Regenerative Turboprop (Allison T78)

[Apophenia]

The 12 Nov 1964 edition of Flight describes the 4000shp Allison T78-A-2 regenerative turboprop.
http://www.flightglobal.com/pdfarchive/view/1964/1964%20-%202849.html

Around the same time, Curtiss-Wright were working on a similar concept and, in 1966, GE mentions regenerative turboprops in a patent application.

Some snippets and images from the Flight article follow. Does anyone know if there was later (or current) work done on regenerative turboprops?

--------------------
From Flight

"Allison Division of General Motors is currently working on a $60m US Navy contract for the design and development of a regenerative turboprop, in which a proportion of the waste heat in the exhaust is recovered through a heat exchanger. The application of this cycle is to be to the next generation of ASW and early-warning aircraft, where this type of engine's ability to develop its peak thermal efficiency when throttled to 50 to 80 per cent power gives a low s.f.c. (specific fuel consumption) for long-endurance patrolling. Designated the Allison Model 545 (military, T78), the new engine will be rated at 4,000 s.h.p."

A test rig was created using a modified T56-A-7. An over-sized test regenerator was developed by AiResearch. The weight of this test regenerator was 320 lbs (for "400 lbs/hr fuel savings").

"The T78 resembles the T56 in being a single-shaft engine incorporating a 14-stage axial compressor and a four-stage turbine, and in having a cannular combustion chamber with six flame tubes. as in the earlier engine, the propellor reduction gear is carried remotely with a triagulated tubular brace, and driven through an extension shaft ...

... The hot gases from the turbine flow into a tailpipe, in inner wall of which is a tapered cone and the outer wall the tube block of the heat exchanger, which when closed diverts the exhaust gases into the regenerator and when open gives the greater part of the gases free passage. The T78 is thus a dual-purpose engine; it may be used either for maximum economy at reduced, but regenerated, cruising powers, or it can be opened up for high output at take-off, or attack, or for high-altitude operation where s.f.c. is normally better in any case."

-----------------
Curtiss-Wright Abstract

THE EVALUATION OF A LIQUID METAL REGENERATOR FOR A 3750 HP TURBOPROP ENGINE, 15 MAY 1963

Abstract : The feasibility of a liquid metal heat exchange system as a versatile, high reliability component meeting the compactness and high performance characteristics required for the Navy's regenerative turboprop engine has been demonstrated.

http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0407471
-----------------

 

[red admiral]

I don't know whether you're aware of the earlier Bristol Theseus that also used a heat regenerator section. It was rather heavy and didn't reduce fuel consumption that much.

 

[Jenci]

The Bristol Theseus used a recuperator heat exchanger, not a regenerator. Due to this, its size and weight was significant and the pressure losses were too high which led to its fate.
You can build more compact heat exchangers with the rotating drum-type (Ljungström) regenerator; the size is two-third to half of a recuperator with the same efficiency. But this is not the ultimate solution: there are leaking of compressed air because of its operation.

 

[Apophenia]

Red Admiral: Great to see images of that Theseus.

This engine was mentioned in the Flight article. The article says the Fedden's design "was the first regenerative gas turbine to run" (although I defer to Jenci's superior knowledge on the distinction between regenerator and recurperator).

The Theseus were flown in two Lincolns and two Hermes Vs (noted in the turboprop conversion lists).

Jenci: Are you aware of current work to apply the Ljungström regenerator to aircraft turbines?

 

[red admiral]

A few more pictures of the Theseus. I'm afraid my photo of the one in the Rolls-Royce Collection at Derby isn't very good.

 

[Jenci]

Apophenia:
I don't know if there's any exisitng project on aircraft gas turbine with the Ljungström-regenerator, mainly because I'm not following up the subject. But I've got a picture on a Volvo experimental automotive g/t with this kind of heat exchanger:



Here is an other automotive g/t from a russian book:



The text says: "Schematic drawing of a GT-304, 200 HP gas turbine from General Motors"
I know these are not for aircrafts, but the theory of operation is very similar to a turboprop with power-turbine.

 

[Apophenia]

Red Admiral: Thanks. Love the twin-engined Lincoln. Any idea if that aircraft is RE418 or RA716/G?

On the Hermes V, an image from Italian ebay of HP.82/2 (G-ALEV). Does anyone know why the Theseus B.TH.4 engines failed on the final flight of HP.82/1 (G-ALEU)?

Jenci: Thanks. Interesting that potential automotive applications still dominate.

A glance around the web showed that most work is on recuperators for turbofans (including naval turbine applications) not turboprops. However, there was a 1998 Pratt & Whitney Canada patent application.

I found one mention of current interest in turboprop/turboshaft recuperator applications -- the Frontline MicroFire for R-R/Allison 250s: http://www.frontlineaerospace.com/microfire

Others

http://www.mtu.de/en/technologies/engineering_news/intercooled_recuperated.pdf
http://www.seasdtc.com/events/2007_conference/posters/PPEM001.pdf [UAV applications]

 

[Skybolt]

Ah, regenerative turboprops are a fascinating subject. The Flight article derived froman extensive press tour done by Allison in their plant. AW&ST had a similar article six or seven pages long (I have, but where ?). There were a numer of technical presentation in AIAA and particularly SAE in 1962-65 timeframe. Working from memory, Allison and others had a concept for a regenerator using a molten sodium circuit to use the heat of the burner instead of the exhausts gases alone. Seems a chemical version of the indirect cycle nuclear engine done by P&W. Prospective uses were for Navy long range patrol aircraft and Air Force long-endurance missile carriers and cargos. The technology was abandoned in late '60s due to the success of high-bypass turbofans.

