Characterization of the airflow field in the rotor spinning unit based on a novel experimental approach and numerical simulation

Abstract

It is very challenging to experimentally characterize and verify the airflow in the rotor spinning machine because the process takes place in an enclosure. In an attempt to portray the process, we present a methodology that combines a novel experimental approach and numerical techniques. We developed a model unit and used colored smoke to mimic the airflow behavior practically, measured the air pressure, and compared the results to the simulation data. Three state conditions, namely suction and rotation (the regular rotor spinning operation, (Case 1)), without rotation (Case 2), and without suction (Case 3), were adopted to investigate the formation mechanism of the airflow field in the rotor spinning unit based on two operating conditions. Results show that, in a regular state, the airstream accelerates rapidly in the transfer channel under the dominant action of air suction at the rotor outlet and crashes clockwise to the rotor wall with the joint action of two operating conditions. In the rotor, the airflow flows clockwise with the velocity distribution of a multi-ring gradient due to the dominant action of high-speed rotor rotation. Analytics from the air pressure indicate that while the air pressure in the rotor is mainly controlled by the action of the air suction mechanism, it is also affected by the superposition action of the rotation mechanism. This approach is groundbreaking for rotor spinning machine optimization and is anticipated to trigger more insights that will lead to fundamental research in the spinning industry and beyond.

Keywords

airflow field numerical simulation rotor spinning optimization computational fluid dynamics

Rotor spinning is the most widely used new spinning technology to commercially process fibers into yarns, with the advantages of low cost of production, high productivity, process automation, and flexibility to produce various novel yarns.^1–4 Airflow plays a crucial role in the spinning process of rotor spinning, which is responsible for transporting fibers in the transfer channel, transferring fibers into the rotor interior, and assisting in collecting fibers in the rotor groove. However, airflow may also hamper fiber orientation and eventually result in the deterioration of yarn quality.⁵ Therefore, airflow studies can provide a fundamental benchmark and useful insights toward the understanding and optimization of the rotor spinning process, for instance, the airflow field characteristics and the effects of geometric and spinning parameters on the airflow in the rotor spinning unit and, hence, on yarn properties.⁶

Numerous studies have focused on the airflow field in the rotor spinning unit employing the computational fluid dynamics (CFD) technique. In 1996, Kong and Platfoot⁷ developed a two-dimensional (2D) computational model and simulated the flow patterns in the transportation zone of the rotor spinning machine. Their results revealed that the airflow patterns could be affected once the geometric dimensions of the transfer channel or opening roller velocity varied, then altering the configuration of fibers. Yang et al.⁸ conducted numerical simulation on three-dimensional (3D) flow in the transfer channel and rotating rotor, and the results showed that the vortex generated in the rotating rotor, the airflow speed decreased on the slip wall, and high-pressure regions developed on the slip wall and rotor groove. Further understanding of 3D airflow characteristics in the rotor spinning unit can be found in the research of Lin et al.⁹ and Xiao et al.¹⁰ They simulated 3D airflow in the rotor spinning unit and investigated the effects of significant geometric and spinning parameters on airflow patterns. Their studies and results provide fundamental guidance for the selection and optimization of both the rotor design and spinning parameters.

To understand the effect of the airflow on the fibers and yarns, some researchers have conducted experimental studies. Zeng and Yu,¹¹ Guo and Xu,¹² and Pei et al.¹³ adopted high-speed photography to capture and analyze the fiber motion in the airflow field at different operating conditions of air-jet spinning. Seyedi et al.¹⁴ utilized the tracer fiber technique to investigate and analyze the fiber migration in the airflow field of newly rotor-jet spun yarn with the rotor-jet spinning parameters varying. Seyed et al.¹⁵ and Lin et al.¹⁶ carried on the spinning test to compare the yarn quality and hence verified the effect of the optimized transfer channel airflow field on the yarn properties. Akankwasa et al.¹⁷ also evaluated the blended yarn quality to support the simulated results of the airflow characteristics in conventional and dual-feed rotor spinning machines. There are many simulation works on the airflow in the rotor spinning machine and some experimental methods to figure out and verify the effect of the airflow on fibers and yarns. Nevertheless, no visual experimental findings on the airflow in the rotor spinning unit in the industrial setting have been reported.

