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To develop high-performance non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) is an urgent demand. We herein report an ingenious strategy to develop highly selective and competitive precious metal-free ORR electrocatalyst Co/CoO/SnO encapsulated in N-rich mesoporous carbon (CCS@NPC) nanocubes, via a simple one-step introduction of all functional components. Atomically mixed composites with significantly enhanced catalytic activity were obtained by adopting amorphous mesoporous ternary metal oxides CoSnO3 nanocubes as precursor and template. The synergistic effect of the triphase nanohybrids (Co, CoO & SnO) anchored in nitrogen-doped mesoporous carbon nanocubes promoted the catalytic reactions, owning to the anisotropic morphology, great heterojunction interfaces, and structural stability. The large-scale prepared CCS@NPC exhibited excellent electrocatalytic activity toward ORR with an admirable onset potential (1.01 V) and diffusion-limited current density (5.88 mA cm−2). The CCS@NPC showed stronger stability (20 h) and higher methanol tolerance with the concentration up to 8.0 M, compared to that of commercial Pt/C.

Proton Exchange Membrane Fuel Cell (PEMFC) has attracted widespread interest. In the present work, similarity analysis is adopted for a three-dimensional single-phase isothermal model of PEMFC to derive similarity criteria. Seven kinds of input criteria (Π1 ∼ Π7) are obtained, relevant to the fluid flow, pressure drop, flow resistance in a porous medium, activity loss, diffusion mass transfer, convective mass transfer and ohmic loss in PEMFC respectively. Dimensionless voltage and dimensionless current density are defined as two output criteria. Numerical verifications show that if the seven criteria keep their individual values with their components vary in a wide range, the dimensionless polarization curves keep the same with a deviation about 1%, showing the validity and feasibility of the present analysis. From the effect on the dimensionless polarization curve, sensibility analysis shows that the seven criteria can be divided into three categories: strong (Π4 and Π7, -94.9% ∼ +349.2%), mild to minor (Π5 and Π6, -4.5% ∼ +5.0%), and negligible (Π1, Π2 and Π3, -1.2% ∼ +1.1%). The similarity analysis approach can greatly save computation time in modeling the output characteristics of PEMFC.

Microchannel is helpful for cooling the microelectronic devices with high heat flux. In this study, topology optimization method with three objective functions (pumping power, bottom wall mean temp...

The formation of highly coherent double-shelled L12 nano-precipitates in dilute Al–Er–Sc–Zr alloys was investigated with the combined use of Cs-corrected transmission electron microscopy characterization and first-principles energetics calculations. The double-shelled nano-precipitates are primarily featured with an Er-rich core surrounded by a Sc-rich inner shell and a Zr-rich outer shell. First-principles energetics analyses based on the classic homogenous nucleation theory suggested that once forms, this double-shell structure can be thermally stable. The predominant formation of this double-shell structure has thus both profound kinetic and thermodynamic origins. Its formation and stability preference to all other possible L12 nano-structures would become more pronounced as its size increases, no matter what the solute ratio and aging temperature of interest.

Reducing the Platinum (Pt) loading while maintaining the performance is highly desired for promoting the commercial use of proton exchange membrane fuel cells (PEMFCs). Different methods have been adopted to fabricate catalyst layers (CLs) with low Pt loading, including utilizing lower Pt/C catalysts (MA), mixing high Pt/C catalysts with bare carbon black particles (MB), and reducing CL thickness while maintaining high Pt/C ratio (MC). In this study, self-developed pore-scale model is adopted to investigate the performance of the three Pt reduction methods. It is found that MA shows the best performance while MB shows the worst. Then, effects of Pt dispersion are further explored. The results show that denser Pt sites will result in higher local oxygen flux and thus higher local transport resistance. Therefore, MA method, which shows the better Pt dispersion, leads to improved performance. Third, CLs with quasi-realistic structures are investigated. Higher tortuosity resulting from the random pores produces higher bulk resistance along the thickness direction, while MA still exhibits the best performance. Finally, improved CL structures are investigated by designing perforated CL structures. It is found that adding perforations can significantly reduce the bulk transport resistance and can improve the CL performance. It is demonstrated that CL structure plays important roles on performance, and there are still huge potentials to further improve CL performance by increasing Pt dispersion and optimizing CL structures.

