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• Water desalination performances of COFs membranes were studied by molecular dynamics. • Water permeation of CTF-FUM and TPMA monolayer membranes are 3 orders of magnitude higher than TFC. • CTF-FUM and TPMA membranes could exhibit nearly 100% salt rejection. • Membrane thickness should be considered for designing 2D hydrophobic COF membranes. To explore energy-efficient two-dimensional porous materials for water desalination is an urgent demand for rational utilization of water and sustainable development. In addition, how the pore hydrophilicity influencing the desalination performance remains elusive. Basing on molecular dynamics (MD) simulations, we herein proposed ultrathin membranes of 2D covalent organic frameworks (COFs) with triazine units that are competent to reject ions efficiently while giving access to high water permeability. The ordered porous structure of these two COFs membranes (CTF-FUM and TPMA) enable nearly 100% salt rejection and show remarkable higher water permeation (∼57.2 and ∼ 48.1 L cm−2 day−1 MPa−1, respectively) over 3 magnitude compared to commercial TFC membranes (∼0.03 L cm−2 day−1 MPa−1). As a further step, the influence of layer number on membrane desalination performance was discussed to understand the underlying mechanisms. Compared to TPMA multilayer, it was found that CTF-FUM multilayer with hydrophobic pore shows less water permeability and higher transport resistance, and even has no obvious water transport through the 8-layer membrane. This MD simulation work gives a microscopic insight into 2D COF membranes on desalination and suggests that TPMA multilayer might be a promising membrane material for desalination applications. [ABSTRACT FROM AUTHOR]

Flow boiling stands out as an efficient mode of heat transfer with significant potential for reducing carbon footprint. Based on a coupled Volume-of-Fluid and Level Set (VOSET) method, this numerical study reveals the microlayer and its evaporative heat flux distributions, as well as the effects of microlayer evaporation and depletion on bubble dynamic behavior, flow pattern, wall superheat distribution, and boiling crisis for the flow boiling in a mini-channel. These aspects are firstly comprehensively reported by the numerical method. Results indicate that the rapid evaporation of a thin microlayer positioned beneath elongated bubbles significantly contributes to heat dissipation during flow boiling in a mini-channel. The heat flux through the microlayer surpasses 300 kW/m2, accounting for approximately 42% of heat dissipation at 200 kW/m2. Furthermore, this contribution increases to over 70% as the heat flux exceeds 500 kW/m2. As a result, in comparison to scenarios without microlayers, the presence of microlayers results in an average heat transfer coefficient (HTC) that is 3.97 times larger within the range of 200–500 kW/m2. Additionally, microlayer evaporation delays the happening of flow boiling crisis. Without microlayers, the wall superheat exceeds 180 K at 500 kW/m2, whereas under microlayer influence, the maximum local wall superheat is only 37.09 K at 1000 kW/m2. With further increases in heat flux, the emergence of large dry patches resulting from microlayer depletion induces the beginning of heat transfer deterioration. Finally, the ongoing coalescence of elongated bubbles necessitates a prolonged period for rewetting dry patches at the exit of the mini-channel, triggering the flow boiling crisis at 1300 kW/m2. The study results provide crucial guidance for the development and application of the flow boiling heat transfer technology for the thermal management of data centers. [ABSTRACT FROM AUTHOR]

