Operating conditions impact on flow and acoustics in turbocharger compressors
Abstract: Fluid machines are an integral part in energy conversion with applications from pumps, fans, propellers, compressors and turbines. In the automotive industry, turbochargers are commonly employed to counteract the effect of engine downsizing. However, designing efficient compressors with wide operating ranges and reduced noise emissions consitute a challenge.This thesis investigates flow instabilities and sound generation in turbocharger compressors, utilizing compressible Large Eddy Simulations (LES). The numerical approach is validated through sensitivity studies and comparison with measurement data. Three different compressor designs used in both light-duty and heavy-duty applications are examined with the aim of enhancing the understanding of rotating stall mechanism in real-world configurations and their impact on aerodynamically generated noise.The analysis employs compressible Navier-Stokes equations with a scale-resolving model, evaluating its robustness in comparison to other computational methods under various operating conditions. The system's response to time-varying boundary conditions is assessed, and the effect of pulse amplitude is quantified.Subsequently, the mechanism for aerodynamically generated noise, focusing on the broadband components are explored through analysis of the recirculation region. Resolving the Taylor micro-scale in the recirculation region enhances the understanding of the dynamics in this zone. It is demonstrated that an inlet recirculation zone develops near surge conditions, which is highly sensitive to the choice of boundary conditions and turbulence formulation. Passive flow control, such as the ported-shroud, are considered to illustrate their influence on performance, stability and noise.Finally, the system is studied using a two-port method, accounting for rotational effects. This provide insights into the transmission poperties at low frequencies (< 3 kHz) and the mechanism of sound generation. It is demonstrated that the use of Computational Fluid Dynamics can improve the understanding of flow-acoustic interaction in complex geometries. Additionally, the developed numerical simulation and post-processing methods have potential application in a range of turbochargr systems, from hybrids to fuel cell application.
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