The rotor spinning operation is mainly controlled by the air suction mechanism and rotation mechanism, that is, air suction at the rotor outlet and high-speed rotor rotation. It is based on these two operating conditions that an appropriate airflow field is generated in the rotor spinning unit, therefore achieving fiber transmission, transfer, and accumulation. There are a few publications that have investigated the effects of the two operating conditions on the airflow field in the rotor spinning unit. Xiao et al.¹⁰ studied the influence of rotor speed on the airflow, and their results showed a significant change of the flow structure in the rotor as the rotor speed varied. The research of Lin et al.^9,18 indicated that more vortices around the rotor groove could be produced with lower rotor speed, and much higher rotor speed can cause more yarn breakage. Their studies on rotor outlet pressure revealed that it was beneficial to yarn production to decrease the rotor outlet relative pressure. Still, the vortex intensity on the transfer channel inlet can increase, resulting in a decrease in fiber straightening and yarn properties. However, there are no previous studies that have performed experimental verifications focusing on these two operating conditions and given attention to their contribution to producing the airflow field, namely, the formation mechanism of the airflow field in the rotor spinning unit.

This work aims to experimentally and numerically investigate the dynamic and static characteristics of the airflow field (i.e., airflow behavior and air pressure) in the rotor spinning unit. The formation mechanism of the airflow field in the rotor spinning unit is also analyzed by comparing the experimental findings and simulation results in three cases with two operating conditions being varied. Our work provides a further understanding of the airflow field characterization from a practical perspective, and the technique used in this work can be extensively utilized in other flow problems.

Experimental details

Experimental setup

The general rotor spinning unit of the air suction open-end rotor spinning is shown in Figure 1. An air pump is connected with the aspirating hole and sucks air out of the spinning unit. The air in the rotor is also sucked from the rotor outlet and then two airstreams are fed into the rotor from the transfer channel and doffing tube, respectively. Simultaneously, the rotor rotates at high speed. Thus, the specific airflow field and twisting environment of rotor spinning are generated.

Figure 1.

Schematic diagram of the rotor spinning unit.

Since the rotor spinning unit is opaque and sealed in the actual spinning state, it is difficult to observe the airflow behavior and obtain the characteristics of the airflow field in the rotor spinning unit. To address this problem, a model rotor spinning device with a model rotor spinning unit (the model unit, for short) was set up based on the geometrical parameters of the actual rotor used in the open-end spinning (see Figure 2). The model unit consists of a rotor, a lid, and a cavity, all of which are made of plastics, transparent polycarbonate and resin, thus making it easy to view the rotor interior (see Figures 2(a) and (c)). The dimensions of the model unit are presented in Figure 2(d). Furthermore, in our device setting, an air pump was used to implement the air suction, thus providing the negative pressure required for spinning. The model rotor was driven by an electric motor through a flat belt to accomplish the high-speed rotor rotation.

Figure 2.

(a) Schematic diagram of the experimental setup. (b) Model rotor spinning device. (c) Model unit. (d) Main dimensions of the model unit.

Experimental arrangement

In a bid to obtain the dynamic and static characteristics of the airflow field in the rotor spinning unit, we observed the airflow behavior and measured the air pressure in the model unit. Furthermore, for purposes of exploring the formation mechanism of the airflow field in the rotor spinning unit based on the air suction mechanism and rotation mechanism, three state conditions were designed. Case 1 was under the real operating conditions of the rotor spinning device at a regular spinning state, both with the air suction at the rotor outlet and high-speed rotor rotation. Cases 2 and 3 were carried out under the condition without high-speed rotor rotation and air suction at the rotor outlet, respectively. The denotation and parameters of the three state conditions are described in Table 1. Therein, the outlet pressure was measured by a digital pressure gauge with a resolution of 1 Pa. The rotation speed of the rotor rotating clockwise was obtained using a light-emitting diode (LED) stroboscope with a 1 rpm resolution.

