Min Suk Cha

Lifted flame stabilization in developing and developed regions of coflow jets for highly diluted propane

Stabilization of lifted flames has been investigated experimentally for highly diluted propane with nitrogen in coflow. For various fuel mole fractions and jet velocities, three distinct flame types are observed: nozzle attached flames, stationary lifted flames, and oscillating lifted flames. When fuel jet velocity is much smaller than coflow velocity, the base of a nozzle attached flame is observed to have a tribrachial structure. Based on the balance mechanism of the propagation speed of a tribrachial flame with flow velocity, jet velocity is scaled with stoichiometric laminar burning velocity. Results show two distinctive lifted flame stabilization modes—stabilization in the developing region and stabilization in the developed region of jets—depending on the initial fuel mole fraction. It has been found that a lifted flame can be stabilized for a fuel velocity even smaller than the stoichiometric laminar burning velocity. This behavior has been attributed to the buoyancy effect, and flow visualization supports it. Oscillating lifted flames are observed with the frequency range of 3–4.5 Hz by the repetitive action of buoyancy due to increased and decreased burning rates during falling and rising periods of flame oscillation.

Characteristics of Laminar Lifted Flames in a Partially Premixed Jet

Abstract The liftoff and blowout characteristics of laminar flames in a partially premixed jet are studied for propane and n-butane fuels mixed with air. As the flow rate increases, flame lifts off from a nozzle attached flame and liftoff height increases highly nonlinearly with flow rate, and then blowout occurs. The jet velocities at liftoff and at blowout decrease linearly with the increase in air dilution and are independent of nozzle diameter. The liftoff height at blowout is proportional to the square of nozzle diameter and to the square of fuel mass fraction. Correlations for the liftoff heights and blowout conditions are derived by using a cold jet theory for velocity and species concentrations, based on the tribrachial nature of lifted flames. That is, at the flame anchoring point, the flame has the characteristics of a stoichiometric flame and its propagation speed balances axial flow velocity. The experimental findings of liftoff and blowout are successfully predicted.

Effect of electric fields on the liftoff of nonpremixed turbulent jet flames

The effect of electric fields on the liftoff of nonpremixed turbulent jet flames has been investigated by applying high-voltage alternate current (ac) to the nozzle of propane fuel. Flame liftoff velocities and liftoff heights were measured as functions of applied voltage and frequency. The fuel jet velocity at flame liftoff increased and flame liftoff height decreased with increasing voltage, implying that the range of flame stability can be extended with the ac charging. Meanwhile, the effect was minimal when applying direct current (dc). This stabilization effect with ac charging was also influenced by the frequency. As the applied voltage increased, a streamer corona was generated between the flame edge and the nozzle. When the jet velocity and, thus, the liftoff height becomes large, then the ac charging effect disappeared in such a way that the flame liftoff height became comparable to that of a free jet without applying voltage. The liftoff velocity was correlated linearly with voltage in the corona-free electric field enhanced regime.

Characteristics of lifted flames in nonpremixed turbulent confined jets

Characteristics of lifted flames in nonpremixed jets were studied experimentally with emphasis on the effects of the entrained flow field which was varied by placing a plate near the nozzle and by confining the jet. Results show that lifted flame behavior in a confined jet is drastically different from that of a free jet. In the confined jet, the liftoff height is linearly proportional to the nozzle diameter and the flow velocity, while the liftoff height is independent of the nozzle diameter in the free jet. The ratio of the liftoff height at blowout to the nozzle diameter maintains a near-constant value of 50 for both the free and confined jets. The blowout velocity is linearly proportional to the nozzle diameter in the free jet, whereas it is independent of the nozzle diameter in the confined jet. The jet velocity at liftoff maintains a near-constant value for the free jet, while the liftoff velocity decreases with the increase in the nozzle diameter for the confined jet. The blockage effect of the plate near the nozzle exit systematically reduces the liftoff height, and a criterion is proposed to include such an effect in interpreting liftoff behavior.

The reformation of liquid hydrocarbons in an aqueous discharge reactor

We present an aqueous discharge reactor for the reformation of liquid hydrocarbons. To increase a dielectric constant of a liquid medium, we added distilled water to iso-octane and n-dodecane. As expected, we found decreased discharge onset voltage and increased discharge power with increased water content. Results using optical emission spectroscopy identified OH radicals and O atoms as the predominant oxidative reactive species with the addition of water. Enriched CH radicals were also visualized, evidencing the existence of cascade carbon–carbon cleavage and dehydrogenation processes in the aqueous discharge. The gaseous product consisted primarily of hydrogen, carbon monoxide, and unsaturated hydrocarbons. The composition of the product was readily adjustable by varying the volume of water added, which demonstrated a significant difference in composition with respect to the tested liquid hydrocarbon. In this study, we found no presence of CO2 emissions or the contamination of the reactor by solid carbon deposition. These findings offer a new approach to the reforming processes of liquid hydrocarbons and provide a novel concept for the design of a practical and compact plasma reformer.

