1.25 A common procedure for measuring the velocity of anairstream involves the insertion of an electrically heatedwire (called a hot-wire anemometer) into the airflow,with the axis of the wire oriented perpendicular to theflow direction. The electrical energy dissipated inthe wire is assumed to be transferred to the air by forcedconvection. Hence, for a prescribed electrical power, thetemperature of the wire depends on the convection coef-ficient, which, in turn, depends on the velocity of the air.Consider a wire of length L = 20 mm and diameterD = 0.5 mm, for which a calibration of the formV = 6.25 × 10-5 h² has been determined. The velocity Vand the convection coefficient h have units of m/s andW/m².K, respectively. In an application involving air ata temperature of T = 25°C, the surface temperature ofthe anemometer is maintained at T₁ 75°C with a volt-age drop of 5 V and an electric current of 0.1 A. What isthe velocity of the air?=

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Engineering

Mechanical Engineering

age drop of 5 V and an electric current of 0.1 A. What is the velocity of the air?

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter8: Natural Convection

Section: Chapter Questions

Problem 8.9P

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Question

100%

1.25 A common procedure for measuring the velocity of an
airstream involves the insertion of an electrically heated
wire (called a hot-wire anemometer) into the airflow,
with the axis of the wire oriented perpendicular to the
flow direction. The electrical energy dissipated in
the wire is assumed to be transferred to the air by forced
convection. Hence, for a prescribed electrical power, the
temperature of the wire depends on the convection coef-
ficient, which, in turn, depends on the velocity of the air.
Consider a wire of length L = 20 mm and diameter
D = 0.5 mm, for which a calibration of the form
V = 6.25 × 10-5 h² has been determined. The velocity V
and the convection coefficient h have units of m/s and
W/m².K, respectively. In an application involving air at
a temperature of T = 25°C, the surface temperature of
the anemometer is maintained at T₁ 75°C with a volt-
age drop of 5 V and an electric current of 0.1 A. What is
the velocity of the air?
=

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