FR2883411A1 - METHOD AND DEVICE FOR GENERATING A THERMAL FLOW CHARGED WITH PARTICLES - Google Patents

Abstract

The invention relates to a method and a device for generating a charged heat flux of particles. This device comprises a plasma source (1) comprising an end having a main axis (2) along which is directed an outward plasma jet (3) and at least one particle injector (5) having at least one outlet orifice (6), this injector (5) being intended to inject a carrier gas and particles into the plasma jet (3). According to the invention, the device comprises a support that can be moved in two directions to position axially and radially the injector (5) with respect to said main axis (2) and tilting means for controlling the angular position of said injector (5) with respect to a perpendicular axis (7) to said main axis (2), a first optical detector (9) and display means for detecting the average trajectory of the particles from the outlet of the injector (5) and in the plasma jet (3), and means for determining the average speed (10,11) of said particles .

Description

METHOD AND DEVICE FOR GENERATING A THERMAL FLOW

CHARGE OF PARTICLES

  The present invention relates to a method and a device for generating a particle-laden heat flux for characterizing materials subjected to severe thermal environments.

  It is known that spacecraft such as probes are subjected to various assaults (micrometeorites, ultraviolet radiation and ionizing agents, etc.). A particularly severe source of aggression for the structure of such a probe appears during atmospheric re-entry. Indeed, the probe is then subjected to abrasive thermal aggressions related to the presence of solid or liquid particles in the atmosphere of the star and the speed of reentry of these machines. These attacks are particularly sensitive when returning a space shuttle in the Earth's atmosphere since it dissipates its kinetic energy that has allowed it to remain in low orbit by friction with the air of the atmosphere. These abrasive thermal aggressions also occur in propulsion systems such as powder propellants that have a relatively large share of alumina particles in their jet.

  It is therefore essential to be able to simulate these laboratory-induced environments to predict by tests and simulations the evolution of the behavior of materials or objects such as a thermal coating intended to become the structural element of a re-entry shield. a spacecraft or a divergent nozzle.

  Until now, in order to characterize materials, very hot gases that can reach or exceed 3000 C have been used on small samples that are suitably arranged, for example "stopping points", ie the gases arriving perpendicular to the surface of the samples, or "inclined board", ie the gases arriving substantially tangentially to the surface of the samples.

  These gases are typically generated either by glow discharge type sources or by inductively coupled plasma sources. The higher the energies of these sources, the more they can provide a very hot gas in large quantities, controlled and constant for carrying out tests on representative samples, i.e. of large sizes.

  These extreme environment simulation methods have proved satisfactory for the development of space launcher and space probe systems known to date.

  Nevertheless, the permanent search for an optimization of the structures implemented in spacecraft, for example to obtain structural elements having an ever higher resistance to mechanical and / or thermal stresses for a minimum mass, requires a refinement of these simulation methods to characterize these structures.

  Indeed, the known simulation methods do not take into account, or improperly, abrasion phenomena by solid or liquid particles encountered either on the stars having atmospheres, or in the aforementioned powder propellants.

  US Pat. No. 3,893,335 (Jonhson et al.) Discloses a device for simulating the conditions of re-entry into a particle-laden atmosphere. This device supplies a hot air flow generated by an arc plasma to a nozzle, the flow at the outlet of this nozzle being directed towards the sample to be analyzed. To charge this particle stream, particles with high velocities of up to 6000 m / s are injected into the hot air stream downstream of this nozzle. However, these particles are accelerated independently of the hot air flow at a speed determined by the operator. This simulation method therefore requires particularly complex and bulky means to accelerate the solid particles at such speeds but also poses problems mixing two very hypersonic gas flows.