Premixed Combustion Under Electric Field in a Constant Volume Chamber

The effects of electric fields on outwardly propagating premixed flames in a constant volume chamber were experimentally investigated. An electric plug, subjected to high electrical voltages, was used to generate electric fields inside the chamber. To minimize directional ionic wind effects, alternating current with frequency of 1 kHz was employed. Lean and rich fuel/air mixtures for both methane and propane were tested to investigate various preferential diffusion conditions. As a result, electrically induced instability showing cracked structure on the flame surface could be observed. This cracked structure enhanced flame propagation speed for the initial period of combustion and led to reduction in flame initiation and overall combustion duration times. However, by analyzing pressure data, it was found that overall burning rates are not much affected from the electric field for the pressurized combustion period. The reduction of overall combustion time is less sensitive to equivalence ratio for methane/air mixtures, whereas the results demonstrate pronounced effects on a lean mixture for propane. The improvement of combustion characteristics in lean mixtures will be beneficial to the design of lean burn engines. Two hypothetical mechanisms to explain the electrically induced instability were proposed: 1) ionic wind initiated hydrodynamic instability and 2) thermodiffusive instability through the modification of transport property such as mass diffusivity.

Measurements of Electron Energy by Emission Spectroscopy in Pulsed Corona and Dielectric Barrier Discharges

Abstract Electric field distributions and average electron energies are measured by an optical emission spectroscopic method to investigate streamer characteristics in a pulsed corona discharge (PCD) and a dielectric barrier discharge (DBD) in atmospheric air. In PCD, average electron energies appear to be in the range of 10 ~ 12 eV along the streamers. Time-resolved measurements show that streamers in DBD have a relatively low value of average electron energy of 9 ~ 10 eV. Enhancement of the electron energy is observed when DBD is operated in a non-uniform geometry, such as dielectric barrier with a hole.

Efficient Use of

Carbon dioxide reforming of methane has drawn the attention of many researchers for years, because of both its theoretical yield of hydrogen to carbon monoxide, i.e., H2/CO = 1, and the use of greenhouse gas CO2 . Although catalytic processes have been widely investigated, plasma-based techniques were recently considered, because catalytic processes have problems, such as coking and slow start-up time. However, since there is a lack of concrete information about plasma-induced processes, we systematically tested a dry reforming of methane by using an arc-jet plasma reactor. The effects of supplied power for plasma generation, CO2/CH4 ratio, O2 addition, and the amount of CH4 + CO2 in the reactant were experimentally investigated in nitrogen balance. As a result, the H2/CO ratio of a product can be controlled in the range of 0.8-2.5, and the direction for efficient use of the dry reforming process was proposed. Detailed mechanism and characteristics of the dry reforming of methane were also discussed.

\hbox{CO}_{2}

Here, we present the microscopic physical characteristics of nanosecond discharges with an array of bubbles in distilled water. In particular, applying a single high-voltage pulse, four delayed intensified charge-coupled device cameras successfully visualized four successive images during a single discharge event. We identified three distinctive modes of ignition inside a bubble, depending on the relative location of the bubble with respect to pin-to-hollow needle electrodes when a single bubble was located in an inter-electrode gap of 1 mm: anode-driven ignition, cathode-driven ignition, and co-ignition near both electrodes. Anode- and cathode-driven ignitions evolved into either a complete propagation of the streamer or an incomplete propagation, which were limited in location by proximity to an ignition location, while co-ignitions consistently showed complete propagation. When we increased the gap to 2 mm to accommodate multiple bubbles in the gap, an ignited bubble near the cathode was able to cause the ignition of an upper adjacent bubble. Bubble–bubble interface zones can also be spots of ignition, such that we observed simultaneous co-ignitions in the zones of bubble–bubble interfaces and near electrodes with triple bubbles. We compared the experimental results of discharge propagation with different ignition modes between Ar, He, and N2 bubbles. In addition, numerical simulations for static electric fields reasonably supported observed ignition behavior such that field intensity was locally enhanced.

Reforming of Methane With an Arc-Jet Plasma

A Dielectric barrier discharge (DBD) is a promising candidate to remove carbon tetrafluoride (CF/sub 4/) because the DBD has been successfully used for generating ozone (O/sub 3/) and decomposing nitrogen oxide (NO). A streamer- and glow-mode operations of DBD were carried out in a coaxial cylinder reactor by coupling 10-kHz alternate current (ac) power. The effect of oxygen on the CF/sub 4/ decompositions was investigated and the optimum oxygen concentrations were found. We compared N/sub 2/-streamer, N/sub 2/-glow, and He-glow modes operated in a DBD to investigate the effect of plasma mode on CF/sub 4/ removal. The results showed that the most efficient CF/sub 4/ removal was obtained in He-glow mode, while N/sub 2/-streamer and N/sub 2/-glow modes demonstrated similar performances. With a practical point of view, we investigated an optimized geometrical configuration of streamer discharge. A micro hole-size perforated DBD showed significant increases in CF/sub 4/ removal efficiency compared to the normal N/sub 2/-streamer mode.