# Potentials for Pressure Gain Combustion in Advanced Gas Turbine Cycles
## 3 Results and Discussion
### Simple Cycle Gas Turbine  
Detailed results of the simple cycle optimizations at a turbine blade temperature of 1100 K can be found in [19]. Therefore, emphasis will be put on the effect that different blade temperature constraints have on the optimized gas turbine specifications and thermal efficiency for the simple cycle GT. Figure 7 presents the relative key performance parameters for each combustor model after the optimization. The reference values for each combustor model are summarized in Table 4. First, consider a blade temperature of 1000 K in Figure 7 (bottom). Relative changes compared to the optimization for a blade temperature of 1100 K are shown. The 9% reduction in blade temperature led to lower thermal efficiencies. This reduction had a stronger effect on a cycle with isobaric combustion (\(-8.6\%\)) than on a cycle with isochoric combustion (\(-5\%\)). PGC could enable lower blade temperatures with increased creep life at an acceptable efficiency penalty. Furthermore, the specific work of the cycle was also reduced for a lower blade temperature. Here, the PGC cases were more affected than the isobaric case. The compressor pressure ratios were most affected according to Figure 7 and were reduced by up to 25%. However, this sensitivity for optimum thermal efficiency was already present for isobaric combustion and was not altered by realistic PGC. Furthermore, the TET did not decrease proportional  
Figure 6: Specific work for various models and cycles. to the decrease in blade temperature. Instead, TET was reduced by only \(\approx\)3% for the isobaric and realistic PGC cases. Here, the reduced temperature constraint was met by a small reduction of the TET and an increase of the blade cooling. In the isochoric case, TET was reduced by almost 10%, which was more than the reduction of the blade temperature limitation. This larger reduction decreased the cooling mass flow requirements, which eventually resulted in a higher cycle efficiency. Apparently, the power requirement for the compression of cooling air had a huge penalty on thermal efficiency for the ideal isochoric case. This was because ideal isochoric combustion gave pressure ratios across the combustor, which were much higher compared to the other combustor models (compare Figure 2). In contrast to a reduction in blade temperature as discussed above, Figure 7 (top) depicts the results at a higher blade temperature limitation of 1200 K. The design space was enlarged, resulting in higher thermal efficiencies. The isochoric cycle was not very sensitive to this constraint, as already shown. Indeed, it only resulted in a thermal efficiency gain of 4.4%. The efficiency of the isobaric cycle, in turn, improved by more than 7%. Consequently, technology improvements leading to higher acceptable blade temperatures or improved cooling technologies will not raise cycle efficiency as much for PGC as for conventional combustion. Comparing the behavior of the PGC models in Figure 7, ideal isochoric and all Mix models behaved consistently for different temperature constraints. For the PGC models by [PERSON] and [PERSON], that was not the case. Lower and higher temperature constraints led to completely different cycles. For a blade temperature of 1200 K, the compressor pressure ratio was only increased by 10%, whereas for a blade temperature of 1000 K, the pressure ratio was reduced by almost 30%. That was because for optimum cycle efficiency, the combustor operating points differed significantly. The corresponding temperature-entropy diagrams with [PERSON]'s and the Mix ref combustor models for all three blade temperature constraints are depicted in Figure 8. [PERSON]'s model favored high combustor temperature ratios because the pressure ratio rose exponentially (see Figure 2), whereas the Mix ref model showed an asymptotic behavior with a rising combustor temperature ratio. The blade temperature constraint of 1200 K drove [PERSON]'s model to very high temperature ratios.