Question

A spherical thermocouple junction 1.0 mm in diameter is inserted in a combustion chamber to measure the temperature ?T? of the products of combustion. The hot gases have a velocity of V= 5 m/s. (a) If the thermocouple is at room temperature, Ti,when it is inserted in the chamber, estimate the time required for the temperature difference, ?T?-T, to reach 2% of the initial temperature difference, ?T?-Ti. Neglect radiation and conduction through the leads. Properties of the thermocouple junction are approximated as k= 100 W/m·K, c=385 J/kg·K, and ?=8920 kg/m³, while those of the combustion gases may be approximated as k=0.05 W/m·K, ?=50 x 10^-6 m²/s, and Pr=0.69.

(b) If the thermocouple junction has an emissivity of 0.1 and the cooled walls of the combustor are at Tc=400 K, what is the steady-state temperature of the thermocouple junction if the combustion gases are at 1000 K? Conduction through the lead wires may be neglected.

Assuming that the gas properties are constant, what is the value of the Nusselt number?

What is the average convection heat transfer coefficient for the gas flowing over the thermocouple, in W/m²·K?

What is the Biot number for this problem?

How long does it take for the temperature difference (?T?-T) to get within 2% of the initial temperature difference (?T?-Ti), in s?

What is the final steady state temperature of the thermocouple, in K, if the combustion gases are at 1000 K?

Answer 1