Indian Journal of Fibre & Textile Research Vol. 32, December 2007, pp. 414-420

Air flow behaviour in commingling nozzles and their influence on properties of commingled yarns

R Alagirusamya, Vinayak Ogaleb & Manick Bhowmickc

Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India Received 20 December 2006; revised received and accepted 6 June 2007

Two types of commingling nozzles have been used to study the air flow behaviour inside the nozzle and their effects on commingling performance of glass/nylon yarns. The air flow behaviour has been analysed with computational fluid dynamics software FLUENT 6.1 and actual air flow velocities are calculated using measured air pressure. It is observed that there is a good correlation between the simulation and the measured air flow pattern. The air flow pattern in the Nozzle-1 configuration shows more turbulent zones than that in Nozzle-2. The commingled yarn properties also clearly indicate that the effectiveness of commingling is much better with Nozzle-1 configuration than that with Nozzle-2 configuration.

Keywords: Air-jet nozzle, Commingled yarn, Composites, Computational fluid dynamics, Glass fibre, Hybrid yarns, Nylon IPC Code: Int. Cl.8 D02G3/00

1 Introduction Studies on composites from commingled yarns with different reinforcing fibres and matrix material combinations are reported in the literature.1,2 Although high levels of properties are attained with this technique, there is no work reported on optimization of commingling process in order to get best composite properties with less severe consolidation conditions. The heart of the commingling process is the air nozzle used for mixing of the constituent filaments. The design of the nozzle and the air flow behaviour inside the nozzle play critical role in deciding the quality of commingling. Iemoto and Chono3 measured air pressure distributions in yarn duct wall of mingling nozzles with various sizes of yarn channel and air entry orifice. They observed that the pressure in yarn duct does not always increase with supplied yarn pressure, but it predominately depends on the diameter ratio between the yarn channel and the air entry orifice. Chono and Iemoto4 also studied the yarn motion in an intermingling nozzle and found that the yarn motion is almost symmetric in the yarn duct axial direction. In another study, Weichun et al.5 developed sixteen mingling nozzles with different diameters of yarn

channel and air entry orifices and studied the effect of diameter of yarn channel in jet on the number of nips in mingled yarns. They found that the maximum number of nips per unit length does not monotonously change with the change in diameter of yarn channel and air entry orifices. They also commented that although the angle between the main channel and the axis of the single hole is always aimed at 90°, a slight inclination towards yarn entry side of the main channel would increase the nozzle effectiveness due to the forwarding effect. Iemoto et al.6 have produced nip and opened parts in a yarn without using conventional mingling jet but by running the yarn along a track where it is fastened at both ends and then exposing the yarn to an air-jet for a moment. They observed that with the increase in nozzle length, air-jet force acting on yarn as well as the diameter of opened part reduces. In an earlier investigation, the researchers have studied the influence of different nozzle configurations for commingling7 and found that a configuration having two air inlets, one acting perpendicular to the yarn channel and another acting at 45° opposite to the yarn flow direction, gives the best results. In this study, the nozzles with the above-mentioned configuration and another configuration where two air inlet holes acting perpendicular to yarn channel from opposite directions, are taken for investigation.

______________ aTo whom all the correspondence should be addressed. E-mail: alagiru@gmail.com bPresent address: Health Care Division, 3M India Limited, Bangalore. cPresent address: Technological Institute of Textile & Sciences, Bhiwani 127 021.

In the present study, the air flow behaviour in two types of nozzles has been simulated with FLUENT

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6.1 computational fluid dynamic software and the actual air velocities are measured with an experimental setup. Glass/nylon commingled yarns are prepared to relate the air flow profile with the commingling performance. 2 Materials and Methods 2.1 Preparation of Commingled Yarn Samples A laboratory model of commingling equipment has been developed for the present study. A 600 tex glass multifilament yarn and 176 tex nylon multifilament yarns from separate packages were combined and fed to a pair of feed rollers. After passing the yarn through supporting guides and an air nozzle, the commingled yarn was wound onto a package. Air nozzles with two different geometries were used to study the effect of air flow profile on commingling. The nylon yarn volume fraction was kept at three different levels, namely 40%, 57 and 67%. The air pressure was kept at 700 kPa. 2.2 Air Pressure Measuring System Figure 1 shows the experimental setup to measure the air pressure in the commingling nozzles. A nozzle holder is developed to hold the nozzle at a fixed position without interfering the air circulation inside and around the nozzle. The system is able to measure the air pressure at points along and across the nozzle (diam.5 mm) separated by 1mm precisely. The system has two screws with a pitch of 0.5 mm which will give the needle (pressure carrying duct to the mercury manometer) a precise movement of 0.5 mm vertically or horizontally as required. For measuring the air pressure inside the nozzle at different points with sufficient accuracy a ‘differential mercury manometer’ was used. The manometer can measure with an accuracy of 1mm Hg pressure. One side of the ‘U’ tube of the manometer was open to

