Extrusion instability in an aramid fibre spinning process

The efficiency of polymer extrusion processes can be severely limited by the occurrence of viscoelastic extrusion instabilities. In a para-aramid fibre spinning process, for example, a ?m-scale extrusion instability is responsible for the waste of tons of polymer per year. At present, a considerable amount of research literature is available on such viscoelastic extrusion instabilities. However, this literature largely applies to isotropic polymers, whereas the polymer solution that is used for the production of para-aramid fibres is liquid crystalline. Liquid crystalline polymers (LCPs) are anisotropic at rest, and their flow behaviour is known to deviate from that of other viscoelastic fluids. Therefore, more research was needed to characterise the extrusion instability in the para-aramid fibre spinning process. The work presented in this thesis deals, first, with the question to which extent the contraction flow of a nematic aramid solution is similar to contrac- tion flow of isotropic polymeric fluids. More specifically, we looked into the flow stability of nematic contraction flow, and its influence on the extruded aramid jet. Second, as a fibre spinning process typically involves the extrusion of around 1 000 closely spaced jets, the influence of the presence of neighbouring outlets on the behaviour of viscoelastic contraction flow is addressed. For the first part of the research, the aramid fibre spinning process was modelled by a 100 ?m deep 100:1 planar contraction flow with free outflow, which was designed to capture the essential features of the extrusion process. At a depth of 100 ?m the optically anisotropic aramid solution is sufficiently transparent to allow for flow visualisation, while at the same time the pressure drop over the geometry permits flow velocities that are realistic for fibre spinning, without damaging the glass flow cells in which the planar contraction geometries were etched. The contraction flow of a nematic aramid solution was compared with the behaviour of a PEG-PEO Boger fluid in the same geometry, using flow visualisation and Particle Image Velocimetry (PIV). It was shown that, under fibre spinning conditions, a nematic aramid solution shows viscoelastic vortex growth. Like in contraction flows of isotropic polymeric fluids, the vortex size in the aramid solution increases with increasing flow rate, and decreases when the contraction entrance is made more gradual (e.g. tapered, or rounded). The velocity field in the aramid solution was demonstrated to be characteristic of its shear-thinning behaviour. The influence of the defect structure in the aramid solution was visible in a wavy instability in the upstream channel, and in the occurrence of regions with a higher velocity than the surrounding flow, in the first minutes after starting the flow. The oscillation of the extruded jet was shown to be coupled to asymmetric velocity fluctuations in the upstream channel. Although no extrusion instability was encountered in the experiments, the existence of a relation between the jet behaviour and the upstream velocity field implies that the stability of the upstream velocity field is important for the stability of the jet. The similarity between the contraction flows of a nematic aramid solution and an isotropic viscoelastic fluid justifies the use of experiments with model fluids in the study of para-aramid fibre spinning. Therefore, the study of the influence of the presence of multiple outlets was carried out with a model fluid. Experiments using a PEG-PEO Boger fluid, and numerical simulations using a FENE-CR model (Finitely Extensible Non-linear Elastic, Chilcott-Rallinson closure), were performed in contraction geometries with one or three outlets, and a large or small distance between the outlets in the three-outlet geometries. The experiments and simulations show that in the three-outlet geometries, the curvature of the streamlines towards the outlets causes a horizontal pressure gradient in the upstream channel, resulting in the flow rate being distributed unequally over the outlets. Because the streamline curvature changes with increasing lip vortex size, the distribution of the flow rate over the outlets depends on the Weissenberg number of the flow. Furthermore, the vortex size was observed to decrease due to the presence of multiple outlets, with a smaller distance between the outlets leading to smaller lip vortices. This was demonstrated to result in a higher maximum elongation rate, a higher pressure drop over the geometry, and a decreased stability of the flow. The fluctuations in vortex height in what is classified here as unstable flow seems to lead to fluctuations in flow rate and outflow direction in the outlets. The results presented in this thesis are relevant for fibre spinning pro- cesses, but also for other production processes featuring multi-outlet extrusion of viscoelastic fluids. A logical next step would be to do experiments with a transparent model fluid in a more complex, three-dimensional extrusion geometry, to study extrusion instability in a multi-outlet geometry and to optimise the efficiency of such extrusion processes.Fluid mechanicsMechanical, Maritime and Materials Engineerin

