Cavitation and flow instabilities in a 3-bladed axial inducer designed by means of a reduced order analytical model

The present paper illustrates the main results of an experimental campaign conducted using the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The tests were carried out on the DAPAMITO inducer, a three-bladed axial pump designed and manufactured by Alta S.p.A. using a simplified analytical model for the prediction of geometry and noncavitating performance of typical space rocket inducers. The transparent inlet section of the facility was instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet and the middle of the axial chord of the blades. At each axial station at least two transducers were mounted with given angular spacing in order to cross-correlate their signals for amplitude, phase and coherence analysis. However, probably because of the high value of the blade tip clearance, very few flow instabilities have been detected on the inducer, including: steady asymmetric cavitation caused by the different extension of the cavitating regions on the blades; cavitation surge at a frequency equal to 0.16 times the inducer rotational frequency; a higher-order axial phenomenon at 7.2 times the rotational frequency.http://deepblue.lib.umich.edu/bitstream/2027.42/84278/1/CAV2009-final92.pd

A New Cavitating Pump Rotordynamic Test Facility

The present paper illustrates the operational characteristics of the CPRTF (Cavitating Pump Rotordynamic Test Facility), an experimental apparatus specifically designed for the measurement of rotordynamic fluid forces acting on turbopump impellers in fluid dynamic and inertial/thermal cavitation similarity conditions. The realization of the CPRTF is currently in progress under ASI (Agenzia Spaziale Italiana) funding and consists in the upgrade of the CPTF (Cavitating Pump Test Facility), already available at Centrospazio, Consorzio Pisa Ricerche, Pisa, Italy. The experimental apparatus, operating in water, will be capable of carrying out the measurement of the steady and unsteady forces exerted by the flow on the impellers of cavitating/noncavitating turbopumps. More generally, the facility is designed as a flexible, versatile and inexpensive apparatus that can be readily be adapted to carry out detailed experimental investigations on practically any kind of fluid dynamic phenomena relevant to high performance turbopumps. The main operational requirements, development choices and design trade-offs that led to the final configuration of the facility are illustrated and its performance in testing of cavitating/noncavitating turbopumps under fluid dynamic and thermal cavitation similarity are discussed. Experimental results from a number of turbopump configurations and operational conditions are presented to illustrate the present capabilities of the facility

A hybrid modulation technique for the dc-bus voltage balancing in a three-phase npc converter

In this paper a new pulse width modulation technique for three-phase neutral point clamped (NPC) converter is presented, with the aim to actively control the dc-bus capacitors' voltages. To meet this requirement, usually NPC modulation techniques are either based on the sole common mode voltage injection (CM1) or on the sole multi-step (MS) switching mode of operation. Contrarily, the presented approach combines these two strategies, taking advantages of all their main benefits while keeping the switching transitions to the minimum required number. The approach has been numerically tested and compared with some of the other strategies, showing an overall better behaviour, especially for high modulation indices

Assessment of the Propulsive Performance of Fuel Vapor Pressurized Hydrogen Peroxide-Ethane Rocket Engines

In the last years low-toxicity “green” storable liquid propellants have become considerably more attractive as possible substitutes for nitrogen oxides and hydrazines. The main advantage of “green” propellants is represented by the significant cost savings associated with the drastic simplification of the health and safety protection procedures necessary during propellant production, storage and handling. Fuel Vapor Pressurization (FVP) technology of “green” bipropellant rocket engines potentially offers very significant additional advantages in terms of system cost, complexity, reliability, safety and mass, with practically no penalty in propulsive performance compared to traditional storable propellants such as mixed nitrogen oxides and hydrazines. Pioneering FVP experiments were carried out by Goddard, Wyld and others. Detailed studies have been conducted and several tests have been successfully performed since 1994 in the US, but no such experience is presently available in Europe, nor FVP has ever attained flight readiness anywhere in the world. The main characteristics of the FVP system examined in this work consist in the use of storable, non-toxic, inexpensive, non hypergolic, high-energy propellants such as hydrogen peroxide (HP, H2O2) and ethane (C2H6) and in the storage of these propellants in a single lightweight tank, using a flexible diaphragm or a bladder to separate the fuel from the oxidizer and a catalytic reactor to decompose the hydrogen peroxide before mixing and combustion with ethane. This configuration therefore yields a very simple and yet highly efficient and reliable propulsion system by eliminating the cost, the weight and complexity of propellant tanks and pressurization bottles, pressure and flow regulators and ignition systems. These advantages are of special relevance in low- or mediumthrust rocket engines for the rapidly expanding market of “small” space missions and led the authors to focus on the analysis and assessment of propulsion systems operating according to this concept. The present paper reports therefore the preliminary evaluation of fuel vapor pressurized H2O2-C2H6 rocket propulsion systems. The results of the analysis confirm that the development of FVP technology may represent a significant contribution to the containment of the propulsion cost of small- and medium-size spacecrafts

