bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ensemble averaging parallel-in-time approach for industrial LES
Authors: Plana-Riu Josep , Xavier Trias F , Alsalti-Baldellou Àdel , Colomer Guillem , Oliva Asensio 
Published: June 2024

Abstract
Computational Fluid Dynamics for industrial applications usually relies on RANS modeling instead of LES, as the latter is more expensive and requires much longer simulation times. In order to reduce the computational cost of the simulation, an ensemble averaging parallel-in-time approach is presented so that the simulation time of LES simulations can be reduced up to overnight lengths by exploting the benefits of using sparse matrix-matrix products, SpMM, instead of the classic sparse matrix-vector products, SpMV, or SpMV-like stencil-based kernels, as it might lead to a speed-up given the proper conditions. These conditions are tested for a classical differentially heated cavity benchmark, to apply later on to the simulation of a CSP collector, as it is a well-known academic case that resembles the final geometry.
State of the art of micro vapour-compression systems
Authors: Blatnik Mihael , Klinar Katja , Kitanovski Andrej 
Published: June 2024

Abstract
Thermal management is becoming more challenging due to the increasing compactness and computational power of appliances. Affordable and efficient solutions include active liquid cooling methods, thermoelectric cooling, and the use of heat pipes. From these, vapour-compression technologies achieve the highest power density. However, the downscaling of vapour-compression for cooling of personal computers or workstations is rather problematic from several different reasons. The first problem concerns the low second law efficiency of miniature vapour-compression systems. The second problem concerns the available space occupation of such a cooling system. Moreover, miniature and high-power density condensers and evaporators have not yet been adequately researched and developed to date. In this review, we present several vapour-compression refrigeration systems that are based in some way on the miniaturisation of systems or individual components – evaporators, condensers, and compressors. These are presented and compared in terms of cooling power, COP, and the second law efficiency. At the end of the manuscript, guidelines for future improvements and the implementation of miniature vapour-compression systems are presented.
Direct air capture with renewable heating based on the technical lime cycle
Authors: Klockow E , Linder M 
Published: June 2024

Abstract
The industry and space heating demand contributes significantly to the CO2 emissions in Germany, as it is mainly supplied by the combustion of fossil fuels. In this contribution a concept is presented that not only provides heat based on renewable sources, but is also able to actively capture CO2 from the atmosphere. Thereby, the reversible reaction of lime with water is utilised to store and provide heat on demand – all based on abundantly available resources. By closing the technical lime cycle this long-term energy storage could additionally be combined with a Direct Air Capture technology.
Prediction of the minimum heat loss coefficient for safe operation of a Li-ion cell: A machine learning approach
Authors: Akula Rajesh , Kumar Lalit 
Published: June 2024

Abstract
The operating conditions, either C-rate or DoD, of all the Li-ion cells in the battery pack of EVs are subject to continuous change. In addition, the cells’ ambient temperature (Tamb
) is also not constant due to geographical or day/night conditions. As the generation and transfer of heat from the cells are vital functions of C-rate, DoD, and Tamb
, choosing an appropriate heat loss coefficient for the given conditions is imperative to maintain the operating temperature of the cell below a specified Set Point Temperature (SPT). The selected heat loss coefficient must be the minimum possible such that overcooling of the cells can also be eliminated. The present study employed a machine learning based surrogate model called Gaussian Process Regression (GPR) to achieve this objective for an AMP20M1HD - A0 Li-ion pouch cell. The training and validation of the surrogate model are conducted with the samples generated using Latin Hypercube Sampling and simulated using the NTGK model available in the Ansys Fluent. The model’s accuracy is further tested for three new combinations of the operating conditions, which are not used for training or validation. Using the present model, the predicted minimum heat loss coefficient successfully regulates the cell’s maximum temperature below a user-specified SPT for the same user-given operating conditions. The developed model immensely helps in designing a cost-effective battery thermal management system with optimum cooling capacity by predicting the nature of heat loss coefficients for all plausible combinations of the operating conditions.
Condensation of water on PDMS-coated copper surfaces for fresh water harvesting
Authors: Pfeiffer T , Kappl M , Butt H-J , Stephan P , Gambaryan-Roisman T 
Published: June 2024

Abstract
With an increased need to solve fresh water scarcity for humanity in many places around the world, dew water harvesting is one promising approach to improve the situation. While condensation from humid air has been a research topic for many years, it is still unclear how surface wettability influences the condensation efficiency and the fresh water harvest. In this work, we contribute to bridging this knowledge gap by comparing the condensation efficiency of PDMS-coated hydrophobic and uncoated hydrophilic surfaces. We characterize the humid air velocity field to enable comparability of the present results on condensation efficiency with the results gained at other conditions. In addition, the surface wetting behavior prior to and after the condensation experiment is investigated to analyze the PDMS-coating longevity and allow to draw conclusions on their suitability for long term use in condensation applications.
Confined boiling on an immersed heated corner: influence of one wall on the other
Authors: Volpe Antonio Della , Baudin Nicolas , Roux Stéphane , Yu Robert , Fiard Jean-Michel , Bellettre Jérôme 
Published: June 2024

