Serhat Yesilyurt

Nitrogen front evolution in purged polymer electrolyte membrane fuel cell with dead-ended anode

In this paper, we model and experimentally verify the evolution of liquid water and nitrogen fronts along the length of the anode channel in a proton exchange membrane fuel cell operating with a dead-ended anode that is fed by dry hydrogen. The accumulation of inert nitrogen and liquid water in the anode causes a voltage drop, which is recoverable by purging the anode. Experiments were designed to clarify the effect of N2 blanketing, water plugging of the channels, and flooding of the gas diffusion layer. The observation of each phenomenon is facilitated by simultaneous gas chromatography measurements on samples extracted from the anode channel to measure the nitrogen content and neutron imaging to measure the liquid water distribution. A model of the accumulation is presented, which describes the dynamic evolution of a N2 blanketing front in the anode channel leading to the development of a hydrogen starved region. The prediction of the voltage drop between purge cycles during nonwater plugging channel conditions is shown. The model is capable of describing both the two-sloped behavior of the voltage decay and the time at which the steeper slope begins by capturing the effect of H2 concentration loss and the area of the H2 starved region along the anode channel.

A numerical investigation of the effect of thermoelectromagnetic convection (TEMC) on the Bridgman growth of Ge1−xSix

Thermoelectric currents at the growth interface of GeSi during Bridgman growth are shown to promote convection when a low-intensity axial magnetic field is applied. TEMC, typically, is characterized by a meridional flow driven by the rotation of the fluid; meridional convection alters the composition of the melt, and shape of the growth interface substantially. TEMC effect is more important in micro-gravity environment than the terrestrial one, and can be used to control convection during directional solidification of GeSi. In this work, we report on the numerical simulation of the effect of TEMC on the growth of GeSi.

Surrogates for numerical simulations; optimization of eddy-promoter heat exchangers

Abstract Although the advent of fast and inexpensive parallel computers has rendered numerous previously intractable calculations feasible, many numerical simulations remain too resource-intensive to be directly inserted into engineering optimization efforts. An attractive alternative to direct insertion considers models for computational systems: the expensive simulation is evoked only to construct and validate a simplified input-output model; this simplified input-output model then serves as a simulation surrogate in subsequent engineering optimization studies. We present here a simple “Bayesian-validated” statistical framework for the construction, validation, and purposive application of static computer simulation surrogates. As an example, we consider dissipation-transport optimization of laminar-flow eddy-promoter heat exchangers: parallel spectral element Navier-Stokes calculations serve to construct and validate surrogates for the flowrate and Nusselt number; these surrogates then represent the originating Navier-Stokes equations in the ensuing design process. A validation-based a posteriori error framework serves to quantify the effect of surrogate-for-simulation substitution.

Modeling and Experiments of Voltage Transients of Polymer Electrolyte Membrane Fuel Cells With the Dead-Ended Anode

Operation of PEM fuel cells (PEMFC) with the dead-ended anode (DEA) leads to severe voltage transients due to accumulation of nitrogen, water vapor and liquid water in the anode channels and the gas diffusion layer (GDL). Accumulation of nitrogen causes a large voltage transient with a characteristic profile whereas the amount of water vapor in the anode is limited by the saturation pressure, and the liquid water takes up very small volume at the bottom of the anode channels in the case of downward orientation of the gravity. We present a transient 1D along-the-channel model of PEMFCs operating with periodically-purged DEA channels. In the model, transport of species is modeled by the Maxwell-Stefan equations coupled with constraint equations for the cell voltage. A simple resistance model is used for the permeance of nitrogen and transport of water through the membrane. Simulation results agree very well with experimental results for voltage transients of the PEMFC operating with the DEA. In order to emphasize the effect of nitrogen accumulation in the anode, we present experimentally obtained cell voltage measurements during DEA transients when the cathode is supplied with pure oxygen. In the absence of nitrogen in the cathode, voltage remained almost constant throughout the transient. The model is used to demonstrate the effect of oxygen-to-nitrogen feed ratio in the cathode on the voltage transient behavior for different load currents. Lastly, the effect of small leaks from the anode exit on the voltage transient is studied: even for leak rates as low as 10 ml/h, nitrogen accumulation in the anode channels is alleviated and the cell voltage remained almost constant throughout the transient according to the results.

