1. The numerical analysis of flame stability in case of premixed hydrogen-air combustionMarc Jaeger, Matjaž Hriberšek, Niko Samec, Yang Guo, Xuebin Wang, Filip Kokalj, 2025, original scientific article Abstract: This study addresses the urgent need for decarbonization of heating sector, focusing on the potential of hydrogen-air combustion in gas condensing boilers. In the field of gas condensing boilers, groups of holes or slits can be used in perforated burner surfaces, providing a possible solution to the challenge of hydrogen air combustion. Real burners are always adjacent to an area with non-existent combustion, which can lead to a disturbed flame pattern. For this reason, a comparison between single-slit and multi-slit perforated flame holders with a significantly reduced structure temperature with a hydrogen-air premix is carried out as part of this study. Using Ansys Fluent, a 2D simulation approach is employed to analyze laminar flame stability behind a perforated flame holder. Inlet conditions consider fully premixed and homogenous distributed mixtures as well as nonhomogeneous premixed mixtures, with variations in pressure, velocity, and mixing degree. The model examines various slit-burner configurations and flashback behaviors based on an actual geometric shape used in a gas heating device. It has been found that with finite multi-slit burners, the bulk flow rates at the critical flashback point are 30–50% higher compared to infinitely symmetrical burners. A further deterioration of 32% is to be expected due to real mixture quality fluctuations and inhomogeneous inflow conditions. Finally, the findings demonstrate the model’s potential for designing efficient surface-stabilized burners and lay the groundwork for future 3D simulations in the pursuit of sustainable heating solutions.
Keywords: hydrogen combustion, flashback, flame stability, perforated flame holder, gas condensing boiler
Published in DKUM: 19.05.2025; Views: 0; Downloads: 2
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2. Ellipsoidal soft micro-particles suspended in dilute viscous flowJana Wedel, Matjaž Hriberšek, Jure Ravnik, Paul Steinmann, 2025, original scientific article Abstract: Soft particles in viscous flows are prevalent both in nature and in various industrial applications. Notable examples include biological cells such as blood cells and bacteria as well as hydrogels and vesicles. To model these intriguing particles, we present an extension of our recent, efficient, and versatile pseudo-rigid body approach, originally developed for initially spherical soft particles suspended in arbitrary macroscale viscous flows. The novel extension allows modeling the barycenter and shape dynamics of soft initially non-spherical, i.e. ellipsoidal particles by introducing a novel shape and orientation tensor. We consider soft, micrometer-sized, ellipsoidal particles deforming affinely. To this end, we combine affine deformations (as inherent to a pseudo-rigid body) and the Jeffery-Roscoe model to analytically determine the traction exerted on a soft ellipsoidal particle suspended locally in a creeping flow at the particle scale. Without loss of generality, we assume nonlinear hyperelastic material behavior for the particles considered. The novel extension of our recent numerical approach for soft particles demonstrates that the deformation and motion of the particles can be accurately reproduced also for ellipsoidal particles and captures results from the literature, however, at drastically reduced computational costs. Furthermore, we identify both the tumbling and trembling dynamic regime for soft ellipsoidal particles suspended in simple shear flow again capturing results from the literature. Our extended approach is first validated using experimental and numerical studies from the literature for quasi-rigid as well as soft particles, followed by a comparison of the effects of particle deformability for some well-known fluid flow cases, such as laminar pipe flow, lid-driven cavity flow, and a simplified bifurcation. We find that taking particle deformability into account leads to notable deviations in the particle trajectory compared to rigid particles, with increased deviations for higher initial particle aspect ratio. Furthermore, we demonstrate that our approach can track a statistically relevant number of soft particles in complex flow situations.
