The effects of the tip clearance on the performance of small-scale turbopumps for ORC applications; analysis and modeling

In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters as considered by most correlations but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor and its influence is non-linear. The head rise decreases as the tip clearance ratios increased, however, for radial impellers, the higher head rise was observed for small tip clearance ratios.

Testing and modelling of a novel oil-free co-rotating scroll machine with water injection

Luis Carlos Mendoza Toledo, Jürg Alexander Schiffmann

Efficient compressed air energy storage requires reversible isothermal compression and expansion devices. The isothermal compression and expansion processes can either be approached by several stages with intercooling or by the more convenient injection of a liquid, often water. While volumetric machines are readily available for dry processes the compression and expansion of a gas with the presence of liquid is still problematic. The concept of a co-rotating scroll has been identified as a promising technology to cope with the presence of liquid. The current paper discusses the first experimental results of an oil-free co-rotating scroll prototype tested in expansion mode on a wide range of rotational speeds, varying water injection flow rates and with different nominal flank clearances. A maximal overall isothermal efficiency of 34% and a maximum output power of 1.74 kWelwere measured with this first prototype, providing the proof of the technical feasibility of the oil-free co-rotating scroll expander concept. The experimental data indicate a positive effect of water injection suggesting good heat transfer behaviour between the water and the air in the individual chambers, which is a result of the relatively long residence time compared to other volumetric concepts. The experimental sensitivity analysis yields a strong dependency of the machine performance on both the nominal flank clearance and on the injected water rate. The analysis through a semi-empirical model suggests the inversion of a classical trend, i.e. the increase in total leak-age area with rotor speed. This is resulting from the centrifugal loads acting on the flanks and deforming them to produce increased radial and flank clearances. The injection of water is suggested to significantly decrease the leakage. A deterministic reduced order model of the co-rotating scroll expander was devel-oped in order to better understand the governing phenomena within the machine and to provide design guidelines for further prototypes. A novel leakage model takes into account for the structural deformation of the flanks and the scroll involutes as a result from the rotor speed. By means of this comprehensive thermodynamic model, mechanical power, mass flow rate and exhaust temperature were predicted within a range of ±12% and ±4 K respectively compared to experimental data. The calibrated model sug-gests an achievable isothermal efficiency of 87% for an improved co-rotating scroll concept, thus offering promising perspectives not only for compressed air storage, but also for wet expansion in Absorption Power Cycles, trilateral flash cycle and Organic Rankine Cycles.

Elsevier2017

Testing and modelling of a novel oil-free co-rotating scroll machine with water injection

Luis Carlos Mendoza Toledo, Jürg Alexander Schiffmann

Elsevier2017