Methods and apparatus to enhance paper and board forming qualities

Abstract: Methods and apparatus to enhance paper and board forming qualities with insert tubes and/or a diffuser block in the paper forming machine headbox component which generates vorticity in the machine direction (MD) which is superimposed on the streamwise flow to generate a swirling or helical flow through the tubes of the diffuser block. Tubes of the diffuser block are designed such that the direction of the swirl or fluid rotation of the paper fiber stock may be controlled and the direction thereof is controlled in such a way to provide effective mixing, coalescence and merging of the jets of fluid emanating from the tubes into the converging section, i.e., nozzle chamber of the headbox. Also disclosed is the effective mixing of the jets generating cross-machine direction (CD) shear between the rows of jets that form at the outlet of the tubes inside the nozzle chamber of the headbox to align paper fibers in the cross-machine direction, thus increasing CD strength of the manufactured sheet. Vortex forming means are disclosed for a plurality of tubular elements in the diffuser box for generating controlled axial vortices in the machine direction promoting mixing of the jets of paper stock from the tubular elements as the jets flow into the nozzle chamber to a uniform flow field of stock at the slice opening for the rectangular jet.

Description: Background of the Invention 1. Field of the Invention The present invention relates generally to increased productivity and formation quality in paper forming machine headbox components by hydrodynamic optimization of paper and board forming. More particularly, the invention relates to the generation of defined vortices in jets of paper fiber stock enhancing the mixing of the stock as jets of paper fiber stock emanate from a diffuser block for coupling a distributor to a nozzle chamber in a paper forming machine headbox for discharging paper fiber stock upon a wire component promoting uniform flow of the stock from the diffuser block or insert tubes. Advantageously the generation of small scale turbulent flows with the defined vortices of the jets avoids large scale hydrodynamic problems of secondary flows, flow instabilities, boundary-layer separation and other hydrodynamically-induced nonuniformities in the forming section nozzle chamber of the paper forming machine headbox component, avoiding the problems of: twist/warp in board grades; non-uniform basis-weight; non-uniform fiber orientation; non-uniform moisture profile; cockling and diagonal curl in printing paper; and streaking (jagged) dry line on the forming table or wire component. 2. Description Of The Related Art DE 93 04 736 U discloses a diffuser block according to the preamble of claim 1 for coupling a distributer to another chamber in a paper forming machine headbox for discharging paper fiber stock upon a wire component moving in a machine direction, the diffuser block comprising a multiplicity of individual tubular elements for communication of the paper fiber stock between the distributer and the nozzle chamber, with the tubular elements being orientated axially in the machine direction and arranged as a matrix of rows and columns for generating multiple jets of the stock flowing in the nozzle chamber. US-A-3 846 229 discloses a method of mixing jets of paper fiber stock emanating from a multiplicity of axially lined tubes arranged at a matrix of rows and columns in a diffuser block coupled to a nozzle chamber in a paper forming machine headbox for discharging a uniformed flow field of stock upon a wire component moving in a machine direction, which method includes producing fine-scale turbulence in the stock, whereas relatively large eddy currents and vortices are to be avoided. B. Felsch: "Papiermacher-Taschenbuch", 1998, Dr. Curt Haefner Verlag, Heidelberg, pages 129-136 discloses replaceable inserts for diffuser blocks for adapting controlled micro-turbulences to the manufacturing conditions. The quality of paper and the board forming, in manufacture, depends significantly upon the uniformity of the rectangular jet generated by a paper forming machine headbox component for discharging paper fiber stock upon the wire component of the paper forming machine. Attempts to establish uniform paper stock flow in the headbox component, particularly the nozzle chamber, and to improve paper fiber orientation at the slice output of the headbox have involved using a diffuser installed between the headbox distributor (inlet) and the headbox nozzle chamber (outlet). The diffuser block enhances the supply of a uniform flow of paper stock across the width of the headbox in the machine direction (MD). Such a diffuser box typically includes multiple conduits or tubular elements between the distributor and the nozzle chamber which may include step widening or abrupt opening changes to create turbulent flows for defloculation or disintegration of the paper fiber stock to ensure better consistency of the stock. High quality typically means good formation, uniform basis weight profiles, uniform sheet structure and high sheet strength properties. These parameters are affected to various degrees by paper fiber distributions, fiber orientations, fiber density and the distributions of fines and fillers. Optimum fiber orientations in the XY plane of the paper and board webs which influences MD/CD elastic stiffness ratios across the width is of significant importance in converting operations and end uses for certain paper grades. Conventional paper forming apparatus used primarily in the paper and board industry consists of a unit which is used to transform paper fiber stock, a dilute pulp slurry (i.e., fiber suspended in water at about .5 to 1 percent by weight) into a rectangular jet and to deliver this jet on top of a moving screen (referred to as wire in the paper industry). The liquid drains or is sucked under pressure through the screen as it moves forward leaving a mat of web fiber (e.g., about 5 to 7 percent concentration by weight). The wet mat of fiber is transferred onto a rotating roll, referred to as a couch roll, transporting the mat into the press section for additional dewatering and drying processes. The device which forms the rectangular jet is referred to as a headbox. These devices are