bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical investigation of different cooling technologies during heatwaves in Catalan Mediterranean buildings
Authors: Garrido Núria , Capdevila Roser , Tontodonati Enrico , Borghero Luca
Published: June 2024

The Mediterranean area is particularly affected by heatwaves, which are extremely critical for health and productivity of people inside the dwellings as well as for energy consumptions related to cooling demand. This study numerically investigated the resilience of different cooling technologies in the most common construction in Catalan building stock during present and future climatic conditions, heatwaves. The studied technologies include green roof, blinds, advanced windows, air conditioning and their combinations. Meteorological data of heatwaves for Barcelona city have been created for three time periods, Present, Mid Future and Long Future. They have been modelled using dynamical downscaled Regional Climate Models, climate projections based on IPCC scenarios. Resilience was assessed using indicators that consider the linear correlation between the grade of overheating inside a dwelling and the severity of outdoor conditions. Thermal comfort and survivability have been also investigated, evaluating sensations of building’s occupants related to indoor air temperature and relative humidity. The original building presented acceptable levels of resilience with a resilience index α below 1, while thermal comfort conditions resulted poor, especially during heatwaves, being for more than 60% of the time in Extreme Caution conditions or worst. The application of the different cooling technologies significantly helped in the improvement of resilience and survivability inside the dwelling. However, their performances reduced in the future scenarios, reaching more than 90% of the time under Extreme Caution conditions or worst. Air conditioning resulted the best solution for avoiding overheating, especially in heatwaves constraining it to a maximum of 1.8°C. Combinations of several passive measures resulted necessary for maintaining suitable conditions.
Comparative analysis of the behaviour of the hot water production system of an indoor swimming pool
Authors: Velasco F.J.S. , Giménez-Villa J. , García-Cascales J.R. , Illán-Gómez F. , Delgado Marín J.P. , Otón-Martínez R.A.
Published: June 2024

In this paper, we analyse the thermal behaviour and the environmental impact of an indoor swimming pool placed in Pozo Estrecho, Cartagena, in the Southeast of Spain. The existing installation is formed by a group of solar thermal collectors supported by two propane heaters which heat the water of the pool and satisfy the needs of hot water in the changing room of the pool. Here we propose an installation based on the use of a heat pump supported by solar thermal collectors. In this work, we first describe the installations. They are modelled with Trnsys and the models employed briefly described. Later, we compare their primary energy consumption and its environmental impact in terms of their CO2 emissions. A rough estimate of its economic impact is also given. Some conclusions are finally drawn.
Enhancing thermal modelling of electric machines: steady-state verification of a novel methodology
Authors: Nonneman Jasper , T’Jollyn Ilya , De Paepe Michel
Published: June 2024

The increasing power density of electric machines for electric vehicles necessitates accurate thermal analysis in design through simulations with computation times not exceeding one minute. Existing thermal models are either accurate and complex while requiring large computation times or they are fast but lack accuracy. To address these challenges, a novel hybrid modelling methodology is proposed, combining 3D lumped parameter thermal networks with thermal resistances extracted from 2D finite element models (FEM) to reduce computation demands while preserving accuracy. Verification of the modelling methodology against 3D FEM in OpenFOAM shows key component temperature deviations of less than 0.67 °C comparable to the 3D FEM uncertainty of 0.39 °C. The computation time is 18.3 s compared to 44 h for 3D FEM. This highlights the time-efficiency of the proposed methodology while maintaining accuracy for key component temperatures.
Low enthalpy geothermal source for sustainable energy production in small islands: a real case study
Authors: Fraia Simona Di , Massarotti Nicola , Vanoli Laura
Published: June 2024

The global demand for clean and sustainable energy sources has significantly increased in recent years, mainly because of concerns about climate change and the finite nature of traditional fossil fuel reserves. In this context, geothermal energy has gained significant attention as a reliable and eco-friendly alternative. In particular, low enthalpy geothermal resources have emerged as an attractive option due to their accessibility, widespread distribution, and potential for decentralization. Moreover, reliability and flexibility of this source allow for year-round utilization to meet diverse energy demands, resulting in reduced dependency on traditional energy sources and associated greenhouse gas emissions. For this reason, geothermal energy appears to be a promising solution in small islands, where energy production is often a challenge due to the absence of infrastructures that make them energy-dependent on the mainland. Therefore, in this work, low enthalpy geothermal source is considered as effective and sustainable solution for electrical energy production in small islands. As case study a low temperature source available in Ischia island, Southern Italy, is considered. The proposed solution is analysed from energy, economic and environmental point of view.
Experimental investigation of the impact of surface tension and artificial nucleation site geometry on the bubble growth
Authors: Vodopivec M , Berhe MG , Bucci M , Zupančič M , Fontanarosa D , Može M , Castagne S , Golobič I , Vetrano MR
Published: June 2024

