This paper presents the extension of the open source SU2 software suite to perform turbulent Non-Ideal Compressible Fluid-Dynamics (NICFD) simulations. A new built-in thermodynamic library has been developed and tightly coupled with the existing structure of the code, properly re-organized for accommodating arbitrary thermophysical models. The library implements simple models and interfaces to an external software for a more accurate estimation of thermophysical properties of NICFD pure fluids and mixtures. Moreover, the Reynolds-averaged Navier-Stokes (RANS) equations are spatially discretized by resorting to suitably defined convective and viscous numerical schemes for general fluids. The capabilities of the code are finally verified on two-and three-dimensional inviscid and turbulent flow problems against solutions obtained with different NICFD solvers, and known analytical ones. The results prove that SU2 is comparatively accurate and computational efficient with respect to existing codes. Definitively, SU2 can be considered as a trustworthy tool for NICFD-based simulations and the future pillar of advanced automated design techniques involving complex fluid laws.

Organic rankine cycles (ORC) are renowned to be attractive energy conversion systems for the thermal energy sources in the small-to-medium power range. A critical component in the ORC technology is the turbo-expander; the difficulties involved in the accurate thermodynamic modeling of organic fluids and, especially, the complex gasdynamic phenomena that are commonly found in ORC turbines may result in relatively low efficiency and in performance reduction at partial loads. In this perspective, a relevant path of development can be outlined in the evaluation of nonconventional turbine architectures,\ud such as the radial-outward or centrifugal turbine. In the present work, a critical evaluation of the feasibility of multistage transonic centrifugal turbines for ORC systems is presented. To support this study, a two-step design procedure, specifically oriented to\ud ORC turbines, was developed. The methodology includes a 1D mean-line code coupled to an external optimizer to perform a preliminary design of the machine. The selected configurations are then verified with a CFD (computational fluid dynamics)-based throughflow solver, able to deal with any flow regime and to treat fluids described by\ud arbitrary equations of state. The overall procedure is applied to the design of two different turbines of the same target power of about 1 MW, the former representing a transonic six-stage turbine and the latter a supersonic three-stage turbine. The two machines are\ud characterized by very different shape and comparable performances. The results are extensively discussed in terms of both overall data and detailed flow fields.\ud [DOI: 10.1115/1.4023122

Organic Rankine cycle (ORC) power systems are rapidly diffusing as a technology for the conversion of thermal energy sources in the small-to-medium power range, e.g., from 150 kWe up to several MWe. The most critical component is arguably the expander, especially if the power capacity is small or very small, as it is the case for innovative high-potential applications such as waste heat recovery from truck engines, or distributed conversion of concentrated solar radiation. In these so-called high-temperature applications, the expansion ratio is very high; therefore, turbines are the expanders of choice. Recently, multistage radial-outflow turbines (ROT), a nonconventional turbine configuration, have been studied, and first commercial implementations in the MWe power range have been successful. The objective of this work is the evaluation of the radial-outflow arrangement for the turbine of high-temperature mini-ORC power systems, with power output of the order of 10 kWe. To this end, a method for the preliminary fluid-dynamic design is presented. It consists of an automated optimization procedure based on an inhouse mean-line code for the one-dimensional preliminary design and efficiency estimation of turbines. It is first shown that usually adopted simplified design procedures, such as that of the so-called repeating-stage, cannot be extended to minicentrifugal turbines. The novel methodology is applied to the exemplary case of the 10 kWe turbine of an ORC power system for truck engine heat recovery documented in the literature. The expansion ratio is 45. The preliminary fluid-dynamic design of two miniturbines is presented, namely, a five-stage transonic and a three-stage slightly supersonic turbine. The outcome of the preliminary design leads to two turbine configurations whose fluid-dynamic efficiency exceeds 79% and 77%, respectively. The speed of revolution is around 12,400 and 15,400 RPM for the five-stage and the three-stage machine, respectively. These results show that the ROT configuration may allow for compact and efficient expanders for low power output applications

An adjoint-based shape optimization approach for supersonic turbine cascades is proposed\ud for application to organic Rankine cycle (ORC) turbines. The algorithm is based\ud on an inviscid discrete adjoint method and encompasses a fast look-up table (LuT)\ud approach to accurately deal with real-gas flows. The turbine geometry is defined by\ud adopting state-of-the-art parameterization techniques (NURBS), enabling to handle both\ud global and local control of the shape of interest. A preconditioned steepest descent\ud method has been chosen as gradient-based optimization algorithm to efficiently search\ud for the nearest minimum. The potential of the optimization approach is first verified by\ud application on the redesign of an existing converging–diverging turbine nozzle operating\ud in thermodynamic regions characterized by relevant real-gas effects. A significant\ud efficiency improvement and a more uniform flow at the blade outlet section are achieved,\ud with expected beneficial effects on the aerodynamics of the downstream rotor. The optimized\ud configuration is also assessed by means of high-fidelity turbulent simulations,\ud which point out the capability of the present inviscid approach in optimizing supersonic\ud turbine cascades with very limited computational burdens. Finally, the newly developed\ud real-gas adjoint method is compared against adjoints based on ideal equations of state\ud on the same design problem. Results show that the performance gain obtained by a fully\ud real-gas optimization strategy is by far higher than that achieved with simplified\ud approaches in case of ORC turbines. This proves the relevance of including accurate\ud thermodynamic models in all steps of ORC turbine desig

Non-Ideal Compressible Fluid-Dynamics (NICFD) has recently been established as a sector of fluid mechanics dealing with the flows of dense vapors, supercritical fluids, and two-phase fluids, whose properties significantly depart from those of the ideal gas. The flow through an Organic Rankine Cycle (ORC) turbine is an exemplary application, as stators often operate in the supersonic and transonic regime, and are affected by NICFD effects. Other applications are turbomachinery using supercritical CO2 as working fluid or other fluids typical of the oil and gas industry, and components of air conditioning and refrigeration systems. Due to the comparably lower level of experience in the design of this fluid machinery, and the lack of experimental information on NICFD flows, the design of the main components of these processes (i.e., turbomachinery and nozzles) may benefit from adjoint-based automated fluid-dynamic shape optimization. Hence, this work is related to the development and testing of a fully-turbulent adjoint method capable of treating NICFD flows. The method was implemented within the SU2 open-source software infrastructure. The adjoint solver was obtained by linearizing the discretized flow equations and the fluid thermodynamic models by means of advanced Automatic Differentiation (AD) techniques. The new adjoint solver was tested on exemplary turbomachinery cases. Results demonstrate the method effectiveness in improving simulated fluid-dynamic performance, and underline the importance of accurately modeling non-ideal thermodynamic and viscous effects when optimizing internal flows influenced by NICFD phenomena.