Multistage axial compressors have always been a great challenge for designers since the flow within these kind of machines, subjected to severe diffusion, is usually characterized by complex and widely developed 3D structures, especially next to the endwalls. The development of reliable numerical tools capable of providing an accurate prediction of the overall machine performance is one of the main research focus areas in the multistage axial compressor field. This paper is intended to present the strategy used to run numerical simulations on compressors achieved by the collaboration between the University of Florence and Ansaldo Energia. All peculiar aspects of the numerical setup are introduced, such as rotor/stator tip clearance modelling, simplified shroud leakage model, gas and turbulence models. Special attention is payed to the mixing planes adopted for steady-state computations because this is a crucial aspect of modern heavy-duty transonic multistage axial compressors. In fact, these machines are characterized by small inter-row axial gaps and transonic flow in front stages, which both may affect non-reflectiveness and fluxes conservation across mixing planes. Moreover, the high stage count may lead to conservation issues of the main flow properties form inlet to outlet boundaries. Finally, the likely occurrence of partspan flow reversal in the endwall regions affects the robustness of non-reflecting mixing plane models. The numerical setup has been validated on an existing machine produced and experimentally tested by Ansaldo Energia. In order to evaluate the impact on performance prediction of the mixing planes introduced in the steady-state computation, un-steady simulations of the whole compressor have been performed at different operating conditions. These calculations have been carried out both at the compressor design point and close to the surge-line to evaluate the effect of rotor/stator interaction along the compressor working line.

International policies aiming at keeping global warming within safety limits will strongly impact gas-fired power plants. Flexibility is going to be the key-word, hence this paper focuses on possible strategies to increase turndown capability (i.e. lowering Minimum Environmental Load (MEL)) of open and combined cycle power plants (CCPP) through retrofittable compressor service packages. In particular, the following three options have been analyzed: extra-closure of Variable Inlet Guide Vanes (IGVs), Blow-Off (BO) lines opening and inlet bleed heating. All these solutions aim at reducing the compressor outlet mass-flow rate while keeping a safe stability margin. The effect of lowering the minimum load capability by opening the BO lines has been numerically investigated through full compressor 2D throughflow analyses. Moreover, the impact on compressor performance and stability of the extra-closure of IGVs has been analyzed with the support of 3D steady-state CFD modelling. Finally, the overall performance of the power-plant has been included and discussed in order to provide plant managers with a solid starting point for a techno-economic analysis.

The current industrial standard for numerical simulations of axial compressors is the steady Reynolds-averaged Navier–Stokes (RANS) approach. Besides the well-known limitations of mixing planes, namely their inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost, and which is a major accountable for the radial mixing: the transport of streamwise vorticity. Streamwise vorticity is generated for various reasons, mainly associated with secondary and tip-clearance flows. A strong link exists between the strain field associated with the vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for the mixing. In the rear compressor stages, due to high clearances and low aspect ratios, only accounting for the development of secondary and clearance flow structures, it is possible to properly predict the spanwise mixing. In this work, the results of steady and unsteady simulations on a heavy-duty axial compressor are compared with experimental data. Adopting an unsteady framework, the enhanced mixing in the rear stages is properly captured, in remarkable agreement with experimental distributions. On the contrary, steady analyses strongly underestimate the radial transport. It is inferred that the streamwise vorticity associated with clearance flows is a major driver of radial mixing, and restraining it by pitch-averaging the flow at mixing planes is the reason why the steady approach cannot predict the radial transport in the rear part of the compressor.

The axial compressors of power-generation gas turbines have a high stage count, blades with low aspect ratios and relatively large clearances in the rear section. These features promote the development of strong secondary flows. An important outcome deriving from the convection of intense secondary flows is the enhanced span-wise transport of fluid properties mainly involving the rear stages, generally referred to as “radial mixing”. An incorrect prediction of this key phenomenon may result in inaccurate performance evaluation and could mislead the designers during the compressor design phase. As shown in a previous work, in the rear stages of an axial compressor the stream-wise vorticity associated with tip clearance flows is one of the main drivers of the overall span-wise transport phenomenon. Limiting it by circumferentially averaging the flow at row interfaces is the reason why a steady-state analysis strongly under-predicts radial mixing. To properly forecast the span-wise transport within the flow-path, an unsteady analysis should be adopted. However, due to the high blade count, this approach has a computational cost not yet suitable for industrial purposes. Currently, only the steady-state full-compressor simulation can fit in a lean industrial design chain and any model upgrade improving its radial mixing prediction would be highly beneficial for the daily design practice. To attain some progresses in RANS model, its inherent lack of convection of stream-wise vorticity must be addressed. This can be done by acting on another mixing driver, able to provide the same outcome, that is turbulent diffusion. In particular, by enhancing turbulent viscosity one can promote span-wise diffusion, thus improving the radial mixing prediction of the steady approach. In this paper, this strategy to update the RANS model and its application in simulations on a compressor of the Ansaldo Energia fleet is presented, together with the model tuning that has been performed using the results of unsteady simulations as the target.

Due to the generally high stage and blade count, the current standard industrially adopted to perform numerical simulations on multistage axial compressors is the steady-state analysis based on the Reynolds-averaged Navier-Stokes approach (RANS), where the coupling between adjacent rows is handled by means of mixing planes. In addition to the well-known limitations of a steady-state picture of the flow, namely its inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost through a mixing-plane, and which is believed to be a major accountable for the radial mixing: the transport of stream-wise vorticity. Streamwise vorticity arises throughout a compressor for various reasons, mainly associated with secondary and tip-clearance flows. A strong link does exist between the strain field associated with the transported vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for mixing itself. Especially for the rear stages of a multistage axial compressor, due to high clearances and low aspect ratios, only accounting for the development along the meridional path of secondary and clearance flow structures it is possible to properly predict the spanwise mixing. In this work, the results of steady and unsteady RANS simulations on the high-pressure section of an industrial heavy-duty axial compressor are presented and compared with experimental data acquired during a test campaign. Adopting an unsteady full-annulus URANS approach, the enhanced radial mixing in the rear stages of the compressor is properly captured, obtaining a really good agreement with experimental data both in terms of total temperature and pressure outlet radial distributions. On the contrary, with a steady-state modelling, the radial transport is strongly underestimated, leading to results with marked departures from experiments. Examining what occurs across the inter-row interfaces for RANS and URANS solutions, a possible explanation for this underestimation is provided. In particular, as the stream-wise vorticity associated with clearance flows is one of the main drivers of radial mixing, restraining it by pitch-averaging the flow at mixing planes of a steady-state analysis is the reason why this simplified approach is not able to properly predict the radial transport of fluid properties in the rear part of the axial compressor.