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Session: Working fluids II
Session starts: Tuesday 08 October, 14:00
Presentation starts: 14:40
Room: Willem Burger Zaal

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Matteo Pini (Politecnico di Milano)
Andrea Spinelli (Politecnico di Milano)
Giacomo Persico (Politecnico di Milano)
Stefano Rebay (Università di Brescia)

Abstract:
The reliable modeling of real-gases is nowadays of great importance in many industrial applications, especially in the energy field. The prediction of real-gas thermodynamic properties based on the direct use of an equation of state (EoS) and of its derivatives, implies a high computational cost in case of numerical studies, when a set of governing equations is iteratively solved (e.g. detailed CFD calculations, dynamic plant simulations). A different approach is represented by the use of look-up tables. In the thermodynamic regions of interest, a grid of nodal points (storing all properties) is preliminary built. Within the discretized domain, the properties in any point are computed using fast interpolation methods, with a dramatic reduction in computational time. However, a proper technique has to be applied to guarantee the thermodynamic consistency, which is not automatically satisfied as in the case of direct EoS application. Finally the desired accuracy can be addressed by selecting the number of nodes and the interpolation scheme. This paper presents a novel interpolation method for property calculation of real gases using look-up tables. Herein, any grid has been built using accurate EoS implemented in the software FluidProp. The method assigns a selected functional form to the internal energy e as a function of the specific volume v and of the specific entropy per unit mass s (e=e(v,s)). Within any cell of the thermodynamic domain, the coefficients of the functional form are calculated referring to the local grid data; therefore, a fundamental relation is locally established, in such a way that any thermodynamic property of any internal point is intrinsically consistent. The method is here presented for the siloxane MDM and for the carbon dioxide. Both single and two-phase regions close to vapor saturation line have been explored, for reduced temperature ranging between reduced temperature of 0.6 and 1.05. The accuracy and the computational cost of the method have been assessed in comparison with those resulting from direct EoS computation. As an example of application, the throughflow calculation of a centrifugal turbine operating with MDM is also presented.