In this paper, a three-dimensional non-isothermal computational model for predicting indoor SVOC distribution is proposed, considering the effects of turbulence diffusion and suspended particles. The realizable k-e model is introduced for turbulent flow simulation in a room. The Euler-Euler method is adopted to deal with the gas-particle two-phase flow coupled problem. Inertia slip velocity and irreversible first-order absorption boundary are employed for more accurate prediction of particle motion. The simulated curve of outlet gas-phase di-2-ethylhexyl phthalate (DEHP) concentration with emission time is verified by available experimental data. The emission process of DEHP in a 15 m2 room in Beijing during 100 days with or without air cleaner is simulated by the developed model considering air leak through window and door gaps. It is found that if the air cleaner keeps on all the time during 100 days the gas-phase DEHP concentration in the room will tend to be uniform, while the emission process is far from equilibrium without an air cleaner even the emission lasts 100 days. Results also suggest that floor heating, decrease of particle concentration, weaken of heat transfer, enhancement of mass transfer, and air infiltration in window gap contribute to decrease DEHP concentration.

Numerical models that accurately describe the nano/microscale transport phenomena in the proton exchange membrane fuel cells (PEMFC) are highly required. In this study, a multiscale PEMFC model is ...

understanding interactions between multiphase flow and reactive transport processes in catalyst layers (CL) of proton exchange membrane fuel cells is crucial for obtaining better performance and lower cost. In this study, a pore-scale model is developed to simulate coupled processes occurring in CLs, including oxygen diffusion, electrochemical reaction, and air-liquid two phase flow. Simulation conducted in an idealized local CL structures shows that the pore-scale model successfully captures dynamic behaviors of liquid water including generation, growth and subsequent migration, as well as the interaction between multiphase flow and reactive transport. Pore-scale simulation is then conducted in hydrophobic CLs with complicated structures where carbon, platinum, ionomer and pores are resolved. It is found that filling modes of the liquid water in the CLs are different. Before forming the continuous flow paths in CLs, liquid water presents as tiny droplets in pores surrounding relative large pores. After the continuous flow paths are formed, liquid water dynamic behaviors follow the capillary fingering mechanism. The multiphase flow and reactive transport processes are closely coupled with each other, and as liquid water saturation increases the reaction rate decreases. Increasing the hydrophobicity can alleviate the water flooding, accelerate the water breakthrough, and facilitate the water evaporation.

Nucleate pool boiling heat transfer of R1234ze(E), R1233zd(E) and R134a were investigated outside two reentrant cavity tubes. The two tubes have different fin density while the size of subsurface tunnel width and fin height are almost the same. In the experiment, the pool boiling heat transfer is tested at the saturation temperature of 6 °C and heat flux of 10–80 kW/m2. A brief summary of pool boiling experimental data of Hydrofluoroolefins(HFO) refrigerants on different enhanced tubes from recent work is also provided. It is found that the combinations of tube and refrigerant: R134a and R13234ze(E) outside Tube-B1 shows almost equivalent boiling heat transfer performance. The two combinations also yield the highest averaged overall heat transfer coefficient. At the lower heat flux less than 60 kW/m2, for the tube with larger fin density and thinner fin thickness, the number of nucleation cavities is found to be more than that with larger fin thickness. At the heat flux more than 40 kW/m2, the boiling heat transfer coefficient of R134a and R1234ze(E) for the two enhanced tubes nearly merges into a single curve. At the higher heat flux, boiling heat transfer shows weak dependence on the surface structures. The boiling heat transfer coefficient of R1233zd(E) is more than 40 percentages lower than R134a for the two tubes at the nearly identical conditions. The research is helpful for the designer to summarize the boiling heat transfer performance of some typical HFO refrigerants.

This article conducts a two-dimensional numerical model to simulate the ferrofluid droplet formation from microfluidic T-junction under inhomogeneous magnetic fields with diverse strengths. This ex...

The effect of Er on the microhardness and precipitation behavior of the heat-treated Al−Si−Mg alloy was investigated by microhardness tester and TEM. As a comparison, the influence of natural aging was also studied. It is shown that the thermal stability of the over-aged Al−Si−Mg−Er alloy is highly related to the average size of the precipitates. The average size of β’’ precipitates in Al−Si−Mg−Er alloy is smaller than that in Al−Si−Mg alloy, and the distribution is more localized under condition of without introducing natural aging. However, when natural aging is introduced before artificial aging, the Al−Si−Mg−Er alloy has similar average size and distribution of precipitates with the Al−Si−Mg alloy, resulting in similar mechanical properties. The effect of Er on the precipitation kinetics in the alloy was also discussed in detail to explain these phenomena.

