EFFECTS OF OPERATING VARIABLES

The operating variables for the performance of open cycle gas turbine are as follows:

1. Pressure ratio
2. Compressor inlet temperature
3. Turbine inlet temperature
4. Efficiency of the compressor
5. Efficiency of the turbine

These variables affect the thermal efficiency and the work ratio of the gas turbine.

1 Effect of Pressure Ratio

For the simple Brayton cycle,

Thermal efficiency, 

Work ratio, 

The thermal efficiency for various values of γ and r_p are given in Table 16.2.

The variation of thermal efficiency with pressure ratio for different values of ratio of specific heats is shown in Fig. 16.30. It may be observed that the thermal efficiency increases as the pressure ratio and the ratio of specific heats increases.

The work ratios for different values of r_p and T₃/T₁ ratio are given in Table 16.3.

 

Table 16.2 Effect of pressure ratio on thermal efficiency

Figure 16.30 Variation of thermal efficiency with pressure ratio of a gas turbine

 

Table 16.3 Effect of pressure ratio and temperature ratio on work ratio (γ = 1.4)

Figure 16.31 Variation of work ratio with pressure ratio of a gas turbine

The variation of work ratio with pressure ratio for different values of temperature ratio is shown in Fig. 16.31. It may be observed that the work ratio decreases as the pressure ratio increases. However, the work ratio increases with increase in temperature ratio.

2 Effect of Efficiencies of Compressor and Turbine on Thermal Efficiency

From Eq. (16.11), we have

where 

Figure 16.32 Variation of thermal efficiency with pressure ratio

Figure 16.33 Variation of thermal efficiency with temperature ratio

Let  then

For 

The effect of machine efficiencies on thermal efficiency has been shown in Fig. 16.32. The thermal efficiency initially increases, becomes maximum, and then starts decreasing. Figure 16.33 shows the effect of temperature ratio on thermal efficiency. It may be observed that the thermal efficiency increases as temperature ratio increases.