The formative phase of plasma, referred to here as transient plasma, is under investigation as a technology for ignition of pulse detonation engines and other applications. Previously it was reported that greatly reduced delays to detonation can be achieved with low energy cost. During the past year studies have been carried out to better understand the physical processes responsible for this rapid ignition. We report recent work at USC, and in collaborations with the Naval Postgraduate School and the Air Force Research Laboratory at Wright Patterson Air Force Base. These studies showed the importance of ignition over an array of streamers near and at the anode. Some additional work with application to internal combustion engines is also discussed.

Introduction

The use of non-equilibrium, highly transient plasmas, generated in high E/n conditions (E is the applied electric field, and n is the gas density) and applied during their formative phase, allows radicals and other electronically excited species to be generated over a relatively large volume. Under an earlier ONR program this was demonstrated to be an energy effective means of NOx remediation for engine effluent, and under the current program has been applied effectively to ignition of pulsed detonation engines (PDE) and more recently extended to internal combustion engines. Energetic electrons produce excited species through impact dissociation, excitation, and ionization of background gas molecules in the system. At high pressures a non-equilibrium plasma is normally a corona discharge. Typically this is a low power, weakly luminous discharge that appears on sharp points or edges where field enhancement is occurring. In order to improve the power characteristics of the corona while still retaining its ability to generate radicals throughout a large volume, a pulsed corona methodology is used, which is subsequently referred to as transient plasma. This allows high power discharges that increase the active area of the corona and the number of radicals produced. This is distinct from ongoing experimental and modeling work concerning non-thermal plasmas and their effects on the combustion process, such those generated via microwave discharges. Key for transient plasma is the high electric field associated with the streamer head, which may produce electrons in energies around 10 – 15 eV or more, and which increases the probability of molecular dissociation significantly.
In the past year, experiments have been conducted at

the University of Southern California and in collaboration with researchers at the Naval Postgraduate School, Wright Patterson Air Force Research Laboratory, and Nissan Research Center. In these studies it was observed that TPI significantly reduces delay times in both static and flowing systems. This allows for high repetition rates, high altitude operation, and reduced NOx emissions.

Experimental Results

The experiments demonstrated considerable reduction in delay to detonation, and improvement in repetition rate, while retaining low energy cost.

Each experiments was under varying conditions, and in each case a dramatic reduction in delay was observed. The results suggest a potential solution to one of the most serious limitations to the development of PDEs. Desired pulse amplitude depends on the exact geometry of the combustor, ignition chamber as well as corona electrode. Typical voltages employed are 50 kV – 70 kV for these studies, with pulse energies of the order 100 mJ to 1.16 J depending on how well matched the ignition system is to the load. The ignition system typically consists of a pseudospark based pulse generator, a rapid charger or HV DC supply, and an electrode interface assembly.

Studies of OH production

The focus of this work is to determine the extent the discharge volume is populated with free radicals, specifically the hydroxyl radical (OH), how this affects ignition, if transient plasma truly is a volumetric ignition source and the role of streamers in ignition and flame propagation. The effective generation of charged particles mainly takes place in the active corona volume, which for most applications is typically near the electrode that sees the high electric field. In our study this is the anode (center electrode), and the chamber is overvolted enough to extend the active volume all the way to the chamber wall. Most of the excitation and chemical reactions take place in the active volume. It should be noted that the electrode used in most experiments is a threaded rod. Its surface irregularities create a significant field enhancement, resulting in a much stronger field than a smooth anode. Also, we determine the flame propagation speed upon plasma ignition to discern whether the residue plasma, if any, has an effect on flame propagation.

The experiment consisted of two separate components: a Planar Laser Induced Fluorescence (PLIF) measurement system to probe OH development and production induced by the transient plasma and a high speed imaging component to analyze flame propagation.

In the PLIF measurement, streamer induced OH production was confirmed throughout the discharge volume. It was found that while OH was produced throughout the chamber volume, it drops below the detection threshold of the experiment near 100 µs. In addition, very high levels of OH are present near the anode, where OH emission is seen out to 1 µs. Ignition occurs along the length of the anode around 1 ms and propagates towards the chamber wall with a cylindrically-shaped flame front. This front is seen at t = 5 ms (frame 17) when the flame has propagated into the beam path. It should also be noted that the white haze along the bottom of frames 7-16 is a remnant of the normalization process that appears when the image is brightened and does not represent OH emission.
While ignition near the electrode is expected, as this is the region of highest electric field, the OH decay in the PLIF experiment relative to when ignition occurs seems to indicate that there may be additional effects taking place. There are several possible mechanisms to explain why ignition occurs over the length of the anode. The electric field is the highest near the anode and, thus, the highest density of radicals is found near the anode. It is possible that this high electric field is able to create enough radicals to ignite the mixture along the rod; however, the radical pool is not large enough to stimulate enough heat release for ignition away from the anode. Another potential cause of ignition near the anode is a plasma-induced local increase in the concentration of atomic oxygen in the mixture, where the high currents of the transient plasma remove, at least partially, the electrode’s surface oxide layer. This local generation of oxygen would enhance combustion near the anode. A final postulate is that the plasma discharge likely heats the anode. This heating may assist the combustion reactions by providing the heat needed to assist local ignition along the anode. Further study is needed in order to determine why ignition initiates along the electrode and if ignition of the chamber volume is possible with increasing reduced electric field.

A high speed camera was used to observe flame propagation of an expanding cylindrical wave propagating outward from the anode. Local ignition at the anode was determined to occur ~ 0.9 ms after the streamer induced OH signal decayed. The high speed camera was also used to determine the laminar flame speed of a stoichiometric CH4-air mixture to be 39±13 cm/s, which is consistent with the literature values, and supports the notion that residue plasma, if any, has a little effect on flame propagation.

Conclusions

Transient plasma has consistently shown reductions in ignition delay and increased lean burn capability relative to traditional spark ignition. However, there has been relatively little work attempting to explain the physical mechanism behind these effects. An OH PLIF experiment was performed and calibrated such that OH streamer-induced production was confirmed over the streamer volume at peak OH number densities of ~4x1014 molecules-cm-3. Additionally, it was determined that OH produced by the transient plasma decays to negligible values within 100 µs of the applied pulse.

Transient plasma ignition is on the cutting edge of ignition methodologies for combustion engines. The PDE community in particular is interested in this technology for several reasons: 1) the capability to ignite in a manner that achieves detonation in hydrocarbon-air mixtures (without a previously required oxygen supplement); 2) the extension of the lower flammability limit of mixture and 3) the capability to reduce ignition delay times by factors of 2 to 9 offers comparable potential for increase in repetition rates. For these reasons transient plasma has the potential to overcome the traditional capacitive and inductive spark discharge, and laser discharge ignition techniques.
C. Brophy and J. Sinibaldi are with the Naval Postgraduate School, and C. Carter and M. Ryan are with the WPAFRL at Dayton OH.

Acknowledgment
This work has been supported by the Office of Naval Research.