Radiation grey surface

The emissivity of a material is defined as the ratio of the radiation per unit area emitted from a real or from a grey surface (one for which the emissitivity is independent of wavelength) to that emitted by a black body at the same temperature. Emissivities of real materials are always less than unity and they depend on the type, condition and roughness of the material, and possibly on the wavelength and direction of the emitted radiation as well. For diffuse surfaces where emissivities are independent of direction, the emissivity, which represents an average over all directions, is known as the hemispherical emissivity. For a particular wavelength X this is given by ... 

Figure 9.35. Comparison of black body, grey body and real surface radiation at 2000 K.<45 ...

Wliat is the emissivity of a grey surface, 10 nr in area, which radiates 1000 kW at 1500 K Whal would be the effect of increasing the temperature to 1600 K ... 

Radiation arrives at a grey surface of emissivity 0.75 al a constant temperature of 400 K, at the rate of 3 kW/m2. What is the radiosity and the net rate of radiation transfer to the surface What coefficient of heat transfer is required to maintain the surface temperature at 300 K if the rear of the surface is perfectly insulated and the front surface is cooled by convective heat transfer to air at 295 K ... 

Two large parallel plates with grey surfaces are situated 75 mm apart one has an emissivity of 0.8 and is at a temperature of 350 K and the other has an emissivity of 0.4 and is at a temperature of 300 K. Calculate the net rate of heat exchange by radiation per square metre taking the Stefan-Boltzmann constant as 5.67 x 10-8 W/m2 K4. Any formula (other than Stefan s law) which you use must be proved. 

For two large parallel plates with grey surfaces, the heat transfer by radiation between them is given by putting Ai = A2 in equation 150 to give ... 

Two grey. surfaces that form an enclosure exchange heat wilh one another by thermal radiation. Surface I has a... 

Fig. 5.57 Hollow enclosure bounded by isothermal surfaces (zones) each of which is a grey Lambert radiator...

Surfaces with low emissivities often exhibit approximately mirrorlike or specular reflection rather than diffuse reflection. We want to investigate how the assumption of mirrorlike reflection affects the heat transfer. The assumptions regarding the emission of diffuse and grey radiation remain unaltered. Grey Lambert radiators with mirrorlike reflection are therefore assumed. 

W/mK (grey curves) and 14 W/mK (black curves). Dashed curves heat generated from catalytic reactions. Dotted curves heat convected to the gas. Solid curves heat conducted in the solid. Dash-dotted curves surface radiation heat exchange... 

Accuracy of Pyrometers Most of the temperature estimation methods for pyrometers assume that the objec t is either a grey body or has known emissivity values. The emissivity of the nonblack body depends on the internal state or the surface geometry of the objects. Also, the medium through which the therm radiation passes is not always transparent. These inherent uncertainties of the emissivity values make the accurate estimation of the temperature of the target objects difficult. Proper selection of the pyrometer and accurate emissivity values can provide a high level of accuracy. 

In our sample calculations (Example 3 1.1) we have chosen the colour of the outdoors surface as light grey and taking the vveathering effect into account, have considered the coefficient of both absorption and emission as 0.65. The manufacturer, depending on the colour and site conditions, may choose a suitable coefficient. It is, however, advisable to be conservative when deciding the temperature rise due to solar radiation to be on the safe side. 

Assuming the approximate absorption coefficient of solar radiation to be 0.65, for a light-grey external surface, having collected soot and dirt over a period of time, as in ANSI-C-37-24 the approximate temperature rise on account of solar radiation... 

In this way, ihe emissive power of a grey body is a constant proportion of the power-emitted by the black body, resulting in the curve shown in Figure 9.35 where, for example, e = 0.6. The assumption that the surface behaves as a grey body is valid for most engineering calculations if the value of emissivity is taken as that for the dominant temperature of the radiation. 

Brownish, hemispherical, with ten lines on the surface, not radiating from a single point. A grey Thunder Stone, marked with ten lines distributed in pairs. Full of small holes. 

Example 1.2 A poorly insulated horizontal pipe (outer diameter d = 0.100m), with a surface temperature = 44 °C, runs through a large room of quiescent air at = 18 °C. The heat loss per length L of the pipe, Q/L has to be determined. The pipe is taken to behave as a grey radiator with emissivity s = 0.87. The walls of the room are treated as black surroundings which are at temperature = a = 18 °C. 

Here, (5.75) has been used because a grey radiator (s x independent of A) is present. The temperature T of the satellite surface will then be... 

A very long cylinder is struck by radiation that comes from a single direction, perpendicular to its axis (parallel directed radiation). The surface of the cylinder behaves like a grey radiator with the directional total emissivity = 0.85 cos/ . Calculate the reflected fraction of the incident radiative power. 

Two very thin, radiation protection shields, A and B, are positioned parallel to and between two very large, parallel plates at temperatures Ti = 750 K and T2 = 290 K. All surfaces are grey radiators with the same emissivity s = 0.82. 

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