Gases radiation exchange with

Up to this point, the gas-radiation calculation methods discussed were concerned only with black surfaces exchanging heat with the gas. In many engineering applications the enclosure walls are dirty and sooty, the wall emissivity is very high, and the heat-transfer calculation by Eq. (8-53) may be a reasonable... 

Each particle exchanges energy due to radiation with its neighbours. To take into account reflection between the particles it is assumed that each particle contributes its radiative loss to a global radiation field. The radiation field is related to the flow model, and the cell each particle is contributing its radiative heat loss to, is determined by the position of the neighbour particles. It is assumed that no radiation due to short ray travelling distances is absorbed in the gas phase between the particles. 

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. 

The radiative exchange in a gas filled enclosure is more difficult to calculate than the exchange dealt with in 5.5.3, without an absorbing and therefore self radiating gas. In the following we will consider two simple cases, in which an isothermal gas is involved in radiative interchange with its boundary walls that are likewise at a uniform temperature. At the end of this section we will point to more complex methods with which more difficult radiative exchange problems may be solved. 

Tlie fin is the most common fonn of extended surface, associated with both tubular and plate-type heat exchangers - the car radiator is a highly compact finned unit. Most heat exchangers used in gas streams have fins to improve the gas-side heat transfer. Compact units such as the plate-fin heat exchanger have fins, or secondary surfaces , between each pair of plates. Fins are used in both natural and forced convection. They are also used, when they are sometimes called ribs, to aid boiling or to provide drainage... 

The hollow cathode lamp takes the form of a cup-shaped cathode in a space filled with an inert gas. The cathode is made of the same metal as the one which is to be estimated- e.g. when measuring calcium, a cathode made of metallic calcium is used. An electrical potential is applied between the anode and the cathode, resulting in the cathode being bombarded with gaseous ions which exchange energy with the element. The element emits radiation in discrete lines, but some of the secondary lines may be removed with filters. Double beam instruments are available which correct for variations in the output of the lamp. 

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. 

Gas molecules can alter their states of vibration and rotation by exchanging energy with the radiation field. This exchange occurs in discrete quantities, resulting in modifications to the field at specific frequencies associated with resonances in the molecular structure. As a consequence, molecules absorb and emit radiation in a complex pattern of discrete lines that deviate significantly from a blackbody spectrum. Because each type of molecule has a unique structure and, therefore, unique energies of motion, the pattern of observed lines is characteristic of the matter present. 

Isotopic exchange with oxygen gas usually occurs on surfaces at fairly high temperatures (300-2000°C.). Isotopic exchange between oxygen gas with carbon dioxide occurs only above 800°C. (Bank, 1958). The exchange of oxygen with water at low temperature occurs only in the presence of radiation (Hart et al., 1953), free radicals (Kasarnowsky et al., 1956), or ozone (Forchheimer and Taube, 1954). 

Treatment of Refractory Walls Partially Enclosing a Radiating Gas Another modification of the results in Table 5-10 becomes important when one of the surface zones is radiatively adiabatic the need to find its temperature can be eliminated. If surface A9, now called A, is radiatively adiabatic, its net radiative exchange with Aj must equal its net exchange with the gas. 

Dunkle, R.V. Radiation Exchange in an Enclosure with a Participating Gas in Rosenhow. W M. and Hartnett, J.P.. eds. Handbook of Heat Transfer (McGraw-Hill, New York, 1973)... 

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