Experimental characterization and modelling of a cavitating centrifugal pump operating in fast start-up conditions

The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behavior is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods. Three different types of fast start-up behaviours have been identified and have been presented at ISROMAC 12 (Duplaa et al, 2008). According to the final operating point, the start-up is characterized either by a single drop of the delivery static pressure, by several low-frequency drops, or by a water hammer phenomenon that can be observed both a the inlet and outlet of the pump. A physical analysis to explain these three different types of transient flow behaviour has been recently proposed (Duplaa et al, 2010). In the present paper, a modelling of the fast start ups in cavitating conditions is proposed. It consists of a two steps adaptation of fast start-up model in non cavitating conditions proposed by Dazin et al (2007). For that, fast X-rays imaging has been performed in the impeller with the collaboration of the French Atomic Agency (CEA) in order to determinate the high frequency evolution of the volume fraction during fast the start-ups. Although the results of the modelling presented here are not definitive, they are very promising

Experimental and Numerical Investigation of Unforced unsteadiness in a Vaneless Radial Diffuser

The paper reports combined experimental and numerical investigations of unforced un- steadiness in a vaneless radial diffuser. Experimental data were obtained within the diffuser using stereoscopic time resolved Particle Image Velocimetry (PIV) recording three velocity components in a plane (2D/3C), coupled with unsteady pressure transducers. To characterize the inception and the evolution of the unsteady phenomena, spectral analyses of the pressure signals were carried out both in frequency and time-frequency domains and the PIV results were post processed by an original averaging method. Two partial flow rates were investigated in detail in this paper. A single unforced unsteadiness was identified for the lowest flow rate, whereas, two competitive intermittent modes were recognized for the higher mass flow. Numerical analyses were carried out on the same pump by the commercial code CFX. All the computations were performed using the unsteady transient model and the turbulence was modelled by the Scale-Adaptive Simulation (SAS) model. Numerical pressure signals were compared with the experimental data to verify the development of the same pressure fluctua- tions

Cavitation inception in fast startup

The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behaviour is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, mass flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods

Transient behavior of a radial vaneless diffuser

The paper refers to the behavior of a radial flow pump vaneless diffuser during a starting period. Results obtained with a 1D numerical model are compared with some new experimental data which have been obtained using 2D/3C High repetition rate PIV within the diffuser coupled with unsteady pressure measurements. These tests have been performed on a test rig with a radial impeller matched with a vaneless diffuser. They have been made in air, on a test rig well adapted for studies on interactions between impeller and diffuser, as well as for the use of optical methods and especially Particle Image Velocimetry (PIV) as there is no volute downstream of the diffuser. The present study refers to new experiments combining pressure measurements and 2D/3C High Speed PIV at partial flow rates within a vaneless diffuser with a large outlet radius. Four Brüel & Kjaer condenser microphones are used for the unsteady pressure measurements. They were flush mounted on the shroud side of the diffuser wall and on the suction pipe of the pump. The sampling frequency was 2048 Hz. For PIV measurements, the laser sheet was generated by a Darwin PIV ND:YLF Laser at three heights within the diffuser. PIV snapshots have been recorded by two identical CMOS cameras. A home made software has been used for the images treatment. The results consist in fields of 80 x 120 mm2 and 81 x 125 velocity vectors with a temporal resolution of 250 velocity maps per second. For each flow rate and each laser sheet height in the diffuser, two acquisitions of about 1500 velocity maps have been realised. The experimental data are compared with the ones provided by a 1D transient model of the flow within the diffuser

Leakage flow simulation in a specific pump model

This paper deals with the influence of leakage flow existing in SHF pump model on the analysis of internal flow behaviour inside the vane diffuser of the pump model performance using both experiments and calculations. PIV measurements have been performed at different hub to shroud planes inside one diffuser channel passage for a given speed of rotation and various flow rates. For each operating condition, the PIV measurements have been trigged with different angular impeller positions. The performances and the static pressure rise of the diffuser were also measured using a three-hole probe. The numerical simulations were carried out with Star CCM+ 8.06 code (RANS frozen and unsteady calculations). Comparisons between numerical and experimental results are presented and discussed for three flow rates. The performances of the diffuser obtained by numerical simulation results are compared to the performances obtained by three-hole probe indications. The comparisons show few influence of fluid leakage on global performances but a real improvement concerning the efficiency of the impeller, the pump and the velocity distributions. These results show that leakage is an important parameter that has to be taken into account in order to make improved comparisons between numerical approaches and experiments in such a specific model set up

Étude expérimentale et modélisation de l'écoulement de jeu d'une aube fixe isolée

