Moussa Tembely

Transient Multiscale Modeling of Aging Mechanisms in a PEFC Cathode

In this paper we propose a mechanistic model of the electrochemical processes in a polymer electrolyte fuel cell cathode, focusing on aging phenomena. The proposed model is based on a nonequilibrium thermodynamics approach previously developed by us, describing the transient response to current perturbations of an electrochemical double layer at the catalyst/Nafion-electrolyte interface. It describes the internal dynamics of the electrochemical double layer, taking into account the coupling between the transport of protons and Pt 2+ in the diffuse layer, as well as the carbon-supported Pt coarsening, the Pt oxidation, the oxygen reduction reaction, and the water dipoles adsorption in the inner layer. This continuous nanoscale interfacial model is coupled with a microscale model of the oxygen transport through the impregnated Nafion layer and is designed to be coupled with a continuous microscale model of electron, proton, and Pt 2+ transports through the membrane-cathode assembly thickness. The model allows analysis of cathodic potential sensitivity to the operating conditions, the initial Pt loading, and to the temporal evolution of the electrochemical activity from aging mechanisms. In particular, the influence of simulated time on the impedance spectra pattern is studied.

The matching of a "one-dimensional" numerical simulation and experiment results for low viscosity Newtonian and non-Newtonian fluids during fast filament stretching and subsequent break-up

This paper develops a model for fast filament stretching, thinning, and break-up for Newtonian and non-Newtonian fluids, and the results are compared against experimental data where fast filament relaxation occurs. A 1D approximation was coupled with the arbitrary Lagrangian Eulerian (ALE) formulation to perform simulations that captured both filament thinning and break-up. The modeling accounts for both the initial polymer stretching processes from the precise movement of the two moving pistons and also the subsequent thinning when the pistons are at rest. The simulations were first validated for a low viscosity Newtonian fluid matched to experimental data obtained from a recently developed apparatus, the Cambridge Trimaster. A non-Newtonian polymer fluid, with high frequency linear viscoelastic behavior characterized using a piezoaxial vibrator rheometer, was modeled using both an Oldroyd-B and FENE-CR single-mode constitutive models. The simulations of the filament deformation were compared with experiment. The simulations showed a generally reasonable agreement with both the stretch and subsequent relaxation experimental responses, although the mono mode models used in this paper were unable to capture all of the details for the experimental time evolution relaxation profile of the central filament diameter.

Shear driven droplet shedding and coalescence on a superhydrophobic surface

The interest on shedding and coalescence of sessile droplets arises from the importance of these phenomena in various scientific problems and industrial applications such as ice formation on wind turbine blades, power lines, nacelles, and aircraft wings. It is shown recently that one of the ways to reduce the probability of ice accretion on industrial components is using superhydrophobic coatings due to their low adhesion to water droplets. In this study, a combined experimental and numerical approach is used to investigate droplet shedding and coalescence phenomena under the influence of air shear flow on a superhydrophobic surface. Droplets with a size of 2 mm are subjected to various air speeds ranging from 5 to 90 m/s. A numerical simulation based on the Volume of Fluid method coupled with the Large Eddy Simulation turbulent model is carried out in conjunction with the validating experiments to shed more light on the coalescence of droplets and detachment phenomena through a detailed analysis of the aerodynamics forces and velocity vectors on the droplet and the streamlines around it. The results indicate a contrast in the mechanism of two-droplet coalescence and subsequent detachment with those related to the case of a single droplet shedding. At lower speeds, the two droplets coalesce by attracting each other with successive rebounds of the merged droplet on the substrate, while at higher speeds, the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. It is shown that coalescence phenomenon assists droplet detachment from the superhydrophobic substrate at lower air speeds.

The matching of polymer solution fast filament stretching, relaxation, and break up experimental results with 1D and 2D numerical viscoelastic simulation

This paper is concerned with the comparison of two numerical viscoelastic strategies for predicting the fast filament stretching, relaxation, and break up of low viscosity, weakly elastic polymeric fluids. Experimental data on stretch, relaxation, and breakup were obtained using a Cambridge Trimaster for a Newtonian solvent (diethyl phthalate) and three monodisperse polystyrene polymer solutions. Two numerical codes were tested to simulate the flow numerically. One code used a one-dimensional approximation coupled with the arbitrary Lagrangian–Eulerian approach and the other a two-dimensional axisymmetric approximation for the flow. In both cases, the same constitutive equations and mono and multimode parameter fitting were used, thereby enabling a direct comparison on both codes and their respective fit to the experimental data. Both simulations fitted the experimental data well and surprisingly the one-dimensional code closely matched that of the two-dimensional. In both cases, it was found necessary to...

Predictive Model of Supercooled Water Droplet Pinning/Repulsion Impacting a Superhydrophobic Surface: The Role of the Gas–Liquid Interface Temperature

Dynamical analysis of an impacting liquid drop on superhydrophobic surfaces is mostly carried out by evaluating the droplet contact time and maximum spreading diameter. In this study, we present a general transient model of the droplet spreading diameter developed from the previously defined mass-spring model for bouncing drops. The effect of viscosity was also considered in the model by definition of a dash-pot term extracted from experiments on various viscous liquid droplets on a superhydrophobic surface. Furthermore, the resultant shear force of the stagnation air flow was also considered with the help of the classical Homann flow approach. It was clearly shown that the proposed model predicts the maximum spreading diameter and droplet contact time very well. On the other hand, where stagnation air flow is present in contradiction to the theoretical model, the droplet contact time was reduced as a function of both droplet Weber numbers and incoming air velocities. Indeed, the reduction in the droplet contact time (e.g., 35% at a droplet Weber number of up to 140) was justified by the presence of a formed thin air layer underneath the impacting drop on the superhydrophobic surface (i.e., full slip condition). Finally, the droplet wetting model was also further developed to account for low temperature through the incorporation of classical nucleation theory. Homogeneous ice nucleation was integrated into the model through the concept of the reduction of the supercooled water drop surface tension as a function of the gas-liquid interface temperature, which was directly correlated with the Nusselt number of incoming air flow. It was shown that the experimental results was qualitatively predicted by the proposed model under all supercooling conditions (i.e., from -10 to -30 °C).

