Experimental investigation and simulation of the performance of a pre-twister based on the jet vortex field used for the ring spinning frame

Abstract

The ring spun technique has always held a domain position in yarn spinning, thanks to its simple and robust structure, ease of maintenance and good quality in the product. A pre-twist device is arranged between the front rollers and an additional pair of rollers, which are named the holding rollers. In this paper, the simulation and characterization of the airflow field, as well as the fiber movement and deformation, in this pre-twisting zone are carried out by computational fluid dynamics software. The airflow simulation results show that there is a core region formed around the roving in the pre-twister, with a radius of about 1.55 mm. In the core region, the tangential speed is the major velocity component ranging from 0–70 m/s and the axial velocity is the minor one ranging from –13 to about 4.1 m/s. The air velocity inside the whole tube is from 462 to 0 m/s. The two-way coupled simulation verified that the airflow can drive the fibers to wind on the roving. The experiments on the surface morphology verified the wrapping phenomena in the simulation. Due to the employment of the pre-twister, the yarn properties got a little worse, with declines of 6% in strength and 10% in evenness, respectively. Comparing with the Uster Statistics for 2018, these declines are acceptable. Furthermore, it is shown that with this kind of pre-twister, the spinning speed can increase by 27.13% with an acceptable decrease in cotton yarn property (decreases of 13% in strength and 8% in evenness).

Keywords

pre-twisting ring spinning fluid simulation fiber kinematics numerical simulation two-way coupled simulation

Ring spinning was invented in 1828 and is the major yarn producing method all around the world, with its advantages of simple machine structure, better fiber adaptability and steady yarn quality.¹ However, it still has its own disadvantages, such as yarn hairiness and relatively lower manufacturing speed.^2,3 Researchers has done a great deal of work on improving these issues in the ring spinning method, including inventing new forms of twisting and winding devices, or relative accessories.^4–8 Furthermore, as one of the new forms of spinning devices based on ring spinning, sirospinning has been used in industry manufacturing since 1998. Based on that, the Solospun was put forward by the Commonwealth Scientific and Industrial Research Organisation (CSIRO)² later. To reduce the hairiness of yarns produced by ring spinning, the compact spinning system has been applied widely for ring spun yarn manufacturing, such as the compact spinning machines K46 and K42 from Rieter, the EliTe CompactSet from Suessen, the ZinserImpact 72XL from Zinser and so on. New spinning methods, such as rotor type open end spinning, air-jet spinning and vortex spinning, all have their strength and limits in fiber adaptation and product quality, which makes ring spinning irreplaceable. Therefore, improving the yield of ring spinning still has significance. During the spinning process, the main end-breakages happen at the twist triangle and are mostly the result of tension variations. With the speed up of production, the tension and its variation on the roving increase and it also calls for a higher breaking strength on the roving. Adding twists to the roving is a way to help the roving to stand a larger tension, while more twists on the roving means lower producing efficiency and it needs more drafting energy in spinning. So, improving the roving strength during spinning without adding more twists in the roving process could be a better choice.

For issues referring to airflow and fiber mechanical properties during spinning, there have been studies focusing on the airflow characteristics and yarn or fiber mechanical theory. New spinning methods are the main research focus, including the channel of rotor spinning,^9–12 air-jet spinning,^13,14 vortex spinning^15–19 and also other studies on compact ring spinning^20,21 and the application of an air-jet nozzle in siro-spinning.^22,23 Researches on the employment of an air-jet on ring spinning has also been put forward, while the yarn properties still need improvement.²⁴ These studies all emphasize the importance of airflow on the fibers during the yarn formation.

Interactions between fluids (air and solutions) and fibers are common in the textile industry; the fluid and fiber mechanical processes should be calculated with laws and equations of different physical disciplines. In general, there are two options to solve these problems in simulation, which are the one-way coupling solution and the two-way coupling solution. The former method can save more computational resources and time, while the latter one can give more accurate results and guarantee energy conservation at the interface.²⁵ We would like to get as much fiber motion as possible and, therefore, two-way coupling is selected in this study. While three-dimensional simulation with this method has not been used before, this study makes sense in understanding the fiber deformation.

