Loss models for on and off-design performance of radial inflow turbomachinery

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It is critical to accurately predict the performance of radial inflow turbomachinery in the preliminary design stages. Preliminary design of radial inflow turbomachinery uses basic geometry information and does not have access to detailed blade design. Experimentally derived models using an optimised geometry are widely used to develop a correlation between real world performance and basic design parameters of the turbine. Yet, these correlation models are primarily based on ideal working fluids. Modern turbomachinery systems operate with new complex fluids such as CO2 and R143a which have non-linear compressibility and non-ideal viscosity. Additionally, modern turbomachinery systems are expected to run with reliable performance at off-design conditions. This paper develops a new loss model configuration suitable for the analysis of turbomachinery operating with CO2 and R143a working fluids where off-design performance estimation is critical and temperature fluctuation may be expected. The paper systematically analyses over 1.5 million loss model configurations developed through the enumeration of analytical models for each loss mechanism from within literature. Each loss model configuration is compared against computational simulations of a turbine running at a wide range of off-design conditions. Primary findings of the paper are that loss model configurations must capture the appropriate physics. A loss model configuration must account for slip, as well as interaction between the turbine and immediate downstream components. Furthermore, models that were directly derived from experimental data showed better predictive performance. Specifically, due to the inclusion of appropriate physics and models directly derived from experiments, the proposed loss model within this work showed high accuracy, with a maximum error between the model and CFD of less than 2%. Many performance analysis methods focussed on the on-design performance of turbomachinery alone; the present work extends the application to incorporate more in-depth analysis of the off-design performance of turbomachinery. The paper provides a loss model configuration that is immediately useful for the development of radial inflow turbomachinery operating with CO2 and R143a and can be easily adopted to the early cycle design phase to ensure that performance is consistently high.