bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiphase flow simulation in hybrid porous structure
Authors: Nimvari M. E. , Gibbons M. J.
Published: June 2024

Recent research has primarily focused on creating biporous and hybrid porous structures with multiple pore sizes and length scales to optimize capillary pressure and permeability. Despite numerous experimental investigations on biporous and hybrid media, there is a noticeable absence of numerical simulations that explore the multiphase flow within these media. Therefore, the present study aims to conduct a pore-scale numerical simulation of two-phase flow in a biporous structure. The biporous structure is proposed by arranging clusters of solid particles in a staggered regular pattern, with each cluster consisting of closely packed particles. The dimensions and characteristics of the simulated structure are based on previous experimental literature on biporous and hybrid media. A monoporous structure simulation is also included for comparison with biporous results. ANSYS Fluent is utilized to carry out the numerical simulations of capillary pumping flow. The simulation results indicate that the permeability and average capillary pressure of the biporous media are four times and over one and a half times higher, respectively, compared to those of the monoporous media. The presence of smaller pathways within each cluster of a biporous and hybrid porous media enhances the capillary effect in comparison to conventional monoporous structures. Additionally, the larger pores between the clusters contribute to a higher permeability of the hybrid porous structure. As a result, the combined effect of increased capillary action and higher permeability leads to improved performance of the hybrid porous structure. Overall, the proposed simplified biporous geometry accurately models fluid flow in real, complex biporous and hybrid structures.
Study of turbulent wavy annular flow inside a 3.4 mm diameter vertical channel by using the Volume of Fluid method in OpenFOAM
Authors: Zanetti E , Berto A , Bortolin S , Magnini M , Del Col D
Published: June 2024

In annular downward flow, an annular liquid film flows at the perimeter of the channel pushed down by the gravity force and by the shear stress that the vapor core exerts on it. Depending on the working conditions, the vapor-liquid interface can be flat or rippled by waves. The knowledge of the liquid film thickness is very important for the study of annular flow condensation because the thermal resistance of the liquid is often the most important parameter controlling the heat transfer. A new approach for the simulation of annular flow is here proposed using an in-house developed transient solver based on the Volume of Fluid (VOF) adiabatic solver interIsoFoam available in OpenFOAM. With the VOF method, in addition to the standard set of equations (continuity and momentum), a transport equation related to the advection of the volume fraction scalar field has to be solved. The numerical setup consists of 2D axisymmetric domain. An adaptive mesh refinement (AMR) method is added to the solver to better capture the interface position. The k-ω SST model is used for turbulence modelling in both the liquid and vapor phases and a source term (whose magnitude is controlled by a model parameter named B) is included in the ω equation to damp the turbulence at the interface.
Acceleration of flow-focused liquid jets in the presence of a strong electric field
Authors: Zupan B , Bajt S , Chapman Henry N , Šarler B
Published: June 2024

This paper investigates the acceleration of flow-focused micro-jets under an applied external electric field. Such thin and fast jets are needed, for example, to deliver protein crystals in X-ray free electron laser experiments. This contribution focuses on analysing the acceleration of liquid jets produced from 50 % vol water and ethanol mixture. Such jets are focused with nitrogen gas and are subjected to 0 - 7 kV electric potential between a submerged electrode and an external electrode downstream of the nozzle. To gain insights into jet geometric properties an automatic image recognition and computer vision algorithm is utilised on already published data. Jet velocities and acceleration calculated for two jetting regimes show that application of external electric field accelerates the jets up to four orders of magnitude larger than the conventional micro-jets.
A numerical and experimental study of breakdown of falling film on horizontal circular tubes
Authors: Singh Km Jyoti , Gedupudi Sateesh
Published: June 2024

Falling film horizontal tube heat exchangers are used in several industrial applications such as sea water desalination, multi effect distillation, chemical industries, ocean thermal energy conversion systems and refrigeration. Different flow patterns that occur during falling film evaporation on an array of horizontal tubes are droplet, droplet-columnar, columnar, columnar-sheet and sheet modes. From the heat transfer point of view, a sheet mode (continuous falling film spanning the intertube space) is the most desirable falling-film mode. Falling film breakdown leads to the occurrence of dry patches on tubes which cause heat transfer deterioration. In the present study, falling film breakdown on horizontal tubes under adiabatic condition has been investigated both numerically and experimentally. The study focuses on the influence of tube pitch to diameter ratio on the threshold film Reynolds number required to maintain continuous falling film. Results indicate that the threshold film Reynolds number increases with the increase in the pitch to diameter ratio for a given diameter of the tube. The study also shows that as the number of tubes in the column increases, the threshold film Reynolds number required to maintain continuous film over all the tubes increases. The study highlights the need for correlations for the column-sheet mode transition that take into account the effect of number of horizontal tubes in a vertical array.
Numerical contact line behavior prediction for drop-wall impact using Basilisk
Authors: Piskunov M , Piskunova A , Vozhakov I , Misyura S
Published: June 2024

