PRACTICAL 7: Radiation
C1 What does a view factor represent and when is a view factor from a surface to itself not equal to zero?
C2: What is the difference between black and gray bodies and what simplifying assumptions can be made about them?
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1. An opaque horizontal plate is well insulated on the edges and lower surface. The irradiation on the plate is 3000 W/m2, with a reflectivity of 0.167. The plate has a uniform temperature of 700K and has an emissive power of 5000 W/m2. Determine
a) the total emissivity of the plate (Answer: 0.37)
b) the total absorptivity of the plate. (Answer: 0.83)
The same plate is now experiencing uniform irradiation on both the upper and lower surfaces. The ambient air temperature surrounding the plate is 290K with a convection heat transfer coefficient of 30 W/(m2K). Both upper and lower surfaces of the plate have a radiosity of 4000 W/m2, and the plate surface temperature is unchanged. Determine
c) the irradiation of the plate (Answers: 16,300 W/m2)
2. Consider a surface exposed to solar radiation. At a given time, the direct and diffuse components of solar radiation are GD = 400 and Gd = 300 W/m2, and the direct radiation maes a 20 angle with the normal of the surface. The surface temperature is 320K at the time. Assuming an effective sky temperature of 260K, the ambient temperature is 300K, determine the net rate of radiation heat transfer for these cases:
a) s = 0.9, = 0.9 (gray absorber; Answer = 307 W/m2)
b) s = 0.1, = 0.1 (gray reflector; Answer = 34 W/m2)
c) s = 0.9, = 0.1 (selective absorber; Answer = 575 W/m2)
d) s = 0.1, = 0.9 (selective reflector; Answer = -234 W/m2)
e) Explain the meaning of the terms in a-d and how they related to the calculated values of net radiation rates.
f) If we do not neglect convection, and the convective heat transfer coefficient is 20 W/m2K, is this significant in comparison to the net radiative rate? (Answer = 400 W/m2, significant!)
3. Consider a 5-m x 5-m x 5-m cubical furnace whose surfaces approximate black surfaces. The base, top and sides are maintained at 800K, 1500K and 500K, respectively. Determine
a) View factors for the problem
b) net rate of radiant heat transfer from the base to side surfaces (Answer = 394 kW)
c) net rate of radiant heat transfer from the base to the top surface (Answer = -1319 kW)
d) net rate of radiant heat transfer from the base surface (Answer = -925 kW)
4. Consider a cylindrical furnace with r0 = H = 1m. The top and base surfaces have e of 0.8 and 0.4, respectively, and are maintained at T = 700K and T = 500K. The side surface approximates a blackbody with temperature T = 400K. Determine
a) View factors for the problem
b) Net rate of radiation heat transfer at each surface
c) Confirm how these surfaces can be maintained at specified temperatures.
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