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Publication Title | FLUID-DYNAMICS OF THE ORC RADIAL OUTFLOW TURBINE

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Paper ID: 118, Page 1 FLUID-DYNAMICS OF THE ORC RADIAL OUTFLOW TURBINE
Claudio Spadacini*, Lorenzo Centemeri, Dario Rizzi, Massimiliano Sanvito and Aldo Serafino
Exergy SpA,
Via Santa Rita 14, 21057, Olgiate Olona (VA) Italy
e-mail: info@exergy.it
Web page: http://www.exergy-orc.com
ABSTRACT
It is well documented that axial turbines and radial inflow turbines have traditionally been the selected solutions for ORC, both with an overhung configuration. In the last years a different turbine technology for ORC has been developed, engineered, manufactured and tested by Exergy: the radial outflow turbine.
In order to better understand its potential and limits, the present study has the purpose of conducting a fluid-dynamic study of the ORC radial outflow turbine. To pursue this aim, here firstly a summary description of the radial outflow turbine and of its features is given, by means of mechanical and thermodynamic fundamentals.
Secondly, moving from the hypothesis of direct coupling with generator, boundary conditions for a 2 MW case are chosen and a radial outflow turbine is studied, focusing on fluid-dynamic design: after a preliminary mean line study a CFD simulation of the machine is performed.
The described analysis includes also a comparison with an axial ORC turbine with the overhung configuration directed coupled with a generator: this approach could allow to valuate fluid-dynamic losses in both technologies and can explain the reason why the radial outflow turbine shows a higher efficiency than the axial overhung one in many ORC applications.
1. INTRODUCTION
In these days the ORC market is in expansion and as a matter of fact a rising interest about it can be observed in the scientific community. In terms of system performance, turbine is the most important and critical component of ORC systems (Macchi, 2013) and for this reason it is subject to many researches and studies.
An interesting innovation in this panorama is the ORC radial outflow turbine developed by Exergy, which has several unique characteristics qualifying this unconventional configuration as advantageous for many ORC applications, as it ideally matches the process conditions typical for these kinds of uses. In fact it has been demonstrated (Macchi, 2013 – Frassinetti et al., 2013) that this machine is competitive with both axial and radial inflow turbines, solutions usually adopted in ORC applications with the overhung configuration as it allows to have compact machines and to reduce sealing problems (Salucci et. al, 1983).
2. THE ORC RADIAL OUTFLOW TURBINE AT A GLANCE
To better understand the particular features of the radial outflow turbine (see Figure 1) some considerations about turbomachinery fluid-dynamics and about thermodynamics are necessary.
The well-known general Euler equation for turbomachinery, ignoring minor negligible losses, provides a formulation of the specific work for a single stage as (for instance Stodola, 1927):
𝒍=𝒖𝟏 𝒄𝒕𝟏 − 𝒖𝟐 𝒄𝒕𝟐 (1)
3rd International Seminar on ORC Power Systems, October 12-14, 2015, Brussels, Belgium

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MicroAD: This research and pdf compilation was sponsored Infinity Turbine Turn your waste heat into energy to save on grid based power or sell back to the grid Organic Rankine Cycle utilizes waste heat to make power. Infinity Turbine Waste Heat to Power Solutions

RADIAL-OUTFLOW-TURBINE: In a radial outflow turbine the organic fluid enters the disk axially in its center and expands radially through a series of stages mounted on the single disk. At the discharge of the last rotor row the flow passes through a radial diffuser and is then conveyed to the recuperator and or condensa- tion section of the system, through the discharge volute. In the early 20th century, Parsons Siemens and Ljungstrom developed the first steam based radial outflow turbines. These early model turbines required a large number of stages. For very high enthalpy drop fluids, such as steam, a single-disk/multi stage configuration was therefore deemed not suitable due to the very large diameter disk necessary to accommodate all the required stages. No further development of the radial outflow turbines oc- curred, as they were phased out for steam applications by axial turbines.

The Geothermal Radial Outflow Turbine: An innovative turbine configuration for geothermal applica- tions was developed by the Italian turbine manufacturer EXERGY. The technology, known as the organic radial outflow turbine was designed, engineered, manufactured and tested in Italy. A 1 MWe geothermal organic Rankine cycle (ORC) equipped with the EXERGY radial outflow turbine has been in operation since early 2013. The radial outflow turbine is a new type of turbine that have the potential to increase the geothermal binary power plants ef- ficiency by increasing the turbine efficiency. The operational results has been positive and demonstrates the viability of the technology and the possibility to develop it for bigger sizes.

Preliminary Design and Off-Design Analysis of a Radial Outflow Turbine for Organic Rankine Cycles: Recently, the advantages of radial outflow turbines have been outstanding in various operating conditions of the organic Rankine cycle. However, there are only a few studies of such turbines, and information on the design procedure is insufficient. The turbine target performance could be achieved by fine-tuning the blade angle of the nozzle exit. In addition, performance evaluation of the turbine against off-design conditions was performed. Ranges of velocity ratio, loading coefficient, and flow coefficient that can expect high efficiency were proposed through the off-design analysis of the turbine.

Study on applicability of radial-outflow turbine type for 3 MW WHR organic Rankine cycle: The article presents the results of study on the reasonability of using radial-outflow turbines in ORC. Peculiarities of radial-outflow turbine design utilizing modern design technologies and application to ORC was considered in the first part of the paper. For this particular cycle design, turbines of radial-outflow type were chosen. Their application enables the increase of mechanical output power by 11 percent compared to original radial-inflow turbines.

LOSS GENERATION IN RADIAL OUTFLOW STEAM TURBINE CASCADES: Small high-speed technology based radial outflow steam turbines are characterised by ultra-low aspect ratios, which can lead to rapidly growing secondary losses. The prelimi- nary evaluation of turbine performance is usually based on axial turbine loss predictions, which can be a source of error. The main objectives of this work are to find out how the losses are generated in radial outflow turbines when the aspect ratio is markedly below unity and how accurately axial turbine loss models can predict the trends. To achieve these objectives, a radial outflow turbine cascade having a blade shape and aspect ratios comparable with a prototype machine is examined. As a result of the study, it is suggested that for the examined radial outflow cascade the axial turbine loss correlations can predict the trends reasonably well. The rapidly increasing secondary losses are connected to the merging of secondary structures and also incidence at off-design.

PRELIMINARY DESIGN OF RADIAL-INFLOW TURBINES FOR ORGANIC RANKINE CYCLE POWER SYSTEMS CONSIDERING PERFORMANCE AND MANUFACTURABILITY ASPECTS: In order to make organic Rankine cycle power systems economically feasible, it is essential to find a reasonable trade-off between the performance and the initial cost of system. In order to show its relevance in a practical application, the method is applied to two radial-inflow turbines cases: a state-of-the-art turbine using air and a turbine using the working fluid Novec 649 for a heat recovery application. The results indicate that there exists a trade-off between turbine performance and manufacturability, and that it is possible to develop turbine solutions with similar values of efficiency with improved manufacturability indicator by up to 14 to 15 percent.

DESIGN AND FLOW ANALYSIS OF RADIAL AND MIXED FLOW TURBINE VOLUTES: Radial and mixed flow turbines which are an important component of a turbocharger consist essentially of a volute, a rotor and a diffuser. Vaneless volute turbines, which have reasonable performance and low cost, are the most used in turbochargers for automotive engines. Care has to be done in the design of the volute, whose function is to convert a part of the engine exhaust gas energy into kinetic energy and direct the flow towards the rotor inlet at an appropriate flow angle with reduced losses.

An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall: The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.

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