I'll take a stab, from a 'little knowledge' point of view:
1. First up, I'm not quite sure about your assertion, however, using the same amount of energy to accelerate a large mass of air to a lower speed produces more thrust than accelerating a small mass of air very to a much higher speed. So long as your aircraft isn't travelling at too significant a fraction of that gas stream velocity.
To the fan - if you unshroud the fan it becomes a propellor - propellors are wonderfully efficient at low airspeeds, and awful at high airspeeds (see Turboprops). They accelerate a very large mass, but can't get it going too quickly.
What the (divergent) inlet duct allows you to do is condition the airflow to the fan - slowing it. After the fan the duct converges, accelerating the airflow. This means the fan working in a lower airspeed stream. That will mean that the fan doesn't have to spin as fast for a given airspeed as it would if unducted/a propellor, yet you're getting the thrust stream up to a useful speed.
2. Partly see 1. I suspect that is, or will become a limiting factor on the size of fans. The blade angle at the back of the shaft isn't the same as the one at the front - it will be coarser. Note that some companies are working on geared fans - presumably because as the fan gets large, the need to keep RPM down is pushing the tail end RPM down too much to be efficient. I'm quite certain it's organised such that the fan doesn't go transonic, irrespecitve of what's going on with gas velocity.
3. Most likely simple engineering - it's all spinning too fast, too loaded and the cost outweighs the benefit. Also remember it's damn hot in there... you can't really be using hydraulics to move things around..
However, in between the spinning blades, are stationary guide vanes which redirect the gas flow to each disc. These are variable in some engines.
4. The most efficient propellor is one very large blade - it minimises the interference between the blades. However, to absorb a given amount of horsepower, it will be impractically long (bang on the ground), and will only be able to spin slowly before the tip goes transonic. To absorb more power within a given diameter, add blades. Now, I don't know about the fan, but I'd assume the same applies. Also remember the front end of a turbofan is part compressor - I'm not sure how well a low solidity fan would perform the compress part.
5. More smaller engines (with high bypass) vs less larger. There's a certain amount of loss and friction inherent in any engine. I'm guessing, but I rather suspect if you halve the size of the engine, you're very unlikely to halve those losses - so 2 small engines have proportionally more losses than 1 big one. Add in manufacturing cost, complexity, maintenance etc..
But really, the important bit is 1. The fan is not simply a shrouded propellor - the duct is rather important to the operation of the whole thing. For an extreme example, Concorde: While in supersonic flight, the front end of those engines were still seeing subsonic airflow, thanks to a bunch of ducts, ramps and vents.
Last edited by Mark1234; 28th November 2008 at 05:01.