Real-Gas Effects in Organic Rankine Cycle Turbine Nozzles

Abstract

Organic Rankine cycle turbogenerators are a viable option as stationary energy converters for external heat sources, in the low power range (from a fewkWup to a few MW). The fluid-dynamic design of organic Rankine cycle turbines can benefit from computational fluid dynamics tools which are capable of properly taking into account realgas effects occurring in the turbine, which typically expands in the nonideal-gas thermodynamic region. In addition, the potential efficiency increase offered by supercritical organic Rankine cycles, which entails even stronger real-gas effects, has not yet been exploited in current practice. In this paper, real-gas effects occurring in subcritical and supercritical organic Rankine cycle nozzles have been investigated. Two-dimensional Euler simulations of an existing axial organic Rankine cycle stator nozzle are carried out using a computational fluid dynamics code, which is linked to an accurate thermodynamic model for the working fluid octamethyltrisiloxane C8H24O2Si3). The cases analyzed include the expansions starting from actual subcritical conditions, that is, the design point and part-load operation, and three expansions starting from supercritical conditions. Results of the simulations of the existing nozzle for current operating conditions can be used to refine its design. Moreover, the simulations of the nozzle expansions starting from supercritical conditions show that a nozzle geometry with a much higher exit-to-throat area ratio is required to obtain an efficient expansion. Other peculiar characteristics of supercritical expansions such as low sound speed and velocity, high density, and mass flow rate, are discussed

The influence of molecular complexity on expanding flows of ideal and dense gases

“…To ensure a small discretization error, a grid convergence study has been performed by computing the solution on three grids with decreasing element size. 11 The coarse grid is shown in Fig. 12, together with the Mach number field and streamlines obtained on the finer grid for steam.…”

“…Hence, more complex thermodynamic models must be used to complement the fluid dynamic analysis of a dense-gas process. [10][11][12][13] Recent gas dynamic studies have focused on the influence of fluid molecular characteristics on the real-gas behavior of a fluid flow. Investigations in Ref.…”

Non-ideal compressible-fluid effects in oblique shock waves

“…The characterisation of non-ideal compressible-fluid flows is not only of theoretical interest, but also is relevant to applications involving molecularly complex fluids in their dense-gas regime, such as machinery operating in Organic Rankine Cycle (ORC) and supercritical CO 2 power systems, turbogenerators, refrigerators and many others, see reference [14,15,16,17] The present work is organised as follows. In section 2, the Rankine-Hugoniot jump relations across surfaces of discontinuity are recalled along with the admissibility requirements.…”

Similarity Theory Based Radial Turbine Performance and Loss Mechanism Comparison between R245fa and Air for Heavy-Duty Diesel Engine Organic Rankine Cycles

“…After three times of optimization, the best turbine produced 45.6 kW at 56.1% efficiency. Colonna et al [16,17] made the fluid-dynamic design of ORC turbine using CFD tools. The authors numerically investigated the real gas effects occurring in the supersonic ORC stator nozzles.…”

“…As the organic fluid properties are totally different from air, these researches are mainly divided into two technical routes. One route focuses on forward design, including preliminary design of geometry parameters and aerodynamic design of blade profiles [12][13][14][15][16][17][18][19][20][21]. Dolz et al [13] propose that the pre-design of turbomachinery must take real gas equations of state into consideration, because the specific energy deviation between real gas and perfect gas can be as large as 100%, which may lead to total wrong turbine preliminary design result.…”