• A comprehensive review on numerical simulations of boiling heat transfer from atomistic to mesoscopic and macroscopic scales is presented. • Critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling are highlighted. • Potential solutions and future research in the field of boiling heat transfer using the MDS, LB and CFD methods are proposed. Boiling is an efficient heat transfer mode with significant potential for thermal management in high-power electronic equipment. However, a comprehensive understanding of the boiling process, which encompasses bubble nucleation, growth, coalescence, slipping, and detachment across various scales, remains challenging. Molecular dynamics simulation, lattice Boltzmann, and computational fluid dynamics methods are popular and powerful tools for investigating boiling heat transfer phenomena at microscopic, mesoscopic, and macroscopic scales. These methods enable researchers to uncover the underlying boiling mechanisms and propose heat transfer enhancement techniques. Therefore, this paper provides a comprehensive review of boiling heat transfer, spanning from atomistic to mesoscopic and macroscopic scales, utilizing these three numerical methods. It addresses critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling, and proposes potential solutions and future researches, supplementing our previous review [Some advances in numerical simulations of multiscale heat transfer problems and particularly for boiling heat transfer, Annu. Rev. Heat Transf., 6 (2022) 217–269]. Besides, by shedding light on the characteristics of these numerical methods in studying boiling heat transfer, this paper aims to foster their development and advance enhanced heat transfer technologies. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) porous flow fields have been utilized to achieve ultrahigh power density for proton exchange membrane fuel cells (PEMFCs). However, issues such as the severe corrosion and high interface contact resistance of 3D porous flow field limit the effective enhancement in power density. In this work, porous flow fields with various pore structures, i.e., Ni foam and Ti felt are employed and further decorated with carbon coating using the chemical vapor deposition (CVD) method to improve the anti-corrosion capacity. Meanwhile, the through-plane electrical conductivity and hydrophobicity are collectively improved for Ni foam and Ti felt by the carbon coating. Single fuel cell (FC) tests show that both the peak power density (PPD) and limiting current density (LCD) are improved for PEMFCs assembled with C@Ni foam and C@Ti felt flow fields in the cathode. The corresponding electrochemical impedance spectroscopy (EIS) confirms that the improved PPD (C@Ti felt) is attributed to the collective improvements in electrical conductivity and anti-corrosion capacity, and the improved LCD (C@Ni foam) is attributed to enhanced water removal capacity. The triple improvements in the intrinsic property of porous flow field materials obtained in this work are expected to offer novel approaches for developing PEMFCs with ultrahigh power density. [Display omitted] • Carbon coating is introduced in porous flow field by chemical vapor deposition (CVD). • Through-plane contact resistance is reduced for C@Ni and C@Ti porous flow field. • Anticorrosion capacity was enhanced for C@Ni and C@Ti porous flow field materials. • Water removal capacity is improved for C@Ni and C@Ti porous flow field materials. • Electrochemical performance is enhanced for fuel cell with C@Ni and C@Ti. [ABSTRACT FROM AUTHOR]

Global warming due to greenhouse gas emissions has continuously threatened the climate and environment. Zirconium-based metal-organic frameworks (Zr-MOFs) with Zr 6 inner cores represent a subfamily of nanoporous materials with good physicochemical stabilities, showing significant prospects for practical applications in gas separation. A molecular simulation study is reported here to investigate the membrane separation of CO 2 /N 2 and CO 2 /CH 4 mixtures in five Zr-MOFs (Zr-fum, UiO-66, DUT-52, Zr-cca, UiO-67) with similar ligand in their structures and different pore-limiting diameter (PLD) ranging from 0.36 nm to 0.59 nm. The gas permeation and separation performance are evaluated with the concentration gradient-driven molecular dynamics (CGD-MD) method. The results show that the separation of both gas mixtures is dominated by the preferential sorption of CO 2 over N 2 and CH 4 , respectively. Meanwhile, the PLD of MOFs is also a significant factor governing permeation. MOFs with a larger PLD have a higher permeability and a lower selectivity for CO 2 /N 2 and CO 2 /CH 4 separation. Free-energy profiles were calculated to describe the insights into CO 2 separation, that the larger PLD can cause a lower energy barrier when gases transport through the pore of MOFs. The results were compared with the grand canonical Monte Carlo and equilibrium molecular dynamics (GCMC + EMD) approach, a traditional method to predict gas adsorption and diffusion in MOFs. As the GCMC+EMD approach neglected the interaction between components and potential mass transfer resistance at the surface of the membranes, the results obtained from the CGD-MD method in this work are more reliable. This study provides microscopic insights into CO 2 separation in Zr-MOF membranes and suggests their potential use for gas separation. [ABSTRACT FROM AUTHOR]

