Iztok Žun

Phase discrimination in void fraction measurements via genetic algorithms

The paper deals with the difficulties of reconstructing a hydrodynamic structural function from a time‐dependent raw signal produced by a sensor. In the presence of two fluid phases, the deformation of a phase interface at a sensor leads to a lag between the occurrence and the detection of the event, and the probe response characteristics may be altered with time for several reasons. A new phase discrimination technique is presented based on man–machine interaction and utilizing genetic optimization. To discriminate the phases, two‐point threshold discrimination is proposed at first. In addition, a man–machine method is introduced which can mimic a human expert observation of a raw signal and his intuitive discrimination procedure. Finally, genetic algorithms are used to search for those threshold settings that produce solutions closest to those defined by an expert. The proposed new method is capable of adaptation to specific flow conditions.

Gas-filled cavity structures and local void fraction distribution in vessel with dual-impellers

Abstract An experiment was carried out in a pilot-size aerated mixing vessel using dual Rushton impellers. The two-phase mixture was composed of air and deionized water. Different gas-filled cavity structures were recognized close to the outer edge of the impeller blade by frequency transformation of the time-domain structural function. The majority of hydrodynamic regimes for the upper impeller were of VC structure, while the lower impeller operated over the VC, 1L, 2L, S33, L33, and RC structures respectively. Local void fraction α was also measured by a resistivity probe at 250 modes in the vertical half-section plane of the vessel. Spatial distributions of α are presented for the aforementioned hydrodynamics regimes. A transition from quite homogenous local void fraction distribution in a VC structure to a rather nonhomogeneous void distribution with a high concentraion of bubbles around the shaft was detected.

Computational fluid-structure interaction simulation of airflow in the human upper airway

Obstructive sleep apnoea syndrome (OSAS) is a breathing disorder in sleep developed as a consequence of upper airway anatomical characteristics and sleep-related muscle relaxation. Fluid-structure interaction (FSI) simulation was adopted to explain the mechanism of pharyngeal collapse and snoring. The focus was put on the velopharyngeal region where the greatest level of upper airway compliance was estimated to occur. The velopharyngeal tissue was considered in a way that ensures proper boundary conditions, at the regions where the tissue adheres to the bone structures. The soft palate with uvula was not cut out from the surrounding tissue and considered as an isolated structure. Both, soft palate flutter as well as airway narrowing have been obtained by 3D FSI simulations which can be considered as a step forward to explain snoring and eventual occlusion. It was found out that during the inspiratory phase of breathing, at given elastic properties of the tissue and without taking gravity into consideration, velopharyngeal narrowing due to negative suction pressure occurs. Furthermore, soft palate flutter as the main attribute of snoring was predicted during the expiratory phase of breathing. The evaluated flutter frequency of 17.8 Hz is in close correlation with the frequency of explosive peaks of sound that are produced in palatal snoring in inspiratory phase, as reported in literature.

Fluid-bed coater modifications and study of their influence on the coating process of pellets

Objective: In this study, different modifications of bottom spray fluid-bed coater with draft tube inserted were characterized and evaluated. Materials and methods: After coating the neutral pellets with polymeric solution comprising coloring agent pellet batches were characterized for coating variation, yield and degree of agglomeration. Results: Funnel-shaped distribution plate was found to improve process yield and decrease the degree of agglomeration at selected values of process parameters, whereas coating uniformity was worse in all cases when compared to conventional Wurster chamber. Results of the coating chamber with the swirl airflow generator indicate more uniform deposition of the coating material and in some cases an improved process yield and decreased formation of agglomerates when compared to conventional Wurster chamber. In series of experiments using Wurster chamber, having tangentially oriented air intake slots, which enabled introduction of air above the distribution plate, coating layer was more uniformly deposited on the pellet cores and formation of agglomerates was lower compared to the results obtained in a conventional Wurster coating chamber. Conclusion: Modifications of Wurster coating process by introducing swirling air motion within the draft tube or by introduction of air above the distribution plate have at selected values of process parameters resulted in reduced per-particle coating variation, degree of agglomeration and improved process yield.

