Brenton Greska

Chapter 3 – HIGH-SPEED JET NOISE REDUCTION USING MICROJETS

This chapter presents a possible approach for the suppression of the dominant large-scale mixing noise sources in a supersonic jet. One of the dominant sources of noise from a high-speed jet is intimately related to the large-scale vortical structures that convect at supersonic speeds relative to the ambient medium. Microjets are used to reduce screech tones, Mach-wave radiation/crackle, and large-scale mixing noise in supersonic jets. They inject a high-pressure gas at the nozzle exit of supersonic jet. The microjets are effective probably because they inhibit the formation of the large eddies—the consequence of basic jet instabilities—which are responsible for the crackle and the low-frequency component of the mixing noise. When the jet is imperfectly expanded, the commonly observed screech tones are completely suppressed by the microjet injection. The main jet parameters, the nozzle pressure ratio, and the temperature ratio are chosen to correspond with the realistic engine operating conditions.

High-Speed Jet Noise Reduction Using Microjets on a Jet Engine

by a jet engine. Experiments were conducted at the NASA Ames Research Center using a General Electric YJ97-GE-3 turbofan jet engine that was equipped with a 317.5 mm converging nozzle. The engine was operated at conditions that resulted in jets with fully expanded Mach numbers of 0.9 and 1.3. The microjets were generated using up to 48 evenly spaced micro-nozzles that had exit diameters of 1.2 and 2.4 mm. The operating pressure of the microjets was varied from 7.9 to 42.4 bar. Various microjet configurations were used resulting in a total mass flux of the microjets that ranged from 0.5 to 2.3 % of the primary mass flux for the subsonic jet and from 0.3 to 1.0 % of the primary mass flux for the supersonic jet. Through the various configurations it was found that reductions of up to 2 dB in the OASPL could be obtained for both the subsonic and supersonic jets. The reductions for the subsonic jet were seen at all frequencies while they were seen primarily at the higher frequencies for the supersonic jet. A reduction of about 2 dB in the shock noise of the supersonic jet was also observed.

On the Far-Field Propagation of High-Speed Jet Noise

This paper deals with the effects of atmospheric absorption on the propagation of high-speed jet noise. The common practice for determining the far-field jet noise spectra at a distance far from the jet exit (>100D, where D is the nozzle exit diameter) involves extrapolating data that is typically obtained between 35D and 100D from the nozzle exit. The data is extrapolated along a radial line from the nozzle exit by accounting for the effects of spherical spreading and atmospheric absorption. A previous paper discussed far-field measurements that were obtained for a twin engine aircraft at three locations along a radial line in the peak noise radiation direction. The authors were unable to extrapolate the spectra from the nearest location to either of the further locations and the observed differences were attributed to nonlinear effects in the jet noise signal. It is the purpose of this paper to show that the common extrapolation practice is valid for high speed jets, except in the peak radiation direction and its surrounding angles. Mach wave radiation is present at these locations and the common practice will yield unsatisfactory results, similar to those observed in the previous paper. The data used in this paper is taken from experiments carried out at 1/5th-scale and full scale and the experimental conditions of these high-speed jets are quite similar to those of the previous paper.Copyright

The Near-Field Effects of Microjet Injection

Presented herein are the results of an investigation into the effects of gaseous microjet injection on the near-field noise of a high-speed jet. The nitrogen-based microjets were generated through the use of six equally spaced 800-μm converging micro-nozzles, for which the operating pressure was varied from 100 to 400 psig. The experiments focused on both ideally and overexpanded main jets that were generated using a converging-diverging nozzle that was designed for Mach 1.8. The exit diameter of this nozzle was 40.0 mm and both of the resulting jets had a fully expanded velocity, Uj, of 880 m/s. An increase in the frequencies associated with Mach wave radiation was observed in the first few microphone locations for both cases. This was followed by a decrease in these same frequencies in the microphone location indicative of the far-field peak radiation direction. This suggests that the microjets are in fact breaking down the large scale structures associated with Mach wave radiation in the peak direction. In both cases, the reductions in the peak radiation direction were found to be similar but a stark contrast was observed at the shallower angles. While the shock noise in the overexpanded jet was only slightly increased, the microjet injection on the ideally expanded jet resulted in the generation of overexpanded-like shock noise. It was also observed that the noise reductions, as well as the increases, propagate from the near-field into the corresponding direction in the acoustic far-field.

The Off-Grid Zero Emission Building

This paper deals with the Off-Grid Zero Emissions Building (OGZEB), a project undertaken by the Sustainable Energy Science & Engineering Center (SESEC) at Florida State University (FSU). The project involves the design, construction and operation of a completely solar-powered building that achieves LEED-NC (Leadership in Energy and Environment Design-New Construction) platinum certification. The resulting 1000 square foot building will be partitioned such that 750 square feet will be a two bedroom, graduate student style flat with the remaining 250 square feet serving as office space. This arrangement will allow the building to serve as an energy efficient model for campus designers in student living and office space. The building will also serve as a prototype for developing and implementing cutting edge, alternative energy technologies in both residential and commercial settings. For example, hydrogen will be used extensively in meeting the energy needs of the OGZEB. In lieu of high efficiency batteries, the excess electricity produced by the building’s photovoltaic (PV) panels will be used to generate hydrogen via water electrolysis. The hydrogen will be stored on-site until needed for either generating electricity in a Proton Exchange Membrane (PEM) fuel cell stack or combusted in natural gas appliances that have been modified for hydrogen use. Although commercial variants already exist, a highly efficient water electrolysis device and innovative PEM fuel cell are currently under development at SESEC and both will be implemented into the OGZEB. The use of hydrogen in modified natural gas appliances, such as an on-demand hot water heater and cook top, is unique to the OGZEB.Copyright