Table 1.

Details of the operating conditions in the three state conditions

Case	Operating conditions	Outlet pressure (Pa)	Rotation speed (rpm)
1	Suction and rotation	–5980	30,080
2	Without rotation	–5799	/
3	Without suction	/	30,080

Observing airflow behavior

In order to achieve a vivid observation of the airflow behavior in the model unit, we introduced a reddish-violet smoke into the inlet airflow. This colored smoke flows with the airstream in the model unit and thus adheres to the model unit. This phenomenon makes it possible to visualize the airflow due to the distinctive color of the airflow and the transparency of the device components. The transfer channel is the main airflow inlet of the rotor spinning unit and the only entrance for fibers during regular spinning. Hence, this study focused on the flow behavior of the airflow from the transfer channel to the rotor interior. The colored smoke was introduced in the airstream at the inlet of the transfer channel and the introduced amount in all three cases was 0.894 g. We then used a camera (FUJIFILM X-T3, 24 fps) to capture and record the colored airflow behavior in the model unit.

Measuring air pressure

The measurement of air pressure was also conducted based on the developed model unit. Four circular holes were punched on the model lid to insert the pressure gauge probe. While one of the punched holes was in use, the other three holes were blocked. The four holes correspond to four lines locating the points selected for further air pressure measurement and analysis. As shown in Figure 3, the four lines are parallel to the center axis of the rotor and equidistant from the center axis. Line 1 intersects the center axis of the transfer channel at point A, and $y_{A} = 9.94 mm$ . Therefore, Line 1 is opposite to the transfer channel outlet. Point O is located at the center axis of rotor, and $y_{O} = y_{B} = y_{C} = y_{D} = y_{A}$ . The distance between Points A and O is 19.2 mm. Figure 3(b) shows the y-coordinates of the five points selected as the measurement points on four lines, namely $y = 8, 10, 12, 14, 16 mm$ . Also, the measurement of the air pressures at these selected points was carried out in all three cases.
Figure 3.
Schematic diagram of the pressure measurement points and their located lines in the (a) x–z plane and (b) x–y plane.

Experimental results

Observation results of airflow behavior

The process of the colored airflow flowing into the model unit in the three cases is illustrated in Figure 4, where t represents the time of the colored smoke being introduced into the airstream at the transfer channel inlet. The model units in all three cases were all clear at $t = 0 s$ . Figure 4(a) shows the colored airflow distribution process in the model unit in Case 1 (suction and rotation). It can be seen that the peripheral wall of the rotor evenly showed slight reddish violet (at $t = 2 s$ ). There was a concentration spot tapering clockwise on the lid and along the upper side of the transfer channel exit at the rotor outlet, as indicated by the arrow at $t = 4 s$ , which was the traces of the airflow flowing out of the rotor. Then at $t = 36 s$ , a higher concentration of the colored smoke appeared on the rotor bottom. An increase in the introduced time resulted in a proportionate increase of the reddish-violet smoke, which led to the concentration of smoke, resulting in the dark marks (see the arrows in Figure 4(a)). At $t = t_{1} = 1 min 49 s$ , the colored smoke was used up. The appearance time and intensity of all colored marks showed that most of the airflow from the transfer channel flowed clockwise (the same direction as the rotor rotating) toward the rotor wall and then flowed into the rotor interior. Simultaneously, part of the air flowed clockwise away from the rotor at the outlet.
Figure 4.
Sequence images of the colored airflow behavior inside the model unit in (a) Case 1 (suction and rotation), (b) Case 2 (without rotation), and (c) Case 3 (without suction).