The energy harvesting performance of symmetrical flapping hydrofoil varying with thickness, shape and position of pitching axis was numerical investigated via immersed boundary method at a Reynolds number of 1100. The foil performs the given sinusoidal heaving and cosine-curvilinear pitching motions. The center of the hydrofoil is the intersection of the chord line and vertical line at maximum thickness. α is the ratio of the length between the leading edge and the center to the length of chord. β is the ratio of the length between the upper edge and the center to the length of chord. x p is the length between the leading edge and the pitching axis. It is found that enhancing the thickness of the foil can modestly increase the energy harvesting efficiency in the range of 0.03 ≤ β ≤ 0.06 , however, it is in contrast in the range of 0.06 ≤ β ≤ 0.105 . It is also found that the higher the value of α, the higher the extracting energy when α ≤ 0 . 4 . The optimal foil thickness is β = 0.06. The optimal pitching axis is located at the foil center when α ≤ 0 . 35 , and it is x p = 0.35 l when 0.35 α ≤ 0 . 50 .The highest energy harvesting efficiency of 41.71% is achieved when α = 0.4 .

The CaO/Ca(OH)2 thermochemical energy storage system can store heat through reversible reactions for long term and transport energy for long distance, and thus can solve the mismatching between energy supply and demand. In this study, a one-dimensional model is developed for the physical–chemical–thermal processes during the hydration reaction of CaO/Ca(OH)2 system in an indirect fixed bed reactor, and the corresponding governing equations are solved by the tridiagonal matrix method with self-developed program parallelized by Message Passing Interface. The characteristics and complicated coupling mechanisms of the vapor flow, heat transfer, mass transport and reaction processes are analyzed. Then effects of inlet pressure, convective heat transfer, reactant porosity, reactant permeability and reactor size on the reaction performance are discussed, respectively. It is found that higher inlet pressure, heat transfer coefficient, permeability and porosity can enhance the heat and mass transfer processes, thus accelerating the reaction efficiently. Finally, the reaction performance under different conditions is comprehensively evaluated by four indicators including the reaction time, average power, temperature plateau duration and standard deviation.

Dropwise condensation on rough surfaces enhanced with pillars in the presence of non-condensable gas (NCG) including the initial nucleation process is simulated using the multispecies multiphase la...

Water management is crucial to the performance of proton exchange membrane fuel cells (PEMFC). In this work, a pore-scale model is developed based on lattice Boltzmann (LB) method for the simulatio...

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.

This paper studies the catalytic performance and resistance to electrochemical damage of different catalysts in fuel cells by DFT calculations. The most commonly used platinum particle catalysts (Pt (111) surface) and three kinds of graphene-based platinum single-atom catalysts (G-N1-Pt, G-N2-Pt and G-N4-Pt) are selected as research objects. Based on Norskov's classical electrochemical theory, the step diagrams of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) under the standard reaction conditions and the interference with the addition of SO3− groups are calculated. Combined with the actual adsorption situation in the intermediate steps of the reaction, the catalytic performance of the standard reaction and the catalytic performance under the interference of SO3− groups are compared. For HOR reaction and ORR reaction, the catalysts with the best catalytic ability and anti-interference ability are G-N1-Pt catalyst and G-N2-Pt catalyst, respectively. A catalyst selection principle that balances activation performance and anti-interference performance in fuel cells is proposed.

Pre-deformation before aging has been demonstrated to have a positive effect on the mechanical strength of the 7N01 alloy in our previous study, which is rather different from the general negative effects of pre-deformation on high-strength 7XXX aluminum alloys. In order to explain the strengthening mechanism relating to the positive effect, in the present study, the microstructure of the aged 7N01 alloy with different degrees of pre-deformation was investigated in detail using advanced electron microscopy techniques. Our results show that, without pre-deformation, the aged alloy is strengthened mainly by the η′ type of hardening precipitates. In contrast, with pre-deformation, the aged alloy is strengthened by the hierarchical microstructure consisting of the GP-η′ type of precipitates formed inside sub-grains, the ηp type of precipitates formed at small-angle boundaries, and the dislocation introduced by pre-deformation (residual work-hardening effect). By visualizing the distribution of the ηp precipitates through three-dimensional electron tomography, the 3D microstructures of dislocation cells are clearly revealed. Proper combinations of ηp precipitates, GP-η′ precipitates and residual dislocations in the alloy are responsible for the positive effect of pre-deformation on its mechanical properties.

In this study, the authors present a P-band phased array front-end for radar applications, implemented using a 0.18 μ m SiGe BiCMOS process, thereby enhancing robustness over process-voltage-temperature (PVT) variations. It comprises a low noise amplifier, 6-bit attenuator, 6-bit phase shifter, power amplifier, T-R switches as well as control circuits. As the key here is to achieve high resolutions in both gain and phase controls, the 6-bit attenuator and 6-bit phase shifter are optimised by cascading unit cells with different topologies, hence ensuring a good balance between accuracy, silicon area and robustness. Temperature compensation techniques are also used in the low noise amplifier and power amplifier so that the circuit works robustly against PVT variations. Fabricated using TowerJazz 0.18 μ m SiGe BiCMOS technology, the prototype front-end occupies a chip area of 5 × 2.5 mm 2 , including bonding pads and test buffers. It performs with a receiver (RX) gain of 12 dB, a 3 dB noise figure, and a transmitter (TX) gain of 29 dB, while consuming 60 mW (RX mode) and 600 mW (TX mode) from a 3.3 V supply voltage. The chip is also measured in extreme conditions (e.g. temperatures exceeding 100 ° C ) to ensure robust operation and is suitable for low-cost phased array systems.

