Dariusz Asendrych

Self-sustained oscillations in a homogeneous-density round jet

The paper is devoted to a new phenomenon of self-sustained oscillations triggered in a round free homogeneous-density jet. It was shown by the experimental investigations supported by the numerical approach based on extensive-Large Eddy Simulations studies that such a self-sustained regime can be established in a homogeneous-density jet, provided that the boundary layer at the nozzle exit is sufficiently thin and the perturbation level sufficiently low. The growth rate of the naturally amplified unstable modes is high enough to induce backflow leading to self-excited oscillations.

Modelling liquid redistribution in a packed bed

The paper is devoted to the computational fluid dynamics (CFD) simulation of liquid spreading process in a packed bed. The volume of fluid (VOF) approach was applied to simulate the flow in a realistic porous region composed of 6mm Raschig rings. Modelling results were used to determine the probability density function (PDF) distribution of liquid velocity vector orientation angle which was then implemented into 2-fluid Euler-Euler multiphase model of packing column. The simulation showed that the model is capable to simulate adequately the liquid redistribution in a porous region being, however, much more efficient computationally than the VOF method.

Numerical Study of the CO2 Absorber Performance Subjected to the Varying Amine Solvent and Flue Gas Loads

The paper is devoted to the amine-based post-combustion carbon dioxide capture technology. The aim of the paper was to analyze the effect of varying flow conditions on the CO2 capture efficiency of the absorber column. As a research tool, a numerical model of the chemical absorption with aqueous monoethanolamine solution in a packed bed was employed. A complex physio-chemical process including two-phase flow hydrodynamics, heat transfer, and absorption chemistry was simulated by Ansys Fluent commercial software. The parametric study was focused on CO2 capture efficiency in terms of varying loads of amine solvent (liquid) and flue gas. The corresponding changes of liquid holdup, species concentration, temperature and reaction rate distributions are discussed in detail allowing to better understand the absorption column operation. The simulation results have shown clearly the mutual interactions of partial processes and the sensitivity of the system to varying column loads. They have been found to be useful in defining the optimal ranges of operational parameters.

Numerical Modelling of Heat Transfer in Rectangular Microchannels

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel heights, namely 1mm and 0.3mm (0.1mm), were used for 3D (2D) numerical model respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, the thermal entrance effect is analyzed in terms of heat transfer efficiency. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect in heat transfer in microchannel heat sink up to the smallest size considered (0.1 mm).

CFD simulation of a turbulent fiber suspension flow - a modified near-wall treatment

Turbulent Eucalyptus fiber suspension flow in pipes was studied numerically using commercial CFD software. A pseudo-homogeneous approach was proposed to predict the flow behavior of pulp fiber suspensions for medium consistencies and for Reynolds numbers ranging from 4.7÷65.3·103. Viscosity was introduced into the model as a function of shear rate to represent the non-Newtonian behavior of the pulp suspension. Additionally, the existence of a water annulus was considered at the pipe wall, surrounding the flow core, where viscosity is equal to the water viscosity. The near-wall treatment was modified considering an expression for the logarithmic velocity profile in the boundary layer, similar to the one suggested by Jäsberg (2007). The final model could reproduce the drag-reduction effect resulting from the presence of fibers in the flow. Moreover, the numerical results show that a better fit for pressure drop is obtained when the modified near-wall treatment is used and the Jäsberg adjustable parameters are adapted to take into account the flow conditions.

Numerical simulation of turbulent pulp flow of concentrated suspensions: Influence of the non-Newtonian properties of the pulp

ABSTRACT Computational tools such as Computational Fluid Dynamics can be useful to predict the flow of pulp suspensions. The present study aims to simulate numerically the flow of concentrated pulp suspensions using CFD tools. A commercial CFD software was used, Ansys Fluent, to simulate the turbulent flow of Eucalyptus and Pine suspensions. A pseudo-homogenous approach was followed by considering the pulp viscosity as a function of shear rate and pulp consistency. The model takes into account the existence of a very thin layer surrounding the core region, near the pipe wall. It is studied the influence of the presence of fibers in the lubrication layer, by considering that the pulp consistency depends on the distance to the pipe wall. Two low-Reynolds number (LRN) k-ϵ turbulence models, Abe-Kondoh-Nagano and Chang-Hsieh-Chen, implemented in Ansys Fluent by using User-Defined-Functions, were studied. The drag reduction effect observed experimentally could be reproduced by both models. The LRN models were modified taking into account the modifications proposed in literature for the turbulent flow of polymers and solid particles when a drag reduction effect is present. The models were validated by comparing the calculated pressure drop with the experimental values.

Experimental study of gas-liquid heat transfer in a 2-phase flow in a packed bed

The paper deals with an interfacial heat exchange between gas (air) and liquid (water) flowing countercurrently through a packed bed. The experimental investigation was performed with the use of column of 0.1 m inner diameter and filled in with glass Raschig rings. Loads of working media ranged between 0.018÷0.142 m3(m2s)-1 and 0.0007÷0.0053 m3(m2s)-1 for gas and liquid phases, respectively. It was found that interfacial Nusselt number is mainly dependent on the gas load, noticeably influenced by the temperature difference between phases and practically independent of the liquid load.

Numerical and Experimental Investigations of Momentum and Heat Transfer in Microchannels

The paper presents the results of a research performed during recent years at LEGI Grenoble with joint participation of CzUT researchers. The special attention is devoted to experimental and numerical studies devoted to three cases involving various aspects of microchannel flow physics (range of microchannel sizes corresponds to the classification of Kandlikar [19]). The first aspect is related to hydraulic properties of a network of parallel triangular microchannels, where experimental investigations revealed the rapid increase of pressure drop for Re exceeding value of 10. The second aspect of the research was the influence of surface roughness, which was investigated both experimentally and numerically for periodically and randomly distributed surface elements. The third research case was devoted to the numerical modelling of heat transfer. As a result, experimental and numerical analyses showed that there was no scale effect for the microchannels considered, i.e. the relevance of the classical continuum flow model was confirmed.© 2008 ASME