  It is also known from the document "Mars entry simulation with dust using an inductively heated generator", 22nd Aerodynamic Measurement Technology and Ground Testing Conference, 2002, AIAA 2002-3237; a simulation device using an inductively coupled plasma (ICP) source connected to a vacuum chamber discharged by a pumping unit. Injection of the solid particles into the gas stream is performed before the ICP plasma source in the plasma carrier gas. This method is therefore extremely unfavorable since it not only causes accelerated wear of the device but also alters the quality of the gaseous jet charged with particles. In fact, the abrasion products resulting from the interaction between the plasma charged with solid particles and the internal walls of the enclosure pollute the gaseous jet.

  In addition, the particles are injected into the plasma without any particular action to make this gaseous stream loaded with particles representative in terms of temperature and speed, the latter being important parameters for simulating abrasion phenomena.

  Also, the present invention proposes a method and a device for generating a heat flux loaded with simple particles in their design and in their operating mode to simulate in a controlled manner the heat flows loaded with particles representative of powder propellants or planetary reentries in order to characterization of standard size samples for the space and / or aeronautical field.

  For this purpose, the subject of the invention is a method for generating a heat flux charged with particles, these particles being at least partially directed on an object, into which a carrier gas and particles are injected from at least one injector. of particles having at least one exit orifice, in a plasma jet directed from the end of a plasma source outwardly along a main axis, the plasma jet having a core.

  According to the invention, the axial and radial positions of this particle injector are adjusted with respect to the main axis and the inclination of this injector with respect to an axis perpendicular to the main axis, and the momentum is controlled particle average at the nozzle outlet to uniformly drive the particles in the plasma jet core so that said particles acquire at a variable distance D from the end of the plasma source an average velocity maximum, and - this distance D is determined from the end of the plasma source 5 and the object is positioned at approximately this distance D. It is necessary to ensure the characterization of the objects under the closest possible conditions real environments to which these objects will be subjected. By way of example, the particle velocities in propellant jets of solid propellant rockets can reach from 1000 m / s to 3000 m / s. For this purpose, the object to be characterized is positioned in the jet of plasma charged particles, in the vicinity of where the average particle velocity is maximum, this speed being adjustable. This ensures that the sample is placed at a place where the particles have acquired a medium speed sufficient for characterization.

  The object of the invention is therefore achieved on the one hand by adjusting the amount of movement of the particles at the outlet of at least one injector and by controlling the position of said outlet orifice to make the plasma jet charged of particles as homogeneous as possible, and secondly by determining the position D, variable along the main axis, where the particles acquire a maximum average speed so as to position the object to be characterized around this place .

  In various embodiments of the method for generating a heat flux charged with particles, the present invention also relates to the following characteristics which will have to be considered in isolation or in all their technically possible combinations: the average maximum particle velocity is measured and the speed is adjusted from the plasma jet to a determined velocity value, the average trajectory of the particles of the injector outlet orifice and in the plasma jet is determined by using a first optical detector so as to adjust the position and the inclination of this injector and to adjust the average amount of movement of the particles at the outlet of the particle injector, the average velocity of each particle is measured by illuminating this particle at at least three different times using a light source generating pulses of light, and by detecting in a single image the light reflecting corresponding echo using a second optical detector, the second optical detector and the light source being synchronized.

  The invention also relates to a device for generating a particle-laden heat flux comprising: - a plasma source comprising a plasma source end having a main axis along which a plasma jet is directed outward, - at least a particle injector having at least one outlet port, said particle injector for injecting a carrier gas and particles into the plasma jet.

  According to the invention, this device comprises: a support movable in two directions for axially and radially positioning said injector with respect to said main axis and inclination means for controlling the angular position of said injector with respect to an axis perpendicular to said main axis; a first optical detector and display means for detecting the average trajectory of the particles from the exit of the injector and in the plasma jet, and means for determining the average speed of said particles.

