The proposed design for an anemometer to determinethe

Chapter 7, Problem 7.23

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The proposed design for an anemometer to determinethe velocity of an airstream in a wind tunnel is com-prised of a thin metallic strip whose ends are supportedby stiff rods serving as electrodes for passage of currentused to heat the strip. A fine-wire thermocouple isattached to the trailing edge of the strip and serves asthe sensor for a system that controls the power to main-tain the strip at a constant operating temperature forvariable airstream velocities. Design conditions pertainto an airstream at T??25?C and 1u?50 m/s,with a strip temperature of Ts?35?C.(a) Determine the relationship between the electricalpower dissipation per unit width of the strip in thetransverse direction, P?(mW/mm), and the airstreamvelocity. Show this relationship graphically for thespecified range of u?.(b) If the accuracy with which the temperature of theoperating strip can be measured and maintainedconstant is?0.2?C, what is the uncertainty in theairstream velocity? (c) The proposed design operates in a strip constant-temperature mode for which the airstream velocityis related to the measured power. Consider nowan alternative mode wherein the strip is providedwith a constant power, say, 30 mW/mm, and theairstream velocity is related to the measured striptemperature Ts. For this mode of operation, showthe graphical relationship between the strip temper-ature and airstream velocity. If the temperature canbe measured with an uncertainty of?0.2?C, whatis the uncertainty in the airstream velocity?(d) Compare the features associated with each of theanemometer operating modes.

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