bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical analysis of transport phenomena in a steam reforming reactor with optimal multi-segments catalyst distribution
Authors: Pajak M , Brus G , Kimijima S , Szmyd J S
Published: June 2024

The contemporary industrial trends pursue alternative energy sources, to substitute fossil fuels. The current direction is induced by concerns regarding exhausting natural resources and the environmental impact of the technologies rising globally. Conventional technologies have a dominant share of the current energy market. The most crucial issue with current technology is the emission of greenhouse gases and their negative impact on climate. One of the possible approaches to limit the issue of emissions is the steam reforming of natural gas, leading to the production of hydrogen. Fuel cells are a robust technology, able to conduct a catalytic conversion of hydrogen and oxygen, for the direct production of electrical energy. Fuel cells are one of the most environment-friendly technologies to this day, as their exhaust gases mostly consist of steam. Currently, almost 50% of the hydrogen produced is acquired via hydrocarbons reforming. The process described in the presented analysis occurs between methane and steam. The presented numerical analysis regards small-scale reactors, which are more suitable when it comes to the processing of distributed or stranded resources for hydrogen production To optimize the small-scale unit’s performance, the macro-patterning strategy is introduced. Steam reforming has a strong endothermic character and tends to produce unfavorable thermal conditions. The process enhancement is acquired by introducing non-catalytic regions to the catalytic insert geometry. The non-catalytic segments are introduced to suppress the reaction locally, decreasing the magnitude of temperature gradients. Unification of the temperature distribution is proven to increase the reforming’s effectiveness. The presented analysis introduces a new approach to the catalytic insert division, to investigate if a complete temperature field unification is possible. The catalytic insert is simultaneously divided along the reactor’s radius and length, resulting in a set of concentric rings, placed along the reactor’s axis. The calculations are conducted using in-house numerical procedure, coupled with a genetic algorithm. The algorithm optimizes the process effectiveness by modification of the segment’s alignment and porosity.
Simple model of liquid piston compressor
Authors: Cerkovnik N , Čurović L , Prezelj J
Published: June 2024

The study proposes a new 0D thermodynamic model for the fast and simple evaluation of Liquid Piston compressors whose geometry and operation are determined by large number of parameters. With the proposed model, the evaluation of the parameters becomes easier, more dependencies are found and the understanding of the influences of parameter changes is deepened. Therefore, optimal designs for the mentioned machines can be created and tested experimentally. During the study, the model was validated with experimental and CFD data, and the results showed good agreement. It was found that evaluation criteria need to be defined for each set of geometric and operational parameters in order to compare the designs.
CFD design and optimization of a multi-probe device for temperature measurements in Waste-to-Energy plants
Authors: Grossi G , Arpino F , Canale C , Cortellessa G , Ficco G , Lombardi T
Published: June 2024

The temperature of exhaust gases is an essential parameter to monitor in Waste-to-Energy (WtE) plants, since it significantly affects energy efficiency, pollutant emissions and materials’ integrity. Current methods for monitoring gas temperature in such plants (i.e., thermocouples, infrared pyrometers, and aspirated thermocouples) exhibit shortcomings in both accuracy and reliability when used in environments where radiative heat exchange plays an important role. In previous research, by means of Computational Fluid Dynamics (CFD) simulations, the authors designed a new multi-probe device, with the aim of mitigating the issues associated with these commonly adopted techniques. The newly designed measuring device is herein further characterized by means of multi-region CFD simulations, to investigate any local thermal-fluid dynamics effects which may impair the measurement, due to the mutual influence of the ducts where the sensors are located. Results have shown that the mutual influence is not significant, confirming the validity of the designed shape.
Impact of variation of the cooling system operating strategy on energy efficiency and waste heat quality: a preliminary investigation on a hybrid-cooled data centre
Authors: Testa L , Stuart P , O’Donnell C , Persoons T
Published: June 2024

The growing demand for digital services is leading to a rapid increase of the number of data centres worldwide. Because of the large amount of energy required to power these facilities and the consequent challenges for the energy system and the environment, efforts are made to discover new solutions that allow a more prudent use of energy without compromising the quality of the service provided. One of these potentially viable solutions is a less conservative configuration of the operating strategy of the cooling system, which can request up to 30% of the total power absorbed by a data centre. This paper is a first estimation of the benefits in terms of power savings and waste heat quality that would follow the implementation of this solution on the Green DC, a data centre based in Dublin, Ireland. The Green DC is installed within a university and provides a variety of services, like HPC and online lecture streaming.
Experimental analysis of an innovative geo-exchange system installed on the island of Ischia, in southern Italy
Authors: Guida V , Massarotti N , Mauro A
Published: June 2024

