A CRITICAL ANALYSIS ON THE IMPACT OF EXTERNAL LOSSES ON THE PERFORMANCE OF A CENTRIFUGAL COMPRESSOR STAGE - EXPERIMENTAL AND MULTI-FIDELITY NUMERICAL ASSESSMENT

The study presents a combined experimental and numerical analysis of the impact of external losses in defining the real performance of a model stage. This is of particular importance in the early-validation phase, where novel solutions going beyond the state of the art need to be verified for the first time. Due to the number of simulations to be carried out during preliminary design, in fact CFD calculations often focus on main flow path analyses and neglect these losses. If optimization is carried out within a known design space, this does not create any problem and the different solutions can be analyzed in a comparative way. On the other hand, when a new design solution comes into play, possible changes in the impact of losses can lead to unfair comparison between numerical predictions and test-rig experimental data. In particular, the results of detailed experimental tests carried out in the test rig developed by the University of Florence in support of its industrial partner Baker Hughes are presented here. In the test, different seals at the impeller's eye are compared. Results are then compared to numerical simulations of the stage that include cavities and resolve leakage flow, and of those of a newly-developed integrated and computationally-efficient tool that combines CFD simulations of only the main flow path with engineering corrections for loss sources. Results show that indeed a comprehensive CFD model that resolves cavity leakage is able to correctly replicate experimental results, although this is obtained with a notable calculation cost, which is often not affordable at industrial level. On the other hand, almost the same accuracy is reached with the combination of a main flow path-only CFD model and correction method, thus proving the interest for this type of approach in the industrial context.