THREE-DIMENSIONAL SIMULATIONS OF TRANSIENT RESPONSE OF PEM FUEL CELLS

Transients have utmost importance in the lifetime and performance degradation of PEM fuel cells. Recent studies show that cyclic transients can induce hygro-thermal fatigue. In particular, the amount of water in the membrane varies significantly during transients, and determines the ionic conductivity and the structural properties of the membrane. In this work, we present three-dimensional time-dependent simulations and analysis of the transport in PEM fuel cells. U-sections of anode and cathode serpentine flow channels, anode and cathode gas diffusion layers, and the membrane sandwiched between them are modeled using incompressible Navier-Stokes equations in the gas flow channels, Maxwell-Stefan equations in the channels and gas diffusion layers, advection-diffusion-type equation for water transport in the membrane and Ohm’s law for ionic currents in the membrane and electric currents in gas diffusion electrodes. Transient responses to step changes in load, pressure and the relative humidity of the cathode are obtained from simulations, which are conducted by means of a third party finite-element package, COMSOL

Modeling and simulations of deformation and transport in PEM fuel cells

Performance degradation and durability of PEM fuel cells depend strongly upon transport and deformation characteristics of their components especially the polymer membrane. Physical properties of the membrane, such as its ionic conductivity and Young’s modulus depend on its water content, which varies significantly with operating conditions and during transients. Recent studies indicate that cyclic transients may induce hygro-thermal fatigue that leads to the ultimate failure of the membrane shortening its lifetime, and thus, hindering the reliable use PEM fuel cells for automotive applications. In this work, we present two-dimensional simulations and analysis of coupled deformation and transport in PEM fuel cells. A two-dimensional cross-section of anode and cathode gas diffusion layers, and the membrane sandwiched between them is modeled using Maxwell-Stefan equations in gas diffusion layers, Biot’s poroelasticity and Darcy’s law for deformation and water transport in the membrane and Ohm’s law for ionic currents in the membrane and electric currents in the gas diffusion layers. Steady-state deformation and transport of water in the membrane, transient responses to step changes in load and relative humidity of the anode and cathode are obtained from simulation experiments, which are conducted by means of a commercial finite-element package, COMSOL Multiphysics

Simulations of microflows induced by rotation of spirals in microchannels

In microflows where Reynolds number is much smaller than unity, screwing motion of spirals is an effective mechanism of actuation as proven by microorganisms which propel themselves with the rotation of their helical tails. The main focus of this study is to analyze the flow enabled by means of a rotating spiral inside a rectangular channel, and to identify effects of parameters that control the flow, namely, the frequency and amplitude of rotations and the axial span between the helical rounds, which is the wavelength. The time-dependent three-dimensional flow is modeled by Stokes equation subject to continuity in a time-dependent deforming domain due to the rotation of the spiral. Parametric results are compared with asymptotic results presented in literature to describe the flagellar motion of microorganisms

Simulation-based analysis of a biologically-inspired micropump with a rotating spiral inside a microchannel

Microorganisms such as bacteria use their rotating helical flagella for propulsion speeds up to tens of tail lengths per second. The mechanism can be utilized for controlled pumping of liquids in microchannels. In this study, we aim to analyze the effects of control parameters such as axial span between helical rounds (wavelength), angular velocity of rotations (frequency), and the radius of the helix (amplitude) on the maximum timeaveraged flow rate, maximum head, rate of energy transfer, and efficiency of the micropump. The analysis is based on simulations obtained from the three-dimensional timedependent numerical model of the flow induced by the rotating spiral inside a rectangular-prism channel. The flow is governed by Navier-Stokes equations subject to continuity in timevarying domain due to moving boundaries of the spiral. Numerical solutions are obtained using a commercial finiteelement package which uses arbitrary Lagrangian-Eulerian method for mesh deformations. Results are compared with asymptotic results obtained from the resistive-force-theory available in the literature

Validated reduced order models for simulating trajectories of bio-inspired artificial micro-swimmers

Autonomous micro-swimming robots can be utilized to perform specialized procedures such as in vitro or in vivo medical tasks as well as chemical surveillance or micro manipulation. Maneuverability of the robot is one of the requirements that ensure successful completion of its task. In micro fluidic environments, dynamic trajectories of active micro-swimming robots must be predicted reliably and the response of control inputs must be well-understood. In this work, a reduced-order model, which is based on the resistive force theory, is used to predict the transient, coupled rigid body dynamics and hydrodynamic behavior of bio-inspired artificial micro-swimmers. Conceptual design of the micro-swimmer is biologically inspired: it is composed of a body that carries a payload, control and actuation mechanisms, and a long flagellum either such as an inextensible whip like tail-actuator that deforms and propagates sinusoidal planar waves similar to spermatozoa, or of a rotating rigid helix similar to many bacteria, such as E. Coli. In the reduced-order model of the microswimmer, fluid’s resistance to the motion of the body and the tail are computed from resistive force theory, which breaks up the resistance coefficients to local normal and tangential components. Using rotational transformations between a fixed world frame, body frame and the local Frenet-Serret coordinates on the helical tail we obtain the full 6 degrees-of-freedom relationship between the resistive forces and torques and the linear and rotational motions of the swimmer. In the model, only the tail’s frequency (angular velocity for helical tail) is used as a control input in the dynamic equations of the micro-swimming robot. The reduced-order model is validated by means of direct observations of natural micro swimmers presented earlier in the literature and against; results show very good agreement. Three-dimensional, transient CFD simulations of a single degree of freedom swimmer is used to predict resistive force coefficients of a micro-swimmer with a spherical body and flexible tail actuator that uses traveling plane wave deformations for propulsion. Modified coefficients show a very good agreement between the predicted and actual time-dependent swimming speeds, as well as forces and torques along all axes