Figure 4(b) shows an entirely different airflow behavior from that exhibited by Case 1 in the same conditions. The colored marks both on the lid and rotor wall in Case 2 were mostly concentrated at the exit of the transfer channel and then dispersed along the upper and lower rotor wall and lid (see the arrows in Figure 4(b)). The colored spots on the left-hand side of the lid were distributed radially at the transfer channel outlet, and the colored marks were concentrated closer to the transfer channel outlet. This observation revealed that the airflow coming from the transfer channel crashed directly onto the rotor wall. This was followed by two airstreams that flow respectively along the upper and lower rotor wall. Simultaneously, there was also part of the air flowing out of the rotor outlet. Furthermore, some stained blots appeared in zone A on the lid at $t = t_{2} = 1 min 51 s$ , indicating that there was a small amount of the airflow flowing through zone A. In Figure 4(c), it can be observed that the stained marks on the rotor wall in Case 3 were almost the same as those in Case 1 for the same timestamp. However, there were no obvious colored spots on the lid, revealing that most of the airflow from the transfer channel reached the rotor wall but was not rushed onto the rotor wall. The reddish-violet smoke diffused inside and outside the rotor at $t = 10, 36, 59 s$ (see the arrows in Figure 4(c)), indicating that the velocity of the airflow in the rotor was low, thus resulting in an outflow of colored smoke from the rotor. The reddish-violet smoke was depleted at $t = t_{3} = 2 min 11 s$ .

Measurement results of air pressure

Figure 5 illustrates the measurement results of the air pressure in the three cases. It can be seen that the measurement values in Case 1 (–3500 to –7000 Pa) are close to those in Case 2 (–4000 to –9000 Pa), while the pressure values of the selected points in Case 3 are kept in the range from 0 to –500 Pa. This phenomenon reveals that the air suction mechanism mainly generates negative pressure in the rotor at the regular spinning state. From Figures 5(a) and (b), it can be seen that the pressure of the points on Line 1 fluctuates greatly, and shows a trend of descending first and then rising with the increase of y. We attribute this to Line 1 being opposite to the transfer channel outlet. Nevertheless, as demonstrated in Figures 5(a) and (b), the lowest pressure value on Line 1 appears at different selected points. In Case 2, due to the airflow coming from the transfer channel directly crashing to the rotor wall along the direction of the center axis of the transfer channel, the velocity of intersected point A is supposed to be the largest. Given that the gravity of air is negligible, according to Bernoulli’s principle,¹⁹ the pressure at point A is consequently the smallest. Therefore, the negative pressure of the end closest to point A, that is, the point at $y = 10 mm$ , is the smallest (see Figure 5(b)). Meanwhile, in Case 1, the airflow from the transfer channel crashes onto the rotor wall clockwise instead of along the direction of the center axis of the transfer channel, which accounts for the smallest pressure appearing at a point with a higher y-coordinate, namely, the point at $y = 12 mm$ (see Figure 5(a)). The slightest pressure in Case 1 is lower than that in Case 2 under the superposition action of high-speed rotor rotation. Furthermore, the pressure at each selected point on the other lines in Case 1 is also close to that in Case 2. Meanwhile, the former is slightly smaller than the latter with the superposition of the airflow field generated by the rotation mechanism, and the former value stays around –6000 Pa.
Figure 5.
Experimental pressure values in (a) Case 1 (suction and rotation), (b) Case 2 (without rotation), and (c) Case 3 (without suction).

From the above analysis, it can be seen that not only the colored airflow behavior but also the air pressure in Case 1 (suction and rotation) can be roughly regarded as the superposition of those in Case 2 and Case 3 (i.e., without rotation and without suction). To further understand the dynamic and static characteristics of the airflow field in the rotor spinning unit, as well as investigate the formation mechanism of the airflow field based on two operating conditions, air suction at the rotor outlet and high-speed rotor rotation, numerical simulation of the three cases was also carried out.

