Mihai Mihăescu

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 ...

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.

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.

On the Assessment of Centrifugal Compressor Performance Parameters by Theoretical and Computational Models

1D performance prediction modeling and steady-state CFD are applied to assess a high-performance centrifugal compressor. Computed total pressure ratio is compared with experimental data obtained fr ...

Numerical Flow Simulations of a Flexible Plate Attached to an Obstacle in Crossflow

For biomedical applications with relevance to the human upper respiratory tract, the knowledge of the tissue behavior when exposed to a particular flow field is crucial. Moreover, it is important to quantify how the tissue properties affect the biomechanics of the obstruction. Since in-vivo measurements are often not possible, this is assessed computationally by using simplified geometries. The present study is devoted to the analysis of a fluid-structure interaction scenario relevant to obstructive sleep apnea (OSA) and snoring. The dependence of the solution on the most critical parameters of the system is assessed, with the aim of quantifying their influence on tissue self-excitation and obstruction dynamics.

Wall Treatment Effects on the Heat Transfer in a Radial Turbine Turbocharger

Contradicting results about heat transfer effects on the performance of turbine turbocharger motivated this study. It was aimed to assess the effects that the wall treatment in a numerical sense has on the performance of a radial turbine of automotive turbocharger operating under a continuous flow condition. Adiabatic and non-adiabatic conditions were analyzed by using Unsteady Reynolds Averaged Navier-Stokes (URANS), Large Eddy Simulations (LES) and Detached Eddy Simulations (DES) approaches. When considering heat transfer, heat transfer loss at various locations is highly dependent on the near-wall modelling approach employed. Development of thermal boundary layer in the upstream region of turbine affects how the gas is convected in the downstream components, such as the scroll and the rotor. As long as the deviation in predicting thermal boundary layer does not affect the prediction of gas temperature at the inlet and outlet of the rotor, the difference in turbine power prediction by different near-wall modelling approaches was found to be small.

Steady-State and Unsteady Simulations of a High Velocity Jet Into a Venturi Shaped Pipe

A jet pump consists mainly of a convergent-divergent Ven-turi shaped duct where a primary stream is applied with the role of entraining a secondary jet. Due to their simple and reliable concept, jet pumps are used in miscellaneous applications. Performance optimization of a jet pump has to be performed for various operation conditions. Thus, numerically robust and cheap models, able to predict accurately the performance parameters of such devices are necessary. Reynolds Averaged Navier-Stokes based formulations are computationally efficient to predict the performance of a jet pump. However, these simulations rely on turbulence closure coefficients, which need to be validated with experimental observations. Large Eddy Simulation solves the most energetic structures in the flow field and it can be used to capture the flow dynamics. On the experimental side, confined geometries challenge the investigation capabilities to capture the flow field accurately and in all the details. The flow field in the jet pump is investigated using Large Eddy Simulation approach and a steady state Reynolds Averaged Navier-Stokes formulation. The flow field solutions obtained with the two numerical tools are compared. A reasonable agreement for the velocity and pressure contours could be achieved. However, the turbulence kinetic energy distribution and the entrained mass flow rate are predicted to be distinct. The difference in entrained mass flow rate leads to differences in jet pump efficiency estimation.