Atransformer that is 10 cm long, 6.2 cm wide, and 5 cm high is to be cooled by attaching a 10-cm6.2-cm wide polished aluminum heat sink (emissivity 0.03) to its top surface. the heat sink has seven fins, which are 5 mm high, 2 mm thick, and 10 cm long. a fan blows air at 25â°c parallel to the passages between the fins. the heat sink is to dissipate 12 w of heat and the base temperature of the heat sink is not to exceed 60â°c. assuming the fins and the base plate to be nearly isothermal and the radiation heat transfer to be negligible, determine the minimum free-stream velocity the fan needs to supply to avoid overheating.