Simulation of coalescence with stratified sampling

We analyze a stratified strategy for numerical integration and for simulation of coalescence. We use random points which are more evenly distributed in the unit cube than usual pseudo-random numbers. They are constructed so that only one point of the set lies in specific sub-intervals of the cube. This property leads to an improved convergence rate for the variance, when they are used for integrating indicator functions. A bound for the variance is proved and assessed through a numerical experiment. We also devise a Monte Carlo algorithm for the simulation of the coagulation equation. We start with an initial population of particles whose sizes are sampled from some initial distribution, and these sizes evolve according to the coalescence dynamics; the random numbers used are the stratified points described above. The results of some numerical experiments show a smaller variance, when compared to a Monte Carlo simulation using plain random samples.

Numerical simulation of the drop size distribution in a spray

Classical methods of modeling predict a steady-state drop size distribution by using empirical or analytical approaches. In the present analysis, we use the maximum of entropy method as an analytical approach for producing the initial data; then we solve the coagulation equation to approximate the evolution of the drop size distribution. This is done by a quasi-Monte Carlo simulation of the conservation form of the equation. We compare the use of pseudo-random and quasi-random numbers in the simulation. It is shown that the proposed method is able to predict experimental phenomena observed during spray generation.

Impact of Micro-Droplets on Superhydrophobic and Hydrophilic Surfaces

Ice accretion is a major threat to all exposed structures such as wind turbines, overhead power cables, offshore structures and aircrafts. Such deposition starts by an impact of water droplets of different sizes on the surface of the exposed structure. This work aims to shed more light on the difference in the dynamics occurring upon the impact of microdroplets on substrates with various wettabilities, hydrophilic (aluminum) and Superhydrophobic (Aluminum + WX2100) surfaces. Experiments are conducted on a wide range of diameters, between cloud sized droplets with diameters ranging down to 20μm, and 10 times larger droplets with a diameter of 250 μm. A comparison in the impact (through deformation) results is made all through the wide range and explained using the two extremes. This is done experimentally by analyzing the maximum spread diameter on the hydrophillic surface and superhydrophobic surface and maximum height as a function of time on the hydrophillic surface. Both parameters are visualized experimentally, simulated numerically for the same impact velocities and then results are compared for verification.© 2014 ASME

Dynamic of Water Droplet Impacting on a Hydrophilic Surface Accompanied With Stagnation Flow

Droplet impact on solid surfaces has been extensively reported in the literature, however the effect of accompanying air flow on the outcome of impacting droplet has yet to be addressed and analyzed which is similar to real scenario of impacting water droplet on aircraft’s leading edge at in-flight icing conditions. This study addresses the net effect of airflow (i.e. stagnation and the resultant shear flow) on the impacting water droplet with the same droplet impact velocity which is exposed to different airspeeds. In order to provide stagnation flow, a droplet accelerator was built which can generate different airspeeds up to 20 m/s. Droplet impact behavior accompanied with stagnation flow on a polished aluminum surface with a contact angle of 70° was investigated by high speed photography. 2.5 mm water droplet size with impact velocities of 2, 2.5 and 3 m/s which correspond to non-splashing regime of impacts are exposed to three different regimes of air speeds namely 0 (i.e. still air case), 10, and 20 m/s. It was observed that when droplet reaches to its maximum spreading diameter, some fingered shape at the end of spreading lamella (i.e. Rayleigh-Taylor instability) is appeared. When stagnation flow is present these fingered shape droplets are exposed to the generated shear flow close to the substrate (i.e. Homann flow approach) causes a droplet break up while complete non-splashing regime is observed in still air case. In spite of the fact that maximum spreading diameter is not largely affected by air flow compare to still air case, droplet height variation is significantly reduced by about 70 percent for strong stagnation flow (i.e. 20 m/s) which generates non-recoiling condition resulting in the thin film formation.Copyright

Droplet Impact and Solidification on Hydrophilic and Superhydrophobic Substrates in Icing Conditions

The present study investigates the detailed visualization and behavior of a spray (e.g. multiple droplets) impinging on hydrophilic, and superhydrophobic aerodynamic shapes in isothermal room and icing conditions. These experiments can provide a fundamental understanding of in-flight icing. A superhydrophobic coating has a very low surface energy so it can be used to counteract ice accumulation. It also reduces the adhesion strength of ice to the surface which ensures easier removal of the ice during flight. The focus of the experiments primarily lies on the fundamental study of spray impact on an airfoil in room and icing conditions. Under such conditions, important icing features such as rivulets and runback flow are observed. This provides us with the basics of ice formation on an aerodynamic surface. The study also focuses on the comparison of ice accumulation on aluminum and superhydrophobic surfaces. All experiments are carried out in a small scaled icing wind tunnel using high speed photography with frame rates ranging from 4000 to 50000 frames per second. The experimental observations show that ice accumulation on a superhydrophobic surface is significantly less than that on a hydrophilic surface.