P1 Approximation of the Spherical Harmonics Method
Following from: Differential approximations
Leading to: The simplest approximations of double spherical harmonics, Radiation of an isothermal plane-parallel layer, Radiative equilibrium in a plane-parallel layer, Radiation of a nonisothermal layer of scattering medium, An estimate of P1 approximation error for optically inhomogeneous media, Diffusion approximation in multi-dimensional radiative transfer problems
In this article, we consider the well-known simple approximation for radiative transfer in scattering media. The spherical harmonics method was developed by Jeans (1917) in his work on radiative transfer in stars. Further description of the method, as it applies to radiative transfer, was given by Kourganoff (1952, 1963). Application of the spherical harmonics method to neutron transport problems was considered by Davison (1957) and Murray (1957). With the use of the spherical harmonics method, the spectral radiation intensity is presented in a series of spherical functions. In the first approximation of this method, P1, which is the usual version of the diffusion approximation, the following linear dependence is assumed:
By multiplying the radiative transfer equation
with , and integrating it over a solid angle, one can find that the spectral radiation flux is related to the spectral radiation energy density Iλ0 by the following equation:
Note that Eq. (3) is obtained for the arbitrary scattering function but it is the same as that for the transport approximation. This means that the P1 approximation is insensitive to the details of the scattering function, and the asymmetry factor of scattering μλ is the only characteristic of the scattering anisotropy taken into account in this approach. The combination of Eq. (3) with the radiation balance equation
leads to the following equation for the spectral radiation energy density:
The Marshak boundary condition (Marshak, 1947) is usually employed in the P1 approximation. In the absence of walls and external sources of radiation, this condition has the following form (Case and Zweifel, 1967):
where is the external normal to a boundary surface. According to Pomraning (1964) and Shokair and Pomraning (1981), we consider also the following corrected boundary condition:
Hereafter, condition (7) will be called the Pomraning boundary condition. Both variants of the boundary condition will be considered below in analysis of P1 approximation accuracy. It should be noted that the approximation, which is equivalent to P1 for one-dimensional problems, was developed independently by Milne (1930) and Eddington (1959), and this model is also known as the Milne–Eddington approximation.
We do not consider high-order approximations (PN) of the spherical harmonics method in this article. The reader can refer to the available literature on this subject (Kofink, 1959; Bayazitoglu and Higenyi, 1979; Yücel and Bayazitoglu, 1983; Karp et al., 1980; Ou and Liou, 1982; Khan and Thomas, 2005; Atalay, 2006). Note that the PN approximation is equivalent to the discrete ordinate method based on the corresponding order of the Gaussian quadrature for the integral term of the radiative transfer equation (RTE) (Barichello and Siewert, 1998).
The P1 approximation is a simplified approach, which is expected to be fairly good with absorbing and highly scattering media at large optical distances from boundaries or interfaces that have a strong variation of temperature and radiative characteristics of the medium. Nevertheless, the simplicity and clear physical sense of P1 have attracted the attention of many researchers who have suggested various modifications of this approach as applied to some specific radiative transfer problems. One can remember the modified differential approximation (MDA) by Olfe (1967, 1968, 1970) [see also Glatt and Olfe (1973) and an improvement of the MDA by Wu et al. (1987)] and the improved differential approximation (IDA) by Modest (1974, 1975, 1990). Both methods separate the radiation emitted by the walls from the thermal radiation of the medium. However, these methods are not as simple as the ordinary P1 and require the evaluation of some integral correction factors (Modest, 2003). Formal attempts to improve the P1 accuracy by modifying the boundary conditions were made by Liu et al. (1992a,b) and Su (2000). We will not consider the possible modifications of the boundary conditions for P1 and high-order approximations of the same method here. Some additional results on this subject can be found by Lii and Özişik (1973) and Chien and Wu (1991), and also by Modest (2003). In one-dimensional problems, the accuracy can also be improved by applying the P1 approximation separately to different solid angle ranges, as was done by Mengüç and Subramaniam (1992).