anywhere from 1 to 9 meters wide depending on the width of the paper machine. There are different types of headboxes used in the industry. However, there are some features that are common among all of these devices. The pulp slurry (referred to as stock) is transferred through a pipe into a tapered section, the manifold, where the flow is almost uniformly distributed through the width of the box. The pipe enters the manifold from the side and therefore, there must be a mechanism to redirect the flow in the machine direction. This is done by a series of circular tubes which are placed in front of the manifold before the converging zone or nozzle chamber of the headbox. This section is referred to as the tube bundle, the tube bank or the diffuser block of the headbox. These tubes are either aligned on top of each other or are placed in a staggered pattern. There are anywhere from a few hundred to several thousand tubes in a headbox. The tubes in current headboxes have a smooth surface starting from a circular shape in the manifold side and going through one or two step changes to larger diameter circular sections. Some tubes converge into a rectangular outlet (some with rounded edges) at the other end opening to the converging zone of the headbox. Analysis relating to the present invention shows that the flow entering the tube may start to recirculate generating vorticity in the machine direction. The sign of the vorticity vector depends on the location of the tube. Very often, there is a pattern that develops as a natural outcome of the tube pattern structure and the structure of the headbox. In current machines, there is no control on the direction or strength of the vortices in the tubes. The tubes all have flat smooth internal surface and the flow pattern and secondary flow inside the tubes is governed by the inlet and outlet conditions. The machine direction vorticity could be positive or negative depending on the inlet and outlet conditions which in turn depend on the location of the tube in the tube bank. Summary of the Invention A new concept in accordance with the invention is to control the formation of secondary flow in the tubes in order to achieve a superior flow field inside the converging zone of the headbox. Any mechanism used to control or enhance the secondary flow inside the tubes and in the tube bank region to achieve a certain flow property in the converging zone of the headbox is part of this concept. Thus, the concept relates to the modification of the flow inside the tube bank by altering the internal surface geometry of current tubes or tube inserts. The internal surfaces of all of the current tubes or tube inserts are either circular and therefore axisymmetric (type I), or, they start from a circular inlet and eventually converge into a rectangular outlet (type II) with a four fold symmetry (i.e., the entire tube can be divided into symmetric regions by two diagonal cross-sectional planes, one vertical cross-sectional plane and one horizontal cross-sectional plane. The new concept is to modify the geometry of the type I and/or inserts such that the internal surface is no longer axisymmetric or non-axisymmetric, and to modify the internal geometry of the type II tubes such that the internal geometry of the tube or the insert is no longer four fold symmetric. One described embodiment modifies the internal geometry of each tube in order to generate machine-direction (MD) vorticity and subsequently to arrange the tube or the insert in such a manner so that all the jets in each row of the tube bundle form with the same sign of MD vorticity vector and the jets in each column form with alternating sign of the MD vorticity. This generates shear layers which would result in cross-machine orientation of fibers and therefore would increase the strength and other physical properties in the CD while providing effective mixing and turbulent generation between tubes adjacent to each other in each row. Another described embodiment modifies the internal geometry of each tube insert or tube in order to generate machine-direction (MD) vorticity and subsequently to arrange the tubes or the inserts in such a manner so that all the jets in each row and column of the tube bundle form with the same sign of MD vorticity vector. This results in strong mixing and dispersion of the fibers and fillers and therefore better uniformity in fiber and filler distribution in the sheet. Another mechanism to generate axial vorticity inside the tubes of a headbox is to have a device, a tube insert, wherein a flat section at the manifold side is followed by a converging curved section, followed by a straight tube section, and where, one or more inclined fins or grooves are placed on the flat section or on the flat and the converging curved section of the headbox tube or insert nozzle of the headbox tube. The purpose of inclined fins or grooves is to control the defined direction or orientation of the axial vortices generated inside the tubes. The converging section of the insert nozzle or tube will accelerate the fluid and increase the angular velocity of the fluid, consequently, increasing the strength of the vortex as the fluid moves toward the straight (constant diameter) section of the tube. It is an object of the present invention to provide methods and apparatus to enhance paper and board forming qualities which overcomes the various problems of the prior art by providing vortex forming means for a plurality of tubular elements for generating controlled axial vortices in the machine direction promoting mixing of the jets of stock from the tubular elements as the jets flow into the nozzle chamber to a uniform flow field of stock. It is another object of the present invention to provide a paper forming machine headbox component for receiving a paper fiber stock and generating a rectangular jet therefrom for discharge upon a wire component moving in the machine direction. It is a further object of the present invention to provide a diffuser block for coupling a distributer to a nozzle chamber in a paper forming machine headbox for discharging paper fiber stock. It is a still further object of the present invention to provide a method of mixing jets of paper fiber stock emanating from a multiplicity of axially aligned tubes arranged as a matrix of rows and columns in a diffuser