The temperature of an air stream flowing is measured by a copper – constantan thermocouple which may be approximated as a cylinder of 1.7 mm diameter. Initially the junction and air at a temperature of 25 oC. The air temperature suddenly changes to and is maintained at 215 oC. The thermal junction properties are: ρ = 8750 kg/m3 ; Cp = 380 J/kgoC; k(thermocouple) = 28W/moC; h = 145 W/m2 - oC. a. Determine the time required for the thermocouple to indicate temperature of 165 oC. Also, determine the thermal time constant and the temperature indicated by the thermocouple at that instant. b. Discuss the stability of this thermocouple to measure unsteady state temperature of a fluid when the temperature variation in the fluid has a time period of 3.6 s
The temperature of an air stream flowing is measured by a copper – constantan
thermocouple which may be approximated as a cylinder of 1.7 mm diameter. Initially
the junction and air at a temperature of 25 oC. The air temperature suddenly changes
to and is maintained at 215 oC. The thermal junction properties are: ρ = 8750 kg/m3
;
Cp = 380 J/kgoC; k(thermocouple) = 28W/moC; h = 145 W/m2
-
oC.
a. Determine the time required for the thermocouple to indicate temperature of 165
oC. Also, determine the thermal time constant and the temperature indicated by
the thermocouple at that instant.
b. Discuss the stability of this thermocouple to measure unsteady state temperature
of a fluid when the temperature variation in the fluid has a time period of 3.6 s.
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