The present work provides an experimental assessment of the impact of liquid surface tension on the single bubble dynamics generated by artificial nucleation cavities during boiling in a quiescent liquid. De-ionized water is used with and without added non-ionic surfactant (Dynol 800) to modify the fluid’s surface tension while keeping the other thermo-physical fluid properties unchanged. Furthermore, two different cavity geometries are investigated: a vertical and a 30°-inclined cone produced via femtosecond laser texturing. This paper shows preliminary results related to the influence of surface tension and cavity geometry on the single bubble dynamics. A methodological baseline is provided to enable future studies aiming to answer still-open questions about vapour bubble nucleation phenomena.
Direct Numerical Simulation of single bubble dynamics in nucleate pool boiling with micro-region modeling and thermal coupling to a solid wall
Authors: Wei Linkai , Bois Guillaume , Nikolayev Vadim S.
Published: June 2024

In this study, we conduct two-dimensional, axisymmetric simulations to investigate the growth and departure of a single bubble, with a particular focus on the conjugate heat transfer between the fluid and the heating wall. A multiscale modeling approach is employed to account for nanoscale effects near the liquid-vapor-solid triple contact line (CL). At the macro (bubble size) scale, the interface dynamics is captured with the front-tracking method by using the open-source code TRUST/TrioCFD. The macro scale algorithm is coupled with a sub-grid micro-region model. It is driven by the wall superheating at the CL as input, and predicts the macroscopic apparent contact angle and heat fluxes. To validate our modeling approach, we first conduct quantitative comparisons using the data on the bubble growth experiment RUBI and its simulations. Next, an investigation of the case of water under atmospheric pressure and gravity is performed. Notably, we observe a significant spatial variation in temperature near the nucleation site during the expansion and contraction of the bubble base. This variation greatly affects the thermal boundary layers in the fluid and solid domains and the bubble growth, demonstrating the need to resolve the conjugate heat transfer in the considered cases.
Multi-scale boiling heat transfer investigation on micro-thin aluminum heaters
Authors: Bucci M , Zupančič M , Golobič I
Published: June 2024

Nucleate boiling is a highly efficient heat transfer mode distinguished by the liquid-vapor phase change, which occurs through the formation, growth, and detachment of vapor bubbles from a heated surface. Its crucial role in various industrial applications, such as nuclear power plant operation and effective heat management in small electronic devices, has driven significant research efforts. However, despite extensive research dedicated to boiling investigations, there are still substantial knowledge gaps that hinder our ability to accurately predict heat removal rates. These knowledge gaps arise from the complex nature of small-scale boiling phenomena, which are further complicated by their strong dependence on operating conditions and the interactions between walls and fluids. In an effort to address some of these gaps, we conducted multi-scale investigations during pool boiling of de-ionized water on micro-thin aluminum heaters. We captured bubble dynamics through multiple synchronized diagnostic sources, including high-speed backlit imaging to track bubble growth, synchronized high-speed infrared thermometry to capture the corresponding thermal footprint on the boiling surface, and in-house developed fast-response micro-thermocouples to measure temperature at multiple locations within the fluid. Our study reveals peculiar aspects of heat transfer mechanisms occurring at single bubble level (low heat fluxes) and in fully developed nucleate boiling regimes (high heat fluxes).
Influence of subcooling conditions on pool boiling in microgravity conditions: Single artificial case
Authors: Ronshin F , Kabov O , Rednikov A , Tadrist L
Published: June 2024

A series of boiling experiments on a single artificial nucleation site has been carried out on the International Space Station (ISS) in the framework of the Multiscale Boiling (RUBI) setup installed in space in 2019. The aim of these unique experiments is to understand the bubble nucleation-growth mechanisms involved in heat transfer during boiling under well-controlled operating conditions without being masked/impeded by gravity and natural convection. The bubbles can grow to large sizes that are inaccessible in the presence of gravity. They are observed by a side-view black-and-white camera and by an infrared camera through a transparent heated substrate. In this paper, we present the results of a single-bubble pool-boiling experiment with an emphasis upon the role of the surrounding liquid subcooling, several values of which are investigated. The experimental results are complemented by numerical simulations using a previously developed model. However, the model is slightly modified to account in a simplest way for possible non-condensable residuals, without which we would have a hard time to explain certain observed behaviors.
Drag effect of steam flow on droplet removal during dropwise condensation at different surface inclinations
Authors: Abbatecola A , Tancon M , Bortolin S , Del Col D
Published: June 2024

Dropwise condensation is a quasi-cyclic process characterized by the nucleation, growth, and removal of discrete liquid droplets on a subcooled surface. The removal of condensate is a critical aspect, usually achieved by exploiting the gravity force, the drag force of vapor or the surface wettability gradient. This paper presents an experimental study of the vapor drag action on condensate removal, with focus on droplet’s departing radius (rmax
). Specifically, for the experimental campaign, vapor velocity was varied from 3 to 14 m s−1 considering three different surface inclinations: vertical, 45° inclined, and horizontal. The results showed that, as the velocity increases, the difference in departing radii among the three different configurations decreases and, consequently, the difference in heat transfer coefficients decreases too. In fact, at the highest vapor velocity (~14 m s−1), rmax
was almost equal for all the inclinations leading to similar heat transfer coefficients (~120 kW m−2 K−1). Interestingly, on a horizontal surface considering vapor velocity equal to 3 m s−1, despite the lack of gravity’s contribution to droplet removal, no transition to filmwise condensation was observed.
Grooved hybrid copper surfaces for condensation heat transfer.
Authors: Goswami Amit , Pillai Suresh C. , McGranaghan Gerard
Published: June 2024

