This invention relates to an apparatus and method for aerodynamic levitation. More specifically, the invention pertains to an apparatus and method for aerodynamic levitation of a liquid drop using a profile generator to configure the velocity profile of the air flow impinging upon the liquid drop.
The invention has application for containerless processing of materials. This is especially true for small size samples such as those for prototype applications.
The invention also has usefulness as an analytical tool to study the properties of liquid materials. One example of such an analytical use is the determination of the thermal conductivity of a molten material.
Although there may be a number of applications for levitation techniques, one principal use of levitation techniques is for containerless processing. This may be due, at least in part, to the availability of orbital space environments where containerless processing can be of advantage.
In space, there exists a reduction of gravitational force which significantly reduces the hydrostatic pressure, buoyant forces, natural convection and sedimentation that occurs in an earth bound environment. Because of its nature, containerless processing can take advantage of such an environment.
However, because a truly zero-gravity environment cannot be completely realized in an orbiting space craft due to accelerations associated with space craft, trajectory alterations, atmospheric drag and movement of astronauts within the vehicle, it is still necessary to spatially localize the material using externally generated forces. Levitation techniques are able to generate these external forces.
These levitation techniques, which are useful for containerless processing, provide the ability to manipulate materials without physical contact with a container. Because of the absence of this contact, containerless processing eliminates container-induced contamination and heterogeneous nucleation.
Some of these techniques have the inherent characteristic to melt and levitate the material at the same time. Oftentimes, this is not a desirable feature because of the advantage of separating the levitation and melting functions.
Containerless processing has been used for the manufacture of ceramics, such as shown and described in U.S. Pat. No. 4,966,737 to Werner et al. entitled "Method And A Device For Manufacturing A Powder Of Amorphous Ceramic Or Metallic Particles". This patent describes the use of an acoustic levitation field acting with an inert cooling gas in the area in front of the nozzle projecting a substance in the liquid state. The inert cooling gas cools the liquid material while in a levitated state.
Levitation techniques have also been utilized to isolate a sample from its surroundings for spectrometric measurements at very high temperatures as shown and described in U.S. Pat. No. 4,958,126 to Brevard et al. entitled "Probe For Magnetic Resonance Spectrometric Measures At Very High Temperatures". This patent concerns a probe for a spectrometer resonator for very high temperatures that uses a laser bean focused on a sample as a means to heat the sample. The sample is levitated in the interior volume of the resonator.
Levitation techniques used to produce products were also shown and described in U.S. Pat. No. 4,929,400 to Rembaum et al. entitled "Production Of Monodisperse Polymetric Microspheres". In this patent larger size microspheres can be produced by the levitation of larger sizes of droplets of polymerizable material during the polymerization by radiation.
U.S. Pat. No. 4,378,209 to Berge et al. shows a gas levitator used in containerless processing. The gas levitator supports the material as the levitator axis is rotated from vertical to horizontal to inverted to vertical. This levitator can be used on earth at any angle of inclination with respect to an earth reference and in space.
As previously mentioned, this invention has application as an analytical tool for low-gravity processes. In the past, persons have tried to study low-gravity processing of materials. The National Aeronautics and Space Administration has utilized various vehicles to study containerless processing. NASA has utilized the KC-135 Air Force cargo plane which achieves a low-G state for 25 seconds by flying a prescribed parabolic trajectory. NASA has also utilized an F-104 aircraft which flies a parabolic trajectory and attains a "free fall" period of about 60 seconds.
Beginning in 1975, NASA conducted space processing applications with the help of the rocket program in order to provide short duration, for example, five to seven minutes, flight opportunities for research in a low-gravity environment.
A convenient and economic device for the study of low-gravity solidification of materials is a drop tube. A drop tube is ideally suited for investigations of super-cooling and free fall solidification of high temperature refractory metals and alloys.
One such drop tube is located at NASA's Marshall Space Flight Center. This drop tube consists of a ten centimeter inside diameter, about one hundred fifty feet long, stainless steel tube. A bell jar is at the top and contains an apparatus that melts and releases the material. The tube and bell jar are vacuum tight and may be evacuated or back-filled with various gases during an experiment. A pyrometer and infrared detectors provide a thermal history of the melted sample as it drops and solidifies in the tube. Viewing and instrumentation ports are located on each floor. A sample is decelerated and caught by a detachable catcher at the bottom of the tube. This apparatus provides up to about 4.2 seconds of free fall time.
In regard to various levitation techniques, there are four basic levitation techniques. These techniques are electromagnetic levitation, electrostatic levitation, acoustic levitation, and aerodynamic levitation. This invention pertains to aerodynamic levitation. But, in order to provide a background for the invention, there follows a brief discussion about each of the previous three levitation techniques.
Electromagnetic levitation techniques only provide for the melting and levitation of electrically conductive materials in a containerless environment. Early research on this technique attempted to eliminate crucible contamination of samples and provide studies of reactive high melting point metals.
In this method, the levitating force and heating power are produced by the induction of eddy currents in a metallic sample by an alternating electromagnetic field. This technique may be used in a high vacuum environment. However, this technique requires the simultaneous melting and levitation of the material.
