Sharath Nagaraja

Multi-scale modelling of pulsed nanosecond dielectric barrier plasma discharges in plane-to-plane geometry

An integrated theoretical and numerical framework is developed to study the dynamics of energy coupling, gas heating and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBDs) in air. The work represents one of the first attempts to simulate, in a self-consistent manner, multiple (more than 100) nanosecond pulses. Detailed information is obtained about the electric-field transients during each voltage pulse, and accumulation of plasma generated species and gas heating over ms timescales. The plasma is modelled using a two-temperature, detailed chemistry scheme, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. The analysis is conducted with pressures and pulsing frequency in the range 40‐100Torr and 1‐10 5 Hz, respectively. The input electrical energy is directly proportional to the number density, and remains fairly constant on a per molecule basis from pulse to pulse. Repetitive pulsing results in uniform production of atomic oxygen in the discharge volume via electron-impact dissociation during voltage pulses, and through quenching of excited nitrogen molecules in the afterglow. The ion Joule effect causes rapid gas heating of ∼40K/pulse in the cathode sheath and generates weak acoustic waves. Conductive heat loss to the walls during the time interval between voltage pulses prevents overheating of the cathode layer and development of ionization instabilities. A uniform ‘hat-shaped’ temperature profile develops in the discharge volume after multiple pulses, due to chemical heat release from quenching of excited species. This finding may explain experimentally observed volumetric ignition (as opposed to hot-spot ignition) in fuel‐air mixtures subject to NS DBD. (Some figures may appear in colour only in the online journal)

Nanosecond Pulsed Plasma Activated C2H4/O2/Ar Mixtures in a Flow Reactor

The present work combines numerical and experimental efforts to investigate the effect of nanosecond pulsed plasma discharges on the low-temperature oxidation of C2H4/O2/Ar mixtures under reduced pressure conditions. The nonequilibrium plasma discharge is modeled using a one-dimensional framework, employing separate electron and neutral gas temperatures, and using a detailed plasma and combustion chemical kinetic mechanism. Good agreement is seen between the numerical and experimental results, and both results show that plasma enables low-temperature C2H4 oxidation. Compared to zero-dimensional modeling, the one-dimensional modeling significantly improves predictions, probably because it produces a more complete physical description (including sheath formation and accurate reduced electric field). Furthermore, the one- and zero-dimensional models show very different reaction pathways, using the same chemical kinetic mechanism and thus suggest different interpretations of the experimental results. Two kine...

Parallel On-the-fly Adaptive Kinetics for Non-equilibrium Plasma Discharges of C2H4/O2/Ar Mixture

To enhance the computational efficiency for the simulation of plasma assisted combustion (PAC) models, three new techniques, on-the-fly adaptive kinetics (OAK), point-implicit stiff ODE solver (ODEPIM), and correlated transport (CoTran), are combined together to generate a new simulation framework. This framework is applied to non-equilibrium plasma assisted oxidation of C2H4/O2/Ar mixtures in a low-temperature flow reactor. The new framework has been extensively verified by both temporal evolution and spatial distribution of several key species and gas temperature. Simulation results show that it accelerates the total CPU time by 3.16 times, accelerates the calculation of kinetics by 80 times, and accelerates the calculation of transport properties by 836 times. The high accuracy and performance of the new framework indicates that it has great application potentials to many different areas in the modeling and simulation of plasma assisted combustion.

Multiscale modeling and general theory of non-equilibrium plasma-assisted ignition and combustion

