FR2947813A1 - SYNTHETIC JET GENERATING MICROSYSTEM, MANUFACTURING METHOD AND CORRESPONDING FLOW CONTROL DEVICE. - Google Patents

Abstract

This microsystem (10) for generating a synthetic fluid jet comprising an enclosure defining a cavity (12) and provided with an outlet orifice (14) of the jet and at least one deformable membrane (16) associated with actuating means for causing at least one vibration of the membrane (16) and thus a suction and a discharge of fluid into and out of the cavity (12) to generate the synthetic jet is characterized in that the means of actuation comprise electromagnetic means (18, 20).

Description

The present invention relates to a microsystem for generating a synthetic jet of fluid comprising an enclosure defining a cavity for receiving the fluid and equipped with a fluid jet. outlet orifice of the jet and at least one deformable membrane associated with actuating means for causing at least one vibration of the membrane and thus a suction and a discharge of fluid into and out of the cavity to generate the jet synthetic. It also relates to a method of manufacturing such a microsystem and a corresponding flow control device. More particularly, the invention relates to the field of devices generating synthetic jets. Synthetic jets are jets of fluids, generally gaseous, expelled through an orifice and produced by the alternative emptying and filling of a cavity consisting of one or more vibrating walls. In the state of the art, various means for generating volume variations of the cavity have been used. A first example is the use of a piston actuated by a motor. The synthetic jets, generated with such an actuation are powerful but at the cost of a low frequency of operation and a significant consumption. In addition, they are bulky systems. A second example of actuation commonly applied is the use of one or more piezoelectric elements. These piezoelectric devices are less efficient in terms of speed than the devices using a piston, but the frequency ranges are more versatile. Nevertheless, the performances of the synthetic jets are dependent on the resonances of the piezoelectric elements, the best results being obtained at the resonant frequencies. The size, although reduced compared to devices using a piston as a means of actuation, is still important (of the order of several cubic centimeters). Currently, devices for generating synthetic jets interest more and more researchers and manufacturers especially in the field of flow control.

Flow control is a technique that manipulates a flow (free or near a wall) using devices (passive or active) to produce favorable changes. These changes are, for example at the boundary layer, the retardation or advancement of a laminar / turbulent transition and the prevention of detachment. These changes in flow have positive consequences for the user, such as drag reduction, increased lift, noise reduction, or improved boundary layer mixing. The main problem at present is the congestion of the flow control devices. Indeed, the two most interesting control solutions, namely the boundary layer control and the detachment control, act on scales where the orders of magnitude of the structures on which we want to produce an effect are of the order of a millimeter. see submillimetric. This means that the means of action used must be of the same order of magnitude. Microsystems called MEMS (Micro Electro Mechanical Systems) provide answers to the stated problems, namely a small footprint and suitable scaling for the problems of control delamination and boundary layer. These systems are of small size which allows a very localized operation but also information taking using microsensors with improved spatial resolution compared to conventional sensor elements. In addition, their collective machining techniques and low consumption help improve their performance in flow control applications. This benefit is indicated at the end of the description on page 8.

In the state of the art, there are today very few examples of devices for generating synthetic jets made using microtechnologies. In addition, none of the current devices has performance satisfactory specifications of flow control applications for air vehicles (aircraft, missiles, rockets, ...) or terrestrial (cars, trains, ...). Moreover, the existing devices are not integrable on realistic macroscopic configurations of vehicle elements.

The object of the invention is to solve these two problems. For this purpose, the subject of the invention is a microsystem of the aforementioned type, characterized in that the actuating means comprise electromagnetic means.

