• Cavitation
• Combustion
• Design
• Instabilities
• Liquid rocket engines
• Rocket propulsion
• Rotordynamic
• Turbopump

A comprehensive experimental campaign has been carried out on a centrifugal pump and on the same pump with an inducer upstream. The centrifugal stage was designed by means of a reduced order model for preliminary design and non-cavitating performance prediction of radial turbopumps, illustrated in this thesis.
The model expresses the 3D incompressible, inviscid, irrotational flow through helical blades with slow axial variations of the pitch and backsweep by superposing a 2D cross-sectional axial vorticity correction to a fully-guided flow with axisymmetric stagnation velocity in the meridional plane. Application of the relevant governing equations yields a set of constraints for the axial evolution of the blade pitch and backsweep that allows for the closed form definition of the impeller geometry and flow field in terms of a reduced number of controlling parameters. In turn, mass and momentum conservation are used to account for the mixing of the flow leaving the impeller and its coupling with 2D reduced order models of the flow in the diffuser (if any) and the volute. The generated information is sufficient for completing the geometric definition of the machine and for determining its ideal non-cavitating performance in accordance with the resulting flow field. However, the evaluation of the main additional sources of losses in the impeller, diffuser, and volute is necessary for a more realistic prediction of the non-cavitating performance of the machine. The major contributions to these losses typically arise from flow incidence and mixing at the leading edges of the impeller blades and at the tongue of the volute, as well as from viscous effects in the blade channels; these contributions are here described and added to the ideal model.
A test machine named VAMPIRE, equipped with a six-bladed radial impeller, a vaneless diffuser and a single-spiral volute has been designed by means of the proposed model. The pumping and suction performances of the machine have been determined in a series of tests in SITAEL’s (previously Alta) Cavitating Pump Rotordynamic Test Facility (CPRTF). The measured pumping characteristic proved to be in excellent agreement with the model predictions, thus effectively validating the design model.
Moreover, the six-bladed centrifugal stage has been interfaced with a three-bladed inducer upstream, which is a typical configuration in space applications. Non-cavitating and suction performance tests have been carried out also on this configuration, named VAMPDAP. Results highlights the higher resistance to head breakdown induced by cavitation of this configuration than the centrifugal pump alone. However, lower non-cavitating performance characterizes the VAMPDAP configuration with respect to the VAMPIRE one.
The tests carried out on the two configurations under whirl motion have investigated the influences of the flow coefficient, cavitation number, liquid temperature together with the imposed whirl motion of the rotor on the machine performance. A recently developed procedure (called chirp), able to continuously characterize the rotordynamic forces behavior, has been first applied on a centrifugal stage. Together with the continuous method, also the classic discrete procedure of data reduction at constant whirl frequency has been applied. Based on the available experimental evidence, the recently developed procedure, which gives a continuous spectrum of fluid-induced rotordynamic forces, demonstrated to be very useful to capture the unforeseeable complexity of the rotordynamic forces and their consequences on stability of turbopumps.
Furthermore, the chirp technique has been first adapted in order to provide continuous description of the rotordynamic forces during classic cavitating and non-cavitating performance tests under whirl motion. Results highlighted that the flowrate has a highly unpredictable effect while cavitation starts to affect the rotordynamic forces when pumping degradation begin. Therefore, all the obtained results confirmed a major role of the pumping performance and, in turn, of the blade loading on the intensity and stability behavior of the rotordynamic forces.
The second part focuses on an experimental study on droplet combustion characteristics for ethanol during exposure to external acoustical perturbations by means of a standing wave generated within an acoustic waveguide in normal gravity. The study examined combustion during excitation conditions in which the droplet was located in various positions with respect to a pressure node (PN). High level of excitation corresponding to periodic partial extinction and reignition of the flame front have been explored. Flame characteristics were observed via phase-locked OH* chemiluminescence imaging, which revealed a flame deflection with an orientation depending on the droplet’s relative position with respect to the PN, variation of the burning rate characteristics, and oscillations in the flame standoff distance from the droplet as well as in the chemiluminescent intensity. The observed flame deformation has been found in good qualitative agreement with theoretical acoustic acceleration. However, large discrepancies have been found between the experimental and the theoretical values.
Simultaneous imaging and pressure measurements enabled quantification of combustion-acoustic
coupling via the Rayleigh index and the phase-difference between the flame intensity and the pressure oscillations. Unstable combustion for excitation level not leading to partial extinction has been estimated by both the metrics, while partial extinction cases have been found always in the stable region. In fact, the lack of combustion process during flame extinction led to out-of-phase pressure and flame intensity oscillations, thus to theoretical stable condition.