SE1400467A1 - Jet engine comprising a fluidic injection system for shock and mixing noise mitigation - Google Patents

Abstract

Abstract: The present invention relates to a jet engine, designed to be operated at supersonic speed, comprising a fluidic injection system for shock and mixing noise mitigation. The noise mitigation is achieved by specific combinations of the location of the injectors along the engine axis, the mass flow rate ratio of the injection and the angle of the injection. The combinations are different for different operational modes of the engine nozzle.

Description

Abstract: The present invention relates to a jet engine, designed to be operated at supersonic speed, comprising a fluidic injection system for shock and mixing noise mitigation.

The noise mitigation is achieved by specific combinations of the location of the injectors along the engine axis, the mass flow rate ratio of the injection and the angle of the injection. The combinations are different for different operational modes of the engine nozzle.

I Jet engine comprising a fluidic injection system for shock and mixing noise mitigation The present invention relates to a jet engine comprising a fluidic injection system for shock and mixing noise mitigation and especially such a jet engine designed to be operated at supersonic speed.

Modern aircraft engines are progressively more powerful and operate at higher pressure ratios and higher temperatures resulting in more noise emissions and stronger IR and RF signatures. Noise mitigation of supersonic jets continues to be a challenging issue since current noise reduction technologies such as passive mixing devices have provided only modest noise reduction with drawbacks such as drag penalties and increased noise at higher frequencies. Many methods to reduce jet noise have been proposed and investigated since jet engines have become widely used in commercial and military aviation sectors. This includes modifying the jet flow field by placing obstructions at the nozzle exit including tabs, lobed mixers, and chevrons. Chevrons have been especially successful at reducing jet noise in subsonic jets by creating streamwise vortices that promote increased mixing of the jet with the surrounding fluid.

Supersonic jets, unlike subsonic jets, not only contain broadband turbulent mixing noise but other tonal and broadband components. These additional components of supersonic jet noise are classified as screech tones and broadband shock-associated noise (BBSN), respectively. BBSN has been linked to large scale coherent structures interacting with the quasi-periodic shock cells located in the jet.

Screech tones are discrete tones that always appear at a lower frequency than BBSN. Screech tones arise when a feedback loop is created between perturbations of the shear layer and upstream traveling acoustic waves. The feedback loop is initiated by flow instabilities at the nozzle lip that propagate downstream along the shear layer. These instabilities, when interacting with the shock waves, create acoustic fluctuations that propagate upstream and further perturb the shear layer at the nozzle lip where it is very thin and susceptible to disturbances.

Microjets injection has been suggested as a noise control method for jets operating at high subsonic speeds. An attractive feature of microjets injection is that it can be conditionally implemented when required as a noise control method in contrast to chevrons which are perpetually in the jet flow. Microjets injection has been shown to reduce screech and BBSN pressure levels without shifting the frequency, showing 2 that mass injection actively reduces the interaction of the large scale coherent structures with the stationary features (shock waves) in the jet similar to chevrons. Most previous work with microjets has been performed using steady injection. However, pulsed microjets have also been considered.

One of the difficulties that have been encountered thus far is that the design space for microjets injection is very large. Microjets steady injection flow parameters include mass flow, pressure, velocity, and momentum ratio, while pulsed injection adds frequency, duty cycle, and phase. Injector configuration parameters include injector diameter ratio, pitch and yaw angle with respect to the jet engine axis, alignment of non-symmetric injectors, injector shape, number, spacing and location. The injection angle relative to the jet axis has been studied with injection angles between 0 and 90° being the most common, however few parametric studies on injection angles have been conducted. Most injectors have been placed at the nozzle exit injecting inwards through the outside of the jet shear layer.