Condensation is commonly utilized in numerous thermal management applications and has two modes termed filmwise and dropwise, the latter providing superior heat transfer performance. However dropwise condensation is usually limited by its dependence on gravitational shedding of condensate. To combat this, hybrid surfaces consisting of regions of different wettability have emerged as a promising solution. Of such designs, hybrid grooved surfaces consisting of grooves with varying wettability in the groove valleys and ridge tops are of current interest. To date, most such surfaces have been silicon-based and over flat substrates. In this work, condensation experiments are performed on a copper tube with fabricated grooved hybrid surfaces. The aim is to take advantage of anisotropic wetting due to the presence of grooves and condensate drainage from higher wettability regions. The experimental results quantify the heat transfer with the degree of subcooling. At the same time, droplet dynamics were studied on the hydrophobic-hydrophilic surfaces, showing several droplet growth and departure mechanisms leading to effective heat transfer.
Cryogenic quenching process enhancement through coating and microstructure optimization
Authors: Graffiedi Marco , Dent Francis J. , Khodaparast Sepideh , Bucci Matteo
Published: June 2024

In this work, we explore the impact of coatings and microstructures on heat transfer during a cryogenic quenching process. An easily reproducible quenching test is presented as a benchmark for testing different solutions. The study involves two different flat polymeric coatings as well as three porous microstructures. The results show that pairing a low-conductive coating with an appropriate porous surface microstructure on top of a stainless-steel plate can reduce the chill down time, accelerating the transition from room temperature to liquid nitrogen temperature, by a factor of five. High speed video recordings have been used to analyse the quenching process with different coatings and microstructures showing how the suppression of the film boiling regime is the key to enhancing the quenching process.
Spray cooling of micropillared steel plates: Two-stage quenching phenomenon
Authors: Kita Y , Matsunaga T , Ariyoshi T , Fukuda H , Takata Y , Kohno M
Published: June 2024

Spray cooling is a crucial process of steel manufacturing, nuclear power plants and electronics thermal management. However, its precise control is challenging due to the various boiling regimes involved at different temperature levels. Transition from film boiling to nucleate boiling, namely, the onset of quench is our particular interest as it remarks the thermal history of the hot object. Based on our previous findings that quench occurs at a specific temperature defined at the droplet-surface interface we demonstrate that micropillars on steel plates can increase the quench temperature Tq
. We performed water spray cooling tests with stainless-steel samples, on which various configurations of micropillars were fabricated using laser texturing. Interestingly, we observed two stages of quench for a specific sample: a drastic increase in the cooling rate at ca. 450 °C, followed by another increase at ca. 250 °C. The first cooling rate increase is attributed to the initiation of stable liquid-solid contact at the pillar tops. Eventually, water wets the entire pillars and the bottom surface, further increasing the cooling rate. Samples with higher pillars showed no film boiling regime in the experiment, implying higher quenching temperature (Tq
> 600 °C). We attribute the increased apparent Tq
to the narrow heat conduction path within the micropillar, resulting in a significant temperature drop at the tip which enhances liquid-solid contact.
Effect of hydrophobic coating on optimization of dropwise condensation of steam on hybrid surfaces
Authors: Suzzi Nicola , Croce Giulio
Published: June 2024

Dropwise condensation of pure vapor on hybrid surfaces, characterized by alternate hydrophobic-hydrophilic regions, is numerically investigated via phenomenological, Lagrangian modelization of dropwise condensation on hydrophobic regions. The drop size distribution over the hydrophobic domain, which knowledge is crucial for development of accurate simplified, statistically based models, is computed. Comparison with literature correlations shows that the theoretical correlation for size distribution of small droplets gives an inaccurate estimate, while the improved correlation, derived from the droplet population balance, better describes the numerical size distribution and, if incorporated in the statistically based model, allows to accurately predict the condensing flux.
Flow boiling heat transfer in a plate heat exchanger with mixed chevron angle plates
Authors: Yang Chien-Yuh , Hsieh Mu-Ting
Published: June 2024