Based on the constant amplitude CF test of oil well tube material TP140small specimen, the procedures of predicting the variable amplitude corrosion fatigue (VACF) life of given reliability of TP140 were developed. Through the multi-sample sinusoidal loading constant amplitude CF life texts, the reliability distribution of CF life was analyzed. The CF life prediction of the specified reliability was obtained under 4 different stress levels, and the corresponding P-S-N curve expressions were obtained. Using VACF loading block spectrum and P-S-N curve expressions, the VACF life was calculated without considering the loading sequence effect. Furthermore, 5 VACF life tests were carried out using the same loading block spectrum. The VACF life with reliability was predicted by the principle of reliability and statistics. VACF life was calculated and compared with the predicted life. Test results and analysis show that the predicted results agree well with the experimental results, and CF life of TP140 casing steel follows a lognormal distribution at the given equivalent stress level, which has been substantiated.

The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 × 3 (column × row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm–2, saturation temperatures of 6 to 16 °C and film flow rates of 0.016 to 0.18 kgm–1s–1. Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm–2 and two film flow rates of 0.035 and 0.07 kgm−1s−1, and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms−1. It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123.

Exploring the origins of local transport resistance and characterizing the oxygen transport resistances in the catalyst layer (RCL) are critical for cost reduction. In this paper, a comprehensive mesoscopic model for simulating coupled transport processes of oxygen and water vapor for different structural parameters under different operating conditions in reconstructed microstructures is proposed. The local transport resistance is calculated after achieving the limiting current density and the effective diffusivity of oxygen. The results demonstrate that RCL increases greatly with decreasing platinum loading (LPt) and the transport resistances in other components of the cell dominate for high-loadings. Both the reduced oxygen permeation coefficient in the ionomer thin-film and the adsorption resistance account for the origins of local transport resistance. The local transport resistance increases with the bare carbon ratio for a constant LPt and Pt/C ratio due to the decreased effective ionomer surface, and increases with the I/C ratio due to the increased ionomer thickness and decreased Knudsen diffusivity. Due to the presence of liquid water, a slight decrease followed by an increase of the local transport resistance versus relative humidity is obtained. The contribution of ionomer thin-films to RCL is more sensitive to liquid saturation compared with that of pores. This journal is

In the present work, a numerical method is utilized to simulate the nucleate pool boiling of R134a outside the horizontal plain tube. Eulerian multiphase model integrated with Rensselaer Po...

Temperature distribution in proton exchange membrane fuel cell (PEMFC) is of great importance to water and thermal management. In this study, 25 μm thick Type-T thin film thermocouples (TFTCs) were fabricated using micro-electro-mechanical system (MEMS) to realize the real-time measurement of temperature distribution inside PEMFC. Besides, fifteen sheathed thermocouples were used to measure the temperature distribution outside the cell. The effect of inserting TFTCs on the cell performance was confirmed to be negligible. The experimental results show that the embedded TFTCs can rapidly response to temperature regime while have a minimal interference to the performance of the fuel cell. The temperature variation of the inside TFTCs with current generally has the same trend with the outside sheathed thermocouples, but with a more frequent fluctuation due to their small thermal inertia and inside complex environment. In this work, the details of MEMS-based techniques of making TFTCs and measured distribution of inside cell temperature are provided.

In this paper, a novel semi-empirical model is developed for predicting the thermal conductivity of Diathermic Oil (DO)-based nanofluid for solar thermal application. First, a corrected thermal conductivity model of nanofluid is developed based on the widely used Yu-Choi model. In this corrected model, the thermal conductivity distribution of nanolayer is treated as a quadratic form instead of a constant value along with the distance from nanoparticle. The corrected model shows the predicted thermal conductivity of nanofluid is significantly influenced by nanolayer thickness. Then, the semi-analytical nanolayer thicknesses for different DO-based nanofluid are calculated utilizing the corrected model and the experimental data in literatures. It is found that the nanolayer thickness of DO-based nanofluid is not a constant value but varies with the volume fraction of nanoparticles, nanoparticle radius, and nanofluid temperature. According to the semi-analytical results, the empirical equations between the above variables and nanolayer thickness are summarized. Finally, by adopting the empirical equation of the nanolayer thickness in the corrected model, a novel semi-empirical thermal conductivity model is developed for predicting the thermal conductivity of DO-based nanofluid. The semi-empirical thermal conductivity model is validated by a large number of experimental data from literatures.