This paper reports the results of research done using a pre-twisting device based on the jet vortex field, which helps to twist the fiber strand before entering the twisting section, consisting of a ring and a traveler, so that end-breakages during the spinning process can be reduced and the spinning process efficiency can be improved. Previously, airflow simulation and bead-chain model fiber have been used in pre-twister analysis, while in the bead-chain model, only the tangential component of the air velocity has been considered with the interpolation method.^26,27 In this study, the airflow feature and the fiber kinematics in the device have been simulated simultaneously with the help of two-way coupled simulation on Ansys Workbench. In addition, corresponding experiments have also been carried out for verification.

Fluid dynamics simulation

Pre-twist ring spinning device

The pre-twist ring spun device is designed to achieve additional twisting after the fiber leaves the front rollers and has not yet reached the yarn guide (Figures 1(a) and (b)). In this figure, the roving was firstly drafted by three pairs of rollers, which are marked as the front rollers, middle rollers and the back rollers. After drafting by the rollers, the fibers inside the roving are more orientated. Then the roving goes into the pre-twisting device, with one end held by the front rollers and the other end held by the fourth pair of rollers, which are named the holding rollers. The pre-twister is mounted in the fiber strand path and is tangent to the front roller nip.

Figure 1.

Pre-twisting device in ring spinning: (a) diagram of the pre-twisting devices; (b) pre-twisting devices in experiment.²⁷

In the pre-twister, four sliding pressure jets result in a rotating vortex flow field, which blows the free ends of the fibers to wrap on the fiber strand surface. With this process, the roving is supposed to attain a sheath–core structure before its core fibers get twisted within the ring and traveler region. Since the breakages arise at the spinning triangle, this pre-twisting device is designed to improve the strength of the fiber strand in this area by inserting additional twists on the surface of the roving.

Three-dimensional geometric model

The cross-section of the pre-twister and its flow area are separately shown in Figures 2(a) and (b). As the fibers are affected by airflow in the highlighted region shown in Figure 2(b), in this study, the three-dimensional model is reduced to this region (5.5 mm ≦ y ≦ 8.0 mm). y is in the height direction in Figure 2(b).

Figure 2.

Pre-twister description: (a) cross-section of the pre-twister; (b) flow area in the pre-twisting device.

According to the changes in the roving cross-section shape, the pre-twisting process can be divided into three steps. Firstly, when the roving is going into the pre-twister, the upper end is held by the front rollers and its cross-section has a flat ribbon shape (Figure 3). Secondly, when the roving gets inside the pre-twister, its width gradually shrinks and it moves forward to the outlet, and the free ends of the fibers are driven by the vortex in the tube and wrap on the surface of the roving. Finally, when the roving is leaving the pre-twister, its cross-section (the lower end of the roving) becomes round.

Figure 3.

Geometry model.

Figure 4.

Mesh for the geometry model.

Numerical simulation

Numerical simulations are useful to obtain detailed information on the interaction process and engineering design. In this study, a two-way coupled simulation was proposed to calculate the motion of the fiber in these specific flows with a free surface. Because of the circular jet and vortex flow inside the pre-twister, the airflow was solved by the Realizable κ-ε model in the Fluent analysis system. The motion of the fiber was determined by the transient structural analysis system. Dynamic mesh setting is needed for fluid simulation in a two-way coupled system. The generated mesh is displayed in Figure 4. The node number of the fiber is 39,207, and the node number for the fluid domain is 116,873.

The boundary condition is shown in Figure 3. The inlet is set as positive pressure of 250 kPa based on the previous research on fluid simulation.²⁷ The outlet is set at a pressure of 100 kPa. The walls on the fiber are set as fluid–solid-interface, and the walls inside the fluid area, which connect with the solid fiber, are also responsible for transferring data between the fluid and solid. The SIMPLE algorithm is applied in the simulation. The residuals on parameters in this work are set as 10⁻³.

