# Potentials for Pressure Gain Combustion in Advanced Gas Turbine Cycles
## 3 Results and Discussion
### Thermal Efficiency and Specific Work  
Absolute thermal efficiency after the optimizations are depicted for the different cycles and combustor models for a blade temperature constraint of 1100 K in Figure 4. For all cycles, isobaric combustion resulted in the lowest efficiencies (right-hand side). Instead, the highest efficiencies were found for ideal isochoric combustion (left-hand side). While [PERSON]'s model gave the highest efficiencies within the realistic PGC representations ([PERSON], Nalim, and Mix ref) followed by [PERSON]'s model and the Mix ref model, they only differed by 1.5 PP. If different cycle setups were compared, the simple cycle resulted in the lowest efficiencies. The intercooled and recuperated cycle were almost level, and the intercooled-recuperated cycle (Int-rec) achieved the highest efficiencies. The ideal isochoric combustion in an intercooled-recuperated cycle could increase the efficiency by almost 20 PP compared to a simple cycle with constant pressure combustion. Realistic representations of PGC still achieved an efficiency gain in the intercooled-recuperated cycle of 15.6 PP. Figure 5 presents possible synergies of PGC in advanced thermodynamic cycles at different blade temperature constraints. Here, thermal efficiency gains are depicted for the different types of combustion and cycles. First, the blade temperature constraint of 1100 K was analyzed. The efficiency gain was defined as the difference between simple cycle thermal efficiency and thermal efficiency of the advanced cycle using the same type of combustion. Figure 4 illustrates the efficiency for various models and cycles at \(T_{mat}\) of 1100: intercooling, for example, increased the efficiency for isobaric combustion from 37.36% to 40.89%. This efficiency gain of 3.52 PP is depicted in Figure 5. The efficiency gain for PGC modeled by [PERSON] in an intercooled cycle was slightly higher (from 44.04%-47.41%). Therefore, applying PGC in an intercooled cycle can lead to higher efficiency gains, and it can create synergies. This is marked exemplary by the green arrow in the zoom up. No synergies were created with the model of [PERSON] (red arrow). In general, the intercooled cycle was favorable for PGC since it had a lower combustor entry temperature, which allowed for more heat input in the combustor, resulting in higher pressure ratios. However, taking ideal isochoric combustion, the efficiency gain was smaller. Above a relative proportion of the combustor pressure ratio to overall pressure ratio, the efficiency  
Figure 4: Thermal efficiency for various models and cycles at \(T_{mat}\) of 1100 K.  
gains due to intercooling were reduced. A possible explanation was that ideal isochoric combustion favored low TET and OPR, but had a high pressure increase over the combustor (compare with the detailed results in Section 3.3). Hence, the compressor pressure ratios and powers were low, reducing the potential of intercooling. Looking at the recuperated cases in Figure 5 (middle), isobaric combustion yielded the highest efficiency gain. Applying PGC did not create synergies, but subtracted some of the efficiency gain present at isobaric combustion. Applying PGC reduced both turbine exit temperature and compressor exit temperature. However, the former was more affected. As a result, the driving temperature difference for recuperation was reduced for PGC (compare with the detailed results in Section 3.4). The intercooled-recuperated engine setup improved gas turbine thermal efficiency for isobaric combustion (11 PP) the most. Using realistic PGC representations, the efficiency rose by \(\approx\)9.8 PP. For ideal isochoric combustion, the improvement was only about 6.6 PP. Hence, applying combined recuperation and intercooling was more beneficial for isobaric combustion than for PGC. For a blade temperature constraint of 1100 K, it can be concluded that intercooling seemed favorable for PGC because thermal efficiency gains could be increased.