Rotordynamic Forces on a Four Bladed Inducer

The present paper illustrates the results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at ALTA S.p.A. aimed at characterizing the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch inducer, designated as DAPAMITO4. The forces acting on the impeller have been measured by means of a rotating dynamometer mounted just behind the inducer. The roles of the imposed whirl motion of the rotor, flow coefficient, cavitation number and liquid temperature have been investigated. The destabilizing role of cavitation has been confirmed. The experimental results are consistent with previous findings obtained by the authors, as well as with former data published by Caltech researchers. The observed dependence of the tangential and normal components of the rotordynamic force on the whirl-to-rotational speed ratio does not follow the quadratic functional behavior often assumed in the open literature. Rotordynam..

A Simplified Analytical Model for Evaluating the Noncavitating Performance of Axial Inducers

The present paper describes an analytical model for the preliminary prediction of the noncavitating flow field and performance of helical inducers. The proposed model is based on the traditional troughflow theory approximations with empirical corrections for outlet flow deviation and hydraulic losses due to inlet incidence effects and friction in the blade channels. Unlike most classical models, it allows – even if under still rather restrictive assumptions – for the prediction of the radial and circumferential flow velocity fields at the inducer exit section and for the approximate evaluation of the head coefficient as a function of the flow coefficient in terms of the static pressure rise generated by the inducer. The results are presented of the model validation by comparison with the experimental data obtained for several inducers tested in different facilities worldwide

Hybrid Modulation Technique with DC-Bus Voltage Control for Multiphase NPC Converters

The article presents a novel carrier-based pulsewidth modulation technique for multiphase neutral point clamped converters. The technique is aimed to actively control the neutral point (NP) potential while supplying the desired set of line-to-line voltages to the load. Standard techniques are either based on the sole common mode voltage injection or on the sole multistep switching mode; contrarily, the proposed algorithm combines these two approaches to take advantage of their main benefits. The technique performs well for each number of phases, for each modulation index, and for each type of load. It can control in closed-loop the NP voltage to any desirable value with a reduced number of switching transitions. The proposed approach has been experimentally validated and compared with other carrier-based algorithms

General Approach for Modeling and Control of Multiphase PMSM Drives

This article presents a modeling approach and a control strategy for multiphase surface-mounted permanent magnet synchronous machine drives. The mathematical model is completely general with respect to the machine parameters and to the winding configuration. It also intrinsically considers the effects of eventual constraints for the phase currents, generated from the electrical connections among the phase windings or resulting from faults. The current controller is entirely formalized in the phase variables domain. It is based on a pseudoinverse decoupling algorithm and on a linear decoupled controller. The current references are computed by means of a maximum-torque-per-ampere strategy, which can be also easily adapted for torque sharing purposes. The proposed controller requires minimum changes with respect to system reconfigurations or parameters variations and, therefore, it is suited both for healthy and for faulty operations. An extensive set of experimental results has been conducted to validate the proposed approach in several testing scenarios

Cavitation and Flow Instabilities in a 4-Bladed Axial Inducer Designed by Means of a Reduced Order Analytical Model

A reduced order model for preliminary design and noncavitating performance prediction of radial turbopumps is illustrated. 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 flowfield 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, thus generating the information necessary for completing the geometric definition of the ..

Hybrid modulation technique with dc-bus voltage control for multiphase NPC converters

The paper presents a novel Carrier-Based Pulse Width Modulation (CBPWM) technique for multiphase Neutral Point Clamped (NPC) converters. The technique is aimed to actively control the Neutral Point (NP) potential while supplying the desired set of line-to-line voltages to the load. Standard techniques are either based on the sole Common Mode Voltage Injection (CMI) or on the sole Multi-Step (MS) switching mode; contrarily, the proposed algorithm combines these two approaches to take advantage of their main benefits. The technique performs well for each number of phases, for each modulation index and for each type of load. It can control in closed-loop the NP voltage to any desirable value with a reduced number of switching transitions. The proposed approach has been experimentally validated and compared with other carrier-based algorithms