New test facility for sand erosion studies

New test facility for sand erosion studies

Wear 233–235 Ž1999. 712–716 www.elsevier.comrlocaterwear New test facility for sand erosion studies M. Lemistre ) , D. Soulevant, F. Micheli, A.A. De...

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Wear 233–235 Ž1999. 712–716 www.elsevier.comrlocaterwear

New test facility for sand erosion studies M. Lemistre ) , D. Soulevant, F. Micheli, A.A. Deom ´ (ONERA), BP 72-29 aÕenue de la DiÕision Structures and Damage Mechanics Department, Office National d’Etudes et de Recherches Aerospatiales ´ Leclerc, F-92322 Chatillon Cedex, France

Abstract Erosion by sand of aircraft and rotorcraft engines being a real problem, ONERA has developed a test facility to reproduce and analyse in laboratory, the impact of solid particles on targets. This test facility, based on aerodynamic particles acceleration, allows to generate a continuous flow of particles having a mean diameter of 80 to 600 mm, with a velocity up to 180 m sy1. A remarkable characteristic of this set-up is that the particles impact on the target is achieved without any residual airflow. During tests with metallic targets, a vectorial mapping of the reflected particles has been reconstructed for several incidence angles Ž15 to 758., by measurement of two components of the velocity and statistical signal processing. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sand erosion; Aerodynamic accelerator; Laser velocimetry

1. Introduction In order to characterise sand erosion resistance by sand, a new test facility has been designed at ONERA. This facility allows to produce a continuous flow of calibrated solid particles having a velocity up to 180 m sy1 and to impact a solid target without any residual airflow. The 2-D velocity distribution of impacting and reflected particles is measured. This test facility includes two main parts: an aerodynamics set-up generating the flow of particles which impact on the target without residual airflow and a velocity measurement set-up. This article first describes the two main parts of the test facility, then gives results obtained during the qualification tests; several examples of vectorial mapping of the reflected particles velocity are shown.

2. Experimental set-up

2.2. Velocity measurement set-up

2.1. Aerodynamics set-up Concerning the aerodynamics set-up Žsee Fig. 1., the low velocity calibrated particles Ž20 m sy1 ., penetrate

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author.

upstream of the supersonic nozzle, supplied by an absolute pressure of 2 bars. At the nozzle exit, the air flow velocity being greater than Mach 1, the particles are carried along the accelerating pipe and the diphasic flow Žair q particles. penetrates in the low pressure chamber ŽLPC on Fig. 1. connected to a vacuum pump. Inside the low pressure chamber, a small pipe located exactly in the axis of the diphasic flow, is connected to an experimental enclosure ŽEE in Fig. 1. where the absolute static pressure is 1 bar. The dynamic pressure of the air flow being 1 bar, in the entrance of the small pipe, there is equality of the two pressures Žair flow dynamic pressures static pressure. and the air velocity is null. So, only the particles penetrate into the experimental enclosure and impact the target. The air flow is entirely evacuated by the vacuum pump. Note that, since the experiment enclosure is entirely sealed Žexcept the small pipe., there is no air flow inside, and the particles impact without turbulence.

Tel.: q33-1-46-73-48-95;

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The velocity measurement set-up is a fringes laser velocimeter, shown on Fig. 2. Two beams of coherent light Ž1,2. stemming from the same laser ŽL. and covering optical paths of similar length, are self-crossing in the VMP Žvelocity measurement point., where a system of interference fringes appears. When a particle is crossing this fringes system, a modulated light signal is produced. The measurement of the frequency of the modulation

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 2 1 0 - 0

M. Lemistre et al.r Wear 233–235 (1999) 712–716

The presence, at the same point, of two fringe systems of different wavelengths and directions allows to measure simultaneously the two components of the particle velocity. The two pairs of beams are constituted of the two most intense wavelengths, green Ž l s 0.5145 mm. and blue Ž l s 0.488 mm. of an ionised argon laser. The measurement volume Žthat can be displaced in the particle flow., corresponding to what photodetector sees, appears like a sphere having a diameter of some hundreds of microns. The photodetector is made of a telescope coupled with two photomultipliers each tuned on one used wavelength. The precision order of this process is "1.6% on each component, from 20 to 200 m sy1 . For each measurement point, one performs the acquisition of 1000 to 2000 velocity vectors, the processing of these data allowing then to compute average velocity components and different statistical values. The whole of the velocimeter Žlaser, optical setting and photo detector. being slaved in position, it is possible to perform several locations of VMPs, which allows to draw a mapping of the field velocity. The accuracy of the position setting is "50 mm, the accuracy of the position measurement is "10 mm.

Fig. 1. Schematic diagram of aerodynamics set-up.

allows to determine one component of the particle velocity Vi given by: Vi s f I

3. Qualification tests

Ž 1.

where f is the measured frequency and I is the interfringes distance. The sign of the considered velocity component can be found by an artificial fringe displacement obtained by a Bragg cell ŽBC. inserted in the path of one of the two beams, so that the relationship Ž1. becomes: Vi s f I y Vd i

Ž 2.

The velocity qualification test of the set-up has been performed with three diameters of particles: 80, 200 and 600 mm. In order to determine the divergence of the particle flow, three measurement points in the incident flow have been investigated Žsee Fig. 3., one on the axis of the particles flow Žpoint A. and one on each side of this

where Vd i is the magnitude of the fringe displacement velocity.

Fig. 2. Schematic diagram of a one-component laser velocimeter.

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Fig. 3. Location of the VMPs in the injected particle flow.

