Modifications of the basic gas turbine cycle

The simple Brayton cycle has a low work ratio and, at moderate pressure ratio, a high exhaust temperature. These modifications address that.

1. Regeneration (heat exchanger)

Hot turbine exhaust pre-heats compressor delivery air before the combustor, which reduces the fuel needed.

• Work output is unchanged; heat supplied falls, so efficiency rises.
• Useful only when the turbine exhaust is hotter than the compressor exit, i.e. at low pressure ratio. Effectiveness $\epsilon = (T_x - T_2)/(T_4 - T_2)$ is 0.6 to 0.85.
• For aircraft it adds too much weight; used in stationary and marine plants.

2. Intercooling

Compression is done in two stages with cooling in between to near the inlet temperature.

• Compressor work falls (work per kg $\propto$ absolute inlet temperature), so net work and work ratio rise.
• Efficiency alone may fall because more fuel is needed to heat the cooler air, unless a regenerator is also used.

3. Reheating

The gas is expanded in two turbine stages with an additional combustor between them.

• Turbine work rises, so net work increases (and the exhaust temperature too).
• Efficiency without regeneration usually falls slightly; with regeneration it improves.

 Arrangement with all three modifications:

 air->[LPC]->[Intercooler]->[HPC]->[Regenerator]
                                      |
        [Comb 1]->[HPT]->[Comb 2 (reheat)]->[LPT]
                                      |
                       exhaust <------+ (heat to regenerator)

4. Afterburning (thrust augmentation)

Extra fuel is burnt in the turbine exhaust, which still contains much unburnt oxygen, before the propelling nozzle.

• Jet velocity rises; thrust increases by 40 to 70%.
• Fuel consumption rises sharply (TSFC two to three times higher), so used only for take-off, climb and combat.
• A variable-area nozzle is needed.

Combined arrangement

Intercooling + reheating + regeneration approach the Ericsson cycle (isothermal compression and expansion) and so give the highest efficiency of all, at the price of weight, cost and complexity.