International audienceThe tip-leakage flow has detrimental effects on the performance of compressors. In this paper the effects of gap height and incoming casing boundary layer thickness are analyzed. Velocity and total pressure measurements are carried out in a plane behind the trailing edge of an isolated fixed blade. The total pressure loss is decomposed in a vortex loss and a wake loss. It appears that the increase of total pressure losses with the gap height comes essentially from the vortex part. This observation motivated the development of a model based on an analogy with a jet in crossflow to estimate the tip-leakage vortex circulation. The predictions of this model are consistent with the experimental data for gaps smaller than 4 % of chord

Modeling of Hydro-Pneumatic Energy Storage Using Pump Turbines

Modelling of a hydro-pneumatic energy storage system is presented in this paper. Hydro pneumatic storage aims to combine the good efficiency of hydraulic energy conversion and the space flexibility of pneumatic storage. The project aims to model a prototype which uses a rotodynamic multi-stage pump-turbine to displace a virtual liquid piston to compress air. To facilitate mass and heat transfers between both phases, there is no separation between the water and the air. A dynamic model of the storage system is developed using block diagram methodology. It takes into account characteristic curves of the pump-turbine and thermodynamic equations. Modelling results show that vapour diffusion contributes to reducing compression final temperature. This implies an increase of storage efficiency. A test rig construction will begin at the end of autumn 2011. It will be electrically connected to the “Distributed Energies” platform of ‘’Arts et Métiers Paristech’’ in Lille.adem

Contrôle du décollement dans un diffuseur aubé de turbomachine centrifuge

L'amélioration de la performance des turbomachines fonctionnant loin de leur point d'adaptation passe par la maîtrise des phénomènes instationnaires qui se produisent dans ces différents organes. L'étude présentée ici se concentre sur les interactions entre une roue de ventilateur centrifuge et son diffuseur. Elle vise, par des mesures de pression sur les différentes parois du diffuseur, ainsi que par des sondages dans les canaux inter-aubages à analyser finement ces phénomènes instationnaires et notamment les décollements fluctuants apparaissant sur les aubes à sur débit, et à mesurer leur influence sur la performance du diffuseur. Dans un second temps, un dispositif de contrôle passif de ces décollements utilisant des générateurs de vortex est proposé. Plusieurs configurations sont testés et leurs résultats sont comparésThe performance improvement of turbomachinery operating at off-design conditions can be achieved by the understanding of unsteady phenomena which are occuring in its components. The present study is focussing on the interaction between a centrifugal impeller fan and its vaned diffuser. It aims at analysing precisely these unsteady phenomena (and especially the fluctuating separated region identified on the vanes wall) and their consequences on the diffuser performance by three holes probe and unsteady pressure measurements. In a second step, devices for a passive control of the separation are introduced. Several configurations are tested and their results are comparedVILLENEUVE D'ASCQ-ECLI (590092307) / SudocSudocFranceF

Experimental and Numerical Investigation of Unforced unsteadiness in a Vaneless Radial Diffuser

The paper reports combined experimental and numerical investigations of unforced un- steadiness in a vaneless radial diffuser. Experimental data were obtained within the diffuser using stereoscopic time resolved Particle Image Velocimetry (PIV) recording three velocity components in a plane (2D/3C), coupled with unsteady pressure transducers. To characterize the inception and the evolution of the unsteady phenomena, spectral analyses of the pressure signals were carried out both in frequency and time-frequency domains and the PIV results were post processed by an original averaging method. Two partial flow rates were investigated in detail in this paper. A single unforced unsteadiness was identified for the lowest flow rate, whereas, two competitive intermittent modes were recognized for the higher mass flow. Numerical analyses were carried out on the same pump by the commercial code CFX. All the computations were performed using the unsteady transient model and the turbulence was modelled by the Scale-Adaptive Simulation (SAS) model. Numerical pressure signals were compared with the experimental data to verify the development of the same pressure fluctua- tions

Modeling dynamic systems : contribution to the unsteady behavior of a condenser based on the pseudo-bond graph approach

This article is devoted to the dynamic study of the brazed plate condenser (BPC). The proposed model is based on the bond graph theory. The proposed model is based on the bond graph theory because of its energetic approach and multi-physics character of the studied system. The model is discretized into five control volumes. The resolution of mass and energy equations is done by Runge-Kutta method embedded in 20sim software. Analyses of simulation results show that the model has a good ability to transcribe the time evolution of the temperature and pressure in both regimes, transitional and permanent. Also, the model is experimentally validated without any fitting of the set of thermal exchange coefficients.FUI Thermodluid-R