A long, cylindrical, electrical heating element of diameter 10 mm, thermal conductivity 240 W/m·K, density 2700 kg/m3, and speci
fic heat 900 J/kg·K is installed in a duct for which air moves in cross flow over the heater at a temperature and velocity of 27°C and 15 m/s, respectively. (a) Neglecting radiation, estimate the steady-state surface temperature when, per unit length of the heater, electrical energy is being dissipated at a rate of 2000 W/m. (b) If the heater is activated from an initial temperature of 27°C, estimate the time required for the surface temperature to come within 10°C of its steady-state value. Evaluate the properties of air at 450 K.
1 answer:
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Answer:d
Explanation:
Given
Temperature
Also
R=287 J/kg
Flow will be In-compressible when Mach no.<0.32
Mach no.
(a)
Mach no.
Mach no.=0.63
(b)
Mach no.
Mach no.=0.31
(c)
Mach no.
Mach no.=1.27
(d)
Mach no.
Mach no.=0.127
From above results it is clear that for Flow at velocity 200 km/h ,it will be incompressible.