HW4.docx - 4–27 The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a 1.2-mm-diameter sphere The

HW4.docx - 4–27 The temperature of a gas stream is to be...

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4–27 The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a 1.2-mm-diameter sphere. The properties of the junction are k= 35 W/m·K, = 8500 kg/m 3 , and c p = 320 J/kg·K, and the heat transfer coefficient between the junction and the gas is h = 90 W/m 2 ·K. Determine how long it will take for the thermocouple to read 99 percent of the initial temperature difference. Answer: 27.8 s 4–29 A thermocouple, with a spherical junction diameter of 0.5 mm, is used for measuring the temperature of hot air flow in a circular duct. The convection heat transfer coefficient of the air flow can be related with the diameter ( D ) of the duct and the average air flow velocity ( V ) as h = 2.2( V / D ) 0.5 , where D , h , and V are in m, W/m 2 ·K and m/s, respectively. The properties of the
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