Timothy Ombrello

Kinetic Ignition Enhancement of

Kinetic ignition enhancement of H₂ diffusion flames by a nonequilibrium plasma discharge of H₂- and CH₄-blended oxidizer was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species as well as computations of rates of production and sensitivity analyses were conducted to identify the important kinetic pathways. It was found that the competition between the catalytic effect of NO_x and the inhibitive effects of H₂O and CH₄ governed the ignition processes in the system. With air as the oxidizer, ignition was enhanced from the plasma-produced NO_x. With H₂ addition to the oxidizer, H₂O formation significantly increased the ignition temperature. However, with plasma activation, the inhibitive effect of H₂O was significantly reduced because of the dominant role of NO_x. With CH₄ addition to the oxidizer, the ignition temperatures increased due to the radical quenching by H₂O or CH₄, depending upon the strain rate. The results showed that the inhibitive effects were significantly decreased with plasma activation. Unlike vitiated air ignition, plasma-enhanced ignition for fuel-air mixtures can suppress the inhibitive effects of H₂O and CH₄ because of the overwhelming catalytic NO_x effect at low temperatures.

\hbox{H}_{2}

The effects of inflow distortion on ignition within a cavity based flameholder in a supersonic flow were investigated experimentally. A canonical form of distortion was established by forming a shock with a wedge in a Mach 3 freestream and impinging upstream and over a cavity. The cavity was both actively and passively fueled with injection from the cavity closeout ramp and through an upstream jet along the streamwise centerline, respectively. Two ignition devices were used to provide different forms of energy deposition: a spark discharge and a pulse detonator. The sensitivity of each ignition device was determined by varying the cavity fueling rates. The spark discharge provided a passive means of ignition that was heavily dependent on the flowfield, and therefore was much more sensitive to the effects of distortion. The pulse detonator was more forgiving, allowing for ignition success across a wide range of fueling conditions. The investigation emphasized that careful placement of an energy deposition device, especially a passive one, is required to provide a robust ignition solution across a range of fueling and inflow distortion conditions.

Versus Fuel-Blended Air Diffusion Flames Using Nonequilibrium Plasma

O2(a 1 Δg) were observed for each flame by comparing flame stabilization locations with and without the plasma generated species. Atmospheric pressures were utilized to investigate the effects of O3 and showed up to a 10% enhancement in the flame speed for 1300 ppm of O3 addition to the O2/N2 oxidizer of lifted C3H8 flames. Numerical simulations showed that the O3 decomposition early in the preheat zone of the flame produced O which rapidly reacted with C3H8 to abstract an H, which led to OH production. The subsequent reaction of the OH with fuel fragments produced H2O and other stable species, yielding chemical heat release to enhance the flame speed. The effect of O2(a 1 Δg) was studied at low pressure (27 Torr) and was isolated by adding NO to the plasma afterglow to eliminate O3. For transport times on the order of one second in the presence of NO, the only remaining oxygen species were O2(X 3 Δg) and O2(a 1 Δg). Under these conditions, the enhancement of O2(a 1 Δg) could be studied in isolation, becoming an ideal source for combustion experiments. It was found that O2(a 1 Δg) was a better oxidizer than O2 by significantly enhancing the propagation speed of C2H4 flames. The present experimental results will have a direct impact on the development of elementary reaction rates with O2(a 1 Δg) and O3 at flame conditions to establish detailed plasma-flame kinetic mechanisms.

Effects of Inlet Distortion on Cavity Ignition in Supersonic Flow

The effect of O3 on flame propagation of C2H4-air Hencken flames at sub-atmospheric pressure was investigated through detailed experiments and simulations. The Hencken burner provided an ideal platform to interrogate flame speed enhancement because of the steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and nearly adiabatic properties of the flame, as well as good comparison to simulations. The results showed significant enhancement from O3 with more than an 8% increase in flame speed for 12,500 ppm of O3 at stoichiometric conditions and even more off-stoichiometric. Flame speed enhancement by O3 was also found to increase with increased axial stretch rate, suggesting significant enhancement is possible under highly stretched conditions, such as in turbulent flows. Two-dimensional simulations agreed well with the experiments in terms of flame speed, as well as the trends of enhancement. Rate of production analysis showed that the primary pathways for O3 enhancement were through increased production of OH and O in the pre-heat zone.

Lifted Flame Speed Enhancement by Plasma Excitation of Oxygen

An experimental investigation of the effectiveness of a nanosecond duration repetitivelypulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C2H4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies ( ≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.