The need to use efficient and eco-friendly air conditioning systems is a fundamental requirement for today’s society. In this context, low and medium enthalpy geothermal energy plays a crucial role. The research and development activities carried out in the present work have made it possible to successfully design and analyze an innovative technology capable of supplying thermal energy to environments without pumping fluid from the subsoil. The proposed system is based on the use of a downhole heat exchanger (DHE). The authors have developed an innovative system for the sustainable use of low and medium enthalpy geothermal energy, installed on the island of Ischia, near Napoli, in southern Italy. The proposed system is based on the use of an ad hoc designed DHE, capable of optimizing the heat transfer with the subsoil, without the need to withdraw fluid from the aquifer. A numerical finite element model was developed to study the interaction between the DHE, the well and the surrounding aquifer. The experimental set-up consists of the heat exchanger and an above ground system, necessary to test the efficiency of the exchanger. The DHE is inserted inside a geothermal well made with a steel casing, equipped with a filtering section in correspondence of the DHE, in order to increase the heat transfer performance due to increased convection with the surrounding aquifer. The experimental data show that the DHE allows to exchange more than 40 kW with the ground, obtaining overall heat transfer coefficient values larger than 450 W/m2 K.
Simulation of non-Newtonian gas-focused micro-jets in chocked gas flow regime
Authors: Zahoor R , Bajt S , Šarler B
Published: June 2024

Stable liquid jets are crucial for successful serial crystallography experiments. They are produced from gas dynamic virtual nozzles (GDVNs), where the liquid from an inner capillary is focused by a co-flowing gas from an outer converging capillary. Our previously investigated non-Newtonian jets with incompressible and compressible focusing gas under atmospheric conditions were extended towards compressible chocked gas under vacuum conditions. An axisymmetric GDVN was considered with a fixed gas flow rate of 15 mg/min and liquid flow rate of 40 µl/min. A mixture formulation of the laminar compressible multiphase problem was solved within finite volume method and volume of fluid framework. The jet lengths, diameters, velocities, and temperatures were analysed as a function of the power-law non-Newtonian modification of reference water. It is observed that the jets under vacuum conditions are thinner for Newtonian and shear-thickening fluids than those in the atmosphere. The jet length increases from shear-thinning to shear-thickening rheology but is not affected by the pressure. The shear produced at the nozzle outlet is similar for both pressure conditions but increases in downstream directions for vacuum conditions. Gas expanding into vacuum cools by ~100-150 K while the temperature of the liquid jet drops only by a few K.
Investigation of unsteady friction effects in transient two-phase pipe flow
Authors: Bergant A
Published: June 2024

Water hammer and transient cavitating flow may induce large pressure fluctuations in pipelines. Transient events in pipeline may cause a drop in pressure large enough to break liquid homogeneity and continuity (liquid column separation, distributed vaporous cavitation). Trapped air pockets in liquid may be a major problem in piping systems. The paper deals with mathematical tools for modelling unsteady skin friction, transient vaporous cavitation (liquid column separation) and transient gaseous cavitation (trapped air pockets). The method of characteristics transformation of the unsteady liquid pipe flow equations gives the water hammer solution procedure. A convolution-based unsteady skin friction model is explicitly incorporated into the staggered grid of the method of characteristics. Incorporating discrete cavities into the water hammer model leads to the discrete cavity model. An advanced discrete gas cavity model (DGCM) with consideration of unsteady pipe flow friction effects is presented in the paper. The DGCM is capable to simulate vaporous and gaseous cavitation at the same time. Pipeline apparatus for measurement of transient liquid flow (water hammer) and transient two-phase flow (vaporous and gaseous cavitation) is briefly described. The paper concludes with two distinct case studies showing profound effects of unsteady skin friction on pressure histories.
The kinetic energy transfer analysis between the gas and the liquid in flow-focusing of the micro-jet
Authors: Kovačič K , Šarler B
Published: June 2024

The present study uses computational fluid dynamics to analyse the kinetic energy transfer from the gas to the liquid phase, considering the significant influence of surface tension. The considered situation is the gas dynamic virtual nozzle, where the co-flowing gas focuses and accelerates the liquid jet. The experimentally validated half-space three-dimensional gas-liquid mixture model addresses the unsteady, incompressible, isothermal, Newtonian, low-turbulent two-phase flow. The continuity, momentum and the k-ω SST turbulence model are employed to resolve the fluid flow. The numerical solution is based on the finite volume method and volume of fluid approach with a geometric reconstruction scheme for tracking the gas-liquid interface. The total pressure of the gas, an indication of its energy, is tracked along streamlines and analysed spatially and temporarily. It is found that around 50 % of the focusing gas energy is transferred to the liquid jet before its breakup for the nozzle with Weber number 3.5, and gas and jet Reynolds number 1842 and 108, respectively. The linear regression between jet length and energy transfer efficiency is discovered. The presented methodology represents an essential tool for analysing and understanding the energy transfer process between the focusing gas and the liquid jet.
Numerical modelling of a hybrid vapor compression refrigeration assisted closed loop liquid cooling system for high-performance computing systems
Authors: Naduvilakath-Mohammed F M , Lebon Michel , Robinson A J
Published: June 2024

