Surface diffuse radiating

No radiator exists that has a spectral intensity Lx independent of the wave length. However, the assumption that Lx does not depend on j3 and ip applies in many cases as a useful approximation. Bodies with spectral intensities independent of direction, Lx = Lx(X,T), are known as diffuse radiators or as bodies with diffuse radiating surfaces. According to (5.9), for their hemispherical spectral emissive power it follows that... 

The double integral here has the value n, so that for diffuse radiating surfaces... 

Fig. 5.10 Intensity L = Ia(T) and directional emissive power I = In(T) cos (3 of a diffuse radiating surface...

For a diffuse radiating surface, it follows from (5.19) that... 

The body has a diffuse radiating surface (Lambert radiator) then eA = eA(A, T) is valid. 

When e x = e x(X,T), (diffuse radiating surface), it holds that... 

T he total or global solar radiation has a direct part (beam radiation) and a diffuse part (Fig. 11.31). In the simulation, solar radiation input values must be converted to radiation values for each surface of the building. For nonhorizontal surfaces, the diffuse radiation is composed of (a) the contribution from the diffuse sky and (b) reflections from the ground. The diffuse sky radiation is not uniform. It is composed of three parts, referred to as isotropic, circumsolar, and horizontal brightening. Several diffuse sky models are available. Depending on the model used, discrepancies for the boundary conditions may occur with the same basic set of solar radiation data, thus leading to differences in the simulation results. 

Gebhart B. Surface temperature calculations in radiant surroundings of arbitrary complexity—for gray, Diffuse Radiation. Int.. Heat Mitss Transfer, vol. 3, no. 4, 19iil. 

The term Qsh is the net solar radiant energy absorption rate on the basin bottom. It is equivalent to total radiation incident on the basin cover minus reflection from the cover, the water surface, and the basin bottom, and minus loss due to structural shadowing. Its determination from Weather Bureau records of total daily radiation on a horizontal surface is complicated by many factors such as variation in angle of incidence, and resulting transmissivity of cover, hourly and seasonally, intensity change due to cloudiness, and different properties of direct and diffuse radiations. Detailed explanation of these meteorological and optical calculations is beyond the scope of this paper, but may be found in the literature (6). 

Perez R., Stewart R., Arbogast C., Seals R., Scott J., An anisotropic hourly diffuse radiation model for sloping surfaces description, performance validation, site dependency evaluation. Solar Energy 1986 36 481M97. 

Although the relation is derived for black surfaces, it holds for other surfaces also as long as diffuse radiation is involved. 

All the preceding discussions have considered radiation exchange between diffuse surfaces. In fact, the radiation shape factors defined by Eq. (8-21) hold only for diffuse radiation because the radiation was assumed to have no preferred direction in the derivation of this relation. In this section we extend the analysis to take into account some simple geometries containing surfaces that may have a specular type of reflection. No real surface is completely diffuse or completely specular. We shall assume, however, that all the surfaces to be considered emit radiation diffusely but that they may reflect radiation partly in a specular manner and partly in a diffuse manner. We therefore take the reflectivity to be the sum of a specular component and a diffuse component ... 

Equation (8-77) expresses the diffuse radiation leaving 1 which arrives at 2 and which may contribute to a diffuse radiosity of surface 2. The factor 1 - ps represents the fraction absorbed plus the fraction reflected diffusely. The inclusion of this factor is most important because we are considering only diffuse direct exchange, and thus must leave out the specular-reflection contribution... 

Part of the diffuse radiation from 2 is specularly reflected in 3 and strikes 1. This specularly reflected radiation acts like diffuse energy coming from the image surface 2(3). Thus we may write... 

In summary, we outline the computational procedure to be followed for numerical solution of radiation heat transfer between diffuse, gray surfaces. This basic procedure is the same for a hand computation, calculation with a minicomputer, or a large computer. 

It is also necessary to take sky radiation into account, that is, sunlight scattered by the atmosphere and reflected diffusely and which reaches all surfaces of the tank, including those not hit by direct sunlight because they are in shadow. This diffuse radiation Gs varies greatly but is generally small, between about 2.2 Btu/(h)(ft2) (6.93 W/m2) on a clear day and 44.2 Btu/(h)(ft2) (139 W/m2) on a cloudy day. For the day as described, assume that Gs is 25 Btu/(h)(ft2). This value must be added to all surfaces, including those in shadow. 

The conditions where net erosion turns into net deposition for carbon irradiation may further be complicated by effects of surface temperature, influencing the self-sputtering yield by radiation-enhanced sublimation (RES) as well as the diffusion and surface segregation of implanted impurities. An example is introduced in Fig. 9.4b, where the erosion of W due to C+ bombardment... 

The intensity of radiation emitted by a surface, in general, varies with direction (especially with the zenith angle 0). But many surfaces in practice can be approximated as being diffuse. For a diffusely emitting surface, the intensity of the emitted radiation is independent of direction and thus I, = constant. 

The direct and diffuse radiation incident on a horizontal surface on earth s surface. 

C How i.s the intensity of emitted radiation defined For a diffusely emitting surface, how is the emissive power related to (he intensity of emitted radiation ... 

The net rate of radiation transfer between any two gray, diffuse, opaque surfaces that form an enclosure is given by... 

Table II gives values of Jx for an intermediate set of conditions details are in Leighton s report (18). This equation uses the weak absorption approximation it neglects surface reflection, and it takes no account of the effects of diffusion within the absorbing layer. The weak absorption approximation will make the calculated rates too high, and neglecting the surface reflection will make them too low as a result these two errors partially cancel each other. Any accounting of internal diffusion on absorption rates will depend on the directional distribution of the diffused radiation and on whether the rate near the surface on the average rate throughout the layer is the more important. Until more is known of these factors, the effects of diffusion within the layer must be regarded as indeterminate.

Enclosure with N Diffuse-Gray Surfaces—The Net Radiation Method.579... 

Sometimes it is important to consider the direction of reflected irradiation exitent from a surface. A property called the bidirectional reflectance distribution function (BRDF) is used to specify the directional distribution of the reflected intensity for a specified direction of incident radiation [2-4]. A specular surface is a mirrorlike surface for which the incidence angle is equal to the reflection angle. For a diffusely reflecting surface, the reflected intensity is the same in all directions, and if perfectly reflective, the BRDF is l/n sr. ... 

The sum of the direct and the diffuse radiation on a surface. Total solar radiation is sometimes used to indicate quantities integrated over all wavelengths of the solar spectrum. The most conunon measurement of solar radiation is total radiation on a horizontal surface, often referred to as global radiation. 

The absorptivity of a gray body, like the emissivity, is the same for all wavelengths. If the surface of the gray body gives diffuse radiation or refleption, its monochromatic absorptivity is also independent of the angle of incidence of the radiant beam. The total absorptivity equals the monochromatic absorptivity and is also independent of the angle of incidence. 

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