A Theoretical and Experimental Investigation on Ejector Acoustics and Ejector Silencer Design

Abstract

Ejectors are common devices used across many industries, however, they are often plagued by the presence of low frequency pressure pulsations causing both broadband and tonal noise. This thesis presents a theoretical and experimental investigation into the acoustics of an ejector-silencer combination with the emphasis placed on silencer design. More specifically, the noise generated by the jet, the diffuser and natural modes is explained and estimated. The noise generation mechanisms of impingement tones and edgetones are presented but accurate predictions are found to be hard to make. An outline of the experimental facility is followed by the mechanical, acoustical and aerodynamic design details of the approximately 70:1 ejector scale model. A detailed analysis on the primary nozzle control and calibration is presented before beginning an aerodynamic and acoustic characterization of both the ejector and the wind tunnel facility. From experimentation, it is found that the placement of a perforated cone in front of the solid cone is beneficial in reducing the noise generated without overly affecting the entrainment ratio. Other configurations tested tend not to be as acoustically effective or to decrease the entrainment ratio below an acceptable level. Experiments prove that the low frequency noise generated by the ejector is mainly caused by natural mode excitation.iv

An experimental investigation of the use of an outlet silencer to quiet ejectors

“…Source: reproduced with permission from Desmarais (2018). 9 increased from 0.7 in small intervals, it is found that the tone grows significantly, with the maximal levels measured somewhere between approximately Mach 0.76 and Mach 0.8. At higher Mach numbers, the amplitude of this tone reduces, resulting in a spectrum which shows no dominant tone.…”

“…Source: reproduced with permission from Desmarais (2018). 9 numbers is attributed to the jet containing more energy at higher Mach numbers, thus being better able to excite the acoustic natural modes of the ejector. As mentioned earlier and detailed in equation 1, the acoustic efficiency of a jet markedly increases with jet velocity.…”

“…Table 1 identifies the spectral peaks of Figure 9, including those not mentioned above (The calculations of the natural modes are omitted for brevity, but the interested reader can find the details in Desmarais. 9 ). This modal decomposition is arbitrarily performed at Mach 0.9.…”

“…Full details of the Schroeder frequency calculations, including the reverberation time estimation, can be found in Desmarais. 9 It is interesting to compare the spectra for locations upstream of the silencer (location B) with locations downstream of the silencer (location F for example) in Figure 9. In a general sense, it can be seen that the SPL is lower at location F than B.…”

“…Source: reproduced with permission from Desmarais (2018). 9 as the baseline configuration. For this reason, the two natural frequencies of slightly different values are not equally affected, thus reducing the width of the peak.…”