This study presents a numerical model of a miniature vapor compression refrigeration (VCR) system that is used to cool liquid coolant in a secondary pumped loop that is deployed to cool a high-powered CPU. The model employs a physical approach and iterative algorithms to solve coupled non-linear equations for both the refrigeration cycle and pumped single phase cooling loop. Experimental tests were conducted to verify the model using an in-house test facility. This paper outlines the modelling and solution approaches taken, discusses the efficacy of the model in terms of agreement with experimental measurements, and highlights the utility of the model as a design tool for cooling high powered electronics.
Experimental study of erosion in heat exchangers immersed in fluidized beds
Authors: Domínguez-Coy P , Córcoles J I , Almendros-Ibáñez J A
Published: June 2024

Fluidized beds can be used as solar energy receivers in beam-down concentrated solar power plants. The fact that solid particles can withstand temperatures over 1,000°C is promising because it would mean higher thermal efficiencies than conventional concentrated solar power plants (CSP, that use molten salts, whose maximum operating temperature is limited to about 560°C). Nonetheless, immersed heat exchangers are exposed to erosion because fluidized particles impinge on their surfaces. The multiple variables of a multiphase flow and the uncertainties of the material detachment process make erosion a complex problem that stills not fully solved.

The aim of this study is to conceive an experimental procedure to measure erosion on cylindrical probes immersed in a lab-scale high temperature fluidized bed (able to operate up to 1,000°C and atmospheric pressure), simulating a tube of a heat exchanger in a fluidized bed working at expected temperatures in CSP applications.

Preliminary results at ambient temperature are presented in this study. Silica sand particles were fluidized with air and erosion was measured on cylindrical probes made of aluminium 5027. Silica sand was characterized with a microscope before and after each test to determine its size distribution and shape. Several parameters (diameter, mass, and circumferential profile) of each probe were also studied before and after each test. Different fluidization velocities were implemented to assess the influence of the flow rate on erosion.
Visualization of inclusions in a water model for continuous casting of steel
Authors: Perpar M , Mramor K , Hodej B , Cotič M , Vertnik R , Vengust V , Šarler B
Published: June 2024

With the water model, we experimentally confirmed the possibility of using a suspension to imitate the molten casting powder (slag) at the upper edge of the melt during the continuous casting of steel. The project requires that the slag particles do not leave the suspension layer during operation or that they return to it from the molten steel. It turns out that this is possible if the selected suspension particles mimic sufficiently large inclusions in the real system. The imitation of slag particles in a real system with suspension particles in water model is ensured by the equal terminal flotation velocities of both. Among the discussed polyethylene and cork particles, only cork particles, with a size of 0.2 mm to 0.5 mm, and a density of around 400 kg/m3, meet this condition. It was shown that the presented methodology could be used to estimate the proper position of the submerged entry nozzle.
A preliminary model of water and salt transport in a laboratory scale pressure-retarded osmosis (PRO) module
Authors: Wachnicki J , Łapka P , Furmański P
Published: June 2024

Nowadays, more and more attention is being paid to the production of green energy. Recently, energy from salinity gradients has also attracted interest. One such process that can generate useful work is the pressure-retarded osmosis (PRO), which uses a semi-permeable membrane to separate the feed and pressurized draw solutions. The semi-permeable membrane allows the transport of solvent (water) from the feed solution (diluted low pressure) side to the draw solution (concentrated high pressure) side, and thus, the excess of pressurized water on the draw side can be used to generate power, i.e., by expansion in the turbine. This work presents a preliminary numerical model developed to study the water and NaCl salt transport through the semi-preamble membrane and in the PRO module designed to perform experimental studies. The model was first verified for simple 2D module geometry. It was then used to study the flow through a simplified 3D module, mimicking the real one used in laboratory-scale experiments. The influence of the hydrodynamic pressure and the NaCl draw solution’s mass flow rate in the module on the energy generation efficiency was examined. The maximum power density obtained for half the osmotic pressure of the NaCl draw solution (i.e., for 28 bar) was found to be equal to approximately 4 W/m2. An increase in the flow rate of the draw solution causes a decrease in the thickness of the boundary layer at the membrane, which reduces the effect of the external concentration polarization. This results in an increase in the concentration difference on either side of the membrane, contributing to a non-linear increase in power density depending on this mass flow rate. Increasing the average velocity from 0.02 m/s to 0.1 m/s increases the power density by up to 80%, from approximately 2.6 to 4.7 W/m2.
Optical characterization of a solar concentrating dish system
Authors: Suter C , Tembhurne S , Holmes-Gentle I , Haussener S
Published: June 2024

