No abstract

Blade vibrations resulting in alternating stresses are often the critical factor in determining blade life. Indeed, many of the failures experienced by turbomachinery blades occur due to high-cycle fatigue caused by blade vibrations. These vibrations can arise either through self-excited oscillations known as flutter or through aerodynamic forcing of the blades from factors such as periodic wakes from up and/or downstream vanes or unsteady flow phenomena such as compressor surge. The current paper deals with the design and the analytical and experimental verification of the axial blading for a new generation of industrial compressors, a hybrid axial compressor that combines the advantages of conventional industrial compressors — broad operating range and high efficiency — with the advantages of gas turbine compressors — high power-density and high stage pressure ratios. Additionally, the surge robustness of this novel compressor blading has been greatly improved. During the development phase extensive efforts were made to ensure safe operation for future service life. This was achieved by designing blades that will not flutter, do not have high resonance amplitudes throughout their entire operating range and are extremely robust against surge. This strongly increased robustness of the new compressor blading was achieved by the implementation of a “wide-chord” blade design in all rotor blade rows in combination with a proper tuning of resonance frequencies throughout the entire operating range. For the verification of the new blading well-established methods accepted by industry were used such as CFD and FEA. Furthermore, coupling of the two into a method referred to as Fluid Structure Interaction (FSI) was used to more closely investigate the interaction of flow and structural dynamics phenomena. These analytical techniques have been used in conjunction with extensive testing of a scaled test compressor, which was operated at conditions of dynamic similitude (matching of scaled blade vibration frequencies, flow conditions, and Mach number) with full-scale operational conditions. Strain gauges placed on the blades and a state of the art technique known as “tip timing” were used to verify blade vibrations over a wide range of combinations of guide vane positions and rotational speeds. No propensity was found of any of the blades to develop high vibration amplitudes at any of the operating conditions investigated in the rig tests. The comparison of non-linear forced response analyses and the rig test results from strain gauges and tip timing showed close agreement, verifying the analysis techniques used. In conclusion it can be stated that the blade design exhibits a very high level of safety against vibrations within the entire operating range and during surge.

Large axial compressors have been used in various industrial applications for decades. They are not only operating as air compressors, but also for other process gases that need to be compressed with large volume flows. Today, CO2-intensive processes certainly have a special position, which, in connection with the current climate situation, will experience a major change in the coming years. MAN Energy Solutions focusses on alternative technologies and new solutions, such as LAES, CAES or CCS, to master the entrepreneurial, technical, and operational challenges of decarbonization. The internally called “MAX1” axial compressor blading, with its basic technology developed between 2007–2015, is an important part of this strategy and its technical and economical realization. The “FlowCut technology”, discussed in this paper, enables the usage of axial compressors in large-scale production plants for compressing CO2, hydrocarbons, natural gas, nitrogen oxides or other gas mixtures. FlowCut technology means the adaption of the annulus contour for gas compression by means of shortening the blades — without changing the airfoil contour — in such a way, that it leads to an almost full recovery of the compressor performance compared to air compression. Each gas differs from air individually in its gas properties, molecular weight, molar heat capacity and polytropic exponent. These parameters influence the pressure and temperature rise in the compressor and therefore require an individual design of the flow channel in order to keep the flow angles in the inlet and outlet planes of the various stages and thus the essential overall performance of the compressor as similar as possible. The underlying “FlowCut” procedure is performed with a calibrated in-house S2 streamline curvature program. When the flow path is adapted, the blades need to be shortened or extended. The basic blading design of the MAX1 blading already includes the ability of such adaptations. Nevertheless, changes in both aerodynamic and structural-mechanical behavior occur that differ from the baseline design. For this reason the “FlowCut” technology was verified in 2020 in an 11-stage test rig with a nominal volume flow of approximately 90.000 m3/h for air and a pressure ratio of 8. The design flow and the machine Mach number were changed in such a way that the resulting flow path is similar to a CO2 compressor for CCS applications. The rig was heavily instrumented with thermocouples and pressure probes, strain gauges, tip-timing probes, fast pressure transducers; as well as inlet and outlet flange-to-flange instrumentation to collect performance data under a variety of operating conditions. With a focus on aerodynamic and mechanical aspects the test results for the compressor behavior will be presented and compared to the results for the baseline flow path, which were gained in 2015. Analytical and experimental data will be discussed in detail.