In gas-turbine engines, the temperature of the exhaust gas leaving the turbine

is often considerably higher than the temperature of the air leaving the com-pressor. Therefore, the high-pressure air leaving the compressor can be heated

by transferring heat to it from the hot exhaust gases in a counter-flow heat ex-changer, which is also known as a regeneratoror a recuperator.A sketch of

the gas-turbine engine utilizing a regenerator and the T-sdiagram of the new

cycle are shown in Figs. 8–34 and 8–35, respectively.

The thermal efficiency of the Brayton cycle increases as a result of regener-ation since the portion of energy of the exhaust gases that is normally rejected

to the surroundings is now used to preheat the air entering the combustion

chamber. This, in turn, decreases the heat input (thus fuel) requirements for

the same net work output. Note, however, that the use of a regenerator is rec-ommended only when the turbine exhaust temperature is higher than the com-pressor exit temperature. Otherwise, heat will flow in the reverse direction (to

the exhaust gases), decreasing the efficiency. This situation is encountered in

gas-turbine engines operating at very high pressure ratios.

The highest temperature occurring within the regenerator is T4

, the temper-ature of the exhaust gases leaving the turbine and entering the regenerator.

Under no conditions can the air be preheated in the regenerator to a tempera-ture above this value. Air normally leaves the regenerator at a lower tempera-ture, T5

. In the limiting (ideal) case, the air will exit the regenerator at the inlet

temperature of the exhaust gases T4

. Assuming the regenerator to be well in-sulated and any changes in kinetic and potential energies to be negligible, the

actual and maximum heat transfers from the exhaust gases to the air can be

expressed as: qregen= h5-h2

A regenerator with a higher effectiveness will obviously save a greater

amount of fuel since it will preheat the air to a higher temperature prior to

combustion. However, achieving a higher effectiveness requires the use of a

larger regenerator, which carries a higher price tag and causes a larger pres-sure drop. Therefore, the use of a regenerator with a very high effectiveness

cannot be justified economically unless the savings from the fuel costs exceed

the additional expenses involved. The effectiveness of most regenerators used

in practice is below 0.85.

Therefore, the thermal efficiency of an ideal Brayton cycle with regeneration

depends on the ratio of the minimum to maximum temperatures as well as the

pressure ratio. The thermal efficiency is plotted in Fig. 8–36 for various pres-sure ratios and minimum-to-maximum temperature ratios. This figure shows

that regeneration is most effective at lower pressure ratios and low minimum-to-maximum temperature ratios.