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Title:Velocity, temperature, and electric field measurements on a plasma-assisted hydrogen diffusion flame
Author(s):Retter, Jonathan Eric
Director of Research:Elliott, Gregory S
Doctoral Committee Chair(s):Elliott, Gregory S
Doctoral Committee Member(s):Dutton, J. Craig; Freund, Jonathan B; Kearney, Sean P
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
laser diagnostics
Abstract:A plasma-assisted hydrogen diffusion flame, denoted as a coaxial AC DBD burner, was designed and analyzed for comparisons with simulations and to better understand the coupling of the plasma and reacting flow. Emission, PLIF, and schlieren imaging of the burner revealed three distinct operating regimes of the flame, from a reference flickering flame with no applied potential, through a regime of wide flames at moderate potentials from 4-7 kV peak, to the fully collapsed or flattened flame at the highest potentials considered in this work at 9 kV peak voltage all at 18 kHz. Particle image velocimetry measurements revealed the formation of a toroidal vortex within the flame, beginning as an internal toroidal vortex in the wide flame regime, and switching to an outer toroidal vortex in the collapsed flame regime, producing a near counter flow flat diffusion flame on the burner surface that is ventilated by this toroidal vortex. Thermal and species measurements of the flame, by means of Rayleigh or Raman scattering techniques, in different regimes demonstrated a change in the fuel mixing structure and location of the reaction zone due to the position and intensity of the plasma-induced toroidal vortex. Finally, optical electric field measurements, from an electric field induced second harmonic generation technique, revealed the spatial-temporal structure of the horizontal and vertical components of the electric field over the top surface of the burner, thus completing the velocity, temperature, and electric field measurements on this plasma assisted burner. These electric field measurements, coupled with PIV measurements, produced new insight into the body force effect of AC DBD discharges. The electric field measurements revealed period-averaged biases in the electric field as a function of position above the burner surface. Mapping the response of positively charged ions to this period biased electric field showed the ions travelled in the same direction as the bulk flow measured with PIV, suggesting the period-averaged momentum transfer of the ions to the neutral molecules integrated over many AC periods is the leading cause of this induced flow velocity. This was true for both quiescent air operation of the DBD, as well as cold flow operation which resulted in a complete reversal of the bulk flow direction. The specific diagnostic tools utilized to examine the fundamental characteristics of the DBD burner are not new techniques, but the application of them to the complex environment of the burner led to additional developments that allowed for the actual measurement of the quantities of interest. For the PIV measurements, the collapsed flame configuration led to issues with the alumina solid particle seeding momentum dominating the flowfield. This led to utilizing flame synthesis of silica to seed the flame, thus having a self-seeding collapsed flame for velocity measurements. Filtered Rayleigh scattering signals were a combination of species and temperature, thus relying on spontaneous Raman measurements to correct for the species contribution to back out temperatures from Rayleigh scattering. Hybrid fs/ps CARS measurements at long probe delays led to issues with the dephasing of O2 molecules, leading to a new model incorporating finely split Raman transitions to account for the non-trivial decay. Finally, single-shot simultaneous temperature, species, and electric field measurements were demonstrated in a flame for the first time by means of coupled CARS-EFISHG, allowing for corrections to the EFISHG signal to be made for temperature and species concentrations of the measurement region.
Issue Date:2018-11-08
Rights Information:Copyright 2018 Jonathan Eric Retter
Date Available in IDEALS:2019-02-06
Date Deposited:2018-12

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