Xing Rao

Combustion Dynamics of Plasma-Enhanced Premixed and Nonpremixed Flames

Combustion dynamics are investigated for plasma-enhanced methane-air flames in premixed and nonpremixed configurations using a transient arc dc plasmatron. Planar laser-induced fluorescence images of hydroxyl (OH) and carbon monoxide (CO) radicals are obtained over a range of equivalence ratios (φ = 0.7 - 1.3), flow rates (6-18 LPM), and plasma powers (100-900 mA) to monitor radical propagation and in situ fuel reforming. The flow rates presented here are outside the range of normal flame stability. In the nonpremixed mode, the fuel is injected separately as a coflow around the plasma discharge, resulting in a unique two-cone flame front geometry, and the flame stability is mainly dependent on the flow dynamics. For premixed flames, partial oxidation occurs inside the chamber, resulting in higher energy conversion efficiencies, and stability is shown to be sensitive to the combustion chemistry. Both configurations are significantly influenced by in situ fuel reforming at higher plasma powers.

Microwave-Plasma-Coupled Re-Ignition of Methane-and-Oxygen Mixture Under Auto-Ignition Temperature

The re-ignition phenomenon is observed when fuel/oxidizer is re-introduced into an atmospheric-pressure plasma discharge generated by cutting off the gas flow in a re-entrant microwave-plasma applicator system used for plasma-assisted ignition and combustion research works. Results indicate that, for re-ignition to occur, the electric field must be strong enough to fully establish a weakly ionized and self-sustained plasma discharge, and with elevated radical concentrations. The re-ignition was possible at gas flow speeds higher than typical flame propagation rates, and temperature measurements (thermocouple and N2 emission) reveal that re-ignition occurs under auto-ignition temperatures. The high-speed imaging of the flame propagation shows that it is a two step process of initiating a fast pyrolysis flame, which, in turn, stabilizes and starts the direct coupling process of the plasma energy into the flame for full re-ignition to occur.

Nitric Oxide Formation in a Premixed Flame With High-Level Plasma Energy Coupling

This paper presents quantitative planar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO) in a transient-arc direct-current plasmatron igniter using premixed air/fuel mixtures. Quantitative measurements of NO are reported as a function of gas flow rate (20-50 standard cubic feet per hour), plasma power (100-900 mA, 150-750 W), and equivalence ratio (0.7-1.3). Images were corrected for temperature effects by using 2-D temperature field measurements obtained with infrared thermometry and calibrated by a more accurate multiline fitting technique. The signals were then quantified using an NO addition method and spectroscopic laser-induced fluorescence modeling of NO. NO PLIF images and single-point NO concentrations are presented for both plasma-discharge-only and methane/air plasma-enhanced combustion cases. NO formation occurs predominantly through N2(v) + O ? NO + N for the plasma-discharge-only case without combustion. The NO concentration for the plasma-enhanced combustion case (500-3500 ppm) was an order of magnitude less than the plasma-discharge-only case (8000-15 000 ppm) due to the reduction of plasma reactions by the methane. Experiments show the linear decay of NO from equivalence ratio 0.8-1.2 under the same flow condition and discharge current.

Laser Diagnostic Imaging of Energetically Enhanced Flames Using Direct Microwave Plasma Coupling

Quantitative images of temperature and hydroxyl (OH) concentrations are presented in plasma-enhanced flames, where a nonthermal microwave plasma discharge is coupled directly with the reaction zone of the flame. The plasma jet is generated through a novel microwave (2.45 GHz) waveguide based a coaxial reactor system. Planar laser-induced fluorescence is used to generate the OH fields, and planar Rayleigh scattering thermometry is used for the temperature. Plasma-enhanced flames present new possibilities for ignition and flame holding under harsh operating conditions, including stabilization of combustion in hypersonic flame conditions.

