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The cavitation performances and its internal flows in a centrifugal turbopump with and without a three-bladed axial inducer have been studied both experimentally and numerically. A 3D steady numerical model has been applied to simulate the cavitating flow from the inlet to the outlet ducts of the turbopump with and without an inducer by using ANSYS CFX code. In the present work, to clarify this relationship, we conducted experiments in both cold water ( T = 20°C) and hot water ( T = 80°C) with a focus on the development of vapor volume fraction and head degradation by gradually reducing the inlet pressure from atmospheric conditions to the minimum allowable value at a constant rate of about 3 mbar/s. The measured and predicted cavitation performances of the turbopump with and without an inducer have been compared under different operating conditions and the temperature of the operating fluid.

The article illustrates the application of Bayesian estimation to the identification of flow instabilities, with special reference to rotating cavitation, in a three-bladed axial inducer using the unsteady pressure readings of a single transducer mounted on the casing just behind the leading edges of the impeller blades. The typical trapezoidal pressure distribution in the blade channels is parametrized and modulated in time and space for theoretically reproducing the expected pressure generated by known forms of cavitation instabilities (cavitation auto-oscillations, n-lobed rotating cavitation, higher-order surge/rotating cavitation modes). The Fourier spectra of the theoretical pressure so obtained in the rotating frame are transformed in the stationary frame, frequency broadened to better approximate the experimental results, and parametrically fitted by maximum likelihood estimation to the measured auto-correlation spectra. Each form of instability generates a characteristic distribution of sidebands in addition to its fundamental frequency. The identification makes use of this information for effective detection and characterization of multiple simultaneous flow instabilities with intensities spanning over about 20 dB down to about 4 dB signal-to-noise ratios. The same information also allows for effectively bypassing the aliasing limitations of traditional cross-correlation methods in the discrimination of multiple-lobed azimuthal instabilities from dual-sensor measurements on the same axial station of the machine. The method returns both the estimates of the model parameters and their standard deviations, providing the information needed for the assessment of the statistical significance of the results. The proposed approach represents therefore a promising tool for experimental research on flow instabilities in high-performance turbopumps.

This paper reports the experimental characterization of the cavitation-induced flow instabilities of a high head three-bladed inducer at design condition detected simultaneously by means of piezoelectric pressure transducers located at different axial and azimuthal stations on the casing of the pump and strain gages mounted on the pressure side of each blade. The simultaneous analysis performed in the stationary and rotating frames, supported by high-speed movies, suggests that the mechanism of formation of the detected subsynchronous rotating cavitation resembles that of modal stall in compressors, being the cavitation region the source of compliance. In fact, at decreasing cavitation number, a strong attached cavitation, developed preferentially on one blade and capable of completely surrounding and unloading the following blade, starts to be destabilized as consequence of the progressive intensification of a modal oscillation. The developed complex instability, consisting in a strong oscillation of the attached cavitation, is detected in both the frames as the simultaneous presence of a subsynchronous rotating cavitation and a cavitation surge.

The introduction of splitter blades (splitters) in the impeller represents a possible way to improve the head and efficiency of centrifugal turbopumps. Splitters decrease flow slip, deviation, blockage and losses at the impeller discharge by reducing the displacement flow and the adverse pressure gradient it induces on the boundary layers (BLs) developing on the pressure sides of the full blades. These benefits are to be balanced against the introduction of additional flow blockage and losses. It is therefore of interest to investigate which splitter blade geometry (position and length), if any, is most effective to best exploit the potential advantages of adding splitter blades to a centrifugal impeller. To this purpose a quasi-3D, reduced-order method, previously developed for preliminary prediction of the geometry and performance of centrifugal turbopumps, is extended to account for the introduction of splitter blades for the purpose of investigating their influence on the hydraulic performance of these machines. Splitters with radial leading edges and the same shape as the full blades are introduced in the six-bladed backswept impeller of the VAMPIRE turbopump, a high-efficiency machine previously used for experimental validation of the original version of the flow model. The results highlight the most relevant action mechanisms of splitter blades in affecting the turbopump performance and provide general indications on their most effective design for performance improvement. As a consequence of the evolution of the BLs in the blade channels, relatively longer splitters located closer to the pressure sides of the full blades are more efficient at and above design flow, while the reverse is the case at high blade loadings. The results also indicate that the introduction of splitter blades only marginally improves the performance of the already optimized VAMPIRE pump in view of its small residual margins for performance gains.

