Natural convection
1. Consider a flat-plate solar collector placed horizontally on the flat roof of a house. The
collector is 1.5 m wide and 6 m long, and the average temperature of the exposed surface
of the collector is 42°C. Determine the rate of heat loss from the collector by natural
convection during a calm day when the ambient air temperature is 15°C. Also, determine
the heat loss by radiation by taking the emissivity of the collector surface to be 0.9 and the
effective sky temperature to be -30°C.
2. Flat-plate solar collectors are often tilted up toward the sun in order to intercept a greater
amount of direct solar radiation. The tilt angle from the horizontal also affects the rate of
heat loss from the collector. Consider a 2-m-high and 3-m wide solar collector that is
tilted at an angle  from the horizontal. The back side of the absorber is heavily insulated.
The absorber plate and the glass cover, which are spaced 2.5 cm from each other, are
maintained at temperatures of 80°C and 40°C, respectively. Determine the rate of heat
loss from the absorber plate by natural convection for  = 0°, 20°, and 90°.
3. Consider two concentric horizontal cylinders of diameters 55 cm and 65 cm, and length
125 cm. The surfaces of the inner and outer cylinders are maintained at 46°C and 74°C,
respectively. Determine the rate of heat transfer between the cylinders by natural
convection if the annular space is filled with (a) water and (b) air.
4. Consider a wall-mounted power transistor that dissipates 0.18 W of power in an
environment at 35°C. The transistor is 0.45 cm long and has a diameter of 0.4 cm. The
emissivity of the outer surface of the transistor is 0.1, and the average temperature of the
surrounding surfaces is 25°C. Disregarding any heat transfer from the base surface,
determine the surface temperature of the transistor. Use air properties at 100°C.