Radiation

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1 HEAT TRANSFER & THERMAL ANALYSIS 2011-12 RADIATION-1 Problem 1 Consider an opaque horizontal plate that is well insulated on its back side. The irradiation on the plate is , of which is reflected. The plate is at 227 °C and has an emissive power of . Air at 127 °C flows over the plate with a heat transfer coefficient of . Determine the emissivity, absorptivity, and radiosity of the plate. What is the net heat transfer rate per unit area? Solution For an opaque surface
= 0, so J Net heat transfer 2 Problem 2 A small surface of area is known to emit diffusely, and from measurements the total intensity associate with emission in the normal direction is . Radiation emitted from the surface is intercepted by three other surfaces of area , which are 0.5 m from A1 and are oriented as shown. Calculate 1. the intensity of emission in each of the three directions; 2. the solid angles subtended by the three surfaces; 3. the rate at which radiation is intercepted by the three surfaces 4. the irradiation , at each of the three surfaces. EXAMPLE12.1 A small surface of area A1/H1100510/H110023m2is known to emit diffusely, and from measurements the total intensity associated with emission in the normal direction is In/H110057000 W/m2/H18528sr. Radiation emitted from the surface is intercepted by three other surfaces of area A2/H11005A3/H11005A4/H1100510/H110023m2, which are 0.5 m from A1and are oriented as shown. What is the intensity associated with emission in each of the three directions? What are the solid angles subtended by the three surfaces when viewed from A1? What is the rate at which radiation emitted by A1is intercepted by the three surfaces? SOLUTION Known:Normal intensity of diffuse emitter of area A1and orientation of three surfaces relative to A1. Find: 1.Intensity…