NUMERICAL LOSS BREAKDWOWN STUDY FOR A SMALL SCALE, LOW SPECIFIC SPEED SUPERCRITICAL CO2 RADIAL INFLOW TURBINE

Radial inflow turbines, characterised by a low specific speed are a candidate architecture for the supercritical CO2 Brayton cycle at small scale, i.e. less than 5MW. Prior cycle studies have identified the importance of turbine efficiency to cycle performance, hence well designed turbines are key in realising this new cycle. With operation at high Reynolds numbers, and small scales, it is uncertain as to the relative importance of loss mechanisms in supercritical CO2 turbines. This paper presents a numerical loss breakdown study of a low specific speed radial inflow turbine operating on supercritical CO2. A combination of steady-state and transient calculations are used to determine the source of losses within the turbine stage. Losses are compared with preliminary approaches. Geometric variations to address high loss regions of stator and rotor are trialled. Analysis shows stage losses to be dominated by endwall viscous losses in the stator when utilising stator geometry definitions derived from gas turbines. These losses are more significant than predicted using preliminary methods. A reduction in stator-rotor interspace and use of a foiled blade showed a significant improvement in stage efficiency, without detriment to stator-rotor interaction. An investigation into rotor blading shows favourable performance gains through the inclusion of splitter blades. Through these modifications, a stage performance improvement of 7.5 points is possible over the baseline design. INTRODUCTION Thermal power conversion cycles, using supercritical carbon dioxide (sCO2) were first proposed by Angelino (1968). Benefiting from the non-linear thermo-physical properties of sCO2, these cycles achieve thermodynamic efficiencies better than comparable steam Rankine cycles at temperatures above 500-6000C as shown by Dostal (2004). With appropriate cycle enhancements, and consideration of realistic component performance, cycle efficiencies in excess of 50% can be attained, with the ability to maintain acceptable turndown performance (Jahn and Keep, 2017). Further benefits of the supercritical fluid are high densities and low viscosities, which results in compact and power dense components. In Australia, the Australian Solar Research Institute (ASTRI) is investigating concentrated solar thermal power in conjunction with thermal energy storage with the sCO2 power cycle. The aim of this program is to develop a power generation technology that addresses energy despatchability at a competitive cost in the 1-25MW range, as a solution to provide electricity to rural communities (ASTRI 2012). A number of other sCO2 power system demonstration projects are currently under way in the USA (Held 2014; Moore et al., 2015; Wilkes et al., 2016). For applications in this power range, specific speeds in the range 0.2 to 0.6 are typical and radial turbomachinery is a preferred solution (Balje 1962). Recent works have shown that cycle efficiency is more sensitive to turbine efficiency than compressor efficiency (Brun et. al, 2017), hence it is clear that design efforts should be focused on improving the performance of the turbine. In order for efficient turbines to be realised, an understanding of their internal loss mechanism is vital. Present preliminary turbine design methodologies utilise loss models developed for gas turbines. Whilst some of these are physics based and should provide satisfactory performance with a change in fluid, others are empirical or experience based. In order to enhance preliminary design, and hence speed up the design process, it is necessary to verify preliminary methods for new applications and update models where appropriate. Two recent studies (Wheeler and Ong, 2014; Pini et al., 2017) use steady-state and transient CFD calculations to This work is licensed under a Creative Commons Attribution 4.0 International License CC-BY 4.0 elucidate loss mechanisms in Organic Rankine cycle (ORC) turbines. Wheeler and Ong (2014) shows discrepancies between CFD calculations and a typical preliminary method, particularly in the proportional loss split between stator and rotor. Whilst the turbines of previous numerical studies on ORC turbines utilise similarly dense fluids and operate at a similar scale and specific speed to those of sCO2, some key differences are that ORC machines operate with fluids with low acoustic speed, high volumetric flow variation and higher pressure ratios. Hence these turbines cannot be considered similar according to similitude theory as presented by White and Sayma (2015). In the present paper, a numerical loss breakdown study is performed for a low specific speed 300kW sCO2 radial inflow turbine. METHODOLOGY The present turbine design is based on conservative supercritical CO2 cycle conditions with a target shaft power of 300kW. Design constraints are listed in Table 1. Table 1: Stage operating conditions