Thermochemical energy storage has drawn great attention due to its advantages such as high energy density, little heat loss, and long term and distance storage. The present study aims to enhance reactive transport processes during the dehydration process of Ca(OH)2/CaO. A physicochemical model is developed for the dehydration process occurring in porous reactant, including vapor flow, heat transfer and endothermic chemical reaction. Distributions of temperature and Ca(OH)2 concentration in cylinder reactors are discussed, and complicated coupling mechanisms are revealed between multiple processes. In addition, the results show that increasing temperature can efficiently enhance the reactive transport. Effects of reactant porosity are also explored and different reaction patterns are observed. Finally, porous channels with high thermal conductivity and permeability are embedded into the reactor. On the one hand, the porous channel with high permeability serves as highway for the vapor generated to be discharged out of the reactor, thus pushing the reaction towards dehydration direction. On the other hand, the porous channel with high thermal conductivity also enhances local heat transfer, thus accelerating the dehydration chemical reaction. In summary, with porous channels added, the dehydration process is greatly enhanced.

The bubble nucleation of liquid argon over patterned surfaces with different wettabilities is investigated by using molecular dynamics (MD) simulations. In order to study effects of different wall temperatures on bubble nucleation process, bubble nucleation positions and bubble dynamics behaviors are observed. Simulation results show that at low wall temperatures bubble nucleation tends to occur over the hydrophobic part of the wall. With increasing wall temperature, the bubble nucleus gradually changes its position from the hydrophobic to hydrophilic part. When the wall temperature increases to a certain value, bubble nuclei first appear on both the hydrophobic and hydrophilic parts. By analyzing heat flux and change trend of argon state points, differences in bubble nucleation process at different wall temperatures are explained. Moreover, the effect of area fraction of hydrophobic part on bubble nucleation temperature is investigated. It is found that there exists an optimal area fraction which makes the bubble nucleation temperature the lowest.

Semi-volatile organic compounds (SVOC) exist in indoor air at very low concentrations (usually less than 100 µg/m3), but do great harm to human health. SVOC pollutants transfer in indoor air with suspended particles so complicatedly that traditional experimental methods are hardly to reveal the mass-transfer mechanism. This study employs Lattice Boltzmann method (LBM) to investigate the mass transfer behaviours of two-phase SVOC pollutants (gas- and particle-phase), and compare two widely-used gas/particle partition models (the instantaneous equilibrium model and the dynamic model). LBM is used to simulate the fluid flows and SVOC mass transfer, and the cell automation (CA) probabilistic model is applied to simulate the particle transport. The interaction between the gas- and particle-phase of SVOC is considered in the source term of the Lattice Boltzmann equations. One particle is firstly simulated in order to focus on the particle-phase concentration, then a large number of particles are simulated to focus on the effect of particles on the concentration field of SVOC. The simulation results show: (i) particles in the instantaneous equilibrium model for gas/particle partition always help gas-phase SVOC transport in indoor air by particle-mediation effect, while such an effect in the dynamic model is greatly determined by Kp and residence time of particles; (ii) particle-mediation effect can also increase the emission rate, and a non-uniform distribution of particles predicted by our numerical model indicates that such an additional emission rate increases exponentially with particle concentration, because a small number of particles have little possibility of approaching emission surface. (iii) Our numerical model can effectively estimate SVOC concentration in the presence of airborne particles by tracking each particle. These results highlight the roles of airborne particles in the transport of indoor SVOC pollutants, and are expected to be useful to consider dust deposition and resuspension when particles are in a realistic size distribution.

Friction stir welded lap joints of 5083 aluminum alloy plates with thickness of 2mm and T2 pure copper plates with thickness of 3mm were prepared, and the microstructures and properties of lap joints were investigated. The results showed that Al and Cu interdiffusion occurred at the interface of the lap joints, the grains in the stir zone of Al were refined and the microhardness increased sharply. The joint of Cu can be divided into four parts, including the nugget zone (NZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM). Using the optimum welding parameters (rotation speed of 900rpm and welding speed of 100mm/min), the lap joints were well formed. There were fewer welding defects and the average shear tensile force reached 3.155KN. Also, the ductile-brittle hybrid fracture occurred at the transitional zone of the advancing side of the Al plates. XRD results showed that intermetallic compounds (IMCS) mainly composed of Al2Cu and AlCu4 were produced in the interface of the lap joints during welding.