Abstract
An experimental test section with horizontal and vertical heated plates forming an inverted L-shape is filled with 3M Novec7000 dielectric fluid at atmospheric pressure. Both plates of 64 × 40 mm
2 dimensions are heated with wall heat fluxes ranging from 0 to 20 W cm
−2. There is natural convection for low heat fluxes, medium heat fluxes trigger boiling and for higher power, the critical heat flux is reached. The wall temperature and heat flux are calculated from in-wall thermocouple measurements in steady state. The bubbly flow on the vertical surface is observed with a high-speed camera through a window.
The confinement between the vertical heated surface and the wall is varied from 0.5 to 15 mm. In the range of this study, the heat transfer coefficient in natural convection and nucleate boiling regimes increases when the spacing decreases. However, since the critical heat flux decreases when it is more confined, the film boiling regime is reached faster. From the visualizations and the thermal measurements we try to model these different behaviors.
Depending on which plate is switched on or off we observed different behaviors. The confinement of the vertical plate does not have any influence on the horizontal one since the plate width is much bigger than the gap change. Also, the horizontal plate heating does not influence the heat transfer on the vertical plate in this study. However, there is an influence of the vertical boiling on the horizontal one, stronger when there is a confinement than in free configuration.
Nucleation, growth and bubble detachment in liquid-vapor phase change on structured surfaces
Authors: Arhar Klara , Može Matic , Zupančič Matevž , Golobič Iztok 
Published: June 2024

Abstract
Comprehensive grasp of heat and mass transfer, particularly in applications involving liquid-vapor phase change, hinges on management of nucleation, growth, and detachment of vapor bubbles. Various parameters influence the dynamics of phase-change heat and mass transfer and thus dictate the interactions between the surface, the liquid, and the vapor, profoundly impacting the underlying processes. To tailor these phenomena and harness them for technical applications involving high heat flux densities and intense mass transfer, such as boiling and electrolysis, surface functionalization is under intense development. By designing structured surfaces and creating preferential nucleation sites that promote heterogeneous nucleation, it is possible to exert control over the location and density of active nucleation sites on the surface. This, in turn, enables the regulation of bubble growth and detachment from the surface. With the aim of identifying optimal surface treatments for functionalizing surfaces and enhancing their performance in phase-change applications, we evaluated the nucleation, growth and detachment of a single bubble in a liquid-vapor phase change on untextured and laser-textured surfaces during water electrolysis. Platinum was chosen as the preferential material due to its favourable electrochemical properties for the hydrogen evolution reaction in acidic media. Electrolysis was performed at various voltages and the bubble dynamics were evaluated through high-speed imaging to investigate the intricacies of bubble growth and their mutual interactions. At 3.5 V using the laser textured surface, hydrogen bubbles detaching from the electrode surface had on average 40% smaller diameter, while the frequency of their detachment was 2,5-times higher compared to the untextured surface. This opens the possibility for further research that could lead to improving the efficiency of electrolysis.
The effective use of focused shadowgraphy for single bubble nucleate pool boiling investigations
Authors: Everts M , Welzl M , Brüggemann D 
Published: June 2024

Abstract
Nucleate pool boiling is known for its high heat transfer coefficients. Despite being widely implemented, the prediction of nucleate pool boiling mechanisms remains complex and single bubble heat transfer analysis is helpful to simplify the problem. Owing to the interesting bubble behaviour during nucleate pool boiling, publications tend to focus on the bubble dynamics while the experimental setup is only briefly discussed. Small but critical information on the experimental setup is often omitted, which makes it challenging or even impossible to reproduce or compare experimental data. Therefore, the purpose of this study is to provide a systematic approach to using focused shadowgraphy for the investigation of single bubble dynamics using R245fa. A pool boiling experimental setup has been built and equipped with temperature, pressure and heat flux sensors, as well as a high-speed camera and light source. The nucleate pool boiling takes place at a single cavity on a copper block. The influences of the heat transfer surface area, light source, and diffuser films were investigated to provide a systematic approach to the use of focused shadowgraphy for the investigation of single bubble dynamics.
Application of the localized method of fundamental solutions to heat transfer problems
Authors: Gáspár Csaba 
Published: June 2024