Solar dishes concentrate the quasi-collimated sun light towards a focal point. Typically, the optical characterization consists of experimental flux map measurements and Monte-Carlo ray-tracing (MCRT) simulations, which are fitted based on experimental data and serve for the prediction of the distribution of the irradiation in receivers or reactors. However, the MCRT simulations usually rely on idealized mirror geometries (i.e. parabolic mirrors), neglecting the actual dish geometry resulting from its construction method (such as petals, facets or segments). Thus, the parameter fitting might yield unrealistic parameters and local radiative flux peaks (“hot spots”), which are often experimentally detected behind the focal plane, cannot be predicted. Here, we characterized the 7m diameter solar dish at EPFL comprising 27 petals with a nominal focal length of 3.8 m and a rim angle of 50.3°. The measured peak concentration was 1781 suns and the received integrated solar power was 20.0 kW over an 18 cm diameter spot. We proposed an advanced geometry approach for the MCRT model considering the curvature of the dish, i.e. geometries mathematically described by a parabola with an exponent not equal, but close to 2. The advanced geometry approach was then applied for predicting experimentally measured flux distribution in focal and off-focus planes. The fitted reflectivities were 70% and 86% for the idealized and advanced geometry approach, respectively. The idealized geometry approach found an unrealistically low value whereas the advanced geometry approach’s value was within 1% of the manufacturer’s specification. The advanced geometry approach predicted a parabola exponent of 1.94, which emphasizes that the best geometry to describe the solar dish deviates from a perfect paraboloid. We conclude that MCRT models should consider non-perfect parabolic geometries for solar dishes in order to obtain more realistic values for the fitted parameters.
Energetic and financial evaluation of a district heating network under varying supply temperatures and booster technologies
Authors: Degelin A , Tassenoy R , Vieren E , Demeester T , T’Jollyn I , De Paepe M
Published: June 2024

Low-temperature district heating (LTDH) enables the use of various renewable energy sources, reduces heat losses and increases the energy efficiency of the distribution network. LTDH is especially applicable in energy-efficient buildings as the supply temperature for space heating can be reduced. However, urban areas consist of energetically refurbished and non-refurbished buildings. In these scenarios a LTDH network with a central heat pump (HP) and decentral booster units, such as a booster HP or electrical heater, can be a solution. This study investigates and compares the energetic performance and levelized cost of heat (LCOH) of eight concepts for refurbished and non-refurbished buildings for a district heating network in the city of Ghent, Belgium. The simulations consider supply temperatures ranging from 10 °C to 75 °C. Results show that the primary energy use is lowest when using booster HPs, for both refurbished buildings (402 MWh/year) and non-refurbished buildings (139.6 MWh/year). The LCOH, however, is lowest when booster units are not necessary as the LCOH is mainly driven by the high investment cost of the network and the booster units. This results in a LCOH of 213 €/MWhth for non-refurbished buildings at a network temperature of 75 °C and 297 €/MWhth for refurbished buildings at 55 °C.
Influence of baffle height on the in-cylinder mixing and performance characteristics of a gasoline direct injection engine - a CFD investigation
Authors: Vishal V , Mallikarjuna J M
Published: June 2024

In modern GDI engines, in-cylinder flows play a crucial role in the performance and emission characteristics. Adding baffles to the cylinder head is one way to enhance these flows, as baffle effectiveness is linked to its height. This study aims to investigate the influence of baffle height on the in-cylinder flows and its impact on the performance and emission characteristics of a four-stroke, four-valve, spray-guided GDI engine using computational fluid dynamics (CFD) analysis. The maximum baffle height is determined based on the geometry of the engine. Throughout the analysis, the engine operates at a constant speed of 1000 rev/min., under part-load conditions. From the results, the baffle heights of 2 mm and 3 mm significantly improve mixture stratification compared to the base engine. Furthermore, the indicated mean effective pressure (IMEP) for the engine with 2 mm and 3 mm baffle height improves by about 3% and 4%, respectively, compared to the base engine. In addition, with the increase in the baffle height, there is a significant reduction in the HC emissions with a marginal increase in NOX emissions.
Subgrid moving contact line model for direct numerical simulations of bubble dynamics in pool boiling of pure fluids
Authors: Nikolayev Vadim S. , Wei Linkai , Bois Guillaume
Published: June 2024

This contact line vicinity model is conceived as a subgrid model for the DNS of bubble growth in boiling. The model is based on the hydrodynamic multiscale theory and is suitable for the partial wetting case. On the smallest length scale (distance from the contact line) ∼ 100 nm, the interface slope is controlled by the Voinov angle. It is the static apparent contact angle (ACA) that forms due to evaporation, similarly to previous models neglecting the contact line motion. The calculation of the Voinov angle is performed with the generalized lubrication approximation and includes several nanoscale effects like those of Kelvin and Marangoni, vapor recoil, hydrodynamic slip length and interfacial kinetic resistance. It provides the finite values of the heat flux, pressure and temperature at the contact line. The dynamic ACA is obtained with the Cox-Voinov formula. The microscopic length of the Cox-Voinov formula (Voinov length) is controlled mainly by the hydrodynamic slip. The integral heat flux passing through the contact line vicinity is almost independent of the nanoscale phenomena, with the exception of the interfacial kinetic resistance and is mostly defined by the dynamic ACA. Both the dynamic ACA and the integral heat flux are the main output parameters of the subgrid model, while the local superheating and the microscopic contact angle are the main input parameters. The model is suitable for the grid sizes > 1 µm.
Remaining useful life prognostics during boiling-induced surface degradation
Authors: Berce Jure , Arhar Klara , Može Matic , Zupančič Matevž , Golobič Iztok
Published: June 2024