atmospheric pressure and the other side was connected to the measuring needle with telescopic tube arrangement without any leakage. 2.3 Commingling Nozzles Nozzle-1 This nozzle has 4mm diameter for the main yarn channel. Two air-jets are impinging on the channel at 900 to the main channel and another air-jet is impinging on the air channel at an angle of 45° to the channel opposite to the direction of yarn flow. The air holes have a radius of 0.5mm. The structure of the nozzle-1 is given in Fig. 2. The position 1 is a straight line running from top to bottom of the yarn channel and the top point is marked in the top view of Fig. 2. It is the position, where the lines joining the two straight air-jets meet the periphery of the circle of main channel seen from above, when the 45° air-jet is kept upward. Positions 2, 8 are the straight lines spaced at 45° intervals along the main channel, clockwise from the position 1 at the periphery of the main channel. The middle position is the straight line going through the centre of the main channel. This convention has been followed throughout the paper.

Fig. 2 — Top and front views of Nozzle-1 (all dimensions are given in mm)

Fig. 1 — Experimental setup to measure air pressure in commingling nozzles

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Nozzle-2 This nozzle has a 4mm diameter for the main yarn channel. One air-jet is impinging on the channel at 90° to the main channel at the middle and another air-jet is impinging on the air channel from opposite side. The air-jets have a radius of 0.5mm. The structure of the nozzle-2 is given in Fig. 3. There is a 1.5mm wide cut in the main air channel cylinder which opens it to the atmosphere to facilitate the threading of the yarn. 2.4 Test Methods Ten specimens of 1 m length each were selected form different parts of package for each commingled yarn sample to obtain nip frequency. As the nip frequency alone does not describe the extent of nips in commingled yarn, other parameters like the degree of interlacing and average nip length8 (cm) were also obtained, as shown below:

Degree of Total length of nips in the yarn =interlacing (%) Length of yarn specimen

´

2.5 Simulation Method In actual practice the fibrous strands are permeable. As the high pressure air-jets strike the tow, the filaments get separated from each other and being flexible they also move with air flow. Due to their flexibility and small diameter as compared to yarn channel, they offer least resistance to air flow, thus the air flow pattern is not expected to be affected by the presence of fibres and hence simulation of air flow can be carried independent of filaments. Therefore, in the present work, the undisturbed air flow through the commingling nozzles has been studied since the flow pattern inside the yarn chamber has most significant influence on the process of mixing of the reinforcing filaments and matrix forming filaments.

100 …(1)

An attempt has been made to investigate the effect of nozzle configuration on commingling of glass and nylon filaments. In all the configurations, the flow is turbulent and hence the standard k-ε model of turbulence along with standard wall functions is used. As high range of pressures (~ 700-800 kPa) was used for commingling, the model used in FLUENT is turbulent, compressible flow model. Typically, the Reynolds’ averaged momentum and continuity equations along with energy equation, turbulent kinetic energy equation and ε equation are solved in segregated solver. The boundary conditions include inlet boundary and outlet boundary. In all configurations, air inlet boundaries are assumed to be pressure inlet type while outflow boundaries are assumed to be pressure outlet type. Following governing Reynolds’ averaged equations are used for solving a compressible turbulent flow9,10:

Fig. 3 — Top and front views of Nozzle-2 (all dimensions are given in mm)

Top View

Yarn

Mass conservation equation:

0i

i

ρuρt x

¶¶ + =¶ ¶

…(2)

Momentum conservation equation:

( ) 23

ji li j ij

i i j j i

uu upρu u μ δlx x x x x x

é ùæ ö¶¶ ¶¶ ¶ ¶ ÷çê ú÷ç= - + + - ÷ê úç ÷÷ç¶ ¶ ¶ ¶ ¶è øê ¶ úë û

( ' 'i jj

)ρu ux¶+ -

¶ …(3)

Turbulent kinetic equation:

Front View ( )i t

i j k j

ρku μ kμ G ρεx x σ x

é ùæ ö¶ ¶ ¶÷çê ú÷= + + -ç ÷ê úç ÷ç¶ ¶ ¶è øê úë û …(4)

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Rate of dissipation of the turbulent kinetic energy:

( ) 2

1 2i t

i j ε j

ρεu μ ε ε εμ c G c ρx x σ x k

é ùæ ö¶ ¶ ¶÷çê ú÷= + + -ç ÷ê úç ÷ç¶ ¶ ¶è øê úë û k …(5)

where ρ is the density; G, the rate of generation of turbulent kinetic energy; t, the time; u, the velocity; ε, the rate of dissipation of turbulent kinetic energy; p, the static pressure; ui’, the fluctuation in ui; k, the turbulent kinetic energy; tμ , the turbulent viscosity; and 1 2, , , ,kc c cε μσ σ , the constants of the k – ε model. 3 Results and Discussion 3.1 FLUENT Simulation study 3.1.1 Nozzle-1 The solver was first chosen as FLUENT 5/6. The main yarn channel and the air-jets were created from the volume command. The 45° angled air-jet was created a little bigger than original size and then subtracted from the main channel to properly add them together. Then meshing of the volumes was carried out with hexagonal mesh and cooper meshing scheme. The pressure input faces were defined as ‘pressure inlet’ for FLUENT 6.1 software. Figure 4 shows the Gambit modeling of the Nozzle-1. Then the mesh was exported for further processing. The mesh file was read in FLUENT software and the grid was checked for any fault in meshing. The

minimum volume should not be negative. The grid was properly smoothed, swapped and then scaled to the original scale unit for length. The length unit of FLUENT was also changed to the same unit. The solver was kept at the default one and the energy equations were activated. The viscous equations were chosen as ‘k-epsilon (2 equations)’ and the default settings were kept. FLUENT has pre-defined material ‘air’. So for material, default settings were kept and then the boundary conditions were defined. The pressure for pressure inlet and outlet was given in Pascal unit and the temperature was provided. For this nozzle pressure, inlet has a pressure value of 700 kPa and the outlet has 500 kPa. For wall and interior, the default settings were kept. The default operating conditions were selected. The energy relaxation factor was changed to 0.8. Default residuals were monitored during solution. After about 53 iterations the solution emerges. Air velocity profiles at different annular positions along the length are given in Fig. 5. Air velocity profiles at three planes near the jet impinging are shown in Fig. 6. It is observed that there is a very good amount of turbulence in the region where jet is impinging to the main channel. As there are three jets present in this nozzle, it has three good turbulence areas. Another aspect to be noted is that the place where the angled jet is impinging has a broader area for commingling turbulence though the air velocity is less, compared to the straight jets where the velocity is very high but the commingling area is less.

Fig. 5 — Air velocity profile for Nozzle-1 with FLUENT

Fig. 4 — Gambit modeling of Nozzle-1

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3.1.2 Nozzle-2 The procedure of modeling for the second nozzle was the same as in the previous one. To make the slot cut in main channel, first a brick of proper dimension was made and then it was subtracted from the main yarn channel properly. To get the slot, the remaining faces of the brick was totally deleted. Here also the hexahedral mesh was used with cooper scheme. The open slot in main channel was defined under pressure outlet. The mesh was exported for further processing. The FLUENT simulation is found to be the same as that in the previous nozzle. Here also inlet pressure was 700 kPa and the outlet pressure was 500 kPa. Figures 7 and 8 show the velocity profiles for Nozzle-2 obtained with FLUENT simulation. In this nozzle, there are two turbulence areas mainly near the jet impinging. The air velocity on other region of the jet is less due to the fact that the main yarn channel is open to atmosphere. Turbulence near the jet impinging area is quite high and the velocity is also high. 3.2 Experimental Measurement of Velocity Profiles 3.2.1 Nozzle-1 In this nozzle, 700 kPa pressure was applied in the inlet jets irrespective of mass flow rate and velocity was calculated. Figure 9 shows the velocity profile for Nozzle-1. The velocity is given to the upward direction and hence a negative value in velocity signifies that it is in the downward direction. If the coefficient of variation of velocity magnitudes is an indication of turbulence inside the nozzle, then this nozzle has a quite good turbulence. The maximum value of CV% of this nozzle is 98.8%, which is quite high and this value is obtained at the plane 38 mm downward from the top of the nozzle. This indicates

Fig. 6 — Velocity profiles at three planes near the jet impinging

Fig. 7 — Air velocity profile for Nozzle-2 with FLUENT

Fig. 8 — Velocity profiles at two planes near the jet impinging the high level of turbulence present in the air flow. The average CV% value for all the planes is also quite high (42.85%). Error in velocity magnitude obtained from CFD analysis with respect to experimental values is 8.51%.