Fiber model

Fibers are particles of large aspect ratio length to diameter, with flexibility and elasticity, and they can present complex motion and deformation in the fluid. Our pre-twister tube only aims at adding twists on the roving, with a maximum height of 3.5 mm. In our work, we have discussed the fiber movement in two cases for a better understanding. Two fiber models of 1 mm (case 1) and 3 mm (case 2) were built to represent the fiber deformation in the fluid and look into its specific movement. Firstly, for case 1, a short fiber section of 1 mm is applied with the transient fluid forces. The analysis of the short free fiber can present the direct effect of the air on fiber, as its small size can be quick and nimble to reflect the fluid force variation. Secondly, a fiber with the length of 3 mm and one end fixed has been applied with the numerical fluid flow. As hairiness longer than 3 mm is considered as harmful hairiness in yarn, we chose this length in the model design. So case 2 is designed to show the deformation variation of the fiber ends that stick out from the roving. In the model, one end of the fiber is fixed to present the holding force from other fibers, while the other end is free in the fluid. The density of fibers used in this model was set as 1540 kg/m³, and the Young's modulus is set as 0.8 GPa.²⁸

In this study, the partitioned method is used with a pressure-based solver and a transient solver. The interface between the solid and fluid is used for information sharing. The fluid governing equations are first calculated and then the fluid pressure acting on the solid is transferred to the structure solver. Then, the displacement of the solid is also transferred to the fluid solver. The mesh is updated in each step. The information sharing is dependent on the coupling system.

To get more practical results, before the transient simulation, we also carried out simulation on the fluid under steady-state. The transient simulation is initialed with this steady result.

Results and analysis

Fluid velocity distribution

It can be seen in Figure 5 that the streamlines in the tube are forming a vortex around the yarn wall. The air velocity inside ranges from 462 to 0 m/s. It can be established that at the bottom of the pre-twister, the air near the roving goes upward and the air near the wall of the pre-twister goes outward. The upward air near the roving helps blow the fiber ends from the roving, so that they can be involved in the air vortex.

Figure 5.

Streamlines of velocity in the pre-twister.

To be specific, the analysis of the tangential velocity component and axial velocity component is shown in Figure 6. In Figure 6, the x-axis refers to the coordinate position of the z-axis in the model's three-dimensional coordinate in Figure 6, and the y-axis refers to the velocities. In Figure 6(a), the airflow tangential velocity develops from 0 mm/s at the wall of roving to the maximum when the radius equals 2.5–3.00 mm. This is because both the roving and the tube wall are steady in the tangential direction, so they prevent the airflow velocity developing. It can be seen that in the core region (–1.5 mm ≤ x ≤ 1.5 mm), the absolute value of the tangential velocity ranges from 0 to 70 m/s, while the axial velocity ranges from –13 to about 4.1 m/s. So, the tangential velocity is the major component of the airflow velocity and it is supposed that it makes the free fiber ends wrap on the roving.

Figure 6.

Tangential (a) and axial (b) velocity distribution in the pre-twister.

