5. Alternative Propulsion System Configurations
5.2 Propulsion System Optimisation
Previous configurations have simulated the propulsor array assuming that each propulsor is sized using the same key design variables and for the same net propulsive force per propulsor.
However, as each propulsor is subject to different flow conditions, a more efficient configuration may be found by sizing each propulsor individually to minimise the power consumption. A number of these design variables have been addressed individually in Section 3.3. Based on these simulations, a number of conclusions can be drawn regarding the influences and benefits of the design variables. The main design variables for propulsor array that can be used as part of an optimisation are:
• Propulsor fan pressure ratio
• Propulsor inlet aspect ratio
• Net propulsive force per propulsor
• Number of propulsors
• Array length
Given the modelling assumptions, some of the design variables will lead to similar results from a performance analysis perspective. Inlet aspect ratio, the number of propulsors, and the propul-sor array length are all interrelated variables that determine the propulpropul-sor array’s size. They therefore tie into the overall trade-off between extending the propulsor array either up into free-stream flow, or along to the outer edges of the fuselage. Differences will arise between the design variables when considering factors such as the propulsion system weight (due to the number of motors/propulsors) or the pressure loss in the duct (following Seddon’s formulation [107], or due to the transition from a rectangular to circular cross-section duct).
For the following analyses, a fixed number of propulsors was assumed. In addition, as re-ducing the array length would reduce the ingested drag percentage, the array length will be fixed to equal the length of the baseline propulsor array. The combination of these two as-sumptions removes the propulsor’s inlet aspect ratio as a design variable. Inlet aspect ratio will
5. Alternative Propulsion System Configurations
Table 5.4: Performance of a propulsion system configuration with array sized to equal the boundary layer thickness (Turbofan BPR = 4.0, Array FPR = 1.7)
ADP Cruise RTO SLS
Altitude (ft) 30000 40000 0 0
Mach Number 0.84 0.84 0.25 0
Engine
TET (K) 1811 1728 1895 1922
Net Thrust (kN) 11.8 7.4 34.0 48.8
Power (MW) 15.1 9.2 28.4 28.4
Mass Flow (kg/s) 131.89 84.56 234.12 220.10
Fuel Flow (kg/s) 0.733 0.443 1.471 1.490
Array
NPF (kN) 95.3 59.8 288.5 379.2
Mass Flow (kg/s) 669.9 428.1 1340.1 1321.9
Power Consumption (MW) 30.2 18.4 56.8 56.8
Specific Power Consumption (W/N) 317.2 308.1 197.0 149.7
Propulsor RPM 100.0% 100.2% 90.5% 91.9%
Length (m) 20.1
Propulsion System
eSFC (mg/Ns) 12.32 11.88 8.25 6.25
eBPR 16.7 16.6 16.9 16.7
eST (N/kg) 127.5 124.9 197.1 270.5
NPF (kN) 118.9 74.6 356.5 476.7
Thrust Split 80.1% 80.2% 80.9% 79.5%
Table 5.5: Overall of a propulsion system configuration with array sized to equal the boundary layer (Turbofan BPR = 4.0, Array FPR = 1.7)
Component Weight % of Total
Distributed Propulsors (total) 1430 kg 18%
Turbofan (×2) 1650 kg 41%
HTS Generators (×2) 630 kg 16%
Motors (total) 1550 kg 19%
Misc. HTS 545 kg 7%
Total Weight 8085 kg
instead be determined by changes to the propulsor size resulting from changes to the remain-ing design variables. For the followremain-ing propulsor array optimisations, a turboshaft configuration will be assumed with all thrust produced by the propulsor array. This enables the optimised configuration to be compared most easily against the baseline propulsor array configuration.
As the propulsion system’s thrust requirement is fixed, the net propulsive force is a constraint for the optimisation process.
