Bernhard Semlitsch

Numerical Flow Analysis of a Centrifugal Compressor with Ported and without Ported Shroud

Turbochargers are commonly used in automotive engines to increase the internal combustion engine performance during off design operation conditions. When used, a most wide operation range for the turbocharger is desired, which is limited on the compressor side by the choke condition and the surge phenomenon. The ported shroud technology is used to extend the operable working range of the compressor, which permits flow disturbances that block the blade passage to escape and stream back through the shroud cavity to the compressor inlet. The impact of this technology on a speed-line at near optimal operation condition and near surge operation condition is investigated. A numerical study investigating the flow-field in a centrifugal compressor of an automotive turbocharger has been performed using Large Eddy Simulation. The wheel rotation is handled by the numerically expensive sliding mesh technique. In this analysis, the full compressor geometry (360 deg) is considered. Numerical solutions with and without ported shroud for a near optimal operation condition and near-surge operation condition. The flow-field of the different cases is analyzed to elucidate the functionality of the ported shroud. In agreement with previous observations, it was found that the ported shroud reduces the flow disturbances in the blade passage for all operating conditions. However, the compressor efficiency for the off-design operation condition was found to be higher without the ported shroud, supporting the findings reported recently by an experimental investigation. The computational results are validated with experimental measurements in terms of the performance parameters and available Particle Image Velocimetry data. Copyright

Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers

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Centrifugal compressor: The sound of surge

When the centrifugal compressor operates at low mass flow rates (close to the unstable operating condition called surge), flow instabilities may develop and severe flow reversal may occur in the wh ...

Investigation of the Surge Phenomena in a Centrifugal Compressor Using Large Eddy Simulation

The flow through a ported shroud compressor of an automobile turbocharger is simulated using Large Eddy Simulations. Generally, the compressor is subjected to work within certain range of the mass-flow conditions. Reduction of the operation mass-flow below a certain minimum limit, leads to breakdown of the complete compressor operability. Flow reversal occurs in the compressor wheel, which results in amplification of velocity and pressure fluctuations. Consequentially, large vibratory stresses are induced into the blades under off-design condition and thereby affect the blade life duration detrimentally. The aim of this study is to understand the generation of flow-structures during extreme operable conditions (surge condition) in a centrifugal compressor. The investigation of the appearing flow-structures with the surge phenomenon is essential to explore new methods that improve the stability or the flow-operating regime of the compressor. The complete 360° compressor geometry is utilized in the computational simulations. Further, the transient sliding mesh technique is applied to account for an accurate prediction of the mesh motion and thus, the geometrical interaction between the impeller and the stationary diffuser. The numerical results are compared with available experimental measurements obtained under the same operating conditions (design and near-surge condition). The rotating stall instability is predicted using FFT data analysis. Furthermore, the numerical study captures the low frequency peak characterizing the global instability of the surge condition.Copyright

Numerical flow analysis in centrifugal compressor near surge condition

This numerical study presents data relevant to the flow characteristics inside of a centrifugal compressor, at design and near-surge conditions. The main objectives were to characterize the flow st ...

Flow Structures and Losses in the Exhaust Port of an Internal Combustion Engine

A numerical study of the flow in the exhaust port geometry of a Scania heavy-duty Diesel engine is carried out mainly by using the Large Eddy Simulation (LES) approach. Unsteady Reynolds Averaged N ...

Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.

Similarities and differences concerning flow characteristics in centrifugal compressors of different size

The aim of the present investigation is to explain the differences in the compressor map by analyzing the compressible flow in two compressor designs, i.e. a small passenger car design and a sportive car design. The two different compressor designs are assessed with steady state Reynolds Averaged Navier-Stokes simulations for several operating conditions. Similar flow features are observed near optimal efficiency operating conditions when the flow field parameters are scaled properly. The present investigation elucidates the reason for the different limiting shapes of the surge line for the two compressor maps at high speed lines.

Analyzing the Impact of the Inlet Temperature on the Acoustic Noise Production from a Supersonic Jet using LES

Non-ideal expanded supersonic jets emerging from a nozzle produce three different types of noise, i.e., shock-associated broadband noise,screech noise, and the turbulent mixing noise. The screech tone occurs due to self-excitation in a feedback-loop of flow propagating downstream and acoustic wave interaction. In downscaled laboratory experiments often the screech noise occurs, while the real applied exhaustjet of a gas turbine engine does not show this phenomena. Apart from a geometric scaling difference, usually a lower temperature is employed in experimental studies. The compressible Navier-Stokes equations are simulated numerically by a large eddy simulation approach to investigate the effect of jet operation temperature onto the noise development in a supersonic jet originating from a convergent-divergent nozzle. The jet-exit Mach-number is 1.56, while the total temperature ratios are 1.27, 2.46, and 3.65. The differences in the acoustic near-field will be presented and the interaction of flow-field with acoustic waves will be analyzed and compared to each other.

Large Eddy Simulation of Fluidic Injection into a Supersonic Convergent-Divergent Duct

Convergent-divergent (C-D) ducts operating in supersonic flow-regimes provoke choked flow conditions in the region of the narrowest cross-section.