Probe measurements of local carrying gas fraction in trickle-bed reactor

The possibility of measuring local carrying gas fraction in a pilot-size trickle-bed reactor was explored. The reactor bed was randomly packed with equal-size spherical particles. The two-phase mixture was composed of air and water. The carrying gas phase was detected by resistivity probes mounted in pores. To reduce the eventual bias caused by a localized flow the regime in a single pore, the local carrying gas fraction was defined by the allowability criterion, which relies on the correlation of the flow regime in a single pore with those of the surrounding pores. For this purpose the probes were fixed in an arrangement with the scale length greater than the particle diameter. Phase discrimination in different two-phase flow regimes was carried out by using the principles of artificial intelligence. A statistical analysis of probe signals revealed possible two-mode reactor operation. The results showed that local carrying gas fraction measurements in a pilot-size trickle-bed reactor over different modes of operation are possible. A comparison of the integrated carrying gas fraction profile with the global gas saturation showed rather good agreement.

Phase discrimination vs. multiscale characteristics in bubbly flow

Abstract Effective progress toward a better understanding of turbulent bubbly flow demands a strategy that comprises several space and time dependent phases of work priorities. The current work focuses upon experimental methods that can be used to collect the information on interfacial structures in bubbly flow and bubble to slug flow transition. Two experimental methods are analyzed in details: probe detection of phase conductivity and visualization. Although different experiments by nature, they utilize a common approach that can be described crudely as a two-part process: first, the scale information is broken into parts, and once the information is analyzed by the discrimination system lower level, it can be reassembled by the system higher level to tell us what is where in the environment. The uncertainty and applicability of phase discrimination are discussed for frozen interfacial structures as well as time-space evolution. In order to identify characteristics that are required by cascade modeling, macro-, meso- and microscales are considered. The assessment is given for 1D, 2D and 3D applications. Several attributes that are needed in spatio-temporal modeling at the reassembling stage are pointed out.

Laser ultrasonics for bubbly flow detection

Laser ultrasonics was used to study flow of air bubbles in still water. An experimental system was developed utilizing a Nd:YAG laser to generate ultrasonic pulses in the MHz range and a He-Ne laser beam to detect echoes from the bubble-water interfaces. Several methods were applied to compare ultrasonic waveform parameters with air flow rates. The results show a correlation between the number and magnitude of amplitudes in the waveform and the bubble production air flow rate.

Skill-based interpretation of noisy probe signals enhanced with a genetic algorithm

The paper presents the design and evaluation of an adaptive signal processing procedure based on human skill. The focus is on interpreting probe signals detected in gas?liquid flow in the presence of noise where existing signal interpretation techniques may encounter difficulties. Interpretation of a probe signal requires construction of a corresponding two-state signal that denotes the presence of the phases, i.e. gas and liquid, at the probe tip. To develop a computer procedure that would imitate a skilled operator in probe signal interpretation, manual knowledge acquisition and evolutionary optimization were employed. First, a prototype signal interpretation procedure based on operator skill was designed, and the procedure parameters were then optimized with a genetic algorithm. In the optimization process, a two-state signal reconstructed from the probe signal by an operator was used as a reference. The robustness of the approach was tested in a series of numerical experiments. They included local evaluation on training and test signals, calculation of global void fraction values, and an assessment of variability among different experts. The results showed that the developed technique is highly consistent with the operator way of signal interpretation and represents a reliable prerequisite for gas?liquid flow measurements.

Modeling the Effect of Red Blood Cells Deformability on Blood Flow Conditions in Human Carotid Artery Bifurcation.

The purpose of this work is to predict the effect of impaired red blood cells (RBCs) deformability on blood flow conditions in human carotid artery bifurcation. First, a blood viscosity model is developed that predicts the steady-state blood viscosity as a function of shear rate, plasma viscosity, and mechanical (and geometrical) properties of RBCs. Viscosity model is developed by modifying the well-known Krieger and Dougherty equation for monodisperse suspensions by using the dimensional analysis approach. With the approach, we manage to account for the microscopic properties of RBCs, such as their deformability, in the macroscopic behavior of blood via blood viscosity. In the second part of the paper, the deduced viscosity model is used to numerically predict blood flow conditions in human carotid artery bifurcation. Simulations are performed for different values of RBCs deformability and analyzed by investigating parameters, such as the temporal mean wall shear stress (WSS), oscillatory shear index (OSI), and mean temporal gradient of WSS. The analyses show that the decrease of RBCs deformability decrease the regions of low WSS (i.e., sites known to be prevalent at atherosclerosis-prone regions); increase, in average, the value of WSS along the artery; and decrease the areas of high OSI. These observations provide an insight into the influence of bloods microscopic properties, such as the deformability of RBCs, on hemodynamics in larger arteries and their influence on parameters that are known to play a role in the initiation and progression of atherosclerosis.