  In various embodiments of the device for generating a heat flux charged with particles, the present invention also relates to the following characteristics which should be considered in isolation or in all their technically possible combinations: the particles are chosen from the group comprising Al 2 O 3, SiO 2, FeOH , Fe3O4 and combinations thereof, the injector comprises a flow regulator for controlling the flow of the carrier gas used to inject the particles so as to control the average amount of movement of these particles at the outlet of the injector, the particle size is between about 20 and 40 micrometers, the particle concentration is between about 0.001 and 40 percent by weight of the plasma jet, - the particle concentration is between about 20 and 40 percent by weight of the jet of plasma. plasma.

  In various possible embodiments, the invention will be described in more detail with reference to the accompanying drawings, in which: FIG. 1 schematically represents the device for generating a charged particle flux according to one embodiment of the invention; FIG. 2 is a schematic representation of the device for generating a charged particle flux according to another embodiment of the invention; FIG. 3 schematically shows the average trajectories followed by the particles as a function of the arc current associated with a volume flow rate of air for a particular mode of implementation of the invention; FIG. 4 schematically shows the distribution of the mean particle velocities (m / s) along the principal axis as a function of the applied arc current (A), the distance on the x-axis being measured from the end of the plasma source (in mm).

  Figure 1 shows a device for generating a particle-laden heat flux according to one embodiment of the invention. This device comprises a plasma source 1 comprising a source end having a main axis 2. The plasma source 1 is advantageously a plasma torch. In this particular embodiment, the plasma torch is an AQTIL plasma torch marketed by EADS ST. This high-power torch comprises two coaxial copper tubular electrodes between which the plasma gas is injected with a large tangential velocity. The electrodes are cooled with water. This torch can advantageously operate stably in a wide range of current intensity and air flow rates.

  The plasma source 1 produces a plasma jet 3 directed outwards along this main axis 2. A sample 4 placed along this main axis 2 receives the plasma jet 3.

  The device comprises at least one particle injector 5 having at least one exit orifice 6. This particle injector 5 is intended to inject a carrier gas and particles into the plasma jet at the outlet of the plasma source 1 The carrier gas of the particles is used to entrain and allow the penetration of the particles into the plasma jet 3. The flow rate of the carrier gas is fixed according to the nature of the particles, its particle size distribution but also the power dissipated in the However, this flow rate remains very low compared to the flow rate of the plasma jet 3 so that the perturbation generated by the penetration of the particles is negligible. By way of illustration, for an air volume flow rate of between 1500 and 8000 l / min, the volume flow rate of the carrier gas is less than 20 l / min. In a preferred embodiment, the device comprises several injectors 5 distributed homogeneously around the plasma jet 3. The number of injectors 5 is advantageously between 2 and 8.

  The injector 5 may comprise a flow regulator for controlling the flow of the carrier gas used to inject the particles so as to control the average amount of movement of these particles at the outlet of the injector 5.

  A support 8 displaceable in two directions makes it possible to position the injector 5 axially and radially with respect to the main axis 2 of the end of the plasma source 1, and tilting means make it possible to control its angular position with respect to an axis perpendicular to the main axis 2 7 (Figure 2). The support 8 is, for example, an arm mounted on a displacement table in a plane xy parallel to the main axis 2. It also makes it possible to adjust the position of the outlet orifice 6 of this injector 5 along the axis perpendicular to the main axis 7 2. The movement of this arm can be motorized or not. The tilting means of the injector make it possible to incline it at an angle of between 0 and 90 towards the end of the plasma source 1.

  Preferably, the size of the particles is between about 20 and 40 microns and their concentration is between about 0.001 and 40 percent by weight of the plasma jet. To simulate propellant jets, the particle concentration will rather be between about 20 and 40 percent by mass of the plasma jet.

  The particles are preferably selected from the group consisting of Al 2 O 3, SiO 2, FeOH, Fe 3 O 4 and combinations thereof. Particles AI2O3 and SiO2 are preferred in simulations of a powder propellant, while FeOH particles Fe3O4 are preferred for simulations of atmospheric re-entry and, in particular, re-entry into the Martian atmosphere. Other particles could be implemented to simulate other environments.