Drop-wall impact and spray cooling have a wide technical application. There are still fundamental problems associated with wettability and its effect on the drop spreading. Experiments and direct numerical simulations are performed in a wide range of initial drop velocities (0.2–3.6 m/s). The 3D simulation is implemented by solving the incompressible Navier-Stokes equations along with the volume-of-fluid method in the Basilisk software. An adaptive mesh refinement near the interfacial surface provides a cell size of 5 µ m. The effect of the contact angle calculated using the Hoffman function on the minimum drop height and its maximum spreading is explored. The contact angle affects the dynamics of the contact line. Moreover, the mesh cell size and the initial velocity of a water drop before impact define the development of the drop rim instability when compared with experimental results. Experimental and numerical findings are compared to estimate quantitatively numerical model capabilities.
Estimating temperature fields in flows from sensors
Authors: Speetjens MFM , Lensvelt R , Nijmeijer H
Published: June 2024

Active control of scalar transport (e.g. temperature or chemical species) in engineering flows by feedback of intermediate scalar fields to the controller to determine the control action can substantially improve the performance of such processes. Key for practical application of such state-feedback control is the fast and accurate estimation of scalar fields from discrete sensor data (e.g. thermocouples in case of heat transfer), since the full field can usually not be measured. This study develops a state observer that enables such sensor-based state estimation for a representative problem: the temperature evolution during heat transfer in the 2D Rotated Arc Mixer (RAM). State observers with this capability are to the best of our knowledge non-existent in the literature. The performance of the observer is investigated by its ability to reconstruct known temperature fields from the corresponding values at the sensor locations using fields obtained from (i) numerical simulations and (ii) infrared thermography in a RAM laboratory set-up. This shows a good performance (for typical engineering applications) that under realistic conditions most likely depends mainly on the modelling accuracy for the physical system.
Effect of Acoustics on Droplet Grouping Behaviour in a Single Stream of Droplets
Authors: Kumar M. , Vaikuntanathan V. , Ibach M. , Arad A. , Bar-On R. , Weigand B. , Katoshevski D. , Greenberg J. B.
Published: June 2024

Droplet and particle grouping can be influenced by applying an acoustic field and have practical applications such as particle scavenging and aerosol filters of engine exhaust and air purifiers. The present work experimentally investigates the influence of a standing acoustic wave on a single stream of droplets. The experimental setup consists of an acoustic transducer and a reflector plate through which the droplet stream passes in the presence or absence of an external pressure field generated by a standing acoustic wave. A droplet stream is generated with the help of a nozzle connected to a pressurized working fluid supply and piezoelectric transducer to control the spacing between droplets. The effect of the acoustic pressure field on the droplet stream generated by the nozzle operated at different piezoelectric excitation frequencies and fluid pressures is investigated. Droplet stream characteristics at every nozzle excitation frequency are observed with a high-speed camera when the acoustic field is switched OFF and ON. The competing effect of nozzle excitation frequency and acoustic field is observed. At lower nozzle frequencies, the nozzle generates an unstable stream of droplets having different sizes and spacings between them. When the acoustic field is applied at these lower frequencies, the stream of droplets becomes organized, and in some cases, it becomes equispaced and of the same size. However, an opposite behavior is observed at higher frequencies. In these cases, as the acoustic field is applied, an equispaced mono-disperse droplet stream becomes unstable due to the coalescence of droplets within the stream.
RANS representation of transition and separation over a low-Re number blade section at high angle of attack
Authors: Pagliarini L , Corsini R , Stalio E , Bozzoli F
Published: June 2024

Systems based on wind energy harvesting can successfully meet part of the increasing green energy demand worldwide. However, wind turbines operation might be undermined by varying atmospheric conditions, which could result in an increase of angle of attack and consequent onset of flow separation phenomena, especially at low Reynolds numbers. Such conditions are strongly influenced by blades geometry, and they negatively affect structural integrity and power output of wind turbines. For this reason, it is crucial to define a tool capable of swiftly allowing numerical investigations on different geometrical configurations to delay and mitigate flow separation occurrence. The present work aims at modelling laminar-turbulent transition and turbulent flow separation over a wind turbine blade section operating at angle of attack = 15°, Re = 66000 and Pr = 0.71 by means of a steady RANS approach. Turbulence is treated by means of the Transition SST k-ω and the Transition k-kL-ω models. The main aerodynamic and thermal coefficients are evaluated and compared against a high-order accurate DNS database for validation. The results highlight, for the present test case, a better capability of the Transition SST k-ω of perceiving the main thermo-fluid dynamic features of the separated flow over the blade section.
Numerical investigation of methane-air jet flame with hydrogen addition in industrial kiln burners
Authors: Liu Jiannan , Rigola Joaquim , Schillaci Eugenio , Ruano Jesus , Perez-Segarra Carlos D.
Published: June 2024

