# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## 3 Results and Discussion
### Impact of Heat Exchanger Effectiveness on Cycle Performance  
The impact of heat exchanger effectiveness on the cycle thermal efficiency is shown in Figure 3, for constant \(OPR\) of 40 and pinch temperature difference in the saturator of 5 K. The \(x\)-axis and \(y\)-axis of the contour plots are, respectively, the aftercooler and economizer effectiveness, and the contour lines show the estimated cycle thermal efficiency. Moving vertically and horizontally through the contour plots, the effectiveness of the recuperator and intercooler vary respectively. Therefore, the top-right point of the top-right contour corresponds to the cycle \(A_{5}^{40}\) shown in Table 4.  
In all contours with intercooler effectiveness of 0.75 (left column of contours in Figure 3), there is a change in in the variation rate of the thermal efficiency when the \(\varepsilon_{AC}\) falls below 0.85. This is due to the notably high value of aftercooler heat capacity ratio \(C_{AC}^{*}\) = 0.95 for aftercooler effectiveness of 0.95 and 0.85, which it decreases to \(C_{AC}^{*}\) = 0.64 when the effectiveness of the aftercooler is 0.75, in order for the outlet water temperature to remain within the limits imposed to avoid steaming in the saturator (\(T_{w,out}=T_{sat}(0.9\times p_{SAT})\)). The dependency between the thermal efficiency and the heat exchanger effectiveness remains linear when changing the effectiveness of all heat exchangers together. The comparison between cycles \(A_{5}^{40}\) and \(G_{5}^{40}\) highlights the impact of the recuperator effectiveness and results in a reduction of 2.28 pp in cycle thermal efficiency when recuperator effectiveness becomes 0.8 from 0.9. The impact of intercooler effectiveness is shown by comparing cycle \(A_{5}^{40}\) against \(F_{5}^{40}\). A reduction in \(\varepsilon_{IC}\) from 0.95 to 0.75 causes a reduction of 2 pp in the cycle thermal efficiency. The impact of the aftercooler and economizer is shown by the comparison of the cycle \(A_{5}^{40}\) with the cycle \(D_{5}^{40}\), and of \(A_{5}^{40}\) with the cycle \(E_{5}^{40}\), with, respectively, a reduction of 0.48 pp and 1 pp on the thermal efficiency. The combined effect of the reduction in effectiveness of all the heat exchangers is shown by the comparison of the cycle \(A_{5}^{40}\) with the cycle \(B_{5}^{40}\) and \(C_{5}^{40}\) with, respectively, a drop of 2.7 pp and 5.5 pp in thermal efficiency. Figure 4 shows the effect of the heat exchanger effectiveness on the absolute humidity at the inlet of the combustion chamber for constant values of \(OPR\) (40) and saturator pinch temperature difference (5 K). The maximum absolute humidity in the design space is 24.6%, which ensures stable combustion in a modern combustion chamber as discussed by [PERSON] in [30]. Figure 4: Absolute humidity at the inlet of the combustion chamber across the design space as a function of the heat exchanger effectiveness—\(OPR\) = 40 & \(\Delta T_{ps}\) = 5 K.  
The impact of heat exchanger effectiveness on the optimized saturator bypass ratio, on the consequent absolute humidity (\(\omega\)) at the inlet of the combustion chamber and on cycle thermal efficiency is illustrated in Figure 5 for constant \(OPR\) = 40 and \(\Delta T_{ps}\)= 5 K.  
The reduction in effectiveness of the aftercooler and economizer has an impact on the amount of heat and mass transfer that occurs in the saturator and, therefore, on the thermal efficiency of the cycle. For an aftercooler effectiveness of 0.75 (\(A_{5}^{40}\)\(\rightarrow\)\(D_{5}^{40}\)--see Figure 5), the water mass flow in the saturator is reduced by 0.5% while a 1.2% reduction is observed in its temperature (\(C_{AC}^{*}\) = 0.95 across the range). The temperature of the gas at the inlet of the saturator increases by 13.8%, reducing the temperature leap in the saturator and, therefore, the heat and mass transfer that occurs within the device.