2
$\begingroup$

A recuperator is a device which extracts heat from the exhaust of a turbine and transfers it to the front air. This heats the air, which means the turbine uses less fuel to heat the combustion chamber to its max operating temperature.

They are used in gas turbines for power generation, because they can supposedly double the fuel economy.

Is this viable in a turboprop? Basically you need 2 heat exchangers per recuperator, so I could understand if they're too heavy. The other thing is potentially blocking airflow too much at the intake, causing too much drag.

Are these problems surmountable in aircraft engines? Is there a way to make them light enough and streamligned enough for aircraft?


The related question doesn't really answer this for turboprops. For jet engines, that related question does explain why it's no good---extracting heat from the exhaust means less jet speed. For turboprops however, it's all about shaft power, just like a electrical generator (before applying power to a prop instead of pure generator).

$\endgroup$
1
3
$\begingroup$

An aircraft engine is a thermodynamic device, that converts heat to mechanical power. A 'virtual' air parcel entering the engine goes through a thermodynamic cycle, which determines the bulk of the power plant efficiency, which include quite a big chuck of irreversibele air friction, with a minor role for bearing losses.

The engineering optimisation process however includes a lot more than just power plant efficiency. At the bottom line, the optimisation is tailored towards generating the most dollars of profit per unit of time, but a more optimistic view is that the engineer will optimise the aircraft fuel efficiency as a whole. In practice, this results in finding a balance between power-to-weight ratio and fuel efficiency, as additional weight increases the power required.

A recuperator is a heat exchanger between the compressor discharge air and the engine exhaust air*. Its goal is to replace part of the energy required from combustion of fuel by energy extracted from the exhaust, and does not modify the thermodynamic cycle in any way. I looked up some (approximated) data from a PT-6 turboprop at 850shp: [source, copied from training manual]

  • Ambient: 300K
  • Compressor discharge: 600K
  • Turbine inlet: 1200K
  • Exhaust: 800K

A recuperator could in theory cool the exhaust to 600K and heat the compressed air to 800K (ideal infinitely long counterflow heat exchanger) for a delta of 200K. The total desired temperature increase from compressor discharge to turbine inlet is 600K (1200K-600K), so we can reduce the fuel flow required by 1/3rd (only 400K delta still required). That seems excellent!

In practice, heat exchangers are bulky (air is a terrible fluid to transport heat). Furthermore, it's only operating on a 200K difference. The real question then is whether a real-world air-to-air heat exchanger (likely of cross-flow type, like an automotive intercooler, so less efficient than a counter flow heat exchanger) can be made so efficient that it does not introduce more losses than gains. Historic evidence suggest this is unlikely, especially if cost is considered (the material must be able to withstand 800K).

Furthermore, if we're going down the road of heat exchangers, different options are available. For example, an intercooler (cooling between compressor stages, which actually improves the thermodynamic cycle) could work on a higher temperature differential as well as lower absolute temperature, and would only present a single flow obstruction when the air is still nicely compact (power required is volume flow times pressure drop!) inside the engine. However, historic evidence again suggests that this is not cost-efficient, and instead seems to favour increasing the combustion chamber temperature and pressure.

*You may wonder, why not heat the intake air with exhaust air? We would work on a much larger temperature difference! In fact, this would be counterproductive, as the thermodynamic cycle operates on the difference between high and low temperature. Increasing the temperature at the inlet disproportionately increases the power required to compress the air, and does not result in an increase in turbine power (which in the end operates on a pressure differential, and cannot extract work from hot air at ambient pressure). There's a reason we use intercoolers, not inter-heaters in compressors.

$\endgroup$
1
  • $\begingroup$ Adding heat at the intake is wrong because we get power/thrust from the increase in volume of the high-pressure air exiting the combustion chamber, which can push on more stuff than the small volume of air coming from the compressor. Heating and expanding the high-volume low-pressure air at the intake means there's less expansion we can do in the combustion chamber, since, as the above answer states, there's less temperature difference, thus reducing the power output. $\endgroup$ – Abdullah Sep 11 '20 at 17:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.