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Morales, Jesse

University of British Columbia

Vacuum ejectors are devices commonly used in industries such as mining to collect and transport process wastes such as sludge. Ejectors operate using a high-pressure primary fluid accelerated through a nozzle to entrain and mix with the secondary fluid which is discharged, creating a region of low pressure as secondary fluid is evacuated. The performance of an ejector can be defined in several ways including the quality of suction produced as the compression ratio, and the ratio of primary fluid to secondary fluid used as the entrainment ratio. The ejector’s performance is highly sensitive to primary stream pressure and flow rate which can produce complex flow structures in the mixing chamber as the secondary flow is entrained. One possible cause of poor performance is boundary layer separation from the walls of the mixing chamber, which is referred to as “mixing chamber stall”. In addition to degrading the performance of the ejector, mixing chamber stall may interfere with design enhancements intended to improve the ejector’s entrainment ratio. This study aims to identify and experimentally investigate the mixing chamber stall and characterize its impact on the vacuum ejector performance. A rectangular vacuum ejector experimental setup is constructed and incorporated with pressure diagnostics. The current study uses a primary nozzle with a design Mach number of 2.28. The nozzle exit position, presence of secondary flow, and primary nozzle stagnation pressure are the controlled parameters during experiments. The output variables in this study are the stagnation pressure ratio, compression ratio, and normalized wall pressures in the ejector mixing chamber and diffuser. Pressure data shows that mixing chamber stall occurs over a range of nozzle operating conditions. The stall initiation point is relatively constant around a nozzle operating stagnation pressure ratio (SPR) of 5.9 for all tested nozzle exit positions, but is reduced when secondary flow is introduced. The stall endpoint is shown to increase as nozzle exit position is increased and ranges from 7.5 < SPR < 11.0. The stall endpoint SPR is similarly reduced with the presence of secondary flow. Peak stall strength is most significant in the constant area section of the ejector mixing chamber and remains relatively constant for all nozzle exit positions. Changes in the mixing chamber pressure profiles such as the stall initiation SPR, stall endpoint SPR, and the peak stall strength SPR correspond with changes in acoustic behaviour including a pronounced increase in broadband acoustic intensity in the mixing chamber at the onset of stall. The diffuser experiences a broadband decrease in acoustic intensity at the onset of stall. Dominant frequencies of pressure oscillations at the tested locations along the ejector walls are around St ≈ 2.1×10⁻³ and 9.3×10⁻³ and are subject to a minor frequency shift that begins while the ejector mixing chamber is stalled.

Text

2022-06-06

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/81704

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/