Papers by Author: Oh Chae Kwon

Abstract: A new burner configuration for a compact fuel-cell reformer with a high-temperature air combustion concept was studied. The burner was computationally designed for a 40 Nm3/hr hydrogen-generating reformer using natural gas-steam reforming method. In order to satisfy the primary requirements for designing a reformer burner (uniform distribution of temperature along the fuel processor walls and minimum heat losses from the reformer), the features of the present burner configuration included 1) a self-regenerative burner for an exhaust-gas-recirculation to apply for the high-temperature air combustion concept, and 2) an annular-type shield for protecting direct contact of flame with the processor walls. For the present design conditions, the predicted temperature distributions along the processor walls were found uniform within 100 K variation. Thus, the present burner configuration satisfied the requirement for reducing temperature gradients along the processor walls, and consequently demonstrated that the high-temperature air combustion concept could be applied to the practical fuel reformers for use of fuel cells. The predicted uniform temperature distributions along the processor walls were experimentally demonstrated for a test burner.

Abstract: Stability limits of premixed microflames were experimentally and computationally studied in order to understand the fundamental behavior of the flames when applied for micropower generation. Single microflames were generated on microtubes with inner diameters of 300-420 μm for methane-air mixtures at temperatures of 298-400 K and atmospheric pressure. For all the microflames at normal temperature, the stability limits were observed in a fuel-rich region, which is different from conventional macroflames exhibiting fuel-lean stability limits. Similar to the macroflames, however, the stability limits of the microflames show C-shaped curves in a tube exit Reynolds number (Re) – fuel equivalence ratio diagram, due to insufficient residence times and heat losses. For elevated temperature that is realistic condition for micropower generation using a heat-recirculation concept, the stability limits were extended toward the fuel-leaner conditions. Numerically predicted structure of microflames near the critical point (that is defined as the fuel-leanest condition among the C-shaped fuel-rich stability limits) showed significant fuel-dilution immediately near the tube exit due to a low Re effect, explaining why the stability limits of microflames are observed only in the fuel-rich region. Microcombustors for micropower generation should be designed to completely consume fuel for better performance.

Abstract: To study nanoparticles generated within the high-density plasma system, it is necessary to know the particle concentration (#/cm3), which is typically measured using laser light scattering of particles trapped inside the plasma. This technique has limitations because particles are localized due to the forces that act on the trapped particles inside the plasma and the localization point varies as the particles grow. Unless spatially averaged particle concentrations are obtained by scanning through the plasma, laser light scattering measurements of particle concentration might represent only the local variation of particle concentration. In this paper, novel method is presented to measure the particle concentration employing TEM measurement results and the simulation of particle transport for calculation of transport efficiency from the plasma region where the particles are generated to the TEM grid. As the particles were collected on the TEM grid after the plasma was extinguished, the simulation includes the effects of Brownian diffusion, aerodynamic drag and gravitational sedimentation but not electrostatic or ion drag force. Simulation results were obtained for particles ranging from 5 to 100 nm. For each particle size, transport efficiencies from 56 different starting positions were evaluated. It was found that transport efficiencies of particles in the 20 to 50 nm diameter range were highest, since these particles tend to follow the gas flow. Sampling efficiencies of particles smaller than this decreased due to Brownian diffusion. For larger particles, sampling efficiencies also decreased, due to gravitational sedimentation. The measured particle concentrations were found to be ~108 #/cm3 and roughly constant over time.

Abstract: Velocity and density distributions of a high-speed and initial CO2 jet flow have been analyzed simultaneously by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Three high-speed cameras have been used for the tomography and the PIV since a shape of a nozzle for the jet flow is asymmetric and the initial flow is fast and unsteady. The speckle movements between no flow and CO2 jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO2 jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method simultaneously.