block coupled to a nozzle chamber in a paper forming machine headbox for discharging a uniform flow field of stock upon the wire component. It is yet another object of the present invention to provide an insert tube insertable in a diffuser block for coupling a distributer to a nozzle chamber in a paper forming machine headbox including vortex forming means for generating the controlled axial vortices in the machine direction to promote the mixing of paper fiber stock jets flowing into the nozzle chamber. Briefly, the present invention relates to a diffuser block according to independent claims 1; a paper forming machine headbox component according to independent claim 5; a method of mixing jets of paper fiber stock according to independent claim 15; and an insert tube according to independent claim 23. Preferred embodiments of the invention are indicated in the dependent claims. The appended claims set forth the features of the present invention with particularity. The invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings. Brief Description of the Drawings FIG. 1A shows a paper forming machine headbox component used with a diffuser block exposed to show vortex forming means provided for a plurality of the tubular elements of the diffuser block in accordance with the invention; FIG. 1B shows a cross-sectional view thereof; FIG. 1C shows an insert tube embodying vortex forming means also in accordance with the invention for insertion in the diffuser block of a conventional paper forming machine headbox component; FIG. 1D illustrates a tubular element of a step diffuser block for generating controlled axial vortices therein; FIGS. 2A and 2B show an additional embodiment of the invention wherein fins or grooves at the inlet of the tubular element may be utilized to generate vortices and converging section which can also be formed as a curved section forming an elongated portion near the inlet also generate controlled axial vortices within tubular elements; FIGS. 3A through FIG. 3H illustrate various controlled vortices configurations as positive and negative defined vortices emanating from the diffuser block to generate small scale turbulence between adjacent tubes for improved formation, and predetermined cross flows to achieve uniform stock flow in the nozzle chamber according to the invention; FIGS. 4A-4H illustrate stock flow irregularity associated with conventional paper forming machine headbox components; and FIGS. 5A-5H illustrate the use of controlled axial vortices in the paper stock jets to provide more uniform paper stock flows in the nozzle chamber approaching the slice of the paper forming machine headbox component in accordance with the invention. Description of the Preferred Embodiments Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. FIG. 1A illustrates an embodiment of a paper forming machine headbox component 10 for receiving a paper fiber stock and generating a rectangular jet therefrom for discharge upon a wire component moving in a machine direction (MD). A distributor 12 is provided for distributing the paper fiber stock flowing into the headbox component 10 in a cross-machine direction (CD) which would be generally perpendicular to the machine direction of the wire component in a conventional hydraulic headbox. It is important to note however, that the present invention may also be embodied in a conventional air-cushioned headbox as well as the hydraulic headbox. The distributor 12 is provided to supply a flow of paper fiber stock across the width of the headbox 10 in the machine direction. A nozzle chamber 14 is shown having an upper surface and a lower surface converging to form a rectangular output lip defining a slice 22 opening for the rectangular jet at opening 24. As shown in cross section in FIG. 1B, the paper fiber stock flows as indicated by the arrows in the nozzle chamber 14 to output the rectangular jet 30 upon the wire 32 partially shown in FIG. 1B. A diffuser block 16 is provided to couple the distributor 12 to the nozzle chamber 14. As illustrated in FIGS. 1A and 1B, the diffuser block 16 includes a multiplicity of individual tubular elements 18 disposed between the distributor 12 and the nozzle chamber 14, and in accordance with the invention, the presently described embodiment includes vortex forming means 20 provided for a plurality of the tubular elements 18. The vortex forming means 20 embodied herein may be provided for a subset or a plurality of the multiple tubular elements 18 for generating controlled axial vortices in the machine direction promoting mixing of the jets of the stock from the tubular elements 18, as the jets flow into the nozzle chamber to a uniform flow field of stock at the slice opening 22 for the rectangular jet 30 from the rectangular opening 24 at the slice 22. As FIG. 1B illustrates in cross section, steps 26 and 28 as might be found in a conventional diffuser block for the purpose of breaking up, defloculating or disintegrating the paper fiber stock to enhance the uniformity thereof. As already described a step diffuser block is generally provided in conventional headboxes, and the present invention may or may not require the use of such a step diffuser, but for the purpose of the described embodiment, the step diffuser is provided as shown. In accordance with an embodiment of the invention, FIG. 1C shows an insert tube 34 which is insertable in a diffuser block for coupling the distributor to the nozzle chamber in a paper forming machine headbox for discharging paper fiber stock upon a wire component moving in a machine direction. The diffuser block in conventional machines includes a multiplicity of individual tubular elements as already discussed and also provide for the ability for such inserts, typically smooth cylindrical tubular inserts for varying diameter of the individual tubular elements. However, in accordance with the invention, the inserts of the described embodiment and shown herein are typically used to generate vortices within such tubes and thus, asymmetric or non-axisymmetric surface with ridges or fins or grooves as opposed to smooth axisymmetric inner surfaces are employed. The tubular elements and the insert tubes are oriented axially in the machine direction and arranged as a matrix of rows and columns for generating multiple jets of paper fiber stock flowing into