This study designed a 120-plates PHE with combination of 60 plates with 65° chevron angle and 60 plates with 35° chevron angle. The plates with 65° chevron angle were assembled in the half part of the PHE which near the inlet port to provide higher flow resistance in comparing to the other 35° plates. The 35° chevron angle plates were put away from the inlet port to reduce the flow resistance caused by the longer flow path. It is aimed to reduce the flow maldistribution in the multiplate heat exchanger and improve its heat transfer performance. Refrigerant R-410A flow boiling heat transfer performance in the mixed angle PHE and another PHE with 65° chevron angle was tested for comparison. The test results show that heat transfer coefficient in the mixed chevron angle PHE is 20% to 70% higher than that in the 65° chevron angle PHE with 25% lower pressure drops. The mixed angle plates apparently reduced the liquid refrigerant flow deficient region and provided a better heat transfer performance.
Boiling of binary mixtures on superhydrophobic surfaces with artificial cavities
Authors: Zhang Ningxi , Orejon Daniel , Liu Jionghui , Sefiane Khellil
Published: June 2024

This study experimentally investigates bubble behaviour from an isolated artificial cavity on a superhydrophobic substrate, the working fluid is a binary mixture of Novec649 and Novec7000 with mole fractions between 0.05 – 99.5 as well as their pure counterparts. The results show an earlier onset of bubble departure as well as deceasing trends in bubble departure diameter in the presence of the binary mixture compared to pure cases. The bubble departure diameter is at its minimum and fastest growth rate at a mole fraction of approximately 0.50. Theoretical models from literature were tested and satisfactory agreement with different models were achieved depending on the concentration.
Development of physically based modelling of bubble behaviour for subcooled boiling applications
Authors: Cranmer J. , Giustini G. , Skillen A. , Tunstall R.
Published: June 2024

A methodology for the modelling of phase change phenomena in two-phase flow is presented, based on interface capturing simulation and the mechanistic modelling of interfacial heat and mass transfer. The volume of fluid approach is adopted, utilising a compressive scheme to maintain interface sharpness, allowing a continuum representation of mass and heat source terms to be used. The methodology is verified against analytical solutions for both evaporation and condensation phase change problems, demonstrating excellent agreement even in the case of a large phase density ratio and high rate of evaporation.
Prediction of heat transfer coefficient and pressure drop of flow boiling and condensation using machine learning
Authors: Galicia Edgar Santiago , Matamoros Andres Hernandez , Miyara Akio
Published: June 2024

Heat transfer coefficient and pressure drop are two critical parameters in diverse thermal engineering applications, and the prediction of these parameters plays a crucial role in the development of heat exchangers. Traditional methods are often reliant on empirical correlations based on limited refrigerants and experimental conditions. The present research explores the application of machine learning methods to predict the heat transfer coefficient and pressure drop of a wide range of pure and mixed refrigerants. The methodology involves the compilation of over 20, 000 experimental observations encompassing an extensive range of operating conditions, heat exchanger geometries, and refrigerants. A total of 27 machine learning algorithm models, such as regression tree, support vector machines, and deep neural networks, are trained on this diverse dataset to discern intricate patterns and dependencies. After dividing the dataset into 80% for training and the rest for validation, the machine learning prediction shows an ability to predict the heat transfer coefficient and the pressure drop with high accuracy in most of the refrigerants. Notably, the accuracy achieved by the machine learning models surpassed that of conventional correlations, highlighting their superior predictive capabilities. Furthermore, the accurate prediction of heat transfer enhancement methods added another layer of validation to the model’s effectiveness. It is noteworthy that the dataset employed in this study has been made publicly accessible online.
Advancements in laser-based spatiotemporal measurements of flow boiling
Authors: Chen Zengchao , Narayan Surya , Lobasov Aleksei S , Pervunin Konstantin S , Falsetti Chiara , Bucci Matteo , Markides Christos N
Published: June 2024

Recent advancements in laser and imaging systems, as well as in computational processing capabilities, have made quantitative optical imaging, which is often combined with laser illumination, highly adaptable, robust and reliable. Laser-based diagnostic techniques, such as planar laser-induced fluorescence (PLIF), offer the possibility of simultaneous spatiotemporally resolved measurements of temperature fields in the liquid phase at boiling conditions. In this paper, we examine the applicability of two-colour PLIF (2cPLIF), where the ratio between individual fluorescent emissions from uniformly dispersed dyes is used to take temperature measurements in the liquid phase in the presence of moving vapour-liquid interfaces typical of boiling flow. The implementation of 2cPLIF necessitates uniformity in the concentration of different dyes across the flow field. However, in the case of a multiphase flow such as boiling, thermophoresis can lead to inhomogeneous dye distributions. To overcome this challenge, a single-dye multispectral planar laser-induced fluorescence (SDMS-PLIF) method has been developed, which employs fluorescent emissions in different spectral bands of the same dye (Nile Red). The spectral characteristics of Nile Red were measured using a spectrometer to identify its temperature-sensitive bands over a wide range of dye concentrations, from 0.3 to 30 mg/L. Following this, we demonstrate the measurement capabilities of SDMS-PLIF thermography as applied to a boiling flow in a miniaturised vertical square channel, gaining insight into the thermohydrodynamic interactions between vapour bubbles and a heated wall.
Investigation of dropwise condensation of water through an efficient individual-based model
Authors: Tancon M , Abbatecola A , Mirafiori M , Bortolin S , Del Col D
Published: June 2024

