The heat loss from the hairdryer duct enclosure was neglected. To a designer, while this may appear to be reasonable assumption it should really be checked bu order of magnitude calculations even if all the parameters are not known. In order to determine whether the heat loss from the exterior of the duct enclosure is neglected or not, assume a high surface emissivity of 0.8, an average surface temperature of 400C, and consider an estimate for natural convection coefficient at a high reasonable value. If the length of hair dryer is 20 cm, calculate the expected heat loss per unit length of the dryer duct, compare it with heat dissipation per unit length from the internal electrical heating, and comment on validity of negligible heat loss assumption.
GIVEN
- High surface emissivity of 0.8, an average surface temperature of 400C
- Length of the hear drier L=20 cm =0.2 m
FIND
- Expected heat loss per unit length of dryer duct
- Compare it with heat dissipation per unit length from the internal electrical heating.
ASSUMPTIONS
- Steady state conditions prevail
- Heat loss from outside of dryer duct is neglected.
PROPERTIES AND CONSTANTS
the air properties are
Convection heat transfer coefficient for air (considering higher value) (hc)=20 W/(m2 K)
Specific heat capacity(Cp)= 1.0049 kJ/kgK
Heat radiation from body grey body of average temperature of 400C=313 K to ambiance of 220C
Heat transferred by natural convection is given by
Heat dissipation per unit length of dryer from internal electrical heating is given by
Q/L=110.5/0.2= 552.2 W/m
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