Analysis of the Weft Insertion Process and Development of a Relay Nozzle Concept for Air-Jet Weaving

Air jet weaving is the most productive method for the production of fabrics. However, the energy consumption of air jet weaving machines is signifi cantly higher than the consumption of other weaving machines. Almost 80% of the energy consumed can be attributed to the losses at the relay nozzles. At the Institut für Textiltechnik of RWTH Aachen University, Aachen, Germany diff erent novel relay nozzle geometries with lower air consumption have been developed on the basis of simulations and trials. The simulations have shown potential energy savings up to 50% compared to conventional relay nozzles. Furthermore, practical validations of the results of these simulations have been done. The velocity, stagnation pressure and volume fl ow were measured in the reed channel. In addition, an energetic and economic evaluation of the best relay nozzle geometry was done. The evaluations have shown that up to 50% of the energy can be saved.


Air Jet Weaving
Th e textile industry is an energy intensive industry.Th e increasing energy costs represent a challenge for textile manufacturers as well as for the developers of textile production machines.As example, the air jet weaving is the most productive but also the most energy consuming weaving method [1].In the air jet weaving process the weft yarn is inserted into the shed with compressed air by using diff erent types of nozzles.Figure 1 shows a schematic view of the weft insertion components.Th e current state-of-the-art air jet weaving machines employ a tandem and main nozzle combination for the purpose to provide the initial acceleration to the weft yarn, and a series of relay nozzles along the reed channel to keep constant yarn velocity of about 55-80 m/s.A profi led reed provides guidance for the air.At the end of the insertion process a nozzle catches and stretches the yarn at the right side of the machine.A cutter is used to cut the yarn when the insertion is completed, and the beat-up movement completes the fabric production process [3].

Infl uence of the Energy Consumption
Th e air-jet weaving machine combines high performance (Table 1) with low wear, because no mechanical parts are directly involved in the weft insertion process [3].However, the main drawback regarding this technology is a very high energy consumption (Table 1) due to the compressed air usage which is required during the weft insertion process.Since the cost of energy has a systematic increasing trend, power consumption is still a challenging issue.In particular it is a limiting factor of the air jet weaving technology in the countries with high energy and manufacturing costs.An overview of the manufacturing costs of a woven fabric can be seen in Table 2 [5].
Table 1: General characteristics of air-jet weaving machine [3,4] Air jet weaving machine

Weft ınsertion rate 2000 m/min
Average specifi c energy consumption of woven fabric 3-5 kWh/kg For instance, in Italy the total manufacturing costs are 0.665 USD/m of woven fabric and power costs correspond to 23% (0.56 USD/m).In other countries such as India and China, the total manufacturing costs are lower, 0.235 USD/m and 0.274 USD/m respectively, but on the other hand the power consumption is responsible for 27% (0.064 USD/m) and 34% (0.092 USD/m) respectively of the entire value.
In order to decrease the energy consumption and to increase the energy effi ciency, air-jet weaving machines are under constant development.At the Institut für Textiltechnik of RWTH Aachen University (ITA), Aachen, Germany, a novel method based on energy balances has been applied for the purpose of reducing the power costs while keeping constant the fabric quality.Th e study focused on the air fl ow fi eld of the relay nozzles [6].

