Turbofan

The turbofan is a combination of the turbine and a ducted fan. Reducing the propeller diameter and increasing the number of blades allows the fan to be aligned along the turbine axis. A shroud or the cover is used to improve the quality of flow through the fan. There are two airflow streams. One stream through the engine that is mixed with fuel, burnt and drives the turbine. The other over the engine.

An important parameter characterizing the turbofan is the bypass ratio:

Higher bypass ratio are associated with

lower specific fuel consumption

reduced noise

higher structural weight

higher engine drag

higher cost