Turbofan engine technology

This report reviews current turbofan engine noise and engine noise reduction technology, specifically focusing on the engine technology of larger passenger jetliners which have entered into service within the last ten (10) years.
Important factors in turbofan engine design from a community noise perspective and the sources of noise along with their relative importance are also presented. A review of different engine noise reduction technologies is presented, as well as an estimate of the technology’s readiness level.
Finally, potential trade-offs, challenges, and future technology directions are outlined.

1. CURRENT TURBOFAN ENGINE TECHNOLOGY Presented by Sammy Jamar Presented to Dr. Kamel Shorbagy

2. Introduction This report reviews current turbofan engine noise and engine noise reduction technology, specifically focusing on the engine technology of larger passenger jetliners which have entered into service within the last ten (10) years. Important factors in turbofan engine design from a community noise perspective and the sources of noise along with their relative importance are also presented. A review of different engine noise reduction technologies is presented, as well as an estimate of the technology’s readiness level. Finally, potential trade-offs, challenges, and future technology directions are outlined.

3. CONT.. Turbofan engines are commonly used on commercial transports due to their advantage for higher performance and lower noise. The noise reduction comes from combinations of changes to the engine cycle parameters and low-noise design features. Turbofan engine noise technology has seen significant advances, generally dominated by improvements in the engine cycle with the bypass ratio (BPR) increasing and the fan pressure ratio (FPR) decreasing significantly. Details in engine design like the detailed fan/OGV design play a significant role and, in some cases, can even overwhelm the general trend of lower noise with higher BPR and lower FPR. Looking ahead, it is unclear if the trend of increasing overall propulsive efficiency with the corresponding fuel burn reduction and lower community noise will continue.  

4. 1.CURRENT ENGINE TECHNOLOGY Fig. 1 Range of BPR and FPR of current engines

5. CONT.. Figure 1 shows the cycle range and specific thrust trend of current engines. For a given amount of thrust, to increase the BPR and drop the FPR at maximum takeoff power, the specific thrust (thrust/mass flow) is reduced and the fan diameter is generally increased.

6. CONT… Fig. 2 Noise and fuel burn trend with fan diameter

7. CONT.. Fig 2: Even on new airplanes, as the fan diameter increases, the overall wetted area increases and integration issues increase, which can increase weight and drag and lead to a fuel burn minimum and, at some value of fan diameter, the fuel burn benefit goes away and becomes a penalty.

8. CONT.. Fig. 3 Typical sources of noise from a turbofan engine

9. CONT.. Fig. 4 Typical source breakdown of a current (~ BPR10) engine

10. CONT.. The amount of community noise which a given engine makes is determined not only by the engine’s FPR and BPR but also; Detailed fan and fan OGV design, The details of the LPT and low-pressure compressor (LPC) design, How oil cooling is done As the fan diameter increases, the weight and integration issues increase and the impact on things like the fan/OGV spacing become significant. Reduced fan/OGV spacing leads to an increase in noise. Increasing BPR and lowering FPR results to an increase in engine heat and thus cooling is required through the introduction of bleeds which are a great source of noise again.

11. CONT.. Recently, combustors have been designed for high efficiency and low nitrogen oxide (NO2) emissions. To achieve these requirements, the engines tend to operate at higher temperatures and at, or close to the fuel-lean limit. This makes the combustion process unsteady and increases the combustion noise. The design of the low-pressure system including the compressor and turbine greatly depends on whether a gearbox is used to drive the fan. In the case of a geared fan, both the low-pressure compressor (LPC) and turbine (LPT) are designed to run at a higher speed, generally closer to their peak efficiency. For the geared fan engine, unless low blade counts are used, the LPT noise will be close to the high end of the audible range.

12. CONT.. Fig. 5 Cumulative noise relative to Sect. 3 of the recently certified first of model twin airplanes

13. CONT.. Figure 5 shows the ICAO-certified noise levels [8] of twin turbofan powered first of model airplanes recently delivered as a function of the published bypass ratio. Even though the published BPR is notoriously inaccurate and is usually quoted at the cruise condition, it does show that there is more than one example where the noise delta between the engines is opposite of the expected trend

14. 2.REVIEW OF NOISE REDUCTION TECHNOLOGIES Fan and jet noise are still the most dominant noise sources and this section will focus on technologies that reduce these sources.