Emission reduction and decarbonisation in the industry are crucial for low-carbon industry and energy transition at the global level. Replacing traditional fossil fuel with clean energy is an effective approach to reduce carbon emissions and optimize energy efficiency in manufacturing processes. Industrial kiln, which requires high natural gas fuel consumption, typically releases amounts of harmful combustion products. In this paper, the objective is to study the influence of hydrogen addition into methane-air jet flame in industrial kiln burners. The industrial burner analysed in this study is a cylinder vessel with axial orifices and swirl turbulent co-flow air jets in the fuel-air inlet structure. A more recent reduced chemical kinetic mechanism for methane-hydrogen combustion is utilized in the present flame simulation and validated in benchmark flames. The chemical mechanism involves 45 reactions and 18 species. Various methane-hydrogen blending fuels are studied in the jet flame, where flame structure and flame characteristics including chemical species, temperature, and velocity are predicted.
Building a reduced order model from CFD data on leaves to evaluate optimal climate conditions
Authors: Plas Wito , Demeester Toon , De Paepe Michel
Published: June 2024

Climate control in vertical farms or greenhouses is crucial in order to have the optimal climate for the plants. This climate can be simulated using Computational Fluid Dynamics (CFD). This way, different ventilation methods can for example be compared. Previously, plants were modeled as a porous zone, in which additional source terms are added to the energy or transpiration transport equation. In this study, ten different leaves under different orientations are studied. The heat and mass balance for each leaf is solved and the boundary layer is adequately resolved. Different leaf orientations and positions to the flow are used, so that each leaf has its own distinct boundary layer. Leaf temperature and humidity are a function of the incoming flow, but also of the incoming temperature, humidity and stomatal resistance. In order to be able to quickly assess the effects of changing boundary conditions, on the temperature and humidity of the leaves, a Reduced Order Model (ROM) is made from the CFD-data. This method allows to quickly assess the effects of a stomatal resistance, the incoming radiation or a changing inlet temperature or humidity. A ROM was built using the data from one simulation. In order to see whether the ROM is able to predict other configurations, two additional cases were solved in CFD and were compared to the ROM. Leaf temperatures and humidities only differed maximally 3.14% and were quite good at predicting this. Predicted leaf transpiration rates differed slightly more, as these differed maximally 10.5%. Overall, the ROM has proven to be a valuable and numerically cost-effective tool to predict overall leaf climate and can be a good tool when later the climate of a full plant is simulated.
State of the art of heat pumps for heating above 150 °C
Authors: Cimermančič Matevž , Klinar Katja , Kitanovski Andrej
Published: June 2024

In current aim to catch long-term energy and climate policies, the role of the heating sector cannot be overstated. Heating, accounting for 50% of final global energy consumption, stands as a huge target for transformation. In this context, heat pumps are emerging as a potential decarbonization tool that promises to significantly reduce emissions through the use of renewable or waste energy, as they can utilize waste heat to elevate temperature to higher levels more efficiently compared to any other existing technology. Despite their potential, heat pumps are currently only used to a limited extent for industrial high-temperature heating, which makes it necessary to investigate viable options for improving their effectiveness in this area. This paper offers an insightful exploration into the current state of the art in high-temperature heat pumps (HTHPs) with heat sink temperatures above 150 °C. Our primary focus centers on the examination of vapour-compression systems, examining factors such as heating capacity, efficiency, and environmental considerations. Additionally, we provide valuable recommendations and guidelines for the future exploitation of industrial HTHPs.
Experimental characterization and numerical modelling of an air to water CO2 heat pump under different outdoor and operating conditions for domestic hot water generation
Authors: Velasco FJS , Illán-Gómez F , García-Cascales JR , Biyogo-Obono RE , Otón-Martínez RA
Published: June 2024

This work assesses the influence of different outdoor and operating conditions on the performance of an air-to-water CO2 heat pump for domestic hot water (DHW) generation. For that purpose, an air to water CO2 heat pump with internal heat exchanger (IHX) was developed and characterized in an experimental campaign under well-controlled lab conditions. The system includes an in-house control system implemented to operate at the optimal gas cooler pressure. The heat pump and the control system are tested for DHW generation. The heat pump uses a finned-tubes heat exchanger as evaporator and plate heat exchangers as gas cooler and internal heat exchanger. The numerical model used a 1dimensional, cell by cell discretization method to characterize the performance of both, the gas-cooler and the evaporator under a wide range of values of their key variables. The IHX is also modelled with the same strategy. ARHI equations are considered to describe the compressor performance. The experimental data obtained are used to validate a numerical model of the heat pump under different air velocities, air temperatures and air humidity at the evaporator. The effectiveness of the control system proposed is experimentally confirmed during the experimental tests. Results show a good agreement between the numerical simulations and the experimental data what permits to validate the model and to assess the performance of the transcritical heat pump under different climate conditions.
Seasonal energy performance assessment of a hybrid HVAC system driven by solar and biomass energy for space heating and cooling in residential buildings
Authors: Dongellini M , Naldi C , Moser C , Varga S , Soares J , Morini GL
Published: June 2024

