Jae Wan Park

Experimental investigation on cellular breakup of a planar liquid sheet from an air-blast nozzle

The cellular breakup phenomenon is investigated experimentally for a planar liquid sheet from an air-blast nozzle. The dominant sinuous wave growing spatially downstream forms complicated cellular structures of perforated thin films and surrounding ligaments. Several characteristic parameters are measured from photographic images and compared with linear temporal analysis. The dominant wavelength is proportional to the inverse square of the relative velocity between air and liquid. The estimated breakup time matches the growth time of the most unstable wave, while the breakup length corresponds to a product of breakup time and liquid velocity. Numerical simulation shows a substantially reduced mean effective velocity near flow reattachment region of the air stream. Air turbulence seems to play a major role on initial perturbations of cellular breakup in the given nozzle configuration. The measured spatial growth rates are always less than linear predictions due to deviation from the linear regime at higher amplitudes.

Application of electrokinetic instability for enhanced mixing in various micro-T-channel geometries

We present simple effective micromixer designs utilizing electrokinetic instability (EKI) with downstream cavity structures in a micro-T channel. The amount of free charge increases at each corner of the cavities where inhomogeneous flow stream meets the electric field curved along the concentration gradient. The resulting Coulombic force enhances the instability and leads to a repetitive evolutionary flow pattern from one cavity to the next, yielding a higher mixing efficiency. The mixing efficiency is found to increase by about 15% for the channel with herringbone-shape cavities as compared with the straight channel.

In situ neutron radiography analysis of graphite/NCA lithium-ion battery during overcharge

Overcharge of lithium-ion batteries can lead to the deposition of lithium ions on the surface of graphite electrodes. The phenomenon of lithium deposition causes reduced electrochemical performance and presents safety concerns for lithium-ion batteries in high-power applications. This study presents a technique using neutron radiography (NR) for in situ visualization of the effects of overcharge in a graphite/NCA (LiNi0.8Co0.15Al0.05O2) lithium-ion cell. Patterns of deposition of solid material on the surface of the graphite electrode observed in the radiographs were confirmed by direct observation of the electrode. Inductively coupled plasma mass spectrometry was used to verify the elemental contents of the deposited material. NR is shown to be a promising tool for the study of lithium-ion batteries in high-power applications.

Lattice Boltzmann simulation of the movement of droplets on stripe-patterned surfaces having different wettability

We performed Lattice Boltzmann simulations of droplet movement of patterned surface.Bond number and interparticle strengths are determined for the simulation.The critical angle which droplet starts to move is affected by Bond number and thickness of the stripes. A stripe-patterned surface with different wettability can be effective for the passive control of droplet movement on a vertical surface. We have performed three-dimensional Lattice Boltzmann (LB) simulations to investigate the effect of the pattern characteristics and liquid properties on the droplet movement. The simulation was initiated by imposing gravity on the droplet formed on the surface. The droplet moves along the direction of the pattern when the angle between the gravity and the pattern is small; however, it starts to overrun the stripes when the angle is greater than a certain value, i.e., critical angle. It is shown that the critical angle decreases as the Bond number increases while it increases as the strength of the adhesion/repulsion force increases. The droplet forms a curved asymmetric boundary on the stripe-patterned surface due to gravity and surface forces. The critical angle is also affected by the thickness of the stripes.

Comprehensive Battery Equivalent Circuit Based Model for Battery Management Application

This paper presents a comprehensive control oriented battery model. Described first is an equivalent circuit based battery model which captures particular battery characteristics of control interest. Then, the model categorizes the battery dynamics based on their different time constants (transient, long-term, life-time). This model uses a 2-D map representing the temperature and state-of-charge dependent model parameters. Also, the model uses new battery state-of-charge and state-of-health definitions that are more practical for a real battery management system. Battery testing and simulation on various types of batteries and use scenarios was completed to validate that the model is easy to parameterize, computationally efficient and of adequate accuracy.Copyright

Flow uniformity in and its effect on the performance of polymer electrolyte membrane fuel cell stacks

In this work the effect of flow uniformity on a PEM fuel cell stack performance has been investigated to optimize the stack design. Based on the hydraulic resistance network method, a correlation for the ratio of pressure drop in flow channel, to that in stack manifold, has been derived analytically as a measure of flow uniformity among the cells in the stack. It has been shown that the amount of flow variation can be predicted via the pressure drop ratio, and is found that they are inversely proportional to each other. The results indicate that the output voltage degrades rapidly as the amount of flow variance is increased. Sufficient flow uniformity is crucial to minimize the cell‐to‐cell voltage variation. However, the space available for the manifold is limited on bipolar plate and excessive flow uniformity may result in net performance degradation either due to a reduction in the active cell area or excessive pumping power. Optimization has been carried out based on net output power which is obtained by subtracting the pumping powers for the anode and cathode streams from the stack output power. The effect of minor loss on cell‐to‐cell voltage variation as well as stack output voltage has been investigated and it may become considerable when the number of flow channels per bipolar plate is small.