Abstract
Compact-siro spinning is a new kind of spinning technique that combines compact spinning and siro spinning, and is widely put into practice. In this paper, a simulation and characterization of the flow field in the condensing zone are made by computational fluid dynamics software. It mainly includes the distribution rules of static pressure and velocity. According to the relative references and factory condition, several orthogonal and control experiments have been done. Through testing the strength, evenness and hairiness properties and analyzing and comparing the results with the data of three different suction slots, we select the compact-siro spinning suction device whose yarn has the best overall performance. The theory analysis gives foundation and explanation for the experiment, and also provides a theoretical basis for optimizing the properties of compact-siro yarn in production practice.
Compact-siro spinning 1 is a new kind of spinning technique that combines compact spinning 2 and siro spinning, 3 and is widely put into practice. Siro spinning parallelly feeds two rovings with a certain pitch into the same drafting zone of the spinning frame. Then the two rovings are twisted on one spindle after the draft parallelly. The structure of siro spun yarn is similar to plied yarn, with good strength, hairiness 4 and evenness properties. However, it has disadvantage of worse twist stability and indistinctive plied yarn characteristic. As previous research shows, the compact spinning system 5 can decrease the twisting triangle zone and make the fiber tangled. Therefore, considering the advantage of the two spinning techniques, compact-siro spinning can give the yarn a stable and obvious twisting structure under negative pressure. 6 Yang 7 studied an advanced spinning technology that combined compact spinning and siro-spinning in a traditional ring spinning machine. Wang 8 studied the spinning mechanism and the yarn structure of compact-siro spinning. Wang 9 made a systematic analysis for the causes of the increase in compact-siro spun yarn strength, evenness and the decrease in hairiness in theory. Lu 10 studied influences of airflow on fiber movement on compact-siro spinning with a lattice apron. If the technique is mature enough, it will result in savings in cost, energy and labor force, leading to better market prospects. In this paper, the numerical simulation of airflow in the condensing zone of compact-siro spinning is studied, and the impact factors of the agglomeration effects are analyzed. The comprehensive performances of three different suction slots are compared, and the compact-siro spinning suction device whose yarn has the best overall performance is selected.
Fluid dynamics simulation
Compact equipment of compact-siro spinning
Compact-siro spinning is based on the ring spinning machine in order to achieve a combination of compact spinning and siro spinning technology. Two rovings are fed parallel into drafting mechanism at a constant pitch through the double toroidal of the ring spinning frame, while being drawn by the parallel state. The two strands get into the air, gathering from the front roller nip zone. There is a lattice apron on the outside surface of the shaped tube. The lattice apron is driven by the friction of the output pressure top-roller. It opens the suction slots on the spinning surface of the shaped tube. Inside the shaped tube is in negative pressure. The two strands output from the front roller are adsorbed on the surface of the lattice apron at the position of the corresponding suction slots. The two strands go forward with the lattice apron, controlled by the agglomeration at the same time, and are output from the output pressure top-roller jaw. The two fiber strands obtain a more compact structure after gathering and after initially being twisted. The two fiber strands combine together, then are twisted strongly, and obtain a similar strands structure. The compact equipment of compact-siro spinning is shown in Figure 1(a). The type of compact-siro spinning frame is a Toyota RX240. In this paper, the three suction slots mainly studied are re-equipped based on this frame.
Equipment of compact-siro spinning: (a) compact equipment of compact-siro spinning; (b) machine drawing of compact equipment.
Three-dimensional geometric model
Before modeling, every part of the compact equipment should be measured. Here the measurement is the same, except the suction slot. Figure 1(b) shows an assembly drawing of every part in the condensing zone. The special curved surface structure of the shaped tube makes great impact on the distribution of airflow in the condensing zone. Any treatment will not be done in order to make the simulation more accurate.