The H2020 project Hybrid-BioVGE aims to develop a sustainable HVAC system concept for buildings’ climatisation entirely based on renewable energy sources. The Hybrid-BioVGE system provides space heating/cooling and DHW production of small- and medium-scale buildings and deploys solar and biomass heat as energy inputs. During the heating period, thermal energy is provided directly by solar thermal collectors and a biomass boiler. The cooling energy demand is met by a thermally-driven ejector cycle based on a Variable Geometry Ejector (VGE), which adjusts its geometry according to two degrees of freedom. A demonstrator of the Hybrid-BioVGE system was designed and installed in a residential building in Porto (Portugal). The demonstrator’s seasonal energy performance was assessed numerically by a simulation model implemented in TRNSYS. Simulation results show that up to 98% of the system energy input is provided by renewables during the heating season, with a solar fraction higher than 60%. Conversely, the system energy performance is lower than expected during the cooling season due primarily to the limited performance of the heat rejection loop.
Physically based heat exchanger sizing method for the thermal management system of all-electric regional aircraft
Authors: Nozinski Marius , Benam Behnam Parizad , Servi Carlo De , Kabelac Stephan , Falsetti Chiara
Published: June 2024

Fully electric propulsion systems integrating hydrogen-powered fuel cells and batteries are promising options to reduce the overall climate impact of regional aircraft. However, the increase in low-temperature heat sources aboard the aircraft calls for advanced thermal management system solutions. To address this challenge, this study presents a sizing methodology for ram air heat exchangers in the nacelle-integrated cooling loop of an all-electric regional aircraft based on the ATR-72 platform. Different discretization schemes are compared to identify an optimal sizing method. The results highlight the simplicity and efficiency of the 0D ε-NTU model. Geometric design variables are optimized with respect to drag and mass during a hot-day take-off. The resulting Pareto front reveals a tendency for low airflow outlet temperatures and large diffuser area ratios to result in lightweight designs but in turn, induce high drag and require a large installation space. Comparative analyses of specific optimal ram air duct designs and equivalent skin heat exchangers demonstrate the potential of a second heat sink over a flight mission. The limited heat transfer area of the skin heat exchanger proves insufficient for hot-day take-off and climb but offers advantages during cruise and descent thanks to the reduced drag.
Assessment of internal and external disturbances on the fuzzy-based thermal control of a sub-scaled building testbed
Authors: Saguilan Anayely , Pacheco-Vega Arturo
Published: June 2024

This experimental study follows on our previous work on the development of robust fuzzy-based thermal control strategies of a multi-room sub-scaled building testbed. In the present analysis, the focus is placed on testing the robustness of the fuzzy controller under internal and external disturbances, as it deals with maintaining specific setpoint values of room temperatures. The testbed has eight rooms, distributed on two floors, with a cooling unit that supplies cool air to each room, and eight 40 W light bulbs serving as heat sources. T-type thermocouples gather the temperature data, and eight dampers deliver the airflow. The controller uses information about the difference between setpoint and actual temperatures, their derivative, and their cumulative integral. The fuzzy sets and if-then rules are built based on experimental data, and a Mamdani inference method is used to provide the inputs to the actuators. Results from experimental tests show that the fuzzy control strategy can handle the different types of disturbances while maintaining the room setpoints.
Investigation of the operating characteristic of a demand-controlled 368 m deep CO2 thermosyphon geothermal borehole heat exchanger for building heating
Authors: Hagedorn Janina , Kahlfeld Robin , Nageler Malte , Kabelac Stephan
Published: June 2024

This paper examines a 368 m deep geothermal probe for heating an existing building. The geothermal probe based on the thermosyphon principle works with CO2. The working fluid evaporates and condenses in the probe so that no pump is required for transportation. The geothermal probe has an output of up to 22 kW at an underground temperature of up to 18°C. The demand-controlled use of the geothermal probe was investigated and the installed heat pump was optimized for its use. The average COP of the last heating season was between 2,3 and 4,6. For the second heating season the temperature of the soil around the probe is also measured via a distributed temperature sensing system, that covers the whole depth of the probe. It has been shown that the ground temperature only cools down to a certain point and does not fall below this point after a short regeneration period. It was shown that the heat pump with geothermal probe operates at constant temperatures at the evaporator compared to air heat pumps, even at cold outside temperatures.
Effect of water injector location on combustion and performance of an HCCI engine - A CFD analysis
Authors: Naik Bharat , Mallikarjuna J M
Published: June 2024

A homogeneous charge compression ignition (HCCI) engine is capable of producing high thermal efficiency with very low emissions. However, because of the limited upper load level constrained by knocking, the HCCI engine is not yet commercialized. Water injection can tackle this issue, enhancing the upper load limit without excessive heat release rate. However, the effectiveness of water injection depends on in-cylinder water evaporation, which in turn depends on water injection parameters. Therefore, this study is conducted to study the effect of water injector location on the combustion and performance characteristics of an HCCI engine by a computational fluid dynamic (CFD) analysis. The engine specifications and boundary conditions are taken from the literature. A three-dimensional computational fluid dynamics (3D CFD) model was developed using CONVERGE CFD coupled with detailed chemical kinetics to gain a better understanding of the phenomena of water injection in an HCCI engine. The CFD models used are validated from the available experimental data in the literature for the engine considered. Here, the water injector location is optimized based on the Water vapor distribution within the cylinder, nitric oxide (NOx) emissions, and maximum rate of pressure rise (MRPR). The results show that the water injector at 37 mm from the center of the cylinder gives better results than in other cases considered. With the optimum water injector location, the indicated mean effective pressure is increased by about 35.91% than without the water injection case. This study will be helpful in the development of HCCI engines.
3D CFD simulation of room air temperature and velocity with ceiling fan and exhaust fan and the resulting thermal comfort
Authors: Thomas Trino , Gedupudi Sateesh
Published: June 2024