Simulation scheme

The computational domain of the model unit is presented in Figure 6(a). There are two inlets, that is, the velocity-inlet and the pressure-inlet, and a wall boundary with the computational model. The velocity-inlet locates at the transfer channel inlet, and the inlet velocities measured by Pitot tube in Cases 1–3 are 28.87, 28.31, and 4.19 m/s, respectively. Since the doffing tube is open to the atmosphere, the corresponding inlet is set as the pressure-inlet with the pressure values equal to the atmospheric pressure (1.01 × 10⁵Pa). The condition values of the outlet pressure and wall boundary in Cases 1–3 are the same as those in Table 1. Therein, the outlet pressure in Case 3 is –129 Pa.
Figure 6.
(a) Meshed computational model and boundary conditions. Mesh independency test for three different grid schemes: velocity magnitude (b) along the x-axis at y = 10 mm and (c) along the y-axis in the direction of the center axis of the transfer channel.

The airflow in the rotor spinning unit can be assumed as an isentropic, viscous, incompressible, and turbulent flow.^6,7,9 There is minimal heat transfer in the rotor spinning process; hence, the governing equations considered here only consist of two equations. One is the mass conservation equation. Since the fluid density ρ is time-independent here, this equation can be expressed as
$\nabla \cdot (ρ u) = 0$
(1)
where u is a velocity vector. The other is the momentum conservation equation, articulated as
$\frac{\partial (ρ u_{i})}{\partial t} + \nabla \cdot (ρ u u_{i}) = \nabla \cdot (μ grad u_{i}) - \frac{\partial p}{\partial x_{i}} + S_{i}$
(2)
where t, $μ,$ and p are the time, coefficient of dynamic viscosity, and air pressure, respectively. u_i is the component of u on x_i, and S_i is the general source term on x_i. Given the high Reynolds number of the turbulent airflow in the rotor spinning unit, the realizable k-ɛ turbulence model was adopted, and the transport equations for k (kinetic energy) and ɛ (dissipation rate) in this turbulence model are
$\frac{\partial (ρ k)}{\partial t} + \frac{\partial (ρ {ku}_{i})}{\partial x_{i}} = \frac{\partial}{\partial} [(μ + \frac{μ_{t}}{σ_{k}}) \frac{\partial k}{\partial x_{j}}] + G_{k} - ρ ɛ$
(3)

$\begin{matrix} \frac{\partial (ρ ɛ)}{\partial t} + \frac{\partial (ρ ɛ u_{i})}{\partial x_{i}} = \frac{\partial}{\partial x_{j}} [(μ + \frac{μ_{t}}{σ_{ɛ}}) \frac{\partial ɛ}{\partial x_{j}}] \\ + ρ C_{1} S ɛ - ρ C_{2} \frac{ɛ^{2}}{k + \sqrt{v ɛ}} \end{matrix}$
(4)
where $C_{1} = max (0.43, \frac{η}{η + 5})$ , $η = \sqrt{(2 S_{ij} \cdot S_{ij})} \frac{k}{ɛ}$ , $S_{ij} = \frac{1}{2} (\frac{\partial u_{i}}{\partial x_{j}} + \frac{\partial u_{j}}{\partial x_{i}})$ . Here, $μ_{t}$ is the turbulent viscosity, $σ_{k}$ and $σ_{ɛ}$ represent the turbulent Prandtl numbers in the equations of k and ɛ, respectively, which are constants here, $σ_{k} = 1.0$ , $σ_{ɛ} = 1.2$ , G_k is the generation of k caused by the mean velocity gradient, and v is the kinematic viscosity. C₂ is also a constant in this model, and $C_{2} = 1.9$ .

The wall functions are employed in the near-wall regions. The computational domain of the model was meshed into the unstructured tetrahedral cells with ICEM CFD 15.0, as illustrated in Figure 6(a). The governing equations were solved based on the finite volume method and the second-order upwind scheme using the commercial CFD package ANSYS Fluent 15.0. We achieved grid independence of the mesh by evaluating three grids with 842,637 (Grid 1), 1,193,867 (Grid 2), and 1,889,641 (Grid 3) elements (shown in Figures 6(b) and (c)). The results of the mesh independence test show that Grid 2 was more suitable for further simulation.

