The fan is connected to the low pressure turbine. After the fan comes the high pressure compressor which is driven by the high pressure turbine with the combustion taking place between HP compressor and turbine. The HP part of the engine is also called the gas generator.
You are right sorry the HP turbine drive the HP compressor
Thrust growth on turbofans is usually obtained by increasing fan airflow, which is commonly achieved by
increasing its bypass ratio. The
bypass ratio
is the ratio of the air that goes around the engine to the air
that goes through the core –
Figure 3
. In high bypass engines, the core engine primarily acts as a gas
generator providing high energy gas flow to drive the fan turbine. The fan alone produces anywhere from
50% - 85% of the total engine thrust depending on the engine model. In addition, high bypass engines
burns fuel far more economically relative to lower bypass ratio engines.