The airflow field pattern in the condensing zone plays a vital role in the pneumatic compact spinning, which significantly affects the yarn's qualities. This study aimed to analyze the effects of the different negative air pressures on fiber condensing in compact spinning with lattice apron using ANSYS. The results of airflow simulations reveal that by increasing the negative pressure, the flow velocity increases, leading to a more tremendous increase in the transverse condensing effects. Additionally, a better convergence led to reduced fiber width and eliminated the spinning triangle. Experimental results showed that the three yarns spun with the highest negative pressure had better strength, hairiness, and evenness than those spun with lower negative pressure.

#### Keywords

- Compact spinning
- lattice apron
- airflow
- negative pressure
- numerical simulation

The air negative pressure plays a very important role in compact spinning as it is utilized to condense the fiber bundle in a pneumatic compact spinning mechanism [1, 2]. To eliminate the spinning triangle, which leads to improving the quality of spun yarn in terms of hairiness and strength, a few studies have been reported [3, 4, 5]. Numerical simulation technique is one among the significant approaches that are used to solve numerous problems in traditional ring spinning [6]. The compact spinning system was built on the foundation of the traditional ring-spinning system [7]. However, numerous researchers consider it to be a modern spinning technique [8, 9, 10]. The first compact spinning system was introduced to industrial application at ITMA in 1995 [11, 12]. Compact spinning is classified into pneumatic and mechanical compacting based on the condensing principle [13]. Recently, the dominant type is the pneumatic compact spinning systems [14, 15] and it is mainly classified into perforated drum and lattice apron compact spinning systems [16, 17].

Compact spinning with lattice apron is the most widely used pneumatic compact spinning system, and it has three-line compact spinning (TLCS) and four-line compact spinning (FLCS) [4, 16]. The condensing zone's flow field pattern plays an essential role in the pneumatic compact spinning, which directly affects the qualities of yarn [18]. Therefore, the flow field investigations in the condensing zone of pneumatic compact spinning have received more attention recently [19]. Recent research on the condensing zone in pneumatic compact spinning use computational fluid dynamics (CFD) [20]. CFD is one of the powerful methods to investigate flow field-based problems [1, 21, 22] because of its ability to investigate theoretical phenomena of fluid flow based on the physical model [23, 24, 25]. Employing the self-designed MATLAB procedure, Dou

In this article, we investigate the condensing zone of compact spinning systems with lattice apron and the effects of different negative air pressure on fiber condensing as well as the mechanical properties of yarn. In the next sections, numerical simulation steps, model setup, simulation data, and analysis are presented. A comprehensive conclusion is then drawn based on the results.

Figure 1 represents the side view of the condensing zone of the compact spinning system with a lattice apron. The 3D flow field's numerical simulations in the condensing zones of three different negative air pressure will be studied. The physical parameters of the model are shown in Figure 2.

In these models, the fiber strand's output direction is defined as the negative direction of the

The flow field in the condensing zone is assumed as incompressible and the turbulence model adopted is the standard k-epsilon two-equation model. As shown in Figure 2, there are three pressure inlet points that are assigned the static pressure values equal to the atmospheric pressure (101,325 Pa). The collecting pipe is connected to the centrifugal fan, and the air inside the collecting pipe is sucked from one side of the pipe; therefore, the side is set as a pressure outlet boundary, that is, −1,000 pa, −2,000 pa, and −3,000 pa (see Figure 2) concurrently.

To simulate the effect of the lattice apron on the airflow characteristics, the plane covered by the grid circle is set to a porous jump boundary (see Figure 2). The pressure changes (

In the above formula, _{2} is the pressure jump coefficient, ^{−07} and 0.09 mm, respectively.

The other faces of the calculation area are solid walls. The condition of the nonslip boundary is observed on the solid wall, which means that the velocity on the wall is zero. The solvers were implemented with a single-precision implicit split operator, and thereby the coupling problem between pressure and velocity was resolved. SIMPLEC algorithm is used, and grid spacing 0.5 mm and convergence precision 1e^{−4} are set to reduce the error to acceptable levels.

In this section, the effects of three different negative pressures on flow velocity were simulated. Figures 3 and 4 indicate that by varying the negative pressure from −1,000 pa to −2,000 pa and −3,000 pa, the airflow velocity will increase from 37.32 m/s to 53.97 m/s and 67.54 m/s, respectively.

Figure 3 shows the streamline diagram of flow velocity in the X–Z section when Y is equal to zero (along the

The Geometrical parameters of the condensing zone

28 | 40 | 20 | 1.5 |

Figure 4 indicates that with respect to the central line of the air-suction slot, the flow field distribution is symmetric and the flow velocity reaches the maximum. This means that the fibers in the area of air suction slot are undergoing the largest air transverse condensing force, and the fibers have more ability to move toward the center strand and the effects of transverse condensing are obtained. Additionally, with increasing negative pressure, the flow velocity increases, and this increase leads to a greater increase in the transverse condensing effects and a better convergence. This phenomenon reduces fiber width and eliminates the spinning triangle.

