLUENT predictions with stack performance data. The FLUENT model uses the electrode exchange current densities to quantify the magnitude of the activation overpotentials via a Butler-Volmer equation [1], A radiation heat transfer boundary condition was applied around the periphery of the model to simulate the thermal conditions of our experimental stack, situated in a high-temperature electrically heated radiant furnace. The edges ofthe numerical model are treated as a small surface in a large enclosure with an effective emissivity of 1.0, subjected to a radiant temperature of 1 103 K, equal to the gas-inlet temperatures. 

The dissipation of heat from a battery to its surroundings normally proceeds via three mechanisms (i) heat flow through the components of the battery and the container walls (ii) heat radiation (iii) free convection of air. In practice, the cooling of a battery takes place mainly through the side walls of the container. The bottom surface is usually in contact with a solid surface, which attains the same temperature as the battery and then ceases to be an effective heat sink. The upper surface plays little part in heat exchange the lid has no direct contact with the electrolyte, and the intermediate layer of gas, which has low thermal conductivity, hinders heat exchange. 

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