Oh Chae Kwon

Dynamic Properties of Outwardly Propagating Spherical Hydrogen-Air Flames at High Temperatures and Pressures

Computational experiments on fundamental unstretched laminar burning velocities and flame response to stretch (represented by the Markstein number) of hydrogen-air flames at high temperatures and pressures were conducted in order to understand the dynamics of the flames including hydrogen as an attractive energy carrier in conditions encountered in practical applications such as internal combustion engines. Outwardly propagating spherical premixed flames were considered for a fuel-equivalence ratio of 0.6, pressures of 5 to 50 atm, and temperatures of 298 to 1000 K. For these conditions, ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), similar to the flames at normal temperature and normal to moderately elevated pressures, implying that the “local conditions” hypothesis can be extended to the practical conditions. Increasing temperatures tended to reduce tendencies toward preferential-diffusion instability behavior (increasing the Markstein number) whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior (decreasing the Markstein number).

Injection Condition Effects of a Pintle Injector for Liquid Rocket Engines on Atomization Performances

Effects of injection conditions on a pintle injector which is proper to recent liquid rocket engines requiring low cost, low weight, high efficiency and reusability were studied. The pintle injector with a typical moving pintle was used for atmospheric experiment using water and air. Injection pressures of water were considered 0.5 and 1.0 bar, 0.1 to 1.0 bar for injection pressures of air and 0.2 to 1.0 mm for pintle opening distance. Sauter mean diameters (SMD) of spray was measured at 50 mm distance from a pintle tip and SMD was treated as a representative parameter in this study. As a result, because of shape characteristics of the pintle injector, there was a transient region between the pintle opening distances of 0.6 and 0.7 mm and this region affected to mass flow rates and SMDs. Also, Reynolds numbers for gas, Weber numbers and momentum ratios were adopted as major non-dimensional paramters and the momentum ratio has strong correlation with SMD.

Analysis of Simultaneous Velocity and Density Distributions for High-Speed CO2 Flow Using Particle Image Velocimetry and Digital Speckle Tomography

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.

Combustion Characteristics of Gaseous CH4/O2 Coaxial Jets in a Model combustor

Recently methane (CH4)/oxygen (O2) bipropellants have been considered as a nextgeneration liquid propellant, due to their advantages such as the eco-friendly and non-toxic properties, low cost and the regenerative cooling capability. Since the reaction process of the propellant in liquid propellant rocket engines involves atomization, droplet vaporization, mixing and turbulent combustion, however, understanding the reaction process and an extensive validation of numerical analysis through fundamental, experimental studies are required for the effective design of liquid rocket engines. In this study, a research model combustor for the combustion experiments of CH4/O2 bipropellant is fabricated, the preliminary tests of gaseous nonpremixed CH4/O2 flames in the combustor with a fully opened nozzle are conducted, and the combustion behaviors of gaseous nonpremixed CH4/O2 flames in the combustor with a nozzle (orifice) are investigated. Results show two distinct flame stability behaviors, i.e., stable, attached flame and oscillating, near-blowout flame, and the cooling effects of combustor walls that are confirmed by the direct flame and OH chemiluminescence images.

Studies on a high-temperature air combustion burner for a compact fuel-cell reformer

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.

Frequency-Equivalence Ratio Correlation Analysis of Methane-Air Premixed Flame Influenced by Ultrasonic Standing Wave (II)

An experimental study was performed for the analysis of frequency-equivalence ratio correlation in the methane-air premixed flame influenced by ultrasonic standing wave. The propagating flame was caught by high-speed Schlieren photography, and the variation of flame-behavior including the flame structure was investigated in detail employing a post-processing analysis of the high-speed images. It was found that a structural variation and propagation-velocity augmentation of the methane-air premixed flame by the intervention of ultrasonic standing wave were more caused off around the stoichiometry. Also, a dependency of the flame behaviors on the driving frequency and equivalence ratio of the reactants was confirmed.

An Ammonia/Hydrogen-Fueled Micro-Thermophotovoltaic Device Integrated with a Micro-reformer

The potential of ammonia (NH3)-hydrogen (H2) blends as a carbon-free fuel in a microthermophotovoltaic (micro-TPV) device integrated with a micro-reformer is evaluated experimentally. A micro-emitter as a thermal heat source is a simple cylinder with an annular-type shield to adopt a heat-recirculation concept and an expanded exhaust outlet to control ignition, which provides stable burning in a small confinement and uniform distribution of temperature along the outer wall. The micro-emitter is surrounded with a micro-reformer for converting NH3 to H2 using ruthenium as a catalyst. The micro-reformer is surrounded by a chamber, the inner and outer walls of which have installations of gallium antimonide (GaSb) photovoltaic cells and cooling fins, respectively. Under optimized design and operating conditions, the micro-TPV device integrated with the micro-reformer shows the emitter efficiency up to 37 % with the maximum temperature of the micro-emitter outer surface up to 1468 K and the conversion rate of NH3 to H2 up to 96.0% in the microreformer. Thus, the feasibility of using NH3-H2 blends in practical micro power and H2 generation devices has been demonstrated, implying the potential of partial NH3 substitution to improve the safety of pure H2 use with no carbon generation.

Simulation Assisted Measurement of Nanoparticle Concentration Generated during High-Density Plasma CVD of Poly-Silicon Films

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.

Stability Limits of Premixed Methane-Air Microflames for Micropower Generation

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.