Yen-Cho Chen

A Direct Numerical Simulation of Early Transition Phenomena in a Buoyancy-Opposed Vertical Channel Flow

The early transition phenomena in a buoyancy-opposed vertical channel flow have been investigated. The spectral method with a weak formulation is employed in the direct numerical simulation to solve the coupled transient 3-D momentum and energy equations. Initial perturbation includes a finite-amplitude 2-D TS wave and a pair of 3-D oblique waves in a K-type disturbance. The results show that after a period of transition time, small-sized vortical structures appear in the central regions of the channel. The subsequent development and breakdown of the vortical structures take place in the central regions of the channel where the shear stress of the laminar base velocity for the buoyancy-opposed flow is significantly larger than that of the isothermal flow. The patterns of the vortical structures in the wall region are almost unchanged during the transition. These transition phenomena are different from those in the buoyancy-assisted and isothermal flows, where the formation and development of the vortical structures initiate from the walls. The simulation results such as the local temperature fluctuations and harmonic energy carried by the different wave modes, are consistent in pointing out a general trend of sudden change from the laminar regime to a different state for the buoyancy-opposed flow, which agrees with the experimental observations obtained for pipe and annulus flows.

Premixed Methanol–Air Combustion Characteristics in a Mini-scale Catalytic Combustor

Abstract Methanol catalytic combustion in a mini-scale tubular quartz-made combustor is investigated in this study. An alumina sphere was employed as the support for the platinum catalyst. The experimental results showed that the combustion can be self-ignited at room temperature. Using the combustor wall temperature to characterize the combustor performance, it was found that the combustion temperature can reach a high value within a short time. The experimental results indicated that the combustor performance depends greatly on the fuel/air supply. A higher temperature can be obtained with a higher fuel/air flow rate. The insulated and non-insulated combustor experimental results indicated that heat loss to the environment is an important factor in governing the combustion characteristics due to the large surface/volume ratio. A higher temperature can also be obtained when the combustor is insulated. Because most of the combustion took place at the combustor entrance region, the experimental result suggested that the combustor length can be shortened, leading to a more compact design allowing the combustor integration with various applications. A simple numerical model was built to provide a greater understanding of the combustion characteristics and examine the heat loss effect on combustor performance.

Temperature and Concentration Simulations in the Methanol Steam Reformer

The methanol steam reforming plays an important role for hydrogen supply to the proton membrane exchange fuel cell in the portable power applications. The catalyst coating on the walls of channels is often used in the fabrication of the reactors in the reformer to minimize the pressure loss. In this study, the temperature and concentration fields in the reactors for the methanol steam reforming were investigated numerically. The methanol conversion is usually used to evaluate the performance of the reformer. The effects of the inlet gas temperature in the heat supply channel and inlet velocity in the reforming channel on the performance of the methanol steam reforming are presented.Copyright

Pre-Reformer Design and Optimization for Solid Oxide Fuel Cells

We have conducted optimization for a flow process consisting typical direct internal reforming Solid Oxide Fuel Cell (SOFC) utilizing syngas with anode off gas recycling. The mass and energy balance analysis for the whole system has been carried out. Mass balance (or molar balance) analysis includes optimization for minimum fuel and oxygen consumption rates corresponding to the temperatures of pre-reformer and SOFC, the steam to carbon ratio inside the pre-reformer, recirculation ratio, and rate of CO2 capture. Studies on the reforming chemical reactions and chemical equilibria are presented. The results include CO2 adsorption in the adsorbent bed as well as recirculation. For the molar balance study, we provided methane consumption rate and overall molar balance. With the energy balance analysis, the temperature distributions in the system are calculated. We have also investigated the total system efficiency based on the first law of thermodynamics. The overall efficiency is defined as the total net power output divided by the lower heating value rate of fuel input. We also provided optimal case design parameters. Thermodynamic efficiency is mainly affected by CO2 adsorption percentage under low steam to carbon ratio region, while efficiency is mainly affected by recirculation rate under high temperature operation. In accordance with our simulation, it is recommended high SOFC temperature, moderate SC ratio, moderate CO2 adsorption and high recirculation operation.Copyright