A numerical approach for predicting flow-induced acoustics at near-stall conditions in an automotive turbocharger compressor

Abstract

Airborne turbocharger noise is becoming a topic of major concern for automotive manufacturers. Particularly, a broadband noise named as whoosh is detected during some engine conditions (full load, tip-in and tip-out maneuvers). This thesis aims to contribute to the understanding of turbocharger compressor flow-induced acoustics by means of 3-dimensional CFD.A methodology to compare CFD and experimental spectra is developed so as to solve the issue of the different location of pressure probes. Pressure decomposition and different types of monitors are required by the model in order to provide meaningful spectra. A tip clearance sensitivity analysis is performed, concluding that tip clearance has an impact on compressor global variables but not on noise generation. CAD tip clearance profile is therefore safely used throughout the thesis. Most important set-up parameters such as grid spacing, solver type or time-step size are evaluated. The model is defined so that noise prediction accuracy is guaranteed in the whole hearing range. In any case, the time-step increment sensitivity analysis reflects that a significant save in computational effort could be made but retaining the right prediction of main spectra features.Finally, three operating conditions are studied at same compressor speed; from best efficiency point to a working point close to surge. All the points present backflows (flow moving from the passage outlet towards the inlet) near the shroud, although their thickness increase when reducing the mass flow. This recirculation zone increase the meridional velocity and improve incidence angle, thus extending the stability margin. Noise emission also is more intense as flow is reduced, particularly at low frequency range. At best efficiency point, a periodic flow separation is observed on the blades suction side. Stalled cells are transported towards the exducer producing a pulsating jet-wake structure that may increase the broadband noise slightly below the main blade passing frequency.In the two points with less mass flow, inducer and diffuser rotating stall can be noticed, causing the so-called whoosh noise. Actually, the point closest to surge presents an increase in the low frequency content found which masks whoosh noise. In this working point, a tornado-like structure is identified in the mixing region between the incoming and recirculating flows, increasing secondary flows and causing large pressure oscillations at the inducer. It is v considered that this phenomenon is responsible for the low frequency noise content rise.Keywords: CFD simulation; Aeroacoustics; Whoosh noise; Rotating stall; URANS; DES vi ResumenLos fabricantes del sector automotriz se están empezando a preocupar por el ruido aéreo generado por el turbogrupo. En particular, un ruido de banda ancha denominado whoosh es detectado en ciertas condiciones (a plena carga y en aceleraciones y desaceleraciones. Esta tesis pretende contribuir al entendimiento de la aeroacústica de los compresores de turbogrupos utilizando CFD tridimens...

Experimental approach for the analysis of the flow behaviour in the stator of a real centripetal turbine

“…The SST turbulence model is commonly used in many of the turbomachinery applications found in the literature as observed in the literature. 25–27 These examples demonstrate the model’s ability to reproduce the flow separation in a wide range of operating conditions.…”

“…The turbine used as the model is very common so the values required to calculate the Reynolds number of a real turbine were obtained from a previous CFD study at 50% closure of variable geometry turbine (VGT) and half-load operating point. 20 Commonly, the turbocharger turbine, installed in a diesel or gasoline engine, is around this opening position.…”

Dynamic mode decomposition of the acoustic field in radial compressors

“…A segregated solver was selected to perform a detached-eddy simulation (DES) with a SST k − ω turbulence model. Outlet pressure and inlet mass flow boundary conditions were set, and the chosen time discretization ∆t = 1.046 × 10 −6 s rendered 1º impeller rotation per time step, in accordance with the sensitivity analysis performed by Navarro [54]. Heat transfer with surroundings was neglected, as it was justified [55] that it did not influence the compressor global variables in the chosen operating conditions.…”

Experiments on turbocharger compressor acoustics

“…The development of this numerical model, including the assessment of the influence of spatial and temporal discretization, turbulence models, boundary conditions, tip clearance, etc. was performed in parallel of this thesis, in the framework of the doctoral thesis of R. Navarro [107], where full details about the model can be found.…”

“…It can be seen that all three types of monitors were replicated at three different cross sections in both inlet and outlet duct, in order to mimic the array installed in the real setup and also to assess the influence of the longitudinal distance to the compressor on the results. Full results of this assessment can be found in [1] and [107], but in essence the axial position demonstrated small influence.…”