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14:20   Session 7C: Volumetric Expanders (2) Chair: Antonio Giuffrida 14:20 20 mins Employing a single-screw expander in an Organic Rankine cycle with liquid flooded expansion and internal regeneration Davide Ziviani, Sergei Gusev, Stefan Schuessler, Abdennacer Achaichia, James E. Braun, Eckhard A. Groll, Michel De Paepe, Martijn van den Broek Abstract: Positive displacement expanders are proven to be cost-effective in the low to medium power range for Organic Rankine Cycle (ORC) systems. Among the different types of volumetric expanders, the screw-type presents a favorable combination of relatively high internal volume ratio (up to 7) and isentropic efficiency (up to 80\%) with respect to the optimal pressure ratio at which sub-critical ORCs operate. In particular, single-screw expanders have shown some potential due to their symmetric and balanced configuration that decreases the loads on the bearings. A comprehensive characterization of this type of machine with two working fluids, i.e., SES36 and R245fa, has been carried out in a previous work [1]. Based on the experimental work, friction losses and internal leakages were found detrimental to the expander performance. As the expander requires lubrication during operation, flooded expansion can be beneficial to reduce such losses as well as to improve the expansion process toward a quasi-isothermal behavior. A thermodynamic cycle model has been developed to evaluate the potential improvements on the thermodynamic performance of organic Rankine cycle with flooded expansion and internal regeneration. A semi-empirical model of the expander is included which accounts for the effects of internal volume ratio. The results from the cycle model have used to design an ORC test setup with an independent lubricant oil loop and internal regeneration. The new test rig will be used validate the trends obtained with the cycle model and to further characterize the single-screw expander. The working fluid employed is R1233zd(E) as a replacement for R245fa. 14:40 20 mins Impact of major leakages on characteristics of a rotary vane expander for ORC Vaclav Vodicka, Vaclav Novotny, Jakub Mascuch, Michal Kolovratnik Abstract: Volumetric expanders are used for low to medium power output ORC applications. For low power output ORCs (< 10 kW), rotary vane expanders represent a suitable choice. Their isentropic efficiency is often reported as the most important or even the only metrics for comparison. Such approach however neglects the effect of leakages within the expander on the rest of the cycle, especially on the evaporator pressure. Filling factor of rotary vane expanders may be affected, among other leakages, by delayed closure of working chamber. This work describes a semi-empirical model with two different leakages – lumped leakage area between inlet and outlet and leakage between working chambers due to delayed contact of vane and stator. Primary purpose of the model is to demonstrate the effect of the delayed chamber closure. Results of the model for several case scenarios are presented, showing an impact of these leakages on an overall cycle performance, isentropic efficiency and filling factor. It is demonstrated that isentropic efficiency of the rotary vane expander might not be always sufficient to compare vane expanders or their modifications even within a same ORC. 15:00 20 mins An experimental and numerical analysis of the performances of a Wankel steam expander Marco Francesconi, Gianluca Caposciutti, Marco Antonelli Abstract: In the last decades, the energy market increased its interest towards the smart grids and electrically isolated systems. These systems utilize small size power generator, which often feature volumetric expanders that are very robust and reliable for a wide range of operative conditions. In this work a study that focuses on a Wankel volumetric expanders was carried out. In order to predict the extractable specific work from the expander, a thermodynamic lumped parameters numerical model was developed. The model was validated by means of experimental data obtained using water steam as working fluid. The results proved the sustainability of this kind of expander, as well as the capability of the numerical model to predict the effective performances of the device. 15:20 20 mins Experimental and numerical characterization of an oil-free scroll expander Alessio Suman, Saverio Randi, Nicola Casari, Michele Pinelli, Luca Nespoli Abstract: Micro-ORC systems are characterized by lower efficiency, but at the same time could be useful for recover parts of energy in domestic applications for which reliability and low noise level represent the biggest challenges. In this paper, an integrated Reverse Engineering (RE)-Computational Fluid Dynamics (CFD) methodology is applied in order to study the adaptation of a commercial scroll compressor to be used as an expander in a micro-ORC system. The analyses reported in this paper consist of: (i) the acquisition of the 5-kW oil-free scroll expander through a RE procedure and its CAD reconstruction, (ii) the set-up of fully three-dimensional transient simulations with the Chimera strategy using the Siemens PLM software,(iii) the validation of the computational analysis by means of experimental tests and finally, (iv) the analysis of the geometry-flow features like flank and axial gaps, coupled with the analysis of the scroll volumetric efficiency and overall performance. Chimera strategy is able to move the computational grid at each time step in order to accommodate the shape and size changes of the gas pockets. The scroll characterization was carried out using both experimental and numerical tests. Six different rotational velocities in the range of (400 – 2400) rpm with a fixed pressure level were tested for validating the numerical model using air as a working fluid. The numerical model was then used to calculate the scroll expander performance operating in an existing ORC system with R134a as working fluid. 15:40 20 mins Computational models for the analysis of positive displacement machines: Real gas and dynamic mesh Nicola Casari, Alessio Suman, Davide Ziviani, Martijn van den Broek, Michel De Paepe, Michele Pinelli Abstract: In recent years, computational fluid dynamics (CFD) has been applied for the design and analysis of positive displacement machines (both compressors and expanders) with numerous challenges due to the dynamics of the compression (or expansion) process and deforming working chambers. The relative motion and in turn, the variation of the gaps during machine operation implies several obstacles for the implementation of reliable CFD models. The majority of the studies reported in literature focused on scroll, twin screw and reciprocating machines. The limitation of the developed methodologies to be applied directly to positive displacement machines with more complex meshing such as that of single-screw has been highlighted in literature. In this paper, a single screw expander is studied by means of (i) a moving mesh technique (dynamic mesh in the Key Frame Remeshing approach) and (ii) a real gas model of a R134a (Peng-Robinson model) implemented in OpenFOAM®. On the top of that, all the possible techniques that come with the software are investigated in their application to single screw. An useful review of the state of the art CFD with open-source software is therefore carried out. The reliability of CFD model represents indeed the first step on which the design process and further optimization will be based.