In recent years, researchers have directed their studies towards solutions aimed at enhancing heat exchangers effectiveness. In this context, dropwise condensation (DWC) has been identified among the most promising solutions to increase the condensation heat transfer coefficient (HTC). In fact, DWC provides heat transfer coefficients up to ten times higher than those achievable during filmwise condensation (FWC), resulting in both economic and energy benefits. The DWC phenomenon is usually modelled by combining the heat exchanged through a single droplet and the drop-size distribution. The latter can be divided into a distribution of large droplets N(r), determinable analytically by semi-empirical models, and a distribution of small droplets n(r), typically determined through statistical approaches called population-based models. Another possibility for the determination of the droplet-size density is to simulate the DWC process by an individual-based model (IBM). In this case, each drop is tracked throughout its entire life cycle (nucleation, growth, coalescence, sliding), and the drop-size distribution is obtained as a result. In this paper, a new IBM for the simulation of DWC of steam is proposed. The developed model allows for the simulation of more than 10 million droplets while keeping an acceptable simulation time thanks to the implementation of parallel computing. The predictions obtained from the model, in terms of drop-size distribution and condensation heat flux, are compared against both PBM results and experimental data.
Experimental investigation of condensation heat transfer on vertically inclined grooved aluminium surfaces
Authors: Shatskiy Evgeny , Kumavat Parth S , O’Shaughnessy Seamus M
Published: June 2024

This study details the experimental investigation of surface condensation heat transfer on vertically inclined aluminium surfaces with simple modifications. A bespoke experimental apparatus was constructed to control the saturation conditions inside the condensation chamber where the test surface is located. The degree of subcooling of the test surface was precisely controlled by an external heat exchanger. The meter bar technique was used to evaluate the condensation heat flux. Using distilled water as the working fluid and a plane aluminium test surface as the baseline case, the experimental apparatus was validated by comparison of the determined heat transfer coefficient with Nusselt’s model of filmwise condensation on a vertical plane surface, where an excellent agreement was obtained. 7 different aluminium test surfaces were subsequently investigated, each one modified with grooves measuring 0.5mm by 0.5mm (height and width), with a consistent spacing of 0.5mm between them. Groove angles relative to the vertical axis of 0° (180°), 30°, 45°, 60°, 90°, 120°, and 150° were investigated, where grooves were symmetrically mirrored at the centre of each test surface. Results highlighted a significant influence of groove angle on the heat transfer coefficient, with the best-performing sample (150˚) producing a maximum enhancement of 45% and an average enhancement of 34% compared to the baseline test surface over the respective range of subcooling.
Meshfree RBF–FD methods for numerical simulation of PDE problems
Authors: Larsson Elisabeth , Mavrič Boštjan , Michael Andreas , Pooladi Fatemeh , Tominec Igor
Published: June 2024

The meshfree RBF–FD method is becoming an increasingly popular discretization method for challenging PDE problems due to its flexibility with respect to geometry and its ease of implementation. Using a combination of polyharmonic spline basis functions and polynomial basis functions has proven to be particularly successful with a convergence rate depending on the polynomial degree and good behavior at boundaries due to the contribution of the spline part. A challenge that has been observed is that derivative boundary conditions can give rise to large errors. A known remedy for this problem is to introduce one or more layers of ghost points, which improves the boundary approximations. In a recent paper, Tominec, Larsson, Heryudono (2021), it was shown that using oversampling is another effective way to handle this problem. Oversampling also allows for theoretical analysis of the method, which can then be viewed as a discretization of a continuous least-squares projection. By looking deeper into the error estimates we gain an understanding about how to best implement the method.
Enhancing fluid distribution in additively manufactured heat exchangers using autogenerated geometries
Authors: Kimmich J.
Published: June 2024

The heat exchanger distributor, which connects the tube carrying the medium to the heat transfer geometry, has received less attention in research studies compared to heat transfer geometries. However, the flow distribution is critical to efficiency. Therefore, a model-in-the-loop approach is presented to optimize additively manufactured distributors. A quasi-fractal distributor and all connected geometries, including pipe connections, heat transfer geometry and collector, are meshed and simulated in OpenFOAM using steady-state computational fluid dynamics. The volumetric flow rates determined from the simulation are assigned to the respective flow cross-sections during the distribution process. The flow cross-sections of the geometric model are modified within an optimization process, considering the prevailing volumetric flows. The results indicate a 64 % decrease in the standard deviation of volume flow across all 85 channels with a simple optimizer and few simulation loops. The printable geometry can be extracted directly out of Python using common file formats. The presented workflow has been proven feasible. Future improvements could involve using the automated model-in-the-loop simulation and creating optimizing algorithms considering fluid flow behavior.
Effect of the plates geometry on the performance of a cross-flow recuperator for indirect evaporative cooling systems
Authors: Caruana Roberta , Marocco Luca , De Antonellis Stefano , Guilizzoni Manfredo
Published: June 2024