Several patents describe the use of fluidic injection for thrust vectoring, enhanced performance, and noise reduction. In US patent 2,990,905 a plurality of small jets circumferentially distributed around a circular nozzle are injected at a small angle relative to the main jet. The auxiliary jets alter the vortex formation in the primary jet leading to reduced noise from the primary jet. US patent 6,112, 513 describes an apparatus and method for varying the effective cross-sectional area of an opening in a fixed geometry nozzle. It uses one or more steady or pulsed fluidic injectors at an angle to the subsonic crossflow to partially block the nozzle throat. The application is for improved performance and flow control for thrust modification. It uses slots or circular injectors at the nozzle throat to throttle or vector the main jet by the fluid injection. US patent 6,308,898 shows subsonic jet mixing using periodically pulsed fluid jets, circumferentially arranged, and opposing each other. The jets are pulsed out of phases to enhance jet mixing with ambient air. Results focus on exciting mixing (temperature reduction) with two circumferentially spaced slot injectors.

The previously described patents are broad and non-specific as to the operating conditions and it is possible not only not to reduce noise when implementing them, but instead increase noise. The present invention recognizes the need to adapt the parameters of the injection system to the nozzle operating conditions, which is not a simple thing and has called for demanding inventive activity. The parameters in question include injection location, number of injectors, injection angle, momentum 3 flux ratio, mass flow ratio, and pressure ratio. Various combinations of these parameters yield varying acoustic results since supersonic jets have multiple noise components.

The results are not obvious based on the general knowledge about microjet injection, but are instead specific combinations of parameters giving surprisingly good noise reduction, while a slightly different combination of parameters may increase the noise level.

As has been stated, in order to reduce noise it is necessary to improve the mixing of the main jet with the ambient flow, reduce its turbulence level, and weaken the shock structures. All these components contribute not only to noise but also to IR and RF signatures. When these measures of the main jet are affected not only noise is reduced but also the other signatures. Therefore, although the application presents data only related to noise reduction, the invention described here will be effective in reducing both noise and other signatures.

The invention solves the problem of noise reduction by being designed in the way that is evident from the following independent claims. The other claims define suitable embodiments of the invention.

The invention will now be described with reference to the accompanying drawings, in which: Fig. la illustrates designations for injection angles, Fig. lb illustrates designations for injection locations, Fig. 2 shows in table form specific optimal sets of control parameters optimized for different nozzle operating conditions, Fig. 3 shows a feedback control system including sensors and controllers to vary the injection conditions based upon the jet operating condition, Fig. 4 shows the shock structure for a non-ideal converging-diverging jet along with representative injector locations to generate noise reduction, Fig. 5a shows the difference in overall sound pressure level, AOASPL, for the injector location of Case 2.4 in Fig. 2 with varying injection mass flow, Fig. 5b shows the reduction of jet noise with an injection pressure of 8.2 bars at different angles relative to the jet for case 2.4. The highest reduction is at angles below 90° that are dominated by shock-associated noise, 4 and Fig. 6a-6c show individual spectra for three microphone angles and OASPL.

When discussing microjet injection, the injection location is separated into two distinct regions, internal and external injection. Internal injection uses discrete injection holes or continuous slotted injection on the nozzle surface to inject air into the primary jet. Fig. la and lb illustrates designations for injection angles and injection locations. The pitch angle is the angle of the injection relative to the primary jet axis and the yaw angle represents the swirl angle. The diameter of the primary jet nozzle exit is denoted D. When the injection occurs upstream of the nozzle exit, the location is denoted with a negative (-) sign. External injection refers to injecting air downstream of the nozzle exit indicated with a positive (+) sign. The external injectors are placed outside of the jet shear layer injecting downwards through the shear layer into the primary jet. The external injectors can be mounted in an external casing. The injector centres distance from the nozzle exit is determined by a practical arrangement of the casing and will be less than 0.1Dj, often around 0.05 Di.

The invention described is one that reduces supersonic jet noise by impacting several sources of noise: 1. Destructive shock interference, thus reducing shock associated noise.

Increasing mixing of the jet with ambient flow, thus reducing the main jet velocity.

Disrupting the formation of large-scale structures, thus reducing low frequency mixing noise. 4. Reduce small-scale turbulence, thus reducing high frequency noise.

Some injection strategies have more impact on mixing enhancement while others address shock mitigation, as shown in the table in Fig. 2.

The device requires precise locations, injection angles, and injection mass flows that provide the most acoustic reduction. The device involves placing an array of injectors internal or external to a converging (C) or a converging-diverging (C-D) supersonic jet nozzle to provide the desired acoustic effect.

