Jian-Bang Liu

Transient plasma ignition of quiescent and flowing air/fuel mixtures

Transient plasmas that exist during the formative phase of a pulse-ignited atmospheric pressure discharge were studied for application to ignition of quiescent and flowing fuel-air mixtures. Quiescent methane-air mixture ignition was studied as a function of equivalence ratio, and flowing ethane-air mixture was studied in a pulse detonation engine (PDE). The transient plasma was primarily comprised of streamers, which exist during approximately 50 ns prior to the formation of an equilibrated electron energy distribution. Results of significant reduction in delay to ignition and ignition pressure rise time were obtained with energy costs roughly comparable to traditional spark ignition methods (100-800 mJ). Reduction in delay to ignition by factors of typically 3 in quiescent mixes to >4 in a flowing PDE (0.35 kg/s), and other enhancements in performance were obtained. These results, along with a discussion of a pseudospark-based pulse generator that was developed for these applications, will be presented.

Modified Fourier transform method for interferogram fringe pattern analysis

A modified Fourier transform method for interferogram fringe pattern analysis is proposed. While it retains most of the advantages of the Fourier transform method, the new method overcomes some drawbacks of the previous method. It eliminates the assumptions of slowly varying phase variation in the test section and the constant spatial carrier frequency. It also extends the frequency bandwidth and avoids phase distortion caused by discreteness of the sampling frequency. Both numerical simulation and experimental examination are performed to evaluate the performance of the method.

Experimental and numerical study of flame ball IR and UV emissions

Abstract Near-infrared (IR) and ultraviolet (UV) emission profiles of flame balls at microgravity conditions in H 2 -O 2 -diluent mixtures were measured in the JAMIC 10 s drop-tower and compared to numerical simulations and supplemental KC135 aircraft μg experiments. Measured flame ball radii based on images obtained in the JAMIC, KC135, and recent space experiments (IR only) were quite consistent, indicating that radius is a rather robust property of flame balls. The predicted IR radii were always smaller than UV radii, whereas the experiments always showed the opposite behavior. Agreement between measured and predicted flame ball properties was closer for UV radii than IR radii in H 2 -air mixtures but closer for IR radii in H 2 -O 2 -CO 2 mixtures. The large experimental IR radii in H 2 -air tests is particularly difficult to interpret even when uncertainties in chemical and radiation models are considered. Experimental radii would be consistent with a chemiluminescence reaction of the form HO 2 + HO 2 → H 2 O 2 + O 2 producing an excited state of H 2 O 2 , since HO 2 is consumed at large radii through this reaction and its exothermicity is sufficient to create excited states that could emit at the observed wavelengths, however, no appropriate transition of H 2 O 2 ∗ could be identified.© 1998 by The Combustion Institute

Premixed Edge-Flames in Spatially-Varying Straining Flows

Premixed-gas flames subject to steady spatially-varying straining flows were studied to examine one aspect of premixed flames in strongly turbulent flows where strain rate gradients are present and local strain rates may be high enough to cause local flame-front extinguishment. The spatially-varying straining flows were created using a counterflow slot-jet burner with slightly non-parallel jet exits. When the flow configuration was premixed combustible gas vs. cold inert gas, so that only a single flame was produced, steady flame “edges” could be created where the flame would exist in the low-strain region but would be extinguished in the high-strain region. When the flow configuration was premixed gas vs. premixed gas, twin flames would exist in the low-strain region that converged to a corner-like tip in the high-strain region. For both configurations the local strain at the location of the stationary flame edge was somewhat lower than the strain required to extinguish flames in the same mixture subject...

Transient plasma ignition

This paper presents images of transient plasma applied in varied circumstances to fuel ignition and subsequent combustion. The transient plasma occurs during the formative phase of an arc discharge, and persists for /spl ap/50 ns. The results demonstrate that transient plasma ignition initiates combustion efficiently, produces excited species distinct from traditional spark ignition, and that for some applications, including reduced delays to ignition, the approach is advantageous.

Numerical simulation of diluent effects on flame balls

The effects of various diluent gases on steady, source-free spherical premixed flames (“flame balls”) were modeled using a time-dependent numerical code with detailed chemical, transport, and radiation submodels. The diluent gas affects the Lewis number ( Le ) and radiative properties of the flames. Numerical solutions for the steady properties and stability limits were obtained for lean H 2 -air, H 2 −O 2 , H 2 −O 2 −CO 2 , and H 2 −O 2 −SF 6 mixtures. When results were nondimensionalized by the properties at the dynamic stability limit, all results collapsed onto one of two curves, depending only on whether the dominant source of radiative loss is from the product H 2 O, whose concentration decays to zero in the far-field, or diluent gas, whose concentration is constant in the far-field. No significant Le effect was found. Numerical predictions were compared with recent Space Shuttle experiments. For CO 2 and SF 6 diluents, experimental results lie between computational predictions obtained with diluent radiation included and with diluent radiation artificially suppressed, indicating that radiation models including reabsorption effects are needed in these cases. Significant influences of the chemical mechanism were found, even for mechanisms that properly predict the burning velocities of H 2 -air mixtures away from extinction limits. Sensitivity analysis showed that the discrepancies are due mainly to differences in the H+O 2 +H 2 O→HO 2 +H 2 O rate parameters for these mechanisms.

EFFECT OF DISCHARGE ENERGY AND CAVITY GEOMETRY ON FLAME IGNITION BY TRANSIENT PLASMA

Effects of number of ignition site on burning times and comparison between pulsed corona and spark discharges with single ignition site and the same energy show that there are two possible mechanisms— chemical and geometric effects, which contribute to shortening delay and rise times of pulsed corona ignitions, respectively. Burning time shortening has also be demonstrated in a geometrically IC engine like combustion chamber at elevated pressure. Discharge efficiency of pulsed corona discharge is observed much higher than spark discharge.

EFFECT OF FUEL TYPE ON FLAME IGNITION BY TRANSIENT PLASMA DISCHARGES

Rise and delay times of mixtures of methane, propane, n-butane, iso-butane and isooctane mixed with air ignited by transient plasma discharge were investigated and compared with spark discharge ignition. Multi-ignition site effect and high electron energy are suggested to contribute to shortening of rise and delay times.

Corona Discharge Ingnition for Internal Combustion Engines

The use of Corona (Transient Plasma) discharge as an ignition source alternative to spark ignition (SI) for a Natural Gas fired engine presents an opportunity for increased efficiency and reduced NOx emissions. The multiple ignition sites created by a Corona discharge produce a faster and more complete burn than conventional SI. Bench tests show a faster combustion rise time (∼3×) and higher peak combustion pressures with corona discharge compared to SI. There are challenges encountered in retrofitting a corona discharge ignition system onto an IC engine. Electrode design and placement are critical in preventing arc formation and engine knock.Copyright