The integration of energy-efficient solutions remains an essential hurdle in the pursuit of sustainable and comfortable indoor settings. As building energy consumption continues to climb, there is an urgent need to implement methods that reduce environmental effects, and at the same time, maintain adequate thermal comfort for occupant well-being and productivity. Ceiling fans, a standard fixture in many indoor settings, have gained popularity for their ability to provide better air movement and temperature adjustment while using substantially less energy than standard HVAC systems. This work presents a 3D Computational Fluid Dynamics (CFD) investigation of thermal comfort that can be achieved with a ceiling fan and an exhaust fan. A 3D fan zone model available in ANSYS Fluent was used to model the ceiling fan, considering the ceiling fan as a solid disc. Numerical methodology has been validated by verifying the transient simulation results against the experimental data available in the literature. The work analyzes thermal comfort parameters - temperature and velocity - in a room with a typical ceiling fan or an exhaust fan available in India. The room temperature and velocity distribution are presented for different times of a day in summer for a warm and humid climatic zone in India. The results show that the thermal comfort obtained with the exhaust fan is better than that with the ceiling fan for the evening, night, and early morning.
On the validation of numerical simulation with experimental results on compressible turbulent flow in an inertial particle separator device
Authors: Bahramian Linda , Amani Ahmad , Rigola Joaquim , Oliet Carles , David Pérez-Segarra C
Published: June 2024

To remove particles from entering the engine or Environmental Control System (ECS), an Inertial Particle Separator (IPS) can be situated in the intake. In IPS, a rapid change in flow direction is induced, and particles separate from the core outlet because of their inertia. Small particles with a low Stokes number (St) follow the flow and often enter the core outlet, while the large particles with a high St are thrown into the scavenge by their high inertia. Accurately capturing the unsteadiness of the fluid is crucial, especially with low scavenge mass flow fraction for the particles with low St. The numerical simulation of the compressible flow is validated here, implementing the 3D Improved Delayed Detached Eddy Simulation (IDDES) and the Reynolds-averaged Navier–Stokes (RANS) methods against experimental data. It is shown that the IDDES method can capture correctly the unsteadiness of the flow. After validation of the fluid phase, the particle separation efficiency was studied for different particle diameters using these turbulence numerical methods along with three different ambient temperatures: standard, hot, and cold. The separation efficiency values were in agreement with the experimental data.
Magnetocaloric refrigerator with magnetic pump and liquid metals
Authors: Rajamani Keerthivasan , Stolwijk Bob , Shahi Mina
Published: June 2024

One-seventh of the worldwide electricity is used for refrigeration related activities. When compared to the predominantly used vapor compression refrigeration system, magnetocaloric refrigeration provides an energy-efficient and eco-friendly alternative. It utilizes a magnetocaloric material which undergoes temperature changes when exposed to magnetic field changes. Nearly 60% of the total system cost is due to the permanent magnets used. For a given cooling power, the amount of permanent magnet needed is determined by the residence time required by the heat transfer fluid in each cycle. The start-of-the-art systems utilizes water-alcohol mixture as the heat transfer fluid, and a mechanical pump for its circulation. By utilizing non-toxic and non-hazardous gallium-indium-tin based liquid metals as heat transfer fluid, which has three order of magnitude higher thermal diffusivity, the residence time and the permanent magnetic material is reduced nearly by a factor of 10. Even accounting for the cost of the liquid metal, it results in considerable savings in the system cost. We present experimental results on the entropy change of using liquid metal with (Mn,Fe)2(P,Si)-based magnetocaloric material, and numerical results on the system level analysis. Further, to improve the reliability of the system, we propose to use magnetic pumping for heat transfer fluid circulation, which does not have any moving parts. The state-of-the art magnetic pump works at frequencies in the range of 100 to 1000 Hz. However, they cause detrimental heating effects due to eddy current losses. To overcome this, we have designed a pump that operates at 1 Hz and still achieves comparable flow rates. We present numerical results on the pump design by studying the dynamics of ferrofluid rise in a vertical pipe with multiple electromagnetic coils.
Characterization of different darken sand particles and behavior under fluidized and irradiated conditions
Authors: Cerutti-Cristaldo LM , Díaz-Heras M , Pérez-Flores JC , Canales-Vázquez J , Almendros-Ibáñez JA
Published: June 2024