In-channel experiments on vertical swimming with bacteria-like robots

Bio-inspired micro-robots are of great importance as to implement versatile microsystems for a variety of in vivo and in vitro applications in medicine and biology. Accurate models are necessary to understand the swimming and rigidbody dynamics of such systems. In this study, a series of experiments are conducted with a two-link cm-scale bioinspired robot moving vertically without a tether, in siliconefilled narrow cylindrical glass channels. Swimming velocities are obtained for a set of varying tail and wave geometries, and employed to validate a resistive force theory (RFT) model using modified resistance coefficients based on measured forward velocity and body rotation rates

Numerical simulations of a traveling plane-wave actuator for microfluidic applications

Continuous forming and propagation of large planar deformations on a thin solid elastic film can create propulsion when the film is immersed in a fluid. Microscopic organisms such as spermatozoa use similar mechanisms to propel themselves. In this work, we present a numerical analysis of the effect of traveling plane-wave deformations on an elastic-film actuator within a fluid medium inside a channel. In particular, we analyzed a micropump that consists of a wave actuator, which is placed in a channel to pump the fluid in the direction of the planedeformation waves. The unsteady flow over the moving boundary between the parallel plates has very low Reynolds number, and, hence, is modeled using the two-dimensional time-dependent Stokes equations. The fluid-structure interaction due to moving boundary is modeled with the arbitrary Lagrangian Eulerian (ALE) method incorporating the Winslow smoothing. COMSOL is used to solve two-dimensional timedependent Stokes equations on a deforming mesh, and to carry out simulations of the flow. Effects of the deformation amplitude, wavelength, frequency and channel height on the flow rate are presented

Numerical analysis of the 3D flow induced by propagation of plane-wave deformations on thin membranes inside microchannels

Propulsion mechanisms of microorganisms are based on either beating or screw-like motion of thin elastic biopolymers. Arguably, this motion is optimal for propulsion at very low Reynolds numbers. Similar actuation mechanisms can be utilized in the design of an autonomous microswimmer or even a micropump. In principle, propagation of plane-wave deformations on a thin-membrane placed inside a channel can lead to a net flow in the direction of the wave propagation. In this study we present effects of the amplitude, frequency, and the width of the membrane on the time-averaged flow rate and the rate of work done on the fluid by the membrane by means of threedimensional transient simulations of flows induced by plane-wave deformations on membranes. Navier- Stokes and continuity equations are used to model the flow on a time-varying domain, which is prescribed with respect to the motion of the membrane. Third party commercial software, COMSOL, is used in to solve the finite-element representation of the 3D time-dependent flow on moving mesh. Numerical simulations show that the flow inside the microchannel depend on the square of the amplitude and is proportional to the excitation frequency. Lastly, characteristic flow rate vs. pressure head curve and efficiency of a typical pump are obtained from 3D transient simulations, and presented here

Numerical analysis of a planar wave propagation based micropropulsion system

Micropropulsion mechanisms differ from macro scale counterparts owing to the domination of viscous forces in microflows. In essence, propulsion mechanisms such as cilia and flagella of single celled organisms can be deemed as nature’s solution to a challenging problem, and taken as a basis for the design of an artificial micropropulsion system. In this paper we present numerical analysis of the flow due to oscillatory planar waves propagating on microstrips. The time-dependent three-dimensional flow due to moving boundaries of the strip is governed by incompressible Navier-Stokes equations in a moving coordinate system, which is modeled by means of an arbitrary Lagrangian-Eulerian formulation. The fluid medium surrounding the actuator boundaries is bounded by a channel, and neutral boundary conditions are used in the upstream and downstream. Effects of actuation parameters such as amplitude, excitation frequency, wavelength of the planar waves are demonstrated with numerical simulations that are carried out by third party software, COMSOL. Functional-dependencies with respect to the actuation parameters are obtained for the average velocity of the strip and the efficiency of the mechanism

Experiments on in-channel swimming of an untethered biomimetic robot with different helical tails

Experiments are carried out with a cm-scale bio-mimetic swimming robot, which consists of a body and a rigid helical tail and mimics typical eukaryotic micro organisms, inside circular channels filled with viscous fluids. The body of the robot is made of a cylindrical capsule, which includes an onboard power supply, a dedicated DC-motor, and a driving circuitry with IR-receiver for remote control purposes. In experiments geometric parameters of the helical tail, wavelength and amplitude, and the diameter of the circular channels are varied to understand the effect of those parameters on the swimming speed of the robots. Models, based on slender body theory (SBT) and resistive force theory (RFT), are implemented to predict the swimming speeds, which are then compared with experimentally measured values. A simple model for the DC-motor dynamics is included to account for the contact friction effects on the body rotation rates. Model results agree reasonably well with experimental measurements