For the first case (Figure 7), it can be seen that the air from the four inlets forms a rectangular vortex in the tube at the start, and then turns into a round vortex. Meanwhile, because of the existence of the roving, the flow in the core area is affected and shows different characteristics. The air in the tube is of relatively low velocity firstly. Driven by the supplement air from the inlets, the fluid firstly generates into the same four parts as the blocking of the roving in Figure 7(a), then forms a rectangular flow around the roving and the fluid squashes to the center and forms four vortexes (b). Then the flow forms the round vortex generally, shown in Figure 7(c), and the fiber begins to move, driven by the airflow. Meanwhile, for the air near the roving, its velocity is in the direction from the center to the surrounding. Therefore, the fiber ends at the surface of the roving can be blown away from the roving and participate in wrapping on the surface of the roving. In Figure 7(d), when the fiber is in the core area of the vortex, the one round-shaped core vortex turns to become distorted and separated into three vortexes. In Figures 7(e) and (f), after leaving the core area, both the circling motion and outward movement of the short fiber continues and finally the fiber approaches the wall of the pre-twister (Figures 7(g) and (h)). This centrifugal and circular movement indicates that although the tangential air velocity helps the fiber to form twists on the roving, the fibers still have a tendency to move away from the roving, which may lead to some fibers running off and fewer fibers in the cross-section of the roving. So, the pressure should be set appropriately to provide forces to produce pre-twists on the roving surface and also to control the fly away of the fibers at the same time. Besides, as the air velocity differs when it is on the upper part and lower part of this fiber section, the fiber upper part has a higher speed than the lower part. Therefore, the fiber section finally lies down. This laying down configuration is similar to the twisting configuration. The fiber trace during this period is shown in Figure 7(i). The trail of the fiber section proved the twist effects of the pre-twister. For case 2, the fiber is supposed to be held by the other fibers in the roving by one end. Due to the fixed support, the fiber deformation is as shown in Figure 8 and it differs from that of case 1.

Figure 7.

The two-way coupling simulation results for case 1.

Figure 8.

The two-way coupling simulation results for case 2.

Figures 8(a)–(d) present the fiber movements and fiber deformation based on the chronological order. From Figures 8(a)–(d), it can be seen that the fiber moves from the original position to the maximum deformation. In addition, the free end also moves toward the roving. It can be proved that although the fiber is in the vortex core, it can still be driven by the airflow, with a length of 3 mm. The deformation and movement show the trend that the fiber approaches and wraps the roving. This result is different from the previous research on fiber deformation under the action of an annular flow field.²⁶ In the previous research, the airflow velocity was simplified as a tangential velocity that scales down from the tube wall to the center. Actually, in this study, it is shown that the roving has its effects on the airflow and the air velocity near the roving has a more complicated fluid velocity distribution instead of unidirectional diminishing. Besides, as the velocity near the roving is relatively low, the fiber effect on the fluid should also been considered. Therefore, the two-way coupling here can reflect the fiber deformation accurately.

Experimental details

Experimental design

A lab Spintester with six spindles was used to for the experiments to spin the cotton yarn. Test A is the application of traditional ring spinning, as the control group. Test B is the pre-twister device modified on the same ring spinning machine. The tests were conducted twice. Firstly, we made both Tests A and B with the same spinning speed, in order to find the pre-twister effect on yarn quality. The related parameters are listed in Table 1. As this pre-twister device is aimed at reducing the breaking down frequency during the spinning process, we increased the production speed for both Tests A and B, to see their differences when the tension increases (the production speed increasing can result in tension increasing). For a better illustration, we increased the production speed to the maximum for both Tests A and B, separately, on the condition that the frequency of the ends down was controlled within 40 times/1000 spindle hours for both tests. Both Tests A and B are fed with the same patch of carded cotton roving (370 tex). For yarn property tests, its properties, including tensile strength, evenness and yarn faults, were observed. The tensile properties of the yarns were measured on an USTER TENSORAPID (Uster, Switzerland), and the gauge length and the crosshead speed are 500 mm and 200 mm/min, respectively. The USTER ME100 (Uster, Switzerland) was used to test the yarn evenness, that is, to measure the yarn evenness of 1000 m with a test speed of 200 m/min.

Table 1.

Parameter settings for experiments

Parameters	Control test	Experimental test
Roving density (tex)	370
Total draft	25
Main draft	20
Front roller speed (m/min)	12.5
Holding roller speed (m/min)	12.5
Spindle speed (r/min)	8125
Inlet pressure (kPa)	–	150–300
Yarn twist design (tpm)	650
Yarn size (tex)	15.3

Experimental results

Surface morphology

Some fibers on the surface of the roving are pre-twisted by the pre-twister, and the yarns are observed under a microscope (Figure 9). For the roving from Test A, all fibers are aligned and the fibers around the surface are in loose arrangement. Meanwhile, in Test B, it can be seen that fibers in the roving can be divided into two kinds, which are the core fibers and the wrapping fibers. The core fibers inside the roving are in a similar arrangement as in Test A, while the wrapping fibers on the surface are winding on the core fibers, as shown in Figure 9(b). Therefore, it can be verified that this pre-twister work to add surface twists.