According to particular embodiments, the microsystem comprises one or more of the following characteristics, taken separately or according to all the technically possible characteristics: the electromagnetic means comprise a coil connected to a source of electric current and a permanent magnet; - The magnet is fixed on the membrane by means of a rigid pad fixed thereto opposite the orifice and the coil is placed above the magnet; the outlet orifice and / or the rigid pad have a circular shape; the coil is a solenoid of inner radius greater than the radius of the rigid pad; the cavity and the orifice are machined from two silicon substrates; the membrane is made of elastomeric material; the elastomeric material is polydimethylsiloxane (PDMS); the magnet is made of NdFeB; the magnet is made of SmCo; the rigid pad is made of silicon; The subject of the invention is also a method for manufacturing a microsystem for generating synthetic jets as defined above, characterized in that it comprises: a) a thermocompression bonding step of a first and second a second silicon substrate; b) a step of deep reactive ion etching (DRIE) of the first and second silicon substrates to respectively form the cavity and the outlet; c) a step of deep reactive ion etching (DRIE) of a third silicon substrate; d) a step of depositing polydimethylsiloxane (PDMS) on the third substrate to form the membrane; e) a step of wet etching the third substrate to form the rigid pad; f) a step of bonding the third substrate provided with the membrane on the first substrate containing the cavity; and g) a step of fixing the electromagnetic actuation means of the membrane. According to a particular embodiment, the manufacturing method is characterized in that the step of deep reactive ion etching uses the Bosch method.

The invention further relates to an aerodynamic flow control device, characterized in that it comprises a synthetic jet generation microsystem as defined above. Thus, the invention makes it possible to overcome the disadvantages of existing synthetic jet generation microsystems by providing an operational integrated actuation device meeting the needs, in terms of flow control, of aeronautical or automobile manufacturers. The device for generating synthetic jets proposed by the invention provides an effective means for inducing, in a flow, an amount of movement greater than that obtained by the devices of the state of the art. Embodiments of the invention will now be described more precisely, but not limitatively, with reference to the accompanying drawings, in which: FIG. 1 is a diagram illustrating the structure of a synthetic jet generator of the state; of the technique, - Figures 2a and 2b are diagrams illustrating the operation of the synthetic jet generator of Figure 1, - Figure 3 is a diagram illustrating the structure of a synthetic jet generator according to the invention, and FIG. 4 is a diagram illustrating the steps of the manufacturing method of the synthetic jet generator of FIG. 3. FIG. 1 illustrates the structure of a generator 2 of synthetic jets of the state of the art. This structure comprises three basic elements, namely a cavity 4, an outlet orifice 6 of the synthetic jet and a wall or membrane 8 mobile. The movable membrane 8 is associated with actuating means (not shown) for causing this membrane 8 to vibrate. FIGS. 2a and 2b illustrate the operation of the generator 2 of synthetic jets.

The moving of the movable membrane 8 by the actuating means causes a variation of volume in the cavity 4. On an oscillatory movement, the progression can be divided into two half-periods. When the change in volume is positive, the fluid is sucked into the cavity 4 and when it is negative, the fluid is expelled from the cavity 4 by creating a swirling structure. The alternation of ejections and aspirations of fluid through the orifice 6 creates interactions of a series of vortices which thus generate a synthetic jet. The synthetic name of such a jet is due to the fact that the mass flow rate resulting over a period (suction / ejection) is zero. This implies that such a jet is formed without external fluid supply and can therefore inject momentum into the system without net mass input. In the state of the art, two types of synthetic jets are known according to the shape of the outlet orifice. If the orifice is circular, swirl rings are generated. The jet thus obtained is called an axisymmetric jet. If the orifice is a rectangular slot, the jet obtained is a two-dimensional jet generating pairs of swirling layers. However, different experiments show that the performances of axisymmetric jets in terms of speed are greater than those of two-dimensional jets. FIG. 3 illustrates the structure of a generator 10 of synthetic jets according to the invention. The generator 10 synthetic jets comprises an enclosure defining a cavity 12 for receiving a fluid, generally gaseous, including air in the intended applications. The chamber is also provided with an outlet orifice 14 and a deformable membrane 16.

The membrane 16 is associated with electromagnetic actuation means comprising a coil 18 connected to a source of electrical current, not shown, and a permanent magnet 20. The actuation of the membrane 16 thus rests on the interaction of the magnetic field created by the coil 18 on the magnet 20. The magnet 20 is fixed on the membrane 16 by means of a rigid pad 22 fixed thereto opposite the orifice 14, the coil 18 being placed on the above the magnet 20. The experiments conducted by the inventors have indeed shown that the inclusion of the rigid pad 22 on the membrane 16 substantially improves the performance of the jet while providing more technological latitude to integrate the electromagnetic actuation means on the generator 10 of synthetic jets. According to one embodiment of the invention, the cavity 12, the orifice 14 and the rigid pad 22 have a circular shape. Furthermore, the coil 18 is a finned solenoid of inner radius greater than the radius of the rigid pad 22, so as not to hinder the displacement of the membrane 16.

