Patent Application
20090078003
1. Technical Field of the Invention
The present invention is directed to a method and apparatus for mixing viscous liquids, and particularly for mixing a glass melt without the use of rotating stirring blades.
2. Description of Related Art
In a conventional glass making process, glass precursors are mixed in appropriate proportions to form a batch material. The batch is then melted in a furnace to form a molten glass or glass melt that is flowed or delivered through a delivery system to a point where the molten glass is used in the manufacture of a final product. Unfortunately, the glass melt flowing from the melting furnace is not typically homogeneous: a variety of process conditions can be responsible for causing variations in density and chemical composition of the melt. For example, temperatures can vary or variations in batch proportions may occur. Level fluctuations may wash different levels of the furnace walls with molten glass. Pipe flow phenomena may result in different flow rates through the delivery system used to transport the melt. These and other process variations may therefore result in physical and/or chemical inhomogeneities that are both temporally and spatially dependent. These homogeneities will hereinafter be referred to as cord.
Consider a tube adapted to transport the molten glass. A cross section of the tube transverse to the longitudinal axis of the pipe will reveal a cross section of the molten glass. At any instant in time, the chemical composition of that cross section may vary spatially—the chemical composition of the melt over the cross sectional area is not uniform at that instant. Thus, spatial inhomogeneity exists. Moreover, that spatial distribution itself may also vary over time, thus creating a temporally dependent inhomogeneity. As used herein the temporal inhomogeneity may be viewed as the time dependent spatial variation. That is, how the cross sectional inhomogeneity varies as a function of time.
The aforementioned compositional variations may exhibit a variety of time constants. For example, temporal variations (e.g. cross sectional chemical inhomogeneity as a function of time) may occur quite rapidly, on the order of minutes, to quite slowly—on the order of hours or days.
Chemical inhomogeneities may result in variations of refractive index within the molten glass, or melt, that in some applications produce undesirable optical anomalies in the finished glass product, such as in an optical display.
To mitigate these chemical inhomogeneities, or cord, the melt must be conditioned, typically by actively stirring to mix and homogenize the melt. Stirring is most generally accomplished by flowing the melt through a stirring vessel in which one or more stirrers are rotated through the melt. The stirrers may include paddles or vanes that stretch, cut and fold the glass as the stirrer rotates, reducing the cord to dimensions small enough that they are of less concern, though not completely eliminated. However, fabrication of devices comprising moving parts in a very high temperature environment can prove challenging.
Because the glass is relatively viscous, as compared to say, liquid water, and the stirrer blades extend close to the walls of the stir chamber, significant shear stress is developed at the surfaces of the stirring vessel and the stirrer(s). Moreover, the molten glass is chemically aggressive in its molten state, particularly at the high temperatures used to process the material (typically in excess of 1400° C.). Although chemically stable, high temperature refractory metals are often used in the construction of the stirring vessel and the stirrers, e.g. platinum-rhodium alloys, the high temperature and corrosive nature of the molten glass, coupled with the high shear stress, can result in erosion of the metal and the subsequent release of metallic particulate (inclusions) into the melt that may render the finished glass article unusable. Thus, while the use of active stirring processes are reasonably effective at reducing cord, they may produce undesirable side effects—the release of metallic particulate which is accompanied by its own set of problems when present in the final product.
U.S. Pat. No. 2,577,213 to G. Slayer, et al. describes a method and apparatus for mixing glass that uses a vertical free falling stream of molten glass that is reoriented to a horizontal plane by catching the stream on a moving platform/container. That is, the stream falls into a receiving container that is reciprocated to lap the stream within receiving container. While capable of mixing the molten glass, the method relies on moving components and mechanically forced reorientation of the molten glass. Thus, interaction between high temperature surfaces (e.g. bearing surfaces) may become issues over extended periods of operation.
The present invention provides a method and an apparatus for mixing a viscous liquid. The present invention may be used on a variety of viscous liquids, but is well suited for the mixing and homogenizing of a glass melt (i.e. molten glass) and eliminates the rotating members and incumbent high shear stress typically found in conventional glass stirring operations.
