Angelo Cervone

Thermal Cavitation Experiments on a NACA 0015 Hydrofoil

The present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Centrospazio/Alta S.p.A. Experiments were carried out on a NACA 0015 hydrofoil at various incidence angles, cavitation numbers, and freestream temperatures. in order to investigate the characteristics of cavitation instabilities and the impact of thermal cavitation effects. Measured cavity length, surface pressure coefficients, and unsteady pressure spectra are in good agreement with the data available in the open literature and suggest the existence of a strong correlation between the onset of the various forms of cavitation and instabilities, the thermal cavitation effects, and the effects induced by the presence of the walls of the tunnel. Further analytical investigations are planned in order to provide a better interpretation of the above results.

Testing and Characterization of a Hydrogen Peroxide Monopropellant Thruster

In the present paper, the use of advanced catalytic beds on ceramic supports as a cost-effective alternative to metal screen reactors for the decomposition of high-concentration hydrogen peroxide in small monopropellant rockets is investigated. For this purpose, a reconfigurable test bench for the characterization of the operation and the propulsive performance of small rocket thrusters has been designed and realized. The present paper illustrates the experimental campaign carried out on a 5 N thruster prototype operating with two platinum catalysts on γ-alumina supporting spheres. The results indicated that Pt/Al 2 O 3 is an effective catalyst combination for the decomposition of 87.5% propellant-grade hydrogen peroxide, with good stability and performance comparable to silver screen beds of equal geometric envelope and operational conditions. Incomplete hydrogen peroxide decomposition and the onset of flow oscillations in the reactor were observed at the tested levels of bed loading, residence time, and flow pressure. Thermal stresses due to the large temperature gradients occurring during the decomposition of high-grade hydrogen peroxide (87.5 % by weight) caused the ceramic pellets to break and the progressive occlusion of the bed. Based on the analysis of the test results, several ways to overcome these problems in future investigations have been tentatively identified, together with the necessary modifications to the present experimental setup.

Development of hydrogen peroxide monopropellant rockets

Alta S.p.A. (Italy) and DELTACAT Ltd. (United Kingdom) are conducting a study, funded by the European Space Agency, into the development of hydrogen peroxide monopropellant thrusters using advanced catalytic beds. The present paper focuses on the design of two different demonstration thrusters with nominal ratings of 5 N and 25 N. Design requirements and specifications are presented, followed by the main results of a concept study, which was conducted to define the approximate dimensions needed. Some details about the specific design of the two prototypes and the choice of the main components are provided, with particular regard to the sensors and transducers to be used during the experimentation. Different catalytic bed configurations, including pure silver gauzes and pellets coated with manganese oxide or platinum, are going to be tested in the prototype thrusters, in order to find the optimum one for further industrial development. A dedicated test bench, designed and realized by Alta S.p.A. for tests on the thruster prototypes, is also illustrated.

Setup of a high-speed optical system for the characterization of flow instabilities generated by cavitation

The present paper illustrates the setup and the preliminary results of an experimental investigation of cavitation flow instabilities carried out by means of a high-speed camera on a three-bladed inducer in the cavitating pump rotordynamic test facility (CPRTF) at Alta S.p.A. The brightness thresholding technique adopted for cavitation recognition is described and implemented in a semi-automatic algorithm. In order to test the capabilities of the algorithm, the mean frontal cavitating area has been computed under different operating conditions. The tip cavity length has also been evaluated as a function of time. Inlet pressure signal and video acquisitions have been synchronized in order to analyze possible cavitation fluid-dynamic instabilities both optically and by means of pressure fluctuation analysis. Fourier analysis showed the occurrence of a cavity length oscillation at a frequency of 14.7 Hz, which corresponds to the frequency of the rotating stall instability detected by means of pressure oscillation analysis.

Reduced-Order Model for H2O2 Catalytic Reactor Performance Analysis

The present paper describes a steady one dimensional model of the hydrogen peroxide decomposition flow in a pellet-type catalytic bed and its application to the parametric design of a typical reactor for small rocket propellant thrusters. The two-phase liquid-gas-vapor flow through the bed is treated as a homogeneous, adiabatic, chemically reacting flow, for which the properties depend on the local composition. Fast equilibrium hydrogen peroxide adsorption and first-order finite-rate desorption is assumed for the one-step hydrogen peroxide decomposition reaction on the catalyst surface. Standard viscous/aerodynamic correlations for porous media are used to account for pressure losses. The predictions of the model depend on a limited number of uncertain parameters, for which the values can be readily determined by comparison with the available experimental data. Good agreement has been attained between the model predictions and the results of hydrogen peroxide monopropellant thruster firings. The model provides a rational framework for identifying the main operational parameters of catalytic pellet beds, understanding their interactions, and efficiently guiding the reactor sizing and design, using the indications easily obtained from sensitivity analyses.

