Effect of bleed slots on turbocharger centrifugal compressor stability

Bleed slots located in the inducer region of centrifugal compressors have been demonstrated to extend the surge margin with minimal negative impact on performance. This paper describes the impact of bleed slots on surge margin enhancement in a turbocharger centrifugal compressor used for heavy duty diesel engines. The study included evaluation of pressure instabilities throughout the compressor map, characterization of dynamic phenomena occurring at low mass flow rates (mfr), and elucidation of the impacts of bleed slots on the compressor stability and surge line. A comparison of the compressor performance was performed between open and blocked bleed slots. Pressure instability levels were measured throughout the accessible compressor map using a high frequency response pressure transducer at the compressor outlet for the two cases. Static and dynamic pressure measurements were acquired within the diffuser using pressure taps and high frequency response pressure transducers. The compressor with open bleed slot showed lower instability levels at low mfr when not experiencing deep surge. Spectral analysis at low mfr showed that the bleed slot suppresses broadband frequencies linked to piping resonance and below the rpm frequency, thus improving the overall stability. When excited, these frequencies create periodic flow disturbances that originate from the low pressure region below the tongue, at low mfr. The low pressure perturbations are then convected by the flow and propagate around part of or the entire perimeter, depending on the compressor speed.