Why do turbine engines require a compressor section

[jh5speed]

I read all the above and reckon there’s room for another angle on this so I’m going to resurrect this thread and wade in here – this is one of those questions which keeps me awake at night – something that’s generally accepted (e.g. because a computer model or a graph tells you) but there are rarely any good/satisfying answers to be found …

The question was put in terms of gas turbines and also reciprocating engines. I’ll answer in terms of the former because that’s what I know about. I’ll assume that the same principle can be read across to reciprocating engines. That exercise is left to the student...

So why do we compress? I distinctly remember answering this with ‘because it packs more wallop’ at a university interview nearly 30 years ago - not a bad answer really, I got a degree in the end….

Let’s put a few things to bed:

1. It is nothing to do with rate of combustion.

I can show (below) that you can get more thrust (and/or better thermal efficiency) from a turbojet with a 14:1 OPR (overall pressure ratio) than from a turbojet with a 12:1 OPR both designed for the same inlet airflow and operating at the the same fuel flow.


2. It is nothing to do with keeping the air going in the right direction.

Mr Whittle figured he’d need a compressor for cycle efficiency reasons and was probably jolly glad that this also kept things going aftwards – so that was something he didn’t have to worry about. I can also show that an engine with hardly any OPR (1.1:1) will develop some thrust but will have a shocking thermal efficiency. You don’t need much pressure ratio to keep things going rearwards – a slightly inclined tube with a fire in it does the job pretty well - think of the Mt Blanc tunnel fire of 1999…

Essentially, the answer is that for the higher OPR engine, the nozzle pressure ratio (NPR) is higher so you get more pressure thrust. Momentum thrust is the same (in our example).

For a convergent nozzle: Gross Thrust = W8.v8 + (Ps8-Pambient).A8

... where 8 is the nozzle throat plane.
W is flow
Ps is static pressure
v is velocity
A is area

Momentum thrust (W8v8) is the same because W8 is the same (we fixed that at the start in the way we set up the comparison for both engines). Exhaust jet velocity v8 is the same because T8 is the same and the nozzle is choked (Mach 1).

But why is T8 the same for the second engine, and indeed, and why is NPR higher i.e. why is nozzle entry pressure (P7) higher?

Furthermore why, for the higher compressor PR, doesn’t the turbine operate at a similarly higher pressure ratio to make the NPR the same as before?

Burning questions indeed ....

Let’s look at two design points as described above. I ran these on a proprietary Gas Turbine simulation program (GasTurb – you can download a free reduced functionality version from the website). This saved me doing the calcs myself. The first engine had 12:1 OPR and the second 14:1 at the same airflow and fuel flow (for this we need a smaller turbine throat area and final nozzle area – these areas fall out of the thermo calcs).

Sure enough – we get a higher NPR in the 14:1 case at much the the same T8 because the extra T we’ve added at the compressor is taken off by the turbine, with about the same combustor heat addition in each case. Thrust is higher (due to increased NPR) and so is thermal efficiency (obviously, because we are getting more thrust at the same fuel flow).

Furthermore, Gasturb allows you to throttle back the second engine (i.e. run slightly slower at fixed geometry using nominal component maps) to get the same thrust as the first engine. Lo and behold, the thermal effy was slightly higher in the second engine case (throttled back – so no longer operating at 14:1).

The 1.1 OPR example gave quite a bit of thrust by virtue of the momentum term (v was significantly increased due to the temp rise) but the cycle was at ~1% thermal effy!
Compare this with Engine 1 at 40% and Engine 2 at 42%.

I haven’t answered those questions yet though - all the GasTurb run showed me is what we know happens, though it does illustrate very clearly that it is not a rate-of-combustion thing...

Essentially the answer is in the relationship:

Pressure Ratio = Temp Ratio ^ (gamma/gamma-1)

Because the turbine has to develop a deltaT (starting at a higher T due to the combustion process) to drive the compressor, the turbine temp ratio is smaller than the compressor temp ratio and consequently the pressure ratio is smaller. For an increase in OPR, the turbine therefore satisfies the extra power requirement with some pressure ‘to spare’ thus NPR increases.

You can do these calcs by hand, just using the equation above – making them very simple by assuming 100% component efficiency and assuming an identical combustor temp rise for each engine (not far from the truth in these two examples – remember we set the airflow and fuel flow to be the same). You can also assume constant gas properties (gamma and Cp) – realities such as varying gamma and Cp just make the sums trickier but don’t change the basic principle.

I realise that I haven’t really explained things all that satisfactorily as my ‘explanation’ still relies on an unchallenged thermodynamic equation (I’m happy to accept it!). But it does illustrate where the benefit comes from, and possibly more clearly than ‘explaining’ things in terms of Temperature-Entropy charts where the pressure lines ‘conveniently’ diverge! (for the same reason of course).

So, a slightly different, and perhaps long-winded - angle on the subject to what’s gone before. And hopefully useful !