This experimental study explored the response of burning liquid fuel droplets to one-dimensional acoustic standing waves created within a closed, atmospheric waveguide. Building upon prior droplet combustion studies quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation (Sevilla-Esparza, et al., Combustion and Flame, 161(6):1604–1619, 2014), the present work focused on higher amplitude acoustic forcing observed to create periodic partial extinction and reignition (PPER) of flames enveloping the droplet. Detailed examination of ethanol droplets exposed to a range of acoustic forcing conditions (frequencies and amplitudes in the vicinity of a pressure node) yielded several different combustion regimes: one with sustained oscillatory flames, one with PPER, and then full extinction at very high excitation amplitudes. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allowed quantification of the combustion-acoustic coupling through the local Rayleigh index. Similar behavior was observed for JP-8 and liquid synthetic fuel derived via the Fischer-Tropsch process, but with quantitative differences based on different reaction time scales. Estimates of the mean and oscillatory strain rates experienced by the flames during excitation assisted with interpreting specific relationships among acoustic, chemical, and fluid mechanical/straining time scales that can lead to a greater understanding of PPER.

The effects of thermal cavitation on the flow instabilities developing in a three-bladed, tapered-hub, variable-pitch space turbopump inducer are described by illustrating the results of similarity experiments in water carried out at Sitael in the Cavitating Pump Rotordynamic Test Facility (CPRTF). The operating conditions have been varied as required for investigating the onset and response of the fluid dynamic instabilities occurring in the test inducer over a wide range of cavitation numbers from noncavitating to heavy cavitation conditions, both at design and off-design flow. The CPRTF water temperature has been varied from room conditions to 80 °C ca. in order to induce thermal cavitation effects of increasing intensity capable of reproducing in the experiments the occurrence of typical cavitation phenomena in cryogenic space propellants. A wide variety and number of axial and rotating fluid dynamic instabilities have been detected, identified and characterized by spectral analysis of the pressure measurements obtained from statoric and rotoric transducers flush-mounted on the casing and the hub of the test inducer. The results indicate that thermal cavitation significantly affects the types of flow instabilities developing in cavitating inducers and the range of flow and suction conditions for their occurrence.

The article illustrates the results of an exploratory study on the effectiveness of maximum likelihood Bayesian estimation in the identification of cavitation instabilities in axial inducers using the blade-to-blade pressure measured by a single transducer flush-mounted on the impeller casing. The typical azimuthal distribution of the pressure in the blade channels is parameterized and modulated in space and time for theoretically reproducing the expected pressure generated by known forms of cavitation instabilities (cavitation surge auto-oscillations, n-lobed synchronous/asynchronous rotating cavitation, and higher-order surge/rotating cavitation modes). The power spectra of the theoretical pressure so obtained in the rotating frame are transformed in the stationary frame, corrected for frequency broadening effects, and parametrically fitted by maximum likelihood estimation to the measurements of the pressure on the inducer casing just downstream of the blade leading edges. In addition to its fundamental frequency, each form of instability generates a characteristic spectral distribution of sidebands. The identification uses this information for successfully discriminating flow oscillation modes occurring simultaneously with intensities differing by up to one order of magnitude. The method returns the estimates of the model parameters and their standard errors, allowing one to assess the accuracy and statistical significance of the identification. The results first demonstrate that elementary maximum likelihood Bayesian identification is indeed capable to effectively detect and characterize the occurrence of flow instabilities in cavitating inducers at a fraction of the experimental and postprocessing costs and complexities of traditional cross-correlation methods.

