A Study on the Influence of Reduced Electric Field on Plasma Assisted Combustion Kinetics

Abstract

Plasma assisted combustion (PAC) technology has attracted much attention in the recent years as an innovative technique to improve the combustion efficiency. Plasma can enhance the combustion process mainly through two modes - thermal and kinetic. In the thermal enhancement mode, plasma increases temperature and accelerate chemical reactions which follows the Arrhenius law. In the kinetic mode, plasma generates high energy electrons which in turn enhance the combustion by producing active radicals (e.g. O, H, OH) and electronically and vibrationally excited molecules via electron impact excitation, dissociation, ionization and recombination reactions, the rate of which has a strong exponential dependence on the reduced electric field (E/N). Plasma can facilitate better ignition, improved flame propagation and stabilization in domains where it normally seems to be difficult. PAC has been studied to improve combustion performance of various engines like scramjet engines, I.C. engines, gas turbine engines and new combustors technologies like MILD or lean burn combustion. Sustaining combustion in scramjet engine is one of the major challenges due to the supersonic flow speed and low resistance time. Various thermal plasmas including plasma torch, gliding arc and microwave and nonthermal plasma like DBD, corona and nanosecond plasma discharge have been investigated in the past to control ignition and flame in scramjet combustors. In the last few years several efforts have been made to understand the kinetic enhancement mode by using non-thermal plasma produced using high voltage, short duration and high repetition rate pulses. Recently, nanosecond pulse discharge has been demonstrated to maximize kinetic combustion enhancement due its effective energy loading into dissociative and ionization reactions. Radicals can be produced at such rate that fundamental properties of a fuel like flame speed, ignition threshold temperature and ignition delay can be improved which is not possible through thermal pathways. This thesis presents an experimental study on the effect of non-thermal plasma on methane/air premixed combustion. Combustion parameters like flame speed and low temperature ignition have been studied with respect to various parameters like applied voltage, pulse repetition rate (PRR), reduced electric field (E/N) and total energy input for plasma generation. A coaxial plasma reactor with a dielectric barrier discharge (DBD) was built in house. This versatile reactor could be configured for different plasma discharge gaps by varying the inner electrode diameter such that a wide range of electric fields can be obtained. Voltage upto 30 kV and pulse repetition rate (PRR) from 1 Hz to 3.5 kHz could be varied. Proper EM shielding against possible electromagnetic interference (EMI) was incorporated in the experimental setup. Up to 70% increment in methane flame speed and ignition of upto 0.4 equivalence ratio mixture at low temperature (<500 K) have been observed in the plasma region for specific discharge gaps (D.G.). Applied voltage and discharge current waveforms are captured for input electric energy quantification. An online gas chromatograph (GC) was employed to identify and quantify the products and evaluate the extent of methane conversion for different reactor configurations. The possible reasons for ignition at specific discharge gaps have been proposed. Optical emission spectroscopy for atmospheric air plasma has been done and E/N is estimated using relative irradiance method. The estimated E/N values have been related to the extent of CH4 conversion observed in GC studies and conversion was maximum at 200 Td (± 25 Td) E/N values. Finally, plasma kinetic simulations were done to predict the radicals / active species production rate at different E/N conditions. Computer simulation were done using an open source software package which incorporate BOLSIG+ for EEDF and rate coefficient calculation for electron impact reactions. Results suggest that density of the important radicals (O, O(1D), O(1S), O3, H etc.) per unit power input also follows similar trend as experimental results of low temperature oxidation for various E/N. The results of the study could aid in developing efficient plasma assisted combustors