Figure 9.

Surface morphology of rovings in Tests A and B before getting into the ring and traveler section.

We tested the yarn breaking strength under different air pressures, and some of the results are listed in Table 2. It can be seen that the mechanical properties are best when the pressure is set at 250 kPa and the corresponding values are shown in boldface. We decided to choose 250 kPa to compare with the ring spinning.

Table 2.

Comparison of yarn properties under different pressure sets

Air pressure (kPa)	150	175	200	250	300
Strength (cN)	239.9	259.3	262.2	259.3	237.6
CV of strength (%)	8.9	8.1	8.0	6.5	12.6

Note: The corresponding values are shown in boldface.

The yarn properties are compared between Tests A and B in Table 3. The yarn strength of Test B has an average decrease of less than 6%. In addition, the strength evenness values are almost at the same level in the evaluation standard and the corresponding level values for each index have been in boldface in Table 3. Due to the pre-twist fibers on the surface in Test B, the fiber orientation would not be better that the common ring spinning yarn, so its strength can be decreased. Secondly, the gauge between the front roller and the holding roller is 40 mm, which is much longer than the principle length of the cotton fibers. Thereby, when the airflow works on the roving, the roving cannot be held tightly and located fixedly in the center of the pre-twister. Instead, the roving rotates itself slightly by the vortex flow in the pre-twister sometimes (Figure 10), with two ends held by the front rollers and holding rollers, separately. This self-rotating may lead to some false twists on the roving. Meanwhile, when the roving leaves the holding rollers, its false twists disappear by themselves. This untwisting process of the false twist leads to a worse evenness (a decrease of 10%), more yarn faults (including the thin and thick places as well as the neps), worse hairiness and the decrease of fiber orientation in the yarn and then the breaking strength of the yarn. To be specific, the increase of the frequency of the thin places can be also related to the airflow in the tube. As shown in Figure 7, the free fiber rotates around the roving and also has the tendency of moving away from the roving, so some free fibers can leave the roving by air, which may lead to more thin places in the yarn. In addition, the running off fiber can be also shown by the simulation (an increase of 100% in thin places). Besides, the local distributed additional entanglements (the left-hand side in Figure 9(b)) helps the roving to bear more stress during the spinning, while it cannot be denied that these entanglements condense the fiber density in some part of the yarn, and lead to the unevenness to the yarn product, especially the thick places (with an increase of 21%). The nep increase in Test B is only 2.8%, which could result from the additional friction from the pre-twister. Therefore, the yarn properties are acceptable. Further investigation can be done for the yarn quality improvement of the pre-twister.

Figure 10.

The self-rotation of the roving in Test B.

Table 3.

Comparison of yarn properties

Parameters	Test A	Test B	Uster Statistics 2018²⁹
Parameters	Test A	Test B	5%	25%	50%	75%
Strength (cN)	275.3	259.3	285	256	229	–
CV of strength (%)	6.5	6.5	7.7	8.5	9.1	–
Elongation (%)	5.65	5.50	6.5	5.9	5.5	5.0
Unevenness, CV (%)	15.7	17.3	13.32	14.79	16.04	17.40
			5%	50%	75%	95%
Thin places (–50%)	37.5	75	8	27	52	100
Thick places (+50%)	515	625	90	234	387	610
Neps (+200%)	535	550	194	588	1066	1767
			25%	50%	75%	90%
Hairiness	5.1	6.0	4.3	4.8	5.3	6.0

Note: The corresponding values are shown in boldface.