The desorption behavior of methane in slit was studied in this paper. We build a differential equation which describe dynamic desorption process, and propose a correction method. The mass transfer resistance in free region is considered and the accuracy of the result is improved using this correction method. Based on differential equation, the effects of pressure, temperature and solid material on dynamic desorption speed are analyzed. Desorption becomes faster when pressure increases. When pressure is low, desorption speed increases with increasing temperature; oppositely, desorption speed decreases with increasing temperature. When pressure is low, the desorption speed at different wall material is quartz > kaolinite > graphite; when pressure is high, the desorption speed is quartz > graphite > kaolinite.

Porous structures of agglomerates in cathode catalyst layers (CLs) of proton exchange membrane fuel cells are reconstructed, in which all the four phases are resolved including Platinum, carbon, ionomer and pore. A pore-scale reactive transport model based on the lattice Boltzmann method is developed, in which oxygen dissolution reaction at pore-ionomer interface, oxygen diffusion inside ionomer, and electrochemical reaction at ionomer-Pt interface are considered. Emphasis is put on structural parameters, especially Pt/C mass ratio, on the reactive transport process and the volumetric reaction rate (or current density). Pore-scale results show that while under high Pt loading oxygen is depleted quite close to the surface of the spherical agglomerate, it has to penetrate deep into the porous agglomerate before it is completely consumed under low Pt loading which is not captured by classical agglomerate model based on homogeneous mixture assumption. Pore-scale results also found that effects of transport inside the agglomerate decreases as reaction rate, porosity or ionomer thickness increases. Finally, local transport resistance inside the agglomerate is evaluated, and it increases as the agglomerate size increases or the dissolution reaction rate decreases.

A hot-rolled medium Mn (0.2C5Mn) steel is annealed at 650 °C to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation (ART) annealing, and the orientation relationship strictly obeys K–S orientation relationship before deformation. Tensile tests are carried out in a temperature range from − 196 to 400 °C to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite stability. Microstructural observations reveal that the metastable austenite gradually transformed into α-martensite, which is controlled by the deformation strain, the temperature and the austenite volume fraction. Both strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature significantly. The stress–strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation temperature. Based on the experimental data, a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.

Falling film evaporation heat transfer of R134a and its potential substitutes R290 and R600a outside a single horizontal plain tube is experimentally investigated, and the effects of the saturation temperature, film flow rate and heat flux on heat transfer coefficient are studied. Heat transfer performance of R290 is slightly superior than that of R134a, while R600a is inferior than that of R134a. The threshold film Reynolds number is determined to separate the variation trend of HTC with film Reynolds number into full wetting and partial dryout regimes. Increase of heat flux benefits the heat transfer in both full-wetting and partial dry-out regimes. New heat transfer correlations based on the present data and data for R32 and R1234ze(E) in the authors’ group are suggested for two regimes. The correlation for full wetting regime fits 96.7% of the total 542 correlated data within ±30% while fits 73.4% of the total 289 data in references from −30% to +15%, the correlation for partial dryout regime fits 97.5% of the total 162 correlated data within ±30% while fits 76.8% of the total 95 data from references within ±30%.

Dropwise condensation on rough structures with different wettability are numerically simulated using Lattice Boltzmann method. The latent heat during condensation process together with the energy equation is taken into consideration in this paper by solving the temperature distribution equation with a source term for the phase change. Typical droplet nucleation positions and wetting states are found out at different surface wettability through numerical results. It is concluded that with the increase of strength coefficient of the fluid-solid interaction or surface hydrophobicity, the nucleation position rises from the bottom to top of the pillar, and the wetting state of droplet changes from the wetting Wenzel state to the nonwetting Cassie one.

A new hybrid system is proposed to capture CO2 as well as generate electricity with the low CO2 inlet condition of confined space. Within the system, a novel photocatalytic porous framework coated by g-C3N4/TiO2 is prepared to avoid the inhibition of microalgae growth caused by the direct addition of photocatalyst. Under 0.8% v/v CO2 inlet condition, chemical oxygen demand (COD) yields from the photocatalytic framework immersed in the phosphate buffer and the algae suspension are 1.63 mg L−1 h−1 and 1.90 mg L−1 h−1, respectively. CO2 sequestration rate of a 60L cylindrical photobioreactor increases from 12% to 22%. The combination modes between photobioreactor and photocatalytic framework can be selected flexibly depends on the demands of application. This hybrid system not only benefits to enhance the CO2 sequestration rate of photobioreactor but also has the potential to be served as the power source in a confined space.

