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Publication Title | Design and Testing of a Radial Flow Turbine for Aerodynamic Research

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S iL
The Society shall not be responsible for statements or opinions advanced in papers or in dis- cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting.
Printed in USA.
Copyright © 1991 by ASME
The Design and Testing of a Radial Flow Turbine for Aerodynamic Research
This paper describes the design of a high-speed radial inflow
turbine for use as part of a gas-generator, and the design of a large- scale (1.2 m tip dia.) low-speed model of the high-speed turbine. Stream-line curvature throughflow, two-dimensional blade-to-blade and fully three-dimensional inviscid and viscous calculation methods have been used extensively in the analysis of the designs. The use of appropriate scaling parameters and their impact on turbine performance is discussed. A simple model shows, for example, how to model the blade lean in the inducer which serves to balance the effect of meridional curvature at inlet to the rotor and can be used to unload the rotor tip. A brief description of the low speed experimental facility is followed by a presentation and discussion of experimental results. These include surface flow visualisation patterns on both the rotor and stator blades and blade row exit traverses.
Radial inflow turbines offer several advantages for use in small
turboshaft applications when compared with axial turbines for the same duty. This is because the radial inflow turbine offers greater work extraction per stage at comparable or higher efficiencies, increased ruggedness, lower costs of manufacture and improved packaging when used in conjunction with a reversed flow combustor.
Studies at Rolls-Royce have shown that a cooled, high-efficiency radial turbine could offer significant improvements in performance as the gas-generator turbine of a high technology turboshaft engine, if small improvements in current levels of technology could be achieved. An un-cooled radial turbine of a similar aerodynamic duty would also present an attractive proposition in smaller power-plants. However, the problems facing the designers of today's radial turbines are not inconsiderable, particularly in the areas of rotor cooling and rotor aerodynamics.
Many of the published design methods are largely based on the design rules developed by NASA and others (e.g. Hiett and Johnston (1963), Rohlik (1975) and Glassman (1976)). These methods or their adaptations for specific applications are essentially based on observations of the overall performance of radial turbines. Very few are based on observed physical processes even though many of the flow and loss models purport to model the details of the flow. In such circumstances, the possession of one-dimensional information and models places potentially unnecessary restrictions on the design process. Today, commercial organisations are replacing costly experimental development programmes by the use of modern CFD codes such as the viscous analysis code of Dawes (1986) but until the reasons behind such phenomena as 'incidence shock-loss' or tip clearance losses are understood trial and error will play a large part in any design process.
This paper describes the initial stages of a research and development programme in radial turbines which addresses the identification and understanding of the major sources of loss and the assessment of aerodynamic design and analysis methods. A large- scale, low-speed radial inflow model turbine has been constructed at the Whittle Laboratory, Cambridge University as part of the research programme. The model is based on a low cost, high pressure ratio, un-cooled turbine which is designed to be scaled for turboshaft applications in the range of 50 to 300 kW. This paper describes the design of the base turbine, the scaling of this turbine which is required to produce the aerodynamic model and presents the results of an initial investigation using the model turbine.
c „1
Re U V W Yp
p 0
3 4shroud 4hub
rotor span
radial chord
Reynolds number
blade speed
absolute velocity relative velocity pressure loss coefficient:
U32) - (pOre1pU
P03 - P3 number of blades
dynamic viscosity kinematic viscosity density
boundary layer momentum thickness
rotational speed
tangential (pitchwise) stagnation conditions stator inlet
stator exit
rotor inlet
rotor exit shroud rotor exit hub
Whittle Laboratory Cambridge University England
Rolls-Royce plc Leavesden England
Presented at the International Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991
This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 30, 1991
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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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