 

[LowObservable]

Land-based, sea-based and static gas turbines routinely use recuperators, and Allison's advanced-projects division was working on them for various needs in the 1990s. Then there's this:

http://www.flightglobal.com/pdfarchive/view/1979/1979%20-%200587.html

and the more recent Euro CLEAN project

http://www.flug-revue.rotor.com/frheft/FRHeft04/FRH0410/FR0410c.htm

 

[Apophenia]

From improving fuel economy (turboprop in the '50s and '60s, turbofans in the '70s) to reducing NOx and CO2 emissions in the 21st Century.

Reducing heat emissions also features in the naval recuperator designs. I wonder if that aspect has any future application to military aircraft?

 

[flacoman]

First post :
Here goes! The Chrysler turbine cars used a recuperator to help fuel economy and lower exhaust temperature.

 

[Nicknick]

Jes, but the pre heating caused much higher NOx emissions which were on reason to ban the turbine power for cars. It also didn't offer any significant archievment considering weight and buil space, since the recuperators were very big.

Still very intresting car!

 

[acegeek9992]

The 12 Nov 1964 edition of Flight describes the 4000shp Allison T78-A-2 regenerative turboprop.

http://www.flightglobal.com/pdfarchive/view/1964/1964%20-%202849.html

Around the same time, Curtiss-Wright were working on a similar concept and, in 1966, GE mentions regenerative turboprops in a patent application.

Some snippets and images from the Flight article follow. Does anyone know if there was later (or current) work done on regenerative turboprops?

--------------------
From Flight

"Allison Division of General Motors is currently working on a $60m US Navy contract for the design and development of a regenerative turboprop, in which a proportion of the waste heat in the exhaust is recovered through a heat exchanger. The application of this cycle is to be to the next generation of ASW and early-warning aircraft, where this type of engine's ability to develop its peak thermal efficiency when throttled to 50 to 80 per cent power gives a low s.f.c. (specific fuel consumption) for long-endurance patrolling. Designated the Allison Model 545 (military, T78), the new engine will be rated at 4,000 s.h.p."

A test rig was created using a modified T56-A-7. An over-sized test regenerator was developed by AiResearch. The weight of this test regenerator was 320 lbs (for "400 lbs/hr fuel savings").

"The T78 resembles the T56 in being a single-shaft engine incorporating a 14-stage axial compressor and a four-stage turbine, and in having a cannular combustion chamber with six flame tubes. as in the earlier engine, the propellor reduction gear is carried remotely with a triagulated tubular brace, and driven through an extension shaft ...

... The hot gases from the turbine flow into a tailpipe, in inner wall of which is a tapered cone and the outer wall the tube block of the heat exchanger, which when closed diverts the exhaust gases into the regenerator and when open gives the greater part of the gases free passage. The T78 is thus a dual-purpose engine; it may be used either for maximum economy at reduced, but regenerated, cruising powers, or it can be opened up for high output at take-off, or attack, or for high-altitude operation where s.f.c. is normally better in any case."

-----------------
Curtiss-Wright Abstract

THE EVALUATION OF A LIQUID METAL REGENERATOR FOR A 3750 HP TURBOPROP ENGINE, 15 MAY 1963

Abstract : The feasibility of a liquid metal heat exchange system as a versatile, high reliability component meeting the compactness and high performance characteristics required for the Navy's regenerative turboprop engine has been demonstrated.

http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0407471

-----------------

Looks like an interesting engine, shame it never entered service!

 

[riggerrob]

More recently, the British start-up: Hill Helicopters is offering a helicopter, turbo-shaft with regenerative combustor cans.

 

[Basil]

Solutions | TURBOTECH | Innovative turbines, Turboprops, Turbogenerators & Range-Extenders

TURBOTECH designs, develops and manufactures innovative regenerative turbines & propulsion systems. Combining proprietary heat-exchangers with state-of-the-art turbines, TURBOTECH's recuperated turbines power conventional and innovative aircraft architectures.

www.turbotech-aero.com

 

[publiusr]

With engines no longer having to be directly hooked to propellers—as opposed to electric drives—-might some of the engines of yesterday get new life?

 

[1635yankee]

Regenerative or recuperative gas turbines have one big problem with modern, high-pressure ratio engines: the air exiting the compressor is hotter than the air exiting the power turbine. This means that the regenerator (or recuperator; there is a difference but it's not important thermodynamically) will cause an increase in fuel consumption, which defeats the entire purpose. The AGT-1500*, which has a recuperator, gets very little benefit from the recuperator at high powers, but it does help maintain a good sfc (iirc, the sfc of the AGT1500 is about .4 lb/hp-hr) down to near-idle power. The aeroderivative of the AGT1500, the PLT-27, got rid of the recuperator because it was not a net benefit in aircraft operation.

 

[Nicknick]

The main benefit of a regeneratic cycle, ist the much better part load efficiency. Without recuperation, the maximum efficiency is only archieved with full power, but with recupeation, the lower pressures at part load enables more recuperation which keeps the efficiency quite high down to about 25 % power. For twin engine Helicopters this is very usefull, because the turbines usually allways rund below 50 % load.