the nozzle chamber 14. The insert tube 34 includes a flat section inlet 36, i.e. a circular inlet flange with a flat surface followed by a tapered section, for receiving the stock from the distributor, which also serves as a shoulder or rim for securing the insert tube 34 in the diffuser block 16. The insert tube 34 embodiment also includes an elongated section outlet 38 connected to the flat section inlet 36 for directing the jets of the paper fiber stock through the tubular elements of the diffuser block 16 as the jets flow towards the nozzle chamber 14. Also in accordance with the invention, vortex forming means 40 are provided for the insert tube 34 for generating the controlled axial vortices in the machine direction to promote mixing of the jets from the elongated section outlet as the jets flow toward the nozzle chamber 14. Herein, the vortex forming means include an asymmetric interior surface as shown in FIG. 1C within the elongated section outlet 38 for generating the controlled axial vortex therein. More specifically, the asymmetric interior surface has a spiral pitch defining a helical path as shown within the tubular elements to generate the controlled axial vortices as the stock travels along the helical path in the elongated section outlet 38. Thus, as described, for tubes in existing headboxes, the insert tube 34 may be constructed of plastic, metal, ceramic or composite inserts with the spiral-shaped grooves, fins, ridges or guides of various form at the inner surface. One such feature is to form spiral-shaped grooves or patterns through the inner surface of the insert as shown. These inserts can be easily placed inside the tubes to generate the desired machine-direction vorticity in the tube. The inserts such as tube insert 34 may be placed inside the tubes at the distributor or manifold side of a headbox 10. The initial section of the insert at the inlet may start with a smooth surface before the vortex generating means, discussed above. Turning now to FIG. 1D, there are several ways to implement the concept described in accordance with the invention. The tubes have the feature of directing the flow in a manner to generate machine direction vorticity in a specific direction (i.e., with a specific vorticity vector sign, defined as positive (+) or negative (-) based on a right-hand rule). Thus, the sign of the secondary flow of the vorticity inside the tube is controlled by the spiral-shaped grooves, fins, ridges or guides of various form in the inner surface or such means for generating the vorticity. One such feature is to form spiral-shaped grooves or patterns through the inner surfaces of the tubes as shown in FIG. 1D, in a step diffuser box. As the fluid enters the tube from the manifold, the spiral grooves direct the flow in a recirculating manner generating or increasing the controlled vorticity in the machine direction. The grooves have increasing or decreasing pitch depending on the type of tube and the headbox design. As shown in FIG. 1D, the pitch of the spiral-shaped grooves may gradually change through the step diffuser tube as indicated by reference numerals 42, 44 and 46; note particularly the increased pitch between the groove 44 and the groove 46. The pitch of the grooves depends on the average MD velocity through the tube. If the MD velocity is very large, then the pitch may be considerably smaller than shown in the figure. As another means for generating the controlled vortices in addition or in place of the spiral grooves or fins, discrete sections of fins or ridges can be used to direct the stock in a helical pattern inside the tubes generating controlled MD vortices. The spiral-shaped grooves, fins or guides allow the fluid to gradually flow in the spiral-shaped pattern of the tube surface. With reference to FIGS. 2A and 2B, additional tube insert embodiments are shown including vortex forming means as an inclined fin or groove 56 and 70 on flat section inlets 48 and 62, i.e. circular inlet flanges, respectively. Such inclined fins or grooves facilitate the generation of the controlled axial vorticity as the stock flows toward the elongated section outlet from the distributor 12 of the headbox 10. The mechanisms of FIGS. 2A and 2B generate axial vorticity inside the tubes of the headbox wherein the flat section at the manifold or distributor side is followed by a converging curved section, i.e. a tapered section with decreasing cross-sectional area in the direction of flow, herein curved sections 50 and 64 and converging portions 52 and 66 are provided as portions of the elongated section outlet connecting to elongated sections 54 and 68 respectively, in the two embodiments of FIGS. 2A and 2B. Where the inclined fin or groove, e.g., 56 or 70, is placed on the flat section, e.g., 48 or 62, or on the flat and the converging section of the headbox tube or insert nozzle of the headbox tube, the purpose of the inclined fin or groove is to control the direction of the vortex generated inside the tube as shown wherein inlet flow 58 is directed as a vortical flow pattern indicated by reference numeral 60 in FIG. 2A; and incoming flow 72 is directed as vortical flow 74 in the embodiment of FIG. 2B. The converging sections 52 or 66 of the insert tube will accelerate the fluid and increase the angular vorticity of the fluid, consequently increasing the strength of the vortex as the fluid flows towards the straight edge 54, or 68 of the tube. FIG. 2 shows the groove 56 as residing within the elongated outlet portion of the tube as well as on the flat section 48; while FIG. 2B provides the groove or fin 70 as residing solely on the flat surface 62. It should be noted that while a single fin or groove is shown on the tubes more fins may be desirable for creating the axial vorticity within the tubes as well as for ease of placement, orientation independence and the like for fitting such tubes into the diffuser block of conventional headboxes. The curved sections 50 and 64 may be incorporated into the elongated section and disposed between the flat section 48 and converging section 52 in FIG. 2A to facilitate the axial vorticity, and as such, provide additional vortex forming means as a curved section included along a portion of the converging section near the flat section for generating the controlled axial vortices as the paper fiber stock