As bypass-ratio in modern aero engines is continuously increasing over the last decades, the radial offset between low pressure compressor (LPC) and high pressure compressor (HPC), which needs to be overcome by the connecting s-shaped intermediate compressor duct (ICD), is getting higher. Due to performance and weight saving aspects the design of shorter and therefore more aggressive ducts has become an important research topic. In this paper an already aggressive design (with respect to current aero engines) of an ICD with integrated outlet guide vane (OGV) is used as a baseline for an aerodynamic optimization. The aim is to shorten the duct even further while maintaining it separation free. The optimization is broken down into two steps. In the first optimization-step the baseline design is shortened to a feasible extent while keeping weak aerodynamic restrictions. The resulting highly aggressive duct (intermediate design), which is shortened by 19 % in axial length with respect to the baseline, shows separation tendencies of low momentum fluid in the strut/hub region. For the second step, the length of the optimized duct design is frozen. By implementing new design features in the process of the optimizer, this optimization-step aims to eliminate separation and to reduce separation tendencies caused by the aggressive shortening. In particular, these features are: a nonaxisymmetric endwall contouring and parametrization of the strut and the OGV to allow for changes in lift and turning in both blade designs. By comparison of the three designs: Baseline, intermediate (separating flow) and final design, it can be shown, that it is possible to decrease length of the already aggressive baseline design even further, when adding a nonaxisymmetric endwall contouring and changes in blade shape of the strut and OGV. Flow separation can be eliminated while losses are kept low. With a more aggressive and therefore shorter duct the engine length and weight can be reduced. This in turn leads to lighter aircrafts, less fuel consumption and lower CO2 and NOx emissions.

The flow in the blade tip vicinity of the transonic first stage of a multi-stage axial flow compressor with variable inlet guide vane (IGV) and casing treatment (CT) above the rotor is investigated experimentally and numerically with focus on the effects of the CT on flow structures and compressor performance. For the experimental part of this study, conventional performance instrumentation is used to estimate the operating condition of the compressor. Radial distributions of total temperature and total pressure are taken at the leading edges of the stators for comparison with simulations as well as for adjusting the operating conditions of the compressor. The velocity field in the rear part of the first-rotor is determined with Particle Image Velocimetry (PIV) at 90% and 96% radial height using two periscope light sheet probes. The employed PIV setup allows a spatial resolution of 0.7 mm × 0.7mm and thus a similar resolution as the spatial discretization in the simulation. For the numerical part of the study, time-accurate simulations are conducted for the same operating conditions as during experiments. Additional simulations of the same configuration with smooth casing are conducted in order to estimate the effect of the CT on the flow. The examination of PIV measurements and corresponding simulations exposes complex vortical structures originating from the interaction of the rotor bow shock with the IGV trailing edge, CT, IGV wake and the tip leakage vortex. The associated induced velocities together with the general passage flow form a complex flow field with significantly altered blockage compared to a common flow field in the tip vicinity. Position and trajectory of the tip leakage vortex are deduced from interactions between tip leakage vortex and IGV wake / CT. The detailed comparison of the tip region of simulations with and without CT shows that the CT influences pressure rise and flow parameters in a wide radial range due to a radial redistribution of the flow. Correspondingly, a rotor with CT can achieve an increased total pressure rise compared to a rotor with smooth casing, with only minor effects on the efficiency.

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.

It has been shown in many cases that a notable aerodynamic stability enhancement can be achieved using casing treatments (CTs) on transonic compressors. This advantage, however, often involves degradation in efficiency at design point conditions. In order to analyze the correlations between efficiency, surge margin and other flow quantities on the one hand and the geometric parameters related to axial slots on the other, an automated multi objective geometry optimization of axial slots is performed. This involves the usage of time accurate URANS simulations for each new CT design the optimization tool proposes. The axial slots are generated using a parametric design, which can produce slots of different size, shape and position. Three operating points are simulated. One at design point (ADP) conditions, a second at reduced speed working line conditions and a third at reduced speed close to the stability limit. Based on the results of the CFD simulations two objective values are calculated. These are, first, an increased efficiency at working line conditions and, second, an increased surge margin at reduced speed. The test case used for the study is the first stage of DLR’s transonic research compressor Rig250. The rig is representative for the front stages of a heavy duty gasturbine compressor. The computational domain includes the IGV as well as the first rotor and stator. The rotor of the configuration is tip-critical for the studied part speed condition. The result of the optimization is a Pareto front with all optimal geometries regarding surge margin and efficiency. It is found that efficiency at design point can be exchanged against surge margin at reduced speed. The working principles and flow phenomena of the Pareto-optimal axial slots are analyzed in detail to obtain a better understanding of the mechanisms leading to the extension in surge margin.

Automated CFD-based optimization procedures have become an essential part of modern aerodynamic compressor design. Although time-accurate CFD provides a higher physical accuracy, due to limited resources still mainly steady state CFD is used. With a constantly growing computing power the question arises, whether it is worth it increasing the computing effort per evaluation using more accurate CFD codes, in order to improve the optimization results. This work investigates how the results of an automated aerodynamic compressor optimization depend on the simulation procedure used to calculate the flow solutions during the optimization. Two configurations of a counter-rotating fan stage with different axial inter-blade spacing have been optimised using a Q3D approach for the midspan airfoil sections. The configurations were chosen, as to represent the two possibilities of low and high unsteady flow interaction between the blade rows. In each case two automated optimizations have been performed. One based on a steady simulation procedure (RANS), the other on a time-accurate (URANS). In addition, the configuration with low axial spacing has been optimized using a RANS procedure with a determinsitic stress model (RANS-DS). A dependency of the optimization results on the CFD method used has been observed for cases showing high unsteady interaction between the blade rows. The best optimization results were obtained using a time-accurate URANS CFD-solver. A comparison between RANS and RANS-DS showed an advantage of using RANS-DS.