Keywords: soft particles, Lagrangian particle tracking, pseudo-rigid bodies, point-particle method
Published in DKUM: 19.05.2025; Views: 0; Downloads: 2
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3. Energy intake models for intermittent operation of dead-end microfiltration filling lineJure Ravnik, Gorazd Bombek, Aleš Hribernik, Timi Gomboc, Matej Zadravec, Aleks Kapun, Grega Hrovat, Jure Gradišek, Matjaž Hriberšek, 2022, original scientific article Abstract: In filling lines equipped with membrane separation devices in the form of filters energy,
consumption is only one of the important working parameters, the other being sustainable filter
performance in terms of separation efficiency. As the filling line is typically equipped with a valve,
intermittent operation of the filter is an important form of its use. Whereas the overall energy
consumption of the filtration process is governed by the continuous operation mode, the intermittent
mode, characterised by opening/closing of the valve, contributes most to problems of filter failure,
i.e., the breakthrough of filtered particles through the membrane. A model for determination of the
energy intake of a microfiltration membrane during the opening and closing of a valve is presented
in this work. The model is based on computational analysis of the pressure wave signals recorded
during the opening/closing of the valve using Fourier transform, and expressed in a nondimensional
filter area specific energy intake form. The model is applied to a case of constant pressure dead-end
microfiltration with three filter types: a single membrane filter, a stacked filter and a pleated filter
with filtration surface areas ranging from 17.7 cm2
to 2000 cm2. Both clean filters, as well as partially clogged filter cases are taken into account. Second order polynomial models of the energy intake are developed and evaluated based on extensive analysis of the experimental data. The analysis of energy intake results show that the largest energy intake was observed for the clean filter case. When membrane fouling occurs at the constant flow rate values it leads to larger energy intake, however,
due to a decreasing specific flow rate during fouling these values do not exceed the clean filter case.
Keywords: membrane filtration, water hammer effect, membrane energy intake, filter clogging
Published in DKUM: 28.03.2025; Views: 0; Downloads: 1
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4. Experimental and mathematical investigation of anaerobic granulate density via settling velocityAljaž Klasinc, Gregor Drago Zupančič, Matjaž Hriberšek, Aleš Hribernik, Aleksandra Lobnik, Matej Zadravec, 2022, original scientific article Abstract: The objective of this study was to determine the density of anaerobic granules on different
heights of a full-scale Upflow Anaerobic Sludge Bed reactor. The density was defined through the
settling velocities of anaerobic granules, measured in a full-scale Upflow Anaerobic Sludge Bed
reactor. In this study, granular density was calculated with the measured settling velocities and
developed mathematical model. The developed mathematical model is based on the Stokes model.
In the experiment, granules were taken from different heights of an Upflow Anaerobic Sludge Bed
reactor, from 0.6 to 7.6 m. The granules’ diameters varied between 1 and 5 mm. The granules were
taken from six different heights through the reactor. The settling velocity of the active granules
(with gas in the granule pore and on the surface of the granule) was measured first. After the active
granules’ settling velocity measurement, the granules were placed in a vortex to obtain degassed
granules (granules without any gas in the pores or on the surface), for which the settling velocities
were also measured later. It is shown that granules’ densities at different heights are independent of
the reactor height.
Keywords: Upflow Anaerobic Sludge Bed reactor, microstructure, anaerobic granular sludge, settling velocity, density
Published in DKUM: 26.03.2025; Views: 0; Downloads: 5
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5. Analiza in modeliranje adsorpcijske naprave z nihanjem tlaka za čiščenje vodika v sistemih elektrolize vode : diplomsko deloJure Čas, 2024, undergraduate thesis Abstract: V sistemih za proizvodnjo vodika z elektrolizo vode je potrebno zagotoviti neprekinjeno delovanje procesa adsorpcije za eliminacije vodne pare. To zagotovimo z uporabo ločevalnega postopka adsorpcije z nihanjem tlaka (PSA). PSA je ciklični postopek z vsaj dvema adsorpcijskima enotama, ki vsebujeta trden adsorbent. V nalogi so prikazane ravnotežne karakteristike adsorbentov za adsorpcijo vodne pare, karakteristike sproščanja adsorpcijske toplote, brezdimenzijski model PSA v simulacijskem orodju AVL CRUISETM M in rezultati numeričnih simulacij. Med preučevanimi materiali so se tipi silikagela izkazali za primerno izbiro, medtem ko zeoliti kažejo slabše rezultate za primer uporabe PSA v sistemih za elektrolizo vode. Model predvideva delovanje PSA z izkoristki nad 90 % in stopnjo čistosti vodika nad 99,99 %.