Much theoretical analysis and experimental verification has indicated that, for compact-siro spinning, the outlet spacing of the suction slots D is an important parameter. The principle is the same with the roving spacing in siro spinning. The outlet spacing of the suction slots D affects the size of the twisting triangle, which is formed by the two strands output from the control roller nip to a focal point. Yang's
7
self-made compact-siro spinning system belongs to an air-compact spinning system, and combines the siro spun technology; the process of spinning is stably reliable. By optimized design to the compact organ, she develops 15 kinds of suction slots of different structures for compact-siro spinning installation. The results of the above-mentioned suction slots are evaluated by means of fuzzy mathematics, and finally the result is that a suction pipe whose suction notches are of symmetrical arrangement and tilted at an angle of 6° with yarn exportation spacing of suction notches at 6 mm and a small caliber size best. In the V-shaped suction systems, the agglomeration effect of the two fiber strands has been enhanced, so that the whole agglomeration effect of yarn is better than that of a straight parallel suction system.
10
In this paper, we aim to compare the V-shaped suction systems, which have different outlet spacing, with the oblique parallel suction systems. Based on the literature and existing models, values are obtained as follows. The shape and dimension of the suction slot can be seen in Figure 2: suction slots 1 and 3 are of symmetric configuration; suction slot 2 is of parallel configuration; L1 = L2 = L3 = 22 mm of suction slots 1, 2 and 3; R1 = R2 = R3 = 0.75 mm; D1 = 4.5 mm, D2 = D3 = 6.5 mm. The inclination angle γ is 6°.
Shape and dimension of the suction slot.
The excess parts, which have nothing to do with the condensing zone, should be eliminated before latticing. This follows the design of the three-dimensional geometric model by GAMBIT, which is a kind of mechanical design software, as shown in Figure 3(a). When calculating, we take point O shown in Figure 3(b) as the origin of coordinates, the opposite delivery direction of the strand in the condensing zone is the X-axis and the width direction of the slot is the Y-axis. The Z-axis is perpendicular to the direction of strand movement.
Schematic diagram of the structure of the compact zone: (a) schematic diagram of the flow field; (b) diagram of the direction of the velocity components.
Numerical simulation
Latticing
Considering the condensing zone is a composite structure of regular and irregular zones, the latticing should be combined with structural and non-structural lattices. 11 The spacing of lattices is based on Interval Size, and considering the time of simulation and the computer equipment, it is chosen ass 0.35. After the three-dimensional geometric model is latticed, the lattice number of the suction slot 1 model is 1351337, the lattice number of the suction slot 2 model is 1347852 and the lattice number of the suction slot 3 model is 1353459. Finally, the document is exported in MASH format from GAMBIT.
Importing, checking and modifying the lattice
The lattice is tested and refined, after importing the document MESH of GAMBIT to Fluent, which can solve a three-dimensional turbulent fluid-flow problem in a mixing elbow. Because the simulation mainly focuses on the condensing zone, so this zone is the aim which is refined. Automatic control is used when refining by Fluent. The range of the X-axis [0, 19], Y-axis [–6, 12] and Z-axis [0, 3] is set in REGION. 12 After the three-dimensional geometric model is refined, the lattice number of the suction slot 1 model is 1935480, the lattice number of the suction slot 2 model is 1941039 and the lattice number of the suction slot 3 model is 1947213.
Boundary condition setting
After finishing the geometry modeling and latticing, the boundary conditions need to be set. As shown in Figure 4, the faces 1, 2, 3, 4 and 5 are specified as the pressure entrance faces, face negative pressure area of shaped tube as the pressure exit faces, and the others as wall.
13
Static pressure contour distribution at Z = 1 mm: (a) suction slot 1; (b) suction slot 2; (c) suction slot 3.
Solved model
In this paper, the fiber strands and lattice apron cannot be considered to affect the flow field in the condensing zone, in order to simplify the calculation. Because of the small negative values actually used, suppose the airflow in the condensing zone is sticky and incompressible without considering the heat exchange and the airflow is recognized to be enthalpy. So when calculating, the standard k-ε model is applied. 14 Depending on the airflow velocity below 50 m/s tested in the spinning mill, the separating solver and implicit scheme are determined.