Methods
Th e results of the research led to the development of a new geometry of the relay nozzle which is able to provide the same value of propulsive force to the weft yarn at a lower operating pressure level.Th is new concept enables that the relay nozzle works at 2 bar inlet overpressure in place of 5 bar as the relay nozzles available on the market [5].In such a way, the productivity is kept constant and the costs associated with compressors to pump up the air are decreased.Th e force on the yarn in the reed channel is provided by the friction between the air and the yarn surface and is given by the following Equation 1 [7]: Th e parameter A is the yarn surface, ρ is the air density and c w is the skin friction coeffi cient.Th e force is proportional to the square of the relative velocity between the air stream c and the yarn c F .So the air velocity is one of the most important infl uences on the propelling force.Assumptions for the further hypothesis: steady state fl ow negligible yarn fl exibility constant yarn velocity across the shed lossless fl ow no shockwaves no change of fl ow direction round free stream.-Th e thrust provided by the relay nozzles to the yarn has the highest infl uence on the productivity of the machine and on the quality of the product.Th e study carried out at ITA focused on the development of a new geometry of the nozzle in order to reduce the acting pressure without negatively aff ecting the productivity.Th erefore, an analytical mathematical model was set up at ITA which calculates the maximum velocity along the free stream of an ideal nozzle.Th is calculation should help to estimate the amount of air velocity a nozzle is able to reach under ideal circumstances.Th is calculation helps to benchmark the new designed nozzle.Th e mathematical model calculates the fl ow parameters under ideal conditions and is shown in Equation 2: Th e parameter c 0 is the air velocity in the core zone and d is the diameter of the nozzle.Th e parameter m describes the mixing factor.Th e position of the yarn is given by the parameter r F and the position x.Th e parameters are shown in Figure 2.  blowing angle of 6-8°.In Figure 4 the simulated velocity of the air stream up to a distance of 120 mm behind the nozzle is shown and compared to the analytical model.Figure 4 shows the velocity of the air on the vertical axes, and the distance downstream the nozzle on the horizontal axes.Th e nozzle is located in the origin.

Figure 4: Comparison between the velocity of the simulated air stream of the nozzle and the ideal analytical model
It can be seen that the velocity of the air is higher than the velocity of 50 m/s for a distance of around 120 mm in the simulation and the theoretical model.Th e fl ow of the nozzle is guided and, therefore, the developed nozzle works eff ectively.Th e diff erence between the simulation and the theoretical model is resulting from the condition that the model calculates with a lossless nozzle without change of the fl ow direction.Th e simulation cannot give any information about the fl ow condition inside the reed without raising the simulation time.Because of the two-dimensional projection of the analytical model, the calculation of the fl ow fi eld at the position of the yarn in the free stream is possible.Th is calculation gives an estimation of the fl ow fi eld which is aff ecting the yarn.Figure 5 shows the calculated fl ow fi eld at the position of the yarn compared with the simulation in the central layer of the nozzle.At a distance of 50 mm the velocity of the fl ow fi eld goes under a value of 50 m/s (see Figure 5).Despite of the lower reachable distance the energy saving potential of the nozzle is about 50%, due to the low operating pressure of 1 bar.Measurements in the reed channel show similar results compared to the analytical model.Only the distance behind the nozzle is diff erent.Th is diff erence can be explained by the infl uence of the reed which is not considered in calculation.Indeed, this diff erence has no eff ect on the energy balance of the weaving machine.

Conclusion
At the Institut für Textiltechnik of RWTH Aachen University, Aachen, Germany, a novel method has been developed to identify potentials for saving energy during textile production processes [1].Th e air-jet weaving process is the most productive but also the most energy intensive weaving process.Th e pneumatic components of the machine have been identifi ed as the biggest energy consumer.Based on a theoretical model of the weft insertion, a new concept of the relay nozzle has been drawn.By means of CFD simulations, the potential of the nozzle is shown which enables the energy reduction up to 50%.Nevertheless, the simulation includes a faithful reproduction of the free fl ow fi eld of the relay nozzle, without taking into account the interaction with the profi led reed and, therefore, it gives a fi rst insight on how to reduce the power consumption of the weft insertion process.Th e simulations were validated by a theoretical model and the measurement of the air velocity.

Figure 2 :
Figure 2: Schematic view of a free stream and the position of the yarn

Figure 3 :
Figure 3: Simulated fl ow fi eld of the new nozzle concept

Figure 5 :
Figure 5: Comparison between the simulated air stream of the nozzle and the calculated fl ow in the free stream at the position of the yarn