The device can be actively controlled using a feedback control system including sensors and controllers to vary the injection conditions based upon the jet operating condition. Fig. 3 describes the concept of the feedback active controller. A microphone embedded in the nacelle or airframe near the nozzle exit monitors the sound level which is compared to a desired level. The difference is fed into the controller which gets information from the engine computer regarding the operating conditions including the Nozzle Pressure Ratio, NPR, and the ambient pressure. With this information the controller determines whether the nozzle is operating at over- expanded, design, or underexpanded conditions. It is then directs the actuator which microjets to activate and what is the required injection pressure (setting the desired, mass flow rate ratio, MFRR). The microjets location (both axially and circumferentially), angle of injection, and number of activated microjets are thus set.

In another embodiment, a fully annular slot can be used to provide the desired effect in which shock deconstruction occurs. The slot injection would be near conditions indicated for the discrete injectors and would reduce noise through the same shock cancellation method.

In another embodiment the device can be used in combination with any vortex generating mechanism such as chevrons or fluidic vortex generators to achieve a combined effect of mixing and shock noise reduction. Chevron or vortex generators reduce jet noise by producing axial vortices that increase mixing and disrupt shocks. However, these vortices are dissipated after a short distance. The addition of fluidic injection strengthens these vortices and thus increases their effectiveness.

The invention relies on specific implementation of fluidic injection at different jet operating conditions in order to achieve the desired effect. The best conditions found are summarized in the table in Fig. 2. The shock structure for a non-ideal converg- ing-diverging jet is shown in Fig. 4 along with representative injector locations to generate noise reduction for this shock structure. There are two sets of shocks that exist in the jet flow, one that originates from the nozzle throat, and one that originates from the nozzle lip. This results in a double shock structure at some conditions. The shock waves reflect in the supersonic jet flow until the jet velocity reduces to subsonic velocity and can no longer sustain the shock waves. The injectors are placed at some location downstream of the throat either internal (upstream of nozzle exit) or external (downstream of nozzle exit).

All of the tests leading to this patent application have been conducted with circular fluidic injectors that were placed at various locations and at various injection angles.

Controlling the amount of injected air can be discussed in terms of mass flow, 6 pressure, or momentum flux ratio j = ,\I p,*piv;, where p is density, V is velocity, and i denotes injection conditions and] denotes jet conditions. Fig. 5a shows the difference in overall sound pressure level, AOASPL, for the injector location for Case 2.4 in Fig. 2 with varying injection mass flow.

The impact of injection at a pressure of 8.2 bars is shown in Fig. 5b for all microphone angles from ° (upstream) to 10 (downstream). Although the figure shows injection pressure, increasing injection pressure means increasing pressure ratio, mass flow, and momentum flux ratio. It is clearly seen that for lower injection pressures there is no effect or an increase in noise. However, at an optimum injection pressure (7.5 bars) corresponding to the conditions in Case 2.4, there is a strong drop in AOASPL (-4.5 dB) at the upstream ° microphone. The highest suppression occurs for angles below 90°, a direction that is dominated by shock associated noise. Increasing pressure causes amplification of BBSN and screech until the optimum condition is reached which suppresses the shocks noise and screech noise.

Individual spectra for three microphone angles and OASPL are shown in Fig. 6a-6c.

The shock noise suppression is clearly seen for 0 and 90° spectra while the turbulent mixing noise that dominates the 1° microphone is relatively unaffected.

This indicates that for this case the noise reduction mechanism is not due to vorticity production that most jet noise reduction devices rely on. Computational results using Large Eddy Simulation (LES) and Computational Aeroacoustics (CAA) support these findings. The CAA predicts the same noise reduction at the sideline and upstream angles from this injection configuration. The noise reduction methodology of this invention aims to reduce both mixing noise and shock-associated noise. Some injection parameters are more effective in reducing mixing noise (for example cases 1.4, 1.6, 2.6 in Fig. 2), some mitigate shock noise (for example cases 1.3, 1.7, 2.7), and some affect both (cases 1.4+1.8, 2.4+2.8).