In one embodiment, a method of mixing a glass melt is disclosed comprising flowing the glass melt having a viscosity μ through an aperture defined by a first surface as a first stream, the first stream falling a distance d through a first free space volume to a second surface that is stationary with respect to the first surface, and wherein d and μ are selected to produce fluid coiling of the stream at the second surface that mixes the glass melt.
In another embodiment, a method of mixing a glass melt is described comprising a) flowing the glass melt having a viscosity μ through a first aperture defined by a first surface, the glass melt falling a distance d1 as a first stream through a first free-space volume to a second surface stationary with respect to the first surface; b) after the flowing of step a), flowing the glass melt through a second aperture defined by the second surface, the glass melt falling a distance d2 as a second stream through a second free-space volume to a third surface stationary with respect to the second surface, and wherein d1, d2 and μ are selected to produce fluid coiling of the first and second streams at the second and third surfaces, respectively, that mixes the glass melt.
In still another embodiment a method of mixing a molten glass is described comprising dividing the molten glass into at least one molten glass stream that falls through a first free space volume, the molten glass stream undergoing viscous buckling that mixes the glass melt.
In yet another embodiment, an apparatus for mixing molten glass is presented comprising a first surface defining a first aperture through which a glass melt having a viscosity μ is flowed to produce a first stream that falls through a first free space volume, a second surface disposed a distance d below and stationary with the first surface, the second surface defining a plurality of apertures through which the molten glass is flowed as a plurality of streams that fall through a second free space volume, and wherein d is selected to result in fluid buckling of the first stream at the second surface for the viscosity μ.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings are not necessarily to scale, and sizes of various elements may be distorted for clarity. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention.
The present invention provides a method and apparatus for mixing a viscous liquid without the incumbent shearing stress typically found in processes using rotating mechanical stirrers. For example, in a glass making process, shearing stresses resulting from active stirring of molten glass in a platinum mixer can be responsible for introducing undesirable erosion products (e.g. platinum) into the glass. The present invention develops the viscous liquid into a free-surface stream that stretches and folds into itself. The present invention is useful for mixing all manner of viscous liquids, and in particular, molten glass.
Shown in
Aperture 12 may be any shape, but is preferably generally circular in cross section. However, aperture 12 may be in the form of a slot, in which case folding rather than coiling occurs. A circular aperture, for example, yields a generally cylindrical stream of viscous liquid flowing from the aperture, whereas a slot yields a flattened stream of viscous liquid (i.e. a sheet). As shown, aperture 12 may be defined by a tube or pipe, or a plurality of tubes or pipes, in fluid communication with a supply of viscous liquid (not shown). By free space volume what is meant is that the volume comprises a region free of liquid through which the stream may fall. Naturally, the stream, by extension, comprises a free surface—a surface that is exposed to the free space volume. The free space volume may comprise a gas or gases, or the free space volume may be a vacuum.
Advantageously, stream 16 buckles without the need to artificially (e.g. mechanically) translate the stream (or the surface that the stream contacts). That is, the stream buckles according to the physical phenomenon described below, thereby eliminating the need for moving parts within apparatus 10. This is particularly beneficial when mixing high temperature viscous liquids, such as molten glass having a temperature in excess of about 1400° C.
As used herein, fluid buckling is a phenomenon that may occur when a free-falling viscous fluid (i.e. liquid) encounters a surface (e.g. a horizontal plate or floor for example). That is, a viscous liquid issuing from, for example, an aperture, and falling a sufficient distance through a free-space region and then contacting a surface may exhibit any one or more of a variety of steady state or chaotic motions at the surface, including, for example, snaking (generally horizontal “S” shapes), coiling (generally circular shapes), lapping (generally vertical “S” shapes) and combinations thereof. For the sake of simplifying discussion, and not limitation, the present description will be directed to the coiling behavior of a stream of viscous liquid having a generally circular transverse cross section. As used herein, such coiling may be referred to as liquid rope coiling, or fluid coiling whereas a more general term that includes lapping or folding may be referred to as fluid buckling.