Aeroacoustics of a Heated Mach 2.0 Jet

Results of an experimental study to determine the aeroacoustic characteristics of a heated supersonic jet operating at an ideally expanded condition are presented. The heated jet was operated at an exit Mach number of 2.0, which resulted in an exit velocity of 782 m/s. The Reynolds number based on the jet exit conditions is about 5x10 5 . The jet characteristics are given in terms of mean velocity and temperature profiles that cover the range from the nozzle exit to about 30 diameters downstream. The acoustic near and farfield measurements were obtained to characterize the sound field. The details of the data provided here are sucient enough to be used in the validation of numerical simulations of supersonic jets. The near-field acoustic data show distinct broadband peaks corresponding to the mixing noise at St = 0.25 and the Mach wave radiation at St = 0.7. The corresponding far-field spectra show a dominant peak at St = 0.2 in the peak radiation direction. A broad spectrum is observed in the direction normal to the jet axis, which is consistent with many previous observations.

A Comparative Study of Heated Single and Equivalent Twin Jets

This paper compares the flow characteristics and noise generated by a single axisymmetric nozzle and parallel and canted twin nozzle configurations. The twin nozzles were designed such that they had a cumulative exit area equivalent to that of the single nozzle. PIV images obtained for the jets operating at cold conditions reveal that the parallel jets begin to merge at x/D = 4 (D = single nozzle diameter) while the canted jets begin to merge at x/D = 2. Of note is that for x/D > 12, the merged canted jets appear to have a jet width that is similar to that of the single jet. Acoustic measurements were made using heated jets operating at comparable conditions. As expected, the parallel twin nozzles produced less noise, up to 3 dB, than the single nozzle at all of the angles examined here. Canting the twin nozzles together reduced the noise suppression benefits and the resulting jet noise was similar to that of the single jet at some angles, depending upon the twin jet orientation. A dependence on the orientation of the jets was also exhibited by the frequency spectra for both the parallel and canted twin jets in both the peak radiation and normal directions. The eect of water microjet injection on the twin jet configurations is also discussed and reductions of up to 7 dB have been observed.

CONSTRUCTION AND IMPLEMENTATION OF THE OFF-GRID ZERO EMISSIONS BUILDING

This paper deals with the construction and implementation of the Off-Grid Zero Emissions Building (OGZEB), a project undertaken by the Energy Sustainability Center (ESC), formally the Sustainable Energy Science and Engineering Center (SESEC), at the Florida State University (FSU). The project involves the design, construction and operation of a completely solar-powered building that achieves LEED-NC (Leadership in Energy and Environment Design-New Construction) platinum certification. The 1064 square foot building is partitioned such that 800 square feet is a two bedroom, graduate student style flat with the remaining 264 square feet serving as office space. This arrangement allows the building to serve as an energy efficient model for campus designers in student living and office space. The building also serves as a prototype for developing and implementing cutting edge, alternative energy technologies in both residential and commercial settings. For example, hydrogen is used extensively in meeting the energy needs of the OGZEB. In lieu of high efficiency batteries, the excess electricity produced by the buildings photovoltaic (PV) panels is used to generate hydrogen via water electrolysis for long term energy storage. The hydrogen is stored on-site until needed for either generating electricity in a Proton Exchange Membrane (PEM) fuel cell stack or combusted in natural gas appliances that have been modified for hydrogen use. The use of hydrogen in modified natural gas appliances, such as an on-demand hot water heater and cook top, is unique to the OGZEB. This paper discusses the problems and solutions that arose during construction and includes detailed schematics of the OGZEBs energy system.Copyright

Near-field Noise Characteristics of a Pulse Jet

3The results of an experimental investigation to study the near-field noise characteristics of an axisymmetric pulse jet are presented. Measurements were made over a range of Strouhal numbers (St = fd/Uj; f: pulsing frequency, d: nozzle exit diameter and Uj: nozzle exit mean velocity) of 0 - 0.106. The mean exit Mach numbers varied from 0.3 to 0.8. Near-field acoustic measurements were made using an array of 6.35mm microphones spanning over an axial distance of 0 – 18 jet diameters. The linear array was positioned at transverse distances of 5 and 6 diameters from the jet centerline. The near field pressure spectra of pulse jets feature large amplitude pulse frequency tone and its harmonics with corresponding broadband amplification. Within the range of parameters studied, the OASPL (Over All Sound Pressure Level) increase of about 32 dB as compared to the steady jet is observed in the nozzle exit plane. The magnitudes of OASPL increases with axial distance, reaches a maxima and then decrease at larger distances downstream. The axial distance corresponding to peak OASPL coincides with that of the primary vortex ring maximum circulation.

On the Limitations of High Speed Jet Noise Suppression (Keynote Paper)

This paper deals with an experimental investigation on the suppression of high-speed jet noise using air/water microjet injection at the nozzle exit. The far-field acoustic measurements from a high temperature Mj = 1.38 and Mj = 0.9 axisymmetric jet issuing from a converging nozzle show the suppression of screech tones, Mach wave radiation/crackle and mixing noise due to the use of microjets. Estimations of the contributions of different noise sources to the far-field sound are made using the current data supported by observations of previous investigators. It appears that the mixing noise reduction due to elimination of large eddies is found to be about 3–5 dB. Any further reduction of noise may only be accomplished by significant turbulence suppression and thermodynamic changes in the jet.Copyright