Mapua Institute of Technology School of Chemical Engineering and ChemistryHeat and Mass Transfer Exam No 3

CHHK 445: HEAT AND MASS TRANSFERExam No. 3: Radiation1.5 hrs

I. Multiple Choice

Direction: Write the letter that corresponds to the best answer on the space provided. Answers must be written in CAPITAL letters.

______1. The emissivity of a gray body is alwaysA. 1.0. C. < 1.0.B. >1.0. D. 1.0 > 10.

______2. Which of the following is true of a black body?A. It absorbs all the radiant energy falling upon it.B. It reflects all the radiant energy falling upon it.C. It transmits all the radiant energy falling upon it.D. A fraction of the radiant energy is being reflected, absorbed, and transmitted by the body.

______3. At thermal equilibrium, the emissivity of a body is equal to its absorptivity. A. True B. False

______4. The exchange of radiation between two surfaces is dependent on A. size the surfaces. C. relative orientation of the surfaces.B. shape of the surfaces. D. all of these.

______5. For radiation with two infinite parallel planes, the view factor is A. 1.0. C. < 1.0B. > 1.0. D. indeterminate.

______6. The energy radiated to a surface, q, at absolute temperature is related as A. q T, C. q T4.B. q T2. D. q T3.

______7. The loss of heat from unlagged steam pipe to the ambient air is byA. conduction C. radiationB. convection. D. conduction, convection, and radiation.

______8. Which of the following holds true for opaque bodies?A. absorptivity = reflectivity C. absorptivity + reflectivity = 1.0B. absorptivity + reflectivity = 0 D. emissivity = 1.0

______9. When two planes are connected by nonconducting but reradiating walls, the fraction of radiation intercepted by surface 2 from 1 A. increases. C. remains the same.B. decreases. D. cannot be defined.

______10. The transfer of heat from a surface by radiation will always be accompanied by convection unless the surface is A. in an isolated space. C. in a very large surrounding. B. in a very hot oven. D. in a vacuum.

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Mapua Institute of Technology School of Chemical Engineering and ChemistryHeat and Mass Transfer Exam No 3

II. Problem Solving

Direction: Solve the following problems on your test booklet. Highlight the final answers.

1. Two parallel rectangles measuring 1.2 m wide and 2.4 m high are 0.6 m apart are connected by reradiating walls. Plate 1 is maintained at 1000 K and the other at 420 K. Determine radiant energy interchange between the two plates if the plates are gray with emissivities of 0.6 and 0.90, respectively.

2. A black solar collector, with a surface area of 60 m2, is placed on the roof of a house. Incident solar energy reaches the collector with a flux of 800 W/m2. The surroundings are considered black with an effective temperature of 30C. The convective heat-transfer coefficient between the collector and the surrounding air, at 30C is 35 W/m2-K. Neglecting any conductive loss from the collector, determine the equilibrium temperature of the collector.

3. A small metal test specimen (disk, d= ¼ in) is suspended by very fine wires in a very large evacuated tube. The metal is maintained at a temperature of 2500F, at which temperature it has an emissivity of approximately 0.20. The water-cooled walls and ends of the tube are maintained at 50F. Determine the total net heat transfer rate by radiation from the test specimen.

4. Two plane disks each 1.25 m in diameter are parallel and directly opposed to each other. They are separated by a distance of 0.5 m. Disk 1 is heated by electrical resistance to 833 K. Both disks are insulated on all faces except the two faces directly oppose to each other. Assume that the surroundings emit no radiation and that the disks are in space. Calculate the temperature of disk 2 at steady state and also the electrical energy input to disk 1.

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