15. 2.1 FAN NOISE Fan noise reduction has historically been achieved through a combination of: decreased FPR and tip speed; Improved acoustic treatment (such as the acoustically smooth inlet); Proper selection of fan and OGV blade counts Ample spacing between the fan and OGV rows

16. CONT..

17. CONT..

18. 2.2 JET NOISE Jet noise reduction has historically been achieved through cycle optimization with generally increasing bypass ratios.

19. CONT..

20. CONT..

21. CONT.. Looking at the above lists, it becomes significantly clear that the only technologies that seem progress to a higher TRL and implemented are those that have either a neutral or positive effect on fuel burn and that also do not add a significant amount of complexity. It is also clear that new approaches are needed, and there needs to be significant effort put forth into new technologies to achieve both noise reduction and avoid negatively impacting fuel burn.

22. 3. FUTURE OPPORTUNITIES For every significant technology advancement, there has been a significant movement of where the “optimum” cycle design point is. This technology advancement tends to increase BPR and reduce FPR, and has traditionally depended significantly on material technology including high temperature and lightweight materials and structure.

23. CONT.. Fig. 6 Effects of technology on engine noise and fuel burn

24. The ‘short and slim nacelle’ with compact nacelle reduces the overall length/diameter ( L / D ) of the nacelle and, therefore, mitigates both installation impacts and weight associated with a larger fan diameter. This unfortunately can have a significant community noise impact and can reduce or potentially eliminate the noise reduction that would typically be seen with the reduction in specific thrust. The desire for a short engine tends to reduce the number of LPT stages, increasing LPT speed. When there is not fan gearbox, this, in turn, can increase the fan speed over what it would otherwise be, leading to an increase in the fan source noise. Another idea that can have a fairly dramatic effect on community noise is the open rotor. Recent studies have shown that, even with rather dramatic compromises for noise, the best noise levels which the open rotor could achieve are to meet chapter 14 which is significantly louder than the current generation of turbofans.

25. Fig. 7 Certified cumulative noise levels  

26. Figure 7 shows the noise reduction in the certified noise levels [15] seen for the last generation versus the current one, due to the pressures just discussed, this trend is not likely to continue without a significant increase in community noise technology funding. High TRL technologies would need to be available in the near future due to the time scales involved in the development of large commercial airliners

27. There has been a significant effort going into the Blended Wing Body concept and other concepts that hold this potential. It would seem, however, that these types of configurations are beyond the 2030 timeframe at least to be in significant commercial service. There is another concept called distributed propulsion which has at least some potential in the longer term. This concept reduces the specific thrust by increasing significantly the number of propulsion units.

28. Fig. 8 Distributed propulsion concept

29. This is illustrated in Fig. 8. As shown, the idea would be for a single engine core to drive several fans, thus, further reducing the specific thrust and potentially improving fuel burn while also having the potential to reduce community noise (depending on the integration). The integration for this concept would be key to whether or not it would, in fact, result in a noise reduction. On one hand, you have increased the potential noise sources, but, on the other, you have likely significantly reduced the fan pressure ratio and tip speeds. There is also the potential to significantly improve the installation of the fans onto the airframe and thereby help use the airplane for shielding. Finally, it may be possible to phase the fans in such a way as to reduce the noise

30. Although this concept may be possible with mechanical shafts and gears, to be viable from a complexity stand point, the core would likely need to consist of a smaller high-speed core (also significantly integrated into the airframe) and a generator with a battery and the fans would each need an electric motor. This is what is typically termed a hybrid electric concept. Due to the limitations in battery energy density and power limitations of a conventional distribution system, this concept is likely limited to smaller airplanes for the foreseeable future.

31. CONCLUSION This review has focused on the technology of engines on larger passenger jetliners whose entry into service has been within the last 10 years, and showed the sources of noise and their general relative importance. The noise sources of these engines, mostly dominated by fan noise with other sources still being consequential, were shown. In addition, a review of different engine noise reduction technologies was shown as well as an estimation of the technologies’ overall technology readiness level. Finally, possible paths forward were explored and potential future technological advancements within engine technology and engine noise were discussed. Looking ahead, it is unclear if the trend of increased overall propulsive efficiency with the corresponding fuel burn reduction and lower community noise will continue. In fact, it seems more and more likely as a significant fuel burn reduction is achieved; lower community noise will not be and may even increase. Therefore, continued reduction in community noise will likely only be achieved if there is a significant increase in overall technology funding and/or a significant change in the overall airplane configuration.

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