1. A thermocouple junction, which may be approximated as sphere, is to be used for temperature...

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1. A thermocouple junction, which may be approximated as sphere, is to be used for temperature measurement in a gas stream. The convection coefficient between the junction surface and the gas is h = 400 W/m.K and the junction thermophysical properties are k = 20 W/m K, c = 400 J/kg-K and p = 8500 kg/m. Determine the junction diameter needed for the thermocouple to have a time constant of 2 s. If the junction is at 25 C and is placed in a gas stream that is at 200 C, estimate the temperature of the junction after 10 minutes. ASSUMPTIONS: 1. Temperature of junction is uniform at any instant 2. Radiation exchange with the surroundings is negligible 3. Losses by conduction through the leads are negligible 4. Constant properties T_=200C h=400 W/mK Gas stream Leads HD Thermocouple junction T= 25C k= 20 W/m-K =400 J/kg-K p== 8500 kg/m 1. A thermocouple junction, which may be approximated as sphere, is to be used for temperature measurement in a gas stream. The convection coefficient between the junction surface and the gas is h = 400 W/m.K and the junction thermophysical properties are k = 20 W/m K, c = 400 J/kg-K and p = 8500 kg/m. Determine the junction diameter needed for the thermocouple to have a time constant of 2 s. If the junction is at 25 C and is placed in a gas stream that is at 200 C, estimate the temperature of the junction after 10 minutes. ASSUMPTIONS: 1. Temperature of junction is uniform at any instant 2. Radiation exchange with the surroundings is negligible 3. Losses by conduction through the leads are negligible 4. Constant properties T_=200C h=400 W/mK Gas stream Leads HD Thermocouple junction T= 25C k= 20 W/m-K =400 J/kg-K p== 8500 kg/m

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