2. (20 pts) Hot wire anemometers are devices used to measure flow velocity. These devices have...
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2. (20 pts) Hot wire anemometers are devices used to measure flow velocity. These devices have a thin heated wire filament in the flow and a second unheated sensor to measure the flow temperature. The flow speed can be determined based on the amount of electrical power required to keep the heated filament at a specific temperature above the bulk flow temperature. 4115 123 Use the following information for this problem: The heated filament is tungsten (electrical resistivity pe = 6.46 10-8 m) with diameter D = 10 m and length L = 7.5 mm. The anemometer is operated such that the filament is always maintained 40C above To = 20C. The filament voltage (E) is measured and mapped to known free-stream velocities. a. (8 pts) Calculate the required filament voltage (E) when the free stream velocity is 5 m s. Tip: At steady state, the filament cooling rate by convection equals the power provided to the anemometer, i.e., q conv=Rel. Also, E = Relc and Re = PeL/Ac where Ac is the cross- sectional area of the anemometer. b. (6 pts) Generate a plot of E vs the free stream velocity from 0.1 to 10 m s. c. (6 pts) Would radiation heat transfer from the filament ever become significant, or can the heat transfer always be modeled as convective? 2. (20 pts) Hot wire anemometers are devices used to measure flow velocity. These devices have a thin heated wire filament in the flow and a second unheated sensor to measure the flow temperature. The flow speed can be determined based on the amount of electrical power required to keep the heated filament at a specific temperature above the bulk flow temperature. 4115 123 Use the following information for this problem: The heated filament is tungsten (electrical resistivity pe = 6.46 10-8 m) with diameter D = 10 m and length L = 7.5 mm. The anemometer is operated such that the filament is always maintained 40C above To = 20C. The filament voltage (E) is measured and mapped to known free-stream velocities. a. (8 pts) Calculate the required filament voltage (E) when the free stream velocity is 5 m s. Tip: At steady state, the filament cooling rate by convection equals the power provided to the anemometer, i.e., q conv=Rel. Also, E = Relc and Re = PeL/Ac where Ac is the cross- sectional area of the anemometer. b. (6 pts) Generate a plot of E vs the free stream velocity from 0.1 to 10 m s. c. (6 pts) Would radiation heat transfer from the filament ever become significant, or can the heat transfer always be modeled as convective?
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Related Book For
Fundamentals Of Momentum Heat And Mass Transfer
ISBN: 9781118947463
6th Edition
Authors: James Welty, Gregory L. Rorrer, David G. Foster
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