flows in the elongated section outlet. In accordance with the invention, FIGS. 3A, 3B and 3C illustrate various methods of mixing jets of paper fiber stock emanating from a multiplicity of axially aligned tubes arranged as a matrix of rows and columns in a diffuser block coupled to a nozzle chamber in a paper forming machine headbox for discharging a uniform flow field of stock upon the wire component moving in the machine direction. As indicated, the MD components of vortices of the jets emanating from the tubes are indicated as positive defined or negative defined axial vortices in accordance with the convention of the right-hand rule and where here we use the convention that positive MD points into the surface of the figures. One could also use the convention that MD is the negative direction. Positive or negative jets refer to jets with positive or negative MD vorticity, respectively. A method in accordance with the invention provides for the generation of positive jets of paper fiber stock emanating from the diffuser block in controlled axial vortices in the machine direction for a first plurality of the tubes, the direction of the vortex being directed in a first positive-defined direction about the axes of each of the first plurality of tubes and positioning at least one of the positive jets adjacent another one of the positive jets promoting mixing as the jets flow into the nozzle chamber. This is illustrated in FIG. 3A where the first row 76 of FIG. 3A and the bottom row 80 of FIG. 3A whereby small scale turbulence is introduced between the individual positively oriented jets of rows 76 and 80 as the fluid flow emanates from the tubes promoting mixing thereof. Small scale turbulence is also introduced between the individual negatively oriented jets of row 78 in FIG. 3A. In addition to the secondary vorticity of the jets promoting mixing of the fluid emanating from the tubes, the configuration of FIG. 3A also generates shear layers which would result in cross-machine orientation of fibers and therefore, would increase the strength and other physical properties in the cross-machine direction, as indicated by shear layers in between 82 and 84 with the inner-posed layer of negative defined rows of vorticity as indicated by reference numeral 78. The jet orientation of row 78 is provided according to the method by generating negative jets of paper fiber stock emanating from the diffuser block in controlled axial vortices in the machine direction for a second plurality of tubes, the direction of each vortex being directed in a second negative-defined direction about the axes of each of said second plurality of tubes and positioning at least one of the negative jets adjacent another one of the negative jets promoting mixing as the jets flow into the nozzle chamber, herein row 78. FIG. 3A illustrates desired flows for enhancing the strength of paper or board because the shear layers in the CD provide CD strength by the alternating MD vorticity direction of the secondary flow of the jets from the tubes in each row of tubes resulting in shear layers which align more fibers in CD. An alternate concept of modifying the internal geometry of each tube in order to generate machine direction vorticity and subsequently arrange the tubes or inserts in a manner such that all the jets of each row and column of the tube bundle form the same sign of MD vorticity vector is shown in FIG. 3B. This results in strong mixing and dispersion of the fibers and fillers and therefore better uniformity in fiber and filler distribution in the sheet and enhanced formation. As shown in FIG. 3B all of the rows and columns have the same orientation indicated by reference numeral 86, namely a positively defined orientation of vorticity which results in turbulent shears as indicated by reference numeral 88 and 90. FIG. 3B shows an orientation best for mixing where uniform dispersion is a criterion having emphasis over strength; such as in tissue or light-weight paper applications. FIG. 3C illustrates alternating sign vorticity 92 and 94 throughout the rows and columns of the tube bank which provides the configuration of Case 2 discussed below in connection with FIGS. 5A-5H wherein the described counter-rotating pattern of adjacent jets provides better mixing over jets lacking vorticity discussed further below. Computer analysis for headboxes employing the configuration of FIG. 3C shows the ability to achieve more uniform flow of the paper fiber stock within the nozzle chamber making secondary jets at the slice weaker and thus noticeable improvement in uniformity. FIGS. 3D, 3E and 3F show additional patterns of the tubes for generating vortices of defined orientation, herein the matrix of rows and columns in the diffuser block being either vertical or inclined columns and introducing the vortex patterns in staggered tube arrangements. FIGS. 3D, 3E and 3F respectively provide patterns similar to those discussed above in connection with FIGS. 3A, 3B and 3C, wherein the individual secondary vorticity of the jets emanating from the tubes is provided in a staggered pattern in FIGS. 3D-3F. In FIG. 3D, the alternating MD vorticity direction of the secondary flow of the jets from the staggered tubes results in shear layers which would align more fibers in the CD. In FIG. 3E, the MD vorticity direction of the secondary flow of the jets from the staggered tubes results in enhanced fiber dispersion and mixing of the fillers in the paper fiber stock. In FIG. 3F, the alternating checkerboard MD vorticity direction of the secondary flow of the jets from the staggered tubes results in effective mixing and fiber dispersion. Additionally, FIG. 3G illustrates plural row pairs of common secondary vorticity of the jets from the tubes in a staggered pattern, herein a pair of negatively oriented rows 96 being provided above a pair of positively oriented rows 97 in a repetitive pattern. Accordingly, the alternating MD vorticity direction of the secondary flow of the jets from the staggered tubes in FIG. 3G results in shear layers which would align more fibers in the CD. From the foregoing, it is appreciated to those skilled in the art that the tubes arranged as a matrix of rows and columns in the diffuser block are provided either vertically or inclined and the rows or columns may be provided as staggered for enhancing fiber