Rudolf Žitný

Viscosity and electrical conductivity of concentrated solutions of soluble coffee

Abstract Viscosity of concentrated aqueous solutions ( ω =0.5–0.8) of soluble coffee was measured in the temperature range 25–95°C. After some time of the shear application in the viscometer and passing a temperature of 95°C, the rheological behaviour was found to be Newtonian. The viscosity was correlated by a five parameter function of coffee mass fraction and temperature. Specific electrical conductivity of coffee solutions in tap water ( ω =0–0.8) was measured in the temperature range 25–72°C. A seven parameter model based on the assumption that the solution is composed of a partially dissociated species and water describes very well the measured data. A modified Casteel–Amis model equation has been identified for comparison. The conductivity dependence on mass fraction exhibits maxima, which are shifted towards higher concentrations the higher is the temperature. Viscosity, refractivity index, density and thermal conductivity of aqueous coffee solutions ( ω =0–0.5) are reviewed.

Radiotracer applications for the analysis of complex flow structure in industrial apparatuses

Abstract Complex flow analysis by gamma-radiotracer utilizes signals of detectors situated around the apparatus. In this case the problem of the relation between detected values and radiotracer concentration has to be solved. Analytical relation was already published for collimated scintillation detector (with several simplifying assumptions) which was used for analysis of parallel flows assuming uniform concentration in both streams. The suggested algorithms were used in parametrical analysis of residence time distribution (RTD) model by non-linear regression. For a general distribution of the tracer concentration another algorithms were suggested and tested by using MC code as well by experiments with point radioactive source. These algorithms are successfully used in verification of complex flow visualisation by computer fluid dynamic method. While the verification on the basis of measurement RTD is sufficient for simple flow structure, the verification on the basis of local responses should be used in the more complex flow structure. Application of suggested algorithms in the verification procedure is demonstrated on results of analysis of “parallel” flow realized on a test rig of the ohmic heater.

Simple rheological models of food liquids for process design and quality assessment

Abstract This paper presents a short review of simple rheological models which can be used for describing rheological properties of liquid food materials. The applicability of these models is discussed from the viewpoint of the purpose for which the rheological description serves. Distinction is made between technological and engineering purposes. For technological purposes, the model is used as a tool for investigating the influence of various technological variables upon the rheological behaviour and, if possible, upon the quality of the product. As an engineering tool a rheological model is helpful in designing new, or improving the efficiency of existing, types of production equipment. If used for an engineering purpose, a rheological model should be evaluated at shear rates identical with those existing in the production equipment. Values of shear rates in some types of equipment together with ranges of shear rates attainable in commercial rheometers are given. The use of rheological models is demonstrated by two examples in dairy technology: variation of rheological properties of mayonnaise with time and frictional pressure drop in pipe-line transportation of quark.

Constitutive Modeling of Human Saphenous Veins at Overloading Pressures

In the present study, inflation tests with free axial extension of 15 human vena saphena magna were conducted ex vivo to obtain data suitable for multi-axial constitutive modeling at overloading conditions (pressures up to approximately 15kPa). Subsequently the data were fitted with a hyperelastic, nonlinear and anisotropic constitutive model based on the theory of the closed thick-walled tube. It was observed that initial highly deformable behavior (up to approximately 2.5kPa) in the pressure-circumferential stretch response is followed by progressive large strain stiffening. Contrary to that, samples were much stiffer in longitudinal direction, where the observed stretches were in the range 0.98-1.03 during the entire pressurization in most cases. The effect of possible residual stress was evaluated in a simulation of the intramural stress distribution with the opening angle prescribed to 0°, 10°, 20°, 30°, 40°, and 50°. The result suggests that the optimal opening angle making the stress distribution through the wall thickness uniform is about 40°. The material parameters presented here are suitable for use in mechanobiological simulations describing the adaptation of the autologous vein wall after bypass surgery.

