Jeongjae Hwang

Effect of Particle Loading Ratio and Orifice Exit Velocity on a Particle-Laden Jet

In order to design a shear coaxial injector of solid particles with water, basic experiments on a particle laden jet are necessary. The purpose of the present study is to understand the effect of particle loading ratio on the particle spray characteristics (i.e. spreading angle, distribution of particle number density, velocity profiles, and particle developing region length). Hydro-reactive Al2O3 particles with a primary particle diameter of 35~50 μm are used in this experiment. An automated particle feeder was designed to supply constant particle mass flowrates. Air is used as the carrier gas. To determine the air velocity at the orifice exit, tracers (aluminum oxide, 0.5~2 μm primary diameter) are also supplied by a tracer feeder. A plain orifice type injector with 3 mm diameter, and 20 mm length was adopted. Particle image velocimetry is used to measure the mean and fluctuating velocity components along the axial and radial directions.

NOx Scaling of Syngas H2/CO Turbulent Non-Premixed Jet Flames

NOx emission characteristics and EINOx scaling of hydrogen and H2/CO syngas non-premixed jet flames under turbulent condition are investigated. Flame length and NOx concentration were measured simultaneously for four different syngas compositions (100/0, 75/25, 50/50, and 25/75 H2/CO% vol.) and three different fuel nozzle diameters (2.5, 3.0, and 3.5 mm). The jet flames were in the buoyancy-momentum transition regime. NOx emission is reduced with increased Reynolds number and increased CO content in the syngas fuel, which result from a decreased flame residence time. The previous EINOx scaling correlation, which is based on flame residence time (flame volume divided by volume flow rate of fuel), does not apply to a hydrogen jet flame in the buoyancy-momentum transition region. The previous scaling applies only to momentum-dominated jet flames. A modified scaling based on a simplified flame residence time (flame length divided by fuel jet velocity) considered the buoyancy effect as proposed. This scaling aligns the 1/2-power with the jet flame in both momentum-dominated and transition regions. The modified scaling also satisfies the 1/2 slope with H2/CO syngas jet flames. Furthermore, the proposed modified scaling adopts the nitrogen amount as a reference, instead of the fuel amount. Finally, the modified scaling collapses the all syngas compositions into a single line.

Effect of Acoustic Excitation on Lean Blowoff in Turbulent Premixed Bluff Body Flames

ABSTRACT The lean blowoff characteristics of a premixed air-methane flame were investigated in a ducted combustor with a bluff body according to acoustic excitation. The blowoff equivalence ratio increases with the Reynolds number and changes depending on the extent of the recirculation zone. Using the relation between the Damköhler number and the Reynolds number, it was confirmed that the flow velocity at the downstream tip of the bluff body and the laminar flame speed are decisive blowoff factors. Although a periodic flame hole appeared far from the blowoff only with acoustic excitation, the blowoff observed by OH radical chemiluminescence occurred using a similar process regardless of the excitation. The recirculation zone collapses and the flame becomes small when it is close to the blowoff. Then, the flame is locally extinguished downstream from the bluff body and the recirculation zone completely collapses. Eventually, the unburned gas does not ignite and the flame is extinguished. The blowoff equivalence ratio rapidly increases at specific acoustic excitation frequencies. This was investigated using proper orthogonal decomposition analysis, the two-microphone method, and phase-lock particle imaging velocimetry measurement. Resonance occurs when the excitation frequency approaches the harmonic frequency of the combustor and it increases the velocity fluctuation in the combustor and the infiltration velocity of the unburned gas in the shear layer of the recirculation zone. Consequently, because the burning velocity must have a larger value corresponding to the enhanced mixture velocity for a sustained flame, the blowoff occurs at a higher equivalence ratio.

An Experimental Study on the Flame Dynamics with V- Gutter Type Flameholder in the Model Combustor

The flame dynamics and the longitudinal combustion instability in the duct combustor with the v-gutter type flameholder were investigated experimentally. The combustor has a long duct shape with a cross section area of 40 x 40 mm. The v-gutter type flameholder is mounted at the side wall of combustor. CNG were used as fuel, and the fuel was injected transversely into air crossflow. In order to study flame characteristics related to longitudinal combustion instability, various measurement techniques were applied: direct-photography, dynamic pressure measurement, Schlieren technique, CH and OH radical chemiluminescence and article image velocimetry (PIV). First, it is observed that the unstable flame shows a very different shape compared with a typical flame generated behind the flameholder. This longitudinal combustion instability is affected by the air mass flow rate, the flameholder geometry and nozzle open ratio. Flame dynamics near the flameholder was observed when the longitudinal combustion instability occurs in the duct combustor having a bluff-body flameholder. All images obtained from various imaging techniques shows flame moves back and forth near the flameholder due to the mixture velocity fluctuation near the flameholder.. Therefore, it is considered that it is liable to induce the longitudinal combustion instability in the duct combustor with a bluff-body flameholder. Nomenclature Aduct = duct area Anozzle = nozzle throat area f = frequency ma = air mass flow rate mf = fuel mass flow rate Va = inlet air velocity Vmixture = mixture velocity Φ = equivalence ratio

An Experimental Study on Combustion Instability Characteristics of Various Fuel-Air Mixing Section Geometry in a Model Dump Shape Combustor

The main objective of this study was investigation of natural gas flames in a lean premixed swirl-stabilized dump combustor with an attention focused on the effect of the various fuel-air mixing section geometry on the combustion instability characteristics. The combustor and mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1800 mm in combustor and 470, 550, 870 mm in mixing section. We observed two dominant instability frequencies in this study. Lower frequencies were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were related to secondary longitudinal mode of coupled with the combustor and mixing section. As a result, combustion instability was strongly affected by acoustic characteristics of combustor and mixing section geometry.

Flame dynamic behavior stabilized by a V-gutter type flameholder

Mechanism of combustion frequencies occurring during combustion is experimentally investigated in model combustor with v-gutter flame-holder. This combustor has a long duct shape with a cross section area of 40 x 40 mm. The v-gutter type flameholder with 14mm width is mounted at the bottom of combustor. Kerosene and methane were used as fuel, and these fuel were injected transversely into air cross-flow. It is found that combustion frequencies were considered as 1L longitudinal mode caused by combustor geometry and vortex shedding mode of flame-holder. And fuel phase effect and nozzle effect were also observed in the low frequency range.