Motohiro Saito

Super-mixing combustion enhanced by resonance between micro-shear layer and acoustic excitation

In terms of the application of high-speed and high-load combustion to isothermal-expansion combustion proposed by the authors, flow and reaction control of relatively small diffusion flames ejected from nozzles 0.5 and 1 mm in width are investigated using acoustic resonance; this study is the first step for the mixing control of a micro-diffusion flame which is considered to be a final goal of the relevant studies. In nonreacting jet experiments, Schlieren visualization and flow measurement by a hot-wire anemometer show that marked amplification of turbulence is induced by acoustic excitation with frequencies proportional to the nozzle-exit velocity. Analogously, visual observation, temperature and measurements of concentration for the reacting jet demonstrate that combustion is significantly enhanced under the resonant condition.

Quantitative Evaluation of Transport Properties of SOFC Porous Anode by Random Walk Process

Microstructural properties of an SOFC anode, such as tortuosity, surface-to-volume ratio and permeability, are quantitatively evaluated using three-dimensional data obtained by a system consisting of a focused ion beam and scanning electron microscope, FIB-SEM. A method employed to evaluate these properties is based on a random walk process in which the diffusion of imaginary particles is considered. In addition, the connectivities of three phases, i.e., the Ni, YSZ and pore phases are also analyzed. The relationships among these properties and their effects on transport phenomena in a porous anode are discussed. It is revealed that the low connectivity and complex microstructural configuration of the porous anode result in large tortuosities in the Ni and YSZ phases, respectively.

Quantification of Ni-YSZ Anode Microstructure Based on Dual Beam FIB-SEM Technique

The three-dimensional microstructure of an SOFC anode is quantified using a dual beam focused ion beam scanning electron microscopy (FIB-SEM) system equipped with an energy dispersive X-ray spectroscopy (EDX) unit. The microstructure of the Ni–YSZ anode is virtually reconstructed in a computational field using a series of acquired two-dimensional SEM images. The three-phase boundary (TPB) density and tortuosity factors are carefully evaluated by applying two different evaluation methods to each parameter. The TPB density is estimated by a volume expansion method and a centroid method, while the tortuosity factors are evaluated by a random walk calculation and a lattice Boltzmann method (LBM). Estimates of each parameter obtained by the two methods are in good agreement with each other, thereby validating the reliability of the analysis methods proposed in this study.

Combining structural, electrochemical, and numerical studies to investigate the relation between microstructure and the stack performance

In this study, the static characteristics of a planar solid oxide fuel cell (SOFC) stack with a standard power output of 300 W were investigated. After a power generation experiment, the microstructure parameters of the tested cell were quantified. SOFC has a complex composite structure for both the anode and the cathode. The electrode microstructure is an important factor determining the electrochemical performance of the entire cell and consequently the entire stack of cells. The most precise information about a cell microstructure can be derived from real structural analysis. A method such as a combination of focused ion beam and scanning electron microscope (FIB–SEM) can provide very detailed information about the electrode microstructure. The presented studies provide a detailed microstructure analysis coupled with stack level electrochemical measurements. The obtained microstructure is implemented into numerical simulation and compared with the stack level simulation. This study proves the import role of microstructure in predicting current–voltage characteristic and the power output of a stack. The presented research can also be used as a benchmark for increasing a number of stack numerical simulations.Graphical Abstract

Development of a charge-transfer distribution model for stack simulation of solid oxide fuel cells

An overpotential model for planar solid oxide fuel cells (SOFCs) is developed and applied to a stack numerical simulation. Charge-transfer distribution within the electrodes are approximated using an exponential function, based on which the Ohmic loss and activation overpotential are evaluated. The predicted current-voltage characteristics agree well with the experimental results, and also the overpotentials within the cell can reproduce the results obtained from a numerical analysis where the distribution of the charge-transfer current within the electrodes is fully solved. The proposed model is expected to be useful to maintain the accuracy of SOFC simulations when the cell components, consisting of anode, electrolyte and cathode, are simplified into one layer element.

