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Unformatted text preview: Chapter 12 Radiation Heat Transfer Review Problems 1288 The temperature of air in a duct is measured by a thermocouple. The radiation effect on the temperature measurement is to be quantified, and the actual air temperature is to be determined. Assumptions The surfaces are opaque, diffuse, and gray. Properties The emissivity of thermocouple is given to be =0.6. Analysis The actual temperature of the air can be determined from K 1111 =  + = + = C W/m 60 ] ) K 500 ( ) K 850 )[( K W/m 10 67 . 5 )( 6 . ( K 850 ) ( 2 4 4 4 2 8 4 4 h T T T T w th th th f 1289 The temperature of hot gases in a duct is measured by a thermocouple. The actual temperature of the gas is to be determined, and compared with that without a radiation shield. Assumptions The surfaces are opaque, diffuse, and gray. Properties The emissivity of the thermocouple is given to be =0.7. Analysis Assuming the area of the shield to be very close to the sensor of the thermometer, the radiation heat transfer from the sensor is determined from 2 4 4 4 2 8 2 1 4 2 4 1 sensor from rad, W/m 9 . 257 1 15 . 1 2 1 7 . 1 ] ) K 380 ( ) K 530 )[( K W/m 10 67 . 5 ( 1 1 2 1 1 ) ( =  +  =  +   = T T Q Then the actual temperature of the gas can be determined from a heat transfer balance to be K 532 = = = = f f th f T T T T h q q 2 2 2 sensor from conv, sensor to conv, W/m 9 . 257 ) 530 C( W/m 120 W/m 9 . 257 ) ( Without the shield the temperature of the gas would be K 549.2 =  + = + = C W/m 120 ] ) K 380 ( ) K 530 )[( K W/m 10 67 . 5 )( 7 . ( K 530 ) ( 2 4 4 4 2 8 4 4 h T T T T w th th th f 1268 Air, T f T w = 500 K Thermocouple T th = 850 K = 0.6 Air, T f T w = 380 K Thermocouple T th = 530 K 1 = 0.7 = 0.15 Chapter 12 Radiation Heat Transfer 1290E A sealed electronic box is placed in a vacuum chamber. The highest temperature at which the surrounding surfaces must be kept if this box is cooled by radiation alone is to be determined. Assumptions 1 Steady operating conditions exist 2 The surfaces are opaque, diffuse, and gray. 3 Convection heat transfer is not considered. 4 Heat transfer from the bottom surface of the box is negligible. Properties The emissivity of the outer surface of the box is = 0.95. Analysis The total surface area is 2 ft 67 . 3 ) 1 1 ( ) 12 / 1 8 ( 4 = + = s A Then the temperature of the surrounding surfaces is determined to be F 43 = =  =  = R 503 ] ) R 590 )[( R Btu/h.ft 10 1714 . )( m 67 . 3 )( 95 . ( Btu/h ) 41214 . 3 100 ( ) ( 4 4 4 2 8 2 4 4 surr surr surr s s rad T T T T A Q 1291 A doublewalled spherical tank is used to store iced water. The air space between the two walls is evacuated. The rate of heat transfer to the iced water and the amount of ice that melts a 24h period are to be determined....
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This note was uploaded on 08/25/2009 for the course AET AET432 taught by Professor Rajadas during the Spring '06 term at ASU.
 Spring '06
 Rajadas

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