The use of solid particles in a fluidized bed with concentrated solar irradiation from the top is a promising technology for the next generation of concentrated solar power (CSP) plants. Sand is an inexpensive and abundant material easy to fluidize, but it has a low absorptivity, which is around 0.5 according to different previous works. This optical property is a key parameter for CSP applications with solid particles. This work presents a novel methodology to induce darkening of the sand surface by inducing solid state diffusion of Mn in SiO2, rendering a stable material resistant to abrasion upon the fluidization process. For this study, two different samples considering different MnCO3:SiO2 weight ratios (1:30 and 1:50) were analyzed. The objective is to compare the two samples and optimize the concentration of MnCO3 to get the desired darkening of the sand and high absorptivity. First, the main properties of the particles were analyzed (particle size, morphology, color and absorptivity). Second, the samples were tested in a lab-scale fluidized bed directly irradiated from the top with a beam-down 4kW Xe lamp. Both samples were tested under three different fluidized velocities: 1.5, 2.0, and 2.5 times the minimum fluidization velocity (Umf). In both cases, there is a significant increase in the maximum temperature reached during the process, with temperatures exceeding 260 °C. This is clearly higher than the case of raw sand, which reaches 230 °C under the same conditions. Furthermore, these values exceed the highest temperature reached by SiC in the same facility, which is 250 °C, and it is considered as one of best tested materials for CSP applications.
Study on the difference in subcooling flow boiling of materials in a vertical rectangular channel at a low flow rate
Authors: He Sihong , Zhao Jiyun
Published: June 2024

Alloy materials with excellent properties are widely used in different industrial fields. Conducting flow boiling studies on alloy surfaces will help explore their applications in flow boiling, especially in heat exchange equipment. Five alloys were selected, including 304SS, Inconel600, Monel400, Zr4 and FeCrAl. Bubble dynamics in a vertically upward-flowing rectangular channel were observed using a high-speed camera under the condition of atmospheric pressure, 400 kg·m^-2·s^-1 mass flowrate and 5 K inlet subcooling. Results show that roughness and wettability cannot individually define material-sensitive heat transfer coefficient (HTC) and critical heat flux (CHF). Thermal effusivity shows a linear relationship to HTC. Material-sensitive CHF seems to be a power-law function of the nondimensional parameter of the product of the maximum nucleation site density and the minimum bubble nucleation area.
Analysis of microscale inner-grooved tubes during condensation of low-GWP R1234ze(E): Flow regime visualization and enhancement factors
Authors: Irannezhad N , Rossetto L , Diani A
Published: June 2024

In an attempt to attenuate the global warming contribution in the refrigeration industry, several regulations have been legislated. HydroFluoroOlefins (HFOs) could be construed as feasible candidates from which R1234ze(E) with GWP approximately equal to unity for various applications has been subjected to experimental investigations. Owing to its prevalence in many industrial applications, flow condensation and boiling inside tubes have received particular attention in such experimental campaigns. The current experimental investigation aims at determining the degree of augmentation of the thermal performance with the usage of inner grooved tubes. In this case, tubes with various geometrical properties are employed for flow condensation of R1234ze(E) at a saturation temperature of 30°C. Measuring the experimental heat transfer coefficient (HTC) allows obtaining the enhancement factor of the enhanced tube which is defined as HTC of an inner grooved tube to that of a conventional smooth tube of the same diameter. Furthermore, since the efficacy of enhanced surfaces is directly linked to prevailing flow patterns, for different operating conditions (G = 100 to 400 m−2 s−1, x = 0.01 to 0.95) the flow pattern has been identified with the aid of a high-speed camera. Lastly, results of flow pattern visualizations and enhancement factors are provided on a non-dimensional graph of EF (enhancement factor) versus JG
(non-dimensional gas velocity) that allows a comprehensive performance analysis of enhanced surfaces.
Effect of an artificial cavity on the microlayer and contact line dynamics during bubble growth in nucleate boiling
Authors: Tecchio C , Regoli I , Cariteau B , Zalczer G , Cabarrocas P Roca i , Bulkin P , Charliac J , Vassant S , Nikolayev V S
Published: June 2024

We present an experimental study on the near-wall phenomena during the growth of a single bubble in saturated pool boiling of water at atmospheric pressure. Our focus is on the dynamics of triple contact line and liquid microlayer that can form between the heater and the liquid-vapor interface of the bubble. The microlayer thickness, the wall temperature distribution and the bubble shape are measured simultaneously and synchronously at 4000 fps by white light interferometry, infrared thermography and sidewise shadowgraphy, respectively. To study the effect of cavities (artificial nucleation sites) we compare two experiments using different heaters. In the first experiment, the bubble grows on a smooth surface of nanometric roughness whereas in the second, the bubble grows over a cylindrical cavity of 25 µm diameter and 50 µm depth. We found that the cavity reduces three times the required wall superheating to trigger the bubble growth. Moreover, the radii of the bubble, microlayer and dry spot are smaller by half and the macroscale bubble dynamics is also slower. The microlayer is thinner and is measurable in a larger portion of its extent. Based on the absence of interference fringes near the contact line (due to high interface slopes) and on recent numerical simulations, we conclude that the microlayer consists in two regions: a dewetting ridge near the contact line that grows over time and a flatter and wider region that thins over time. The microlayer can be seen as a film deposited by the receding meniscus and its profile is controlled by the viscous and surface tension effects; its thinning over time is due to local evaporation only. The ridge is a result of liquid accumulation due to contact line receding and strong viscous shear in the film.
Flow boiling in microgravity
Authors: Mbaye Modou , Qaisar Raza Md. , Sebilleau Julien , Colin Catherine
Published: June 2024