The most widely used approximations are the P1 and P3 approximations of the spherical harmonics method. However, high-order PN approximations are also employed in solving one-dimensional problems. For example, Mengüç and Viskanta (1988) employed the P9 approximation to solve the RTE for a stratified fly-ash cloud near the walls of a pulverized-coal furnace to study the effect of spectral radiative properties on the blockage of radiation to the walls. High-order solutions, up to P11, for gray medium between concentric spheres have been considered by Tong and Swathi (1987) and by Li and Tong (1990). For solving the atmospheric problems, Karp et al. (1980) employed the spherical harmonics approximation up to 99th order (P99) with up to 100 terms of the scattering function and considering up to 15 homogeneous layers with any optical thickness. In solving one-dimensional radiative transfer problems, many researchers are not limited by diffusion approximation, but employ it only for one of the solution stages–such as at the initial approximation, as reported by Adzerikho and Nekrasov (1975) and Sutton and Özişik (1979), or at the first iterative step, as was done by Abramson and Lisin (1985). Mengüç and Viskanta (1985, 1986) limited their development to the P3 approximation but considered the three-dimensional problem in Cartesian coordinates and a two-dimensional axisymmetric problem. The diffusion approximation has been used in similarity analysis of radiative transfer problems in scattering media (Popov, 1980; McKellar and Box, 1981; Lee and Buckius, 1982; Kim and Lee, 1990; Liu et al., 1992a,b; Tagne and Baillis, 2005) and in obtaining analytical solutions for simple geometrical forms of a radiating volume (Popov, 1980; Adzerikho et al., 1979).
Currently, many commercial CFD codes have the P1 approximation as an optional solution technique for radiation calculations. The engineering calculation of heat transfer in combustion is one of the most well-known applications of P1 and P3 approximations (Viskanta and Mengüç, 1987; Onda, 1995; Dombrovsky, 1996a; Marakis et al., 2000; Viskanta, 2005; Klason et al., 2008). The P1 application is widely used in solving various combined heat transfer problems because this approximation usually gives a good estimate of the radiation energy density in scattering media. As examples of the use of the diffusion approximation in modeling of radiative–conductive heat transfer in thermal insulation, one should refer to the studies by Petrov (1993) and Dombrovsky (1996b). In the case of buoyant flow of an optically thick fluid, it was shown by Derby et al. (1998) that the P1 approximation yields surprisingly accurate results compared with the solutions obtained from the rigorous treatment. The same approach is employed in modeling radiative transfer in arcs and discharges (Freton et al., 2002; Dixon et al., 2004; Segur et al., 2006), and even in calculations of the radiation field around blunt-body vehicles in the atmosphere (Hartung and Hassan, 1993).
The spherical harmonics approximation is used in analysis of thermal radiation from rocket plumes (Baudoux et al., 2001), in calculations of neutron transport in nuclear engineering (Ackroyd et al., 1999; Ziver et al., 2005), and in complex atmospheric problems (Nakajima and Tanaka, 1988; Evans, 1993, 1998; Trasi et al., 2004) including in image formation by observing some discrete light sources through clouds (Zardecki et al., 1986). In the last case, even P1 may give fairly accurate predictions for highly scattering optically dense media. Some additional references concerning the use of the diffusion approximation in solving multi-dimensional problems including present-day biological and medical studies will be given in the article Diffusion Approximation in Multi-Dimensional Problems.
It should be noted that spherical harmonics and the simplest diffusion approximation are considered as a present-day tool for unsteady problems, including the case when a pulsed laser beam illuminates a highly scattering medium (Olson et al., 2000; Aydin et al., 2005). In the latter case, the diffusion approximation can be employed to the radiation scattered from the laser beam. Some modifications of spherical harmonics and the P1 approximation as it applies to unsteady problems can be found in the studies done by Morel (2000) and Frank et al. (2007).
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