As this pre-twister device is aimed at reducing the breaking down frequency during the spinning process, we increased the production speed for both Tests A and B, to find their differences when tension increases (the production speed increasing can result in the tension increasing). For a better illustration, we increased the production speed to the maximum for both Tests A and B, separately, on the condition that the frequency of ends down was controlled within 40 times/1000 spindle hours for both tests. The maximum production speed for both Tests A and B is been recorded in the first row of Table 4. Tests A and B shared the same spindle speed at 10,000 r/min. It can be seen that, when other parameters are the same, this pre-twister device helps to increase the production speed, by 27.13%, which is significant in production efficiency. This means that with pre-twisting, the roving can stand more tension during spinning than before.

Table 4.

Comparison of yarn properties

Parameters	Test A	Test B	Uster Statistics 2018²⁹
Parameters	Test A	Test B	5%	25%	50%
Max yarn production speed (m/min)	12.68	16.12	–	–	–
Spindle speed (r/min)	10,000		–	–	–
Yarn twist level (tpm)	800	650	897	–	1039
Elongation (%)	5.79	5.37	6.5	5.9	5.5
Tenacity (cN/tex)	20.66	18.02	18.4	16.5	14.8
			50%	75%	95%
Unevenness, CVm (%)	17.28	18.64	16.04	17.40	19.06
Thin places (–50%)	42.5	90	27	52	100
Thick places (+50%)	417.5	522.5	234	387	610
Neps (+200%)	565	655	588	1066	1767

Note: The data comes from the cotton yarns for weaving application in Uster Statistics 2018.

Note: The corresponding level values for each index are in boldface in Table 4.

The yarn properties of the yarns from Tests A and B are also compared in Table 4. As we know, the spindle rotating motion leads to the twist on yarns and the yarn winding on the spindle. In Test B, its yarns show less twist than in Test A. So, this means that in Test B, the spindle rotating motion can contribute more to the yarn winding efficiency. This can explain the production increase. The tenacity of Test B is also lower than that of Test A (a decrease of 13%), which could be the result of its smaller yarn twists. The evenness of Test B is also a little bit worse that of Test A, with a decrease of 8%. Referring to the Uster Statistics 2018, all these properties are at or above the level of 95% and the corresponding level values for each index are in boldface in Table 4. For faults in yarn, including the thin places (+112%), the thick places (+25%) and the neps (16%), this comparison result get worse than the comparison in Table 3. As we have mentioned before, the worsening in yarn faults is the result of multiple reasons, including the additional friction from the pre-twister, the self-rotation, etc. The speed up intensifies the yarn faults in both Tests A and B. Therefore, the yarn properties are acceptable, while the yarn faults need further work to be improved.

Conclusion

A kind of pre-twister device has been analyzed in this study. A two-way coupled computational fluid dynamics (CFD) model has been used to simulate the airflow distribution in the pre-twister tube, as well as the movement and deformation of the fibers inside driven by the vortex airflow. It shows that the pressure decreases from the inlet to the middle and there is a vortex core region around the roving, with a radius of 1.55 mm. The airflow speed inside are from 462 to 0 m/s, while for the regions near the roving, there is a core region with a radius of about 1.5 mm where the tangential speed ranges from 0 to 70 m/s. Besides, in the core region, the flow varies complicatedly rather than the linear diminishing velocity distribution assumed in the past researches. The fiber finite simulation shows that the fiber can be driven by the airflow to move in a circular track to be wrapped on the roving outer layer. The experiments show, under a microscope, that fibers wrap on the surface of the roving after the pre-twister device. The pre-twister ring spun yarn shows slightly declined yarn properties in both strength and evenness at the same parameter setting, with decreases of 6% and 10%, respectively, which are almost at the same level in the Uster Statistics. Besides, with the help of the pre-twister device, the production speed increases by 27.13%, with a reasonable decrease in yarn properties, including the decline in mechanical properties (tenacity decrease of 13%) and evenness (decrease of 8%). Therefore, in future work, more should be done to improve the yarn quality.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities (Grant Number 2232018D3-12), the Applied Basic Research Programs of “Glory of Textiles” (Grant Number J201807), the Fundamental Research Funds for the Central University (Grant Number CUSF-DH-D-2018030) and the National Key R&D Program of China (Grant Number 2017YFB0309100).

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