Elsevier

Combustion and Flame

Volume 162, Issue 1, January 2015, Pages 50-59
Combustion and Flame

Ignition of ethanol-containing mixtures excited by nanosecond discharge above self-ignition threshold

https://doi.org/10.1016/j.combustflame.2014.07.014Get rights and content

Abstract

The kinetics of ignition in lean and stoichiometric C2H5OH:O2:Ar mixtures by a high-voltage nanosecond discharge are studied experimentally and numerically for gas temperatures above self-ignition threshold. Autoignition delay time and ignition delay after a high-voltage nanosecond impulse discharge are measured using reflected shock-wave technique. It is shown that the additional excitation by the discharge plasma leads to an order of magnitude decrease in ignition delay. Discharge processes are simulated on the basis of the measured time–resolved discharge characteristics. The densities of atoms, radicals and excited and charged particles produced in the discharge phase are calculated and used as initial conditions for chemical modeling. Calculated ignition delay times agree well with measurements with and without the discharge plasma. Calculations show that the effect of the discharge plasma on ignition of the ethanol-containing mixtures is primarily associated with active species production in the discharge phase; the role of fast gas heating during the discharge and in its afterglow is relatively small. A method is suggested to compare the effect of nonequilibrium pulse discharge plasma on ignition in different fuel–air and fuel-oxygen mixtures above self-ignition temperatures. It is shown that this effect is more profound at low gas temperatures when the thermal production of initial radicals is not efficient. Due to O2 dissociation in the discharge phase, the influence of nonequilibrium discharge plasma on ignition is more pronounced for fuels with high activation energy of thermal dissociation (like H2 and CH4). The role of N2 in favoring plasma-assisted ignition owing to additional O2 and fuel dissociation is demonstrated.

Introduction

Ethanol oxidation recently attracted considerable attention of researchers due to its application as an alternative fuel. Using ethanol in transportation fuels, it is possible to meet octane quality demands and reduce undesirable emissions simultaneously. Applications of oxygenated fuel like ethanol is expected to increase as regulations on pollutant emissions become stricter. In addition, ethanol possesses the advantage of being produced from renewable sources like biomass.

To evaluate the feasibility and relation between the ethanol combustion process and emissions, an understanding of the reaction mechanisms of ethanol oxidation and pollutant species production is required. For this purpose, detailed chemical kinetic models of ethanol oxidation have been developed and validated against available experimental data [1], [2], [3], [4], [5], [6]. These experiments include laminar flame speed measurements, ignition delay time measurements in shock tubes and measurements of ethanol oxidation product profiles in jet-stirred and turbulent flow reactors.

Today, applications of nonequilibrium plasma for plasma-assisted ignition (PAI) and plasma-assisted combustion (PAC) have aroused considerable interest (see reviews [7], [8], [9], [10]). Observations and numerical simulation showed that nanosecond discharge plasmas have profound effect on ignition delay reduction and flame stabilization in various combustible mixtures. As a consequence, fuel can be ignited under conditions when ignition does not occur in the absence of discharge plasma. In particular, ignition by nonequilibrium gas discharges seems to be perspective for a number of applications under conditions of high speed flows and under conditions similar to automotive engines [10].

Most studies of PAI and PAC have dealt with H2, alkanes from CH4 to C5H12, ethylene and acetylene. Studies above self-ignition threshold were usually conducted in flow reactors and laminar flow burners (see, for instance, [11], [12], [13], [14], [15], [16]). To identify the mechanisms of PAI and PAC, a numerical simulation and comparison of calculated results and observations is required. When simulating the processes in flow reactors and burners, it is necessary to consider not only plasma and combustion kinetic processes, but the transport of active species and heat under strongly non-uniform conditions. The mechanisms of PAI and PAC are easy to study experimentally and numerically using a shock tube with a nanosecond discharge section [7], [10]. In this case, nonequilibrium quasi-uniform plasma is generated under well controlled conditions and ignition delay time of combustible mixtures can be measured behind a reflected shock wave with and without a high-voltage nanosecond discharge. Using the shock-wave technique, simultaneous measurement of discharge characteristics and numerical modeling of discharge and ignition processes allow the development of kinetic schemes for PAI and PAC in combustible mixtures above the self-ignition threshold [17], [18], [19], [20], [21], [22].

Ignition and combustion of ethanol with discharge plasma has been studied experimentally only when the initial state of ethanol was liquid [23], [24]. Plasma was generated by a repetitively pulsed, nanosecond discharge [23] or a surface microwave discharge combined with a DC discharge [24]. These measurements are hard to use for investigating PAI kinetics because fuel evaporation is difficult to control and plasma can be non-uniform under the conditions studied. The purpose of this paper was to study experimentally and numerically the ignition of lean and stoichiometric gaseous C2H5OH:O2:Ar mixtures behind a reflected shock wave under the action of a high-voltage nanosecond discharge producing a quasi-uniform nonequilibrium plasma. To clarify the effect of discharge plasma on ethanol ignition, measurements were also made in the absence of the discharge. A kinetic mechanism of the effect of nonequilibrium plasma was studied by simulating numerically the discharge and ignition characteristics and comparing calculated results with observations.

Section snippets

Experiment

Shock tube technique has been widely used to measure autoignition delay times in combustible mixtures [25]. In this work, to show the effect of discharge plasma, ignition in ethanol-containing mixtures was studied in a shock tube after the discharge and in its absence. Nonequilibrium plasma was created by a high-voltage nanosecond discharge. Gas mixtures under study were heated by a reflected shock wave just before discharge initiation. In this experiment, the discharge processes occurred on a

Numerical simulation of active species production during discharge and in its afterglow

The effect of the discharge plasma on ignition is generally associated with the production of chemically active species that favor the following ignition processes [7], [8], [9], [10]. We simulated numerically the formation of active species in the discharge phase and in the discharge afterglow using the balance equations for species densities. During the discharge phase, the equations were solved semi-analytically because for this short period of time only electron-impact reactions are

Simulation of autoignition and plasma-assisted ignition. Comparison with experiment

We studied the temporal evolution of the mole fractions of chemically active species in the discharge afterglow and in the ignition phase one after another. It was assumed that the duration of the afterglow phase is 0.5 μs. Ignition in ethanol-containing mixtures was simulated with the CHEMKIN-II code, by analogy with our simulation of the processes in the discharge afterglow, in the zero-dimensional approximation at constant pressure. However, as opposed to the case of the discharge afterglow,

Conclusions

Ignition in lean and stoichiometric C2H5OH-containing mixtures under the action of a high-voltage nanosecond discharge has been studied experimentally and numerically. It was shown that additional excitation in the discharge leads to a noticeable decrease in ignition delay time. There is reasonable agreement between calculated and measured ignition delay times. Under the conditions considered, the main mechanism of the effect of gas discharge on the ignition of ethanol is associated with the

Acknowledgments

This work was partially supported by the Russian Ministry of Education and Science under the program “5Top100”, by the Russian Foundation of Basic Research under the project No. 14-03-31449 and by the AFOSR MURI program “Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma-Assisted Combustion”.

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