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.
Deep Dive into Multiscale modeling strategy 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 dissociatio
the remaining gap between pulses, with timescales of O(1-100 ฮผs). 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 -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 pre-heat zone, which could trigger upstream auto-ignition.
Keywords: plasma assisted combustion, plasma fluid modeling, nanosecond plasma discharge, low temperature chemistry, ignition.
๐ถ ๐,๐ = specific heat at constant pressure of the k-th species (Jโขkg -1 โขK -1 ) ๐ท ๐ = effective diffusion coefficient of the k-th species (cm 2 โขs -1 )
๐ท๐๐ ๐ = decay rate of the k-th species (kgโขcm -3 โขs -1 )
= electric field (Vโขcm -1 ) ๐ธ = total energy (Jโขkg -1 )
๐ฌ/๐ = reduced electric field (Td) ๐ ๐ = number density of the ๐-th species (cm -3 )
๐ ๐ = electron energy density (eVโขcm -3 ) ๐ +,-= sum of number densities of positive and negative ions, respectively (cm -3 )
๐ ๐ ฬ = production rate of electron energy density (eVโขcm -3 โขs -1 )
๐ ฬ๐ฝ๐ป = energy release rate from Joule heating (kgโขcm -1 โขs -3 )
๐ ๐ = energy flux from heat conduction and diffusion (kgโขs -3 )
๐ ๐ = charge number of the k-th species (-1 for negative ions and electrons, +1 for positive ions, and 0 for neutral species)
๐ ๐ = reaction rate of the i-th electron impact reaction (cm -3 โขs -3 ) ๐ ๐ = mobility of the k-th species in the electric field (cm 2 โขV -1 โขs -1 )
๐ ๐๐ = elastic collision frequency of electrons (s -1 ) ๐ = density of the plasma mixture (kgโขcm -3 )
๐ ๐๐๐๐ ๐๐ = ignition delay after plasma pulse burst (s)
๐ ๐ ๐๐๐ = auto-ignition delay without plasma (s) ๐ = electric potential (V) ๐ฬ๐ = production term of the ๐-th species (cm -3 โขs -1 )
Over the past decade, non-equilibrium plasma has been the subject of significant attention, due to its great potential to enhance ignition and combustion in internal combustion engines, gas turbines, scramjet engines, and pulsed detonation engines [1,2]. Past studies have shown that non-equilibrium plasma can shorten ignition delays [3,4], extend the flammability limits to allow ultra-lean combustion for emission reduction [5], increase flame propagation speed [6,7],
and improve flame stabilization [8]. While the phenomena observed in these investigations are very promising, however, the underlying physio-chemical processes for such enhancement are still not well understood. In order to understand these underlying processes, a combination of experimental and numerical investigations is necessary.
The existing experimental studies can be divided into two categories. The first category reproduces the complicated reacting flow conditions in engines. For example, Zhang et al. [9,10] investigated the interaction between a plasma jet and turbulent flows; Kim et al. [11] investigated the interaction between plasma and a fuel jet in a cross flow; Starikovskaia et al.
[12] and Leonov et al. [13,14] investigated the interaction between plasma and supersonic reacting flow. Due to the complicated thermal, kinetic, and transport coupling contexts of these configurations, however, this line of research can offer only limited elucidation of specific plasma enhancement effects. To tackle individual issues, experiments in well-studied simple configurations have been used to simplify or even eliminate hydrodynamic effects and isolate plasma enhancement from other effects. For example, Uddi et al. [15] conducted two-photon absorption laser induced fluorescence (TALIF) measurements of atomic oxygen in fuel/air mixtures subjected to nanosecond pulsed discharges in a rectangular quartz reactor. Yin et al.
[16] studied the ignition of mildly preheated H2/air mixtures under nanosecond pulsed discharges in a quartz flow reactor. Lefkowitz et al. [17,18] conducted in situ measurements of nanosecond pulsed plasma activated C2H4/Ar pyrolysis and oxidation of C2H4/O2/Ar mixtures in a flow reactor.
Although this second group of experimental studies can provide more insights into the underlying physio-chemical processes of plasma enhancement, the number of measurable quantities is very limited and many conclusions can only be indirectly inferred. For this reason, high-fidelity modeling and simulation of plasma assisted ignition and combustion is vital.
Comprehensive numerical models of plasma have been developed over the last several decades, and the field is relatively mature. For example, Ventzek et al. [19] developed a high dimensional plasma model, and Shigeta [20] reviewed a class of models for plasma-turbulence interaction. In contrast, the modeling of plasma assisted ignition and combustion is very limited, primarily because the range of scales involved makes the computation extremely time-consuming. In addition, most of these studies [21][22][
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