In the following the concrete results of the invention will be presented by way of examples. The invention defines a noise reduction system for a jet engine having a converging or a C-D nozzle. C-D nozzles can operate at three conditions depending on the Nozzle Pressure Ratio, NPR=P0J/Pa, where Poi is the total pressure at which the nozzle is operating and Pa is the ambient pressure. 7 The first mode of operation is the design condition, or fully expanded condition, when the nozzle is operated at NPRdes,g, such that Pe=Pa, where Pe is the pressure at the exit plane. Each nozzle has a specific NPR for which it was designed to operate, and when it operates at this NPR the jet should theoretically not contain any shock waves. For a C-D nozzle this design NPR is determined by the C-D nozzle area ratio between the nozzle exit area and the throat (the minimum area location) area. However, to achieve shock free operation the nozzle has to be designed carefully based on the method of characteristics. Practical supersonic jets have conical convergent and divergent sections and therefore they contain shocks even at design conditions, albeit of lower strength.

The second mode of operation is the overexpanded condition, when the NPReperation is lower than NPRdes,gn such that Pe The third condition is an underexpanded jet, when NPRoperation is higher than NPRdesign such that Pe>Pa. At this condition an expansion wave is formed at the nozzle exit to expand the flow to ambient pressure. The expansion wave bounces off the jet shear layer as shock waves.

Converging nozzles can operate only at design conditions, when the nozzle exit Mach number is exactly sonic, and at underexpanded conditions. Converging nozzles cannot, as C-D nozzles, operate at overexpanded conditions.

Each one of the operating conditions requires a different set of control parameters when applying noise control by fluidic injection to obtain noise reduction. These parameters include: spacing between injectors, location of the injectors relative to the nozzle exit plane, angle of injection, and strength of injection. The latter can be characterized by three different alternative parameters: momentum flux ratio, J, mass flow rate ratio, MFRR, between the injection jet, m„ and the main jet, mi, and pressure ratio between the injector pressure, p,, and the ambient pressure pa. Specific sets of control parameters optimized for different nozzle operating conditions are summarized in the table in Fig. 2. 8 For each one of the NPRoperation conditions the fluidic injection parameters can be divided into two groups: Internal injection (Intl) and lip injection. Lip injection can be either just upstream of the lip inside the nozzle (Lip Injection Inside, LII) or just outside of the lip (Lip Injection outside, L10). In this patent application lip injections means that the distance from the lip to the injector centre is less than ± 0.1 Di.

For overexpanded NPR, only applicable to a C-D nozzle, — case 01 — (cases 1.1 to 1.8 of Fig. 2) both LII and LIO yield the best suppression when operating at substantially 900 relative to the main jet direction. 60-1° give good results and 30-1° acceptable results. Mass flow rate ratio, MFRR, of 5% yield suppression of over 6 dB in shock noise and 4 dB in mixing noise. A mass flow rate ratio, MFRR, of 3% up to maximum available will give good results and MFRR from 2 up to maximum available will give acceptable results.

The injection should take place at least 5 points along the circumference of the nozzle. The injectors should preferably be placed circumferentially at an equal distance between the points of injection. If noise reduction is desired at a specific direction relative to the engine, more injectors can be concentrated in the circumferential section of the engine facing that direction, and the injectors' distribution will not be symmetric for this case. 6 injectors provide the best performance, 8 injectors are good and up to 12 give acceptable result.

When Intl is used, the injectors should be at substantially -0.7Li upstream of the nozzle exit (-0.8L to -0.61Li give good results and -0.9L to -0.2Li are acceptable too).

For a C-D nozzle, Li is the nozzle length from the throat (minimum area) to the nozzle exit, and for a converging nozzle from the beginning of contraction to the nozzle exit. At this location, optimal suppression is achieved — case 02a — with MFRR = 3.6% (good: 3 - 4%; acceptable: 2 - 4%) at an injection angle of substantially ° (good: 25-50'; acceptable: 20-70°), or — case 02b — MFRR = 2.4% (good: 2 - 3%; acceptable: 2 - 5%) at an angle of substantially 60° (good: 50-70°; acceptable: 30-90°). Both cases yield over 6 dB reduction in shock and high frequency noise. A combination of Intl and LII can provide optimal suppression and Intl and LIO are effective too. The number of injectors etc. should be as in the previous case.