The magnet 20 is a cylindrical permanent magnet with high coercivity and substantial remanent magnetization. The material chosen for this magnet 20 is, for example, NdFeB or SmCo. The different manufacturing steps of the generator 10 of synthetic jets in MEMS technologies are described in the following description with reference to FIG. 4. The method of manufacturing micro-system 10 for generating an axisymmetric synthetic jet uses micro-wave techniques. Machining resulting from microelectronics that allow to obtain a high definition and precision machined patterns and a collective machining capacity of several structures on the same substrate, which allows a reduction in the unit cost of manufacture during production. The first step 30 of the manufacturing method according to the invention consists in bonding a first 32 and a second 34 silicon substrates. The first substrate 32 is intended to contain the cavity 12 and the second substrate 34 is intended to contain the orifice 14. The preferred method of bonding is a thermo-compression bonding. The use of this method of assembly allows precise control of the thickness of the cavity 12 and the orifice 14 and a very regular surface state in the etching background, since the metal bonding layer, designated 36 in FIG. 4, acts as a barrier layer. Then, two double-sided metal layers 38, 40 are deposited by sputtering respectively on the first substrate 32 and the second substrate 34, to form a physical mask for the subsequent etching of the silicon. In 42, the physical mask formed by the metal layers is open. At 44, the first 32 and the second 34 substrates are etched by a Bosch deep-reactive ion etching (DRIE) method to form cavity 12 and orifice 14, respectively. Bosch-based DRIE etching is a physicochemical etching of the family of plasma etchings also called dry etchings. This etching is carried out in a chamber fed by a molecular gas in low pressure flow (approximately 1 and 1000 mTorr). Plasma is created from an inert gas (usually argon) that is ionized by electromagnetic excitation. The plasma electrons decompose the molecular gas into several chemically active species that will render the medium corrosive. This combines a neutral ion beam (physical etching) and a reactive species flow (chemical etching). The particularity of the DRIE is the alternation of very short steps (a few seconds) of anisotropic etchings and wall passivations. For the anisotropic etchings, the gases used are SF6 and 1'02 and for wall passivations, C4F8 is used. The main advantages of this mode of engraving are its speed (between 6 and 8 pm / min) and the possibility to control the engraving profile that is realized. It is thus possible to make an anisotropic profile with straight flanks on large depths. At 46, the intermediate metal bonding layer 36 is opened and then at 48, a deposit of a layer 50 of SiO2 PECVD oxide is produced on the first substrate 32 containing the cavity 12. Thus, the steps 30 to 48 allowed the formation of the cavity 12 and the outlet orifice 14.

The following description of the manufacturing process concerns the formation of the membrane 16. The material chosen to form the membrane 16 is a silicone-type two-component flexible elastomer, for example polymethilsiloxane (PDMS). This material makes it possible to obtain a large deformability, an important elastic domain (deformation domain without rupture or plasticity) and compatibility with micromachining techniques. At 52, two double-sided SIXNy break layers 54, 56 are deposited on both sides of a third silicon substrate 58 to form a physical mask for subsequent silicon etching. At 60, the physical mask formed by the layer SIxNY 54 is open. At 62, the third substrate 58 is etched by a DRIE etching method. Then, at 64, a PDMS deposit is made on the third substrate 58 to form the membrane 16. At 66, wet etching using potassium hydroxide (KOH) is performed on the third substrate 58 to release the Finally, at 68, the third substrate 58 provided with the membrane 16 is bonded to the first substrate 32 containing the cavity 12. The bonding method used at 68 is a so-called PDMS bonding, since it makes contact with a surface in PDMS (membrane 16) and an SiO 2 surface (layer 50). This method makes it possible to obtain hydrogen bonds or covalent Si-O bonds by bringing the two surfaces 16 and 32 into contact, the surface 32 being activated beforehand by a surface oxidation (layer 50).