Results and discussion

Airflow behavior

According to the visualization results of the colored airflow mechanism in the model unit in the three cases, the airflow behavior in Case 1 showed an astronomical relationship with Cases 2 and 3. Therefore, the airflow behavior in Cases 2 and 3 was analyzed first, and the results were used as a basis for discussing the airflow behavior at a regular spinning state.

Without rotation

Figure 7 shows the simulation and the final experimental results in Case 2. As can be seen from Figure 7(a), the airstream accelerates once it enters the transfer channel inlet, which is on account of the negative pressure in the rotor generated by the air suction at the rotor outlet. The airstream velocity reaches the maximum v₂, $v_{2} = 154.28 m / s$ , at the transfer channel outlet. Most of the airstream crashes to the rotor wall in zone A, and Figure 7(d) demonstrates the crash at the rotor bottom. This corresponds to the experimental results in zone F, where there was an apparent accumulation of the colored smoke caused by the crash. Due to the geometrical constraints of the rotor, there are two airstreams generated after the crash, one flowing clockwise and the other flowing anticlockwise. Then the two airstreams converge in zone B and the converged airflow flows to the rotor interior but does not flow through the rotor bottom (seen zone E in Figure 7(e)) and finally adheres on the peripheral wall of the rotor. However, since the amount of the converged airflow is small, there was no visible change to the color of the rotor wall, which was already obviously stained (see Figure 7(f)). Meanwhile, the marks in zones D and H can present and verify the existence of the converged airflow flowing out of the rotor outlet under the action of the air suction at the rotor outlet. Besides, the convergence of the two airstreams hardly leads to an obvious change of the air pressure on Line 4 close to the converged location (see Figure 5(b)). We attribute this to the large negative pressure generated by the tough action of air suction at the rotor outlet, which makes it scarcely possible to bring a marked change with a small amount of the converged airflow. Furthermore, during the process of the airflow in the rotor interior, there is also part of the airflow flowing out of the rotor at the rotor outlet, which mostly occurs in zone C surrounding the exit of the transfer channel and agrees well with the experimental result shown in zone G.
Figure 7.
Numerical and experimental results in Case 2 (without rotation). The velocity streamlines of the airflow (a) in the rotor spinning unit, (b) in the rotor ( $y \leq 13.5 mm$ ), (c) reaching the lid, (d) in zone A, and (e) in zone B. The final colored images of the model (f) rotor and (g) lid at $t = t_{2}$ .

Without suction

In Figure 8(a), the airstream accelerates slightly in the transfer channel, indicating that the negative pressure generated by the high-speed rotation of the rotor is small. As shown in zone A in Figure 8(b), the airstream flows clockwise (the same direction as the rotor rotating) after exiting the transfer channel. Simultaneously, under the action of centrifugal force, the airflow keeps approaching the peripheral wall of the rotor and flowing toward the rotor outlet. This tendency results in the intense and uniform colored marks on the peripheral wall of the rotor (see the arrow in Figure 8(d)). Still, owing to the friction between the high-speed rotating rotor wall and the airflow, the velocity of airflow increases as it approaches the peripheral wall of the rotor. Thus, there is almost a multi-ring gradient distribution of the airflow velocity in the rotor, and the airflow closer to the peripheral wall of the rotor possesses a higher velocity, as shown by the blue arrow in Figure 8(b). The maximum airflow velocity v₃, $v_{3} = 84.91 m / s$ , is approximately equal to the linear velocity of the peripheral wall with the maximum inner diameter V
$V = n π R = 85.01 m / s$

Figure 8.
Numerical and experimental results in Case 3 (without suction). The velocity streamlines of the airflow (a) in the rotor spinning unit, (b) in the rotor ( $y \leq 13.5 mm$ ), and (c) reaching the lid. The final colored images of the model (d) rotor and (e) lid at $t = t_{3}$ . (Color online only.)