There are three separate goals that can be used for the optimisation process, looking at either the propulsor array, the propulsion system as a whole, or the entire aircraft system. This means optimising in terms of specific power consumption, effective specific fuel consumption, and fuel burn, respectively. The previous analyses have shown that these may be conflicting goals, as a low specific power consumption may not equate to the minimum effective specific fuel consumption, which may not correspond to a minimum fuel configuration. Assuming that the engine design remains otherwise the same, a lower power consumption array will reduce effective specific fuel consumption. However, this is not the case where the engine design must be changed, such as with the use of thrust split. The following sections therefore compare the difference in outcomes between a configuration optimised in terms of the propulsor array per-formance alone (i.e. power consumption), and the aircraft perper-formance (i.e. fuel consumption).
The optimisation presented in Section 5.2.3 makes use of an NSGA-II optimiser developed in previous research by Nalianda [146].
5.2.1 Array Fan Pressure Ratio
Assuming that all propulsors operate with the same fan pressure ratio, there may be an optimum fan pressure ratio for the propulsors that minimises the specific power consumption of the array as a whole (Section 3.3). However, each propulsor in the array is subject to different flow conditions. By applying the same fan pressure ratio to the entire propulsor array, some fans operate with a lower power demand than the baseline, and others with a higher power demand.
The net outcome is a lower power consumption than the baseline propulsor array configuration.
However, it is instead possible to optimise the fan pressure ratio for each propulsor individually.
In this optimisation, a fan pressure ratio is selected that minimises the individual propulsor’s power consumption and hence the total power consumption of the array. Each propulsor was assumed to produce the same thrust. Therefore, changing the propulsor’s fan pressure ratio will lead to a change in the propulsor size. The minimum fan pressure ratio was capped at 1.1.
The feasibility of very low pressure ratio propulsors is not assessed in this section. However, this may set a higher cap on fan pressure ratio than the values shown here.
The optimisation shows that the ideal fan pressure ratio increases from a low value at the airframe centreline (Propulsor 0) up to a maximum at the far end of the array (Figure 5.10).
The propulsor at the centreline is therefore the largest, whilst the propulsor at the end of the array is the smallest. The fan pressure ratio for propulsors 0–3 meets the FPR cap set in the optimisation. The optimisation would otherwise lead to lower fan pressure ratios for the innermost propulsors. The highest power saving in comparison to the baseline is achieved by the propulsors nearest the centreline, as those deviate the furthest from the baseline fan pressure ratio. With this configuration, the total power saving versus the baseline configuration is 3.0%. In comparison, with a fixed fan pressure ratio of 1.13 for the entire array, the power saving is 2.4% versus the baseline. Individual fan optimisation therefore provides a relatively small performance boost over whole array optimisation. The best benefit is seen for the out-ermost propulsors. With a fan pressure ratio optimised for the array as a whole, this propulsor would have a higher fuel consumption than the baseline array configuration. However, there is a positive saving where fan pressure ratios are individually optimised. A lower fan pressure
5. Alternative Propulsion System Configurations
1.00 1.10 1.20 1.30
0 1 2 3 4 5 6 7
FPR
-2.0%
0.0%
2.0%
4.0%
6.0%
0 1 2 3 4 5 6 7
Power Saving vs Base
Optimised Fan FPR Optimised Array FPR
Figure 5.10: Optimum fan pressure ratio for individual fans and a 100% TS array in comparison to the baseline configuration and an optimum array fan pressure ratio configuration
ratio reduces the enthalpy change across the fan, leading to a decrease in power consumption.
However, it also leads to an increase in the intake height and hence an increase in the ingested free-stream flow. As h/δ is already reasonably low for the inner propulsors, there is less of a penalty to increasing their size than for the outer propulsors. The optimised fan pressure ratio is therefore higher for the outer propulsors, as this balances a change in h/δ against the power demand resulting from a change in fan pressure ratio.
The lower power consumption would lead to a corresponding decrease in the effective spe-cific fuel consumption of the propulsor array. However, although the low fan pressure ratio enables a lower power demand for the array (and hence lighter motors and generators), the low fan pressure ratio also leads to larger, heavier fans. The overall system weight is therefore 22611 kg, close to double the weight of the baseline propulsion system’s weight which leads to a 9% increase in the aircraft’s weight. As a result, the fuel consumption of the aircraft increases by approximately 6% versus the baseline configuration for the N3-X propulsion system. Given the influence of weight over the performance of the aircraft, propulsor weight is a vital part of a complete optimisation, as the weight increase will negate the benefits of the eSFC improve-ment.