In an exemplary illustration of fluid coiling for example, a fluid (i.e. liquid) having a density ρ, a viscosity μ (kinematic viscosity υ=μ/ρ), a characteristic stream radius a1, falls through a height d under the influence of gravity g at a volumetric flow rate Q. If d is sufficiently large, the stream becomes unstable due to pertubating forces (e.g. gravity and viscous resistance to bending of the stream) and may form a regular coil having a substantially constant diameter on a surface beneath the stream characterized by a coil having a radius R at a frequency Ω. The coiling frequency Ω is strongly dependent on the height d through which the stream falls. Indeed, Neil M. Ribe provides a more detailed description of fluid coiling in “Coiling of Viscous Jets”, Proceedings of the Royal Society, v.460 (2004), pp. 3223-3239, the content of which is incorporated herein by reference. Ribe identifies multiple height-dependent fluid coiling regimes: viscous coiling, gravitational coiling and inertial coiling. Viscous coiling occurs when the radius of the coil is approximately equal to the height d through which the stream falls. The coiling frequency Ω in the viscous coiling regime can be approximated by the equation Ωv=a1−2Qd−1. In the viscous coiling regime, the diameter of the stream does not change appreciably as the stream falls through the free space volume, coil, and the rotational period of the resultant coil. The coiling is dependent upon, inter alia, the gravitational force, the flow rate of the viscous fluid, the diameter of the aperture, the drop or free-fall distance and the density, viscosity and surface tension of the fluid. The coiling occurs when the falling filament becomes unstable in the face of bending disturbances. This instability appears when the fluid Reynolds number falls below a critical value. Viscous buckling is described in more detail by Habibi, et al. in “Dynamics of Liquid Rope Coiling”, Physical Review E74 (66306), The American Physical Society (2006), pp. 1-10, and by Neil M. Ribe in Physical Review E68 (036305), The American Physical Society (2003), pp. 1-10 the contents of which are incorporated herein by reference in their entirety.
During gravitational coiling, the stream thins/tapers as it falls, and the resulting coil occupies only a small fraction of the total height d. The gravitational coil radius can be approximated as g−1/4(υQ)1/4 and the coiling frequency in the gravitational coiling regime ΩG is approximated by g1/4υ1/4a14/3Q3/4. Finally, during inertial coiling, the coiling radius can be represented by υ1/3a14/3Q−1/3, and the frequency of coiling in the inertial regime ΩG is approximated by υ−1/3a1−10/3Q4/3.
As used herein, fluid coiling will be interpreted to denote the spontaneous coiling of a viscous fluid stream when contacting a surface below the stream, including the regimes described above.
Returning to
As an example of the foregoing, assume the inhomogeneity is a variation in concentration of a single constituent of a multiconstituent viscous liquid. As the viscous liquid stream descends from aperture 12, the concentration varies vertically along the length of the falling liquid. However, as the viscous liquid begins to coil, the concentration variation is oriented horizontally. Moreover, the horizontal orientation varies rotationally due to the circular nature of the coil. Over time, the coil may slump, and/or topple, further distributing the chemical concentration variation. Thus, the inhomogeneous viscous liquid is mixed and made more homogeneous by a reorientation of the liquid.
Of course, the concentration variation above was used as a simple illustration. Other inhomogeneities in a viscous liquid may be considered, and the mixing thereof (making more homogeneous), are contemplated by the present invention, including temperature variations, density variations, chemical/composition variations, particulate dispersion variations, etc.
In another embodiment depicted in
For high temperature viscous liquids such as a glass melt, surface 26 is preferably in the form of a plate formed from a refractory material capable of withstanding the high temperature and aggressive chemical nature of the molten glass. Surface 26 will hereinafter be referred to as aperture plate 26. However, it should be apparent that surface 26 need not be in the form of a plate, i.e. planar. Indeed, as shown in
Aperture plate 26 may, for example, be comprised of a refractory metal selected from the platinum group, i.e. platinum, iridium, palladium, rhodium, osmium, ruthenium or combinations thereof. In other embodiments, aperture plate 26 may be comprised of a ceramic refractory material. For low temperature and/or less chemically aggressive viscous liquids, other materials may be substituted for refractory materials. For example, steel, stainless steel or even plastics may be appropriate. That is to say, the choice of material(s) for the aperture-defining surface depends, inter alia, upon the materials being mixed.
