Kwang Chul Oh

Experiments on the transient effect of evolving jet diffusion flames

The unsteady behavior of diffusion flames was experimentally studied with a jet diffusion flame under a configuration similar to counterflow. We designed and conducted our experiments in such a manner that the fuel was ejected downward so that various time histories of strain rate with good reproducibility were obtained. The fuel jet was ignited by means of a residual flame sustained at the nozzle tip, and the developing process was visualized by both Mie scattering and high-speed shadow imaging. The maximum flame temperature was measured by a compensated thermocouple on the flame tip. A new method was adopted for the time constants. The results show that the history of the strain rate has a significant role in the unsteady behavior of laminar diffusion flames, especially close to flame extinction. Regarding of quenching histories, the extinction strain rates extend as the slope of the strain rate increases, which has not previously been explored experimentally. Even in the non-quenched flame, an unsteady effect is shown based on the fact that there are two maximum flame temperatures at the same strain rate and the one experiencing the higher strain rate is always lower.

The effect of oxygen and carbon dioxide concentration on soot formation in nonpremixed flames using Time Resolved LII technique

The influence of oxygen concentration and CO2 as diluent in oxidizer side on soot characteristics was studied by Laser Induced Incandescence, Time Resolved LII and Transmission Electron Microscopy photography in non-premixed coflowing flames. Through the comparison of TEM photographs and the decay rate of LII signal, suitable two delay times of TIRE-LII method and signal sensitivity (ΔSTIRE-LII/Δ) were determined. The effects of O2 and CO2 as diluent in oxidizer side on soot formation are investigated with these calibrated techniques. The O2+CO2, N2, and [Ar+CO2] mixture in co-flow were used to isolate CO2 effects systematically. The number concentration of primary particle and soot volume fraction abruptly decrease by the addition of CO2 to the co-flow. This suppression is resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of CO2 including thermal dissociation. As the oxygen concentration increases, the number concentration of soot particles at the inception region increases and thus this increase of nucleation enhances the growth of soot particle.

An Experimental Study on the Extinction Limit Extension of Unsteady Counterflow Diffusion Flames

In this study, extinction limit extension of unsteady /air diffusion flames was investigated experimentally. A spatially locked flame in an opposing jet burner was perturbed by linear velocity variation, and time-dependent flame luminosity, transient maximum flame temperature and OH radical were measured over time with the high speed camera, Rayleigh scattering method and OH laser-induced fluorescence, respectively. Unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames, and unsteady extinction limits extend as the slope of the strain rate increases or the initial strain rate decreases. We verified the validity of the equivalent strain rate concept by comparing the course of unsteady extinction process and steady extinction process, and it was found that the equivalent strain rate concept represents well the unsteady effect of a convective-diffusive zone. To investigate the reason of the unsteady extinction limit extension, we subtracted the time lag of the convective-diffusive zone by using the equivalent strain concept. Then the modified unsteady extinction limits become smaller than the original unsteady extinction limits, however, the modified unsteady extinction limits are still larger than the steady extinction limits. These results suggest that there exist the unsteady behavior of a diffusive-reactive zone near the extinction limit due to the chemical non-equilibrium states associated with unsteady flames.