Laser Diagnostics of Plasma Enhanced Flames in a Waveguide Microwave Discharge System

An atmospheric high-Q microwave waveguide applicator is used to couple electromagnetic energy into an air flow and directly into the reaction zone of a premixed and non-premixed methane/air flame for energetic enhancement. Absorbed microwave powers range from 60 to 150 W while combustion powers range from 200 to 1000 W. OH number densities and temperature fields were measured using planar laser-induced fluorescence (PLIF) and Rayleigh scattering, respectively. In all conditions, increases in OH radical density and temperature as functions of power are observed. When the plasma energy is coupled into a premixed flame, a much smaller portion of the applied energy goes into heating, with a significant portion allocated to non-thermal effects. For the non-premixed flame, plasma energy is mostly coupled into the air first in terms of heat, which appears to be the main mechanism for flame holding with final flame temperatures exceeding 3000K.

Nitric oxide formation during ignition and combustion of a transient arc plasmatron

Spatially resolved nitric oxide (NO) concentrations in plasma-enhanced flames of a transient arc plasmatron are presented using planar laser-induced fluorescence (PLIF). Quantitative measurements of NO are made by comparing the laser-induced fluorescence (LIF) signal to a well defined flat-flame calibration torch and using a NO addition method based on sequentially increased seeding of NO. For correction of the LIF signal variation, temperature field measurements are made using a combination of IR thermometry and a more accurate multi-line fitting technique of multiple NO transitions. 2-D NO PLIF images and single point NO concentrations are presented for both plasma discharge only and methane/air plasma enhanced combustion. Measurements are reported as a function of gas flow rate (20 to 50 SCFH), plasma power (100 to 900 mA, 150 to 750 watt) and equivalent ratio (0.7 to 1.3). The NO concentration for the plasma coupled combustion (500 to 3500 ppm) case was an order of magnitude less than the plasma discharge only case (8000 to 15000 ppm) due to the reduction of NO from the hydrocarbon chemistry. NO concentration was observed to decrease with less discharge power, decreased flow rate and increased equivalence ratio.

Plasma Enhanced Combustion using Microwave Energy Coupling in a Re-entrant Cavity Applicator

An atmospheric compact high-Q microwave applicator is used to couple electromagnetic energy directly into the reaction zone of a premixed laminar methane-oxygen flame for energetic enhancement. At low microwave powers (1 to 5 W), the flame is influenced by an electromagnetic field only. As power is increased, the reactive gases in the flame break down and ionize into a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) is conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements to monitor excited state species and derive rotational temperatures using OH chemiluminescence. Measurements are made for two equivalence ratios (φ = 0.9 and 1.1) and two gas flow rates (60 sccm and 100 sccm). In the electromagnetic field only phase (1 to 5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH radicals was detected. With the onset of a plasma plume, a significant rise in both excited state species and OH radical number density was observed.

Design of a Superconducting 28 GHz Ion Source Magnet for FRIB Using a Shell-Based Support Structure

The Superconducting Magnet Program at the Lawrence Berkeley National Laboratory (LBNL) is completing the design of a 28 GHz NbTi ion source magnet for the Facility for Rare Isotope Beams (FRIB). The design parameters are based on the parameters of the ECR ion source VENUS in operation at LBNL since 2002 featuring a sextupole-in-solenoids configuration. Whereas most of the magnet components (such as conductor, magnetic design, protection scheme) remain very similar to the VENUS magnet components, the support structure of the FRIB ion source uses a different concept. A shell-based support structure using bladders and keys is implemented in the design allowing fine tuning of the sextupole preload and reversibility of the magnet assembly process. As part of the design work, conductor insulation scheme, coil fabrication processes and assembly procedures are also explored to optimize performance. We present the main features of the design emphasizing the integrated design approach used at LBNL to achieve this result.

Status of ECR ion sources for the Facility for Rare Isotope Beams (FRIB) (invited)

Ahead of the commissioning schedule, installation of the first Electron Cyclotron Resonance (ECR) ion source in the front end area of the Facility for Rare Isotope Beam (FRIB) is planned for the end of 2015. Operating at 14 GHz, this first ECR will be used for the commissioning and initial operation of the facility. In parallel, a superconducting magnet structure compatible with operation at 28 GHz for a new ECR ion source is in development at Lawrence Berkeley National Laboratory. The paper reviews the overall work in progress and development done with ECR ion sources for FRIB.

Correction to “Combustion Dynamics of Plasma-Enhanced Premixed and Nonpremixed Flames” [Dec 10 3265-3271]

In the above titled paper, the funding information is incomplete. The correct acknowledgement is provided here.