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Publication Title | Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines

Organic Rankine Cycle

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21`eme Congr`es Fran ̧cais de M ́ecanique Bordeaux, 26 au 30 aouˆt 2013

Numerical investigation of dense gas flows through transcritical multistage axial Organic Rankine Cycle turbines

L. Sciacovellia, P. Cinnellaa

a. Laboratoire DynFluid, Arts et M ́etiers ParisTech, 151 Bd de l’Hˆopital, 75013 Paris, France

R ́esum ́e :

Des ́etudes r ́ecentes sugg`erent que les cycles de Rankine organiques supercritiques ont un grand potentiel pour les applications de r ́ecup ́eration de chaleur `a basse temp ́erature, car ils permettent d’atteindre une meilleure rendement de r ́ecup ́eration avec une architecture de cycle simplifi ́ee. Dans ce travail, on ́etudie des ́ecoulements de gaz denses `a travers des turbines ORC axiales multi- ́etag ́ees supercritiques, `a l’aide d’un code num ́erique includant des lois d’ ́etat complexes et un sch ́ema de discr ́etisation d’ordre ́elev ́e. Plusieurs fluides de travail sont pris en compte et les performances des turbines supercritiques sont compar ́ees `a celles de turbines subcritiques utilisant les mˆemes fluides.

Abstract :

Many recent studies suggest that supercritical Organic Rankine Cycles have a great potential for low- temperature heat recovery applications, since they allow better recovery efficiency for a simplified cycle architecture. In this work we investigate flows of dense gases through axial, multi-stage, supercritical ORC turbines, using a numerical code including advanced equations of state and a high-order discre- tization scheme. Several working fluids are considered, and performances of supercritical turbines are compared to those of subcritical ones using the same fluids.

Mots clefs : ORC turbine ; dense gas ; supercritical fluid 1 Introduction

Organic Rankine Cycles (ORCs) are Rankine cycles using an organic fluid instead of water working fluid. ORCs have been largely studied for their relatively high efficiency [9] at such unfavorable conditions. ORC technology has been applied in geothermal [11] and biomass [4] fired power plants, bottoming cycles for combined cycle power plants [5], solar reverse osmosis desalination plants, and others, allowing efficiency improvements, reduction of the cycle size and production in modular units. Many studies have demonstrated that supercritical ORCs, i.e., ORCs in which heat is supplied at a pressure greater than the liquid/vapor critical point pressure, have an even greater potential, since they allow better recovery efficiency for a simplified cycle architecture [6]. The selection of the most suitable working fluid is of crucial importance in designing an ORC process, since it requires to take into account thermodynamic, environmental and safety aspects [8].

In this work, we investigate supercritical flows of dense gases/vapors through axial, multi-stage, ORC turbines, using a numerical code with a high-order discretization scheme. Three working fluids are considered : the refrigerants R134a and R245fa, and carbon dioxide (CO2). The two organic fluids are characterized by complex molecules and moderate to high molecular weights, and can be classified as dense gases according to Cramer and Kluwick [3]. At this stage, we restrict our attention to inviscid flow effects related to the peculiar thermodynamic behavior of each fluid. In most cases, the thermo- dynamic working conditions of these gases are such that the ideal-gas approximation is no longer valid and real-gas effects become significant. For this reason, an advanced multiparameter equation of state

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