Date of Award

Spring 2022

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Tanvir I Farouk

Abstract

In recent years, plasma discharge in liquid medium has been a topic of immense interest. Theoretical efforts have been pursued to obtain insight on physicochemical processes being influenced due to trace water vapor either being present as residual or provided at a known concentration. However, studies on discharge at high vapor content are limited. In this study, discharge characteristics of plasma in high water concentration (> 90%) are investigated experimentally for a pressure range of 1 – 15 Torr to maximize vapor loading without condensation. Voltage-current characteristics were obtained over 0- 14 mA of current for each operating pressure; current density was determined to ensure a “normal” glow regime of operation. Spatially resolved optical emission spectroscopy was also conducted to determine OH, O, H2, and H distribution in the interelectrode separation. The normalized intensities of OH and O emission lines are found to be more prominent in the positive column, whereas the emission lines of H are most intense in the cathode glow region. The electric field distribution along the discharge gap was also measured. We envision that the data obtained from this characterization study will provide valuable data for the validation of plasma kinetic schemes associated with water vapor.

In a continuation in the form of an application, a methodology involving plasma optical emission spectroscopy driven by a direct current plasma source is developed to quantify water vapor concentration in a gaseous stream. The experimental setup consists of a dc driven low-pressure plasma cell in which the emission from the plasma discharge is measured by an optical emission spectrometer. The emission from H at 656.2 nm – the first transition in the Balmer series was found to be the most sensitive to the water vapor concentration in the gas stream. Consistent linear trends of the emission signals with respect to variation in concentration of water are observed for multiple combinations of operating parameters. This method has been applied to a vacuum drying process of a mock nuclear fuel assembly to quantify the concentration of water vapor during the drying process.

The third part of this dissertation tried to deduce the effect of water vapor on dc nitrogen plasma stratification. In general, plasma stratification has been studied for more than a century. Despite the many experimental studies reported on this topic, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed kinetics of electrons and vibrationally excited molecules is employed to simulate moderate-pressure (i.e., a few Torr) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model also considers ambipolar diffusion to account for the radial loss of ions and electrons to the wall. The proposed model predicts self-excited standing striations in nitrogen for a range of discharge currents. The impact of electron transport parameters and reaction rates obtained from a solution of local two-term and a multi-term Boltzmann equation on the predictions are assessed. In-depth kinetic analysis indicates that the striations result from the undulations in electron temperature caused due to the interaction between ionization and vibrational reactions. Furthermore, the vibrationally excited molecules associated with the lower energy levels are found to influence nitrogen plasma stratification and the striation pattern strongly. A balance between ionization processes and electron energy transport allows the formation of the observed standing striations. Simulations were conducted for a range of discharge current densities from ~0.018 to 0.080 mA cm-2 , for an operating pressure of 0.7 Torr. Parametric studies show that the striation length increases linearly with increasing tube radius but decreases in a non-linear fashion with increasing discharge current. The predictions from the model are compared against experimental measurements and are found to agree favorably.

The fourth part of this dissertation studies an application, which employs atmospheric pressure dielectric barrier discharge operating in moisture saturated continuous airflow as the discharge medium as a prospective method for bacterial disinfection from soft surfaces. The effect on two different strains of bacteria: E. coli and B. atrophaeus and for three different media for plasma discharge: static, airflow, moisturesaturated airflow was explored. Optical emission spectroscopy showed the generation of OH and reactive nitrogen species in the inter-electrode spacing between the dielectric and substrate for discharge in saturated air. The oxidizing ability of OH and H2O2 is primarily responsible for improved disinfection. The acidity of the agar medium was analyzed after a treatment duration of 25 mins. It was seen that for the case of moisture saturated air as the discharge medium, the pH change was observed for the longest radial distance from the point of influx. Compared to static conditions, the bacterial load reduction efficiency in moisture saturated air was found to be ~1.5 and ~2.5 times higher for E. coli and B. atrophaeus, respectively.

In the final segment of this dissertation, pulsed dielectric barrier discharge in HeH2O and He- H2O-O2 mixture has been studied in near atmospheric conditions using time and spatially resolved photo fragmentation laser-induced fluorescence. The primary goals were to detect and quantify hydroxyl radicals and hydrogen peroxide produced in the 2-D discharge plane between the dielectric and the ground. The gas temperature was also measured via fluorescence spectroscopy of OH (A-X) rotational states and is found to be bounded in the range of 275-300 K. The OH LIF signal is acquired from LIF (using 282 nm laser) whereas LIF from OH generated solely from H2O2 is measured by subtracting the OH LIF signal from the PFLIF signal (using 213 nm+ 282 nm lasers). A known concentration of H2O2 in He was used to calibrate for H2O2 whereas the OH was calibrated with a kinetic model. It is observed that for both gas mixtures, there is a gradual decay of both OH and H2O2 in the afterglow of the discharge. Furthermore, H2O2 has a prolonged existence in the afterglow (> 10 ms) compared to OH radicals, whose fluorescence signals cannot be traced after ~3 ms in the case of pure He. This may indicate that the primary sink route for OH radicals may be recombination reactions, whereas, for H2O2, it is the ambipolar and the convective losses since, unlike OH, H2O2 is not an active free radical. The addition of 5% O2 in the He admixture increases the fluorescence intensity of both OH and H2O2 in the afterglow, signifying the more dissociative recombination reactions involving H, OH, and O radicals produced in the discharge compared to the case without added O2.

Rights

© 2022, Malik Mohammad Tahiyat

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