Therein, n is the rotation speed of the rotor and R is the maximum inner diameter of the rotor. The velocity of the airflow near the center axis of the rotor is lower, which accounts for the visible reddish-violet smoke diffused inside the rotor in Figure 4(c). Furthermore, the airflow streamlines on the lid in Figure 8(c) are evenly distributed at the rotor outlet. This trend is consistent with the uniform slight marks on the lid in Figure 8(e), which indicates that the airflow flows out of the rotor and is also responsible for the visible reddish-violet smoke diffusing outside the rotor in Figure 4(c).

Suction and rotation

Figure 9 presents the simulation results and the final colored images of the model unit in Case 1, the operating conditions in which are used in the actual spinning process. In the transfer channel (see Figure 9(a)), the airflow behavior is similar to that in Case 2. The airstream accelerates rapidly as soon as it enters the transfer channel, hence achieving the fiber transmission, and the maximum airflow velocity v₁, $v_{1} = 160.38 m / s$ , also occurs at the transfer channel outlet but a little higher than v₂ (the maximum airflow velocity in Case 2) on account of the superposition of the airflow field produced by high-speed rotor rotation. However, it can be found that the action of the air suction mechanism is dominant in the transfer channel. As can be seen from zone A in Figure 9(b), after exiting the transfer channel, with the joint action of two operating conditions, the airstream crashes clockwise to the rotor wall, thereby transferring fibers to the rotor slide wall. It is worth mentioning that there is no anticlockwise airstream generated after the crash in Figure 9(b), which is different from that in Figure 7(b) of Case 2. Besides, no traces caused by the converged airstreams appeared at the lower right-hand corner of the lid either in Figure 9(c) or (e). This shows that high-speed rotor rotation eliminates the anticlockwise airstream, which assists in fibers transferring to the rotor slide wall, leading to an orderly arrangement of fibers.
Figure 9.
Numerical and experimental results in Case 1 (suction and rotation). The velocity streamlines of the airflow (a) in the rotor spinning unit, (b) in the rotor ( $y \leq 13.5 mm$ ), and (c) reaching the lid. The final colored images of the model (d) rotor and (e) lid at $t = t_{1}$ . (Color online only.)

In the rotor, the airflow flows in the direction of the rotor rotating (clockwise), and the velocity streamlines inside the rotor are nearly distributed in a multi-ring gradient (see the blue arrow in Figure 9(b)), which is similar to that in Figure 8(b) of Case 2, but it does not look self-evident due to the maximum velocity occurring at the transfer channel outlet, not on the rotor wall. Therefore, it can be seen that the action of the rotation mechanism is dominant in the rotor, as well as in the spinning process. In addition, due to the centrifugal force, there is also a tendency for the airflow inside the rotor to flow toward the peripheral wall of the rotor, which contributes to the fibers suspended in rotor transferring to the rotor slide wall and the fibers accumulating in the rotor groove. Also, the concentrated colored marks on the rotor were uniformly aligned on the peripheral wall, and the rotor base had light-colored marks along the radius to the rotor center (see the arrows in Figure 9(d)). As the traces show in zones B and C in Figure 9, there is part of the airflow flowing out of the rotor. However, the traces only appear on the upper side of the transfer channel outlet instead of surrounding the transfer channel outlet like that in Figures 7(c) and (g) of Case 2. This trend indicates that the airstream exiting the transfer channel crashes clockwise to the rotor wall, and the high-speed rotation of the rotor eliminates the anticlockwise airstream.

