•A multiphase model for PEMFC cold start is developed considering MPL effect.•Effect of MPL hydrophobicity is more significant for startup from −10 °C or higher.•MPL effect is weak for startup from −15 °C or lower because less liquid presents.•MPL hydrophobicity needs to be designed according to CL and GDL properties.•PEMFC with thinner membrane is more sensitive to MPL hydrophobicity change.A transient multiphase model for cold start process is developed considering micro-porous layer (MPL), super-cooled water freezing mechanism and ice formation in cathode channel. The effect of MPL's hydrophobicity on the output performance and ice/water distribution is investigated under various startup temperatures, structural properties, membrane thicknesses and surrounding heat transfer coefficients. Under the maximum power startup mode, it is found that the hydrophobicity disparity of MPL has negligible influences when started from −15 °C, but it strongly affects the overall performance when started from −10 °C, especially after the cell survives the cold start. Decreasing the MPL's hydrophobicity leads to higher current density, meanwhile, it facilitates the super-cooled water's removal, which in turn reduces the ice formation in catalyst layer. However, excessive water accumulation happens if the generated water is hindered from getting into gas diffusion layer (GDL) due to the significant hydrophobicity gap. Weakening the GDL's hydrophobicity contributes to the water removal since the generated water is easier to diffuse out. A thinner membrane benefits the cold start owing to the reduction of ohmic loss and improvement of membrane hydration, and is more sensitive to the hydrophobicity of MPL. Ice formation in cathode channel is identified under various surrounding heat transfer coefficients.

•DNS for diesel injection is conducted with real turbulence inlet profile.•Jet breakup differs from uniform velocities and sinusoidal perturbation injection.•Droplets are detected from long-narrow, flat and curly ligaments along flow.•Surface wave motion, entrainment and shear stress result in jet fragment.•Effects of liquid-gas density ratio and nozzle size are analyzed.Fuel atomization which begins with turbulence perturbation has a significant influence on engine performance. The effect of turbulence on near nozzle field characteristics under various liquid-gas density ratios is performed in present models. First, using three-dimensional DNS (3-D Direct Numerical Simulation) method, a time-varying single-phase fully developed turbulent pipe flow is generated to store time-varying outlet velocity databases which are then mapped as the two-phase jet inlet velocities. After that, the characteristics of near nozzle diesel jet evolution are studied by solving the two-phase turbulent flows and tracking the two-phase interfaces based on DNS and VOF (Volume of Fluid) methods. It shows that with fully developed turbulent inject velocities, the topology of the jet evolves gradually from violent wavy surface, ligaments, to droplet parcels with different dimensions and shapes, which are considered more realistic and different from uniform/sinusoidal perturbation inject velocities, especially in the near nozzle field. Jets surface distortion, stretched and sheared interface are captured. Three different separation processes in different downstream positions are detected. In the initial stage of the jet, long and narrow ligaments act as primary breakup. In the later stage, the whole breakup process evolves more disorderly and more intensively, so that flat and curly ligaments are observed. Also, higher gas densities and narrower nozzle sizes can accelerate the jet break-up process. Furthermore, the present work is reasonably compared with experimental and numerical modeling results.Download high-res image (92KB)Download full-size image

•An analytical model for alkaline anion exchange membrane fuel cell is developed.•Cathode humidification is critical, especially at low operating temperatures.•Proper Liquid humidification for cathode improves the performance.•Increasing operating pressure leads to less cathode water vapor supply.•Decreasing membrane or CL thickness reduces both ohmic and activation losses.An analytical model for hydrogen alkaline anion exchange membrane fuel cell (AAEMFC) is developed in this study. The results show that due to both the electrochemical reaction and electro-osmotic drag, water in cathode is consumed faster than oxygen. Proper liquid humidification in cathode is favorable for performance improvement, especially at low operating temperatures; on the other hand, without liquid humidification, high reactant flow rate is needed. If there is no liquid humidification and the oxygen stoichiometry ratio is fixed, a higher operating pressure increases both the activation loss and ohmic loss, leading to lower cell performance. With the increment of catalyst layer (CL) thickness, the reactant concentration in CL decreases, and the ohmic resistance of electron and ion increases. Decreasing the membrane thickness reduces both the ohmic resistance and activation loss, because more water can transfer from anode to cathode.

•A 3D multiphase model of AAEM fuel cell is developed.•Water is removed in different phases with different anode humidification levels.•Liquid water supply to cathode is needed, especially for high current densities.•Liquid water transport direction is affected by humidification level in cathode.•Decreasing membrane thickness reduces both ohmic and mass transport losses.Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.

A series of sulfonated polyimides (SPIs) were synthesized from a new side group sulfonated diamine, 2,2′-bis(4-sulfobutoxy) benzidine monomer (2,2′-BSBB), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), and common nonsulfonated diamine monomers by polycondensation reaction. Membranes were prepared by casting their m-cresol solutions. The SPI membranes displayed proton conductivity σ values of 0.2–0.4 S cm−1 at 140 °C in liquid water and showed stronger relative humidity (RH) dependence of proton conductivity compared to that of Nafion 112. Most of the SPI membranes were both tough and flexible and thermally stable up to 230–250 °C. XRD spectra suggested the presence of crystalline structure in the membranes. TEM analysis indicated clear microphase separation structure between hydrophobic main chain and hydrophilic side chain. The fully humidified SPI membranes exhibited obvious anisotropic dimensional changes with much larger expansion in thickness than in plane. The long term proton conductivity stability (ca. 500 h) was confirmed at 140 °C. The methanol permeabilities (PM) of the SPI membranes were in the range of 4.0 × 10−7–1.2 × 10−6 cm2/s with a 50 wt% methanol in feed at 30 °C, which were one order of magnitude smaller than that of Nafion 112. As a result, the membranes displayed much larger selectivity towards both high proton conductance and low methanol permeation, compared to Nafion 112, suggesting high potential application for direct methanol fuel cells (DMFCs).Research highlights▶ Sulfonated polyimides with flexible sulfobutoxy side chains. ▶ Polyelectrolyte membranes with high thermal and mechanical stability. ▶ High proton conductivity durable at high temperature (100–140 °C). ▶ Low methanol permeability by one order of magnitude smaller than Nafion.