An implicit rheological model for numerical simulation of generalized Newtonian fluids

Fitting an explicit curve over some discrete data extracted from a rheometer is the usual way of writing a rheological model for generalized Newtonian fluids. These explicit models may not match totally with the extracted data and may ignore some features of the rheological behavior of the fluids. In this paper, a cubicspline curve fitting is used to fit a smooth curve from discrete rheological data. Spline interpolation avoids the problem of Runge's phenomenon, which occurs in interpolating using high degree polynomials. The formulation for applying presented rheological model is described in the context of least squares meshfree technique. One problem is solved to show validity of the scheme: a fluid with rather complex rheology model is considered and solved by both conventional explicit and proposed implicit models to show the advantages of the presented method.Process and EnergyElectrical Engineering, Mathematics and Computer Scienc

A calibrated physical flow standard for medical perfusion imaging

In the medical sector, various imaging methodologies or modalities (e.g. MRI, PET, CT) are used to assess the health of various parts of the bodies of patients. One such investigation is the blood flow or perfusion of the heart muscle, expressed as the (blood) flow rate normalized by the mass of the volume of interest. Currently there is no physical flow standard for the validation of quantitative perfusion measurements. This need has been addressed in the EMPIR 15HLT05 PerfusImaging project. A phantom simulating the heart muscle has been developed with the capability that it can reproducibly generate a flow profile with individual flow rates known with a relative uncertainty of about 10% (k = 2) and total flow rate known with an uncertainty of 1% (k = 2). An overview of the phantom and its validation is given. Next, a new analysis method is presented to analyse the sequence of images which are acquired when using a standard dynamic imaging protocol. It is concluded that the new, alternative approach gives results comparable to the standard analysis method.Multi Phase System

Pixel-wise assessment of cardiovascular magnetic resonance first-pass perfusion using a cardiac phantom mimicking transmural myocardial perfusion gradients

Purpose: Cardiovascular magnetic resonance first-pass perfusion for the pixel-wise detection of coronary artery disease is rapidly becoming the clinical standard, yet no widely available method exists for its assessment and validation. This study introduces a novel phantom capable of generating spatially dependent flow values to enable assessment of new perfusion imaging methods at the pixel level. Methods: A synthetic multicapillary myocardial phantom mimicking transmural myocardial perfusion gradients was designed and manufactured with high-precision 3D printing. The phantom was used in a stationary flow setup providing reference myocardial perfusion rates and was scanned on a 3T system. Repeated first-pass perfusion MRI for physiological perfusion rates between 1 and 4 mL/g/min was performed using a clinical dual-sequence technique. Fermi function-constrained deconvolution was used to estimate pixel-wise perfusion rate maps. Phase contrast (PC)-MRI was used to obtain velocity measurements that were converted to perfusion rates for validation of reference values and cross-method comparison. The accuracy of pixel-wise maps was assessed against simulated reference maps. Results: PC-MRI indicated excellent reproducibility in perfusion rate (coefficient of variation [CoV] 2.4-3.5%) and correlation with reference values (R2 = 0.985) across the full physiological range. Similar results were found for first-pass perfusion MRI (CoV 3.7-6.2%, R2 = 0.987). Pixel-wise maps indicated a transmural perfusion difference of 28.8-33.7% for PC-MRI and 23.8-37.7% for first-pass perfusion, matching the reference values (30.2-31.4%). Conclusion: The unique transmural perfusion pattern in the phantom allows effective pixel-wise assessment of first-pass perfusion acquisition protocols and quantification algorithms before their introduction into routine clinical use.Multi Phase System

eWaterCycle: Building an operational global hydrological forecasting system based on standards and open source software

Water ManagementCivil Engineering and Geoscience

Methanol-fabricage

Document uit de collectie Chemische Procestechnologie. G-groep.DelftChemTechApplied Science

Economische berekening van de methanolfabriek

Document uit de collectie Chemische ProcestechnologieDelftChemTechApplied Science