fan duct
The real reason for the fan duct is to get the fan pressure ratio up so it creates some real velocity going out the fan nozzle. An unducted fan (all the engine guys have experimented with open fans) is great at lower mach, but its propulsive efficiency drops off pretty fast as aircraft speed increases.
Black Baron asked about RPM - for the 747-size engine it's around 3500 RPM on the fan (called N1 in the commercial world, or NF or NL in military markets). Bigger engines turn slower, little ones turn faster.