Consider the following two statements and then select the option below that is correct
i. It is possible for an object to move in the absence of forces acting on the object.
ii. It is possible to have forces acting on an object in the absence of motion of the object.
1.The second statement is the only correct one.
2.Both statements are correct.
3.Neither statement is correct.
4.The first statement is the only correct one.
2
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The temperature of air flowing through a 25-cm-diameter duct whose inner walls are at 320°C is to be measured with a thermocouple soldered in a cylindrical steel wall of 1.2-cm-OD with an oxidized exterior, as shown in the accompanying sketch. The air flows normal to the cylinder at a mass velocity of 17,600 kg/(h m2). If the temperature indicated by the thermocouple is 200°C, estimate the actual temperature of the air.
GIVEN
• Cylindrical thermocouple wall in an air duct
• Duct diameter (Dd) = 25 cm = 0.25 m
• Duct wall temperature (Tds) = 320°C =m 593 K
• Wall outside diameter (Dw) = 1.2 cm = 0.012 m
• Exterior of wall is oxidized
• Air mass velocity /m A
= 17,600 kg/(h m2)
• Thermocouple indicated temperature (Ttc) = 200°C = 473 K
FIND
• Air temperature (T?) ASSUMPTIONS
• Steady state
• Thermal resistance between the thermocouple and the wall exterior surface is negligible
• Inside of duct behaves as a black body enclosure
• Conduction to the thermocouple wall from the duct wall can be neglected
PROPERTIES AND CONSTANTS
the emissivity of oxidized steel (?) = 0.94. From Appendix 1, Table 5, the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
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