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After browsing around a while, I noticed that many...I think almost all...large turbofans have this gap between the LP turbine and HP turbine. I took some screenshots and put a red circle around the gap.

enter image description here

Why do they have this gap? It's certainly not for the combustion chamber. That part comes before the LP turbine. The only thing I can think of is the LP and HP are on different spools so maybe they need some space for the bearings to brace the two shafts. But there should be bearings bracing the shaft all along its length so idk.

I'm guessing it would be more efficient to eliminate this gap and should allow a smooth transition from high pressure to low pressure. At the very least it would save some room at the end of the engine, shortening its length and maybe saving some weight.

EDIT: At first I thought it was totally empty. But it appears that space has stator vanes not shown in the pictures. I'm editing the title with the phrase "gap of turbine blades" because the original spirit of the question was about way there's a big space between the two sets of turbine blades. The space happens to have something else in it, but not turbine blades and I was wondering why those sets are separated.

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    $\begingroup$ Because the fan runs at a much lower RPM than the compressors, so for a un-geared LP turbine to fan link the LP turbine needs to run at the same lower RPM. As a result the HP and LP turbine has a much difference in diameter so you need some space for the diameter transition. Filling the gap with either LP or HP blades doesn't worth the weight or complexity. Some turbofans has geared LP turbine to fan connection so the LP turbine runs at a much higher RPM hence a smaller diameter, then you won't see the gap on them. $\endgroup$ – user3528438 Aug 29 '17 at 20:08
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enter image description here
(Source) Cutaway for the GE9X.

The gap you see is not empty. Marked above is one of the nozzle guide vanes (they have removed most of them for illustration purposes). Those vanes are analogous to the compressor stator stages.*

Labels 91 and 99 below are those vanes—sadly the stators and vanes are almost always omitted from 'drawings', yet they are as important.

enter image description here
(Source) Turbine stages of the RR Trent 500.

The reason why the diameter difference is steeper in the GE compared to the RR, is because the RR Trent 500 is a 3-spool engine.

In short, the vanes in both engines when omitted from drawings, give the illusion of there being a gap (wasted space).


* Nozzle guide vanes have nothing to do with the propelling nozzle at the rear. In turbine lingo, nozzle means the stator stage for the turbine.

On the funny side, here's a thumbnail from RR's own YouTube channel, absolutely bonkers exhaust. Don't trust illustrations.

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@ymb1 is correct that there is an aerodynamic function of the LPT nozzle in that space, but there is also a mechanical function as well. You correctly observed that there will be bearings supporting the shaft. But what supports the bearings? They are not just floating out there in space. The bearings are supported by a frame.

On the GP7200 and CFM LEAP, there is a frame between the HPT and LPT that is referred to as the "turbine center frame". The function of the frame is to provide a stiff loadpath from the bearing that supports the aft end of the HP shaft up to the main structure of the engine that connects to the aircraft. If you check this post, you'll see plenty of examples for the frames.

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