LFM (Laboratory of Fluid Machines)

About the lab

The research group working at the Laboratory of Fluid Machines (LFM) of the Energy Department (Politecnico di Milano) is active in the field of experimental and numerical Fluid Dynamics, with particular emphasis on turbomachinery for both compressible and incompressible fluids.
The several research contracts carried out for major national and international companies (examples are Franco Tosi Meccanica S.p.a., GE Oil & Gas - Nuovo Pignone, EDF, Ansaldo, Turboden) as well as the relevant research funding achieved by the LFM group during its history (PRIN 2007, PRIN 2009, Regione Lombardia 2010, Fp7-RECORD 2013 to cite the most recent) demonstrate the recognized capabilities of the research group.


Featured projects (3)

The project deals with the gasdynamic characterization of turbine cascades operating with molecularly complex fluids, which exhibit severe non-ideal gas effects both from a quantitative (low values of the compressibility factor) and from a qualitative (low values of the fundamental gas-dynamic derivative) standpoint. The analyses are carried out by means of advanced computational tools, ranging from shape-optimization to uncertainty-quantification techniques, coupled with a turbulent compressible flow solver and state-of-the-art equation of states. Both transonic and supersonic blade profiles are investigated. The project aims at revealing the implications of such non-ideal gas behaviors on the aerodynamic operations and performances of turbine profiles commonly employed in organic Rankine cycle (ORC) power systems, filling the gap between pure theoretical works devoted to the assessment of non-ideal gas effects and real-world applications.
The project deals with the design and analysis of turbomachinery used in supercritical carbon dioxide (sCO2) power systems. A particular attention is devoted to the main compressor, which operates in the close proximity of the thermodynamic critical point. In these conditions, compressor operations face sharp thermodynamic variations, which may make critical the compression process. Furthermore, the occurrence of two-phase flows cannot be excluded and, at the moment, its potential implication on compressor performance is still covered by blurriness.
Modern aero-engine combustor outlet results in a large heat release fluctuations which generate Entropy Waves and vorticity disturbances. These are the cause of a more challenging turbine aerodynamic and indirect combustion noise production. Experiments on the High Pressure Turbine Test Rig of Politecnico di Milano will be carried out employing an Entropy Wave Generator (EWG) equipped with a Swirler Generator to simulate aero-engine representative combustor outlet flows. Thanks to this device, combustor-turbine interaction should be studied in terms of both aerodynamic and acoustic. To properly measure combustor unsteadiness, fast response probes have to be used. Experiments outcomes will be used to develop, further extend and validate existing methods to understand entropy wave transport in the turbine stage and for the computation of indirect combustion noise propagation. The aim of the project is to initiate silent turbine stages design thanks to new concepts and dedicated methodologies applied in a coupled way to the aero-thermodynamic design.

Featured research (21)