Abstract
A localized version of the Method of Fundamental Solutions is applied to the 2D steady heat transfer equation with spatially varying thermal conductivity. Though the corresponding fundamental solution cannot be computed in general, the localization splits the original problem into several subproblems defined on small subdomains; in these subdomains, the subproblems are approximately converted to certain convection-diffusion equations with constant coefficients, so that the Method of Fundamental Solutions is applicable. In each subdomain, the corresponding subproblem is solved separately. This results in an iterative method, which mimics the overlapping (alternating) Schwarz method. Due to its advantageous numerical properties, the technique seems a useful generalization of the Method of Fundamental Solutions. The method is illustrated through numerical examples.
A superconvergence result in the RBF-FD method
Authors: Kolar-Požun Andrej , Jančič Mitja , Kosec Gregor 
Published: June 2024

Abstract
Radial Basis Function-generated Finite Differences (RBF-FD) is a meshless method that can be used to numerically solve partial differential equations. The solution procedure consists of two steps. First, the differential operator is discretised on given scattered nodes and afterwards, a global sparse matrix is assembled and inverted to obtain an approximate solution. Focusing on Polyharmonic Splines as our Radial Basis Functions (RBFs) of choice, appropriately augmented with monomials, it is well known that the truncation error of the differential operator approximation is determined by the degree of monomial augmentation. Naively, one might think that the solution error will have the same order of convergence. We present a superconvergence result that shows otherwise - for some augmentation degrees, order of convergence is higher than expected.
On different implementations of boundary conditions in the meshless RBF-FD method for phase-field modelling of dendritic solidification
Authors: Dobravec T , Mavrič B , Šarler B 
Published: June 2024

Abstract
Dendritic morphology is one of the most common microstructures in solidifying metallic materials. The phase-field method represents one of the most suitable approaches for modelling the evolution of dendritic morphology. In this paper, the meshless RBF-FD method and forward Euler scheme are used to solve the partial differential equations arising from the phase-field model for dendritic growth. We consider the growth of a single dendrite into a supercooled pure melt. On the computational domain’s surfaces, we apply symmetric boundary conditions. Special care has to be taken in the RBF-FD method to satisfy boundary conditions accurately. In this paper, we test four different implementations of boundary conditions. In the first one, the boundary conditions are incorporated when constructing an interpolation problem in local support domains containing boundary nodes. In the second one, the interpolation problem in the local support domains does not consider boundary conditions, which are satisfied by solving a system of linear equations for values in all boundary nodes at each time step. The third one complements the second one with the use of ghost nodes. The fourth implementation is an alternative one, where the values in ghost nodes are determined by direct mirroring. The accuracy and computational efficiency of all four implementations are compared. We discuss the advantages and disadvantages of each implementation. We show that using ghost nodes is recommended for implementing Neumann boundary conditions in the RBF-FD method.
Stratified liquid-liquid flow through microchannels with grooved walls
Authors: Dewangan Mainendra Kumar , Persoons Tim 
Published: June 2024

Abstract
Patterned surfaces have numerous applications in the microscale flow regime, such as heat transfer enhancement, mixing, and microfluidics devices. The present study analytically examines the pressure-driven flow of two immiscible Newtonian fluids through a grooved microchannel. The orientation of the channel is defined as the top and bottom walls being flat and wavy surfaces, respectively. A no-slip boundary condition is assumed at both walls. The present problem is investigated by invoking the Fourier theory for a flow along streamwise grooves at the Stokes flow limit. Flow rates and velocities of both fluids are determined analytically and numerically. A finite-element-based numerical study is conducted to understand the accuracy of the theoretical models. Results are generated to show the effects of viscosity ratio, wall undulation amplitude and wavelength of the patterned channel. For both fluids with a small wavelength, hydraulic permeability decreases with increasing the pattern amplitude at various viscosity ratios. Meanwhile, in the case of large wavelengths, hydraulic permeability increases with pattern amplitude at different values of viscosity ratio. This behaviour of permeability is identified for fluid 2 (which is in contact with the grooved surface). Roughness and confinement effects are captured with increasing pattern amplitude at different wavelengths and viscosity ratios. The present analytical model agrees well with numerical values. The findings provide a deeper understanding of the stratified flow through microchannels with undulating surfaces, with potential applications in electronic cooling, skin-friction drag, interfacial fluid dynamics, and enhancement and reduction of heat transfer.
Enhancing the degrees of freedom of topology optimization via variable-porosity metal foams: Design of heat conduction paths in a volume-to-point problem
Authors: Bianco N , Fragnito A , Iasiello M , Mauro G M 
Published: June 2024

