Turbo fan Basics

The  pure jet engine is capable of high thrust from a small envelope and weight,  but comes at the cost of high fuel consumption, getting high velocity flow to make up for low mass flow is expensive. 

Adding a propeller to the front gives a hig efficiency from obtaining high mass flow added to the core, increasing thrust for a given fuel burn.  The relatively low velocity imparted to the propeller wake is a factor in reduced effectiveness at higher speeds, thrust only occurs when the velocity of the wake mass flow is greater than free stream, so propeller thrust is expected to drop off as speed increases, and it does. 

Ducting the propeller increases the exit flow velocity, but also reduces tip losses, usually this is better than the structures weight and drag effects, and this becomes the fan jet engine we know of today.