International audience; The paper describes results of recent experiments carried out under similarity conditions in the Cavitating Pump Rotordynamic Test Facility for the dynamic characterization of cavitation-induced flow instabilities as simultaneously observed in the stationary and rotating frames of a high-head, three-bladed axial inducer with tapered hub and variable pitch. The flow instabilities occurring in the eye and inside the blading of the inducer have been detected, identified and monitored by means of the spectral analysis of the pressure measurements simultaneously generated in the absolute and relative frames by multiple transducers mounted on the casing near the impeller eye and on the inducer hub along the blade channels. The comparison of the results obtained in the two reference frames over a wide range of flow coefficients and cavitation numbers contributed to shed some light on the complex dynamic interactions between the cavitation-induced instabilities in the eye and in the blade channels of typical high-head inducers.

The hydraulic performance of a centrifugal turbopump with and without a 3-bladed axial inducer has been studied both experimentally and numerically. A 3D numerical model has been used to simulate the flow through from the inlet to the outlet ducts of the turbopump with and without an inducer using the ANSYS CFX code. The sensitivity of the numerical results has been analyzed with reference to the adopted turbulent flow models, to the length of the input and output ducts included in the simulations, to the reference positions used for the evaluation of the total pressure rise and to the temperature of the operating fluid. The measured and predicted hydraulic performances of the turbopump with and without the inducer have been compared under different operating conditions. As expected, the predicted hydraulic performance of the turbopump is significantly influenced by the lengths of the inlet and outlet ducts, the turbulence models and, at low flow rates, the reference positions of the total pressure rise measurements. The pressure rise coefficients obtained from the simulations using an inlet duct with length of 3 rTi and 10 rTi were significantly lower than the experimental results, while at low flow rates those referring to the inlet duct with length greater than 10 rTi were significantly higher than those obtained for the shorter inlet duct. With reference to the effect of the pressure measurement locations, the difference between the numerical results of the pressure rise coefficient and the experimental values was much higher when the data were obtained at the locations where the transducers was mounted in the experimental tests at lower flow rates. Moreover, the hydraulic performance of the turbopump at lower flow rates can be significantly influenced by the use of the upstream inducer, with a pressure drop of 20% in particular at 60% of the design flow rate.

International audience; The paper presents an experimental campaign aimed at the characterization of the relationship between cavitation-induced instabilities and forces acting on the shaft relevant to space application turbopumps. The experiments have been carried-out on a 6-bladed, unshrouded centrifugal turbopump. The described apparatus allows for contemporary measurements of the flow instabilities and the force exchanged between the impeller and the shaft. Pressure fluctuations are frequency analyzed allowing for understanding the instability nature (axial, rotating) and their main characteristics (e.g. amplitude, rotating direction). The frequency content of the force components highlights a strong relationship of the z-component (along the rotating axis) with axial instabilities. On the other hand, rotating cavitation may involve force oscillations along all the three components leading to unwanted and dangerous fluctuating unbalances perpendicular to the rotating axis.

A high-head three-bladed inducer has been equipped with pressure taps on the hub along the blade channels with the aim of more closely investigating the dynamics of cavitation-induced instabilities developing in the impeller flow. Spectral analysis of the pressure signals obtained from two sets of transducers mounted both in the stationary and rotating frames has allowed to characterize the nature, intensity, and interactions of the main flow instabilities detected in the experiments: subsynchronous rotating cavitation (RC), cavitation surge (CS), and a high-order axial surge oscillation. A dynamic model of the unsteady flow in the blade channels has been developed based on experimental data and on suitable descriptions of the mean flow and the oscillations of the cavitating volume. The model has been used for estimating at the inducer operating conditions of interest the intensity of the flow oscillations associated with the occurrence of the CS mode generated by RC in the inducer inlet.