Based on the discussion and comparison of the airflow behavior in the three cases varying in two operating conditions, it can be found that the two operating conditions jointly control the airflow behavior inside the rotor spinning unit in Case 1. The air suction mechanism ensures the primary airflow velocity necessary for achieving the fiber acceleration, transmission, and straightening. The rotation mechanism can eliminate the disruptive anticlockwise airstream formed by the geometrical constraints of the rotor under the action of air suction at the rotor outlet, thus assisting in the smooth transfer of fibers from the transfer channel to the rotor slide wall and the ordered arrangement of fibers. Meanwhile, the high-speed rotor rotation can also contribute to transferring the fibers suspended in the rotor to the rotor slide wall and accumulating fibers in the rotor groove. Besides, it can also be found that the numerical simulation results of the airflow behavior agree well with the corresponding experimental findings in all three cases.

Air pressure

Based on the numerical simulation of the airflow field in the rotor spinning unit described above, the simulated pressure can also be obtained. In order to further validated the simulation results, the simulated pressure values were compared with the measurement data in the three cases. As illustrated in Figures 10(a) and (b), the comparison results of the simulated pressures with the experimental data on the four lines in Cases 1 and 2 are similar. There is a deviation between the two outcomes on Line 1 in the two cases. Still, the overall trend of the two outcomes on Line 1 is the same, both descending first and then rising with the increase of y. There are two reasons for the deviation: in the first place, Line 1 is opposite to the transfer channel outlet, where the airflow changes the most drastically and complicatedly; secondly, the distance between the adjacent measurement points is very short, which may result in measurement errors. Regarding the other lines in Cases 1 and 2, the simulated negative pressure of each point is slightly lower than the measured result due to measurement errors. In Case 3 (see Figure 10(c)), the deviation between the simulated and experimental pressure values on Line 1 is also larger than that on the other three lines due to the specific position of Line 1. However, since the airflow field and negative pressure in the rotor spinning unit in Case 3 are generated by the high-speed rotor rotation and the negative pressure is small, the simulated and experimental results on the four lines are generally consistent. On the whole, the simulation values in Cases 1–3 are justified to conform with experimental data quantitatively.
Figure 10.
Comparison of the simulated pressures with the experimental results in (a) Case 1 (suction and rotation), (b) Case 2 (without rotation), and (c) Case 3 (without suction).

Conclusions

In this work, we developed a novel experimental technique to make the airflow behavior in the rotor spinning unit visible. The results from experiments strikingly agree with the numerical simulation data and the related literature. In addition, we measured the air pressure inside the rotor and analyzed the measurement values, as well as the simulated results, quantitatively.

Two operating conditions, air suction at the rotor outlet and high-speed rotor rotation, are experimentally and numerically verified to jointly control the airflow behavior in the rotor spinning unit at the regular spinning state. In the transfer channel, the action of the air suction mechanism is dominant. The airstream accelerates rapidly from the inlet to the outlet of the transfer channel. After exiting the transfer channel, the airstream crashes clockwise to the rotor wall with the joint action of the two operating conditions. In the rotor, the act of high-speed rotor rotation is dominant. The airflow flows in the direction of the rotor rotating with the airflow velocity distributed in a multi-ring gradient from the peripheral wall of the rotor to the rotor interior. Besides, the air pressures inside the rotor, with fluctuation of descending first and then rising near the exit of the transfer channel and the relatively stable values away from the exit of the transfer channel with the increase of y, are mainly controlled by the action of air suction at the rotor outlet but are also affected by the superposition action of high-speed rotor rotation.

The technique employed in this study can accurately predict and analyze the rotor spinning process in more detail. Previously, it has been a classical challenge to experimentally verify the airflow and understand the effect of different parameters. We intend to utilize the visualization results in our study to investigate further other parameters and analyze other spinning techniques, such as dual-feed rotor spinning. This methodology can be adapted for other systems where airflow is a central phenomenon driving the system.

Footnotes

Author's note

Jun Wang is also affiliated to Key Laboratory of Textile Science & Technology of Ministry of Education, College of Textiles, Donghua University, China.

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 and Graduate Student Innovation Fund of Donghua University (CUSF-DH-D-2019058) and the National Natural Science Foundation of China (Grant No. 11802161).

ORCID iD

Nicholus Tayari Akankwasa

Jun Wang

Huiting Lin

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