Abstract
Multi-material topology optimization determines the optimal distribution of different materials within a design domain in order to achieve specific performance goals. This work takes advantage of such technique to enhance the thermal performance of a benchmark heat conduction problem. The aim is to investigate the impact of enhancing the degrees of freedom introducing different materials, i.e., variable-porosity metal foams in addition to the high-conductivity solid, in the design of heat conduction paths under a constant weight constraint. The study leverages a well-established case study – volume-to-point problem – in the field of thermal management, ensuring a consistent basis for comparison. It consists of a square domain with a uniform heat source and a Dirichlet condition at a point on the boundary. Interpolation functions – ordered solid isotropic material with penalization (SIMP) type – allow the properties – in this case thermal conductivity – of materials to be correctly assigned. The distinction between materials is made by means of different thresholds on the projection function. By varying the foam porosity, we investigate the trade-offs between weight and heat dissipation efficiency. The objective is to find the ideal combination of materials that maximizes heat transfer – minimizing the average domain temperature – while conforming to weight constraints. Findings reveal that multi-material topology optimization – when applied to heat conduction problems – can outperform other design approaches, such as the growth-based algorithm, the evolved constructal tree, and the classical topology optimization (with one solid material), thereby paving the ground to innovative heat sink designs in weight-constrained environments.
Numerical investigation of gyroid heat exchanger
Authors: Jena Bhagaban , Choudhari Vipul D , Saravana Kumar G , Arul Prakash K 
Published: June 2024

Abstract
A novel cylindrical gyroid surface-based tube is designed to investigate fluid flow and heat transfer characteristics numerically. The simulations are carried out using finite volume based CFD software Ansys Fluent. The numerical validation is performed to verify the robustness of the solver and grid independence study are also carried out to compute the simulations using optimum mesh. The heat exchanger (HX) tubes with gyroid surfaces are manufactured using additive manufacturing and a crossflow HX is fabricated with a bank of gyroid tubes where the experiments were conducted. The numerical results of the Realizable k-ε turbulence model with enhanced wall treatment is validated with in-house experimental data. The thermal performance of the gyroid tube HX is also compared with a conventional circular tube (CCT). Parametric analysis is also carried out by varying the gyroid design parameter and an optimum HX design is proposed. The results show that due to larger surface area and enhanced fluid mixing, the gyroid tube-based HXs show better thermal performance compared to CCT-based HX.
Forced air flow through a rectangular channel with 3D turbulence enhancers: fluid-dynamics and thermal analysis by Large Eddy Simulations
Authors: Corti M , Vitali L , Gramazio P , Fustinoni D , Niro A 
Published: June 2024

Abstract
Many newer engineering applications, like the cooling of automotive batteries and high-performance CPUs, require heat exchangers (HEs) with large contact areas and very small heights. Performances of this type of HEs can be significantly improved by using surfaces enhanced with optimized 3D structures. The project Mood4hex aims at designing new geometries with optimized morphology, starting from a large database of experimental data on ribbed surfaces. In order to perform an optimization algorithm on such surfaces, analyses are carried out on CFD models with increasing complexity and computational time, to find a fast-running code that preserves the physical meaning. Until recently we used RANS models, and now we began to investigate the use of Large Eddy Simulations (LES) as a higher order validation tool for simpler models. In this paper, we present the results of LES along with experimental data of a forced air flow inside a streamwise periodic rectangular channel of high aspect ratio, i.e., 1:10, equipped with standard transverse ribs (α = 90°). More specifically, the ribs are placed only on the lower wall that is operated at constant heat flux, while air flowing at Re 500 or 1000. Comparison of experimental data vs LES results shows weaknesses and strengths of the model.
A combined experimental and numerical approach for heat transfer enhancement in a minichannel using half rectified pulsating flows
Authors: Kumavat P S , Alimohammadi S , O’Shaughnessy S M 
Published: June 2024

Abstract
The miniaturization of electronic packages and their associated high-power density circuits requires more innovative cooling solutions. Single phase pulsating flows offer a promising solution due to their disruption of the thermal boundary layer. This study aims to bridge the knowledge gap by using experimental and computational methods to investigate the complex flow characteristics of laminar pulsating flows in a heated rectangular minichannel and couple that analysis with an investigation of thermal-hydraulic performance. Experimental analysis involves a uniformly heated thin foil approximating a constant heat flux bottom wall. Wall temperature measurements are recorded using an infrared thermography system. Analogous to the experimental conditions, a three-dimensional conjugate heat transfer computational model is developed with a volumetric heat generation source. Positive and negative half rectified sinusoidal pulsating flow waveforms are studied for dimensionless pulsation frequencies or Womersley numbers of Wo = 2.5 and 5.1 at a fixed flow rate amplitude A
0 = 3. For the positive half rectified waveform, a marginal enhancement in heat transfer of 2.2% for Wo = 2.5, A
0 = 3 was obtained. Whereas the negative half rectified case leads to a heat transfer enhancement of 9% and 6% for Wo = 2.5 and Wo = 5.1 respectively, with a high thermal performance of η = 2.4 over corresponding steady flow.