State-Constrained Shape-Optimization of Multiphase-ORC-Turbines

• Applicants: Professor Dr.-Ing. Hagen Müller; Dr.-Ing. Lars Radtke
• Subject Area: Fluid Mechanics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 562061216

The majority of the world’s energy supply is covered by electricity generated from thermal power plants. In Germany alone, it was around 60% in 2019. Globally, about 52% of primary energy is lost in the form of waste heat, with the majority being released to the environment at temperatures below 600 K. A promising technology to convert at least parts of this immense potential into usable energy is the application of thermal power plants based on the Organic Rankine Cycle (ORC). These devices use alternative organic working fluids to efficiently convert thermal energy at low temperature levels. This is relevant not only for the use of waste heat, but also for biomass combustion, geothermal or solar thermal energy, as well as for the mobility, household or space sector. Driven by climate change, interest in ORC systems has recently increased significantly, leading to an increase in installed capacity as well as intensified development and research work. A novel approach to further increase the efficiency of currently used systems is to feed the working fluid into the turbine in a two-phase state rather than superheating it. With the use of suitable fluids, a complete drying occurs in the first stator row of the turbine. It has been demonstrated that this so-called wet-to-dry process enables a significantly higher conversion of the available thermal energy into work. The corresponding flow conditions differ significantly from those in conventional ORC turbines. Consequently, advanced blade geometries are necessary to provide high efficiencies, which demands for reliable numerical methods taking into account the differing thermodynamic properties. The present project therefore aims to develop an adjoint shape optimization method being capable of automatically translating the promising results of the wet-to-dry process into specific turbine designs. The desired method takes into account the accurate modeling of the thermodynamic state of organic working fluids and nonequilibrium effects at phase boundaries while at the same time satisfying the thermodynamic state constraints to ensure fully dry inflow for the downstream rotor blades. The latter is necessary to avoid erosion on the downstream blade rows. The method provides the basis for an efficient design of ORC turbines operating according to the wet-to-dry concept, and thus significantly increasing the efficiency of existing systems. The results obtained also provide a deeper understanding of the flow, which is still insufficiently understood, and thus form the basis for further projects.

• DFG Programme: Research Grants