After sustaining long-term nucleate boiling, any surface is prone to changes over time, which signifies an important operational characteristic for reliable thermal management; however, recent research of this topic is extremely scarce. In this work, we first present long-term pool boiling experiments on samples of different surface topography and morphology in de-ionized water and in an aqueous salt solution. We show the changes induced by vigorous bubble nucleation over the course of several hundred hours of operation, discussing the underlying degradation phenomena. Following this, we present an adaptive data-driven prognostic approach, capable of estimating the remaining useful life (RUL) of a boiling surface in real time, based on change of surface temperature. The method consists of (i) a Kalman filter to identify the degradation drift model and (ii) a Monte Carlo simulation to propagate the drift to the terminal threshold, obtaining a distribution of predicted RUL values. Its practical applicability, validated on experimental results, is reinforced by its advantageous features of (i) prognostic performance not depending on the boiling surface or its degradation mechanisms, (ii) low computational demand and (iii) numerous options for individual tuning and extensions.
Measuring liquid film thickness during downflow condensation in a minichannel
Authors: Berto A , Azzolin M , Lavieille P , Bortolin S , Miscevic M , Del Col D
Published: June 2024

The availability of experimental liquid film thickness data during in-tube condensation heat transfer is fundamental for the development of reliable correlations for the design of condensers. During annular flow condensation, the heat transfer is directly linked to the liquid film thickness, as the main thermal resistance is due to thermal conduction through the film of condensate. Therefore, understanding the liquid film characteristics is paramount for modelling two-phase annular flows. Although several prediction methods for liquid film thickness are available in the literature, they are mainly related to air-water adiabatic experiments. More liquid film thickness data are required for a better understanding of the condensation phenomena and for the development of more accurate models. In this work, the liquid film thickness characteristics of R245fa, R134a and HFE-7000 have been investigated during condensation inside a vertical channel with 3.38 mm inner diameter. Shadowgraphy and chromatic confocal imaging were used to detect the evolution of the liquid film thickness with high temporal resolution. The experimental data were useful to address the existence of a dependence of the liquid film thickness on dimensionless numbers. The combination of two dimensionless numbers, specifically the vapor Weber number and the Reynolds number of the liquid phase, allows to better catch the liquid film thickness trends with the varying operating conditions.
Wettability patterning of titanium surfaces through pulsed laser melting for enhanced condensation heat transfer
Authors: Chokshi Avnish , Devra Rajdeep Singh , Rahul N , Vadali Madhu , Sett Soumyadip
Published: June 2024

Wettability engineering of different surfaces has been in the spotlight for the last few decades for enhanced condensation heat transfer in various applications. In this study, we experimentally investigated the water vapor condensation on a wettability-tailored Titanium-based (Ti-6Al-4V) grade 5 alloy. We utilize the microsecond laser to texture the surface by melting at various scanning speeds to realize a wide range of scalable surface structures. We further render these surfaces hydrophobic through chemical vapor deposition of silane at atmospheric pressure. Further water vapor condensation experiments are performed on these surfaces. The results show that the increased surface roughness due to laser-based melting altered the surface wettability of the Ti-surface and made it hydrophilic, exhibiting water drop contact angles ranging between 18° and 56° for the scan speeds between 25mm/s and 50 mm/s, respectively. The vapor deposition of silane on laser-melted Ti-surfaces lowered its surface energy and made them hydrophobic, showing contact angles of water drop up to ~106° specifically at lower scan speeds (~ 25 mm/s). Finally, the vapor condensation experiments showed an enhanced amount of condensed water collection with dropwise mode compared to the bare Ti surface due to a change in the wetting nature altered by laser melting.
A hybrid atomistic-continuum framework for multiscale simulations of boiling
Authors: Gennari Gabriele , Smith Edward R. , Pringle Gavin J. , Magnini Mirco
Published: June 2024

Boiling is a multiscale physics process where the nucleation of vapour bubbles occurs due to molecular-scale interactions between the fluid and a heated wall, but it also depends on the larger-scale hydrodynamics and thermal boundary layers determined by the outer system boundary conditions. Modelling boiling from the nanometre up to the millimetre scales at which bubble departure occurs is not possible via state-of-the-art simulation methods: Molecular Dynamics (MD) simulations can capture nucleation from first principles but are limited to nanometre scales due to their computational cost, whereas computational fluid dynamics (CFD) simulations based on the continuum Navier-Stokes equations cannot capture nucleation. Here, we present a novel multiscale simulation method which merges MD and CFD descriptions into a single modelling framework, where MD resolves the near-wall region where molecular interactions are important, and a CFD solver resolves the bulk flow. We model the progressive heating of a Lennard-Jones fluid via contact with a solid wall until a vapour bubble nucleates in the MD region of the domain and grows by entering in the CFD domain. Our results show that an incompressible CFD flow model based on the Volume Of Fluid method with interphase mass transfer calculated via the Hertz-Knudsen-Schrage equation is sufficient to obtain seamless coupling of phase fraction, velocity and temperature fields, with the hybrid MD-CFD framework yielding bubble dynamics closely matching those of MD alone.
Hyperviscosity stabilisation of the RBF-FD solution to natural convection
Authors: Vaupotič Žiga , Rot Miha , Kosec Gregor
Published: June 2024