Boundary conditions and solving the parameter setting
In the simulation, the pressure entrance is in an atmosphere condition and the exit is in a negative pressure P = 2.35 kPa. To simplify the analysis of the problem, movement of the fibers in the flow field is considered a one-way coupling problem. That is, first calculate the flow field, and then determine the force acting on the fibers, regardless of the counterproductive action of fiber to the flow field. By monitoring the residuals and the import and export quality of airflow, adjust the under relaxation factor to a reasonable value to speed up convergence. When the convergence accuracy is 10−3, change the discrete scheme from a first-order to a second-order upwind scheme and the convergence accuracy to 10−4. 15 Continue the operation above until it reaches the convergence accuracy we need.
Results and analysis
Static pressure distribution
In the calculations of Fluent software, extract the plane of Z = 1 mm and import into Tecplot10.0 software for analysis. The static pressure distribution in the condensing zone is clear and is intuitively observed. Figure 4(a)–(c), respectively, show the static pressure distribution in the XY plane at Z = 1 mm of suction slots 1, 2 and 3. The unit of length in Tecplot is meters (m).
It can be seen from the static pressure contour distribution in the XY plane at Z = 1 mm of suction slots 1, 2 and 3, that the static pressure in the Y direction is reduced from the center of the two suction slots to the edge. The static pressure distribution is based on the centerline (L) between the double suction slots symmetrical and along the X-axis direction first increases, then decreases. Compared with the single suction slot, 16 it can be seen that because of the interaction and configuration of the two suction slots, the static pressure distribution of the two suction slots is not based on the centerline of the two suction slots (M1 and M2) symmetrically. Observing each slot in the figure, it can be found that, close to the centerline L (S1, S2 side) between the double suction slots, the static pressure at the same distance y changes more rapidly and the gradient is larger. Instead, away from the center line L (S3, S4 side) between the double suction slots, the static pressure at the same distance y changes more slowly and the gradient is less. The maximum of the static pressure distributes in sides S3 and S4. Compared (a) and (c) in Figure 4, suction slots 1 and 3 are of symmetric configuration. Where D1 = 4.5 mm and D3 = 6.5 mm, the static pressure of suction slot 1 is higher. Comparing (b) with (c) in Figure 4, we find that the decreasing distribution of suction slot 2 is by two parallel slot centerlines (M1 and M2), with a slant of 6°, and that of suction slot 2 is by two symmetrical slot centerlines (M1 and M2) with a slant of 6°.
Peak values of static pressure
Velocity distribution
Velocity distribution of airflow in the Y-axis V y
The direction of V
y
mainly affects the movement and entanglement of the fiber strands in the transverse direction.
17
Figure 5(a)–(c), respectively, show slots 1, 2 and 3, wherein the dotted line represents the boundary line of the slot. Comparing Figure 5(a)–(c), in the suction, it is shown that the distribution of V
y
is symmetrically based on the centerline (L) between the double suction slots and that it is equal and opposite. For the single suction slot, the distribution of V
y
on the both sides of the centerline (L) between the double suction slots, along the X-axis direction, first increases, then decreases. The compact effect of fibers in the condensing zone expresses a weak distribution at both ends and strong distribution in the middle, which also matches the distribution law of static pressure. Compared with a single suction slot, it can be seen that by the interaction of double suction slots, the distribution of V
y
is not based on the slot centerline (M1 and M2) symmetrically. Close to the centerline L (S1, S2 side) between the double suction slots, V
y
at the same distance y changes more rapidly and the gradient is larger. Instead, away from the center line L (S3, S4 side) between the double suction slots, V
y
at the same distance y changes more slowly and the gradient is less. The maximum of V
y
is more distributed in side S3 and S4. So when fibers are going into the condensing zone along two sides, without considering any other acting force, the fibers will compact and deliver in sides S1 and S2 under the air force. The distribution of V
y
is not based on the slot centerline (M1 and M2) symmetrically. The larger V
y
in sides S3 and S4 makes the fibers compact and delivers to sides S1 and S2. Comparing (a) and (c) in Figure 5, suction slots 1 and 3 are symmetric configuration. Where D1 = 4.5 mm, D3 = 6.5 mm, V
y
of suction slot 1 is higher. The difference of V
y
distribution of suction slots 1 and 3 may affect the condensing effect. As fibers enter the condensing zone, they need a larger compact airflow to condense the loose strand, so the condensing effect of suction slot 1 is better. Comparing (b) with (c) in Figure 5, we can find that the decreasing distribution of suction slot 2 is by two parallel slot centerlines (M1 and M2) with a slant of 6° and that of suction slot 2 is by two symmetrical slot centerlines (M1 and M2) with a slant of 6°.