Low order models based on the Blade Element Momentum (BEM) theory exhibit modeling issues in the performance prediction of Vertical Axis Wind Turbines (VAWT) compared to Computational Fluid Dynamics, despite the widespread engineering practice of such methods. The present study shows that the capability of BEM codes applied to VAWTs can be greatly improved by implementing a novel three-dimensional set of high-order corrections and demonstrates this by comparing the BEM predictions against wind-tunnel experiments conducted on three small-scale VAWT models featuring different rotor design (H-shaped and Troposkein), blade profile (NACA0021 and DU-06-W200), and Reynolds number (from 0.8×105 to 2.5×105). Though based on the conventional Double Multiple Stream Tube (DMST) model, the here-presented in-house BEM code incorporates several two-dimensional and three-dimensional corrections including: accurate extended polar data, flow curvature, dynamic stall, a spanwise-distributed formulation of the tip losses, a fully 3D approach in the modeling of rotors featuring general shape (such as but not only, the Troposkein one), and accounting for the passive effects of supporting struts and pole. The detailed comparison with experimental data of the same models, tested in the large-scale wind tunnel of the Politecnico di Milano, suggests the very good predictive capability of the code in terms of power exchange, torque coefficient, and loads, on both time-mean and time-resolved basis. The peculiar formulation of the code allows including in a straightforward way the usual spanwise non-uniformity of the incoming wind and the effects of skew, thus allowing predicting the turbine operation in a realistic open-field in presence of the environmental boundary layer. A systematic study on the operation of VAWTs in multiple environments, such as in coastal regions or off-shore, and highlighting the sensitivity of VAWT performance to blade profile selection, rotor shape and size, wind shear, and rotor tilt concludes the paper.
Compressible two-phase flows of carbon dioxide in supercritical thermodynamic conditions are encountered in many applications, e.g. ejectors for refrigeration and compressors for power production and carbon capture and sequestration to name a few. Alongside the phase change, transonic/supersonic flow regimes and real-gas effects also add additional complexities in the simulations of such flows. In this work, we investigate cavitating and condensing flows of carbon dioxide via numerical simulations based on the two-fluid concept, applying both a mixture model and a barotropic model. In the mixture model, the phase change is modelled with an extra transport equation for the mass of the dispersed phase and a source term introduced via a penalty formulation. The barotropic model reproduces the pressure–density relation of the mixture along the upstream isentrope. Both the models assume thermodynamic and mechanical equilibrium between phases and exclude meta-stability effects. All results are compared against experimental data taken from literature and the main numerical issues of the models are discussed in detail. The agreement between the simulations and the experiments is remarkable qualitatively and quantitatively, resulting in the range 2%–4% for pressure and below 1% for temperature in terms of weighted mean absolute percentage error for supercritical expansions, even though suggesting a further margin of improvement in the physical modeling, especially for subcritical expansions. Finally, we show that the barotropic model yields comparable predictions of the expansion processes at a lower computational cost and with an improved solver robustness.
Closed Joule-Brayton cycles operating with carbon dioxide in supercritical conditions (sCO2) are nowadays collecting a significant scientific interest. However, the technical implementation of sCO2 power systems introduces new challenges related to the design and operation of the components. The compressor, in particular, operates in a thermodynamic condition close to the critical point, whereby the fluid exhibits significant non-ideal gas effects and is prone to phase change in the intake region of the machine. In the present study, we consider an sCO2 compressor operating in proximity to the critical point, with an intake entropy level of the fluid lower than the critical value. In this condition, the phase change occurs as evaporation/flashing, thus resembling cavitation phenomena observed in pumps, even though with specific issues associated to compressibility effects. The flow configuration is therefore highly non-conventional and demands the development of proper tools for fluid and flow modeling. The paper discusses the modeling issues from the thermodynamic perspective and highlights the implications on the compressor aerodynamics. We propose tailored models to account for the effect of the phase change in 0D mean-line design tools as well as in fully 3D computational fluid-dynamic (CFD) simulations. In this way, a strategy of investigation is build-up as a combination of mean-line tools, industrial design experience, and CFD for detailed flow analysis. The application of the design strategy reveals the potential onset of the phase change according to three main mechanisms: incidence effect, front loading, and channel blockage.
In the frame of a continuous improvement of the performance and accuracy in the experimental testing of turbomachines, the uncertainty analysis on measurements instrumentation and techniques is of paramount importance. For this reason, since the beginning of the experimental activities at the Laboratory of Fluid Machines (LFM) located at Politecnico di Milano (Italy), this issue has been addressed and different methodologies have been applied. This paper proposes a comparison of the results collected applying two methods for the measurement uncertainty quantification to two different aerodynamic pressure probes: sensor calibration, aerodynamic calibration and probe application are considered. The first uncertainty evaluation method is the so called “Uncertainty Propagation” method (UPM); the second is based on the “Monte Carlo” method (MCM). Two miniaturized pressure probes have been selected for this investigation: a pneumatic 5-hole probe and a spherical fast response aerodynamic pressure probe (sFRAPP), the latter applied as a virtual 4-hole probe. Since the sFRAPP is equipped with two miniaturized pressure transducers installed inside the probe head, a specific calibration procedure and a dedicated uncertainty analysis are required.
The successful penetration of supercritical carbon dioxide (sCO2) power systems in the energy market largely depends on the achievable turbomachinery efficiencies. The present study illustrates a systematic framework where both the compressor and the turbine are designed via validated (within ±2%pts against experiments) mean-line tools and the subsequent impact on cycle performance estimates is quantitatively and qualitatively assessed. A significant effort is devoted to the analysis of centrifugal compressor performance operating close to the thermodynamic critical point, where sharp variations in the thermodynamic properties may make critical the compression process. The analysis is performed for different compressor sizes and pressure ratios, showing a comparatively small contribution of the compressor-intake fluid conditions to the machine efficiency, which may achieve competitive values (82-85%) for representative full-scale sizes. Two polynomial correlations for both the turbomachinery efficiencies are devised as a function of proper similarity parameters accounting for machine sizes and loadings. Such correlations can be easily embedded in power cycle optimizations, which are usually carried out assuming constant-turbomachinery efficiencies, thus ignoring the effects of plant size and cycle operating parameters. Efficiency correlations are finally exploited to perform several optimizations of a representative recompression sCO2 cycle by varying multiple cycle parameters. The results highlight that the replacement of the constant-efficiency assumption with the proposed correlations leads to more accurate performance predictions (e.g. cycle efficiency can differ by more than 4%pts), besides showing that an optimal pressure ratio exists for all the investigated configurations.

Lab head

V. Dossena
  • Department of Energy

Members (8)

Giacomo Persico
  • Politecnico di Milano
Paolo Gaetani
  • Politecnico di Milano
Carlo Osnaghi
  • Politecnico di Milano
Alessandro Romei
  • Politecnico di Milano
Alessandro Mora
  • Politecnico di Milano
Andrea Notaristefano
  • Politecnico di Milano
Marco Manfredi
  • Politecnico di Milano
Andrea Giuseppe Sanvito
  • Politecnico di Milano

Alumni (2)

Gabriele D’Ippolito
  • Politecnico di Milano
Giacomo Gatti
  • Politecnico di Milano