CREA lab is happy to welcome you!

Visitors from the ORC 2017 seminar are very welcome to visit our lab!

We are in Building B16A (the red brick building across the street from Aula Magna). Please register at the ORC 2017 registration desk for a full visit.

Join us at ORC 2017!

The IV International Seminar of Organic Rankine Cycle Power Systems starts tomorrow!
Join us at 9.00 in Carassa-Dadda for the opening ceremony. More at www.orc2017.com

Presentation of experimental results from TROVA at ORC 2017

221 - Experimental observation of non-ideal expanding flows of Siloxane MDM vapor for ORC applications 

by Andrea Spinelli, Giorgia Cammi, Marta Zocca, Simone Gallarini, Fabio Cozzi, Paolo Gaetani, Vincenzo Dossena, Alberto Guardone 

Session 4C, Thursday at 14.20 

Abstract: Extensive experimental results characterizing the supersonic expansion of an organic vapor in non-ideal conditions are reported in this paper for the first time. The collected data also allowed the assessment of the accuracy of Computational Fluid Dynamic (CFD) tools employed to predict the non-ideal behavior of such flows, including the consistency of thermodynamic models adopted. The investigation has been carried out on the converging-diverging nozzle test section of the Test Rig for Organic VApors (TROVA), at the Laboratory of Compressible fluid-dynamics for Renewable Energy Application (CREA) of Politecnico di Milano. Supersonic nozzle flow has been chosen as the simplest one of significance for organic Rankine cycle (ORC) turbine channels. The working fluid under scrutiny is Siloxane MDM, a widely employed compound for high temperature ORCs. MDM vapor expands through the TROVA nozzle at moderate non-ideal conditions in the close proximity of the vapor saturation curve. This is the region where ORC expanders typically operate, thus proving the relevance of the investigation for the ORC community. Indeed, detailed experimental data representative of typical ORC expansions were lacking in the open literature up to date. Two different nozzle geometries, featuring exit Mach number of 2.0 and 1.5 respectively, have been tested, exploring a wide range of thermodynamic inlet conditions and diverse levels of non-ideality; from moderate non-ideal state, indicated by a compressibility factor Z = Pv/RT ≅ 0.80, to dilute gas conditions, Z ≥ 0.97. Maximum operating total pressure and temperature are PT ≅ 5 bar and TT ≅ 250 °C. The nozzle flow has been characterized in terms of total pressure, total temperature, static pressure at discrete locations along the nozzle axis, and schlieren imaging. In contrast to the well known case of polytropic ideal gas, the vapor expansion through the nozzle is found to be dependent on the inlet conditions, thus proving the non-ideal character of the flow. This influence is found to be consistent with the one predicted by the quasi-1D theory coupled with simple non-ideal gas models. Experimental data at the nozzle centerline have also been compared with those resulting from a two-dimensional viscous CFD calculation carried out using the SU2 software suite and the improved Peng Robinson Stryjek Vera (iPRSV) thermodynamic model. A very good accordance is found, demonstrating the high accuracy of the applied tools.

The first ever assessment of CFD for ORC flows!

Experimental assessment of the open-source SU2 CFD suite for ORC applications 

by Giulio Gori, Marta Zocca, Giorgia Cammi, Andrea Spinelli, Alberto Guardone
Session 3B, Wednesday, at 16.30 

