Aerodynamic Design and Analysis of Centrifugal Turbine Cascades

Abstract

The centrifugal turbine architecture represents a promising solution for Organic Rankine Cycle (ORC) Systems, in the small-to-medium power range. The large volumetric expansion ratios occurring in ORC turbines complicate the design of the turbo-expander, making the centrifugal arrangement worth of interest with respect to conventional architectures. A new-concept centrifugal turbine has been recently proposed by the authors, based on the design of Ljungström but implementing a stator-rotor arrangement, which allows for multi-stage assembly without compromising compactness. To properly evaluate the potential of the centrifugal turbine solution, reliable data on cascade aerodynamic performances are required, but they are still lacking in the open literature. In this paper the aerodynamics of radial-outward turbine cascades is discussed, on the basis of Computational Fluid-Dynamics (CFD) simulations. A weakly transonic operating condition is selected and for that different classes of profiles are tested. Results show that the intrinsic diverging shape of the radial-outward configuration complicates the blade design; if the flow deflection is not properly controlled along the streamwise direction, the bladed duct can result in a converging-diverging channel, leading to unexpected chocked flows and shocks even in weakly transonic configurations. The indications achieved by the comparison between different blade profiles are gathered to define guidelines for the design of novel elliptic profiles, which allow to control the flow acceleration process, providing high aerodynamic efficiency. Off-design performances of the elliptic profiles are finally addressed by studying the response of the cascade to different expansion ratios and high incidence angles.

Modelling and parametric analysis of small-scale axial and radial-outflow turbines for Organic Rankine Cycle applications

“…A mobile small- application market is currently ongoing [35] and it is receiving more analytical studies by [30,31,32]. While the detailed studies are conducted of radial-inflow turbine that combined with mean-line design and ORC cycle analysis and followed by three-dimensional CFD simulation for a number of working fluids and operating conditions as reported by Sauret et al [1,27,36] and other studies [2][3][4][5][6][7][8][9][10][11]15,17,21,22,26,29,33,34].…”

“…The pressure loss coefficient and enthalpy loss coefficient are approximately equal at a small value of enthalpy [39,41] has been used by many researchers working on radial-outflow turbines, such as [30,31,32,42]. This losses model only includes the profile loss and secondary loss [32].…”

“…The maximum total-to-total efficiency was approximately 76%, with approximate power output of 7kW for a pressure ratio of around 3.5. Persico et al [30,31] conducted a 3D CFD aerodynamic analysis of small-scale centrifugal turbine cascades, which found efficiency to be higher than estimated by 1D analysis. Casati [32] proposed a 1D preliminary design of an example 10 kWe centrifugal turbine for a mini-ORC, for power systems driven by heat recovery.…”

Preliminary Design and Off-Design Analysis of a Radial Outflow Turbine for Organic Rankine Cycles

“…Persico et al [10] studied a design technique of a radial outflow turbine for a 1 MW organic Rankine cycle. The study found that the intrinsic diverging form of the radial outflow configuration makes the blade design difficult.…”

Aerodynamic Optimization Design of a Multistage Centrifugal Steam Turbine and Its Off-Design Performance Analysis

“…Abroad, the preliminary hydrodynamic design of a small centrifugal turbine for the ORC was studied by Casati and others of the Delft University of Technology. They introduced the optimization based on the intermediate streamline method for evaluating turbine design and performance [6][7][8][9][10].…”