# Tutorial 5 Convection

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TUTORIAL 5 : CONVECTION 1   What is forced convection? How does it differ from natural convection? Is convection caused by winds forced or natural convection? 2   What is external forced convection? How does it differ from internal forced convection? Can a heat transfer system involve both internal and external convection at the same time? Give an example 3   In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why? 4   Consider a hot baked potato Will the potato cool faster or slower when we blow the warm air coming from our lungs on it instead of letting it cool naturally in the cooler air in the room? Explain 5   What is the physical significance of the Nusselt number? How is it defined? 6   When is heat transfer through a fluid conduction and when is it convection? For what case is the rate of heat transfer higher? How does the convection heat transfer coefficient differ from the thermal conductivity of a fluid? 7   What is the physical significance of the Prandtl number? Does the value of the Prandtl number depend on the type of flow or the flow geometry? Does the Prandtl number of air change with pressure? Does it change with temperature? 8   Will a thermal boundary layer develop in flow over surface even if both the fluid and the surface are at the same temperature?  9   The forming section of a plastics plant puts out a continuous sheet of plastic that is 12 m wide and 2 mm thick at a rate of 15 m/min The temperature of the plastic sheet is 90°C when it is exposed to the surrounding air, and the sheet is subjected to air flow at 30°C at a velocity of 3 m/s on both sides along its surfaces normal to the direction of motion of the sheet The width of the air cooling section is such that a fixed point on the plastic sheet passes through that section in 2 s Determine the rate of heat transfer from the plastic sheet to the air 10   A 15-cm X 15-cm circuit board dissipating 20 W of   power uniformly is cooled by air, which approaches the circuit board at 20°C with a velocity of 6 m/s Disregarding any heat transfer from the back surface of the board, determine the surface temperature of the electronic components ( a ) at the leading edge, and ( b ) at the end of the board Assume the flow to be turbulent since the electronic components are expected to act as turbulators 11   In a geothermal power plant, the used geothermal water  at 80°C enters a 15-cm-diameter and 400-m-long uninsulated pipe at a rate of 85 kg/s and leaves at 70°C  before being reinjected back to the ground Windy air at 15°C flows normal to the  pipe Disregarding radiation, determine the average wind velocity in km/h 12   A 6-mm-diameter electrical transmission line carries an electric current of 50 A and has a resistance of 0002 ohm per meter length Determine the surface temperature of the wire during a windy day when the air temperature is 10°C and the wind is blowing across the transmission line at 40 km/h   13   The components of an electronic system are located in a 15-m-long horizontal duct whose cross section is 20 cm X  20 cm The components in the duct are not allowed to come into direct contact with cooling air, and thus are cooled by air at 30°C flowing over the duct with a velocity of 200 m/min If the surface temperature of the duct is not to exceed 65°C, determine the total power rating of the electronic devices that can be mounted into the duct 14   The components of an electronic system dissipating 180 W are located in a 1-m-long horizontal duct whose cross section is 16 cm X 16 cm The components in the duct are cooled by forced air, which enters at 27°C at a rate of  065 m3/min Assuming 85 percent of the heat generated inside is transferred to air flowing through the duct and the remaining 15 percent is lost through the outer surfaces of the duct, determine ( a ) the exit temperature of air, and ( b ) the highest component surface temperature in the duct Repeat Prob 14 for a circular horizontal duct of 15-cm diameter 15   What is natural convection? How does it differ from forced convection? What force causes natural convection currents? 16   In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why? 17   Consider a hot boiled egg in a spacecraft that is filled with air at atmospheric  pressure and temperature at all times Will the egg cool faster or slower when the spacecraft is in space instead of on the ground? Explain

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