alignment. FIG. 3H similarly shows a repetitive pair vorticity pattern illustrating, e.g., negatively oriented rows 98 and positively oriented rows 99. Turning now to FIGS. 4A-4H and FIGS. 5A-5H, the effect of vorticity in the tubes of the headbox 10 on the flow is illustrated for the slice and the nozzle chamber 14. Here, analysis shows the effect of vorticity in the jets leaving the tubes in the tube bank and entering the converging zone of the headbox. The purpose of this study is to investigate the effect of vorticity at the tube bank on the free surface rectangular jet 30 at the slice 22. Two cases have been considered, case one with no vorticity and the second case with axial vorticity. These cases are shown in FIGS. 4A and 5A, respectively. The tubes in these cases, i.e., case #1 (FIGS. 4A-4H) and case #2 (FIGS. 5A-5H) are arranged in vertical columns, as shown in FIGS. 4A and 5A, respectively. The flow through the tube in case 1 has velocity component only in the machine direction. Wherein case 2, the flow in the tube has an axial vorticity imposed on the streamwise flow. The imposed secondary flows are counter-rotating axial vortices, that is the direction of rotation is clockwise and counter-clockwise in a checkerboard pattern. The cross machine direction, y, and the vertical z, components of the velocity at the tube outlet and the converging zone inlet are given respectively by: v =± A 2π Δ y cos 2π y Δ y sin 2π z Δ z and w =± A 2π Δ z cos 2π z Δ z sin 2π y Δ y These velocity components are super-imposed on the streamwise velocity component of the jet leaving the tubes as shown in FIG. 5A. In equations (1a, 1b) w and v are the vertical (Z) and transverse (CD) components of velocity, A is the magnitude of the secondary flow at the inlet, Δy and Δz are the horizontal and vertical dimensions of the tube outlet, respectively. The magnitude A, of the super-imposed secondary eddy in this study is 1.5% of the average streamwise component. The secondary velocity profile at the inlet to the converging zone is defined by a 4th order function of the y and z coordinates. The Reynolds number, based on the average inflow velocity U, the vertical height of the headbox L, and the kinematic fluid viscosity, v, is given by: Re = UL v The results of the two cases are described herein with the analysis of computational experiments. The flow characteristics at the slice for each case is given by presenting the contour plot of each of the three velocity components (see FIGS. 4C-4H and FIGS. 5C-5H). Since the direction of the secondary flows cannot be identified in the black and white reproduction of the color-coded plots, we have added arrows to the plots to distinguish the flow direction. For the first case, where the tubes are arranged in a straight vertical column, the flow is periodic with a wavelength of one-third of the width of the computation domain. The vertical component of the flow plays an important role in transferring fluid of high streamwise momentum towards the bottom wall of the headbox. Due to the periodicity of the flow, this momentum transfer varies significantly in the CD direction. Where the vertical velocity towards the wall is larger, the faster moving fluid carried from the middle of the slice to the wall forms a liquid jet. Where the vertical velocity is relatively smaller, a streamwise velocity jet of lower speed appears. These liquid jets can be seen in FIGS. 4D, 4F and 4H, where the contour plot of the three velocity components for this case are plotted along a horizontal cross-sectional plane near the lower lip of the slice. Removing the average vertical velocity from the actual vertical velocity reveals the cellular pattern of the secondary flow structure. The secondary flow patterns at the slice for each of the two cases are illustrated in the contour plots. The contour plots of the average velocity components for cases 1 and 2 at a horizontal cross-sectional plane are shown in FIGS. 4D, 4F, 4H and FIGS. 5D, 5F and 5H, respectively. The vertical velocity component contour plot in FIGS. 4G, 4E and 4C show that the flow at the slice has a periodic structure similar to that in Case 1 (i.e., FIGS. 5G, 5E and 5C). However, in this case the deviation of the actual vertical velocity from the average vertical velocity is smaller. Consequently, less fluid with high streamwise momentum is transferred towards the bottom surface of the headbox. Also, less fluid with low streamwise momentum is lifted from the lower surface towards the middle of the slice. Thus, the secondary jets at the slice for Case 2, are weaker and less noticeable. Compared to Case 1, the secondary fluid flow cells created in this case are further away from the bottom and the CD velocity components are smaller than those of the first case. In Case 1, the vertical velocity component changes sign and the variation in streamwise velocity due to the jets from the tubes remain strong up to the slice. As seen from the contour plot of the z component of velocity, there is considerable non-uniformity in the velocity. This kind of flow results in a streak pattern when manufacturing light-weight sheets. In the other case, however, the vertical component, as well as other components of the flow field, are more uniform due to the vortices which result in more effective coalescence and mixing of the jets. The counter-rotating pattern of adjacent jets, as considered in this study, is perhaps not the most effective pattern for mixing of the fluid and suspended particles in jets from adjacent tubes. A more effective method for mixing is to force the jets from the tubes to rotate in the same direction. Depending on the desired properties of the sheet, the rotational pattern of the jets should be accordingly controlled using the special tubes outlined above and the specific pattern arrangement of FIGS. 3A, 3B or 3C, as appropriate. It will be appreciated by those skilled in the art that modifications to the foregoing preferred embodiments may be made in various aspects. The present invention is set forth with particularity in the appended claims. It is deemed that the scope of the invention as defined by the claims encompasses such modifications and alterations to the preferred embodiment as would be apparent to one of ordinary skill in the art and familiar with the teachings of the present application.