Sliding mode control based disturbance compensation and external force estimation for a piezoelectric actuator

In this paper a sliding mode algorithm for total disturbance estimation and control of piezoelectric stack actuator is proposed. The disturbance observer is based on the lumped parameters model of mechanical motion so it allows the summary action of nonlinear hysteresis disturbance, external forces acting on the system as well as parameters variation to be estimated. Furthermore, using a nonlinear differential equation the internal hysteresis disturbance is removed from the total disturbance in an attempt to estimate the external force acting on the actuator. It is then possible to use this external force estimate as a means of observer based force control of the actuator. Simulation and experiments are compared for validating the disturbance and external force estimation technique. Experiments that incorporate disturbance compensation in a closed-loop SMC control algorithm are also presented to prove the effectiveness of this method in producing high precision motion.

Magnetically actuated micro swimming of bio-inspired robots in mini channels

Untethered swimming microrobots have many advantages for biomedical applications such as targeted drug delivery, simple surgical tasks including opening of clogged arteries and as diagnostic tools. In this paper, swimming of microrobots is examined in water and glycerin filled channels. Propulsion of microrobots is enabled by means of an external magnetic field that rotates in the axial direction of the channel and forces robots to rotate about the axis of the helical tail. Rotation of the helical tail resulted in a screw-like motion of the robot reaching speeds up to several millimeters per second for a 2-mm long robot. The results are compared with resistive force theory, which is based on the assumption that the propulsive force resulting from the rotation of the helix is proportional to the local velocity on the helical flagellum in low Reynolds number micro and viscous flows.

Sliding-mode based force control of a piezoelectric actuator

In this paper force control of a piezoelectric actuator is presented. Force control based on a force observer is emphasized; however, results with a force sensor are also presented for comparison purposes. With the help of the proposed force observer, sensorless force control and estimation of environmental forces are realized. The force observer is first compared with the force sensor for different step motions to verify the capability of the force estimation. The observer output is then used for feedback of the force information in the closed-loop system. Experiments are made to validate the method.

Bayesian-validated computer-simulation surrogates for optimization and design: error estimates and applications

We present a Bayesian-validated surrogate framework which permits economical and reliable integration of large-scale simulations into engineering design and optimization. In the surrogate approach, the large-scale simulation is evoked only to construct and validate a simplified input-output model; this simplified input-output model then serves as a simulation surrogate in subsequent engineering optimization studies. The distinguishing features of our approach are: sequential statistical sampling procedures which permit both efficient adaptive surrogate construction and rigorous ‘probably-approximately-correct’ surrogate validation; and validation-based non-parametric a posteriori error estimates which precisely quantify the effect of surrogate-for-simulation substitution on system predictability and optimality. In this paper we discuss recent improvements and extensions to our construction-validation algorithms and a posteriori error framework, and present several illustrative applications in heat transfer and fluid mechanics.

Improved Kinematic Models for Two-Link Helical Micro/Nanoswimmers

Accurate prediction of the 3-D trajectories of micro/nanoswimmers is a key element to achieve high precision motion control in therapeutic applications. Rigid-body kinematics of such robotic systems is dominated by viscous forces. The induced flow field around a two-link swimmer is investigated with a validated computational fluid dynamics model. Force-free-swimming constraints are employed in order to simulate motion of bacteria-like swimmers in viscous medium. The fluid resistance exerted on the body of the swimmer is quantified by an improved resistance matrix, which is embedded in a validated resistive force theory model, based on a complex-impedance approach. Parametric studies confirmed that the hydrodynamic interaction between body and tail are of great importance in predicting the trajectories for such systems.

Characterization and Modeling of Biomimetic Untethered Robots Swimming in Viscous Fluids Inside Circular Channels

Miniaturized robots with bioinspired propulsion mechanisms, such as rotating helical flagella, are promising tools for minimally invasive surgery, diagnosis, targeted therapy, drug delivery, and removing material from the human body. Understanding the locomotion of swimmers inside fluid-filled channels is essential for the design and control of miniaturized robots inside arteries and conduits of living organisms. In this paper, we describe scaled-up experiments and modeling of untethered robots with a rotating helical tail and swimming inside a tube filled with a viscous fluid. Experiments mimic low Reynolds number swimming of miniaturized robots inside conduits filled with aqueous solutions. A capsule that contains the battery and a small dc motor is used for the body of the robots. Helical tails with different geometric parameters are manufactured and used to obtain swimming speeds and body rotation rates of robots inside the cylindrical channel. Three-dimensional incompressible flow around the robot inside the channel is governed by Stokes equations, which are solved numerically with a computational fluid dynamics (CFD) model. Predicted velocities of robots are compared with the experimental results for the validation of the CFD model, which is used to analyze effects of the helical radius, pitch, and the radial position of the robot on the swimming speed, forces acting on the robot, and efficiency.