• A proposed surrogate model can well predict the cathode transport process of PEMFC. • Cathode gas channels with high performance are designed by topology optimization. • A continuity optimized strategy is proposed to increase the inlet velocity to 7 m s−1. • The topology optimized gas channels are superior to the traditional designs. • Topology optimized gas channels are compared with the bio-inspired gas channels. Optimizing the gas channels (GCs) plays an important role on improving the performance of proton exchange membrane fuel cells (PEMFCs). In this study, a topology optimization (TO) model is developed for optimizing the cathode GCs. A reduced two-dimensional (2D) reactive transport model is developed based on the 3D half-cell model, which considers the multiscale oxygen reactive transport processes inside GCs, the gas diffusion layer and the catalyst layer. The reduced 2D surrogate model can predict the polarization curve agreeing well with experimental results. The TO is employed to optimize the GCs with the objectives of reducing flow resistance and increasing the current density. A continuous scheme is developed to conduct the TO under high inlet velocity. GCs with five different inlet/outlet configurations are optimized. As validated by 3D simulations, the TO structures can obtain higher current density with reduced pressure drop compared with parallel GCs. Finally, the TO GCs are compared with existing bio-inspired GCs. The tree-like structure with wider inlet/outlet channels and narrower center channel is considered as a promising design for future investigation. The present study is helpful for the designing of next-generation of GCs for better cell performance. [ABSTRACT FROM AUTHOR]

A full-knowledge of the temperature distribution in a data center is crucial for the design and operation to avoid thermal failure. However, it is a great challenge to conduct full-field simulations using the computational fluid dynamics/heat transfer (CFD/HT) method due to the unacceptable computational cost caused by the multi-scale structure of data centers. In this paper, a novel multi-scale model based on POD and CFD coupling is proposed to predict the thermal and fluid characteristics in data centers from room scale to chip scale. The temperature and flow fields in an air-cooled data center are predicted and analyzed under the scales of room, server, IGBT and chip. Based on the CFD results of Room-Server level, 16 sets of design cases' temperature fields for Server-IGBT level are compared using the POD and CFD methods, which shows that the maximum average absolute deviation and average relative deviation are 0.004 °C and 0.59%, respectively. Subsequently, three sets of off-design cases for Server-IGBT level and IGBT-Chip level are predicted using the aforementioned methods, yielding the maximum relative deviation 7.21% and 0.98%, respectively. The salient outcome is the remarkable 175-fold reduction in CPU processing time achieved with the POD model compared to traditional CFD simulations. [ABSTRACT FROM AUTHOR]

Membrane separation technology has been widely used in the separation and capture of CO 2 to reduce carbon emissions. The performances of facilitated hybrid membranes (FHMs) can be improved by properly adding metal-organic framework (MOF)-based nanoparticles (NPs) and modifying polymers to the polymer matrix. Grafting and blending methods are widely used to introduce modifying polymers to improve the compatibility between NPs and the polymer matrix, and the performance of FHMs for CO 2 separation. However, the separation performance of FHMs decreases significantly when the long chains of the polymer infiltrate into the pores of the MOF at the interface between the modifying polymer and MOF. To in-depth understand the nano-scale polymer-NPs interfacial interaction mechanism of the grafting and blending methods, we herein employed molecular dynamic (MD) simulation method combined with experiments to investigate the interface between NPs of Cu-MOF (Cu-BDC) and four commonly used modifying polymers (PEG, polyethylene glycol; PEI, polyethylenimine; PVA, polyvinyl alcohol; PPy, polypyrrole) with PVDF (polyvinylidene difluoride) as the polymer matrix. The results showed that the blending method for modification can effectively avoid the polymer blockage in Cu-BDC pores by using four modifying polymers, while the grafting method caused polymer infiltration for PEG and PEI. The relationship between the polymer diameter and the pore limiting diameter (PLD) of the Cu-BDC was evaluated to analyze the polymer infiltration effect. FHMs of Cu-BDC/PEG@PVDF were synthesized based on the MD simulation results, which showed excellent CO 2 separation performance due to the good compatibility between the Cu-BDC and the modified matrix polymer. This work offers an insight in nano-scale into the interface design between the MOF and the polymer for FHMs separating CO 2. [Display omitted] • The interface interaction between MOF and modifying polymers was studied by molecular dynamics. • Using blending method to avoid the modifying polymer (PEG and PEI) infiltrating into MOF(Cu-BDC) pores. • The compatibility between modified matrix polymer and MOF was improved. • Cu-BDC/PEG@PVDF was synthesized and exhibited good performance for CO 2 separation. [ABSTRACT FROM AUTHOR]

