Page 33 - Advanced Gas Turbine Cycles
P. 33
10 Advanced gas turbine cycles
T
0 S
Fig. 1.9. Irreversible Joule-Brayton cycle.
Two objectives are immediately clear. If the top temperature can be raised and the
bottom temperature lowered, then the ratio T= (Tmin/Tmm) is decreased and, as with a
Carnot cycle, thermal efficiency will be increased (for given p). The limit on top
temperature is likely to be metallurgical while that on the bottom temperature is of the
surrounding atmosphere.
A third objective is similarly obvious. If compression and expansion processes can
attain more isentropic conditions, then the cycle ‘widening’ due to irreversibility is
decreased, cr moves nearer to unity and the thermal efficiency increases (for a given 7).
Cycle modifications or innovations are mainly aimed at increasing 6 (by increasing & or
decreasing lA).
Fig. 1.10 shows the processes of heat exchange (or recuperation), reheat and
intercooling as additions to a JB cycle. Heat exchange alone, from the turbine exhaust to
the compressed air before external heating, increases & and lowers &, so that the overall
REHEAT
I MAIN HEAT
SUPPLIED , 3 3’
1’ J 1 A, MAINHEAT
INTERCOOLING REJECTED
1
0
S
Fig. 1.10. Temperature-entropy diagram showing reheat, intercooling and recuperation.