Distribution of V
y
at Z = 1mm: (a) suction slot 1; (b) suction slot 2; (c) suction slot 3.
Peak values of V y
In order to study the distribution of V
y
at a specific location X on the three suction slots, scatter plots of V
y
at X = 18, 10 and 2 mm from the entrance to the exit of condensing zone are made, as shown in Figure 6.
Distribution of V
y
at a specific location X at Z = 1 mm: (a) X = 2 mm; (b) X = 10 mm; (c) X = 18 mm.
Comparing the (a), (b) and (c) in Figure 6, it is found that V y at X = 10 mm is higher than at X = 2 mm, 18 mm, so the condensation of fiber strands occurs mainly at the center of the condensing zone entrance. In Figures 6(a)–(c), the first wave crest and trough on the V y curve of the three suction slots are basically at the same position in the Y-axis, but from the second ones, there are some differences. Combined with value of V y at a specific location X on the three suction slots, at X = 2 mm the second wave crest of suction slot 1 with D1 = 4.5 mm occurs first, where the distance between the double suction slots is the least. At X = 10 mm, the second wave crest of suction slot 3 with D3 = 6.5 mm occurs last, where the distance between the double suction slots is the largest. At this specific location, the three suction slots have large differences in the distance between the double suction slots and the airflow velocity difference. When the distance between the double suction slots is less, the percentage speed variation of V y is larger, the airflow velocity difference is larger and the condensing effect of deformed slot is better. At X = 18 mm, the distance between the double suction slots of suction slot 2 is the least, while that of suction slot 3 is the largest. The percentage speed variation of V y in suction slot 2 is the largest, while in suction slot 3 it is the smallest. As fibers enter the condensing zone, they need larger compact airflow to condense the loose strand, so suction slot 1 is much better.
The velocity distribution of airflow in the Z-axis V z
The airflow velocity in the Z-axis direction mainly affects the adsorption of the condensing zone on the fiber strands. In the actual condensing process, there needs to be a larger V
z
to absorb the unstable strands when entering the condensing zone by a relatively stable negative pressure. So according to the analysis of V
z
distribution in the condensing zone, the deformed slot may make a better effect on the strand. Figures 7(a)–(c), respectively, show the velocity distribution in the Z-axis at Z = 1 mm of suction slots 1, 2 and 3, wherein the dotted line represents the boundary line of the slot. Comparing Figures 7(a)–(c), it can be seen that the velocity in the Z-axis and symmetrically based on the centerline (L) between the double suction slots is equal and opposite. For the single slot of the suction slot, velocity in the Z-axis is reduced from the center of the suction slot to the edge. This is based on slot centerline (M1 and M2) symmetrically. The velocities are mostly negative, which attracts the strand to the surface of the lattice apron and makes the largest adsorption at the center of the slot. This is a benefit for compacting and delivering the strand with a proper adsorption.
Distribution of V
z
at Z = 1 mm: (a) suction slot 1; (b) suction slot 2; (c) suction slot 3.
In order to study the distribution of V
z
at a specific location X on the three suction slots, scatter plots of V
z
at X = 18, 10, 2 mm from the entrance to the exit of condensing zone are made, as shown in Figure 8.
Distribution of V
z
at a specific location X at Z = 1 mm: (a) X = 2 mm; (b) X = 10 mm; (c) X = 18 mm.