This study reports some results obtained on nucleate flow boiling on an isolated nucleation site in microgravity. The experiments have been carried out aboard the International Space Station thanks to the Reference Multiscale Boiling Investigation (RUBI) device. Results reveal the pronounced influence of heat flux, subcooling temperature, flow rates on the bubble behavior, and more specifically on its growth rate. Deviations from the classical square-root time law of diffusive growth are observed at high subcoolings and high flow rates.
Superbiphilic hierarchical aluminum surfaces for exceptional pool boiling performance
Authors: Hadžić Armin , Može Matic , Zupančič Matevž , Golobič Iztok
Published: June 2024

The advancement in high-power electronic devices coupled with the need to ensure reliable and efficient heat dissipation of two-phase cooling systems underscores the urgent necessity for breakthroughs in enhancing boiling performance and especially critical heat flux (CHF) to minimize the risk of system failure. In this field, surfaces with tailored wettability have already demonstrated their potential to enhance boiling heat transfer intensity, while surfaces featuring wickable structures like micropillar arrays have shown significant improvements in CHF. In this study, we investigate the use of aluminium micropillar surfaces with tailored wettability to simultaneously enhance nucleate boiling heat transfer performance and specifically increase of the CHF. We fabricated the micropillar surfaces using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while the wettability of selected surfaces was further tailored by application of a fluoroalkyl phosphonic acid and an additional laser texturing step. Three micropillar patterns were tested under pool boiling conditions using saturated twice-distilled water at atmospheric pressure. Importantly, our results revealed that the bottom part of the boiling interface (i.e., the superhydrophilic area) ensured increased liquid supply, while the top parts (i.e., the superhydrophobic area) tend to serve as nucleation sites. When combined, these two effects allowed us to simultaneously improve the CHF and the heat transfer coefficient, resulting in enhancements of up to 113% (2343 kW m−2) and 450% (205 kW m−2 K−1), respectively, compared to the benchmark untreated surface. This research provides a practical and reliable approach to enhancing heat transfer by fabricating hierarchical surfaces, offering potential applications in ultrahigh heat flux thermal technologies.
Flow boiling in microchannels coupled with micro-nano-modified surfaces
Authors: Pontes P , Perez M , Moreira A L N , Moita A S
Published: June 2024

This paper addresses part of a wider project aiming at the development of a microchannel based heat sink to cool photovoltaic panels. The work presented here gives emphasis to working conditions leading to flow boiling in a microchannel and focuses on flow instabilities and on the potential effect of surface microstructuring in heat transfer enhancement and in controlling such instabilities. After identifying the flow boiling regimes, the observed phenomena are qualitatively and quantitatively described. The results show that a regular pattern of microcavities on the surface to cool enhances heat transfer and promotes the occurrence of a more stable flow.
Enhanced Pool Boiling Heat Transfer with Porous Ti-6Al-4V-Coatings Produced by Cold Spray Metal Additive Manufacturing
Authors: Garivalis Alekos Ioannis , Chen Yan , Shatskiy Evgeny , Robinson Anthony , Marco Paolo Di , Lupoi Rocco
Published: June 2024

In advancing industrial heat transfer mechanisms, surface coatings offer significant potential. This research elucidates the efficacy of the metal additive manufacturing Cold Spray deposition technique for producing enhanced boiling surfaces, specifically focusing on Ti-6Al-4V (Ti64) coatings on Aluminium substrates. This offers a rapid and low-cost fabrication method for producing lightweight enhanced boiling surfaces. The Cold Spray method is typically used to create dense metal deposits. Here, the process has been specially tuned to create highly inhomogeneous honeycomb-type porous Ti64 coatings. Critical Cold Spray deposition parameters, such as particle velocity, preheat temperature, and deposition rate have been identified to create repeatable porous coatings, with thicknesses of up to 3.0 mm achievable. Following deposition, several samples were subjected to systematic boiling heat transfer tests in a purpose-built pool boiling apparatus. Boiling curves were generated for the augmented Cold Spray surfaces as well as a bare surface, with the latter acting as a baseline to which enhancement levels were assessed. Initial data analysis shows that some of the tested surfaces exhibit a notable increase in boiling heat transfer coefficient and Critical Heat Flux (CHF). This enhancement is potentially attributed to increased surface area, increased nucleation site density, capillary wicking, and mitigation of lateral bubble coalescence, though excessive coating thickness may degrade heat transfer. In summary, the novel Ti64 surface structures developed using the Cold Spray deposition technique exhibits high potential for industries necessitating superior boiling heat transfer performance. Importantly, the manufacturing process is industrially scalable, offering the capacity to rapidly coat large areas at low cost compared with subtractive manufacturing other metal additive manufacturing methods.
Study of nucleate boiling growth regime on thin surfaces.
Authors: Jaunet Y , Bucci M , Zupančič M , Sebilleau J , Colin C , Golobič I
Published: June 2024

