• centifugal impeller
• girante centrifuga
• impeller design
• inducer
• induttore
• performance prediction
• turbopoma
• turbopump

Propellant feed turbopumps are crucial components of liquid propellant rocket engines because of the severe limitations associated with the design of high power density and dynamically stable machines capable of meeting pumping and reliability requirements. Any improvements of the turbopump efficiency and head directly reflect on the overall system total mass and costs. For this reason, the turbopump design is critical. Moreover, to obtain high power density, the shaft speed can be increased. This design approach could introduce cavitation which introduces severe design limitations and has to be taken into account during the design phase to avoid possible catastrophic events. The addition of an inducer at the beginning of the hydraulic circuit precludes cavitation in the following stages, and could produces an improvement of the suction and pump performances of the turbopump. This allows the possibility to explore and analyse different configurations with lower total mass of the propellant feed system. On the other side, the use of inducers can cause a (small) reduction of the overall efficiency, lead to an increase of the system complexity, and a loss of compactness.
Typically, the inducer could be placed far from the main stage, rotating at different speeds, or close to the centrifugal inlet section on the same shaft of the main stage. In recent times, the design choices tend to the second architecture, which ensures high compactness and reduces the hydraulic losses occurring in the duct connecting the two impellers. However, the design of these types of turbopumps has to consider the correct matching of the flow conditions in the inducer discharge and the centrifugal impeller inlet. The main parameters influencing this aspect are the blades clocking position, and the gap between the two impellers. Incorrect design could lead to the surge of unsteady pressure fluctuation (clocking position), flow inhomogeneity at the impeller inlet, or unnecessary increase of friction losses.
In this thesis, an alternative architecture with a centrifugal impeller integrating in its first part the inducer is studied in order to improve performances, simplicity and compactness of the turbopump. Indeed, by the total elimination of the gap between the inducer and the centrifugal parts, the problem of the inhomogeneity is overcome and the losses associated to the gap (mixing and friction) are avoided. Furthermore, all the complications (stall, pressure losses) related to the high value of incidence at the centrifugal impeller inlet are avoided, substituted only by the incidence on the additional blades along the channel.
In recent times two 3D reduced-order models were developed and experimentally validated by prof. d’Agostino, prof. Pasini and colleagues of ALTA S.p.A. in Pisa, regarding the design and non-cavitating performance predicting of inducers and centrifugal impeller turbopumps. In this thesis an evolution of those models is proposed to describe centrifugal impellers with an integrated inducer.
In particular, a comparison of many of these impeller concepts were performed in order to obtain a final configuration. To do this, a theoretical model for each of the configurations analysed was developed, and requirements concerning the impeller size (small size) and the flow characteristics inside the channels (to increase the predictability of the model) were established to allow an objective comparison.
Secondly, an application of the selected 3D reduced order model was done developing an impeller which integrates an inducer part to the VAMPIRE radial impeller (turbopump developed in ALTA S.p.A.). The machine design was performed by an optimisation, taking into account many aspects including the overall efficiency, the impeller total solidity and the predictability of the model. Parallelly, a study on the single free-parameter effects was performed, validating the optimisation result.
The obtained geometry of the turbopump, and in particular of the impeller, seems to be in line with modern applications of this concept, and the comparison with the VAMPIRE shows a reduction of the overall efficiency, which is more reasonable for a configuration also including the inducer. To complete the description of the final configuration, an adaptation of the model to compute the performance in off-design conditions was developed, and the characteristic curves were generated.