Small-scale turbopumps for waste heat recovery applications based on an organic Rankine cycle: a study from pre-design to experimental validation

The organic Rankine cycle (ORC) system is among the technologies applied to exploit waste heat in vehicles. Since common organic fluids have lower latent heat than water, higher mass flow rates are required to convert the same waste heat, considerably increasing the pump size and energy usage. The weight and size of standard pumps can significantly penalize the benefits of installing waste heat recovery on vehicles. Hence, efficient and compact pumps, such as small-scale turbopumps, would be very welcome for mobile applications. Numerical investigations are conducted on extensive parameter ranges to study their effects on the performance of small-scale turbopumps. A fully parameterized tool is developed to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage and the entire pump. Investigations are conducted under different operating conditions, and the accomplished results are analyzed to characterize the performance under design and off-design conditions. First off, the effect of tip clearance in small-scale turbopumps is studied. The slip factor is suggested to decrease linearly with increasing the flow rate. In addition, the tip clearance ratio influences the slip factor, and its effect is non-linear. The head rise decreases as the tip clearance ratios increase. The numerical data is employed to infer reduced-order models for considering the tip clearance effect in the early-phase design process of small-scale turbopumps. The models predict the CFD data with an average relative deviation of 3.6% and 5.8% for the slip factor and the head loss coefficient, respectively. Likewise, the effects of splitter blades and meridional profiles are studied. The splitters can increase impeller head coefficient and slip factors by 10%-24% depending on the blade outlet angle. At the same time, the total efficiency is not influenced significantly. For an unshrouded impeller with a tip clearance ratio of 0.10, a higher head rise is observed for a placement closer to the suction side of the main blade. Moreover, meridional profiles with a sharper change in the cross-sectional area in the vicinity of the inlet rather than a linear change from inlet to the outlet are found to be more advantageous. The optimization of the splitter blade deviating from the main blade profile is less successful than the optimization of meridional profiles in mitigating the effects of tip clearance, as only a 2.43% improvement in total efficiency is achieved. In a further step, the specific speed-specific diameter diagrams are extended to small-scale high-speed pumps. The CFD computations are conducted across the entire pump stage, and the results are analyzed to characterize the performance of turbopumps in terms of nq, ds, and efficiency. The good agreement between experimental performance characteristics and CFD predictions validates the computational results and reduced-order models.Finally, a high-speed turbopump with a specific speed of 8.9 is studied experimentally. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar. The turbopump's characteristics are utilized to estimate the possible performance improvement of a target ORC. The comparison suggests that the pump increases the efficiency of the ORC by 0.3% and decreases its back-work ratio by nearly 50%. The pump is approximately ten times more compact compared to commercial systems.