Joachim Götz

Visualisation of flow processes in packed beds with NMR imaging: determination of the local porosity, velocity vector and local dispersion coefficients

Abstract Magnetic resonance imaging (MRI) techniques are used to visualise flow phenomena within packed beds. In order to develop the most complex structure of the packed beds step by step from simpler geometric structures firstly the flow in a tube with one cylinder and a bundle of ten cylinders is investigated. It is possible to determine the three-dimensional (3-D) porosity and velocity distribution within the packing. The flow processes in the inlet region (i.e. transition from the tube without installations to the packing), in the packing and at the outlet of the packing can clearly be distinguished. The expected high porosity at the wall, the oscillating transition to the mean porosity in the interior of the tube and the corresponding wall effect on the flow distribution due to the radial distribution of the porosity could be detected. Furthermore, back flow, stagnant volumes and regions with high gradients of single velocity components can be detected. Axial and radial dispersion coefficients, porosity and velocity distributions, averaged in angular φ and axial direction z , can be derived from the nuclear magnetic resonance (NMR) data. This information is necessary for modelling the structure and the flow within a packed bed and for verifying theoretical predictions with regard to the distribution of the porosity and the velocity. Vector representation of the velocity field can be obtained.

Water-holding capacity and structure of hydrocolloid-gels, WPC-gels and yogurts characterised by means of NMR

The objective of this work is to illustrate the possibilities of NMR (nuclear magnetic resonance) for characterising gels. As model food systems, carrageenan-gels/solutions and whey protein gels are studied. The water-holding capacity of gels including sol-gel transitions is investigated. Pore systems of gels are characterised by analysed diffusion experiments. Yogurt is used as an example of a complex food, and is treated with a special wash-out-test. This test allows conclusions concerning the trend to syneresis and the structure of the system. It is shown that NMR provides a powerful tool for tackling practice-relevant problems, such as syneresis, sandy mouth-feel or increased yield and to improve or develop appropriate processes.

Possible optimisation of pastes and the according apparatus in process engineering by MRI flow experiments

In this paper, the spatially resolved determination of velocities in suspensions by means of nuclear magnetic resonance (NMR) imaging (MRI) techniques is described and applied to steady tube flows (with regard to the total flow rate) in different geometries. Three types of suspensions with different solid volume concentrations are examined in order to demonstrate the influence of the material-specific flow-behaviour and of the geometry of the experimental set-up on the observed flow-pattern. MRI offers the possibility to study the flow of multiphase materials with various forms of contrast (e.g. spin density, relaxation and spectroscopic data, diffusive and convective transport) spatially resolved. Thus even optically opaque suspensions can be studied in the interior of the material and the studied device. In this work the local probability distribution of single velocity components is determined. From this probability distribution both the local mean value and the according standard deviation can be derived. The standard deviation can be interpreted as the local dispersion coefficient of the according velocity component.

Flow behavior of pastes with a deformation history dependent yield stress

Abstract A constitutive equation is introduced in order to describe hardening of oxide-ceramic materials subjected to simple shear flow. The material is modeled as a Bingham fluid with a yield stress that depends on the deformation history. This new material law is studied for several flow situations: simple shear flow between two parallel plates (induced via constant velocity motion of the plates relative to one another), pipe flow with a constant pressure drop, and flow through an acutely conical die at a given volumetric flow rate. For flow between parallel plates, the shear stress is examined as a function of the strain. In the case of pipe flow, the time dependent development of the velocity field and volumetric flow rate is determined. The pressure distribution obtained in flow through a rotationally symmetrical conical die is compared for two material classes: fluids with a constant yield stress versus those in which the yield stress depends on the deformation.

Rheo-NMR: Applications to Food

In food process engineering, the handling, pumping, extrusion, or mixing of highly concentrated disperse systems is of importance for initial, intermediate, or final products. Examples are suspensions (beer mashes, chocolate, dough), foams (protein-, carbohydrate-based), emulsions, porous solids (baked products), and dry/wet bulk solids (malt, sugar). Although plenty of information concerning the rheology of multiphase system is available, the modeling of the flow behavior, structure of the produced system, and its stability cannot be considered as totally completed. In spite of its great importance, the determination of viscosities of disperse systems is difficult in principle [1,2]. Defined velocity profiles in rheometers (so-called viscometric flows), the homogeneity of the sample during the experiment, and applicability of continuous mechanics are usually prerequisites for rheometric studies. In disperse multiphase systems, demixing (phase separation: sedimentation, creaming, wall layers), scale effects [3], preparation, time effects (crushing, de-/agglomeration), tearing apart (fat, ointments), and wall slip cause problems when studying the flow behavior and determining flow functions of disperse systems with conventional rheometric methods. Materials with volume fractions of the disperse phase higher than 5 vol.% are usually opaque [4]. NMR offers the possibility to perform measurements non-invasively, non-destructively, and highly selective (in situ, online) in order to study the structure, structural changes, or inner transport processes (without preparations) in flowing materials. The objective of this contribution is to demonstrate the potential of various NMR techniques in monitoring flow processes and the structure of disperse systems. The relevance of NMR for engineering is based on the following possibilities and applications of noninvasive and non-destructive spectroscopic/tomographic sequences with possible temporal, spatial, and chemical resolution:

Characterization of Products Consisting of Synthetic, Amorphous Silica and Water with Different Moistures by Means of NMR

Mixtures of liquid and powder have usually totally different structures dependent on the moisture content. They vary from dry powders with small amounts of liquid to suspensions with a low solid concentration. For all moisture contents the binding mechanisms between the solid particles and the molecules of the liquid play an important role. In order to study these interactions, various products composed of synthetic, amorphous precipitated silica and water were produced using a special spraying technology with supercritical carbon dioxide. Thus, it is possible to produce disperse solid/fluid systems with high homogeneity. All samples were studied by means of nuclear magnetic resonance (NMR) which allows to distinguish between different types of water with regard to their mobility. The aim of this study is to develop a method for the characterization of solid/liquid systems in terms of their loading capacity of the liquid.