Plasma-Assisted Deflagration to Detonation Transition

Abstract

This dissertation examines kinetic enhancement by nanosecond dielectric barrier discharge (ns-DBD) plasma on fuel-lean, dimethyl ether (DME), oxygen (O2), and argon (Ar) premixture during deflagration to detonation transition (DDT) experiments in a microchannel using in-situ diagnostics. Nonequilibrium plasma produces active species and radicals as well as fast and slow heating to promote ignition due to energetic electrons, ions, and electronic and vibrational excitations. Experiments are conducted to examine the influence of the plasma discharge on the premixture and on the resultant DDT.First, high-speed imaging is used to trace the flame front through the DDT process and to observe onset changes as the result of applied ns-DBDs. The applied plasma demonstrates the ability to nonlinearly control DDT onset, and analysis of the velocity time history in comparison with ignition delay time suggests that the fuel’s low temperature chemistry (LTC) is enhanced when the burned gas flow is choked. Next, hybrid fs/ps Coherent Anti-Stokes Raman Scattering (CARS) is used to determine whether the enhancement is due to either rotational or vibrational excitations or due to the production of active species and radicals. It is demonstrated that minimal vibrational and rotational heating is provided by the plasma and that the nonlinear enhancement is due to a competition between the chemical kinetic enhancement via active species production and an excessive partial fuel oxidation which reduces the heat release rate. To better understand the observed kinetic enhancement, laser induced fluorescence is then used to study formaldehyde (CH2O) concentration, which is indicative of DME’s LTC. It is shown that the ns-DBDs are indeed enhancing the fuel’s LTC, specifically in the shock-cluster, which ultimately results in accelerated DDT onset. The electric field strength of the applied plasma is measured with electric field induced second harmonic generation (E-FISH) and modeled for comparison. The modeled results are used to simulate CH2O formation and consumption over time. The comparison between the prediction and experiment of electric field and CH2O time histories shows a good qualitative agreement. Additionally, novel diagnostic techniques to enhance the spatial resolution and signal-to-noise ratio of E-FISH in confined geometries, such as the microchannel, are detailed. Specifically, chirped fs spatially enhanced E-FISH (SEEFISH) demonstrates the ability to eliminate spurious SHG for the first time.