Abstract: The first-ever experimental assessment of a Computational Fluid Dynamics (CFD) software for Non-Ideal Compressible-Fluid Dynamics (NICFD) flows of interest for ORC applications is presented here. Numerical results using the SU2 open-source suite for multi-physics simulation and design-recently extended to deal with complex thermodynamic models of organic fluids-are compared here to experimental results from the Test-Rig for Organic VApours (TROVA) of the Laboratory of Compressible-fluid dynamics for Renewable Energy Applications (CREA), Politecnico di Milano. Experimental results regard supersonic expanding flows of siloxane fluid MDM (Octamethyltrisiloxane, C8H24O2Si3) in non-ideal conditions representative of ORC applications. Three different geometries are considered for the assessment of the CFD solver. The first is a converging-diverging nozzle, representative of ORC supersonic stators, in which the fluid is accelerated to supersonic speed from highly non-ideal conditions, with inlet compressibility factor Z=Pv/(RT), computed using reference Equations Of State (EOS) for MDM fluid, as low as Z=0.81. The second geometry is a diamond-shaped airfoil at zero incidence in a supersonic flow at Mach 1.5 and Z=0.88, in mildly non-ideal conditions. Oblique shock waves are observed at the airfoil leading edge and interact with the wind-tunnel walls and the rarefaction fan from the airfoil. This test case is useful to understand the physics of oblique shock-wall and shock-shock interactions in turbine cascades operating in off-design conditions. The third geometry is a supersonic backward facing step, in which the formation of an oblique shock is observed experimentally at the reattachment point past the step. The Mach number is around 1.1 and the compressibility factor Z ~ 0.89. This geometry is representative of the trailing edge of turbine blades and it is useful to study the formation of fish-tail shock waves. These NICFD flows are fairly well captured by the CFD solver, thus confirming the validity of both the thermodynamic models and of the CFD implementation, using both the Euler equations for inviscid flows with negligible thermal conductivity and the full Reynolds-averaged compressible Navier-Stokes equations for non-ideal compressible turbulent flows. In the considered shocked flows, grid adaptation is found to be key to capture the relevant flow features using a reasonable amount of grid points.

Presentation of oblique non-ideal shocks at ORC 2017

222 - Non-ideal fish-tail shocks in ORC turbine cascades

by Davide Vimercati, Giulio Gori, Andrea Spinelli, Alberto Guardone 

Session 6C, Friday, at 10.30 

Abstract: Non-ideal shock waves at the trailing edge of supersonic high-pressure turbine vanes for ORC applications are studied numerically using the open-source SU2 solver coupled with mesh adaptation. Flow separation at the trailing edge of ORC turbine, where a supersonic Prandtl-Meyer expansion occurs, generates a limited region of separation between the supersonic flows on the pressure and suction sides of the blades. The merging of these two supersonic regions results in the formation of compression waves that eventually form a characteristic shock pattern comprising two oblique shock waves, called fish-tail shocks. The present investigation follow the study of Andrew Wheeler et al.~(NID 2016), where the authors focused on non-ideal compressible-fluid effects on the flow turning angle resulting from the Prandtl-Meyer expansion, which influences the shape and size of the downstream recirculating region. Here, the downstream process of coalescence into oblique shock waves is studied numerically to determine whether non-ideal shock waves can be observed in typical ORC operation. Non-ideal shock waves are possible only if the fundamental derivative of gasdynamics is less than unity and are characterised by the increase of the flow Mach number across the shock wave. The accuracy of the numerical tool is first assessed against a experimental results over a simplified backward-facing step geometry. Then, numerical simulations are carried out to determine the occurrence of non-ideal oblique shocks in conditions typical to ORC applications. Experimental results regard supersonic expanding flows of siloxane fluid MDM (Octamethyltrisiloxane, C8H24O2Si3) around a 90° corner. The Mach number is around 1.1 and the compressibility factor Z ~ 0.75.

Papers at ORC 2017

The CREA team will present five papers at ORC 2017, www.orc2017.com

• 221 - Experimental observation of non-ideal expanding flows of Siloxane MDM vapor for ORC applications, Session 4C, Thursday at 14.20 
• 222 - Non-ideal fish-tail shocks in ORC turbine cascades, Session 6C, Friday, at 10.30 
• 223 - Experimental assessment of the open-source SU2 CFD suite for ORC applications, Session 3B, Wednesday at 16.30
• 224 - Design and commissioning of a thermal stability test-rig for mixtures as working fluids for ORC applications, Poster session, Wednesday at 17.30 
• 225 - An analysis of fast-response pressure probes dynamics for ORC power systems, Poster session, Wednesday at 17.30