Claims: A diffuser block (16) for coupling a distributer (12) to a nozzle chamber (14) in a paper forming machine headbox for discharging paper fiber stock upon a wire component (32) moving in a machine direction (MD), said diffuser block (16) comprising: a multiplicity of individual tubular elements (18) for communication of the paper fiber stock between the distributer (12) and the nozzle chamber (14), said tubular elements (18) being orientated axially in the machine direction and arranged as a matrix of rows and columns for generating multiple jets of said stock flowing into said nozzle chamber (14); characterized by vortex forming means (20) provided for a plurality of said tubular elements (18) for generating controlled axial vortices in the machine direction promoting mixing of the jets of said stock from said tubular elements (18) as said jets flow into said nozzle chamber (14) to a uniform flow field of well dispersed stock. A diffuser block as recited in claim 1 wherein said vortex forming means (20) comprise a non-axisymmetric interior surface within said tubular elements (18) for generating controlled axial vortices therein as the jets of said stock from said tubular elements (18) flow forward said nozzle chamber (14). A diffuser block as recited in claim 2 wherein said non-axisymmetric interior surface has a spiral pitch defining a helical path within said tubular elements (18) generating the controlled axial vortices as said stock travels along said helical path in said tubular elements (18). A diffuser block as recited in claim 3 wherein said spiral pitch (42, 44, 46) changes along the helical path within said non-axisymmetric interior surface of said tubular elements (18). A paper forming machine headbox component (10) for receiving a paper fiber stock and generating a rectangular jet therefrom for discharge upon a wire component (32) moving in a machine direction (MD), the headbox component (10) comprising: a distributer (12) for distributing stock flowing into the headbox component (10) in a cross-machine direction (CD), generally perpendicular to the machine direction of the wire component (32), and supplying a flow of said stock across the width of the headbox component (10) in the machine direction; a nozzle chamber (14) having an upper surface and a lower surface converging to form a rectangular outlet lip defining a slice opening (22) for the rectangular jet; and a diffuser block (16) according to one of claims 1 to 4. A paper forming machine headbox component (10) as recited in claim 5 wherein said tubular elements comprise a circular inlet flange (48) with a flat surface followed by a tapered section (52) for receiving said stock from said distributer (12) and an elongated section outlet (54) for directing the jet of said stock from said tubular elements as said jets flow into said nozzle chamber (14). A paper forming machine headbox component as recited in claim 6 wherein said tubular elements comprise an insert tube (34) insertable in said diffuser block (16) for receiving said stock from said distributer (12). A paper forming machine headbox component as recited in claim 6 wherein said vortex forming means comprises an inclined fin (56) on said circular inlet flange (48) generating the controlled axial vortices as said stock flows toward said elongated section outlet (54). A paper forming machine headbox component as recited in claim 8 wherein said inclined fin on said circular inlet flange (48) extends partially along said elongated section outlet (54) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (54). A paper forming machine headbox component as recited in claim 6 wherein said vortex forming means comprises an inclined groove (70) on said circular inlet flange (62) for generating the controlled axial vortices as said stock flows toward said elongated section outlet (68). A paper forming machine headbox component as recited in claim 10 wherein said inclined groove (70) on said circular inlet flange (62) extends partially along said elongated section outlet (68) for generating the controlled axial vortices (74) as said stock flows toward said elongated section outlet (68). A paper forming machine headbox component as recited in claim 6 wherein said vortex forming means comprise a plurality of inclined fins (56) and/or grooves (70) on said circular inlet flange for generating the controlled axial vortices as said stock flows toward said elongated section outlet (54). A paper forming machine headbox component as recited in claim 6 wherein said vortex forming means comprises a converging section (52) and wherein said elongated section outlet of said tubular elements further comprises a straight section (54) with said converging section (52) intermediate said circular inlet flange (48) and said straight section (54), said converging section (52) of said tubular elements generating the controlled axial vortices (60) as said stock flows from said converging section (52) to said straight section (54) of said elongated section outlet directing the jets of said stock from said tubular elements as said jets flow into said nozzle chamber. A paper forming machine headbox component as recited in claim 13 wherein said vortex forming means further comprise one or more inclined fins (56) and/or grooves (70) on said circular inlet flange (48; 62) for generating the controlled axial vortices (60; 74) as said stock flows toward said elongated section outlet (54; 68). A method of mixing jets of paper fiber stock emanating from a multiplicity of axially aligned tubes (18) arranged as a matrix of rows and columns in a diffuser block (16) coupled to a nozzle chamber (14) in a paper forming machine headbox (10) for discharging a uniform flow field of stock upon a wire component (32) moving in a machine direction (MD), the method characterized by the steps of: generating positive jets of paper fiber stock emanating from the diffuser block (16) in controlled axial vortices in the machine direction for a first plurality of said tubes (18), the direction of each vortex being directed in a first positive-defined direction about the axes of each of said first plurality of said tubes; and positioning at least one of said positive jets adjacent another one of said positive jets promoting mixing as said jets flow into the nozzle chamber (14). A method as recited in claim 15 wherein said positioning step positions said positive jets of each of said first plurality of said tubes (18) adjacent one another in the diffuser block (16) generating small scale turbulent flows in the nozzle chamber (14) as the jets emanate from said tubes (18) promoting mixing of said stock in the nozzle chamber (14). A method as recited in Claim 15 wherein said positioning step positions said positive jets along at least one of the rows (97) of the matrix in the diffuser block (16) generating a first secondary flow in the nozzle chamber in a cross-machine direction (CD), generally perpendicular to the machine direction (MD), as the row (97) of positive jets emanate from the tubes (18). A method as recited in claim 15 comprising the steps of: generating negative jets of paper fiber stock emanating from the diffuser block (16) in controlled axial vortices in the machine direction (MD) for a second plurality of said tubes (18), the direction of each vortex being directed in a second negative-defined direction about the axes of each of said second plurality of said tubes (18); and positioning at least one of said negative jets adjacent another one of said negative jets promoting mixing as said jets flow into the nozzle chamber (14). A method as recited in claim 18 wherein said positioning step positions said positive jets of said first plurality of said tubes (18) adjacent said negative jets of said second plurality of said tubes (18) in the diffuser block (16) promoting uniform flow of said stock emanating from said tubes into the nozzle chamber (14). A method as recited in claim 18 wherein said positioning step position said positive jets along at least one of the rows (97) of the matrix in the diffuser block (16) generating a first secondary flow in the nozzle chamber (14) in a cross-machine direction (CD), generally perpendicular to the machine direction (MD), as the row (97) of positive jets emanate from the tubes. A method as recited in claim 20 wherein said positioning step positions said negative jets along at least another one of the rows (96) of the matrix in the diffuser block (16) generating a second secondary flow in the nozzle chamber in an opposite cross-machine direction as the row (96) of negative jets emanate from the tubes. A method as recited in claim 21 wherein said positioning step positions the row (96) of negative jets adjacent the row (97) of positive jets generating opposing secondary flows in the cross-machine direction as the rows of jets emanate from the tubes (18) providing shear layers aligning the paper fibers of said stock in the cross-machine direction. An insert tube (34) insertable in a diffuser block (16) for coupling a distributer (12) to a nozzle chamber (14) in a paper forming machine headbox (10) for discharging paper fiber stock upon a wire component (32) moving in a machine direction (MD), the diffuser block (14) having a multiplicity of individual tubular elements (18) for communication of the paper fiber stock between the distributer (12) and the nozzle chamber (14), the tubular elements (18) being oriented axially in the machine direction and arranged as a matrix of rows and columns for generating multiple jets of said stock flowing into the nozzle chamber (14), characterized in that the insert tube (34) comprises: a circular inlet flange (36) for receiving the stock from the distributer (12); an elongated section outlet (38) connected to said circular inlet flange (36) for directing the jets of said stock through the tubular elements (18) of the diffuser block (16) as the jets flow toward the nozzle chamber (14); and vortex forming means (40) provided for said insert tube (34) for generating controlled axial vortices in the machine direction promoting mixing of the jets from said elongated section outlet (38) as the jets flow toward the nozzle chamber (14). An insert tube as recited in claim 23 wherein said vortex forming means comprise a non-axisymmetric interior surface within said elongated section outlet (38) for generating controlled axial vortices therein as the jets flow toward the nozzle chamber (14). An insert tube as recited in claim 24 wherein said interior surface has a spiral pitch defining a helical path within said tubular elements (18) generating the controlled axial vortices as the stock travels along said helical path in said elongated section outlet (38). An insert tube as recited in claim 25 wherein said spiral pitch (42, 44, 46) changes along said helical path within said interior surface of said elongated section outlet. An insert tube as recited in claim 23 wherein said vortex forming means comprises an inclined fin (56) on said circular inlet flange (48) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (52, 54). An insert tube as recited in claim 27 wherein said inclined fin (56) on said circular inlet flange (48) extends partially along said elongated section outlet (52, 54) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (52, 54). An insert tube as recited in claim 23 wherein said vortex forming means comprises an inclined groove (70) on said circular inlet flange (62) for generating the controlled axial vortices (74) as said stock flows toward said elongated section outlet (66, 68). An insert tube as recited in claim 29 wherein said inclined groove (56) on said circular inlet flange (48) extends partially along said elongated section outlet (52, 54) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (52, 54). An insert tube as recited in claim 23 wherein said vortex forming means comprise a plurality of inclined fins (56) and/or grooves (70) on said circular inlet flange (48) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (52, 54). An insert tube as recited in claim 23 wherein said vortex forming means comprises a converging section (52) and wherein said elongated section outlet (52, 54) of said tubular elements (18) further comprises a straight section (54) with said converging section (52) intermediate said circular inlet flange (48) and said straight section (54), said converging section (52) of said tubular elements (18) generating the controlled axial vortices (60) as said stock flows from said converging section (52) to said straight section (54) of said elongated section outlet (52, 54) directing the jets of said stock from said tubular elements (18) as said jets flow into said nozzle chamber (14). An insert tube as recited in claim 32 wherein said vortex forming means further comprises a tapered section (50) with decreasing cross-sectional area in the direction of flow included along a portion of said converging section (52) near said circular inlet flange (48) for generating the controlled axial vortices (60) as said stock flows in said elongated section outlet (52, 54). An insert tube as recited in claim 32 wherein said vortex forming means further comprises one or more inclined fins (56) and/or grooves (70) on said circular inlet flange (48) for generating the controlled axial vortices (60) as said stock flows toward said elongated section outlet (52, 54).

Inventors: AIDUN CYRUS K, AIDUN, CYRUS K.
Applicants: PAPER SCIENCE & TECH INST INC, INSTITUTE OF PAPER SCIENCE AND TECHNOLOGY, INC.
Application year: 1996
Main classification: D21F1
IPC codes: D21F1