Electrostatic levitation techniques can levitate and manipulate materials in a high vacuum. An electrostatic levitator operates on the principal of a feedback controlled by electrostatic force. A CCD camera monitors the object's position and a minicomputer provides a real-time air signal that is used to adjust the main electrostatic force between the electrodes. A more detailed description of electrostatic levitation is found on pages 34-35 of the article by M. Barmatz entitled "Overview of Containerless Processing Technologies" published in Materials Processing in the Reduced Gravity Environment of Space by Elsevier Science Publishing Company, Inc., Guy E. Rindone, Editor (1982). This technique also simultaneously couples the levitation and melting functions.
Any material, whether it be a conductor or insulator, magnetic or non-magnetic, may be levitated by acoustic forces. Acoustic levitation techniques require a gas medium for the propagation of the sound waves.
Acoustic forces are associated with non-linear phenomena. Non-linear acoustic theory predicts that in high intensity standing wave sound fields, samples with densities which are large when compared to the surrounding gas will be positioned at acoustic pressure nodal positions which correspond to the minimum of the force potential well. A more detailed discussion of acoustic levitation techniques as found at pages 29 through 33 of the paper "Overview of Containerless Processing Technologies" by M. Barmatz.
Referring now to aerodynamic levitation, one of the earliest works in the atmospheric sciences which used aerodynamic levitation was that of Blanchard, (Blanchard, C., "The Super-Cooling, Freezing and Melting of Giant Water Drops At Terminal Velocity and Air", Proceedings of the First Conference on the Physics of Cloud and Precipitation Particles, Woods Hole, Massachusetts, pages 233-247, Sep. 7-10, 1955). In this research, the levitation of liquid droplets was achieved in a square wind tunnel which discharged to the environment. According to the article, this tunnel arrangement was able to levitate samples, which were unrestrained in the horizontal plane, for many minutes. The primary purpose of this apparatus was to observe the melting and freezing of water drops.
A number of attempts by other researchers have been less than fully successful in the aerodynamic levitation of liquid droplets. Several efforts, notably by Nordine and Atkins (Nordine, P. C. and Atkins, R. M., "Aerodynamic Levitation of Laser-Heated Solid and Gas Jets", Review of Scientific Instruments, 53 (9), pages 1456-1464, Sep., 1982) and Coutures et al. (Coutures, J. P., Rifflet, J. C., Billard, D. and Coutures, P., "Contactless Treatments of Liquids In a Large Temperature Range By An Aerodynamic Levitation Device and Laser Heating", Sixth European Symposium on Material Sciences Under Microgravity Conditions, Dec., 2, 1986) have attempted to use single or multiple jets for levitation. As long as solid spheres are used, jet levitation can be effective. See Krispin, F., Williamson, J. W., Strauss, A. M., "Jet Levitation of Spherical Shapes for Microgravity Research", 38th Congress of the International Astronautical Federation.
On the other hand, past attempts to use jets for liquid levitation have not met with complete success. Others have attempted to levitate liquid aluminum droplets and solid uranium samples. Winborne, D. A., Nordine, P. C., Rosner, D. E., and Marley, H. F., "Aerodynamic Levitation Technique For Containerless High Temperature Studies in Liquid and Solid Samples", Metallurgical Transactions B, Volume 73, pages 711-713, Dec., 1976. In an effort to utilize jet levitation, Rush et al. developed a constricted-tube air levitator. However, stable melting of the samples was not achieved. Rush, J. E., Stephens, W. K., and Ethridge, K. C., "Properties of a Constricted-Tube Air Flow Levitator", Materials Processing In The Reduced Gravity Environment Of Space, pages 131-138, Edited by Guy Rindone, Elsevier Science Publishing Company, New York, N.Y. 1982. In 1982 Ethridge et al. reported the results of a collimated holed structure gas jet levitator used to levitate hollow glass microballoons. Ethridge, E. C. and Dunn, S. L., "Air Jet Levitation Furnace System For Observing Glass Microspheres During Heating and Melting", Materials Processing In The Reduced Gravity Environment Of Space, pages 121-130, Edited by Guy Rindone, Elsevier Science publishing Company, New York, N.Y. 1982. Nordine and Atkins have also done work in jet levitation as reported in 1982. Nordine, P. C. and Atkins, R. M., "Aerodynamic Levitation of Laser-Heated Solids and Gas Jets", Review of Scientific Instruments, 53 (9), pages 1456-1464, Sep., 1982.
Thus, in view of the importance of containerless processing through levitation techniques, there is a need to develop both an improved apparatus and a method for aerodynamic levitation.
In view of the difficulty associated with the levitation of liquid drops, there is a special need to provide an improved apparatus and method for the containerless processing of liquid drops.
It is further important to provide an improved apparatus and method for the levitation of liquid drops that does not possess an inherent characteristic to melt the material.
There is a need to use aerodynamic levitation in the area of aerodynamic interactions between gas flows and liquid drops. There is a further need to use aerodynamic levitation in the area of mass transfer reactions. There is a need to use aerodynamic levitation in the areas of droplet coalescence and droplet deformation.