Suo Yang

Plasma Assisted Low Temperature Combustion

This paper presents recent kinetic and flame studies in plasma assisted low temperature combustion. First, the kinetic pathways of plasma chemistry to enhance low temperature fuel oxidation are discussed. The impacts of plasma chemistry on fuel oxidation pathways at low temperature conditions, substantially enhancing ignition and flame stabilization, are analyzed base on the ignition and extinction S-curve. Secondly, plasma assisted low temperature ignition, direct ignition to flame transition, diffusion cool flames, and premixed cool flames are demonstrated experimentally by using dimethyl ether and n-heptane as fuels. The results show that non-equilibrium plasma is an effective way to accelerate low temperature ignition and fuel oxidation, thus enabling the establishment of stable cool flames at atmospheric pressure. Finally, the experiments from both a non-equilibrium plasma reactor and a photolysis reactor are discussed, in which the direct measurements of intermediate species during the low temperature oxidations of methane/methanol and ethylene are performed, allowing the investigation of modified kinetic pathways by plasma-combustion chemistry interactions. Finally, the validity of kinetic mechanisms for plasma assisted low temperature combustion is investigated. Technical challenges for future research in plasma assisted low temperature combustion are then summarized.

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.

Well-Balanced Central Schemes on Overlapping Cells with Constant Subtraction Techniques for the Saint-Venant Shallow Water System

We develop well-balanced finite-volume central schemes on overlapping cells for the Saint-Venant shallow water system and its variants. The main challenge in deriving the schemes is related to the fact that the Saint-Venant system contains a geometric source term due to nonflat bottom topography and therefore a delicate balance between the flux gradients and source terms has to be preserved. We propose a constant subtraction technique, which helps one to ensure a well-balanced property of the schemes, while maintaining arbitrary high-order of accuracy. Hierarchical reconstruction limiting procedure is applied to eliminate spurious oscillations without using characteristic decomposition. Extensive one- and two-dimensional numerical simulations are conducted to verify the well-balanced property, high-order of accuracy, and non-oscillatory high-resolution for both smooth and nonsmooth solutions.

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.