Yasushi Kakimoto

Investigation of Flooding Phenomena in PEMFC by Two-Phase Flow Numerical Simulation

Water Problem, which is represented by the flooding and the drying in PEMFC, is one of the critical issues relating to the recent trend of high current density operation of it. For spreading the practical use of PEMFC, we must grasp this problem exactly and resolve it. Two-phase flow numerical simulation is useful for this purpose. In this study, we numerically analyzed a unit-PEMFC two-dimensionally considering the two-phase flow based on the multiphase mixture model developed by Cheng and Wang, adding the authors’ equivalent electric circuit. We simulated not only steady state but also unsteady state. A slightly abrupt change in current density distribution appeared on the point where the interface between the one- and the two-phase zone in GDL was appeared. The distribution of water saturation after cell operation start changed with time passed: the liquid water tend to focus on the GDL after once it spread in both GDL and flow channel.Copyright

Natural Convection Heat Transfer in Vertical Fluid Layers of High Aspect Ratio

A two-dimensional numerical analysis is made on the effect of aspect ratio on flow and heat transfer characteristics of natural convection in a slender fluid layer enclosed between two vertical plates of different temperatures. It is shown that the boundary layer instability induces the hook-shaped flows and that its growth disrupts the large-scale circulatory flow into cellular flows of smaller scales, which in part include the renewed boundary layers. Then it is suggested that there exists a transition region where the average heat transfer coefficient increases with the increase of the height of a vertical fluid layer, as is seen in the transition region from laminar to turbulent natural convection along a single vertical plate. Discussion is also made on the nature of instability to yield cellular flows of smaller scales.Copyright

Estimation of Flooding in PEMFC Gas Diffusion Layer by Differential Pressure Measurement

The flooding, especially in gas diffusion layer (GDL), is one of the critical issues to put PEMFC to practical use. However, the experimental data of the flooding in GDL is so insufficient that the optimization design to solve the flooding problem in GDL has not established until now. In this study we show a method to estimate the water saturation, namely the water droplet occupation for unit volume in GDL. We fabricated a simple interdigitated cell where the supply gas is enforced to flow under rib. This structure made it possible to capture the water droplet in GDL with the measurement of differential pressure through the cell. We operated the cell and measured the differential pressure, and estimated the water saturation with assuming that the flow in GDL is Darcy flow and that the GDL can be treated as sphere packed bed. In addition to deferential pressure measurement, we measured the ionic resistance in polymer electrolyte membrane by AC impedance method. We evaluated the effect of the water saturation on the decrease of cell voltage.Copyright

Fluid Flow through a Permeable Porous Obstacle

An experimental and numerical study has been made on forced convection around a porous obstacle concentrating on the effect due to the permeable surface condition. The effects of porous parameter induce a significant change in flow patterns, where the fluid passing through the porous obstacle produces momentum change and suppresses the Karman vortex in a wake region. The Karman vortex disappears with increase of permeability of the porous media and with increase of Reynolds number of the flow. Further it is noted that the drag coefficient of a porous obstacle is reduced compared with that of an impermeable object. The degree of the decrease in the drag coefficient of the porous obstacle depends on its permeability.

Numerical Analysis of Electro-Thermal Phenomena in Metal by Boltzmann Transport Equation for Electron

By the development of micro-fabrication technology, much smaller-size electronic devices will be soon available. In such a smaller device, a non-equilibrium state might appear in metal and/or semiconductor. In this case, it is difficult to estimate the device performance by the macroscopic transport equations that assume quasi-equilibrium distribution. We are developing a numerical simulation based on Boltzmann transport equation (BTE), which can analyze thermal and electric phenomena even when the state is far from equilibrium. In this study, we show a new formulation of BTE for free electron in metal and its calculation result: the thermoelectric power obtained agreed with that of experimental value: the heat flux derived by the non-equilibrium distribution was two-orders small than that estimated by thermal conductivity.Copyright