In this paper, the fouling and convection heat transfer characteristics of various flue gas heat exchangers are investigated, and the weakening degrees of fouling on the heat transfer performance of different heat exchangers are compared. The anti-fouling performance of novel H-type finned tube heat exchangers are discussed. The results show that the elliptical tube arrangement can effectively reduce the effect of fouling on heat transfer performance. Overall, SHETB has the best heat transfer and anti-fouling performance. The results of this paper are helpful for the efficient recovery and utilization of dusty flue gas waste heat.

The ash particles in the flue gas can deposit on the tube bundles of the heat exchangers and form granular fouling layers. The fouling layer will significantly reduce the efficiency of the heat exchangers. A new numerical model is proposed to study the fouling processes on tubes. The immersed boundary method is used to simulate the morphology of the fouling layers. Coupled with lattice Boltzmann method for flow and particle tracking method, the changing of the shapes of the fouling layers can be conveniently achieved during the simulations. Discussions about the influences of flow velocity on the growth of the fouling layer are given according to the numerical results.

As communication systems are gradually transferred to 5G, the system’s heat dissipation is getting larger, and thermal design becomes an important issue. This paper investigates different improvement methods for a radio base station by numerical simulation, including installation method, metal conduction rod, geometric optimization of heat sink, graphene, and vapor chamber. With these methods, the global maximum temperature drops from 103.5 to 80.2 °C.

Questions regarding bubble nucleation on an ideally smooth surface are seemingly endless, but it can not be adequately verified yet because of the scale limitation (microscopic scale). Hence, in this study, bubble nucleation on an ideally smooth substrate is explored using the molecular dynamics simulation method. An ideally smooth hydrophilic platinum substrate at 145 K is conducted to heat the simple L-J liquid argon. Results show that a visible bubble nucleus successfully forms on the ideally smooth substrate without any additional disturbance, which is common in boiling studies using the traditional numerical simulation methods. However, the nucleation position is unpredictable. At the atomic level, the thermal energy transfer from an ideally smooth substrate to liquid atoms is inhomogeneous due to atomic inhomogeneous distribution and irregular movement, which are the key influencing factors for achieving bubble nucleation. The inhomogeneity will be highlighted with the heating process. As a result, some local liquid atoms near the ideally smooth surface absorb more thermal energy to overcome their potential barrier at a specific moment, causing the emergence of a distinct nucleus there. Furthermore, nanostructure substrates are introduced to make a comparison with the smooth substrate in bubble nucleation. There is no significant difference in the inception temperature of nucleation between the ideally smooth and nanostructure substrates, but the latter has better performance in improving the bubble nucleation rate.

Hypothesis Freezing morphologies of impacting water droplets depend on the interaction between droplet spreading and solidification. The existing studies showed that the shape of frozen droplets mostly is of spherical cap with a singular tip, because of much shorter timescale of the droplet spreading than that of the solidification. Here, we create the experimental conditions of extended droplet spreading and greatly enhanced heat transfer for fast solidification, thereby allowing to study such droplet freezing process under the strong coupling of the droplet spreading and solidification. Experiments We design experiments that a room-temperature water droplet impacts on a subcooled superhydrophilic surface in an enclosure chamber filled with nitrogen gas. We thoroughly investigate the freezing processes of impacting droplets under the effects of impact velocity and substrate temperature. Both the droplet impact dynamics and solidification are studied with a high-speed camera. Findings We observed five different freezing morphologies which depend on the droplet impact velocity and substrate temperature. We found that the formation of diverse morphologies results from the competitive timescales related to droplet solidification and impact hydrodynamics. We also develop a phase diagram based on scaling analysis and show how freezing morphologies are controlled by droplet impact and freezing related timescales.

Over the past 30 years, the lattice Boltzmann (LB) method has been developed into a versatile and powerful numerical methodology for computational fluid dynamics and heat transfer. Owing to its kinetic nature, the LB method has the capability to incorporate the essential mesoscopic physics, and it is particularly successful in modeling transport phenomena involving complex boundaries and interfacial dynamics. Up to now, the LB method has achieved great success in modeling fluid flow and heat transfer in porous media. Since the LB method is inherently transient, it is especially useful for investigating transient solid-liquid phase-change processes wherein the interfacial behaviors are very important. In this article, a comprehensive review of the LB methods for single-phase and solid-liquid phase-change heat transfer in porous media at both the pore scale and representative elementary volume (REV) scale. The review first introduces the fundamental theory of the LB method for fluid flow and heat transfer. Subsequently, the REV-scale LB method for fluid flow and single-phase heat transfer in porous media and the LB method for solid-liquid phase-change heat transfer are discussed in detail. Moreover, the applications of the LB methods in single-phase and solid-liquid phase-change heat transfer in porous media are reviewed. The LB modeling and predictions of the effective thermal conductivity of porous materials are also reviewed. Finally, further developments of the LB method in the related areas are briefly discussed.