In a context of energy transition, heat exchanges are a key for energy sobriety. Among those exchanges, nucleate boiling is the preferred heat transfer method for high heat flux. Vaporisation plays an important role in the nuclear industry, but also in thermal machines such as cooling systems in data centers. In spite of decades studying boiling thermodynamics, modelling heat transfers in the nucleate boiling is still a major difficulty because of its numerous parameters. In the 1960s, scientists developed promising models called heat flux partitioning models. These models have been improved over the years, but they need closure laws about bubbles dynamics and nucleation site density. For the purpose of improving those models, we took part in an international collaboration (ANR TraThI) to study interface heat transfer. Project focuses on boiling with a controlled nucleation site density with a strong emphasis on studying isolated bubbles in water pool boiling, and later addressing multiple bubble interactions in pool and flow boiling. This work focuses on the bubble growth regimes on thin metallic foil observed in a water pool boiling experiment, with a distinction between microlayer and contact line growth regime. Pulsed nanosecond laser was used to create active nucleation site, while high-speed infrared thermography and shadowgraphy were implemented to record transient wall temperatures and bubble dynamics, respectively. This work shows that our experimental configuration induced two different bubble growth regimes, depending on the imposed heat flux and the recent past at the nucleation site and its vicinity. Study provides a framework for further in-depth investigations with different experimental configurations.
Bubble dynamics in high-pressure flow boiling conditions
Authors: Kossolapov Artyom , Hughes Matthew T. , Phillips Bren , Bucci Matteo
Published: June 2024

High resolution measurements of key parameters relevant to bubble departure processes in high-pressure water systems has been a major challenge over the past several decades due to the small size and fast motion of vapor bubbles, making conventional optical measurements unreliable. Addressing this difficulty is critical, as accurate bubble departure data and computationally inexpensive models are needed for future two-phase heat transfer models. To close this gap, we use high-speed imaging to track bubble size and velocity in high-pressure flow boiling conditions (10 – 40 bar). It is observed that bubbles are almost always perfectly spherical in shape and slide along the boiling surface immediately after nucleating. To predict sliding, we conduct a force balance on the bubble and derive an equation of motion in the direction of the bulk flow. Interestingly, we find that at high pressure conditions, the equation of motion can be significantly simplified, resulting in an analytical expression for the bubble motion, growth time and departure diameter, allowing us to develop simple and computationally efficient equations. The applicability of the model and departure criterion across a broader range of high-pressure conditions is discussed, and recommendations for further characterization of bubble departure processes are made.
The influence of liquid height to bubble size ratio on the boiling crisis in pool boiling
Authors: Alvarez Rodrigo Cavalcanti , Bucci Matteo
Published: June 2024

We conducted experiments to analyze water boiling under 10 K subcooling and atmospheric conditions in a horizontal pool boiling test setup, focusing on the effect of liquid height above the heating surface. Our main interest was the transition in boiling crisis mechanisms at liquid heights comparable to bubble size. For pool heights above 4.0 mm, we observed a relatively constant critical heat flux (CHF) of approximately 1.21 MW/m^2. Conversely, CHF significantly dropped to around 0.19 MW/m^2 for liquid heights below 2.5 mm. For depths between 2.5 – 4.0 mm the measured CHF values varied within these limits. Our pool boiling facility featured a stainless-steel chamber filled with deionized water heated by external circulating oil. A ceramic cartridge supported a transparent heater at the cell’s center-bottom, which utilized an indium tin oxide layer on a sapphire substrate for heat dissipation. We monitored bubbles’ footprint dynamics during boiling using infrared and high-speed video cameras. Preliminary high-speed data analysis revealed different hydrodynamic mechanisms corresponding to the obtained CHFs. For heights above 4.0 mm, the formation of an irreversible dry patch is triggered by bubble interactions. Conversely, at heights below 2.5 mm, one of the bubbles nucleating shortly after the onset of boiling failed to detach from the surface shortly. Instead, it grew in size and eventually spread over the entire surface, prematurely triggering a boiling crisis. We attributed this behavior to a lack of buoyancy forces to detach the bubble from the surface as it grew above the liquid level. Pool heights between 2.5 – 4.0 mm showed both behaviors. Further investigations will focus on exploring the transition in boiling mechanisms shifting from buoyancy- to surface-tension-dominated processes by varying bubble dimensions through the introduction of surfactants. Ultimately, we aim to analyze fundamental boiling parameters and heat flux to characterize how boiling dynamics, boiling crisis and CHF value vary with the bubble-to-liquid height ratio.