A selfconsistent 1D theoretical framework for plasma assisted ignition and combustion is reviewed. In this framework, a frozen electric field modeling approach is applied to take advantage of the quasiperiodic behaviors of the electrical characteristics to avoid the recalculation of electric field for each pulse. The correlated dynamic adaptive chemistry (CoDAC) method is employed to accelerate the calculation of large and stiff chemical mechanisms. The timestep is updated dynamically during the simulation through a three-stage multitimescale modeling strategy, which takes advantage of the large separation of timescales in nanosecond pulsed plasma discharges. A general theory of plasma assisted ignition and combustion is then proposed. Nanosecond pulsed plasma discharges for ignition and combustion can be divided into four stages. Stage I is the discharge pulse, with timescales of O(1 to 10 ns). In this stage, most input energy is coupled into electron impact excitation and dissociation reactions to generate charged or excited species and radicals. Stage II is the afterglow during the gap between two adjacent pulses, with timescales of O(100 ns). In this stage, quenching of excited species not only further dissociates O2 and fuel molecules, but also provides fast gas heating. Stage III is the remaining gap between pulses, with timescales of O(1 to 100 microsec). The radicals generated during Stages I and II significantly enhance the exothermic reactions in this stage. Stage IV is the accumulative effects of multiple pulses, with timescales of O(1 ms to 1 sec), which include preheated gas temperatures and a large pool of radicals and fuel fragments to trigger ignition. For plasma assisted flames, plasma significantly enhances the radical generation and gas heating in the preheat zone, which could trigger upstream autoignition.

Numerical and Experimental Investigation of Nanosecond-Pulsed Plasma Activated C2H4/O2/Ar Mixtures in a Low Temperature Flow Reactor

The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C2H4/O2/Ar mixtures at 60 Torr pressure. The non-equilibrium plasma discharge is modeled by a two-temperature framework with detailed chemistry-plasma mechanism. The model shows that 75%~77% of input pulse energy was consumed in electron impact dissociation, excitation and ionization reactions, which efficiently produces significant amount of important radical species, fuel fragments and several excited species. The trends of numerical and experimental results agree well. The results from 1D model are compared with 0D model and it show that 1D model in general agrees better with experiments than 0D model. The modeling results reveal that reactions between O(1D) and hydrocarbons are importantly affecting the formation of C2H6, CH2CO, CH2O, CO, CO2, H2O2, H2O, O2(aaΔΔgg) and O2(bbΣΣgg). Due to the persistent relatively high level of O2(aaΔΔgg) and O2(bbΣΣgg), C2H2 converts into HCO directly without the need of going through the intermediate species of HCCO, CH2* and CH2 in the case without plasma. Owing to the long lifetime of O2(aaΔΔgg), this effect can last to 3.1 sec after the finish of all 150 pulses.

Nanosecond Plasma Enhanced Counterflow Diffusion Flames

The present work establishes an integrated theoretical/numerical framework to study the characteristics of plasma enhanced H2-air counterflow diffusion flames. The oxidizer flow is activated by a parallel-plate nanosecond pulsed plasma discharge system with pulse duration ranging between 15 to 130 ns, and pulsing frequency between 5 to 60 kHz. The plasma discharge is modeled using a two-temperature model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. Electron transport and reaction rate coefficients are expressed as functions of mean electron energy, and stored in lookup tables. A large portion of input pulse energy is used in electron impact dissociation and excitation of N2 and O2. Plasma activation significantly increases the flame extinction strain rates, resulting in stable combustion even at lean conditions.

