5th International Seminar on
ORC Power Systems
Athens Greece

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16:00   Session 5A: Turbines-Design & flow simulations
Chair: Vincent Lemort
20 mins
Three-dimensional Unsteady Stator-rotor Interactions in a High Expansion ORC Turbine
Gustavo J. Otero Rodriguez, Stephan Smit, Rene Pecnik
Abstract: Organic Rankine cycle (ORC) power systems are a viable alternative to convert low- to-medium grade heat sources into electrical power, typically at temperatures between 120 to 350 ° C. Instead of using steam as the working fluid, the system operates with an organic compound that can effectively convert waste heat or solar energy into mechanical power, as it allows for fewer turbine stages and a dry expansion. However, this expansion process takes place in the dense-vapour region, where the ideal gas assumption is invalid. In the development of ORC turbogenerators, the expander is the most critical component due to its direct impact in the overall system performance. The present work investigates the three-dimensional (3-D) unsteady simulation of a high expansion radial inflow turbine which operates with toluene (C 6 H 5 -CH 3 ). The unsteady Reynolds-averaged Navier-Stokes equations are solved with a multi-parameter equation of state, to account for the non-ideal fluid properties. To account for the unsteady stator-rotor interactions, a fully conservative flux assembling technique for the treatment of non-matching mesh interfaces is implemented. The expansion process considered has a large expansion ratio (~100) which results in a highly supersonic flow in the stator exit (Mach ~2.7). To capture the shock waves, a finite volume scheme is used with an approximate Riemann solver. The novelty of this work is that it presents for the first time a detailed analysis of the unsteady phenomena (shock waves, viscous wakes, and shockwave-boundary layer interaction) in an ORC turbine, including the stator/rotor interaction, by means of 3-D calculations. The simulations indicate strong three-dimensional and unsteady effects, especially in the rotor blade passage. Unsteady shock waves emanating from the trailing edge of the stator interact with the bow shock at the leading edge of the blade and a separation bubble in the suction side of the blade. These loss mechanisms need to be considered when predicting the stage performance. Moreover, the three-dimensional effects clearly indicate that the blade profile needs to be adjusted at different span-wise locations to reduce entropy losses (produced by e.g. flow separation, shock waves, and/or secondary flows) and increase the power output.
20 mins
Optimal Design of ORC Turbine Blades under Geometric and Operational Uncertainties
Nassim Razaaly, Giacomo Persico, Pietro Marco Congedo
Abstract: Typical energy sources for Organic Rankine Cycle (ORC) power systems feature variable heat load and turbine inlet/outlet thermodynamic conditions. The use of organic compounds with heavy molecular weight introduces uncertainties in the fluid thermodynamic modeling and complexity in the turbomachinery aerodynamics, with supersonic flows and strong shocks, which grow in relevance in the aforementioned off-design conditions. These features also depend strongly on the local blade shape, which can be influenced by the geometric tolerances of the blade manufacturing. This study presents a Robust Optimization (RO) analysis on a typical supersonic nozzle cascade for ORC applications under the combined effect of uncertainties associated to operating conditions, fluid parameters, and geometric tolerances. The geometric variability is described by a finite Karhunen-Loeve expansion representing a non-stationary Gaussian random field, entirely defined by a null mean and its autocorrelation function. Real-gas effects are modeled through the use of the polytropic improved Peng-Robinson equation of state implemented within the open-source CFD solver SU2. The blade is parametrized by moving B-splines control points, allowed to be displaced in the direction locally normal to the blade. Different statistics, according to the RO formulation chosen, of the Quantity of Interest (QoI) are minimized in the framework of mono-objective optimization, constraining the mean mass flow-rate to be within a small range centered at the baseline value, at nominal conditions. Linear combination of mean and standard deviation of the QoI (equivalent to the popular Taguchi multi-objective optimization) are considered, as well a linear combination of mean and high quantile. The robust optimal blades are compared to the deterministic optimal shape. The impact of the choice of the RO formulation on the final blades is highlighted.
20 mins
Performance of a Small-scale Organic Rankine Cycle System Using a Regenerative Flow turbine: a Simulation Analysis
Ramin Moradi, Luca Cioccolanti, Emanuele Habib, Enrico Bocci
Abstract: Organic Rankine Cycles (ORCs) have an interesting potential in small-scale power production using low temperature heat sources. Despite their unique applications in power production from low-temperature heat sources, the widespread use of small-scale ORC systems is still challenging especially due to high primary cost resulting in long-term payback periods. The reason goes back to a number of inherent technical limitations especially related to the available expander machines, which are usually accounted for a significant amount of the total investment cost of such systems. In this paper, a Regenerative Flow Turbine (RFT) adopted in a small-scale ORC prototype is investigated by means of the modeling study. The performance curves of the considered turbine have been taken from a previous CFD analysis carried out by the authors and used in the current study to evaluate the performance of the system with varying operating conditions. Hence, performance of the ORC system has been studied in terms of efficiency and net power. The system shows an overall performance that is comparable to that of conventional systems, however the main advantage of adopting the RFT is indeed low investment cost and high reliability, which are important factors in design of the such small-scale power systems.
20 mins
1D Modelling of some Paradigmatic Non-ideal Compressible Flows
Francesco Tosto, Claudio Lettieri, Matteo Pini, Piero Colonna
Abstract: Systems based on the organic Rankine cycle (ORC) concept for energy harvesting, refrigeration and power generation make use of a variety of working fluids. For instance, complex working fluids and supercritical carbon dioxide are utilized in organic Rankine cycle and supercritical CO2 Brayton cycle power systems. Thermo-fluid characteristics of these vapor flows in expanders or compressors deviate significantly from those of flows of perfect gases and this deviation increases if the processes occur in proximity of saturation or the vapor-liquid critical point. The term Non-Ideal Compressible Fluid Dynamics (NICFD) is utilized to characterize the gas dynamics of dense vapors: a key parameter to study these flows is the fundamental derivative of gasdynamics Γ, which accounts for variations in speed of sound with density over isentropic processes. Although rather extensive theoretical knowledge has been developed on the fundamental aspects of NICFD, these findings have not been thoroughly studied in the context of applications and particularly of energy conversion applications, i.e., flows in turbomachinery and heat exchangers. This study is aimed at evaluating the impact of NICFD effects by analyzing simple 1D flow configurations reproducing some of the main entropy loss mechanisms of turbomachinery. A new general theoretical framework based on quasi one-dimensional control volume analysis is developed: a new matrix of influence coefficients accounting for shaft work, heat addition, wall friction, variation in cross section area and mass injection in the control volume is derived. The approach is based on works by Shapiro (1953). Experimental observations of the expansion of a siloxane vapor in a de Laval nozzle provide a mean for preliminary validation. Simple Rayleigh and Fanno flow configuration are studied to quantify the impact of these phenomena on the performance of turbomachinery (and heat exchangers).