The numerical stability of fluid flow is an important topic in computational fluid dynamics as fluid flow simulations usually become numerically unstable in the turbulent regime. Many mesh-based methods have already established numerical dissipation procedures that dampen the effects of the unstable advection term. When it comes to meshless methods, the prominent stabilisation scheme is hyperviscosity. It introduces numerical dissipation in the form of a higher-order Laplacian operator. Many papers have already discussed the general effects of hyperviscosity and its parameters. However, hyperviscosity in flow problems has not yet been analyzed in depth. In this paper, we discuss the effects of hyperviscosity on natural convection flow problems as we approach the turbulent regime.
Phase-field formulated meshless simulation of Rayleigh-Taylor instability problem
Authors: Rana K B , Mavrič B , Šarler B
Published: June 2024

The interface between two immiscible Newtonian liquids with different densities and the same viscosity, influenced by gravity, is based on the Phase-Field Method (PFM) formulation. The solution of the related governing coupled Navier-Stokes (NS) and Cahn-Hillard (CH) equations is structured by the meshless Diffuse Approximate Method (DAM) and Pressure Implicit with Splitting of Operators (PISO). The variable density is involved in the inertial and buoyancy terms (non-Boussinesq formulation). The related moving boundary problem is handled through single-domain, irregular, fixed node arrangement in two-dimensional Cartesian coordinates. The meshless DAM uses weighted least squares approximation on overlapping subdomains, polynomial shape functions of second-order and Gaussian weights. Implicit time discretisation is performed for the NS and CH equations in the momentum predictor and Phase-Field (PF) variable corrector steps of PISO, while the momentum corrector steps solve the NS equation explicitly. This solution procedure has improved stability compared to Chorin’s pressure-velocity coupling, previously used in meshless solutions of related problems. The Rayleigh-Taylor instability problem simulations are performed for an Atwood number of 0.76. The DAM parameters (shape parameter of the Gaussian weight function and number of nodes in a local subdomain) are the same as in the author’s previous studies on single-phase flows. The simulations did not need any upwinding in the range of the simulations. The results compare well with the mesh-based finite volume method studies performed with the open-source code Gerris.
Joule heating analyses in electrically conductive micro/nano-sized structures
Authors: Sladek J , Sladek V , Repka M
Published: June 2024

Joule heating is occurred in electrically conductive materials. In cracked structures with high concentration of electric current at the crack tip vicinity the Joule heating can be extremely high and even a melting of metal can be observed. It is interesting to investigate thermal stresses caused by this local heating. In micro/nano-sized structures the overheating of structures can be very fast and structure can be damaged. However, in these small structures it is observed the size effect for both heat transport and also for the mechanical balance equation. The thermal transport cannot be described by the classical Fourier’s equation in materials with an internal structure. For mechanical problem due to the size effect the strain gradients have to be considered in the constitutive equations. Then, governing equations for both problems are given by partial differential equations with order of derivatives being higher than in classical case. The collocation mixed FEM is developed here for this multi-physical problem. The present computational method is applied to some crack problem to illustrate compressive stresses at the crack tip vicinity, which are leading to the crack closure.
Fourth-order phase-field simulation of cracks using strong form meshless method
Authors: Ali Izaz , Vuga Gašper , Mavrič Boštjan , Hanoglu Umut , Šarler Božidar
Published: June 2024

The prediction of crack propagation in fracture mechanics is still a fundamental challenge. In recent decades, phase-field formulation emerged as a powerful tool for the simulation of crack evolution, which offers a continuous representation of cracks from intact to broken material. For the first time, the present study combines a fourth-order phase field and a strong-form meshless method to investigate crack propagation in brittle elastic material. The local radial basis function collocation method, structured with the augmented polyharmonic splines, is used to solve the coupled mechanical and phase-field models. Due to the smooth nature of the fourth-order phase-field model, the sharp transition at the crack surface can be avoided. It is found that the presented method successfully predicts the crack propagation for a material subjected to tensile loading. The obtained results agree well with the benchmark case. Further study will include elastoplastic material behaviour and applications to crack formation in different steel processing steps.
Simulation of temperature field in steel billets moving in the reheating furnace by a meshless method
Authors: Liu Q G , Hanoglu U , Šarler B
Published: June 2024