In recent years, Indirect Evaporative Cooling (IEC) is becoming a very interesting technology, as it can guarantee a good cooling effectiveness with low energy demand, thus helping to face the issues of climate change. In this work, a Computational Fluid Dynamics (CFD) model has been used to evaluate the effects of the plates geometry on the performance of a recuperator for IEC systems with dry primary and secondary channels. The model was validated against experimental data for a cross-flow IEC system with dimpled plates, and the validation results were quite satisfactory. Therefore, five types of dimpled plates differing from the original one in shape and size of the dimples were investigated. The results showed that a good trade-off between the improvement of dry-bulb effectiveness and restrained pressure losses along the channels is obtained with dimples in the shape of right prisms with either triangular, or pentagonal, or circular base section, depending on the application.
Numerical study of solar evaporation in 3D-printed structures
Authors: Fillet R , Nicolas V , Celzard A , Fierro V
Published: June 2024

In this study, we used three-dimensional (3D) printed concave geometries filled with activated carbon to create a unique 3D solar evaporator for clean water production. The evaporation process was modeled considering airflow and solar radiation to investigate the advantages of 3D concave geometries. This work encompasses a series of studies, focusing on mass transfer inside the concavity and around the structure, to elucidate specific exchange points. Additionally, the research explores the effects of air velocity and the incorporation of additional holes within the structure, with the aim of improving evaporation.
Thermal management of multi-chip electronic module with wavy fin heat sink-assisted immersion cooling
Authors: Suresh Pratheek , Vasista Kasavajhula Naga , Kushwaha Shailesh , Balaji Chakravarthy
Published: June 2024

Single-phase immersion cooling technology has emerged as a promising solution to effectively manage the thermal needs of electronics in modern high-heat-flux-density data centres. This approach involves submerging electronic components, such as heat-generating chips, into a thermally conductive dielectric liquid. The present study investigates the thermal performance of a multichip server module under single-phase immersion cooling using NOVEC 7500 (C
9
H
5
F
15
O) as dielectric flow media. A wavy fin heat sink design with varying number of fins on each heat sink is proposed to overcome the poor thermal performance of the dielectric fluid. Initially, an optimal plane fin combination is determined; it reduced the maximum temperature (Tmax
) and Root Mean Square Error of all chips by 22.4 and 7.7 0
C compared to the alternative combinations. Following this, the effect of amplitude (A) and wavelength (λ) of the wavy fin surface for the optimal fin combination from the initial result is studied. Average temperature (Tavg
) monotonically reduces with A and increases with an increase in λ. Due to the implementation of a wavy fin surface, Tavg
and Tmax
were further reduced by 3.9 and 5.3 0
C, respectively. The outcomes of the present study offer valuable insights into improving the thermal performance of single-phase immersion cooling systems and mitigating the thermal shadowing effect.
Experimental investigation of the cooling performance of an additively manufactured prototype for nuclear fusion energy application
Authors: Favero G , Bonesso M , Dima R , Doretti L , Pepato A , Righetti G , Zilio C , Mancin S
Published: June 2024

Metal Additive Manufacturing (MAM) is a non-traditional technology recently introduced to manufacture multifunctional mechanical components. In fact, recent developments in Laser Powder Bed Fusion (LPBF) process have enabled the production of materials characterized by high density and high thermal conductivity properties, such as copper alloys and pure copper, making the technology attractive for thermal science. In nuclear fusion energy applications, mechanical components often encounter extremely high heat fluxes. An innovative solution using unconventional integrated cooling channels is therefore required to safely manage the components. However, the high surface roughness in 3D-printed parts represents an intrinsic limitation of the LPBF technology: the cooling channels show high-pressure drops due to the high viscous dissipation generated by the rough surface. To address this challenge, a lab-scale prototype of an e-beam extraction grid for a fusion experiment with an original integrated cooling system was designed and manufactured. Additionally, a novel heating mask was designed and manufactured to reproduce the realistic heat load distribution on the grid during the experimental tests. The prototype was built using additive manufacturing with a CuCrZr copper alloy. The grid underwent heat treatment via solution annealing and age hardening, to increase thermal conductivity from about 100 W m−1 K−1 to almost 300 W m−1 K−1. The prototype was tested at three different constant heat fluxes by varying the water flow rate while measuring the pressure and the maximum temperatures of the grid. A CFD numerical model was also calibrated to estimate the thermo-hydraulic performance of the prototype under test conditions. The experimental and numerical results are presented in terms of overall thermal performance, maximum temperature, and pressure drop.
Measurement of effective thermal conductivity of composite powders of 2D materials and metals for additive manufacturing
Authors: Lee Hyunjong , Koutsioukis Apostolos , Jafari Davoud , Geurts Bernard J. , Wits Wessel W.
Published: June 2024