Quantitative Evaluation of Transport Properties of SOFC Porous Anode and Their Effect on the Power Generation Performance

The three-dimensional microstructure of a solid oxide fuel cell (SOFC) anode is directly observed using a focused ion beam and scanning electron microscope (FIB-SEM) technique. Microstructural parameters, which are closely related to transport phenomena in porous materials, are quantitatively evaluated by a random-walk-based diffusion simulation. Numerical simulation of the SOFC anode with the obtained microstructural parameters is also performed, and the result is in good agreement with the experimental counterparts. Combined with a sensitivity analysis for the SOFC performance, the relationships between the microstructural parameters and the power generation performance are discussed and guidelines for optimizing the anode microstructure are proposed.Copyright

Catalytic partial oxidation of methane on Ni-YSZ cermet anode of solid oxide fuel cells

The effects of oxygen addition to the methane fuels directly supplied to solid oxide fuel cells were investigated. Fundamental experiments were conducted using Ni-YSZ cermet as a typical anode material. The Ni-YSZ catalysts having different streamwise lengths were fabricated on the YSZ flat plates. Premixed gas of methane, oxygen, nitrogen and stream was supplied to the test catalyst set in a rectangular test channel. The exhaust gas compositions and the surface temperature distributions of the test catalyst were measured. It was found that the oxidation of methane prominently proceeded near the upstream edge of the catalyst followed by steam/dry reforming reactions downstream. It resulted in a formation of the high temperature region leading a large temperature gradient in the streamwise direction.

Three-Dimensional Simulation of SOFC Anode Polarization Characteristics Based on Sub-Grid Scale Model

Three-Dimensional Simulation of SOFC Anode Polarization Characteristics Based on Sub-Grid Scale Modeling of Microstructure Author(s) Kishimoto, Masashi; Iwai, Hiroshi; Saito, Motohiro; Yoshida, Hideo Citation Journal of The Electrochemical Society (2012), 159(3): B315B323 Issue Date 2012-01 URL http://hdl.handle.net/2433/154853 Right

Experimental and Numerical Study on Growth of Single Vapor Bubble in Upward Flow Through Vertical Mini-Tube

A slug flow with phase change in a vertical mini-tube is numerically simulated on the basis of a scheme of continuum mechanics. To formulate two-phase flow, the volume of fluid, VOF, method is employed; the advection of the gas-liquid surface is expressed by the piecewise linear interface calculation, PLIC, scheme, while the effect of surface tension is evaluated by the continuum surface force, CSF, model. Since the treatment of liquid film between a bubble and tube wall is crucially important to properly predict both heat transfer and resulting fluid flow in a mini-tube, a semi-empirical approach based on subsidiary knowledge estimated from a preliminary experiment is newly proposed. Further, an additional numerical procedure is introduced to obtain allowable mass conservation even in the thin-film region with intense evaporation. Consequently, by introducing only one parameter, the physical meaning of which is clear, the bubble behavior is reasonably predicted, and its detailed mechanism is clarified.Copyright

Numerical calculation of flow field by SIMPLE method with Excel

齋藤元浩0 (京都大学),吉田英生(京都大学) Numerical calculation of flow field by SIMPLE method with Excel Hiroshi IWAI, Yuki SUKIZAKI, Motohiro SAITO and Hideo YOSHIDA ABSTRACT It is convenient to use Excel spreadsheet for a numerical simulation, because we can get the visual results in real time. In particular, for beginners, the calculation supported with Excel is a good motivator and trainer. In the present study, we use the Excel for a numerical analysis of two-dimensional flow by SIMPLE (Semi-ImpUcit Method for Pressure Linked Equations). The first step is a discretization of basic equations in the. same way with general numerical simulations. Next, the discretized equations are embedded into Excel worksheets in which the iterative calculations are performed. We can apply this method to various shapes of channel easily compared with the conventional numerical calculations.