Modelling of Flames Subjected to Strong Electric Fields and Pulsed Plasmas

Bang-shiuh Chen, Purdue University

Abstract

Sustainability and climate change remain existential challenges due to the global dependence on the fossil fuels. Green energy technologies require additional investment and research before replacing conventional fossil fuels. Therefore, new technologies for increasing fossil fuel combustion efficiency and reducing harmful emissions are of critical importance. One such promising technology is plasma-assisted igition (PAI) and combustion (PAC). However, more research on the fundamental mechanisms of plasma-assisted combustion is needed to better understand the interaction between plasmas and flames for system optimization. This dissertation focuses on developing efficient, simplified models for plasma- and electric field-assisted combustion to investigate the effects of different plasma and electric field parameters on plasma-flame interactions. To model electric field-assisted combustion, a one-dimensional (1D) premixed flame subjected to a microwave electric field is considered. An open-source code, Cantera, is modified to solve the conventional conservation equations together with Poisson’s equation to account for the electric force between charged species. To accurately predict flame speed enhancement, non-thermal electrons are considered for both kinetics and mass transport. The results show that it is critical to use the electron energy distribution function (EEDF) for calculating electron recombination rates to improve the predictions of electron number density and flame speed. The effect of changing the electric field strength on the electron number density distribution and Joule heating efficiency is investigated, and it is found that an electric field strength of E = 0.8 kV/cm has the highest efficiency. For modelling plasma-assisted combustion, a nanosecond repetitively pulsed (NRP) plasma discharge is placed in the oxidizer stream of a counter-flow diffusion flame. A computationally efficient model composed of a zero-dimensional (0D) plasma reactor model and an unsteady one-dimensional (1D) flame model is developed. The 0D plasma model incorporates detailed plasma chemistry including electron collision reactions, charge transfer reactions, dissociative reactions of oxygen, and relaxation of vibrational states. The open-source flame code, Ember, is modified to solve the unsteady 1D flame equations using the inlet gas composition from the 0D plasma to calculate extinction strain rate and ignition delay time. The calcu lations show that the extinction strain rate depends more strongly on the pulse repetition frequency (PRF) of the plasma than the flow rate of air through the plasma reactor. Furthermore, using a higher pulse repetition rate reduces the ignition delay, but makes flame ignition less energy efficient. The low dimensional models developed in this work are useful for elucidating the mechanisms involved in plasma-assisted combustion and providing important flame properties for experiments and simulations involving plasma application in realistic combustion systems. The electric field and plasma-assisted combustion models developed here are much more accessible and computationally efficient than complicated multi-dimensional models. Therefore, the models can be readily implemented and modified by researchers in the wider combustion and energy science communities for investigating and developing plasma-based combustion technologies.

Degree

Ph.D.

Advisors

Garner, Purdue University.

Subject Area

Alternative Energy|Chemistry|Sustainability|Climate Change|Electromagnetics|Energy|Fluid mechanics|Mechanical engineering|Mechanics|Physics|Wildlife Conservation

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