A number of platinum catalysts for the decomposition of 98% hydrogen peroxide, based on different substrates, geometries, and sizes, have been prepared using different procedures. The catalysts have undergone a set of dedicated tests for screening their catalytic activity and their thermomechanical resistance in order to identify the most efficient and suitable catalyst to be used for a pulsed monopropellant propulsion system. An experimental test campaign on a 20 N monopropellant thruster prototype has been carried out with the aim of assessing the capability of the finally selected new Pt/α-Al2O3 catalysts of effectively decomposing 98% hydrogen peroxide and the attainable propulsive performance in steady-state conditions for the future assessment of the propulsive performance of the intrinsically unsteady new propulsion concept. The catalysts have been able to decompose up to 1 liter of 98% H2O2 with very good efficiencies (hc*>95% and hΔT≥90%) and without any pellet breakage or catalytic degradation. ...

After a brief review of the fundamental aspects of cavitation relevant to the operation of high-performance inducers and turbopumps, the article summarizes their application to the analysis of pumping systems, illustrates the scaling of cavitation phenomena from model tests to full-scale operation, describes the occurrence of flow-induced instabilities in turbomachinery, and introduces the concepts of static and dynamic instability of pumping systems and their generalization to cavitating turbopump systems.

A reduced-order model for the preliminary design and performance prediction of radial turbopumps is illustrated. The model expresses the 3D, incompressible, inviscid, irrotational flow through helical blades with slow axial variations of their pitch and backsweep angles by superposing a 2D axial vorticity correction to a fully guided forced vortex flow with axisymmetric stagnation velocity in the meridional plane. Application of the relevant governing equations allows for the closed-form definition of the impeller geometry and flowfield in terms of a reduced number of controlling parameters. Mass and momentum conservations are used for coupling the flow leaving the impeller with the 2D reduced-order models of the flow in the diffuser and/or the volute, as well as for the evaluation of the mixing losses in the transfer between successive components of the machine. This information completes the geometric definition of the turbopump and determines its ideal noncavitating performance in accordance with the resulting flowfield. As a consequence of the neglect of viscous effects, the slip factor predicted by the present model exceeds those obtained from theoretical/semi-empirical formulas reported in literature for centrifugal pumps, but correctly captures their trend.

The article reviews the main forms of flow-induced instabilities detected in the liquid propellant turbopumps systems of modern rocket engines, with special reference to rotating stall, rotating cavitation, cavitation surge and higher order surge modes, illustrating their characteristics, origin and damage potential.

The experimental characterization of the rotordynamic fluid forces acting on a whirling centrifugal impeller has been investigated at different flow rates and cavitating conditions. The recently developed method for continuously measuring the rotordynamic forces at variable whirl ratios has been readapted and successfully applied for measuring the same forces at constant whirl ratio but variable cavitation number. The flowrate has a major influence on the stability of the rotordynamic forces at positive whirl ratios where a threshold flowrate separates the stable zone from the unstable. At negative whirl ratios, the normal force is typically unstable independently on the flow rate. Cavitation has always a destabilizing effect at positive whirl ratio while it can stabilize the rotordynamic forces at negative whirl ratio.

The paper presents the results of an activity carried out on a three-bladed inducer and compares the experimental data from other similar inducers. All the inducers considered in the present study have been designed by means of the simplified analytical model recently developed by some of the authors. The main effects of different geometrical solutions on hydraulic performance and flow instabilities of the pumps under noncavitating and cavitating conditions are presented in the paper.

In the present paper, the results from an extensive experimental characterization of the rotordynamic forces acting on a whirling three-bladed, tapered-hub, variable-pitch inducer are presented. The campaign has been conducted in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at ALTA S.p.A. The forces acting on the impeller have been measured by means of a rotating dynamometer mounted just behind the inducer. The roles of the rotor whirl motion, flow rate, cavitating condition, and liquid temperature have been investigated. The analysis has been conducted by means of the classical rotordynamic approach together with a recent experimental technique, consisting in measuring the rotordynamic forces continuous behavior (spectra) as functions of the whirl excitation frequency. This technique allows for evaluating information from experiments more rapidly and accurately with regards to previous methods. Therefore, it is useful to better capture the complexity of the rotordynamic forces and assess their consequences on the stability of axial inducers.

