Abstract
Turbopumps constitute an essential component of high thrust liquid rocket engines. They are characterized by a compact design, providing a large shaft power at high rotation rates. This is necessary to deliver the propellants at high pressure into the combustion chamber to generate the required engine thrust. Recirculating fluid around the pump has a major influence on axial loads and fluid dynamical losses, impacting turbopump performance and life. Therefore, simplified modelling approaches are required early on for the preliminary design of the pump impeller, and its side cavities and seals. Indeed, past experience within ArianeGroup for pumps and secondary circuits indicates that the coupling between the main flow and the leakage has to be considered at an early stage of the design. The empirical correlations of the flow in the cavities shall be carefully selected, accounting for the particularities of each new configuration. Furthermore, it is also recommended for the impeller design (e.g. for blade leading edge and pressure relieve orifices positioning), that the effects of leakage reinjection into the main flow shall be taken into account.
In order to obtain first estimates for early design optimization without the cost of full scale 360° high fidelity computational dynamic simulations (CFD), a reduced model is developed to predict losses and axial thrust on the rotor, including effects of fluid recirculation and reinjection. A two-step approach is followed: Firstly, an empirical model developed by Gülich et al. [1] is applied to characterize leakage loss analytically. Secondly, a reduced numerical model is implemented which features a single passage impeller geometry including seals and side wall gaps. The accuracy of both the analytical model and the simplified numerical model are verified in comparison to high fidelity CFD calculations, evaluating the loss contributions in the leakage path and axial thrust for a range of operating points.
In line with expectation, the highest impact on the pump performance are the volumetric losses due to the recirculation of pressurized fluid, with and efficiency decrease of up to 20 % in the investigated cases. The implemented analytical model captures the overall loss mechanisms with a 20 % uncertainty in the design point, disk friction is underpredicted and axial thrust is mostly over-predicted. Due to the simplified numerical model with the single passage impeller geometry including side cavities, the uncertainty can be decreased to about 5 %. At part load operation, the accuracy of both models reduces. It is noted, that thrust prediction is subject to the highest uncertainties.
The current work has provided a simplified numerical model that offers the higher flexibility required for the early design phase as compared to a full annulus CFD simulation of the pump, with an increased accuracy as compared to the analytical models.