In this section, numerical simulation of the flow velocity along the motion trajectory of the fiber strand in the condensing zone was studied. In the condensing zone, there are three kinds of forces acting on the fibers generally. First, the transverse condensing force on

The diagram of flow velocity component on

The flow velocity component on the

The diagram of flow velocity component on

The spinning experiments were carried out using DHU X01 multifunction digital spinning machine whose condensing device had three-rollers lattice negative-pressure type. To verify the results obtained from the simulation discussed above, the three different yarn counts used were spun 29.2 tex, 19.6 tex, and 14.6 tex at different negative air pressures. The yarn count range was selected to cover the common of the yarn produced. The cotton fiber properties and the details of the spinning parameters are shown in Tables 2 and 3, respectively. A cotton roving of 600 tex was used.

Cotton fiber properties

Micronaire | 4.33 ± 0.09 |

Maturity index | 0.86 ± 0.03 |

Average length (mm) | 31.4706 ± 1.2192 |

Upper half length (mm) | 35.6108 ± 0.762 |

Uniformity index | 88.3 ± 1.7 |

Short fiber index | 7.8 ± 0.4 |

Strength (cN/tex) | 49.5 ± 3.1 |

Elongation (%) | 6.6 ± 0.2 |

Moisture (%) | 12.0 ± 0.2 |

Reflectance (Rd) | 81.7 ± 0.4 |

Yellowness (+b) | 17.2 ± 0.3 |

Spinning process parameters

29.2 | 10,000 | 630 |

19.6 | 10,000 | 730 |

14.6 | 10,000 | 830 |

After spinning, all the yarn samples were kept in the lab for at least 48 h under standard conditions, namely, 20 ± 2°C and 65 ± 2% relative humidity (RH). To study the effect of different negative pressure, yarn properties such as the yarn evenness (CV), hairiness index, and breaking strength were measured. The yarn hairiness was measured 10 times for each yarn bobbin using a YG172A hairiness tester under 30 m/min speed, and the average value of the 10 tested results was taken as the hairiness of any particular single bobbin yarn. The average values of ten yarn bobbins were taken as the corresponding hairiness of spun yarn; the measured results are shown in Figure 7. The results show that by increasing the negative pressure, the hairiness is reduced, because the total velocity of airflow is increased, which is resulting in the elimination of the spinning triangle as shown in the simulation result.

The breaking force of yarn was also tested 10 times on XL-1A fully automatic single yarn strength tester at a speed of 500 mm/min with a pre-tension of 0.5 cN based on ASTM D2256 international standards (ASTMD2256, 1997) [28]. The average value of the breaking force of each yarn was recorded and the results are shown in Figure 8. Results show that as a result of increasing the negative pressure, the breaking force of yarn is increased, because the total velocity of airflow is increased, which in turn is due to the elimination of the spinning triangle. This further results in additional twist to free fibers, as shown in the simulation result.

The CV was done by CT3000 evenness tester at a speed of 200 m/min based on D1425/D1425 M-14 (ASTMD3822/D3822M-14, 2014) standards [29]. The average values of 10 bobbin yarns were taken as the corresponding evenness of spun yarn; the measured results are shown in Figure 9.

When the negative pressure was increased, the total velocity of airflow is increased as well. Accordingly, decreasing the transfer of border fibers and the fibers’ distribution in the yarn body became more uniform and advantageous for enhancing CV.

To summarize, we have established the 3D physical model of the condensing zone by implementing numerical simulations based on a standard k-epsilon model. The numerical simulations of the 3D flow field in the condensing zone for compact spinning systems with lattice apron and the effects of different negative pressures on fiber condensing were extensively evaluated. The results reveal that the airflow field helps to achieve convergence of the fiber bundle in compact spinning systems with lattice apron. The optimization of the airflow field resulted in significant increase in yarn mechanical properties.

#### The Geometrical parameters of the condensing zone

28 | 40 | 20 | 1.5 |

#### Cotton fiber properties

Micronaire | 4.33 ± 0.09 |

Maturity index | 0.86 ± 0.03 |

Average length (mm) | 31.4706 ± 1.2192 |

Upper half length (mm) | 35.6108 ± 0.762 |

Uniformity index | 88.3 ± 1.7 |

Short fiber index | 7.8 ± 0.4 |

Strength (cN/tex) | 49.5 ± 3.1 |

Elongation (%) | 6.6 ± 0.2 |

Moisture (%) | 12.0 ± 0.2 |

Reflectance (Rd) | 81.7 ± 0.4 |

Yellowness (+b) | 17.2 ± 0.3 |

#### Spinning process parameters

29.2 | 10,000 | 630 |

19.6 | 10,000 | 730 |

14.6 | 10,000 | 830 |

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