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 the flow 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 coefficient, 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 h 2 has been determined. The velocity Vand the convection coefficient h have units of m/s and W/m 2 ⋅ K , respectively. In an application involving air ata temperature of T. = 25°C, the surface temperature ofthe anemometer is maintained at T ∞ = 75 ° C with a voltage drop of 5 V and an electric current of 0.1 A. What isthe velocity of the air?
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 the flow 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 coefficient, 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 h 2 has been determined. The velocity Vand the convection coefficient h have units of m/s and W/m 2 ⋅ K , respectively. In an application involving air ata temperature of T. = 25°C, the surface temperature ofthe anemometer is maintained at T ∞ = 75 ° C with a voltage drop of 5 V and an electric current of 0.1 A. What isthe velocity of the air?
Solution Summary: The author calculates the velocity of air, the temperature of anemometer, and the rate of heat transfer by forced convection.
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 the flow 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 coefficient, 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
h
2
has been determined. The velocity Vand the convection coefficient h have units of m/s and
W/m
2
⋅
K
, respectively. In an application involving air ata temperature of T. = 25°C, the surface temperature ofthe anemometer is maintained at
T
∞
=
75
°
C
with a voltage drop of 5 V and an electric current of 0.1 A. What isthe velocity of the air?
Air at atmospheric pressure and 25oC is heated in a tube with an ID of 25 mm. The tube wall is maintained at a temperature of 200oC. What length of tube is required to achieve an outlet air temperature of 120oC if the air flow is 10 Nm3/hr?
The flow velocity of air over a flat plate of dimension 1160 ×610 ×25 mm at 30 OC is 3.2 m/s.
The top surface of the plate is maintained at 110 OC. Calculate the bottom temperature of the plate
at steady state.
Oil flow in a journal bearing can be treated as parallel flow between two large isothermal plates with one plate moving at a constant velocity of 8 m/s and the other stationary. Consider such a flow with a uniform spacing of 0.7 mm between the plates. The temperatures of the upper and lower plates are 40°C and 15°C, respectively. By simplifying and solving the continuity, momentum, and energy equations, determine (a) the velocity and temperature distributions in the oil, (b) the maximum temperature and where it occurs, and (c) the heat flux from the oil to each plate.
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