• The construction of the enhanced plate used in the AHU is proposed. • The highest enhanced effects are obtained for "H12-R10-h4" (PEC=1.45 at Re=5000). • All nine enhanced channels can enhance heat transfer at the same pressure drop. • The DMTD is proposed for the cross-flow AHU heat exchanger design. • The heat exchanger picked up is "H7.5-R10-h3" which has the maximum ratio of the heat exchange rate (40.5 kW) over the flow loss power (3.25 kW). In this paper, the construction process of the enhanced plate, the heat transfer and fluid flow characteristics, and the domain decomposition method for AHU performance estimation are presented. The AHU heat exchanger 3D model is built from plate modeling to heat exchanger assembling. The heat transfer plate modeling starts from a basic flat plate, elliptic cylindrical dimples (ECDs) are firstly added to the fixed location, and then spherical crown dimples (SCDs) are applied to the flat surface among the ECDs. For the enhanced channels, the effect of channel height is weaker than SCD depth on the Nusselt number and friction factors. For all enhanced channels, the PECs are larger than 1.1 within Re from 5000 to 35,355. The performance of a deeper depth of SCD is superior to that of a shallow one when the channel height is the same. Among the nine channels studied, the highest enhanced effects are obtained for "H12-R10-h4" (PEC=1.45 at Re=5000). And all nine enhanced channels can enhance heat transfer at the same pressure drop among the Reynolds number of 5000 to 35,355. The energy saving performance decreases with Re, and the channels with smaller SCD depth can save more energy when channel height is identical at the higher Reynolds number region. A decomposition thermal design method is proposed for the cross-flow AHU heat exchanger design, and the typical AHU design result is picked based on the heat exchange rate and flow loss power. The recommended heat exchanger is "H7.5-R10-h3" whose heat exchange rate is 40.5 kW when flow loss power is 3.25 kW. [ABSTRACT FROM AUTHOR]

Facilitated transport mixed matrix membranes (FTMMMs) separation technology has shown advantages in improving CO 2 recycling efficiency and reducing the environmental impacts associated with carbon emission. The design of FTMMMs is highly dependent on the selection of a compatible combination of polymer and filler. Herein, a novel grafted nanoparticles (GNPs) filler UiO-66-NM@PEG with multiple groups prone to hydrogen bond was designed to alleviate the phase separation and performance degradation caused by filler/polymer poor compatibility. In the GNPs configuration, by employing the coexistence strategy of coupling reaction and covalent bonds, maleic anhydride (MAH) was coupled with the amino group on UiO-66-NH 2 without clogging or cracking, and the generated carboxyl groups can be cross-linked with the hydroxyl groups on the poly (ethylene glycol) (PEG) by covalent bonds. The UiO-66-NM@PEG demonstrated excellent interface compatibility between the filler and matrix phase of UiO-66-NM@PEG/PVDF, which was further convinced by molecular modelling. The resultant UiO-66-NM@PEG/PVDF achieved a simultaneous enhancement of gas permeability (537.6 Barrer) and ideal CO 2 /N 2 selectivity (47.8), which surpassed Robenson upper bound. Moreover, the UiO-66-NM@PEG/PVDF has a lasting sustainability over 960 h (40 days), and a valid anti-acid/alkali corrosion resistance in harsh environments. More importantly, UiO-66-NM@PEG can also be well mixed with other mixture matrices such as hydroxy-terminated polydimethylsiloxane (PDMS) and polyether-block-polyamide copolymer (PEBAX) to promote permeability/ideal selectivity up to (724.1 Barrer/21.1) and (307.4 Barrer/31.4). We thus concluded that this rational-design GNPs approach provides a general toolbox for controllable engineering in GNPs whereby enhanced favorable filler/matrix compatibility in FTMMMs with optimized CO 2 /N 2 separation properties. By the means of covalent "bridge-crosslinking", the UiO-66-NM@PEG/PVDF FTMMM with favorable phase compatibility between filler/matrix displayed excellent gas separation performance. [Display omitted] • A novel grafted filler UiO-66-NM@PEG was synthesized via Covalent "Bridge-crosslinking" strategy. • The membrane showed excellent sustainability over 40 days and corrosion resistance. • UiO-66-NM@PEG exhibited favorable filler/matrix compatibility in other two matrices. • The performance of UiO-66-NM@PEG/PVDF surpassed Robenson upper bound. [ABSTRACT FROM AUTHOR]