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axis, distant from 5 mm Žpoints B and C.. The distance between the line of the measurement points and the theoretical impact point of the particles is 30 mm, the target being removed from the experiment enclosure. The two measured velocity components are Vz and Vx , Vz being the main component, parallel to the axis of the particle flow. Fig. 4a–c show the histograms of velocities for particles of 200 mm of diameter, measured at the point A and

Fig. 5. Vectorial representation of particle flow.

calculated on 40 ranks. Velocities are in meter per second and the number of particles in each rank is normalised to the number of counted particles. The two channels of the velocimeter being synchronised, each particle simultaneously identified by the two channels, is characterised by its two velocity components. So it is quite possible to compute the velocity vector for each particle. Fig. 4c shows the histogram of magnitudes of velocity vectors. The mean calculated velocity is 195 m sy1 , the standard deviation being 18.5 m sy1 . Fig. 5 is a vectorial representation of the divergence of the particle flow Žfor the same diameter 200 mm., i.e., the mean computed magnitude for each measurement point ŽA, B and C.. The velocity magnitude is given in meter per second, and the angles are in degrees. We can see that the divergence is extremely weak: 48 between points A and B and, 1.68 between points A and C. Table 1 gives the main characteristics of velocity and divergence obtained for each diameter of particles. Several tests with nozzle in function but without injection of particle, have been performed to determine the

Table 1 Velocity performances obtained for three diameters of particles Particle diameter Žmm. Mean velocity Žm sy1 . Standard deviation Žm sy1 . Mean angle Ždegrees. Angle standard deviation Ždegrees. Particle flow divergence Ždegrees. Fig. 4. Histograms of velocities measured at the point A, for particles of 200 mm of diameter: Ža. Vz component, Žb. Vx component, Žc. magnitude.

80 163 25.0 270 6.0 q0.5 y5.5

200 195 18.5 270 4.5 q1.5 y4.0

600 153 23.5 271 5.5 q2.5 y5.0

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4. Rebound of particles study

Fig. 6. Location of the measurement points in the rebound.

residual turbulence in the experiment enclosure, with and without target. In this configuration, the velocimeter performs the velocity measurement on the dust particles present in the enclosure. The main results of this investigation are the following: the measured magnitudes are included between 0 and 5 m sy1 , with an angle that seems perfectly random, and the presence or the absence of target does not change anything to these values. To conclude about the qualification test, we can say that the phenomenon of solid particle impact can be studied in the almost total absence of residual turbulence.

As part of the study of the phenomenon of solid particle rebound on metallic targets, several tests have been achieved with three diameters of particles Ž d s 80, 200 and 600 mm.. For each diameter of particle, different incidence angles b i have been used from 15 to 758 Žthe incidence angle being the angle formed by the direction of the particle flow and the surface of the target.. In each test configuration Ž d and b i ., 15 VMPs have been characterised, three in the incident flow, having the same location during the qualification tests, and 12 in the rebound region Žsee Fig. 6.. Getting the velocity component in the rebound from the measurement is not easy. Indeed, after impact, particles are fragmented and can collide with particles of incident flow, which can be seen on the obtained histograms by the presence of more than one population. So it is necessary, before computing the velocity vector, to perform a signal processing in order to remove the particles having characteristics not representative of the studied phenomenon, mainly the particles having a velocity component Vz ) 100 m sy1 . This process allows then to compute the mean velocity vector for each measurement point, then to calculate, for each configuration Ž d , b i ., the four significant parameters: incident velocity Vi , mean rebound velocity Vr , incident angle b i and mean rebound angle br and finally to draw the diagrams: VrrVi s f Ž b i . and brrb i s f Ž b i .. This process allows also to draw a mapping of the vectorial velocity field, for each measurement configuration.

Fig. 7. Vectorial velocity fields; Ža. d s 600 mm, b i s 158, Žb. d s 200 mm, b i s 458, Žc. d s 80 mm, b i s 758.

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As an example Fig. 7a–c show, respectively, the obtained diagrams in the three following configurations: d s 600 mm, b i s 158rd s 200 mm, b i s 458 and d s 80 mm, b i s 758. Finally, after each test performed in each configuration Ž d , b i ., the experiment enclosure being entirely sealed, the particles having impacted have been collected for several granulometry measurements, in order to determine their fragmentation.

5. Conclusion ONERA has designed and built a test facility to study the erosion phenomenon due to solid particles impact. This facility allows to reach a particle mean impact velocity of 180 m sy1 , up to 195 m sy1 for given working parameters; the great interest of the device is that no residual airflow exists at the target level. Among the different velocity measurement techniques studied w1–4x, the fringes laser velocimetry technique used

is the one that allows the trajectory; moreover, the major advantage of the device is its working easiness. Acknowledgements The authors thank TURBOMECA for partial funding of the development of this test facility. References w1x Y. Kagimoto, S. Matsumoto, T. Noda, M. Etsu, N. Shida, Experimental Study on Relation Between Erosion Wear Rate and Particle Impact Velocity Measurement by LDV, ELSI VII, Cambridge, England, September 1987. w2x C. Geiler, M. Stanislas, H. Royer, T. Fournel, Automatic assessment of aerosols holograms for granulometry and velocity, 6th International Symposium on Flow Visualisation, Yokohama, Japan, October 1992. w3x C. Geiler et al., Depouillement automatique d’hologrammes de mi´ croparticules: application a` la velocimetrie, Congres ´ ´ ` OPTO, Paris, France, May 1992. w4x R. Henry, J. Lefevre, M. Lemistre, Caracterisation du rebond de ` ´ particules de silice, 5eme Congres ` ` Francophone de Velocimetrie ´ ´ Laser, Rouen, France, September 1996.