Keywords: Adsorpcija, adsorpcija z nihanjem tlaka (PSA), adsorpcijska izoterma, AVL CRUISE M, numerična simulacija
Published in DKUM: 06.03.2025; Views: 0; Downloads: 13
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6. Prenos toplote v polici liofilizatorja : diplomsko deloAlen Bihar Bobinski, 2024, undergraduate thesis Abstract: V diplomskem delu je predstavljena analiza temperaturne porazdelitve po polici laboratorijskega kontaktnega liofilizatorja. Predstavljeno je teoretsko ozadje procesa liofilizacije. Z osmimi termočleni smo izmerili temperaturo po polici liofilizatorja pri dveh različnih postavitvah in pri treh različnih nastavljenih temperaturah police liofilizatorja. S pomočjo numeričnih simulacij smo v poenostavljenem in zanesljivem modelu sestava komore in polic liofilizatorja dobili rezultate temperature izmerjene na istoležnih točkah, kakor pri eksperimentu. Sledila je obdelava podatkov in primerjava v ujemanju dobljenih rezultatov. V diplomskem delu smo ugotovili, da v določenih primerih prihaja do neuniformnosti porazdelitve temperature po polici liofilizatorja, kar lahko pomembno vpliva na učinkovitost procesa liofilizacije.
Keywords: termočleni, računalniška dinamika tekočin, liofilizacija, prenos toplote
Published in DKUM: 06.03.2025; Views: 0; Downloads: 28
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7. A novel pseudo-rigid body approach to the non-linear dynamics of soft micro-particles in dilute viscous flowJana Wedel, Matjaž Hriberšek, Jure Ravnik, Paul Steinmann, 2024, original scientific article Abstract: We propose a novel, demonstrably effective, utmost versatile and computationally highly efficient
pseudo-rigid body approach for tracking the barycenter and shape dynamics of soft, i.e. nonlinearly deformable micro-particles dilutely suspended in viscous flow. Pseudo-rigid bodies are
characterized by affine deformation and thus represent a first-order extension to the kinematics
of rigid bodies. Soft particles in viscous flow are ubiquitous in nature and sciences, prominent
examples, among others, are cells, vesicles or bacteria. Typically, soft particles deform severely
due to the mechanical loads exerted by the fluid flow. Since the shape dynamics of a soft particle -
a terminology that shall here also include its orientation dynamics - also affects its barycenter
dynamics, the resulting particle trajectory as a consequence is markedly altered as compared to
a rigid particle. Here, we consider soft micro-particles of initially spherical shape that affinely
deform into an ellipsoidal shape. These kinematic conditions are commensurate with i) the affine
deformation assumption inherent to a pseudo-rigid body and ii) the celebrated Jeffery-Roscoe
model for the traction exerted on an ellipsoidal particle due to creeping flow conditions around
the particle. Without loss of generality, we here focus on non-linear hyperelastic particles for the
sake of demonstration. Our novel numerical approach proves to accurately capture the particular
deformation pattern of soft particles in viscous flow, such as for example tank-treading, thereby
being completely general regarding the flow conditions at the macro-scale and, as an option, the
constitutive behavior of the particle. Moreover, our computational method is highly efficient and
allows straightforward integration into established Lagrangian tracking algorithms as employed
for the point-particle approach to track rigid particles in dilute viscous flow.
