3.1. Tool Validation
For the thrust class and performance most relevant here, the Leap-1A family engines were selected as validation cases. Using component efficiency predictions as described in [
21,
22], cycles are established to match the public data found for Leap-1A26 and Leap-1A32. This includes the performance data, LTO cycle NOx data, and noise levels listed in the ICAO NOx data bank and noise fact sheet [
19,
20,
35]. The LTO take-off ratings are 120.6 kN and 143.1 kN for Leap-1A26 and Leap-1A32, respectively, as defined in the ICAO emissions databank [
20].
Table 2 contains the comparison between the models and the real engines. Regarding the performance and the mechanical design, it is observed that there is generally a good correlation between the simulated engine and the reported data. Public data found for the cruise BPR and SFC do not specify the corresponding variant of the Leap-1A and thus are not included in
Table 2. The estimated engine weight from the model corresponds to the dry weight, which justifies the deviation from the one provided by the literature. The engine length from the public data accounts for the fan casing which is not included in the model. For the fuel flow, it is observed that, although there are small deviations, the trend of the values and the relative difference between the two simulated and the two real engines are similar.
It is observed that the noise from the simulated engine is comparable with the reported measurements. Part of the deviations can be attributed to different trajectories and flight conditions between the simulated engine and the measured data. On the other hand, the predicted NOx at the climb-out point of Leap1-A26 and the take-off point of Leap-1A32 deviate from the ICAO data substantially, despite the fact that the simulated fuel flow is in good agreement. It is then observed that the Leap-1A engine has an atypical characteristic for the climb-out and take-off points NOx emissions. The ratio between the EINOx at the climb-out point and the take off point is around 0.5, whilst the other engines with similar thrust class have this number about 0.8. This characteristic is attributed to the novel GE combustor type (Twin Annular Premixing Swirler) TAPS II, which cannot be captured by the semi-empirical models found in public literature. Nevertheless, the predicted NOx emissions trend is considered reasonable when comparing the models and the real engines’ data.
3.2. Engine Cycle Design Space Exploration
First, a global optimum operating point is determined for minimum installed SFC, defined in Equation (1), by allowing a variation in OPR, FPR, and BPR. The optimum point is demonstrated in
Table 3. Around the optimum, a trade range of 0.5% penalty in the installed SFC is chosen defining three points (the two end points and the optimum). The trade space for OPR variation is depicted in
Figure 4. It is in line with elementary theory to expect that varying the OPR leads to a change in the number of stages of the turbomachines. This appears as discontinuities in the installed SFC curve in
Figure 4. As the OPR increases, while fixing the fan pressure ratio (FPR) and the pressure ratio split between compressors, the stage count increase in the high pressure compressor (HPC) results in the small step increases in the installed SFC. On the other hand, the sudden decrease of installed SFC at an OPR close to 48 is because of a stage count reduction in the low pressure turbine (LPT). Fixed FPR and specific thrust give a lower bypass ratio (BPR) with an increasing OPR, hence a lower power requirement for the LPT and the reduction in LPT stage counts.
The OPR optimum point, displayed by the red triangle in
Figure 4, is found at an OPR of 48.5 at top-of-climb with an installed cruise SFC of 16.81 mg/Ns. The lower OPR point is located at 44.1 and the higher OPR point at 54.0. The key cruise parameters for the three points are presented in
Table 4. Despite the relatively large variation in OPR, both the installed SFC and the uninstalled SFC experience modest change, whereas there are notable differences in the engine characteristics, i.e., number of stages and weight.
The established optimal OPR point is used for the optimization of the low pressure system. Keeping the OPR constant while varying FPR and BPR, the resulting search space is reproduced in
Figure 5. The cycle optimum is located close to an FPR of 1.60 at top-of-climb. The bypass ratio at this point is 8.6. The red triangle point represents the optimal point given in
Table 3 above. As in the previous optimization case, a 0.5% variation in installed SFC is allowed. Thus, the lower FPR point results in a 77 inch fan, whereas the high FPR design has a 68 inch fan. The key cruise parameters for the three points are presented in
Table 5. Similar to
Figure 4, a lower level of discontinuity introduced by the stage counts change can be observed in
Figure 5. The HPC stage increase from the 73 inch fan to the 68 inch fan is mainly due to a lower installation position of the HPC for a smaller fan design. This results from the same height ratio set for the inlet and outlet of the inter-compressor ducts. It is, however, a secondary effect compared to the weight change incurred by the change in fan size. Furthermore, although there is a relatively large variation in fan diameter and engine weight, the difference in installed SFC is minor. Mechanically, the optimal engine is a direct driven single stage fan with a three stage booster, an eleven stage high pressure compressor, a two stage high pressure turbine, and a six stage low pressure turbine.
