Sanna Haavisto

Novel approach to characterizing the growth of a fouling layer during membrane filtration via optical coherence tomography.

Fouling control is one of the critical issues in membrane filtration and plays a very important role in water/wastewater treatment. Better understanding of the underlying fouling mechanisms entails novel characterization techniques that can realize a real-time noninvasive observation and provide high resolution images recording the formation of a fouling layer. This work presents a characterization method based on optical coherence tomography (OCT), which is able to detect the internal structures and motions by analyzing the interference signals. An OCT system was incorporated with a laboratory-scale membrane filtration system, and the growth of the fouling layer was observed by using the structural imaging. Taking advantage of the Doppler effects, the OCT-based characterization also provided the velocity profiles of the fluid field, which are of great value in analyzing the formation of the cake layer. The characterization results clearly reveal for the first time the evolution of the morphology of the cake layer under different microhydrodynamic environments. This study demonstrates that OCT-based characterization is a powerful tool for investigating the dynamic processes during membrane fouling.

Modifying the flocculation of microfibrillated cellulose suspensions by soluble polysaccharides under conditions unfavorable to adsorption

Carboxymethylcellulose (CMC) and xanthan gum were studied as dispersants for microfibrillated cellulose (MFC) suspension using a rotational rheometer and imaging methods. The imaging was a combination of photography and optical coherence tomography (OCT). Both polymers dispersed MFC fibers, although CMC was more effective than xanthan gum. The negatively charged polymer chains increased the viscosity of the suspending medium and acted as buffers in between the negatively charged fibers. This behavior decreased the number and strength of contacts between the fibers and subsequently dispersed the flocs. The stronger separation of the fibers was reflected in the frequency sweep where the MFC/polymer suspensions had lower gel strength than pure MFC suspension. Dispersing effect was also observed in the flow measurements, where the floc size was more uniform with polymers in the decelerating flow and after long, slow constant shear, which normally induces a heterogeneous structure with large flocs into the MFC suspension.

New insight into rheology and flow properties of complex fluids with Doppler optical coherence tomography

Flow properties of complex fluids such as colloidal suspensions, polymer solutions, fiber suspensions and blood have a vital function in many technological applications and biological systems. Yet, the basic knowledge on their properties is inadequate for many practical purposes. One important reason for this has been the lack of effective experimental methods that would allow detailed study of the flow behavior of especially opaque multi-phase fluids. Optical Coherence Tomography (OCT) is an emerging technique capable of simultaneous measurement of the internal structure and motion of most opaque materials, with resolution in the micrometer scale and measurement frequency up to 100 kHz. This mini-review will examine the recent results on the use of Doppler-OCT in the context of flows and rheological properties of complex fluids outside biomedical field.

Shear localisation in interfacial particle layers and its influence on Lissajous-plots

Interfacial rheological measurements often show in their nonlinear Lissajous-plots rhombus or saddle-like shapes indicating complex local deformation behaviour. A strong interacting silica particle and an almost not interacting clay particle were studied in respect to their interfacial rheological properties. Large amplitude oscillation shear measurements were performed with a bicone geometry and combined with optical measurements, from which particle tracks were calculated. A correlation was found between the appearance of shear localisation and Lissajous-plot shapes. Silica particles showed shear localisation at the bicone edge and rhombic plateaus in the Lissajous-plot, while the shear localisation for the clay particles was observed at the cup’s wall as saddle-like shaped Lissajous-plots

Characterization of micro-fibrillated cellulose fiber suspension flow using multi scale velocity profile measurements

Rheological properties and boundary layer flow behavior of Micro Fibrillated Cellulose (MFC) suspended in water was studied using a novel velocity profiling rheometric technique. The method is based on measuring stationary velocity profiles of fluid flow in a straight tube simultaneously by Doppler Optical Coherence Tomography (DOCT) and by Ultrasound Velocity Profiling (UVP). The high resolution DOCT provides velocity profiles near the transparent tube wall, while UVP yields corresponding information in the interior parts of the flow. The data from the two instruments is combined into a comprehensive velocity profile including both the thin boundary layer near the wall and the interior parts of the flow. Within the boundary layer, concentration and thereby the viscosity of MFC is found to decrease towards the wall. At high flow rate, sublayer of virtually pure water is observed next to the wall, giving rise to apparent wall slip. The results from interior part of the flow show shear thinning behavior in qualitative agreement with results from conventional rheological methods. The results indicate that the new method can provide detailed experimental information on the rheology of MFC suspensions and their intricate boundary layer flow behavior, avoiding uncertainties inherent in many conventional rheological

The flow resistance of fiber sheet during initial dewatering

ABSTRACT In this work, a novel experimental filtration device with pressure profiling was used for analyzing water removal from the fiber network in the forming section of a paper machine. The experimental data were used to develop a stratified model for the flow resistance of the developing fiber sheet during the filtration process. The new model was verified with both laboratory- and pilot-scale measurements and excellent agreement was observed between them. It was also observed that there is a strong linear correlation between the maximal attainable solid content of a filtered sheet after the forming section and the instantaneous response to pressure of the flow resistivity of the furnish.

UDV measurements and CFD simulation of two-phase flow in a stirred vessel

Ultrasound Doppler Velocimetry (UDV) was successfully applied for measuring the three-dimensional velocity profiles of sand?water slurry in a cylindrical tank equipped with a 45° pitched blade turbine. Radial, axial and tangential velocity components were determined at the middle angular position between two baffles. The flow field in the tank was simulated by employing CFD with an algebraic slip mixture model combined with a sliding mesh method for the impeller. The simulated and measured velocity profiles were compared, and good general agreement between these results was found.