Comparing the (a), (b) and (c) in Figure 8, it is found that V z at X = 10 mm is higher than that at X = 2 mm, 18 mm, so the condensation of fiber strands occurs mainly at the center of the condensing zone entrance. In Figures 8(a)–(c), the first wave crest and trough on the V z curve of the three suction slots are basically at the same position in the Y-axis, but from the second ones, there are some differences. The second wave crest of suction slot 3 occurs later than at suction slots 1 and 2. Combined with value of V z at a specific location X on the three suction slots, at X = 10 mm the three suction slots have large differences in the distance between the double suction slots and the airflow velocity difference. The percentage speed variation of V z in suction slot 1 is the largest, which in suction slot 2 is the smallest. So according to the analysis of V z distribution in the condensing zone, suction slot 1 may have a better condensing effect on the strand.
Experimental details
Experiment design
Values of V y at a specific location X/(m/s).
Peak values of V z .
Values of V z at a specific location X/(m/s).
Spinning process design
Yarn morphology
The specimen was tested using a DIGITAL MICROSCOPE KH-7700. Before testing, the specimen should be humidified for 48 hours in a constant temperature and humidity laboratory with a temperature of (20 ± 10)℃ and a relative humidity of (65 ± 15)%. The magnification is 250 times. The main observations are (a) not untwisted yarn; (b) partly untwisted yarn; and (c) untwisted yarn. The three morphological features are as shown in Figure 9. It can be seen that the appearance of yarn structure of suction slot 1 is the most compact, while that of suction slot 2 is the least compact.
The morphological features of yarn: (a) not untwisted yarn for suction slot 1; (b) partly untwisted yarn for suction slot 1; (c) untwisted yarn for suction slot 1; (d) not untwisted yarn for suction slot 2; (e) partly untwisted yarn for suction slot 2; (f) untwisted yarn for suction slot 2; (g) not untwisted yarn for suction slot 3; (h) partly untwisted yarn for suction slot 3; (i) untwisted yarn for suction slot 3.
Yarn hairiness properties
The specimen was tested using a YG172 yarn hairiness tester according to FZ/T1086-2000. 18 Before testing, the specimen should be humidified for 48 hours in a constant temperature and humidity laboratory with temperature of (20 ± 2)℃ and relative humidity of (65 ± 3)%. The yarn tension of different counts is adjusted by (0.5 ± 0.1) cN/tex. The tested length is 10 m and the tested speed is 30 m/min. The tested specimens are six bobbins from different spindles that have 10 replications per bobbin when tested. The mean ± sd (standard deviation) is mainly decided by the test equipment and environment.
Results of yarn hairiness properties
Yarn tensile properties
The specimen was tested on a YG068M automatic yarn tensor according to GB/T398-2008, 19 GB/T5324-2009 20 and so on. Before testing, the specimen should be humidified for 48 hours in the constant temperature and humidity laboratory with a temperature of (20 ± 10)℃ and a relative humidity of (65 ± 15)%. The yarn pre-tension of different counts is adjusted by (0.5 ± 0.1) cN/tex. The clamp distance is 0.5 m. The tested speed is 500 mm/min. The tested specimens are six bobbins from different spindles that have 10 replications per bobbin when tested. The mean ± sd (standard deviation) is mainly decided by the test equipment and environment.
Results of yarn tensile properties
Conclusion
In the actual condensing process, as fibers enter the condensing zone, the strand is loose and its movement is unstable, they need larger V y to be condensed, and larger V z to be adsorbed by a relatively stable negative pressure. According to the analysis of the condensing zone, the static pressure difference of suction slot 1 is the largest; the static pressure in the Y direction is reduced from the center of the two suction slots to the edge; the velocity in the Y-axis, Z-axis and symmetrically based on the centerline (L) between the double suction slots is equal and opposite. By comparing yarn evenness, yarn hairiness and yarn strong indicators of the different suction slots, it is found that the yarn of suction slot 1 has the best performance. So, a suction slot with symmetric configuration and smaller D, such as suction slot 1, may make have a better effect on the strand.
Footnotes
Acknowledgments
The authors are grateful to Ningbo Dechang Textile Machinery Limited Company for providing the compact equipment for the compact-siro spinning with lattice apron used in this paper.
Funding
This work was supported by the Shanghai Natural Science Foundation (No 13ZR1400900); the Keygrant Project of the Chinese Ministry of Education (No 113027A); and the Fundamental Research Funds for the Central University (No 14D110109).