The present paper illustrates and discusses the first results of the characterization of the dynamic transfer matrix of cavitating turbopumps by means of a series of forced-vibration tests carried out on a high-head three-bladed inducer in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Alta, Pisa, Italy. The flow has been excited by vertically vibrating with adjustable frequency and intensity the free-surface tank used to close the loop of the suction and discharge lines of the inducer. The pressure and flow rate fluctuations for the determination of the dynamic transfer matrix have been obtained from unsteady pressure measurements at suitable locations on the inlet/outlet lines of the inducer. Two loop configurations with different inlet/outlet impedances to/from the test inducer have been used in order to obtain at each excitation frequency a set of four linearly independent measurements for the detetermination of the four elements of the dynamic transfer matrix. The preliminary results are in qualitative agreement with the scant data available in the literature for comparable inducers.

PulCheR (Pulsed Chemical Rocket with Green High Performance Propellants) is a new propulsion concept in which the propellants are fed in the combustion chamber at low pressure and the thrust is generated by means of high frequency pulses, reproducing the defence mechanism of a notable insect: the bombardier beetle. The radical innovation introduced by PulCheR is the elimination of any external pressurizing system even if the thruster works at high pressure inside the combustion chamber. At each pulse, pressurization of the combustion chamber gases takes place due to the decomposition or combustion reaction, and the final pressure is much higher than the one at which the propellants are stored. The weight of the feeding system is significantly reduced because the propellants are fed at low pressure, and there is no need for turbopumps, high pressure propellant tanks or gas vessels. The feed pressure becomes independent on the chamber pressure and the performance degradation typical of the blow down mode in monopropellant thrusters can be avoided. The PulCheR concept is able to substitute many currently used propulsion systems for accessing space in both mono and bipropellant configurations. The preliminary assessment of this new propulsion concept has been investigated using green propellants with potential similar performance to the current stateof-the-art for monopropellant and bipropellant thrusters.

The article expands on the ongoing assessment of the reduced order model proposed by some of the authors for the geometric definition and noncavitating performance evaluation in the preliminary design and parametric optimization of mixed-flow centrifugal turbopumps. Some of the dynamically most significant predictions of the model are compared with the experimentally validated URANS (Unsteady Reynolds-Averaged Navier-Stokes) simulations of the non-cavitating flow through a typical six-bladed unshrouded mixed-flow turbopump for liquid propellant rocket engines operating at both design and off-design flow conditions and different values of the impeller clearance. The observed discrepancies can be explained in terms of the simplifying assumptions introduced for the development of the model and their relative magnitude (< ±10%) does not adversely interfere with the accurate prediction of the turbopump performance over a wide range of operating conditions above and below design flow rate. Together with earlier experimental validations, the results dramatically confirm the capability of the proposed model to generate useful engineering solutions of the turbopump preliminary design problem at a negligible fraction of the computational cost required by 3D numerical simulations.

The paper illustrates the experimental validation of the reduced order model recently developed at Alta, Pisa (Italy), for the simultaneous and preliminary geometrical definition and non-cavitating performance prediction of centrifugal turbopumps. The proposed model has been employed to size and design the test machine, named VAMPIRE and equipped with a six-bladed radial impeller, a vaneless diffuser and a single-spiral volute. The pumping and suction performance of the machine have been determined in a series of tests in Alta’s 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. In addition, the experimental determination of the pump’s cavitation characteristic is fully consistent with the expected suction performance of comparable machines, confirming the capability of the proposed model to generate hydrodynamically efficient turbopump designs.