Papers by Author: H.S. Ko

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: Three-dimensional density distributions of an impinging and eccentric flame have been analyzed numerically and experimentally by a combined optical system with a digital speckle tomography. The flame has been ignited by premixed butane/air from air holes and impinged vertically against a plate located at the upper side of the burner nozzle. In order to compare with experimental data, computer synthesized phantoms of impinging and eccentric flames have been derived and reconstructed by a developed three-dimensional multiplicative algebraic reconstruction technique (MART). A new scanning technique has been developed for the analysis of speckle displacements to investigate wall jet regions of the impinging flame including sharp variation of the flow direction and pressure gradient. The reconstructed temperatures by the digital speckle tomography have been compared with a temperature photography by an infrared camera and results of a numerical analysis using a finite element method.