The electromagnetic actuation means of the membrane 16 are then placed by fixing the magnet 20 above the rigid pad 22 and then positioning the coil 18 above the magnet 20. The microsystem for generating synthetic jets and obtained can in particular be integrated in an aerodynamic flow control device for air or land vehicles. Thanks to the electromagnetic actuation means, the forces obtained, proportional to the gradient of the magnetic field, are important.

In addition, the geometrical choice made (axisymmetric jet) and the choice of PDMS elastomer material for the membrane has the consequence of associating the large forces obtained with large displacements. The microgenerative micro-generation system of the invention thus provides an effective means enabling it to induce in the flows a greater amount of movement than any other known micro-mechanical device. Thus, the combination of magnetic actuation, geometric options and materials as well as the development of microfabrication processes allow the device according to the invention to be the only one today to be directly integrable into realistic macroscopic configurations. elements of aircraft or automobiles. This device also has the advantage of being able to be used for other applications, for example for gaseous micro-discharges, or even liquid or the small-scale cooling of electronic components. Other applications of this device are also possible in microturbines, micromotors as well as in the micromanipulation of small amounts of materials. In a number of these applications, the industrial interest of these synthetic microjets is based on the mass production typical of low-cost microelectronics technology streams, allowing these devices to be used in the form of giant digital collections. Distributed actuators (upholstering of a rear body of the vehicle, jet air intakes or aircraft wings, ...). Of course, other embodiments and other applications may be contemplated.

Claims (14)

1. microsystem (10) for generating a synthetic jet of fluid comprising an enclosure defining a cavity (12) for receiving the fluid and provided with an orifice (14) for discharging the jet and at least one membrane ( 16) associated with actuating means for causing at least one vibration of the membrane (16) and thus suction and discharge of fluid into and out of the cavity (12) to generate the synthetic jet, characterized in that the actuating means comprise electromagnetic means (18, 20). 2. Microsystem (10) according to claim 1, characterized in that the electromagnetic means comprises a coil (18) connected to a source of electric current and a magnet (20) permanent. 3. Microsystem (10) according to claim 2, characterized in that the magnet (20) is fixed on the membrane (16) by means of a rigid pad (22) fixed thereto. of the orifice (14) and that the coil (18) is placed above the magnet (20). 4. Microsystem (10) according to claim 3, characterized in that the orifice (14) outlet and / or the rigid pad (22) have a circular shape. 5. Microsystem (10) according to claim 4, characterized in that the coil (18) is a solenoid of inner radius greater than the radius of the rigid pad (22). 6. Microsystem according to claim 1, wherein the cavity and the orifice are machined from two silicon substrates. 7. Microsystem (10) according to any one of claims 1 to 6, characterized in that the membrane (16) is made of elastomeric material. 8. Microsystem (10) according to claim 7, characterized in that the elastomeric material is polydiméthilsiloxane (PDMS). 9. Microsystem (10) according to any one of claims 2 to 8, characterized in that the magnet (20) is made of NdFeB. 10. Microsystem (10) according to any one of claims 2 to 8, characterized in that the magnet (20) is made of SmCo. 11. Microsystem (10) according to any one of claims 3 to 10, characterized in that the rigid pad (22) is made of silicon. 12. A method for manufacturing a microsystem (10) for generating synthetic jets according to any one of claims 3 to 11, characterized in that it comprises: a) a step (30) of bonding by thermocompression of first (32) and second (34) silicon substrates; b) a step (44) of deep reactive ion etching (DRIE) of the first (32) and second (34) silicon substrates to respectively form the cavity (12) and the exit orifice (14); c) a step (62) of deep reactive ion etching (DRIE) of a third substrate (58) of silicon; d) a step (64) of depositing polydimethylsiloxane (PDMS) on the third substrate (58) to form the membrane (16); e) a wet etching step (66) of the third substrate (58) to form the rigid pad (22); f) a step (68) of bonding the third substrate (58) provided with the membrane (16) on the first substrate (32) containing the cavity (12); and g) a step of fixing the electromagnetic actuation means of the membrane (16). 13. A manufacturing method according to claim 12, characterized in that the step (44, 62) of deep reactive ion etching (DRIE) uses the Bosch method. 14.- aerodynamic flow control device, characterized in that it comprises a microsystem (10) for generating synthetic jets according to any one of claims 1 to 11.30.