Multi-Scale Modeling of Plasma-Assisted Ignition and Combustion

The effect of temperature on fuel-air ignition and combustion (thermal effects) have been widely studied and well understood. However, a comprehensive understanding of nonequilibrium plasma effects (in situ generation of reactive species and radicals combined with gas heating) on the combustion process is still lacking. Over the past decade, research efforts have advanced our knowledge of electron impact kinetics and low temperature chain branching in fuel-air mixtures considerably. In contrast to numerous experimental investigations, research on modeling and simulation of plasma assisted combustion has received less attention. There is a dire need for development of self-consistent numerical models for construction and validation of plasma chemistry mechanisms. High-fidelity numerical models can be invaluable in exploring the plasma effects on ignition and combustion in turbulent and high-speed flow environments, owing to the difficulty in performing spatially resolved quantitative measurements. In this work, we establish a multi-scale modeling framework to simulate the physical and chemical effects of nonequilibrium, nanosecond plasma discharges on reacting flows. The model is capable of resolving electric field transients and electron impact dynamics in sub-ns timescales, as well as calculating the cumulative effects of multiple discharge pulses over ms timescales. Detailed chemistry mechanisms are incorporated to provide deep insight into the plasma kinetic pathways. The modeling framework is utilized to study ignition of H2-air mixtures subjected to pulsed, nanosecond dielectric barrier discharges in a plane-to-plane geometry. The key kinetic pathways responsible for radicals such as O, H and OH generation from nanosecond discharges over multiple voltage pulses (ns-ms timescales) are quantified. The relative contributions of plasma thermal and kinetic effects in the ignition process are presented. The plasma generated radicals trigger partial fuel oxidation and heat release when the temperature rises above 700 K, after which the process becomes self-sustaining leading to igntion. The ignition kernel growth is primarily due to local plasma chemistry effects rather than flame propagation, and heat transport does not play a significant role. The nanosecond pulse discharge plasma excitation resulted in nearly simultaneous ignition over a large volume, in sharp contrast to hot-spot igniters. Next, the effect of nanosecond pulsed plasma discharges on the ignition characteristics of nC7H16 and air in a plane-to-plane geometry is studied at a reduced pressure of 20.3 kPa. The plasma generated radicals initiate and significantly accelerate the H abstraction reaction from fuel molecules and trigger a “self-accelerating” feedback loop via low-temperature kinetic pathways. Application of only a few discharge pulses at the beginning reduces the initiation time of the first-stage temperature rise by a factor of 10. The plasma effect after the first stage is shown to be predominantly thermal.

Energy Coupling in Repetitively Pulsed Nanosecond Dielectric Barrier Discharges in Plane-to-Plane Geometry

One-dimensional simulations are performed in air to study the dynamics of energy coupling, gas heating and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBD) in plane-to-plane geometry. The plasma is modeled using a twotemperature detailed chemistry model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. The input energy is directly proportional to number density, and remains fairly constant on a per molecule basis from pulse to pulse. At 40 kHz pulsing rate, nearly 75% of the input energy is coupled the vibrational energy mode as compared to 40% of the energy used in vibrational excitation at 1 kHz pulsing rate. As a consequence, we hypothesize that the compression waves generated through fast gas heating in surface nanosecond discharges will be weaker at higher pulsing frequencies because of reduced energy coupling at higher E/N range (above 200 Td). Repetitive pulsing results in uniform production of atomic oxygen in the discharge volume via electron impact dissociation during voltage pulses, and through quenching of excited nitrogen molecules in the afterglow. A uniform “hat shaped” temperature profile develops in the discharge volume after multiple pulses, owing to chemical heat release from quenching of excited species. This may explain recent observations of volumetric ignition of fuel-air mixtures subjected to nanosecond volume discharges. * Graduate Research Assistant, sharath@gatech.edu † William R. T. Oakes Professor and Chair, vigor.yang@aerospace.gatech.edu 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 07 10 January 2013, Grapevine (Dallas/Ft. Worth Region), Texas AIAA 2013-0569 Copyright

Chemical and Transport Effects of Nanosecond Plasma Discharges on H2-Air and C2H4-Air mixtures

*† The effects of pulsed nanosecond plasma discharges on H2-air and C2H4-air mixtures are studied numerically using detailed chemistry. The premixed flow is activated by a parallel-plate nanosecond pulsed plasma discharge system with pulse duration ranging between 15 to 130 ns, and pulsing frequency between 5 to 60 kHz. The plasma discharge is modeled using a two-temperature model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. An energy equation for electrons is solved to obtain electron mean energy self-consistently. Electron transport and reaction rate coefficients are expressed as functions of mean electron energy, and stored in lookup tables. In air, a large portion of input pulse energy is used in electron impact vibrational and electronic excitation of N2, and dissociation of O2. In H2-air mixtures, an additional pathway of H2 dissociation becomes important, and atomic oxygen formation is suppressed. In C2H4-air mixtures, formation of smaller hydrocarbon chains and atomic hydrogen utilizes much of the input pulse energy.