An example of the sensitivity of the noise control outcome to the combination of injection parameters is that if all parameters are the same as described above for the 9 Intl but MFRR = 1.3% the noise level will increase by 1 dB due to significant augmentation of the high frequency noise.

For the design NPR, which applies to both converging and C-D nozzles, (cases 2.1 to 2.8 of Fig. 2), both LII and LIO yield the best suppression when operated at sub- stantially 900 (good: 70-1°; acceptable: 30-1°) relative to the main jet direction. Mass Flow Rate Ratio, MFRR, of 3.5% — case D1a — yields suppression of over 7-8 dB in mixing noise. A mass flow rate ratio, MFRR, of 2.5% up to maximum available will give good results, and MFRR from 2 up to maximum available will give accepta- ble results. Alternatively, with MFRR of 1.2% — case Dlb — up to 5 dB reduction in shock noise is achieved. A mass flow rate ratio, MFRR, of 1 — 1.7% will give good results and MFRR of 1 - 2% will give acceptable results. The number of injectors etc. should be as in the previous case.

When lintl is used, the injectors should be at substantially -0.71_j upstream of the nozzle exit (good: -0.8L1 to -0.6Li and -0.9Li to -0.5L are acceptable too). At this location, optimal suppression is achieved with — case D2a — MFRR = 2.6% (good: 2 - 3%; acceptable: 1 - 4%) at an injection angle of substantially 60° (good: 40-80°, acceptable: 20-90°), or — case D2b — MFRR = 0.6% (good: 0.5- 0.8%; acceptable: 0.4 - 1%) at an angle of ° (good: 20-°; acceptable: 20-60°). Both cases yield over 1-5 dB reduction in shock and high frequency noise. A combination of Intl and LII can provide optimal suppression and Intl and LIO are effective too. The number of injectors etc. should be as in the previous case.

An example of the sensitivity of the noise control outcome to the combination of injection parameters is that if all parameters are the same as described above for the Intl but the location of injection is between -0.25L and -0.451_1, the noise level will increase by 3 dB due to significant augmentation of the shock and high frequency noise.

For underexpanded NPR, which applies to both converging and C-D nozzles, (cases 3.1-3.4) only Intl is effective. The injectors should be at substantially -0.7L upstream of the nozzle exit (good: -0.8L1 to -0.6L1 and -0.9L to -0.51_3 are acceptable too). At this location, optimal suppression is achieved with — case U1a — MFRR = 1.3% (good: 1 - 2%; acceptable: 1 - 4%) at an injection angle of substantially ° (good: 20-°; acceptable: 20-90°), or — case U1b — MFRR = 0.6% (good: 0.5- 0.8%; acceptable: 0.4 - 1%) at an angle of substantially 60° (good: 40-70°; acceptable: 20- 900). Both cases yield over 2-5 dB reduction in high frequency noise. The number of injectors etc. should be as in the previous case.

An example of the sensitivity of the noise control outcome to the combination of injection parameters is that if all parameters are the same as described above for the Intl but the location of injection is between -0.25Li and -0.45Li, the noise level will increase by 2 dB due to significant augmentation of the shock and high frequency noise.

Claims (14)