Keywords: soft particles, pseudo-rigid bodies, Barycenter and shape dynamics, Lagrangian particle tracking
Published in DKUM: 19.09.2024; Views: 0; Downloads: 17
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8. Informacijska varnost v avtonomnih hotelih : diplomsko delo visokošolskega študijskega programa Informacijska varnostMatjaž Hriberšek, 2024, undergraduate thesis Abstract: V diplomskem delu sem želel raziskati kakšna je izkušnja uporabnika pametnega hotela in kakšno je zaupanje v hotelske storitve v takšnih hotelih, da bo varovalo informacije strank. Cilj je preučiti, kako sodobna digitalna tehnologija spreminja hotelirstvo in v kolikšni meri posega v posameznikovo zasebnost. Glede na to, da so pametni hoteli v zadnjih par letih prišli na tržišče, ogromno ljudi niti ne ve, da takšni hoteli obstajajo oz. še niso bivali v njem. V teoretičnem delu sem za lažje razumevanje razložil nekaj pojmov kot so informacijska varnost, pametni hotel. Nato sem predstavil kakšno tehnologijo lahko najdemo v pametnih hotelih. Omenil sem internet stvari, ki je eden od glavnih razlogov, da so se pametni hoteli sploh razvili. Varnost in zasebnost gostov je zelo problematičen del nudenja storitev v pametnih hotelih. Tu je je treba zelo paziti in skrbeti za pametne naprave, ki so na voljo gostu v hotelu ali v sami sobi. V zadnjem delu pa sem se dotaknil še osebnih podatkov in informacijske varnosti v samih hotelih. Zaupanje gosta v hotel, da bo varoval osebne podatke gostov je zelo pomembna naloga hotelskih organizacij. Gost namreč pričakuje, da bo organizacija skrbela za informacijsko varnost in odpravljala napake, ki bi jih sami zaznali, še predno pride do kakšnih resnejših incidentov. Pri raziskovalnem delu je bila z deskriptivno metodo preučena pretežno tuja literatura. V metodološkem delu naloge pa sem s pomočjo anketnega vprašalnika v elektronski obliki iskal odgovore na zastavljene hipoteze. Na podlagi zbranih informacij so se potrdile zastavljene hipoteze.
Keywords: informacijska varnost, pametni hoteli, zasebnost, diplomske naloge
Published in DKUM: 29.08.2024; Views: 109; Downloads: 42
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9. A coupled point particle two-phase heat and mass transfer model for dispersed flows based on Boundary Element MethodsTimi Gomboc, Matej Zadravec, Jure Ravnik, Matjaž Hriberšek, 2024, published scientific conference contribution Abstract: In dispersed multiphase flow processes we encounter coupled heat, mass and momentum transfer between the disoersed and the continuous phase. In the context of the subdomain Boundary Domain Integral Method (BDIM) solution of the Navier-Stokes equations a two-way coupling model is presented based on the use of the elliptic fundamental solution and the Dirac delta function properties which leads to accurate evaluation of the heat and mass point particle source impacts on the continuous (air) phase. In addition, the two-phase flow case under consideration is extended to the case of porous spherical particle drying with internal moving drying front, which is solved by the Boundary Element Method (BEM).
Keywords: heat transfer, mass transfer, Boundary Element Methods
Published in DKUM: 01.07.2024; Views: 113; Downloads: 18
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10. Vial wall effect on freeze-drying speedMatjaž Ramšak, Matjaž Hriberšek, 2024, original scientific article Abstract: The vial wall thermal conductivity and thickness effect on freeze-drying speed is simulated. A 2D axisymmetric numerical simulation of Mannitol freeze-drying is employed using the boundary element method. The originality of the presented approach lies in the simulation of heat transfer in the vial walls as an additional computational domain in contrast to the typical methodology without a vial wall. The numerical model was validated using our measurements and the measurements from the literature. Increasing the glass vial thickness from 1 mm to 2 mm has been found as the major factor in primary drying time, increasing the gravimetrical Kv up to 20 % for all the simulated chamber pressures. The effect of thermal conductivity was simulated using a polymer and aluminium vial replacing the standard glass vial of the same thickness. The polymer vial‘s decreased Kv value is 5.6 % at a low chamber pressure of 50 mTorr, and 12.2 % at 400 mTorr, which is in excellent agreement with the experiment. Using higher conductivity materials, for example, aluminium, only 3.7 % and 2.3 % Kv increase were computed for low and high chamber pressures respectively.
Keywords: freeze-drying, lyophilization speedup, vial heat conductivity, vial wall thickness, boundary element method
Published in DKUM: 16.04.2024; Views: 264; Downloads: 36
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