Why not connect regenerator before the pump/compressor

[AdamA0]

In thermodynamic cycles, (specifically gas turbine power cycles with regeneration) why don't we connect the regenerator before, and not after, the compressor? would this violate the second law?
if not, then is it used? because i couldn't find any evidence or example on this.
Regards, A.Afif

 

[SteamKing]

In thermodynamic cycles, (specifically gas turbine power cycles with regeneration) why don't we connect the regenerator before, and not after, the compressor? would this violate the second law?

No, it wouldn't violate the second law, but it would be silly to do so.

Why is a regenerator used in a gas turbine? The regenerator recycles some of the heat which would otherwise be lost in the turbine exhaust and uses it to heat the compressed air entering the combustion chambers. This increases cycle thermodynamic efficiency.

If you placed the regenerator before the compressor, then you must compress this warmer inlet air before it is sent to the combustion chamber. Warm air expands, so this means the compressor must work harder to compress this warm inlet air to the proper combustion pressure. The compressor also must work at a higher average temperature, which may affect its design and the material used to construct it.

Instead of increasing cycle efficiency, relocating the regenerator before the compressor inlet would reduce efficiency.

 

[jack action]

The regenerator is an extra heater in series with the main heater that uses fuel. The idea is just to reduce the fuel consumption to get the same heat input:

In this gas-turbine engine, gas is compressed, mixed with fuel which is then burned and then produces power in the turbine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel burner stage. Since the gases have been pre-heated, less fuel is needed to heat the gases up to the turbine inlet temperature. Because some of the energy (that would be usually lost as waste heat) is recovered, the recuperator increases the efficiency of this cycle.

Wikipedia​

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After the compressor, it doesn't affect the T-S or P-V diagram in any way:

If it was heated before, starting at point 1, air would be heated up until a point slightly higher than point 6, then compressed to point 3 (vertical line 1-2 shifted to the right), then heated again until point 4, expansion to point 5 and finally cooled back to point 1 on the same isobar line as the regenerator. You can see how the area would be a lot smaller, i.e smaller work output.

Furthermore, when you heat the air, it expands, you would need a bigger compressor (i.e. a bigger engine) to treat the same amount of air if it was pre-heated.

For a thermodynamic cycle to work, the order is always compression, heat addition, expansion, cooling.

 

[AdamA0]

@SteamKing & @jack action Thank you both, great explanations.