In another embodiment shown in
Aperture plate 26 is contained within vessel 30 and divides the interior of vessel 30 into a volume 32 above aperture plate 26 and free space volume 18 below aperture plate 26. Viscous liquid 14 is supplied to volume 32 through inlet 34 as indicated by arrow 36 and flows through the plurality of apertures 12 in aperture plate 26 in an example of local mixing. The viscous liquid that accumulates or pools at the bottom of vessel 30 then exits vessel 30 through outlet 36 as indicated by arrow 38.
It should be apparent to one skilled in the art having had the benefit of the present disclosure that global (single aperture) and localized ( plural aperture) mixing subunits may be combined into various combinations to effect different mixing efficiencies for liquids of different natures and viscosities. For example, in the embodiment illustrated in
Any combination of aperture plate configurations may be employed, depending on the nature of the particular viscous liquid or liquids to be mixed, and the desired level of mixing. Other configurations include, but are not limited to, global-local, local-global, global-global, local-local, and so forth, and for as many iterations as desired.
In some embodiments, the free space volume through which the viscous liquid falls may be evacuated, or the free space volume may contain, for example, a rapidly diffusing, preferably inert gas such as helium. Any voids that may become entrained in the viscous liquid then either collapses, or gas contained in the void rapidly diffuses from the liquid. Voids may become entrained in the viscous liquid, for example, by operating under conditions such that a pool of viscous liquid does not form over an aperture plate, causing a tubing effect as the viscous liquid flows through the aperture and pulling the “atmosphere” above the aperture plate into the center of the flow (even if an atmosphere contained in the volume is a vacuum). Alternatively, voids may become entrained in the viscous liquid if, when coiling onto a surface below the aperture plate the coil builds to a sufficient height that it fall over, thus trapping the surrounding “atmosphere within the collapsed coil. If lapping occurs, the lap may fold over in such a way that the atmosphere is trapped within the folds, thus entraining the atmosphere.
The entraining behaviors described above, while seemingly undesirable, may, in some embodiments, be advantageous. For example, in a typical glass making process, constituent glass forming materials (batch materials) are melted to form a molten glass precursor material, or glass melt. The melting process produces gaseous by-products that for some glass products are undesirable and therefore must be removed. This can be done, for example, by including a fining agent among the batch materials. The fining agent is chosen so as to release a gas (or gases), typically oxygen, into the melt thus forming large bubbles in the melt. Gases resulting from the melting process coalesce into the large bubbles released by the fining agent. In effect, the fining agent gas collects and lends buoyancy to the melting gases, so that the combined bubbles rise to the surface of the melt and can be released. Such fining may, for example, take place is a special vessel wherein the temperature of the glass melt is increased to reduce the viscosity of the melt and make it easier for the bubbles to rise to the surface.
In one embodiment, a free-surface mixing apparatus according to the present invention may be inserted into the flow of a glass melt prior to the melt entering the fining vessel, and operating so as to entrain a specific fining gas into the molten glass to enhance the fining process.
In still another embodiment of the present invention, a forced gas jet may be used to create turbulence in the free space volume through which the glass exiting the aperture plate descends. Accordingly, as illustrated in
In still another embodiment illustrated in
In yet another embodiment a series of bars or rods may be disposed in a vessel for intercepting and deflecting streams of viscous liquid. Shown in
The falling and intersecting streams of viscous liquid eventually make their way to the pool of viscous liquid at the bottom of vessel 30, where the viscous liquid may undergo fluid buckling (e.g. fluid coiling) and then exit through outlet 38 as indicated by arrow 40. In this particular embodiment, the stream of viscous liquid does not undergo fluid buckling immediately after issuing from an aperture, but instead after leaving the surface of one or more of the deflectors 62.
While the invention has been described in conjunction with specific exemplary embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.
