QJB,
For your first question "Why do turbine engines require a compressor section ?", many people have shown the right direction for the answer and many others have indicated parts of the reason.
However, I haven't seen a comprehensive answer.
Here it is.
In short, a turbine engine requires a compressor by design and thermodynamic constraints.
The constraints stem from linking the following facts:
1. A thermodynamic engine requires that at least one phase of its cycle increases the gas pressure.
A thermodynamic machine produces mechanical work when the gas pass through an expansion and it consumes work when the gas is compressed.
This positive or negative work is visible in a pV (pressure/volume) diagram as the integral beneath the line depicting the state change ( dW= p.dV ).
The machine produce a net work if the expansion is done at a higher pressure than the compression. Thus a thermodynamic machine must have at least one phase where the pressure increases.
2. There are only 2 ways of increasing the gas pressure :
a. mechanically (ie, with a compressor)
or b. through putting some heat in the gas whilst keeping it in a limited volume.
3. A turbine is an open flow thermodynamic engine. This implies that the pressure in the combustion chamber keeps a constant value.
An open flow thermodynamic engine means that, by progressing from one place to the next, the state variables of the gas are evolving and complete the transformations of the thermodynamic cycle. However, in any fixed place, these state variables stay constant though time. Thus the combustion chamber pressure is constant.
4. As no pressure increase occurs in the combustion chamber and as it is the only place where heat is added to the gas, then the pressure increase must happen through a mechanical compression.
Hence the requirement of a compressor.
Luc