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Frequently Asked Questions and Problems for Practice (Module 5)
Q.1  Define Grashof number and its physical significance. What is the analogous in forced convection? 
Q.2  What are the criteria to know natural and forced convection? 
Q.3  Define Rayleigh number and its physical significance. 
Q.4  Show the flow pattern of natural convection for a (a) hot surface down, (b) hot surface up, (c) cold surface down, and (d) cold surface up. 
Q.5  Warm air at 65^{o}C, 3 m/s enters into a square (25 cm) duct made up of steel metal. The uninsulated duct is 10 m long and is in contact with the atmospheric air at 25^{o}C. If the heat losses to the atmosphere by natural convection then what will be the exit warm air temperature from the duct. It may be assumed that the natural convective flow at one surface does not affect the others as the two surfaces are horizontal, one at the top and at the bottom, and the two surfaces are vertical. Calculate the heat transfer coefficient for the obviously forced convection inside of the duct. 
Q.6.  A horizontal tube having diameter of 10 mm is heated to a surface temperature of 240 ^{o}C. The tube is exposed to air at 25^{o}C. Calculate the natural convective heat transfer. 
Q.7.  A sphere having diameter of 25 cm is heated to a surface temperature of 240 ^{o}C. The sphere is exposed to air at 25^{o}C. Calculate the natural convective heat transfer. 
Q.8.  A square (1 m) flat plate inclined at 45 ^{o}C with the horizontal is exposed to air at 25^{o}C and 1 atm. The plate receives 750 W/m^{2} from the Sun and dissipated the heat to the atmosphere by natural convection. What will be the average temperature of the plate at steadystate? 
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