Modelling of thermal pressure forming of starch-based materials

Abstract Models describing the pressure baking of starch-based biologically degradable products are reported to explain the surprising temperature and pressure variations observed experimentally. Baking of potato starch suspension in the confined space of a mould is characterised by an initial increase followed by a decrease in temperature of the processed material even if the temperature of the heating walls is constant. The suggested integral model describes processes, which take place inside the liquid core, porous crumb and gradually formed solid crust at the surface. It is assumed, that the temperature of the sample is controlled by steam pressure during the phase of free water evaporation and thus the pressure drop corresponding to the flow of steam through the crumb, crust and along the wall of the heater is analyzed in more details. Volumetric expansion of processed material is attributed to the expansion of the liquid core filled by bubbles of steam.

Heat Transfer Enhancement and RTD in Pipes with Flow Inversion

The inversion of streamlines between the centerline and wall region of a pipe improves residence time distribution (RTD) characteristics and heat transfer in laminar flows. Suggested one-parametrical inversion models predict RTD and Nusselt number values for the flow of a Newtonian liquid in a pipe with one or more flow inverters. These models are used for inverter description based upon the principle of Deans vortices in a local tube bend. Results obtained by CFD programs reveal an optimum inversion effect at Reφ ≈ 110, where φ is the angle of bend. Model identification using first appearance time and heat transfer enhancement is discussed.

Squeezing flow of collagen solution – Mathematical model of shear and elastic behavior

This contribution deals with the problem of the squeezing flow of highly concentrated collagen solution (7%). The sample of collagen is compressed between two parallel disks of the texture analyzer instrument TA-XT2i. The bottom disk is fixed and the upper one is approaching at a constant velocity. During experiment the force acting on the moving plate is measured accurately. Total force – deformation dependence data sets were applied for evaluation of parameters of mathematical model having independent parameters for shear and elongation deformation regimes. Because lubricated Teflon foils are attached to the both plates a partial wall slip on plates is considered in the developed mathematical model assuming prevalence of the elongation flow contribution.

Constitutive Modelling and Histology of Vena saphena

The inflation-extension test was performed in order to obtain the mechanical response (stress-strain curves) of the human vein - vena saphena magna (usually used for coronary artery bypass graft surgery). Tubular samples of the vein were inflated four times up to the pressure approx. 4 kPa (vein pressure) and then four times up to approx. 16 kPa (systolic pressure). The experiments were recorded by the CCD camera. The longitudinal and circumferential deformations of the tube were evaluated using the edge detection method. The experimental data were fitted by anisotropic, nonlinear, constitutive model in order to obtain model parameters, especially the parameter which can be explained as collagen fibres orientation approximation. This parameter was then compared with the findings from histology. The histology analyses based on label-free imaging were performed additionally to the mechanical testing. Collagen (most important load-bearing component of the vein wall) was visualized using second harmonic generation imaging (SHG, excitation at 860 nm by a tunable IR pulse laser, detection at 430±10 nm). This method enabled us to observe collagen through the vein wall. It was found that the collagen fibres are helically aligned within the vein at an angle 37±6° measured from circumferential axis. The results of collagen orientation angle show a good agreement of findings obtained from histology and from constitutive model.

INTEGRAL METHODS FOR DESCRIBING PULSATILE FLOW

This paper presents an approximate solution of the pulsatile flow of a Newtonian fluid in the laminar flow regime in a rigid tube of constant diameter. The model is represented by two ordinary differential equations. The first equation describes the time evolution of the total flow rate, and the second equation characterizes the reverse flow near the wall. These equations are derived from the momentum balance equation and from the kinetic energy equation, respectively. The accuracy of the derived equations is compared with a solution in which the finite difference method is applied to a partial differential equation.

UNSTEADY FLOW OF THIXOTROPIC COLLAGEN SUBSTANCE IN PIPES

Unsteady flow of thixotropic liquid in pipes is solved by 1D and 2D numerical methods using the same constitutive equation — the only difference is in the radial diffusion of the structural parameter. Comparison shows that the neglected diffusion of structural parameter implicates a much stronger effect of thixotropy. The models are applied for analysis of the observed hysteresis of hydraulic characteristic of collagen.