On the Preliminary Design and Noncavitating Performance Prediction of Tapered Axial Inducers

A reduced order model for preliminary design and noncavitating performance prediction of tapered axial inducers is illustrated. In the incompressible, inviscid, irrotational flow approximation, the model expresses the 3D flow field in the blade channels by superposing a 2D cross-sectional vorticity correction to a fully guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow allows for the control of the blade loading and the estimate of the boundary layer blockage at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carters rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation, and Eulers equation are used to derive a simple second order boundary value problem, whose numerical solution describes the far-field axisymmetric flow at the inducer discharge. A closed form approximate solution is also provided, which proved to yield equivalently accurate results in the prediction of the inducer performance. Finally, the noncavitating pumping characteristic is obtained by introducing suitably adapted correlations of pressure losses and flow deviation effects. The model has been verified to closely approximate the geometry and noncavitating performance of two space inducers tested in Altas Cavitating Pump Rotordynamic Test Facility, as well as the measured pumping characteristics of a number of tapered-hub inducers documented in the literature.

Turbulence modeling of cavitating flows in liquid rocket turbopumps

An accurate prediction of the performance characteristics of cavitating cryogenic turbopump inducers is essential for an increased reliance on numerical simulations in the early turbopump design stages of liquid rocket engines (LRE). This work focuses on the sensitivities related to the choice of turbulence models on the cavitation prediction in flow setups relevant to cryogenic turbopump inducers. To isolate the influence of the turbulence closure models for Reynolds-Averaged Navier–Stokes (RANS) equations, four canonical problems are abstracted and studied individually to separately consider cavitation occurring in flows with a bluff body pressure drop, adverse pressure gradient, blade passage contraction, and rotation. The choice of turbulence model plays a significant role in the prediction of the phase distribution in the flow. It was found that the sensitivity to the closure model depends on the choice of cavitation model itself; the barotropic equation of state (BES) cavitation models are far more sensitive to the turbulence closure than the transport-based models. The sensitivity of the turbulence model is also strongly dependent on the type of flow. For bounded cavitation flows (blade passage), stark variations in the cavitation topology are observed based on the selection of the turbulence model. For unbounded problems, the spread in the results due to the choice of turbulence models is similar to noncavitating, single-phase flow cases.

Evaluation of the Dynamic Transfer Matrix of Cavitating Inducers by Means of a Simplified “Lumped-Parameter” Model

The paper will present an analytical model for the evaluation of the pressure and flow rate oscillations in a given axial inducer test facility. The proposed reduced order model is based on several simplifying assumptions and takes into account the facility design and the dynamic properties of the tested inducer. The model has been used for evaluating the dynamic performance of a prototype of the LE-7 engine liquid oxygen (LOX) inducer, in tests carried out under given external flow rate excitations. The main results of these calculations will be shown, including the expected oscillations under a wide range of operational conditions and the influence of facility design. Calculations showed that the only way to obtain the two linearly independent test conditions, necessary for evaluating the inducer transfer matrix, is by changing the facility suction line: Any other changes in the facility design would result ineffective. Some other important design indications provided by the analytical model will be presented in the paper.

Analysis of Nonisothermal Rarefied Gas Flow in Diverging Microchannels for Low-Pressure Microresistojets

Heat transfer and fluid flow through different microchannel geometries in the transitional regime (rarefied flow) are analysed by means of Direct Simulation Monte Carlo simulations. Four types of three-dimensional microchannels, intended to be used as expansion slots in micro-resistojet concepts, are investigated using Nitrogen as working fluid. The main purpose is to understand the impact of the channel geometry on the exit velocity and the transmission coefficient, parameters which are well known to affect directly the thruster performance. Although this analysis can be applied in principle to several possible microfluidics scenarios, particular focus is given to its application in the field of space propulsion for micro-, nano- and pico-satellites, for which the requirements ask for low thrust levels from some μN to a few mN and moderate specific impulse, as well as a low power consumption in the order of a few W. Analysis shows that the thrust produced by one single microchannel can be increased by about 480% with a careful selection of the channel geometry, decreasing at the same time the specific impulse by just 5%, with a power consumption decrease of more than 66.7%.

Multiprobe characterization of plasma flows for space propulsion

Plasma engines for space propulsion generate plasma jets (also denominated plasma plumes) having supersonic ion groups with typical speeds in the order of tens of kilometers per second, which lies between electron and ion thermal speeds. Studies of the stationary plasma expansion process using a four-grid retarding field energy analyzer (RFEA), an emissive probe (EP) and a Langmuir probe (LP), all mounted on a three dimensionally (3D) displaced multiprobe structure are discussed. Specifically, the determination of plasma beam properties from the RFEA current-voltage (IV) characteristic curves is presented. The experimental results show the ion energy spectra to be essentially unchanged over 300 mm along the plasma-jet expansion axis of symmetry. The measured ion velocity distribution function (IVDF) results from the superposition of different ion groups and has two dominant populations: A low-energy group constituted of ions from the background plasma is produced by the interaction of the plasma jet with the walls of the vacuum chamber. The fast-ion population is composed of ions from the plasma beam moving at supersonic speeds with respect to the low-energy ions. The decreasing spatial profiles of the plasma-jet current density are compared with those of the low-energy ion group, which are not uniform along the axis of symmetry because of the small contributions from other ion populations with intermediate speeds.