The flame structure of gasoline engine is complicated and has the characteristic of fractal geometry. A fractal combustion model was used to simulate the engine working cycle. Based on this model, the fractal dimension and laminar flame surface area of turbulent premixed flames were studied under different working conditions. The experimental system mainly includes an optical engine and a set of photography equipment used to shoot the images of turbulent flame of spark-ignition engine. The difference box-counting method was used to process 2D combustion images. In contrast to the experimental results, the computational results show that the fractal combustion model is an effective method of simulating the engine combustion process. The study provides a better understanding for flame structure and its propagation.

Dependent on automatically generated unstructured grids, a comprehensive computational fluid dynamics (CFD)numerical simulation is performed to analyze the influence of nozzle geometry on the internal flow characteristics of a multi-hole diesel injector with the multi-phase flow model based on Eulerian multi-fluid method. The diesel components in nozzle are considered as two continuous phases, diesel liquid and diesel vapor respectively. Considering that both of them are fully coupled and interpenetrated, separate sets of governing equations are established and solved for each phase. The geometric parameters mainly include the length and exit diameter of nozzle, the rounded radius at inlet of nozzle orifice and the angle between axis of injector and axis of nozzle orifice, and they are individually taken into account to analyze the impact on the cavitating flow in nozzle. The results show that the geometrical characteristics of nozzle have a strong influence on the volume fraction of diesel vapor in nozzle and the outlet flow velocity of injector. So cavitation in nozzle orifice should not be neglected for the in-cylinder fuel atomization process, especially for the primary break-up of liquid jet.

•The breakup and atomization characteristics of power law fluid are studied.•The spray patterns of power law fluid can be divided into six types.•The droplet velocity and size are measured by 3-D phase Doppler system.•The droplet axial velocity W is proportional to droplet size.•The particle size distribution fits in the Rosin–Rammler function.To investigate the atomization characteristics of power law fluid, an impingement jet system is developed in this study. High-speed photography and 3-D phase Doppler methods are used to obtain the breakup regimes, 3-D velocities and size distribution of the droplets. The effects of pre-impinging parameters (injection pressure and pre-impingement length), geometry parameters (orifice diameter and impingement angle) and physical parameters (fluid viscosity) on the atomization characteristics are studied. The spray patterns could be qualitatively categorized into six types: open rim without shedding droplet, closed rim, open rim with shedding droplets, rimless sheet, bow shaped ligament and fully developed. With the increment of injection pressure, the droplet velocity of each direction increases, the droplet Sauter mean diameter (SMD) decreases, the non-dimensional mean droplet size (SMD/D) converges to 0.14, and X0/SMD becomes about 1.3–1.5 (D is the orifice diameter, and X0 is the Rosin–Rammler diameter). The axial velocity (W) increases as the impingement angle decreases. Smaller droplets are produced by larger impingement angles, smaller pre-impingement lengths and lower liquid viscosities. The particle size distribution fits in the Rosin–Rammler function.

•The temporal instability of a power law liquid jet is investigated.•A new dispersion relation for the disturbed surface waves is derived.•The breakup mechanisms are different from those of Newtonian liquid jets.•The breakup of jets can be classified into different modes.The temporal instability of a power law liquid jet injected into a static inviscid gas medium is investigated theoretically for axisymmetric disturbances. The corresponding dispersion relation between the growth rate and the wavenumber of disturbed waves is derived after using a linear approximation. It is available for both shear-thinning and shear-thickening liquids. The effects of several dimensionless parameters including the generalized Reynolds number, the Weber number, the density ratio of gas to liquid and the power law exponent, on the instability of the jet are studied. The results reveal that the jet breakup can be classified into Rayleigh Mode and Taylor Mode. And the instability characteristics are different for different modes and power law exponents. For Rayleigh Mode, surface tension promotes the breakup of the jet and the liquid viscosity prevents the jet from breaking up; for Taylor Mode, both surface tension and the viscosity prevent the jets from breaking up, while the interaction between the gas and the liquid significantly promotes the breakup process. Moreover a liquid jet with a smaller power law exponent is easier to disintegrate.

•The VOF model is utilized to simulate the two phase flow in fuel cell anode channel.•The differences of liquid water transport in anode and cathode channel are discussed.•Suggestions on how to improve the liquid water removal in anode channel and advice for channel design are given.In this study, the gas liquid two-phase flow of low-temperature fuel cells is simulated to study the water removal process in the anode channel utilizing VOF (volume of fluid) method. It is found that the water removal process in the cathode channel is easier compared to that in the anode under the same operating condition and inlet velocity. Increasing the humidification rate and contact angle is helpful for water removal. Moreover, further simulations show that in the semicircle-U channel, the water can be removed successfully, but it will be stuck in the corner in the rectangular-U channel, which can be prevented by changing the contact angle of specific walls into extremely hydrophilic or hydrophobic. Extremely hydrophilic wall would make the entire water pave on the wall and smash into little droplets which is not only helpful in water removal and evaporation but also prevents the water from blocking the holes on the Gas Diffusion Layer (GDL). And different hydrophilic/hydrophobic levels also influence the water removal.