  The device comprises a first optical detector 9, for example an infrared video camera and display means such as a screen for detecting and visualizing the average trajectory of the particles from the outlet orifice 6 of the injector 5 and in the jet of plasma 3.

  In addition, the device comprises means for determining the average speed of said particles. These means advantageously comprise a light source 10 generating light pulses and a second optical detector 11. The second optical detector 11 and the light source 10 are synchronized. Preferably, the light source 10 is a semiconductor laser source and the second optical detector 11 is a fast camera for recording images at high speeds. This camera 11 is capable of detecting low light intensities.

  Since this camera 11 is for example a CCD camera having a lignecolumn matrix of pixels, each measurement is assigned at least one coordinate x representing the distance of the particle along the main axis 2 relative to the end of the source of the pixel. plasma 1 at a time t.

  The speed of each particle is measured by illuminating the particles at at least three different times. The average speed of each particle is then obtained by relating the distance traveled by the particle measured between two measurement points by the delay between two successive pulses of the light source 10. The CCD camera remains open during the at least three exposures so as to visualize the three or more positions of the particles on one and the same image by superposition.

  The information collected by this second detector 11 is advantageously used with those of the first detector to determine the average trajectory of the particles from the outlet of the injector (s) 5 and in the plasma jet 3.

  The device may comprise a sample holder 12, which is tiltable so that the surface of this sample 4 forms an angle between 0 and 90 with respect to the main axis 2 of the end of the plasma source 1. Preferably, this sample holder 12 is able to receive samples 4 of standard size, that is to say of a size representative of the elements used as structural elements of a thermal coating of a spacecraft, for example . The present invention thus lends itself to so-called "inclined board" tests, representative of the sides of the reentry vehicles or thrusters when the heat flow is partly tangential to the surface of the materials. In "inclined board", a square sample with a minimum size of 300 mm by 300 mm is used as standard, and a sample with a minimum diameter of 25 mm is used as "breakpoint".

  Figure 2 shows a device of the invention according to another embodiment. Elements with the same references as in Figure 1 represent the same objects. This device differs from that of FIG. 1, in that the end of the plasma source 1 is connected to a vacuum chamber 13 in which the plasma jet 3 is directed. This chamber 13 is pumped by a unit of pumping. This pumping unit comprises for example at least one high-speed primary pump. On the enclosure 13 is mounted at least one metering valve 14 connected to a metering device pumped for example by a primary pump, and a pressure gauge for introducing a gas into this chamber 13 by means of the metering valve 14 and of the dosing device. This gas is, for example, CO2. This device also comprises a diffuser 15 for discharging the plasma jet 3. Finally, the injector 5 is positioned to inject the particles from bottom to top.

  The invention also relates to a method for generating a charged heat flux of particles, the particles being at least partially directed on an object 4. According to this method, a carrier gas and particles are injected from at least one particle injector. 5 having at least one outlet port 6 in a plasma jet 3. This plasma jet 3 is directed from the end of a plasma source 1 outwardly along a main axis 2. This jet of plasma 3 has a heart.

  The axial and radial positions of this particle injector 5 are then adjusted with respect to the main axis 2 and the inclination of this injector 5 with respect to an axis perpendicular to said main axis 2 and the average amount of movement of the particles at the outlet orifice 6 of the injector 5 for homogeneously driving the particles in the core of the plasma jet 3. In one embodiment, the outlet orifice 6 of the injector 5 is positioned in the plasma jet 3, the temperature of the plasma jet 3 being lower than the melting temperature of the constituent material of the injector 5. In order to adjust the positions and the inclination of the injector and to adjust the average amount of movement of said particles at the outlet of this injector 5, the average trajectory of the particles of the outlet orifice 6 of the injector 5 and in the plasma jet 3 is determined by using a first optical detector 9, for example a video camera eo infrared.

