Heat Transfer Problems

ME 341 HEAT TRANSFER FOR ENGINEERING Exam 2

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2) A thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are shielded and welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created that is correlated to temperature (See Seebeck-Peltier Effect for the operational principle). As the engineer on the job, you are tasked to measure the temperature of a gas stream as accurately as possible using a thermocouple whose junction can be approximated as a 1.2 mm diameter sphere (Lc=V/A). The spherical junction has the following properties k = 35 W/m-K, U = 8500 kg/m3, and cp = 320 J/kg-K, and the convection heat transfer coefficient between the junction and the gas is h = 210 W/m2-K. a) Determine the time it will take for the thermocouple junction to reach a

temperature within 1% of the gas stream temperature to be measured. b) Provide an Excel plot of this time as a function of the junction diameter

(Suggestion: vary the diameter as follows: 0.6 mm; 0.9 mm; 1.5 mm; and 1.8 mm). Comment on your findings. (35 points)