The optimum fan pressure ratio is dependent on a wide range of other design and perfor-mance factors. In particular, inlet total pressure loss was identified as a parameter that leads to a lower effective fan pressure ratio, and hence increases the optimum fan pressure ratio.
Higher total pressure loss in the propulsor intakes would therefore increase the optimum fan pressure ratio for each individual fan. As low pressure ratio fans are sensitive to distorted flow, this is a key factor to assess for further research on optimising the propulsor array.
5.2.2 Array Weight
There are two conflicting goals when considering the array design. A low eSFC is achieved by an array design with a low fan pressure ratio, as demonstrated in the previous analysis.
However, a low fan pressure ratio implies larger and therefore heavier propulsors. In contrast, a higher fan pressure ratio will reduce the propulsor size and hence will reduce propulsor weight in exchange for a higher weight electrical system due to the increased power requirement. With
0 5 10 15 20 25
1.05 1.10 1.15 1.20 1.25 1.30 1.35
Weight (tonnes)
Fan Pressure Ratio
Array Motor Generator Engine
Figure 5.11: Influence of array fan pressure ratio on propulsion system weight
1.20
-5.0%
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
1.05 1.10 1.15 1.20 1.25 1.30 1.35
Fuel Saving vs Baseline
Fan Pressure Ratio
Figure 5.12: Influence of array fan pressure ratio on mission fuel consumption for a 7500 nmi flight
the weight relationships used, the decrease in electrical system weight is proportional to the decrease in the array power demand and hence the decrease in the propulsor fan pressure ratio (Figure 5.11). The main engine weight also decreases proportionally with the decrease in power requirement. However, the weight of the propulsors rapidly increases with a decrease in fan pressure ratio. As the propulsors are the dominant part of the overall propulsion system weight for a 100% thrust split configuration, the overall effect is an increase in the total weight of the propulsion system. In combination, there is no clear minimum weight configuration for the propulsor array.
Although a low fan pressure ratio system offers the best eSFC, the significantly higher weight is detrimental for fuel consumption. The optimum configuration from a fuel burn per-spective will therefore be an array with a slightly higher fan pressure ratio than that which gives the minimum effective specific fuel consumption. This configuration has a lower weight at the expense of an increase in eSFC. Assuming each propulsor has the same fan pressure ratio and for the 100% thrust split configuration, the best fuel saving can be obtained for a fan pres-sure ratio of approximately 1.2 (as opposed to FPR 1.13 for minimum eSFC). However, this configuration reduces fuel consumption by less than 1%. Fuel consumption increases sharply with further decreases in fan pressure ratio, due to the higher overall weight and despite a lower eSFC (Figure 5.12).
5. Alternative Propulsion System Configurations
Figure 5.13: Pareto front of individual propulsor fan pressure ratio optimisation for minimum eSFC and array weight
5.2.3 Optimisation for Fuel
Both the effective specific fuel consumption and the aircraft weight will have an impact on the overall fuel burn on the aircraft. However, the previous two analyses have demonstrated that weight and eSFC are two conflicting goals, as a minimum eSFC configuration has a higher weight. Both Section 5.2.1 and Section 5.2.2 demonstrated that a configuration that minimises eSFC or the array’s power consumption is therefore not the same as the configuration that minimises fuel consumption. This trade-off can be demonstrated by optimising the fan pressure ratio of each individual propulsor in the array with low fuel burn as the target, rather than eSFC or weight.
An NSGA-II optimiser developed by Nalianda [146] was used to assess the trade-off be-tween eSFC and weight. As low weight and eSFC are conflicting goals, the combined analysis leads to pareto front of results (Figure 5.13). The minimum weight that can be reached by the optimiser is limited by the maximum power that can be produced by the main engines at ADP, given the engine’s design assumptions. As minimum weight corresponds to high fan pressure ratio and hence high power demand, there are some configurations for which the engine is unable to produce sufficient power. This point is visible as a cut-off point in the upper end of results for eSFC above approximately 12.04 mg/Ns. The results clearly demonstrate the trade-off between minimising the eSFC and minimising weight, as the goals cannot be achieved simultaneously.