3.3. Multidisciplinary Trades
In the previous section, two cases of engine cycle design space exploration were introduced, the low pressure system FPR/BPR space, and the core engine design space varying the OPR. Three points were selected for each case, corresponding to the cycle optimum and a 0.5% variation in installed SFC. For each point, the key performance parameters were presented, and the engine cycle performance was assessed. This section is dedicated to the evaluation of noise and NOx emissions for the chosen points.
Starting from the optimization of the OPR,
Table 4, it is apparent that, despite the relatively large OPR variation, most of the key cruise parameters, including the installed SFC, remain almost constant. The corresponding results with regard to noise and NOx emissions are presented in
Table 6,
Table 7, and
Figure 6.
As can be observed, there is no significant effect on the total noise produced for the varying OPR cases. Since the optimization only influences the engine core of the propulsion system and the same trajectory is used for all cases, the airframe noise remains constant while the differences in total engine noise are essentially negligible. The trend, however, indicates that there is a slight increase in total noise with increasing OPR value, mainly driven by the LPT noise at approach and the LPT and jet noise at cutback and sideline. Even though the mentioned sources are not necessarily the driving noise factors, they are the ones experiencing largest variations.
On the contrary, the variation in NOx emissions is evident, especially in the Take-off and Climb-out points, where there is a deviation of about 15% between the three cases. There is an obvious trend of increasing emissions with increased OPR, which can easily be explained from the correlation presented in Equation (4). More specifically, increased OPR leads to increased compressor outlet temperature and pressure and therefore NOx emissions. The difference in the total LTO cycle NOx mass is also noteworthy, as can be seen from
Table 7. For the lower OPR engine, a 12% decrease in total NOx mass can be traded for a roughly 0.5% increase in installed SFC.
Regarding the low pressure system optimization,
Table 5, although the mass flow drops by 10–14% between the three cruise points and the FPR increases from 1.44 to 1.64; the installed SFC experiences minor variation over this relatively large design range. This indicates that the fuel burn trade is rather weak in this region and that the designer can choose the diameter within a relatively large range in the FPR/BPR space with a very modest fuel burn penalty. Thus, this should allow a relatively large design freedom with respect to noise, as the low pressure system is the major noise source for modern turbofans.
The main output from the semi-empirical noise and NOx emissions models are collected in
Table 8,
Table 9, and
Figure 7. Regarding the NOx emissions, it is clear that the variation is notable between the three fan options. It is mainly the Take-off and Climb-out points that are indirectly influenced by the specific thrust of the three designs. The smaller engine has a higher FPR and, hence, a smaller thrust lapse. In other words, the thrust of the higher FPR engine does not go up as much as the lower FPR engines when running in take-off. Thus, it needs to throttle up and the resulting temperature in take-off remains somewhat higher than for the lower specific thrust configurations. In total, the LTO cycle NOx emissions’ mass for the three engines is presented in
Table 9. The relatively large difference in the total NOx mass is mainly driven by the fuel flow difference.
Due to the parameter variation in FPR/BPR carried out here, only influencing the propulsion system, the airframe noise remains constant. Albeit, the total engine noise goes down with increasing BPR and decreasing FPR [
7], as can be seen from
Table 8. The relative importance of the fan inlet noise increases as expected at cutback while jet noise decreases significantly [
7,
36]. At the sideline, the jet becomes the dominant source of noise while fan inlet noise decreases. The fan discharge noise drops with decreasing FPR, reflecting the decrease in rotational speed as the diameter of the fan is increased (
Table 5). The reduction in jet noise is driven by the fall in jet velocity in the fan stream. LPT noise rises with increasing FPR, despite the decrease in core mass flow. This trend is mainly driven by the increase in the tip velocity of the LPT’s last stage (
Table 5). In general, the trade between noise and engine performance is strongly dependent on the technology level assumed. For the state-of-the-art high BPR engines described in this paper, the rate of noise reduction is diminishing since the jet velocities and jet noise are no longer so predominant. Except for the fan as still the major source of the total noise, the LPT becomes more important than ever [
37].
A summary of the above multidisciplinary evaluation is shown in
Table 10, where the key environmental trades are presented. It is noted that OPR can be traded for reduction in total LTO NOx mass with a minimum effect on fuel efficiency and noise emissions. On the other hand, variation in fan size allows for improvement in both NOx emissions and noise, with a slightly increased penalty in fuel efficiency.