  The particles acquire at a variable distance D from the end of the plasma source 1 a maximum average speed. The distance D of the end of the plasma source 1 is then determined and the object 4 to be characterized is positioned at this distance D. The object to be characterized can also be positioned to a position D 'along the main axis 2, from this position D, the position D 'being such that the particles still have a speed approximately equal to at least 90% of the determined maximum average particle speed.

  Advantageously, this maximum average speed is measured and the speed of the plasma jet is adjusted to a determined speed value. This adjustment of the speed of the plasma jet 3 can be achieved by adding a nozzle to the end of the plasma source 1, or by increasing the electrical operating power of the plasma source 1, or by adapting the composition of the plasma carrier gas generating the plasma. In the latter case, a gas chosen from the group comprising H 2, CO 2 and N 2 is used.

  Figure 3 shows an embodiment of the invention for alumina powders with a plasma torch. The outlet orifice 6 of the injector 5 is placed at a distance 14 mm from the end of the plasma torch 1 along the main axis 2 and at a height 12 of 24 mm along an perpendicular axis 7 to this main axis 2. The injector 5 is not inclined relative to this axis perpendicular to the main axis 2. This FIG. 2 shows the average trajectories of the particles from the outlet of the injector and in FIG. the plasma jet for a carrier gas of 6 L / min, as a function of the arc current (A) used to generate the plasma torch associated with an air volume flow rate of the plasma torch (Ilmin). The first curve C (cross) is obtained for a torque 450 A-7700 Ilmin, the second curve C2 (solid triangle) is obtained for a torque 310 A-3400 Ilmin and the third curve C3 (circle) is obtained for a pair 180 A-1700 I / min. It follows from these curves that the particles do not penetrate the core of the plasma jet for the first curve C ,, contrary to the other two curves C2 and C3 for which the average trajectories intersect the main axis 2 at about 100 mm from the end of the plasma source 1. The amount of movement of the jet is too great compared to that related to the radial flow at the outlet of the injector.

  It is therefore observed that in order to homogeneously entrain particles at the heart of the plasma flow, that is to say to ensure that all the particles are effectively entrained in the center of the flow of the plasma jet, is necessary not only to adjust the axial and radial positions of the injector relative to said main axis and its inclination relative to an axis perpendicular to said main axis but also to control the average amount of movement of these particles to the orifice of output of this injector.

  Figure 4 shows in a particular embodiment the distribution of average particle velocities along the main axis as a function of the arc current (A). The abscissa axis 16, which represents the position of the particles along the main axis (mm), originates from the end of the plasma source and the ordinate axis 18 represents the average speed of the particles. particles (mis). The powders used are particles of alumina and the plasma source is an AQTIL plasma torch. The first curve S, (rhombus) is obtained for an arc current of 450 A, the second curve S2 (rectangle) is obtained for an arc current of 310 A and the third curve S3 (triangle) is obtained for a 180 A arc current. These curves show that the particles have acceleration and deceleration phases and that the axial position 19 of the maximum of the average particle velocity for a given arc current moves towards the downstream of the torch with the increase of the arc current. The distance D from the end of the plasma source at which the particles acquire a maximum average speed is therefore very variable as a function of the applied arc current. It is necessary to determine this position for the characterization of samples under conditions as real as possible. It is also observed that the average maximum particle velocity is approximately four times higher at 450 A (420 +/- 45 m) than at 180 A (125 +/- 15 m / s). It is therefore possible to adjust the maximum average particle velocity by adjusting the plasma jet velocity by increasing the electrical operating power of the plasma source.

  Advantageously, the invention can be implemented as a device for thermal spraying of particles for the deposition of coatings, for example metal coatings, on a surface.