In this study, pore-scale simulations are conducted to study oxygen transport processes around a carbon particle. Four- constituent microscopic structures of catalyst layer are reconstructed, which account for a carbon particle, ionomer, Pt particles, and primary pores inside the carbon particle. A physicochemical model is developed, which focuses on local oxygen transport processes around a carbon particle including oxygen dissolution at the pore/ionomer interface, oxygen diffusion within the thin ionomer film covering the carbon particle and inside the primary pores filled with water, and electrochemical reactions at the Pt interface. The lattice Boltzmann method is adopted to simulate above reactive transport processes at the pore scale, and oxygen transport resistance is calculated based on the concentration filed obtained. Effects of reactive transport condition, interfacial dissolution rate, Pt loading on the local transport resistance are investigated in detail. The simulation results are compared with experimental results and 1D models in the literature.

Proton exchange membrane fuel cell (PEMFC) has been recognized as the most promising energy conversion device due to its outstanding merits, such as high energy efficiency, low pollution, low operating temperature and low noise. Due to the complicated process of water production and transportation in porous media, the water concentration can affect the distribution of species and ionic conductivity, thus, the water management has been regarded as one of the most critical issues for PEMFC. In this paper, a three dimensional multiphase non-isothermal model for the PEMFC performance prediction has been developed to study the effect of ionomer volume fraction on the performance of PEMFC. The numerical results show that the ionomer volume fraction of catalyst layer has an appreciable effect on the performance of PEMFC and water distribution in porous electrodes, the higher ionomer volume fraction in catalyst layer can be helpful to avoid the flooding problem in porous electrodes.

It’s an effective method to judge the operation state of transformer by measuring the composition and proportion of gas in oil-paper insulated power transformer oil. An on-line monitoring system for transformer oil and gas by laser photoacoustic spectroscopy is designed based on resonant laser photoacoustic spectroscopy and wavelength modulation, it can realize the continuous measurement of transformer oil and gas, the light source is semiconductor laser, which can avoid the problem of gas cross interference existing in traditional photoacoustic spectroscopy. At the same time, the system makes use of ubiquitous Internet of things technology to realize real-time data viewing and reading, which can facilitate the management of oil and gas equipment.

Herein, two new water-stable magnetic frameworks, [Cu1.5(L)(H2O)]·2H2O (1) and [Co4(L)2(CH3CN)(OH)2(H2O)4]·3H2O (2) (H3L = 5-((4-carboxypyridin-2-yl)oxy) isophthalic acid), were synthesized by a solvothermal method in similar reaction systems and characterized by infrared spectroscopy, powder X-ray diffraction and thermogravimetric measurements. Due to their different metal ions, 1 shows a trinodal (4,4,4)-connected three-dimensional (3D) framework that possesses 1D channels based on [Cu2(COO)4] paddle-wheel secondary building units (SBUs), whereas 2 is a 2D layered network based on tetranuclear [Co4(COO)6(μ3-OH)2] SBUs and reveals a (3,6)-connected kgd topological structure. The gas adsorption explorations demonstrated that 1 exhibited commendable selectivity for CO2 over CH4. Moreover, both 1 and 2 showed different degrees of antiferromagnetic exchange action.

This article presents a sharp-interface model for solving solid-liquid phase change of pure materials. An interface capturing method combining VOF and level set, VOSET, is employed to track the moving interface during phase change, and the level-set function generated in VOSET is used to consider the effect of the phase boundary when solving the fields of temperature and velocity. In order to deal with the interaction between the solid and liquid phases, an immersed boundary method (IBM) is incorporated into SIMPLER algorithm. The fundamental numerical approaches presented in this article were individually assessed by test problems, all showing good agreements with benchmark solutions. The established sharp-interface model was then applied in simulating melting and solidification without and with fluid flow, where competitive performance in accuracy was demonstrated by the numerical results.

Five new 3D MOFs: {[Zn3(HL)2H2O]·4H2O}n (1), {[Ba2L5H2O]·2H2O}n (2), {[Ba4(L)2·5H2O]·2H2O}n (3), {[Cd3(HL)2·Cl2·2H2O]·2H2O}n (4) and {[Cu2(L)2·2H2O]·4H2O}n (5), were constructed based on 1-(3,5-dicarboxylatobenzyl)-3,5-pyrazole dicarboxylic acid. Structural analysis shows that 1–5 have 3D frameworks, among which 1(Zn) exhibits high luminescence sensing for Fe3+ ions, excellent framework stability and reusability. The sensing mechanism of 1(Zn) is briefly discussed. In addition, 5(Cu) displays antiferromagnetic exchange phenomena.