Wookyung Kim

Plasma-Discharge Stabilization of Jet Diffusion Flames

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

On the role of oxygen in dielectric barrier discharge actuation of aerodynamic flows

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O2−, play a dominant role in plasma actuation.

Bluff Body Flow Separation Control using Surface Dielectric Barrier Discharges

5 . In this flow regime, the separation point behind the bluff body can be moved downstream in the presence of the AC DBD. However, it is found that, as the flow speed increases, the separation delay effect decreases. The flow alteration effect also decreases with rougher surfaces and higher Rex. Finally, skewed electrodes and reversed electrodes relative to the flow direction are utilized as two different electrode configurations. It is seen that the skewed electrode provides a positive effect on separation delay while the reversed electrode induces earlier flow separation.

Formation and role of cool flames in plasma-assisted premixed combustion

The structure of a plasma-assisted laminar premixed flame is studied numerically. The initial radical yield generated by a nonequilibrium discharge serves as the boundary condition for a one-dimensional flame code predicting the formation of a cool flame which pilots the premixed methane/air combustion. The ignition of the surrounding unactivated methane-air mixture by this cool flame is modeled as an opposed diffusion flame. Our findings indicate that the nonequilibrium discharge is an in situ reformer of the fuel for the production of the cool flame, producing primarily H2 and CO, thus, facilitating the burning of the lean methane-air mixture.

On the quenching of excited electronic states of molecular nitrogen in nanosecond pulsed discharges in atmospheric pressure air

Emission measurements are carried out to study the quenching of excited electronic states of nitrogen, N2∗, in nanosecond pulsed discharges in atmospheric pressure air and nitrogen. The results reveal that ground state N2 quenches N2(C) and N2(B) at rates less than dissociative quenching by ground state O2 by a factor of 4 and 2.5, respectively. Kinetic simulations with the inferred quench rates indicate that the dissociative quenching of N2∗ by O2 is responsible for 82% of atomic oxygen production while electron-impact dissociation of O2 is for 5% under these discharge conditions.

Cross-talk in multiple dielectric barrier discharge actuators

Phase locked particle image velocimetry (PIV) is used to study flow alteration by dielectric barrier discharge actuators mounted on a bluff body. Images from these PIV experiments confirm the importance of negative ions in the actuation process and demonstrate cross-talk interactions between adjacent actuators.

A Study of Plasma-Stabilized Diffusion Flames at Elevated Ambient Temperatures

We report on a study of the use of repetitive ultrashort pulsed plasma discharges in stabilizing a lifted methane jet diffusion flame in elevated temperature (855 K-975 K) vitiated coflow. CH chemiluminescence images are used to record the flame liftoff height, which serves as a measure of the flame stability. The results show that, for the same reduced electric field (E /n), the stabilizing ability of the discharge in the investigated temperature range diminishes with increasing coflow temperature. Based on the results of a zero-dimensional transient chemistry simulation, it is conjectured that the reduced propensity for stabilization at elevated temperature is caused by the rapid depletion of H2 and CO formed in the postplasma gas, under high ambient temperature conditions.

Damköhler Number Similarity for Static Flame Stability in Gaseous-Fueled Augmentor Flows

Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluff-body-type flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damköhler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluff-body stabilized flame at low and high speeds. Flame stability in high-speed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Kármán vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in low-speed test facilities under the conditions of similarity.

A Computational and Experimental Assessment of the Damkohler Number Similarity for Static Flame Stability in Augmentor Flows

In this paper, a numerical and experimental assessment of the Damkohler number sim- ilarity is performed. Three blufi-body stabilized ∞ames are computed using large-eddy simulations (LES) and the ∞amelet progress variable (FPV) model. The vitiated in∞ow and the blufi-body shape mimic the arrangement in an augmentor system. The geometry of the blufi-body is presented in the simulations by an immersed boundary technique on a structured grid. A consistent method for deflning the Damkohler number is provided based on the analysis of the modeled governing equations. The in∞ow velocity and the chemical composition are varied such that the studied ∞ames have the same Damkohler number in all the three simulations. The numerical model is validated by comparison with the experimental PIV data and the reported ∞ame lift-ofi height. For the same charac- teristic Damkohler number and constant momentum ratio between the fuel jet and the vitiated cross ∞ow, the three ∞ames show similar mean features for the recirculation zone and the ∞ame shape. The stability characteristics and the ∞ame dynamics are shown to be controlled by the wake/∞ame interactions. The ∞ow fleld is found to exhibit the well known asymmetric von Karman vortex shedding. The average ∞ame lift-ofi height is found to be similar in the high-speed/fast chemistry case and the low-speed/slow chemistry case. These results suggest the possibility to test augmentor stability features in low-speed test facilities under the conditions of similarity.

The role of local base cavities in an augmentor bluffbody flameholder

This study consists of two parts: liftoff and blowout studies. In the liftoff study, the improvement of liftoff height of bluffbody stabilized partially premixed methane flames and the change of flow field in the recirculation zone of bluffbodies, of variously modified base geometries, are investigated in a high temperature (~ 1300 K) vitiated flow. The basic geometry of the bluffbody consists of a two-dimensional rectangular body with a rounded nose with fuel jets being discharged from the body at several locations upstream of the base. Flame liftoff height measurements are characterized by CH chemiluminescence, while the three-dimensional flow field is determined using stereo particle image velocimetry (PIV). The lowest liftoff height is observed when the geometric modifications from the original rectangular bluffbody base are carried out such that the base has three-dimensional local cavities together with two-dimensionally modified geometries. PIV measurements show that multi-dimensional vortex structures and intense recirculations are induced in the presence of the two and three dimensionally modified base. In the second part of the study, the improvement of the lean blowout limit of bluffbody stabilized methane flames is investigated. The flame configurations consist of fully and partially premixed flames and a hybrid combination of the two. The hybrid flame is produced by injecting methane jets from the streamlined shaped bluffbody into a fully premixed methane/air crossflow. We observe that the blowout limit of the hybrid configuration is extended by up to ~12 % (in terms of the equivalence ratio of the crossflow) with the modified geometries. No noticeable improvement was observed in the fully and partially premixed flame configurations. Gas chromatographic sampling and particle image velocimetry (PIV) show that high fuel mole fraction regions coexist with regions of low speed flow for the modified geometries. Further PIV analysis shows that the downstream flow fields of the modified bases generally have a larger number of incoherent vortices and lower strain rate in comparison with those of the unmodified base.