16:30
Session 3B: CFD
Chair: Teemu Turunen-Saaresti
16:30
20 mins
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Towards the validation of a CFD solver for non-ideal compressible flows
Adam Head, Siddharth Iyer, Carlo De Servi, Matteo Pini
Abstract: Mini ORC power systems with the capability to deliver 3-50 kWe are receiving increased recognition for applications such as heat recovery from automotive engines, or distributed power generation from geothermal reservoirs sources, and distributed thermal solar irradiation. Efficient and reliable expanders are the enabling components of such power systems, and all the related developments are currently at the research stage.
In the open literature experimental gas dynamic data is limited concerning the fluids and the flow conditions of interest for ORC expanders.. Therefore, software tools used for the fluid dynamic design of components cannot be validated against reliable test cases. A new experimental facility called the ORCHID will contribute to bridging this gap. The ORCHID has two interchangeable Test Sections (TS): a) one with a de Laval nozzle and b) one with an expander.
The nozzle TS will facilitate a multitude of validation test cases for Non-Ideal Compressible Fluid Dynamic (NICFD) studies and, in particular, investigate the fundamental physics of ORC vapors during expansion processes. This TS is equipped with temperature sensors, wall static pressure taps, and optical access in order to utilize flow visualization techniques (e.g., Schlieren imaging and Particle Image Velocimetry). It can also accommodate shock wave generators of various types to investigate the effect of fluid flow properties on shock patterns. We present a validation framework for NICFD solvers together with initially conceived experimental test cases in the ORCHID.
Expansion processes of organic vapors often exhibit complex thermodynamic behavior, which is especially prominent close to the critical point. Improved thermodynamic models are often employed but require coefficients, such as critical-point properties, which are influenced by large uncertainties. Indeed, these closure parameters need to be calibrated against an experimental database.
We propagate uncertainties in the closure coefficients through reference equations of state to quantify the effects of uncertainties on the computed flow properties, e.g., shock wave angles. In general, we present an analysis of uncertainties related to thermodynamic modeling of non–ideal compressible gas flows. Preliminary results indicate that errors in the shock wave angle increase as the influence of non-ideal effects become more prominent.
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16:50
20 mins
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Real gas expansion with dynamic mesh in common positive displacement machine
Nicola Casari, Alessio Suman, Mirko Morini, Michele Pinelli
Abstract: Fluids that are elaborated by the machinery involved in ORC undergo several transformations among which the expansion in
positive displacement machines. The gas path inside this component is very complicated and gaps play a crucial role. Due to the
importance of this technical detail, gap design and optimization is a decisive step in achieving an high overall efficiency both of
the expander and the whole cycle. In this work the fluid dynamics of a real gas during the expansion through the gap during the
operation is numerically investigated. The e ects of the gap formation and its evolution on the processed fluid is studied thanks to a
dynamic mesh approach. Different geometries have been considered, each one related to different positive displacement machines
used as an expander in ORC applications. In particular, the variable gap between the fixed and mobile spirals of a scroll expander
and the clearance between the star wheel and the screw of a single screw expander are analysed.
The relative motion and in turn, the variation of the gaps during the machine operation, implies the use of particular numerical
strategies able to well represents these localized geometrical features. On the top of that, the modelling of the processed fluids as
a real gas determines an extra effort in the way of the representation of the actual behavior involved in the positive displacement
machine operation. This analyses show the local fluid dynamic phenomena due to the variable clearances. Both air and R134a,
fluid widespread in the ORC cycles, are used in this work. Different operating conditions are considered and effects like separation
and shock wave are highlighted.
This analysis allows the comprehension of how local phenomena could a ect the overall machine operation and efficiency. In
addition, information about flow coefficient related to di erent time instant could be useful to setup the lumped parameter models.
Gaps are the responsible of the volumetric efficiency of the machine and, coupled with (i) time-variable geometry modification, (ii)
relative velocities and (iii) fluid characteristics characterize the global ORC system performance.
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17:10
20 mins
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Experimental assessment of the open-source SU2 CFD suite for ORC applications
Giulio Gori, Marta Zocca, Giorgia Cammi, Andrea Spinelli, Alberto Guardone
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.
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