1. In many cases, we are concerned with free convection involving gases that are contained within sealed enclo- sures. Consider air at 27°C and pressures of 1, 10, and 100 bars. Determine the gure of meritdescribed in Problem 9.4 for each of these three pressures. Which air pressure will provide the most effective cooling? Hint: See Problem 6.22.
2. The heat transfer rate due to free convection from a vertical surface, 1 m high and 0.6 m wide, to quiescent air that is 20 K colder than the surface is known. What is the ratio of the heat transfer rate for that situation to the rate corresponding to a vertical surface, 0.6 m high and 1 m wide, when the quiescent air is 20 K warmer than the surface? Neglect heat transfer by radiation and any influence of temperature on the relevant thermo- physical properties of air.