Consider the electrical space heater, where the electric resistance heating coil is enclosed in a straight horizontal polished metallic cylinder that has an outer diameter of 1 cm and an emissivity of 0.9. If its surface temperature is at 110°C and the room air temperature is at 20°C, calculate the total heat loss per unit length of the cylinder due to both convection and radiation. For the purpose of this calculation, assume that the reflector is so designed that it “collects” all of the radiant heat from the back half of the horizontal heater and that it reflects 95% of the incident radiation.
GIVEN
• An electric resistance heating coil enclosed in horizontal metallic cylinder.
• Emissivity (?) = 0.9
• Diameter of cylinder (D) = 1 cm= 0.01 m
• Surface temperature (Ts) = 110°C= 383 K
• Ambient air temperature (T?) = 20°C= 293 K
• Reflects 95% of incident radiation.
FIND
Total heat loss per unit length of cylinder due to convection and radiation.
ASSUMPTIONS
• Air is still
• Surface temperature is uniform and constant
PROPERTIES AND CONSTANTS
for dry air at the mean temperature of 65°C
Thermal expansion coefficient (?) = 0.00296 1/K
Thermal conductivity (k) = 0.02825 W/(m K)
Kinematic viscosity (?) = 19.92 × 10–6 m2/s
Prandtl number (Pr) = 0.71
The Grashof number is
we have
Thus total heat loss per unit length by both convection and radiation is given by
You might also like to view...
What is binary diffusion?
What will be an ideal response?
What is true about Antares?
A: name means "Rival of Mars" B: ˜700 times the size of the Sun C: possibly egg-shaped D: cooler than the Sun E: all of the above
As you go from a point where a gravitational field is strong to a point where the gravitational field gets weaker, the gravitational field lines get farther apart
a. True b. False Indicate whether the statement is true or false
You ride on an elevator that is moving with constant upward acceleration while standing on a bathroom scale. The reading on the scale is
A) equal to your true weight, mg. B) more than your true weight, mg. C) less than your true weight, mg. D) zero. E) could be more or less than your true weight, mg, depending on the magnitude of the acceleration.