| 14| Revision Questions

Free Convection Questions

1.     Consider a 0.25-m long vertical plate that is at 70°C. The plate is suspended in air that is at 25°C. Estimate the boundary layer thickness at the trailing edge of the plate if the air is quiescent. How does this thickness compare with that which would exist if the air were flowing over the plate at a free stream velocity of 5m/s. Pr = 0.7

[dL,quiescent= 0.024m, dL,5m/s = 0.0047m]

2.     A glass-door fire screen, used to reduce exfiltration of room air through a chimney, has a height of 0.71m and a width of 1.02m and reaches a temperature of 232°C. If the room temperature is 23°C, estimate the convection heat rate from the fireplace to the room. Pr = 0.69

[q = 1060 W]

3.     Airflow through a long rectangular heating duct that is 0.75m wide and 0.3 m high maintains the outer duct surface at 45°C. If the duct is uninsulated and exposed to air at 15°C in the crawlspace beneath a home, what is the heat loss from the duct per meter of length. Pr = 0.71

[q’ = 246 W/m]

4.     A long tube of 0.1m diameter is maintained at 120°C by passing steam through its interior. A radiation shield is installed concentric to the tube with an air gap of 10 mm. If the shield is at 35°C, estimate the heat transfer by free convection from the tube per unit length. What is the heat loss if the space between the tube and the shield is filled with glass-fibre blanket insulation?

[q’ = 100 W/m, q’insulated = 111 W/m]

Worked solutions to these questions can be found as a downloadable file at the bottom of this page. 

For some useful nomenclature and important equations , click the link below.

|15| Helpful Nomenclature and Equations

Or t0 test yourself further, try this Quick Quiz.