A simulation of a reheating furnace in a steel production line where the steel billets are heated from room temperature up to 1200 °C is carried out. In this work, governing equations are solved in a strong form by the meshless local radial basis function collocation method (LRBFCM) with explicit time-stepping. The solution of the diffusion equation for the temperature is formulated on a two-dimensional central slice of the billet. The temperature field is solved by considering the positions of multiple billets in the furnace and the radiative and convective heat fluxes on the boundaries of the furnace and the billets. Ray tracing procedure, in which the view factors are computed with a Monte-Carlo method, is employed to determine the radiative heat flux. A sensitivity study is performed on the influence of two different stopping times of the billets in the furnace on the temperature evolution.
CFD simulation and geometric optimization of a novel baffle heat sink for power electronics cooling
Authors: T’Jollyn I , Rogiers J , Nonneman J , De Paepe M
Published: June 2024

The electric drivetrain of an electric vehicle typically has two fluid circuits: an oil circuit for lubrication and a water-glycol circuit for cooling. Eliminating the water-glycol circuit can result in a more compact and lighter drivetrain. However, this requires the drivetrain components to be cooled with oil as coolant, which has inferior heat transfer properties compared to water-glycol mixtures. This is particularly challenging for the power modules in the power electronics unit, which exhibit some of the highest heat fluxes in the drivetrain. This study focusses on the design and optimization of a novel type of heat sink for laminar flows with baffles to guide the flow in four different directions. This has two main advantages: the baffles act as fins and increase the heat transfer area and they introduce a swirling motion in the oil thereby disturbing the boundary layers continuously. Computational fluid dynamics simulations using Ansys Fluent are performed to evaluate the thermohydraulic performance of the heat sink. The influence of design parameters such as the height and thickness of both fins and baffles and the baffle spacing in horizontal, upward and downward direction was evaluated by simulations with several combinations of the design space, considering manufacturing constraints. The optimization of the design showed that the fin thickness, horizontal baffle spacing and baffle length should be as small as possible, while there were optimal values for fin height, upward and downward baffle spacing, streamwise baffle spacing and baffle thickness. The simulations show that the thermal resistance of the baffle heat sink can be 19% lower than that of a benchmark pin fin heat sink at equal pumping power.
A novel type of additively manufactured high pressure mini-channel heat exchanger for precooling in hydrogen refueling stations
Authors: Kahlfeld R , Müller F , Müntefering H , Gembarski P , Steinhoff R , Kabelac S
Published: June 2024

During refueling supercritical hydrogen into high pressure storage tanks, the fluid has to be cooled down to temperatures between -33°C and -40°C before entering the vehicle fuel tank. This cooling takes place while the hydrogen is at a pressure of up to 87.5 MPa. The requirements for the heat exchanger performing this task are very high. It has to be pressure resistant, compact enough to fit in the dispenser column and provide a high thermal performance to ensure a fast refueling with high mass flow rates. Only few conventional manufactured heat exchangers are able to fulfil these requirements. With the rise of additive manufacturing technology, especially laser powder bed fusion, new heat exchangers produced without use of conventional joining technologies can be realized. This manuscript presents a new type additively manufactured of mini-channel heat exchanger. It is developed in a joint research project involving the Leibniz University Hanover and an industrial heat exchanger manufacturer. The apparatus has a design pressure of 105 MPa and will be suited to be used in hydrogen refueling stations. The thermal requirements and the design of the apparatus are described. Thermal power and pressure drop for the full-size heat exchanger are calculated via a cell model. Scaled smaller heat exchangers made of 1.4404 stainless steel are additively manufactured via laser powder bed fusion (LPBF). The thermofluiddynamical performance of the scaled apparatus is measured in a testbench to verify the applicability of the used correlations. Deviations in hydraulic diameter and surface roughness are taken into account. Existing correlations are fitted to the new geometry.
Numerical modelling of continuous casting of round billets with turbulence in the melt
Authors: Mramor K , Rek Z , Vertnik R , Šarler B
Published: June 2024

The present work aims to solve the continuous casting benchmark problem in axisymmetry with turbulence in the melt by the meshless method. The physical model of the liquid-solid system that involves mass, momentum and energy conservation is formulated in the mixture continuum approximation. The melt is assumed Newtonian and incompressible. A k-epsilon Reynolds averaged Navier-Stokes turbulence model is implemented with Abe-Kondoh-Nagano closures. The mushy region is modelled as a Darcy porous media with the Kozeny-Carman permeability model. The solution of partial differential equations is implemented locally using collocation with radial basis functions and explicit time-stepping. The velocity pressure coupling is performed using the fractional step method. The validation of the model is assessed by comparing its outcomes with the results of the finite volume method. A sensitivity study of the varying casting speed and temperature on the velocity, temperature, and solid fraction fields is shown.
Nucleation study of xylitol as PCM in two different crystallization systems: a rheometer and a lab-scale prototype
Authors: Navarro M , Lázaro A , Diarce G , Delgado M
Published: June 2024

