Radiative Energy Exchange

Several different mechanisms of electronic energy transfer are believed to operate under different circumstances. The first of these is the so-called trivial mechanism of radiative transfer, which can be represented by the processes 

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O Connor and Hargreaves (115) have reported a fluorescent lifetime of 225 sec for neodymium in CeF3. In this host there is both nonradiative and radiative energy exchange from the lattice d bands to the neodymium ions. 

RADIATIVE ENERGY EXCHANGE BETWEEN SURFACES 7.4.1 Radiosity... 

Because of the laws of radiative energy exchange appreciable heat flows can only be transferred by radiation if the absolute temperature of the radiation source is high, see Eq. 4.3-6. If a first body of surface A] and surface temperature T is... 

Rporm shape factor taking into account that the apparent shape of the particle with respect to radiative energy exchange may be different from its geometrical one and must be determined from experiments. 

In the case of a given surface temperature, the amount of energy released is determined by the parameters for the convective and radiative heat exchange. As far as convection is concerned, these are the temperatures ol the heat source surface and room air, respectively, and the heat transfer coefficient. The radiative heat exchange is determined by the view factors and the temperatures of the surrounding surfaces. 

The room models implemented in the codes can be distinguished further by how detailed the models of the energy exchange processes are. Simple models use a combined convective-radiative heat exchange. More complex models use separate paths for these effects. Mixed forms also exist. The different models can also be distinguished by how the problem is solved. The energy balance for the zone is calculated in each time step of the simulation. 

The decay time of the Cr " band of approximately 150 ns is very short for such emission. Radiative energy transfer may not explain it because in such a case the decay curves of each of the ions are independent of the presence of the other. Thus non-radiative energy transfer may also take part, probably via multipolar or exchange interactions. In such cases the process of luminescence is of an additive nature and the lifetime of the sensitizer from which the energy is transferred is determined, apart from the probability of emission and radiationless transitions, by the probability of the energy transfer to the ion activator. 

It appears at this time that one of the most important mechanisms involved in the luminescence of rare earth ions is energy exchange between them. One may clearly differentiate between two distinct mechanisms (a) radiative exchange and (b) nonradiative exchange. In the radiative mechanism, a photon emitted by ion A is captured by ion B. Since the photon has left the A system, the capture of it by B cannot decrease the lifetime of A. However, f the photon is shuttled back and forth between similar or dissimilar ions, the fluorescent lifetime could well be increased by radiation trapping. This is an interesting phenomenon and warrants further discussion. 

A non-radiative energy transfer solely by dipole-dipole interactions - without recourse to electron exchange - from energy donor to an acceptor (Fig. 5.4) is described by the Forster mechanism. 

See electron exchange excitation transfer, Forster excitation transfer, radiative energy transfer. 

Heat transfer or thermal energy exchange occurs if and only if there is a temperature difference. Moreover, thermal energy can only be transferred from a system or substance with a higher temperature to a system or substance with a lower temperature. The phenomenological laws will be discussed here to provide a quantitative relation of a heat flux, as a measure of energy transfer, with a system temperature gradient. Such a relation will be discussed for conductive, convective, and radiative heat transfer mechanisms. 

The concept of view factors is quite convenient in the analysis of diffuse and gray radiation exchanges. Under these assumptions, the view factor, Fu2, is purely a geometric quantity. Physically, it means the fraction of radiative energy leaving surface 1 that reaches surface 2. In other words, it describes how much surface 1 sees surface 2, thus the name view factor. Due to the restricted nature of this chapter, the expression for the view factors will not be derived here. Instead, the expression will be given here and the reader will be referred to a more-detailed discussion in References 2, 18, and 19. Mathematically, the view factor is defined as... 

Fig. 1 a In the presence of an acceptor A, the luminescence of the originally excited donor D is quenched by a non-radiative energy transfer process, b In order for the energy transfer to be efficient, the acceptor must be in the vicinity of the donor. The ensuing interaction can be either multipole-multipole interaction or exchange interaction. Whereas for dipole-dipole interaction Hda= DA H D A) falls off with the third power of the distance Rda, the exchange interaction falls of exponentially with RDA... 

Radiative heat transfer from one small volume or surface element to another is determined by accounting for the energies of photons of all wavelengths, emitted in all directions over a certain time interval. Depending on the location of each element and its orientation with respect to others, the amount of radiant energy exchange between elements will vary. In order to determine the contribution of each element to the radiation balance, we introduce a fundamental and mathematically convenient quantity termed radiation intensity. 

H. B. Keene, Calculation of the Energy Exchange between Two Fully Radiative Coaxial Circular Apertures at Different Temperatures, Proc. Roy. Soc., vol. LXXXVIIIA, pp. 59-60,1913. 

The technology described in this part of the chapter addresses the radiative heat exchange between the building envelope to the outside and to the inside. Every material with a temperature above absolute zero (0 kelvin) emits energy by radiation towards a surface with a lower temperature. The... 

The rate at which radiation exchanges energy with matter is expressed in terms of the spectral radiative heating rate Hv = - da> dlv/ds), which is (minus) the rate of change of the radiative energy per unit volume. In plane geometry, if we substitute dlv/dxs = —p dl jd x in the radiative transfer equation, Eq. (2), use k v) = a(v) + a v), and integrate over the sphere, we obtain = -dF ldz =... 

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