Flame Propagation Enhancement of Ethylene by Addition of Ozone

Pulse detonation combustors applied to a supersonic flow were investigated experimentally and numerically for their ability to enhance mixing. The high-pressure and high-temperature plume of a pulse detonation combustor exhausting into a M=2 flow was characterized through high-speed shadowgraphy and NO planar laser induced fluorescence. The pulse detonation combustor plume showed significant penetration into the flow with blow-down times greater than 4 ms and downstream pressures elevated between 1.5 and 2 times the static tunnel pressure for several milliseconds. Planar laser induced fluorescence measurements of NO captured the spanwise structure of the plume and the large counterrotating vortex structure. Numerical simulations of the pulse detonation combustor exhausting into the supersonic flow showed good agreement with shadowgraph and NO PLIF images. Injection upstream of the pulse detonation combustor showed enhanced mixing and indicated that there was an optimal separation distance between the upstream jet and pulse detonation combustor for maximum penetration and mixing with the core supersonic flow.

The Effects of Repetitively Pulsed Nanosecond Discharges on Ignition Time in a Pulsed Detonation Engine

Ignition of an ethylene fueled cavity in a supersonic flow was achieved through the application of two energy deposition techniques: a spark discharge and pulse detonator (PD). High-frequency shadowgraph and chemiluminescence imaging showed that the spark discharge ignition was passive with the ignition kernel and ensuing flame propagation following the cavity flowfield. The PD produced a high-pressure and temperature exhaust that allowed for ignition at lower tunnel temperatures and pressures than the spark discharge, but also caused significant disruption to the cavity flowfield dynamics. Under certain cavity fueling conditions a multiple regime ignition process occurred with the PD that led to decreased cavity burning and at times cavity extinction. Simulations were performed of the PD ignition process, capturing the decreased cavity burning observed in the experiments. The PD exhaust initially ignited and burned the fuel within the cavity rapidly. Simultaneously, the momentary elevated pressure from the detonation caused a blockage of the cavity fuel, starving the cavity until the PD completely exhausted and the flowfield could recover. With sufficiently high cavity fueling, the decrease in burning during the PD ignition process could be mitigated. Cavity fuel injection and entrainment of fuel through the shear layer from upstream injection allowed for the spark discharge ignition process to exhibit similar behavior with peaks and valleys of heat release (but to a lesser extent). The results of using the two energy deposition techniques emphasized the importance of cavity fueling and flowfield dynamics for successful ignition.

Enhanced Mixing in Supersonic Flow Using a Pulse Detonation Combustor

Enhanced mixing and conditioning of a transverse jet in Mach-2 cross flow was investigated through the application of a staged pulse detonation injector. NO planar laser-induced fluorescence and high-frame-rate shadowgraph imaging provided measurements for comparison to CFD modeling of the interaction. The large momentum flux of the pulse detonator led to significant redistribution of the plume from an upstream injectant for several milliseconds while simultaneously elevating its temperatures. The result was a conditioning of the transverse jet plume that could be related to increased reactivity if the upstream injectant was fuel. Typical PD exhaust times would enable potential quasi-steady conditioning of the transverse jet plume if pulsation frequencies on the order of 100 Hz are used.

Cavity Ignition in Supersonic Flow by Pulse Detonation

O2(a 1 Δg), on flame propagation enhancement. Singlet molecular oxygen was produced in a microwave discharge, isolated from O and O3 via NO injection, and was quantified with absorption measurements. The O2(a 1 Δg) was transported through a silica coated burner to steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and near adiabatic H2, CH4, and C2H4 flames. The flame speed enhancement inferred from the change in flame liftoff height showed the elevated levels of enhancement for lean and rich versus stoichiometric mixtures.

Transient Mixing Enhancement of a Transverse Jet in Supersonic Cross Flow Using Pulse Detonation

An investigation of a pulse detonation combustor applied to a supersonic flow for enhanced mixing was performed both experimentally and numerically. Characterization of the high-pressure and high-temperature plume of the pulse detonation combustor through high-speed shadowgraphy showed significant penetration into the core of the M=2 flow with blow-down times greater than 4 ms. The momentary blockage from the PDC elevated the downstream pressures to between 1.5 and 2 times the static tunnel pressure for several milliseconds. Planar laser induced fluorescence measurements of NO captured the spanwise structure of the plume and the large counter-rotating vortex structure. Injection upstream of the pulse detonation combustor showed enhanced mixing and indicated that there was an optimal separation distance between the upstream jet and PDC for maximum penetration and mixing with the core supersonic flow.