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11:20   Volumetric expanders III
Chair: Prof. Sotirios Karellas
20 mins
Florian Götz, Stefan Hess, Hans-Peter Kollmeier, Dr.
Abstract: ABSTRACT In a combustion engine about one third of the chemical energy of fuel is converted into mechanical energy. About two thirds of the fuel energy dissipates unused with exhaust gas and cooling water [1]. Waste heat recovery offers possibilities to use this thermal energy. One way to recover heat is the (Organic) Rankine Cycle [2]. One of the key elements of this cycle is the expansion device. Hence the research and development of these machines are of great interest to achieve a high efficiency in waste heat recovery applications. In this paper the use of precise pressure indication, with piezoelectric and piezoresistive transducers, is analyzed. While the measurement technique is well established within engine test benches for combustion engines it is new to expansion devices [3]. Deviating boundary conditions are the key factor to look at for precise and reliable measurements. Furthermore error sources such as the determination of the top dead centre are examined. After evaluation and assimilation of the most important parameters the precise pressure indication is tested in two expansion devices. A rotary-piston and an axial-piston expansion-device are put to test. Based on the results of the analysis, carried out with the measurements of the devices, possibilities and benefits of precise pressure indication of expansion devices are presented. Finally a comparison of the ideal reference cycle is carried out to evaluate the potential of each machine. REFERENCES [1] Horenburg, Peter: Abwärme zu Strom veredeln; BINE-Projektinfo 13/2011, Bundesministerium für Wirtschaft und Technologie (BMWi); FIZ Karlsruhe GmbH [2] Rohloff, Kathrin; Kather, Alfons: Geothermische Stromerzeugung, Kraftwerkstechnologien und Technologien zur gekoppelten Erzeugung von Strom und Wärme; Mai 2011, Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU), Referat Öffentlichkeitsarbeit, 11055 Berlin [3] Teichmann, Rüdiger; Schwarz, Christian; Wimmer, Andreas; Winklhofer, Ernst: Verbrennungsdiagnostik. Sonderdruck, Springer Vieweg, Graz 2012
20 mins
Ilaria Guarracino, Richard Mathie, Aly Taleb, Christos Markides
Abstract: The current trend for ever increasing energy prices acts as an important driver for improved efficiency via novel heat integration and energy generation schemes. An Organic Rankine Cycle (ORC) equipped with a low-loss two-stage reciprocating piston expander has been designed and is tested experimentally. The reciprocating expander is a low-cost, low-maintenance, and readily available prime mover option for these engines, with promising performance characteristics (e.g. efficiency). The tested expander is based on a commercially available unit intended for air-compression applications, which was reconditioned for the purposes of the present tests. A novel rotary valve was developed to guarantee a high efficiency and low leakage rate. The test-bed gives a maximum mechanical output of 3 kWe with R245fa as the working fluid at pressure limited to 10 bar. The optimal working-fluid was chosen from 21 possible refrigerants and alkanes based on theoretical efficiency calculations.
20 mins
Mirko Morini, Claudio Pavan, Michele Pinelli, Eva Romito, Alessio Suman
Abstract: The scroll fluid machine has gained popularity since the 1970s as a compressor in air conditioning and refrigeration applications. Its main advantages are the small number of moving parts and its reduced noise and vibrations. Recently, this technology has gained renewed interest due to its potential adaptability to be used as an expander in micro ORC systems. The ever increasing request for higher efficiency in machine operation (e.g. eco-design) has led to the need for designers to thoroughly investigate the kinematic and thermodynamic behavior of these machines by means of geometric, thermodynamic and, very recently, CFD methods. The relationship between the scroll spiral profiles, and, therefore, scroll pockets evolution, and the machine overall performances both in terms of energy and mechanics is the first step towards understanding scroll machine working behavior. In [1], a method for the design of spiral profiles for performance enhancing of the whole refrigeration plant is presented. In [2], particular attention is paid to the stress to which the scroll profiles are subjected as a function of the geometry of the pockets in order to minimize the thickness of the spiral by saving the mechanical integrity of the scroll. Scroll machine performance evaluation as a function of spiral geometry can be performed by means of thermodynamic models by taking into consideration volumetric loss due to leakage flows [3,4,5]. The use of CFD methods for the evaluation of scroll machine performance is not widespread in literature. In [6] an analysis oriented to the evaluation of the pressure distribution in the pockets and in the leakages through the flank gap is presented. In this paper, geometric, thermodynamic and CFD methods for the modeling of scroll machines are presented. The comparison between two geometric models for the design of the scroll spiral profiles is presented. The two methods are then compared by evaluating overall performances by means of a simplified thermodynamic model. Finally, a CFD transient Dynamic Mesh (DM) strategy is implemented and a sensitivity analysis in terms of grid, boundary conditions and time step performed.