Modeling I

In this paper, flow through hydroentangling nozzles is simulated at different time-scales and discussed in details. A clear relation between the predictions of Computational Fluid Dynamics (CFD) simulations and the experimental observation is demonstrated.  The impaction between the resulting waterjets and solid targets are modeled and relevant conclusions for fiber entanglement are drawn with respect to the experimental observations.

Hydroentanglement is the fastest growing nonwoven fabric bonding technology, but currently the research of hydroentanglement process mainly focuses on experimental studies. In this paper, we developed a model of the hydroentanglement process. Three dimensional simulations of the water flow through fiberweb and forming surface are performed.  This model provides an effective and almost no-cost way (only a computer is used) to predict the influence of the jet pressure, jet diameter, web thickness, forming belt geometry, and process speed on the hydroentanglement degree

MRI is a nondestructive technique suitable for the characterization of fluids in opaque materials.  We have been using several variants of the technique for the investigation of various technical aspects of nonwovens:

(1)
Information as to the spatial distribution of a fluid may be obtained under static conditions.  It is even possible to independently determine the locations of two immiscible fluids contained within one single nonwoven sample.

(2)
The spontaneous imbibition of a fluid (wicking) into a nonwoven material can be studied in detail.

(3)
The Brownian motion of individual fluid molecules may be observed as a function of a variable time window.  This provides information regarding the average size of pores, the pore size distribution and the connectivity of pores in a nonwoven structure.  This technique may be implemented into low-cost benchtop NMR spectrometers leading to a time- and cost-effective alternative to existing techniques concerned with the characterization of porous structures of nonwovens. 

(4)
It is possible to detect the velocity vectors associated with a stationary flow through fibrous substrates.  Results from MRI are well suited to validate detailed computational fluid dynamics (CFD) studies. We see a large potential of this combined approach using CFD and NMR for the development and evaluation of filtration materials

Through-air bonding process is known for its product versatility with wide ranging properties.    The process mechanics of the through-air bonding is simple, in that it relies upon passing heated air through an unbonded porous nonwoven web to heat, soften, melt fibres; and bond the web. It is therefore, essential to evaluate the optimum airflow and heat transfer conditions for improved product quality and energy savings.

This paper presents a computational model of the through-air bonding process that has been developed and experimentally verified, as a collaborative research between Loughborough University and Colbond bv. A 2-D computational fluid dynamics (CFD) model incorporating an empirically determined flow resistance is proposed. The model includes several components of a typical industrial machine including the tensioning belt, the nonwoven web, drum cover and drum. Process conditions such as air velocity, air temperature, web thickness and dwell time have been investigated. Simulation results are compared with the experimental data taken from a pilot plant at Colbond bv. There is a good agreement between the experimental data and the CFD simulations. The approach suggested that qualitative benefits such as the ability to consider more parameters, risk reduction, and the availability of valuable information early in the process can be provided. The model is an effective and economic tool for process and product development and optimisation to achieve improved capacity, quality and energy efficiency.

Many fibrous materials such as nonwovens are consolidated via compaction rolls in a so-called calendering process.  Hot rolls compress the fiber assembly and cause them to thermally bond to each other resulting in a strong but yet porous material.  In this paper, we describe our algorithm for generating 3-D virtual fiber-webs and simulate the geometrical changes that happen to the structure during the calendering process.  In agreement with our experimental observations, it was found that the average Solid Volume Fraction (SVF) profile across the thickness becomes U-shape after the calendering.  Considering fiber-webs composed of two different fibers with different rigidities, it was found that increasing the percentage of the rigid fibers causes the minimum SVF to rise resulting in a more homogeneous material. 

Fraunhofer ITWM has been working on the modeling and simulation of fiber dynamics since about ten years. During this period we have developed and implemented the Fiber Dynamics Simulation Tool called FIDYST. The fiber dynamical behavior in FIDYST is modeled on basis of a rod theory balancing all acting forces. There are inner forces like tension or bending, outer forces like gravitation or air drag as well as forces due to fiber-wall or fiber-fiber interactions. One of the most difficult modeling tasks is certainly the consideration of turbulence effects. Hereto, we have developed a mathematically founded methodology to incorporate these effects by means of stochastical aerodynamical forces. With respect to practical usability for the simulation of nonwoven production processes a general concept for fiber-wall interactions is necessary. FIDYST is able to handle arbitrary geometries based on triangulations.

To deal with extremely large numbers of filaments in the simulation of nonwoven production FIDYST makes use of appropriate reduced stochastical models for the laydown structures. The parameters of these models are identified by simulation of single filaments. This concept allows the generation of a 'virtual' nonwoven structure and its analysis with respect to quality criteria like homogeneity (cv-value) and orientation (MD/CD). This kind of simulations and evaluations are the basis for optimization of nonwoven production concerning process and design parameters.

The lecture presents different aspects of modeling fiber dynamics and the structure of simulation software FIDYST in combination with commercial tools like FLUENT or ANSYS CFX for the computation of the surrounding air flow. The potential of using such kind of simulations for the design and optimization of nonwoven processes is presented in different applications. One example is the significant progress in the development of the former ASON technology worked out in direct corporation with Neumag | Saurer.