A combination of low weight and low eSFC will lead to a minimum fuel burn configuration.
The best configurations can be identified by predicting how weight and eSFC influence the fuel consumption for the design 7500 nmi mission (Figure 5.14). Better fuel savings are achieved by the low weight systems, despite a relatively higher eSFC. Whilst a low eSFC does lead to a fuel saving, array weight increases sharply with a decrease in fan pressure ratio (Figure 5.11).
This increase in weight means fuel consumption quickly approaches that of the baseline N3-X configuration.
The best fuel saving is achieved by a configuration that balances a minimum eSFC against a minimum weight configuration. This configuration achieves a 1.4% fuel saving versus the N3-X baseline configuration. From the dataset of simulated configurations, the best fuel saving is achieved by using fan pressure ratios that are higher than those from the previous eSFC optimisation (Figure 5.15). The optimisation results in a relatively higher fan pressure ratio for the end and centre propulsors. Unlike the optimum from an eSFC only perspective, the low-est fan pressure ratio propulsor is in the half-span of the array, rather than the centreline of
9 10 11 12 13 14 15 16 17 18 Propulsion System Weight (tonnes)
11.7 11.75 11.8 11.85 11.9 11.95 12
eSFC (mg/Ns)
-5 -4 -3 -2 -1 0 1 2 3 4
Fuel Saving vs Baseline Config. (%)
Figure 5.14: Pareto front of individual propulsor fan pressure ratio and fuel saving versus baseline N3-X configuration
the array. End propulsors are sized for the greatest mass flow and are hence the largest and heaviest. Therefore, a higher fan pressure ratio will provide the greatest benefit to weight. In addition to a lower weight, the smaller size means a lower ratio of h/δ and hence a small in-crease in efficiency. In contrast, the centreline propulsors are the smallest and most efficient.
As these propulsors have the lowest SPC, a slight increase in fan pressure ratio is less detri-mental to overall performance than the same increase for propulsors further along the span.
Therefore, their fan pressure ratio may be slightly increased to provide a weight improvement at the expense of a slight loss in performance. Propulsors in the middle of the array half-span trade off these two factors and hence have relatively lower fan pressure ratios than the end and centreline propulsors. These propulsors are smaller than the end propulsor, but have a lower efficiency than the centerline propulsor. The optimum fan pressure ratio is therefore relatively lower, as performance can be improved at the expense of an increase in weight.
The two extreme ends of the pareto front provide low weight and low eSFC configurations for the propulsor array, with the minimum fuel configuration combining elements of both. Higher fan pressure ratios leads to smaller, lighter propulsors. The minimum weight configuration from the pareto front therefore has the highest fan pressure ratios. The low eSFC configuration demonstrates the opposite, as a low fan pressure ratio leads to a lower eSFC (Figure 5.10). All three selected configurations demonstrate a similar trade-off between performance and weight as a function of location.
5.2.4 Combining Propulsion System Options
Excluding the boundary layer-only and podded engine configurations, many of the alternative propulsor array configurations presented in this section provide relatively small fuel burn im-provements in the order of 1–3%. However, a more significant improvement can potentially be gained by combining the options assessed in each alternative configurations. Of the alterna-tives that have been assessed, there are three options that it may be useful to combine:
• Thrust split between main engines and propulsor array
• Turbofan replacement for turbogenerators (in combination with thrust split)
5. Alternative Propulsion System Configurations
Figure 5.15: Fan pressure ratio for individual fans for a minimum fuel configuration in comparison to an eSFC minimum configuration
Figure 5.16: Fan pressure ratio for individual fans for a minimum fuel configuration in comparison to minimum weight and minimum eSFC configuration from the pareto front
• Individually sized propulsors
Assuming each configuration option’s fuel saving can be compounded, a turbofan (BPR 4.0) configuration with an optimised array (for both weight and eSFC) would improve fuel consump-tion by 3.5% versus the baseline N3-X configuraconsump-tion. This would lead to a further 1.5% im-provement in energy usage versus the baseline B777-200LR aircraft model.