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Full text release has been delayed at the author's request until December 18, 2025

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An EXPERIMENTAL and COMPUTATIONAL STUDY of INLET FLOW FIELD in TURBOCHARGER COMPRESSORS

Banerjee, Deb Kumar
http://rave.ohiolink.edu/etdc/view?acc_num=osu1669149324717909

Abstract Details

2022, Doctor of Philosophy, Ohio State University, Mechanical Engineering.
Downsizing internal combustion engines along with turbocharging is an effective approach in reducing carbon dioxide emissions from vehicles to combat global warming. A turbocharger comprises a radial turbine driven by exhaust enthalpy flow connected on the same shaft to a centrifugal compressor that provides compressed air to the engine. Under certain engine operating conditions, the turbocharger faces challenges, however, due to instabilities encountered by its centrifugal compressor, primarily stall and surge. While stall adversely affects the compressor’s aerodynamic performance and efficiency, surge, which is characterized by large amplitude pressure and flow rate fluctuations, results in drastic deterioration of compressor performance and may lead to complete mechanical failure of the turbocharger. The extremely loud noise (reaching 170 dB) generated during surge is also a major concern. To mitigate these instabilities, it is critical to analyze the flow structures involved in these processes. The present work therefore focuses on developing a thorough characterization of the turbocharger compressor flow field over its entire characteristic map (pressure ratio versus flowrate) using state-of-the-art experimental as well as computational techniques. The turbocharger bench stand at OSU-CAR allowed the isolation of the turbocharger’s compressor from the complexities of the engine and provided a simplified bench-top environment for studying the compressor instabilities. The facility was modified by incorporating a stereoscopic particle image velocimetry (SPIV) system that facilitated velocity measurements at the compressor inlet. After integrating all the different components of this system including the laser, chiller, cameras, sheet optics, aerosol generator, laser controller, and timing unit, a methodology for stereoscopic calibration, image acquisition, and optimized post-processing was established. Extensive SPIV measurements were then carried out at the compressor inlet on a plane passing through the axis of rotation at different shaft speeds ranging between 80,000 and 140,000 rpm and spanning the entire range of mass flow rates from choke (maximum flow rate at each speed) to surge (minimum flow rate). Experiments were also conducted on a cross-sectional plane 2 mm in front of the impeller using 2D PIV technique by modifying the light sheet delivery system. This set of measurements enabled collecting planar velocity data at a very close proximity to the impeller blades using a ‘phase-locked’ data acquisition technique (all images correspond to a specific impeller position). Some of the key contributions to literature from these experimental studies are: • A fundamental understanding of the compressor inlet flow field was developed with a focus on its variation with mass flow rate along any speedline of its characteristic map. The phenomenon of “inlet recirculation” was studied in detail, where the flow separated from the compressor blades/shroud moves back into the inlet duct before being entrained by the forward moving core flow. Inlet recirculation was found to start near the peak efficiency region of the compressor map and its inception corresponded to an incidence angle of about 15.5° at the blade tips (irrespective of rotational speed), while the compressor efficiency decreased rapidly as this phenomenon grew stronger with decreasing flow rate. • The upstream penetration of the reverse flow from the compressor was quantified. The penetration depth of the reverse flow upstream from the inducer plane was found to increase quadratically with decreasing flow rate. This analysis also helped with the redesign of the engine’s breathing system to prevent the failure of mass air flow sensors due to contamination by oil particles (from the positive crankcase ventilation valve) carried by the compressor’s reverse flow. • A detailed understanding of the compressor operation during surge at different rotational speeds was developed by combining the SPIV measurements with pressure transducer data from the turbocharger gas stand. The instantaneous operating point during the compressor’s unsteady operation at deep surge was identified on its characteristic map, and the growth of the surge loop with rotational speed was illustrated. The mean amplitude of mass flow rate and pressure ratio fluctuations at deep surge increased in nearly a quadratic fashion with rotational speed. • A first and detailed analysis of the turbocharger compressor’s inlet turbulence was performed, highlighting the operating points where the turbulence becomes strongly anisotropic (isotropy being an important assumption of the eddy-viscosity hypothesis in turbulence modeling). A detailed characterization of the turbulent length scales including integral, Taylor, and Kolmogorov are provided at the compressor inlet, which can help with deciding the mesh resolution for large eddy simulations of turbocharger compressors. While the PIV experiments provided valuable data, the optical access was limited to the compressor inlet, thus to study the flow structures within the impeller, three-dimensional Computational Fluid Dynamics (CFD) simulations were conducted at three different mass flow rates at a speed of 80 krpm using the commercial code STAR-CCM+. The predictions agreed well with measurements from the turbocharger stand, including compressor performance and efficiency. The velocity distributions from the CFD predictions were also validated against the PIV data obtained earlier. The current simulations augment the understanding of centrifugal compressor flow fields and the prediction capabilities by: • Illustrating how specific portions of the impeller stall at different operating conditions. For example, at a mass flow rate of 30 g/s (at 80 krpm), the portion of the blade passage near the shroud between the suction surface of the main blades and pressure surface of splitter blades are occupied by stationary stall cells. • Identifying the presence of rotating stall cells near the inducer, their interactions with the blades, and their impact on noise generation. • Characterizing the tip leakage flow and its possible role in stall inception. • Quantifying the entropy generation within the turbocharger compressor at the different operating conditions. • Discussing the advantages and challenges of applying different turbulence models like RANS and LES for predicting the turbocharger compressor’s flow field as well as its acoustics.
Ahmet Selamet (Advisor)
329 p.
Aerospace Engineering; Automotive Engineering; Mechanical Engineering
Turbochargers; Centrifugal Compressors; Compressor Aerodynamics; Particle Image Velocimetry; Flow Visualization; Laser Diagnostics; Computational Fluid Dynamics; URANS; LES; Compressor Acoustics

Recommended Citations

Citations

  • Banerjee, D. K. (2022). An EXPERIMENTAL and COMPUTATIONAL STUDY of INLET FLOW FIELD in TURBOCHARGER COMPRESSORS [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1669149324717909

    APA Style (7th edition)

  • Banerjee, Deb. An EXPERIMENTAL and COMPUTATIONAL STUDY of INLET FLOW FIELD in TURBOCHARGER COMPRESSORS. 2022. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1669149324717909.

    MLA Style (8th edition)

  • Banerjee, Deb. "An EXPERIMENTAL and COMPUTATIONAL STUDY of INLET FLOW FIELD in TURBOCHARGER COMPRESSORS." Doctoral dissertation, Ohio State University, 2022. http://rave.ohiolink.edu/etdc/view?acc_num=osu1669149324717909

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.