# Radiation Contributions by: Janet deGrazia

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Radiation Contributions by: Janet deGrazia
Department of Chemical and Biological Engineering University of Colorado Boulder, CO

There is a long heated duct that has a semi-circle cross-sectional geometry. The view factor (F21) of the curved surface (2) to the flat surface (1) is_________. 0.36 0.5 0.64 1 2 ANSWER: B. 0.36 The view factor between two surfaces can be determined by comparing surface areas. The surface area (A) times the view factor (F) is preserved across surfaces. So A2*F21 = A1*F12 where A2 is the surface area of 2, F21 is the view factor of 2 to 1, etc. Since A2 is greater than A1, F21 will need to be less than F12. That leaves A and B as possible answer. Since there are two surfaces, it cannot be A or E. 1

There is a long heated duct that has a semi-circle cross-sectional geometry. The view factor (F12) of the flat surface (1) to the curved surface (2) is_________. 0.36 0.5 0.64 1 2 ANSWER: D. 0.64 The view factor between two surfaces can be determined by comparing surface areas. The surface area (A) times the view factor (F) is preserved across surfaces. So A2*F21 = A1*F12 where A2 is the surface are of 2, F21 is the view factor of 2 to 1, etc. Since A2 is greater than A1, F12 will need to be greater than F21. That leaves D and E as possible answer. Since there are two surfaces, it cannot be A or E. 1

Two infinitely long parallel disks are in a vacuum
Two infinitely long parallel disks are in a vacuum. Disk 1 is a black body maintained at 25 °C. Disk 2 is diffuse and opaque and also maintained at 25 °C. The net radiation leaving disk 1 is _________ than that leaving disk 2. Disk 1 Disk 2 greater than less than the same as Not enough information ANSWER: A. greater than Disk 1 is a blackbody. No other source can emit as much radiation. While disk 1 will absorb all incident radiation from disk 2, it will only be a portion of disk 2 total emission.

Two infinitely long parallel disks are in a vacuum
Two infinitely long parallel disks are in a vacuum. Disk 1 is a black body maintained at 25 °C. Disk 2 is diffuse and opaque and also maintained at 25 °C. With respect to disk 1, the net radiation exchange between the disks is _________ zero. greater than less than equal to Not enough information Disk 1 Disk 2 ANSWER: B. less than Disk 1 will emit much more radiation to disk 2 than disk 2 emits back to disk 1. Black body emitters will emit much more radiation than an opaque body. While disk 1 will also absorb incident radiation from disk 2, it will be only a portion of disk 2 total emission.

A large hollow tube, S1, encloses a smaller solid tube, S2
A large hollow tube, S1, encloses a smaller solid tube, S2. Which statement is true? Both are at the same temperature and diffuse emitters. Which statement is true? The net radiation leaving S2 will be higher than that reabsorbed into S2. The view factor of S2 to itself will be greater than S1 to itself. The view factor of S1 to S2 will be greater than S2 to S1. All are true radiation S2 S1 ANSWER: D. All are true. Since S2 is a diffuse emitter, only a portion of the radiation emitted will be directed to some other section of S2, so A is correct. The view factor of S1 is 0 since all of the radiation leaving S1 goes to S2 so both B and C are correct.

Which of the following shows the spectral transmissivity for a semi-transparent diffuse material?
B ANSWER: A. For a semi-transparent material, the absorptivity plus the reflectivity plus the transmissivity = 1.

A B D C ANSWER: A. For an opaque material, there is no transmissivity. , the absorptivity plus the reflectivity plus the transmissivity = 1. Which of the following shows the spectral transmissivity for an opaque diffuse material?