Laser Powder Bed Fusion (LPBF), a promising additive manufacturing technique for composites of 2D materials and metals, requires knowledge of thermophysical properties, such as the thermal conductivity of powder, for process optimization. In this study, we measured the effective thermal conductivity of the most representative Cu-graphene composite powder in the field of heat transfer applications. To measure thermal conductivity, we propose a differential scanning calorimetry (DSC) method to measure the thermal resistances of a powder bed under steady-state heat flow conditions. We observe that the thermal conductivity of a composite powder is 1,000 times lower than that of the bulk metal without 2D material addition; e.g., exhibiting ~ 0.30 W/mK in Cu-1wt.% graphene powder. Furthermore, we discuss powder size and morphology impact on thermal conductivity influencing the number of contact points and thermal contact resistance. Our findings contribute to understanding the thermal conductivity of composite powders in a powder bed and aid in optimizing LPBF processes.
Determination of transient heat transfer by cooling channel in high-pressure die casting using inverse method
Authors: Bohacek J , Mraz K , Hvozda J , Lang F , Raudensky M , Vakhrushev A , Karimi-Sibaki E , Kharicha A
Published: June 2024

Complex shapes of aluminum castings are typically manufactured during the short cycle process known as the high-pressure die casting (HPDC). High productivity is ensured by introducing die cooling through a system of channels, die inserts or jet coolers. Die cooling can also effectively help in reducing internal porosity in cast components. Accurate simulations based on sophisticated numerical models require accurate input data such as material properties, initial and boundary conditions. Although the heat is dominantly dissipated through die cooling, indicating the importance of knowing precise thermal boundary conditions, open literature lacks a detailed information about the spatial distribution of heat transfer coefficient. This study presents an inverse method to determine accurate heat transfer coefficients of a die insert based on temperature measurements in multiple points by 0.5 mm K-type thermocouples and a subsequent solution of the two-dimensional inverse heat conduction problem. The solver was built in the open-source CFD code OpenFOAM and the free library for nonlinear optimization NLopt. The results are presented for the commonly used 10 mm die insert with a hemispherical tip and coolant flow rates ranging from 100 l/h to 200 l/h. Heat transfer coefficients reach values well above 50 kW/m2K in the hemispherical tip, which is followed by a secondary peak and then a gradual drop to values around 1 kW/m2K further downstream.
On the conditions for the occurrence of crystal avalanches during alloy solidification
Authors: Shayesteh G , Ludwig A , Stefan-Kharicha M , Wu M , Kharicha A
Published: June 2024

Experimental studies on the solidification of ammonium-chloride-water alloys in relatively large containments reveal conditions that lead to the formation of numerous crystal avalanches. Columnar segments that occasionally slide downwards along a vertical mushy zone further fragmentate and so crystal multiplication occurs. As a condition for this phenomenon solidification-induced solutal buoyancy that leads to a rising interdendritic flow was identified for the present case. The interaction with sedimentation-induced downward flow ahead of a vertical columnar region results in a redirection of the interdendritic flow and thus, to local conditions that slow down further solidification or even lead to remelting. Gravity is then pulling loose segments downwards. In larger containment, the flow in the bulk melt is generally unsteady and even turbulent. Thus, the outlined flow-solidification/melting interplay happens frequently at numerous positions but in a stochastic manner.
Thermal control circuit modelling with TCCbuilder
Authors: Vozel Katja , Klinar Katja , Petelin Nada , Kitanovski Andrej
Published: June 2024

Thermal control elements (TCEs) and thermal control circuits (TCCs) are a promising addition to conventional methods of thermal management, as they enable non-linear, switchable, and active control of heat. However, TCEs and TCCs are still at an early stage of development and are not yet ready for use in commercial applications. The combinations of TCEs in TCCs and the transient operation of TCEs and TCCs have not yet been sufficiently investigated. We have developed an open-source TCC simulation tool with a user-friendly graphical user interface (GUI), TCCbuilder. TCCbuilder enables the design of TCEs and TCCs and offers the possibility to analyse their behaviour in (quasi-)steady state and transient operation. The tool has an extensive library of materials and TCEs (thermal switches, thermal diodes, etc.) compiled from extensive literature research and contributions from fellow researchers. TCCbuilder’s library of materials provides input data (e.g. temperature- and/or external field-dependent thermal properties) for simulations. However, users can also enter their own data. This article demonstrates the simple design and simulation of a TCC with a caloric device using TCCbuilder.
Oscillating thermal switch using electrostatic forces
Authors: Petelin N , Pečar B , Vrtačnik D , Perne J , Tomc U , Kitanovski A
Published: June 2024

Thermal control devices such as thermal switches, thermal diodes, thermal regulators and thermal capacitors can control the intensity and direction of heat flow, which has proven useful in various thermal management applications. Their operation is analogous to that of their electrical counterparts, where temperature differences correspond to voltage differences and heat flow corresponds to electric current. Here we design and fabricate a thermal switch that controls the heat flow from the heat source by changing the thermal conductance when switching ON and OFF. The thermal switch uses electrostatic forces for actuation and makes thermal contact between the heat source and the heat sink when in the ON state, and breaks contact when in the OFF state. The thermal switch is constructed using the commercially available materials. The heat sink and heat source are made of silicon, and the active part of the heat switch is a flexible gadolinium sheet with a thickness of 161 µm. The proof-of-concept device was evaluated for a magnetocaloric application; however, the proposed thermal switch can be used in any thermal circuit for fast thermal regulation and advanced thermal control.
Thermal control of thin films with nano structure
Authors: Sato M , Takahara Y , Matsumoto M , Kajinami N , Hanaoka M , Iwakawa M
Published: June 2024

