2 Literature review on gas turbine performance

Equation 7
is the heat capacity ratio.
For the ideal cycle the pressure ratio can be defined as follows;

Equation 8
Thus;

Equation 9
Finally the efficiency can be shown in term of pressure ratio;
Equation 10
According to the above relation the efficiency of the gas turbine depends only on the pressure ratio and gamma ( . The efficiency of the gas turbine increases with the pressure ratio. For the same pressure ratio the efficiency increases with the increase of gamma.

Figure 7: Relation between pressure ratio and efficiency. (Cohen, et al., 1996)
According to the figure 7 the efficiency increases with the pressure ratio of the gas turbine but there is a limitation for this explanation. According to the Rahman, et al., 2011, the thermal efficiency increases with the pressure ratio, but after certain value of the pressure ratio the efficiency decreases. In the figure 8 the curve of 1000K turbine inlet temperature starts to decrease around pressure ratio 12 and it reaches “zero” efficiency at pressure ratio 30 and the curve of 1200K turbine inlet temperature starts to decrease around pressure ratio 20. From the same research work it was described that the thermal efficiency decreases, with the increased ambient temperature for the same pressure ratio. For the high ambient temperature, the amount of compressor work is higher that the low ambient temperature due to density change in the air. Therefore the efficiency of the machine reduces in high ambient temperature. Figure 9 shows the variation of efficiency with the pressure ratio for several ambient temperatures.

Figure 8: Variation of compression ratio and turbine inlet temperature on thermal efficiency (Rahman, et al., 2011)

Figure 9: Variation of compression ratio and ambient temperature on thermal efficiency (Rahman, et al., 2011)