Characterisation of non-newtonian fluids using a back-extrusion technique

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.At present standard rheometers provide sufficiently precise measurements characterising behaviour of non-Newtonian materials. In practice, this accuracy is not always necessary, and the methods providing relatively cheap, fast and sufficient measurements of the rheological characteristics are fully acceptable. Back extrusion - representing one of these methods - is based on plunging of a circular rod into an axisymmetrically located circular cup containing the experimental sample. Formerly this method was applied for a characterisation of power-law, Bingham and Herschel-Bulkley fluids. The aim of this contribution is to present a sufficiently simple user-friendly procedure how to determine the individual rheological parameters appearing in the Vočadlo model (sometimes called Robertson-Stiff one) - yield stress, consistency parameter and flow behaviour index.cs201

Combatting cyanobacteria with hydrogen peroxide: a laboratory study on the consequences for phytoplankton community and diversity

Experiments with different phytoplankton densities in lake samples showed that a high biomass increases the rate of hydrogen peroxide (HP) degradation and decreases the effectiveness of HP in the selective suppression of dominant cyanobacteria. However, selective application of HP requires usage of low doses only, accordingly this defines the limits for use in lake mitigation. To acquire insight into the impact of HP on other phytoplankton species, we have followed the succession of three phytoplankton groups in lake samples that were treated with different concentrations of HP using a taxa-specific fluorescence emission test. This fast assay reports relatively well on coarse changes in the phytoplankton community; the measured data and the counts from microscopical analysis of the phytoplankton matched quite well. The test was used to pursue HP application in a Planktothrix agardhii-dominated lake sample and displayed a promising shift in the phytoplankton community in only a few weeks. From a low-diversity community, a change to a status with a significantly higher diversity and increased abundance of eukaryotic phytoplankton species was established. Experiments in which treated samples were re-inoculated with original P. agardhii-rich lake water demonstrated prolonged suppression of cyanobacteria, and displayed a remarkable stability of the newly developed post-HP treatment state of the phytoplankton community

Character and environmental lability of cyanobacteria-derived dissolved organic matter

Autotrophic dissolved organic matter (DOM) is central to the carbon biogeochemistry of aquatic systems, and the full complexity of autotrophic DOM has not been extensively studied, particularly by high-resolution mass spectrometry (HRMS). Terrestrial DOM tends to dominate HRMS studies in freshwaters due to the propensity of such compounds to ionize by negative mode electrospray, and possibly also because ionizable DOM produced by autotrophy is decreased to low steady-state concentrations by heterotrophic bacteria. In this study, we investigated the character of DOM produced by the widespread cyanobacteriaMicrocystis aeruginosausing high-pressure liquid chromatography-electrospray ionization-high-resolution mass spectrometry.M. aeruginosaproduced thousands of detectable compounds in axenic culture. These compounds were chromatographically resolved and the majority were assigned to aliphatic formulas with a broad polarity range. We found that the DOM produced byM. aeruginosawas highly susceptible to removal by heterotrophic freshwater bacteria, supporting the hypothesis that this autotroph-derived organic material is highly labile and accordingly only seen at low concentrations in natural settings

Effect of Die Design on Die Drool Phenomenon for Metallocene Based LLDPE: Theoretical and Experimental Investigation

V této práci byla provedena teoretická a experimentální analýza vlivu designu vytlačovací hlavy na jev die drool u LLDPE. Bylo zjištěno, že rozšíření výstupní štěrbiny vede k výraznému snížení tohoto negativního jevu, jehož vznik je možné vysvětlit na základě negativního tlaku vznikajícího na konci vytlačovací hlavy.The effect of die design on the die drool phenomenon was investigated for metallocene based LLDPE. It has been found that die exit opening and the flared die design can significantly reduce the die drool that die drool onset can be explained by the negative/non-monotonic pressure profile generated inside the die and/or at the die exit region

Two Ways to Examine Differential Constitutive Equations: Initiated on Steady or Initiated on Unsteady (LAOS) Shear Characteristics

The exponential Phan–Tien and Tanner (PTT), Giesekus, Leonov, and modified extended Pom–Pom (mXPP) differential constitutive models are evaluated in two ways: with regard to steady shear characteristics and with regard to large amplitude oscillatory shear characteristics of a solution of poly(ethylene oxide) in dimethyl sulfoxide. Efficiency of the models with nonlinear parameters optimized with respect to steady shear measurements is evaluated by their ability to describe large amplitude oscillatory shear (LAOS) characteristics. The reciprocal problem is also analyzed: The nonlinear parameters are optimized with respect to the LAOS measurements, and the models are confronted with the steady shear characteristics. In this case, optimization is based on the LAOS measurements and equal emphasis is placed on both real and imaginary parts of the stress amplitude. The results show that the chosen models are not adequately able to fit the LAOS characteristics if the optimization of nonlinear parameters is based on steady shear measurements. It follows that the optimization of nonlinear parameters is much more responsible if it is carried out with respect to the LAOS data. In this case, when the optimized parameters are used for a description of steady shear characteristics, efficiency of the individual models as documented differs

(Oxyanions of halogens in drinking water )

This paper looks at chemistry of individual oxyhalogens such as chlorite, chlorate, perchlorate and bromates, their origin in drinking water and health effects which restrict their concentrations. It is concluded that with understanding of mechanisms of formation of individual oxyhalogens and undertaking practical steps during water treatment and disinfection processes their concentration can, in the majority of cases, be controlled within the drinking water guideline limits