The design of high efficiency modern centrifugal compressor stages with a wide operating range is challenging and demanding. To achieving the best design, numerical tools and test equipments for performance prediction and validation need to be at the state-of-the-art. Test data are also necessary to validate and continuously improve the numerical techniques used for performance prediction during the design phase. This paper presents the experimental and computational analysis of the flow field in a modern high flow, high Mach centrifugal compressor stage at the design point. Phase-resolved flow measurements for total pressure, static pressure and flow angle were carried out at the exit section of the centrifugal impeller. The measurements were performed using a Fast Response Aerodynamic Pressure Probe (FRAPP), traversed from the impeller hub up to the shroud; the key experimental results are presented as two-dimensional distributions as well as hub-to-shroud profiles. A CFD analysis was also carried out using an in-house CFD code to compare the results of the computational models with the FRAPP. In particular two different approaches, which combined accuracy and reasonable computational time, were used for the CFD computations: one is the standard methodology with only the flow-path modeled; the second also includes modeling of the leakage cavities. In fact the correct prediction of the flow profiles at the impeller exit and downstream, at the diffuser exit, is fundamental for the accurate design of the non-rotating downstream parts. Historically, the standard CFD approach has been found to be weak in capturing these profiles, especially close to the hub wall. Instead the CFD model including cavities was able to match to within a good approximation the profiles from the FRAPP probe.

In the Oil&Gas industry, the quest for turbomachinery high efficiency is continuously pushing technology towards new improvements. Rotor (impeller) optimization is a rather established practice in the centrifugal compressors design both in the scientific literature and in the industrial experience, but the same focus is not always applied to statoric components optimization. This is due to the smaller impact of plenums and return channels on the overall compressor performance in comparison to the impeller. However, further improvements in the design of centrifugal compressors stage have to deal with the loss minimization of statoric parts, also considering the advanced level of aerodynamic detail reached by Original Equipment Manufacturers (OEMs). In the present paper, the optimization of the return channel is performed by means of 3D Computational Fluid Dynamics (CFD) with the objective of loss coefficient minimization. The CFD simulations are run with a non-commercial proprietary software (Tacoma) considering steady flow with Reynolds Averaged Navier Stokes (RANS) approach and turbulence k-ω model with strain correction; full validation of the employed method was performed in the past against experimental campaigns and is available in the literature. In order to reproduce as much as possible full stage operating conditions, simulations should include impeller, diffuser and return channel, but the computational cost of an optimization with many iterations required the reduction of the domain. In order to preserve simulation accuracy at the same time, flow profiles at impeller exit have been imposed at the considered computational domain inlet (i.e. diffuser inlet), whilst cavity effects and secondary flows have been considered adding source terms taken from the full stage simulation of the baseline geometry. Return channel blades have been parameterized in terms of angle distributions with Bézier curves at hub and shroud, for a total of 18 Bézier poles. Each different design has been simulated with its speedline consisting of 7 operating conditions, whereas progressive optimization based on response surfaces have been considered starting from an initial Design of Experiment (DOE). Over 100 designs have been simulated and the most efficient allows a 20% loss coefficient reduction on a real case stage design at design point.

Computational Fluid Dynamics (CFD) is becoming fundamental to predict turbomachinery performance. However, only using advanced numerical models coupled with high fidelity grid generation is possible to reach a very good matching with test data. In this regard, secondary flow modeling plays a critical role in the accuracy of performance prediction for centrifugal compressor stages. This study analyses the effects of cavity models on centrifugal compressor stages performance across the full range of impeller flow coefficients used in common industrial applications. Both bi-dimensional low flow coefficients with splitter and non splitter blades and three-dimensional high flow coefficients stages have been used as test cases to compare the numerical prediction with test data. Furthermore the effects of secondary flows modeling have been assessed when comparing detailed flow features with advanced experimental data both in terms of 1D profiles and 2D maps. The effects of cavity flows modeling is growing, as expected, moving to very low flow coefficients, reaching several points of difference in efficiency calculation with respect to simpler models. Furthermore, the agreement with experimental data is very good both in terms of overall performance and detailed flow features. Finally, the high fidelity CFD is capable to give deep in-sides into the flow evolution inside the machine allowing aero designers to design centrifugal compressor stages with higher performance. It should be remarked here that a good matching of CFD prediction with test data is possible only by using high fidelity models.

During the design of modern high efficiency, wide operating range centrifugal compressor stages, Computational Fluid Dynamics (CFD) plays an increasing role in the assessment of the performance prediction. Nevertheless experimental data are valuable and necessary to assess the performance of the stages and to better understand the flow features in detail. A big effort is currently being made to increase the fidelity of the numerical models and the probe measurement accuracy during both the design and validation phases of centrifugal compressor stages. This study presents the flow analysis of centrifugal compressor stages using high fidelity computational fluid dynamics with a particular attention to the cavity flow modeling and comparison with experimental data, using an advanced fast response aerodynamic pressure probe. Different flow coefficient centrifugal compressor stages were used for the validation of the numerical models with a particular attention to the effects of cavity flow on the flow phenomena. The computational domain faithfully reproduced the geometry of the stages including secondary flow cavities. The availability of a new in-house automated tool for cavity meshing allowed to accurately resolve leakage flows with a reasonable increase in computational and user time. Time averaged data from CFD analysis were compared with advanced experimental ones coming from the unsteady pressure probe, for both overall performance and detailed two-dimensional maps of the main flow quantities at design and off design conditions. It was found that the increase in computational accuracy with the complete geometry modeling including leakage flows was substantial and the results of the computational model were in good agreement with the experimental data. Moreover the combination of both advanced computational and experimental techniques enabled deeper insights in the flow field features. The comparison showed that only with advanced high fidelity CFD including leakage flows modeling did the numerical predictions meet the requirements for efficiency, head and operating margin, otherwise not achievable with simplified models (CFD without cavities).

A multistage frequency domain (Nonlinear Harmonic) Navier-Stokes unsteady flow solver has been used to analyze the flow field in the MIT (rotor/rotor) aspirated counter-rotating compressor. The numerical accuracy and computational efficiency of the Nonlinear Harmonic method implemented in Numeca’s Fine/Turbo CFD code has been demonstrated by comparing predictions with experimental data and nonlinear time-accurate solutions for the test case. The comparison is good, especially considering the big savings in time with respect to a time accurate simulation. An imposed inlet boundary condition takes into account the flow change due to the IGV (not simulated in the computational model). Details of the flow field are presented and physical explanations are provided. Also, suggestions and recommendations on the use of the Nonlinear Harmonic method are provided. From this work it can be concluded that the development of efficient frequency domain approaches enables routine unsteady flow predictions to be used in the design of modern turbomachinery.