Thermoelectric energy conversions have been attracting much attention, which directly generate electric energy from thermal one by utilizing the Seebeck effect. Among various efforts to improve the conversion efficiency, control of phonon propagation with nano-scale structures has been popular, which utilize phonon scatterings on structural interfaces. The concept is based on the difference of mean free path (MFP) between phonons and electrons (charge carriers). In typical cases with silicon-base devices, MFP of phonons is in an order of 100 nm while that of electrons is 1-10 nm. Thus structures of 10-100 nm size are expected to be effective for suppressing the phonon heat transfer without much reducing the electric transport, leading to conversion efficiency improvement. We have developed a numerical scheme to investigate phonon transport based on the Boltzmann transport equation (BTE) for time development of the phonon distribution function. To treat various shapes of nano-scale structures, we have newly introduced a VOF (volume of fluid) like scheme. In the presentation, we will show results of several test systems of nano-structured Si thin films, with evaluation of effective electric conductivity, to discuss how much nano-scale structures improve the conversion efficiency.
Thermal control devices and thermal circuits
Authors: Klinar Katja , Kitanovski Andrej
Published: June 2024

It is becoming evident that conventional thermal management methods like conventional thermal insulation and conventional thermal storage cannot meet the thermal control requirements of advanced, especially small systems with higher power densities or potentially transient, fluctuating, or migrating hot or cold spots, and for temperature-sensitive devices. This challenge is most evident in electronic components that experience degradation and loss of efficiency without constant and effective heat dissipation. To overcome these limitations, thermal control devices have emerged in various areas of thermal management. These small-scale devices provide non-linear, switchable, and active control of heat, similar to the way their electrical counterparts regulate electric current. Among others, notable thermal control devices include thermal conduits (which act as solid-state heat routers), thermal resistors (which provide thermal insulation), thermal switches (which actively control heat transfer through on-off states), and thermal diodes (which rectify heat currents). In this paper, we provide state of the art on the research activities and applications of thermal control devices.
Advancing thermochemical storage: synthesis and characterization of cement-based composite materials
Authors: Mondello A , Morciano M , Lavagna L , Pavese M , Chiavazzo E
Published: June 2024

Thermal Energy Storage (TES) is crucial for sustainability of the energy sector, yet the development of cost-effective, robust materials remains a significant challenge. This study aims at exploring the synthesis and thermal characterization of cement-based composites for seasonal thermochemical energy storage, with the goal to harness the high energy density of hygroscopic salts while mitigating their limitations. We investigate composites with several cement matrices to improve salt-cement compatibility. Furthermore, we investigate the possible incorporation of porous low-cost compounds to enhance porosity and improve economic aspects. As far as the characterization aspects are concerned, we show experimental adsorption isotherms at different temperatures to estimate key material properties like isosteric heat and water uptake, along with the relevant figures of merit such as energy density. Our research leverages on adjustable porosity and affordability of cement as a host matrix for the ‘active phase’. We studied two synthesis approaches: traditional dry impregnation and an in-situ technique suitable for cements. The in-situ method, being straightforward and reproducible, permits greater control over salt content. Preliminary cost analysis positions these composites competitively in the market. Although we are still at sub-optimal stage, potential cost reduction of some less popular cement matrices suggests an opportunity for improvement.
Optimizing MOF properties for seasonal heat storage: a machine learning approach
Authors: Trezza G , Bergamasco L , Fasano M , Chiavazzo E
Published: June 2024

In the quest to enhance thermochemical energy storage using promising sorbents, this work presents a study on the optimization of Metal Organic Frameworks (MOFs) properties for gas sorption, with a focus on CO2 and H2O adsorption. Through the analysis of crystallographic descriptors, the study aims to streamline the selection of MOFs that could potentially exceed the performance of existing water sorbent pairs. A comprehensive comparison of sequential learning (SL) algorithms reveals a method for identifying the minimal set of descriptors that influence adsorption properties of MOFs. The protocol involves constructing and training machine learning (ML) models to determine the number of influential descriptors and utilizing SHAP analysis to evaluate their importance. Findings suggest that including only these critical descriptors in the exploration space reduces computational load. Notably, the COMBO and the FUELS algorithms consistently outshine random guessing, validating their efficacy in materials optimization. The challenge of accessing full adsorption properties across the entire coverage range is addressed by a computational screening procedure requiring minimal input data. This method suggests that some vanadium based MOFs, originally designed for different purposes, could surpass the current leading compounds for thermal energy storage, primarily due to their optimal Henry coefficient values for water adsorption.