The paper aims to give an overview over the keystones of design of the turbomachine for a supercritical CO₂ (sCO2) Brayton cycle. The described turbomachine is developed as part of a demonstration cycle on a laboratory scale with a low through flow. Therefore, the turbomachine is small and operates at high rotational speed. To give an overview on the development, the paper is divided into two parts regarding the aerodynamic and mechanical design. The aerodynamic design includes a detailed description on the steps from choosing an appropriate rotational speed to the design of the compressor impeller. For setting the rotational speed, the expected high windage losses are evaluated considering the reachable efficiencies of the compressor. The final impeller design includes a description of the blading development together with the final geometry parameters and calculated performance. The mechanical analysis shows the important considerations for building a turbomachine with integrated design of the three major components: turbine, alternator, and compressor (TAC). It includes different manufacturing techniques of the impellers, the bearing strategy, the sealing components, and the cooling of the generator utilizing the compressor leakage. Concluding the final design of the TAC is shown and future work on the machine is introduced.

This paper presents experimental data on a centrifugal compressor operated with CO2. Temperature and pressure at the inlet of the compressor are varied to cover the supercritical region from the liquid-like to the gas-like region. In addition, inlet conditions in the two-phase region are also included. Thus, the experimental test campaign considers thermodynamic conditions relevant for the future energy conversion with sCO₂-Joule cycles. Experimental results are presented as compressor pressure ratio vs inlet mass flow rate at different rotational speeds and throttle positions. Reliable conclusions can be drawn from the experimental results since the reproducibility of the measurements has been demonstrated by conducting experiments in two different test rigs, and measurement uncertainties are reported. The entire compressor geometry is disclosed in a data repository, including CAD models, and input files suitable for mean-line and grid generation programs. Thus, the experimental results are exploitable by the scientific community and pave the road for validated analysis and design tools in the context of the sCO₂-Joule cycle. The presented results open the possibility to estimate uncertainties of analysis and design tools with little effort since geometry information can be quickly integrated. The experimental data is already used in this paper to obtain the accuracy of a CFD code and a mean-line program for sCO₂. In addition to quantifying uncertainties, the results presented can be used to identify shortcomings of existing tools. This can be an essential step in the exploration of the sCO₂-Joule cycle.

Direct transfer pre-swirl systems have been investigated by means of 3D computational analysis. Different computational approaches have been utilized and results have been compared with measurements. Based on the validation studies, guidelines for modeling such systems have been proposed. The present results imply that sufficiently accurate predictions can be obtained by means of a quasi-steady analysis applying the “frozen rotor” approach for treating the interface between the stationary and rotating domains. The results indicate that the dimensionless pressure and temperature show a rather weak sensitivity to the relative position of the stator and rotor, using the frozen rotor approach, for the cases studied.

Steam turbine inlet valves are used to control the power output of steam turbines for power generation. These valves may be subject to vibration under certain operating conditions, especially in part-load operation. Several research papers and reports show that elevated valve vibrations can result in damage to parts of a steam turbine installation. A comprehensive literature review considering 43 different valves investigated in 51 studies reveals the effects causing vibrations. The physics of these effects are explained and methods for reducing flow-induced dynamic forces are presented based on the findings published in the literature. A classification scheme for typical valve designs is developed and the design features are evaluated in terms of valve vibration. Numerical methods for analyzing the fluid dynamics of valves are also presented.

The power output of steam turbines is controlled by steam turbine inlet valves. These valves have a large flow capacity and dissipate a huge amount of energy in throttled operation. The dissipation process generates strong pressure fluctuations resulting in high dynamic forces causing valve vibrations. A brief survey of the literature dealing with valve vibrations reveals that the vibrational problems and damages mostly occur in throttled operation when high speed jets, shocks, and shear layers are present. As previous investigations reveal that a feedback mechanism between the dynamic flow field and the vibrating valve plug exists, the vibrations are investigated with two-way coupled simulations. The fluid dynamics are solved with a scale-adaptive approach to resolve the pressure fluctuations generated by the turbulent flow. The finite element model (FEM) solving the structural dynamics considers both frictional effects at the valve packing and contact effects caused by the plug impacting on the valve bushing. As different flow topologies causing diverse dynamic loads exist, the fluid flow and the structural dynamics are simulated at different operating points. The simulations show that differences to the one-way-coupled approach exist leading to a change of the vibrational behavior. The physics behind the feedback mechanisms causing this change are analyzed and conclusions regarding the accuracy of the one-way-coupled approach are drawn.

The unsteady flow in inlet valves for large steam turbines used in power stations was investigated using the method of computational fluid dynamics (CFD). As the topology of the flow depends on the stroke and the pressure ratio of the valve, the flow was investigated at several positions. Various turbulence models were applied to the valve to capture the unsteady flow field. Basic Reynolds-averaged Navier–Stokes (RANS) models, the scale adaptive simulation (SAS), and the scale adaptive simulation with zonal forcing (SAS-F, also called ZFLES) were evaluated. To clarify the cause of flow-induced valve vibrations, the investigation focused on the pressure field acting on the valve plug. It can be shown that acoustic modes are excited by the flow field. These modes cause unsteady forces that act on the valve plug. The influence of valve geometry on the acoustic eigenmodes was investigated to determine how to reduce the dynamic forces. Three major flow topologies that create different dynamic forces were identified.