• A method for studying microlayer effects on flow boiling is proposed. • The availability of combining radius-based and taylor models in describing microlayer formation is verified. • The implementation procedure of the microlayer model is fully demonstrated. • The effects of different microlayer models on flow boiling are illustrated. • The contribution of microlayer evaporation to heat transfer is 75%−83%. The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel. [ABSTRACT FROM AUTHOR]

Self-cleaning porous membrane can serve as condensing surface for its capacity of transporting condensate to the back through the pores thus keeping the surface clean during the whole condensing period. However, the three-dimensional (3D) condensate behaviors including the initial nucleation, growth and permeation in the porous structures have seldom been clarified comprehensively. In this work, the 3D pseudopotential LB model with a phase change model are employed, and the condensation process inside the porous membrane with various porosity, wettability, additional external force and section geometric configurations are investigated. It's found that with the increase of porosity, surface hydrophobicity and additional external force, more rapid penetration of condensate in the porous media and correspondingly faster removal rate of liquid can be obtained, and the subsequent vapor condensation is enhanced. But the nucleation time gets longer with high porosity and surface hydrophobicity. The additional external force has little effect on the nucleation time. Larger porosity and lower flow resistance inside the straight channel lead to higher condensation rate at the initial stage of condensation, while the porous structure with distorted pores with the same porosity contributes to improve the total condensation amount. This work helps to select the porous membranes with different porosity, wettability or pore scale geometric characteristic and external field for different condensation conditions in specific applications. [ABSTRACT FROM AUTHOR]

• Density based topology optimization method is used to design microchannel heat sinks. • Five inlet and outlet structures are considered by bi-objective optimization. • Straight line inlet and outlet with extension area presents best overall performance. • Reynolds number, thermal conductivity ratio and fluid heat generation are studied. • 2D optimized results are stretched to 3D geometry and validated by 3D CFD simulation. Topology optimization can generate better structures of microchannel with improved cooling performance. In this paper, microchannel heat sinks with five different inlet and outlet structure combinations are optimized using bi-objective topology optimization based on the density method. The layouts of channels are optimized to minimize the power dissipation and total heat generation at the same time. The results show that the straight line inlet and outlet with extension areas (SE) can reduce 20%-50% power dissipation when realizing the same heat transfer performance compared with traditional one channel inlet and one channel outlet (OC). Effects of Reynolds number, the ratio of solid and fluid thermal conductivity and the dimensionless heat generation coefficient in fluid domains are also investigated. The simple channels can be obtained with higher ratio of solid and fluid thermal conductivity and/or higher dimensionless heat generation coefficient in fluid domains. With optimization further considered in the inlet and outlet regions, better overall performance can be obtained. Finally, the 2D optimized structures are further validated by three-dimensional numerical simulation. It is found that the optimized design can realize 53.28% increment of Nusselt number with 40.89% reduction in pressure drop compared with traditional microchannel heat sinks. [ABSTRACT FROM AUTHOR]

Three immiscible fluids systems are often encountered in energy and environmental disciplines, especially in the oil exploration field. It is essential to enhance oil recovery (EOR) by identifying the three-phase fluid flow mechanism of oil-gas-water system in complex geological structures. In the present study, a lattice Boltzmann (LB) color gradient model for three immiscible fluid flow in three-dimensional porous media is employed and validated using five benchmarks (i.e., the multiphase spinodal decomposition; the multiphase Young-Laplace test; the liquid lens; contact angles and 3D channel layered three-phase fluid flow). The effects of wettability, viscosity ratio (M) and capillary number (Ca) on the relative permeability curves are analyzed in detail. When the wetting condition is enhanced, the results indicate that the relative permeability of non-wetting phase raises while that for the wetting phase exhibits a decline. When M is not equal 1, the relative permeability of high viscosity phase is significantly increased, while the other two phases are less affected. Meanwhile, the maximal value of relative permeability does not exceed unity because of the low porosity in the porous medium. For the influence of Ca , simulation results indicate the relative permeability shows weak sensitivity to Ca. • A lattice Boltzmann color gradient model for three-phase fluid flow is proposed. • Pore-scale three-phase fluid flow in porous media is studied. • Three-phase relative permeability is predicted. • Effects of wettability, viscosity ratio and capillary number are explored. [ABSTRACT FROM AUTHOR]