Sugar alcohols are promising materials for latent heat-based thermal energy systems. Xylitol stands out among them for its high phase change enthalpy and density. However, xylitol, as well as other sugar alcohols, show undercooling and low crystallization rates, which hinder it potential as a phase change material. This work studies the effect of the seeding and shearing technique on crystallization, in addition to the effect of the operation conditions, such as temperature or shear. To do so, two systems are studied: a rheometer and a stirred tank. Induction times are then calculated from temperature and viscosity curves, and a crystallization model is used to study the influence of operating conditions in crystallization.
Experimental and numerical pore-scale characterization of multi-mode heat transfer in porous ceramics exposed to a transverse heat flux
Authors: Briclot A , Lea P , Henry J F , Suter C , Haussener S , Randrianalisoa J
Published: June 2024

We present experimental and computational approaches to characterize the multimode heat transfer in an open-cell Si-SiC foam exposed to a transverse heat flux. The experimental setup consists of a 5×5×10 cm3 parallelepipedal 10ppi foam (83% porosity) placed within a rectangular channel, in which airflow (0-8 g.s−1) was established. The pressure drop across the foam was measured and infrared cameras were used to obtain the temperature of the heater plate and the thermal maps of the foam at the outlet and on the top. The computational approach is a 3D pore scale model of flow and multi-mode heat transfer in the channel subjected to the same conditions. A X-ray tomography technique (50 µm resolution) was used to scan the foam structure and subsequent segmentation and binarization of the data resulted to a digitalized volume of the 1000×1000×2000 voxels and about 101 million cells after meshing. The computations were performed using ANSYS Fluent. Laminar or k-ε turbulence models with enhanced wall treatment were used depending on the airflow rate. Finally, the comparison of the numerical results with experimental data shows that the relative deviation of pressure drop is less than 7.1% and that of outlet foam temperature is less than the experimental uncertainties (5-20%). Using similarity with fins, the effectiveness of foam is evaluated around 2.2.
Contact resistance effects on current path and thermal characteristics of Ag-nanowire network
Authors: Tatsumi K , Sugihara Y , Tamai K , Kuriyama R
Published: June 2024

In this study, we measured the temperature distribution of Joule-heated Ag-NW network using thermoreflectance imaging (TRI). Numerical computation was conducted for the electric current flow and heat transfer of the Ag-NW network. The contact electric resistance was obtained by solving the optimization problem to minimize the difference between the temperature distributions of the measurement and computation. The results showed that the contact electric resistance varies over the Ag-NW network at certain magnitude. This variation significantly affected the temperature distribution by defining the current path and producing a non-uniform current distribution.
Conjugate heat transfer analysis of the transient thermal discharge of a metallic latent heat storage system
Authors: Nees F , Pai Y S
Published: June 2024

Thermal energy storage systems utilizing metallic phase change materials exhibit great potential as a technology for mobile applications, offering high storage densities and high thermal discharge rates. First experimental investigations show the functionality and performance characteristics of this system. For a deeper understanding of the thermal discharge, this paper presents a numerical model and analysis of the transient conjugate heat transfer. For validation of the numerical model, the results of the simulations are compared to the available experimental data. The investigated storage is based on an aluminum silicon alloy and a box-shaped graphite container design. In this system, heat extraction is achieved by forced convection of ambient air. The transient thermal discharge was simulated from 650 °C to 100 °C, and the solidification of the storage material at around 577 °C was simulated using an enthalpy-porosity approach. The discharge time and total heat flow show good agreement with the experimental data, indicating the model’s successful validation. An empirical study was carried out to determine the thermal contact resistance at the interface between the storage material and the graphite container. The present study contributes new physical insights regarding the thermal discharge of a novel metallic latent heat thermal energy storage system.
Exploring the potential and challenges of phase change materials in future heat storage systems via computations and experiments
Authors: Carnie J T , Hardalupas Y , Sergis A
Published: June 2024

The current work describes computational and experimental efforts to enable new heat storage technologies. Approximately 50% of the energy consumed globally is used to generate and manage heat. This is still predominantly acquired from the combustion of fossil fuels, consequently generating just over 40% of the total global CO2 emissions. To achieve a sustainable future, there is a need for sustainable, decarbonised and dependable energy sources. Heat storage is a key technology that could help achieving this goal. Recently published work by the current authors has shown the importance of heat storage in decarbonising heating (and cooling) of buildings using phase change materials (PCMs). The black-box approach followed evaluated the energetic, cost, and environmental impacts of a novel concept PCM storage scheme with microgeneration. The results obtained showed that this system had advantages over current and future competing heating technologies. However, PCM technology remains at low technological readiness levels because of the lack of understanding of the complex heat transfer physics associated with their phase change transition. For example, understanding and characterizing the propagation of the solid-liquid front during charging and discharging in such heat storage modules will be important in predicting and optimising their performance. In the current work, computations have been performed to understand and optimise the effects of container geometry and size on the charging and discharging heat flux profiles of a simple heat storage module. Furthermore, fundamental experiments have also been implemented to identify the complex governing mechanisms of heat transfer and storage in such materials during transient operation.