11 Claims: 1. Jet engine, designed to be operated at supersonic speed, having a convergent or a convergent-divergent primary nozzle for a primary jet exiting said jet engine, said jet engine comprising a fluidic injection system for shock and mixing noise mitigation that include injection at at least 5 points along the circumference of the nozzle, inside the primary nozzle or near the primary nozzle exit and directed inwards towards the engine axis, characterised in that the location of the injectors along the engine axis, the mass flow rate ratio of the injection and the angle of the injection falling in one of the following cases depending on the operational mode of the engine nozzle, for the primary nozzle being operated at overexpanded conditions, only applicable to a convergent-divergent nozzle, and the injectors being located near the nozzle exit at -0.1 < x/Dj<+0.1, where x is the distance from the nozzle exit and Dj the nozzle diameter at the exit, — case 01 — the mass flow rate ratio is 3% up to maximum available and the injections from the injector openings make an angle with said engine axis between 60-1°, where an injection along the engine axis from upstream is defined as 0° and an injection along the engine axis from down- stream is defined as 1800 , for the primary nozzle being operated at overexpanded conditions, only applicable to a convergent-divergent nozzle, and the injectors being located inside the primary nozzle at -0.6 < x/Li <-0.8, where Lj is the nozzle length from the throat to the nozzle exit, in a first alternative — case 02a — the mass flow rate ratio is 3 - 4% and the injections from the injector openings make an angle with said engine axis between 25-° or in a second alternative — case 02b — the mass flow rate ratio is 2 - 3% and the injections from the injector openings make an angle with said engine axis between 50-70°, for the primary nozzle being operated at design conditions, applicable to both converging and convergent-divergent nozzles, and the injectors being located near the nozzle exit at -0.1

1. 7% and the injections from the injector openings in both cases make an angle with said engine axis between 70- 1°, for the primary nozzle being operated at design conditions, applicable to both converging and convergent-divergent nozzles, and the injectors being located 12 inside the primary nozzle at -0.6 < x/Li <-0.8, where Li for a convergent nozzle is the nozzle length from the beginning of contraction to the nozzle exit and for a convergent-divergent nozzle, as stated, from the throat to the nozzle exit, in a first alternative — case D2a — the mass flow rate ratio is 2 - 3% and the injections from the injector openings make an angle with said engine axis between 40-800 or in a second alternative — case D2b — the mass flow rate ratio is 0.5 — 0.8% and the injections from the injector openings make an angle with said engine axis between 20-°, for the nozzle being operated at underexpanded conditions, applicable to both converging and convergent-divergent nozzles, and the injectors being located inside the primary nozzle at -0.6< x/Li< -0.8, in a first alternative — case U1a — the mass flow rate ratio is 1 -2% and the injections from the injector openings make an angle with said engine axis between 20-° or in a second alternative — case U1b — the mass flow rate ratio is 0.5— 0.8% and the injections from the injector openings make an angle with said engine axis between 40-70°.

2. Jet engine according to claim 1, characterised in that in case 01 the mass flow rate ratio is substantially 5%

3. Jet engine according to claim 1 or 2, characterised in that in case 02a the mass flow rate ratio is substantially 3.6%

4. Jet engine according to claim 1 or 2, characterised in that in case 02b the mass flow rate ratio is substantially 2.4%

5. Jet engine according to anyone of claims 1-4, ch a r act erised in that in case D1 a the mass flow rate ratio is substantially 3.5%

6. Jet engine according to anyone of claims 1-4, ch a racterised in that in case D1 b the mass flow rate ratio is substantially 1.2%

7. Jet engine according to anyone of claims 1-6, characterised in that in case D2a the mass flow rate ratio is substantially 2.6%

8. Jet engine according to anyone of claims 1-6, characterised in that in case D2b the mass flow rate ratio is substantially 0.6% 13

9. Jet engine according to anyone of claims 1-8, characterised in that in case U1a the mass flow rate ratio is substantially 1.3%

10. Jet engine according to anyone of claims 1-8, ch a r a cte ri sed in that in case U1b the mass flow rate ratio is substantially 0.6%

11. Jet engine according to anyone of claims 2, 5 or 6, characterised i n that the injections from the injector openings make an angle with said engine axis of substantially 900 .

12. Jet engine according to claim 3, 8 or 9, characterised in that the injections from the injector openings make an angle with said engine axis of substantially °.

13. Jet engine according to claim 4, 7 or 10, characterised in that the injections from the injector openings make an angle with said engine axis of substantially 60°.

14. Jet engine according to claim any of the previous claims, ch a r a cte r- ised in that it comprises a feedback control system including a micro- phone monitoring the sound level near the nozzle exit and at least one sensor monitoring the jet operating condition, and a control devise that compares the monitored sound level with a desired level and based on the input from said at least one sensor calculates the optimal injection conditions and activates suitable injectors and decides their MFRR in order to achieve the desired sound level.