Claims (1)

13 Claims   A method for generating a charged heat flux of particles, said particles being at least partially directed to an object, wherein a carrier gas and particles are injected from at least one particle injector (5) having at least one outlet port (6) in a plasma jet (3) directed from the end of a plasma source (1) outwardly along a main axis (2), said plasma jet (3) having a core, characterized in that the axial and radial positions of said particle injector (5) are adjusted relative to said main axis (2) and the inclination of said injector (5) relative to a perpendicular axis (7) to said main axis and controlling the average amount of movement of said particles at the outlet (6) of said injector (5) to homogeneously drive said particles into the core of the plasma jet (3) so that said particles acquire a variable distance D from the end of the a plasma source (1) a maximum average speed, and - said distance D of the end of the plasma source (1) is determined and said object is positioned at approximately this distance D. 2. Method according to claim 1, characterized in that said maximum average velocity is measured and the velocity of the plasma jet (3) is adjusted to a determined velocity value.   3. Method according to claim 2, characterized in that the speed of said plasma jet (3) is adjusted by adding a nozzle to the end of the plasma source (1).   4. Method according to claim 2 or 3, characterized in that the speed of said plasma jet (3) is adjusted by increasing the electrical operating power of the plasma source (1).   5. Method according to claim 2 or 3, characterized in that the speed of said plasma jet (3) is adjusted by adding to the carrier gas used to generate the plasma a gas selected from the group consisting of H2, CO2 and N2.   6. Process according to any one of claims 1 to 5, characterized in that the average trajectory of the particles of the outlet orifice (6) of the said injector (5) and in the plasma jet (3) is determined. using a first optical detector (9) to adjust the position and inclination of said particle injector (5) and to adjust the average amount of movement of said particles at the output of said injector.   7. Method according to any one of claims 1 to 6, characterized in that the average speed of each particle is measured by illuminating said particle at at least three different times using a light source (10) generating pulses of light, and detecting on a single image the corresponding reflected light using a second optical detector (11), said second optical detector (11) and said light source (10) being synchronized.   8. Method according to any one of claims 1 to 7, characterized in that the size of the particles is between about 20 and 40 micrometers.   9. A process according to any one of claims 1 to 8, characterized in that the particle concentration is between about 0.001 and 40 percent by weight of the plasma jet (3).   10. The method of claim 9, characterized in that the particle concentration is between about 20 and 40 percent by weight of the plasma jet (3).   An apparatus for generating a particle-laden heat flux comprising: - a plasma source comprising a plasma source end (1) having a major axis along which a plasma jet (3) is directed outwardly, at least one particle injector (5) having at least one outlet orifice (6), said particle injector (5) being intended to inject a carrier gas and particles into the plasma jet (3), characterized in that it comprises: a support movable in two directions for axially and radially positioning said injector (5) with respect to said main axis (2) and tilting means for controlling the angular position of said injector (5) with respect to a perpendicular axis (7) to said main axis (2), - a first optical detector (9) and display means for detecting the average trajectory of the particles from the outlet of the injector (5) and in the plasma jet (3). ), and - means to detain to reduce the average speed (10,11) of said particles.   12. Device according to claim 11, characterized in that said means for determining the average speed of said particles comprise a light source (10) generating light pulses and a second optical detector (11), said second optical detector (11). and said light source (10) being synchronized.   13. Device according to claim 11 or 12, characterized in that the end of the plasma source (1) is connected to a vacuum chamber (13) in which is directed the plasma jet (3), said enclosure ( 13) comprising a pumping unit and said particle injector (5).   14. Device according to claim 13, characterized in that it comprises at least one metering valve (14) and a pressure gauge for introducing a gas into said enclosure (13).   15. Device according to claim 14, characterized in that said gas is CO2.   16. Device according to any one of claims 11 to 15, characterized in that it comprises a plurality of injectors (5) homogeneously distributed around the plasma jet (3).   17. Device according to any one of claims 11 to 16, characterized in that said particles are selected from the group comprising Al 2 O 3, SiO 2, FeOH, Fe 3 O 4 and combinations thereof.