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Fig. 9 — Air velocity profile of nozzles with input pressure of 700kPa

3.2.2 Nozzle-2 Figure 9 shows the air velocity profile for Nozzle-2 with input pressure of 700 kPa. As there is a slot cut in the main channel which is open to the atmosphere, there is not much air movement outside the range of 15 mm from the air-jets. So during the process, commingling will only take place near the air-jets. Though this nozzle shows a high amount of turbulence in terms of coefficient of variation of velocity, it has to be kept in mind that this high value comes due to zero air velocity inside the nozzle in some places, which is mainly due to the static atmosphere and not due to any velocity change inside the air nozzle. Error in velocity magnitude obtained from CFD analysis with respect to experimental values is 7.42%. 3.3 Properties of Commingled Yarns Design of air nozzle controls cyclic production of nips and opening parts in the commingled yarns. Both the sections are equally important for subsequent processing of commingled yarn. The two phase nip

formation theory has been described by various authors.11,12 Firstly, the filaments are opened by the incoming jets and then the air flow from different levels and directions within yarn channel takes the filaments in quick movements similar to braiding or plaiting or false twisting, leading to the formation of nips at both sides of the jets.

Fig. 10 — Properties of commingled yarns [(a) nip frequency (CV 4.2%), (b) average nip length (CV 3.6%), and (c) degree of interlacing (CV 3.1%)]

The opened segment is produced at the position subjected to strong action of an air-jet whereas the nips are formed at its both sides. In opened sections, reinforcing and matrix forming filaments are randomly dispersed. Opened sections are bulkier than nips. Nips in commingled yarns are compact sections which would act as the binding points between open portions. The nip frequency of commingled yarns is the function of the speed of rotation of the vortex. The frequency of air vortex generated mainly depends on the air pressure and the jet design. If one of the vortices is predominant, the yarn will be retained only in that vortex and it may be twisted in one direction without any commingling effect.

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From preliminary microscopic observations the nips found in commingled yarns can be classified as braids, entanglements, entangled braids, wraps and others (consisting of core, braided core and side-by-side). The occurrence of particular type of nip in a jet depends on the condition of filaments when they are acted upon by the jet. The structure of different types of nips and the possible causes for the formation of these types of nips have been discussed elsewhere.13 Figure 10 shows the properties of glass/nylon commingled yarns produced with both the nozzles studied. Figure 10a clearly indicates that the Nozzle-1 produces yarns with higher nip frequency as compared to Nozzle-2. The difference between the nip frequencies of Nozzle-1 and Nozzle-2 is high at lower nylon volume fraction. This is because the nylon volume fraction is increased by increasing the number of nylon strands, keeping the glass content same. This increases the total number of filaments in the combined strand. As the total number of filaments increases, the chances for forming more stable nips increases, thereby increasing the nip frequency as the nylon volume fraction is increased. The percentage increase is more for Nozzle-2, although yarns produced with Nozzle-1 have higher nip frequency even at the highest volume fraction of nylon. There is no significant difference in the average nip length between the yarns produced with Nozzle-1 and Nozzle-2 although there is a slight increase in the average nip length in the yarns having higher volume fraction of nylon. The degree of interlacement shows similar trend as that of the nip frequency as it is proportional to the product of nip frequency and the average nip length.

4 Conclusions The air flow behaviour in commingling nozzles of two different configurations is simulated using a computational fluid dynamic software FLUENT 6.1. An experimental setup is developed to measure the air velocity at different locations inside the nozzle. There is a very good correspondence between the simulated air flow velocities and the actual measured air flow velocities. The air flow pattern in the Nozzle-1 configuration shows more turbulent zones than that are present in the Nozzle-2. Glass/nylon commingled yarns are produced with both of these nozzle configurations with different volume fractions of nylon. Nip frequency, average nip length and the degree of interlacing which are indicators of the effectiveness of commingling are measured. These commingled yarn properties also clearly indicate that the effectiveness of commingling is much better with Nozzle-1 configuration than with Nozzle-2.

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