Total emissive power

Fight or raffinate phase Emissive power, total w Btu/(h) (ft1)... 

Emissive power is the total radiative power leaving the surface of the fire per unit area and per unit time. Emissive power can be calculated by use of Stefan s law, which gives the radiation of a black body in relation to its temperature. Because the fire is not a perfect black body, the emissive power is a fraction (e) of the black body radiation ... 

Emissive power The total radiative power discharged from the surface of a fire per unit area (also referred to as surface-emissive power). 

From equation 0.112, the total emissive power is given by. 

Emissive power E Total thermal radiation energy emitted by a surface per unit time per unit surface area The three terms, Emissive power (E),... 

E Total emissive power of radiation No Total number of particles in a... 

From the previous chapter, the blackbody total intensity is related to the blackbody total hemispherical emissive power by... 

Exponential integral of order n, where n = 1, 2,3,.. . Hemispherical emissive power, W/m2 Hemispherical blackbody emissive power, W/m2 Volumetric fraction of soot Blackbody fractional energy distribution Direct view factor from surface zone i to surface zonej Refractory augmented black view factor F-bar Total view factor from surface zone i to surface zonej Planck s constant, J s Heat-transfer coefficient, W/(m2 K)... 

Conversion of gray-gas total exchange areas GS and SS to their nongray form depends on the fact that the relation between radiative transfer and blackbody emissive power dT is linear and proportional. The gray-gas-equivalent emissivity is applicable only to the... 

The Stefan-Boltzmann law in Eq. 12-3 gives the total blackbody emissive power f. i, which is the sum of the radiation emitted over all wavelengths. Sometimes we need to know the spectral blackbody emissive power, which is the amount of radiation energy emitted by a blackbody at a thermodynamic temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength X. For example, we are more interested in the amount of radiation an incandescent lighthulb emits in the visible wavelength spectrum than we are in the total amount emitted. 

It is left as an exercise to show that integration of the spectral blackbody emissive power over the entire wavelength spectrmn gives the total blackbody emissive power E ... 

Consider a 20-cm-dlameter spherical ball at 800 K suspended in air as shown in Fig. 12-12. Assuming the ball closely approximates a blackbody, determine a) the total blackbody emissive power, (h) the total amount of radiation emitted by the ball in 5 min, and (c) the spectral blackbody emissive power at a wavelength of. 3 (im. 

SOLUTION An isothermal sphere is suspended in air. The total blackbody emissive power, the total radiation emitted in 5 min, and the spectral blackbody emissive power at 3 p.m are to be determined. 

C Define the total and spectral blackbody emissive powers. How are they related to each other How do they differ ... 

When the total emissivity of a gas at temperature Tg is knovvn, the emissive power of the gas (radiation emitted by the gas per unit surface area) can be expressed as Eg = CgaT. Then the rale of radiation energy enulted by a gas to a bounding surface of area/lj becomes... 

In the literature the emissive power is variously called the emittance, total hemispherical intensity, or radiant flux density. 

Consider radiative interchange between two finite black surface area elements Ai and A2 separated by a transparent medium. Since they are black, the surfaces emit isotropically and totally absorb all incident radiant energy. It is desired to compute the fraction of radiant energy, per unit emissive power 1, leaving Ai in all directions which is intercepted and absorbed by A2. The required quantity is defined as the direct view factor and is assigned the notation F12. Since the net radiant energy interchange Q12 = AiFi jEi between surfaces... 

Hemispherical spectral emissive power and total intensity... 

The spectral intensity Lx(X,f3,p,T) characterises in a detailed way the dependence of the energy emitted on the wavelength and direction. An important task of both theoretical and experimental investigations is to determine this distribution function for as many materials as possible. This is a difficult task to carry out, and it is normally satisfactory to just determine the radiation quantities that either combine the emissions into all directions of the hemisphere or the radiation over all wavelengths. The quantities, the hemispherical spectral emissive power Mx and the total intensity L, characterise the distribution of the radiative flux over the wavelengths or the directions in the hemisphere. 

Here d2radiation flow emitted by the surface element into the solid angle element dee in the direction of the angle /3 and total intensity L has units W/m2sr it belongs to the directional total quantities and represents the part of the emissive power falling into a certain solid angle element. 

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