Sergey B. Leonov, Aleksander A. Firsov, Michail A. Shurupov, Dmitry A. Yarantsev
Joint Institute for High Temperature RAS, Moscow, 125412 Russia, d.yaran@rambler.ru
M.A. Bolshov, Yu. A. Kuritsyn, V.V. Liger, V.R. Mironenko Institute for Spectroscopy RAS, Troitsk, Moscow reg, 142190, Russia
Abstract. The paper is focused on study of particular mechanisms of plasma impact on high-speed combustion.
A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in reacted gases. The technique is based on the detection of the spectra of H2O absorption lines with different energies of low levels. Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. Under conditions of high level of flow disturbances an optimal technique for fitting the experimental spectra was developed based on presentation of the transient absorption spectra as 2D images in the first step of data processing. In the most cases the high signal-to-noise ratio enabled the reconstruction of the temporal behavior of temperature with a resolution of ~1ms. The validated TDLAS technique was applied for detection of temperature and H2O concentration in the combustion zone of a supersonic (M = 2) air-fuel flow. Direct wall-injected hydrogen and ethylene were used as the fuels. The combustion process was initiated and sustained by near-surface electric discharge. The data of DLAS measurements have proved the idea of two-stage mechanism of plasma-assisted combustion.
Introduction.
This study is motivated by our previous experimental results [1] and also by experimental results and modeling of other groups [2-4]. It was expected that the main heat release due to combustion will be within the region of the plasma- fuel interaction. Experiments demonstrated that the main pressure rise was observed far downstream of
the electrodes line, with bright luminosity is being observed from discharge region, Fig. 1.
One more result has been recently obtained by group of TRINITY [6]. 1-D simulation of the ethylene/air mixture ignition by the glow discharge and heating at three different initial temperatures predicted two-stage pattern of plasma ignition.
Calculated evolution of the gas temperature during ignition is shown on the Fig. 2.
Fig.2. Calculated evolution of gas temperature at three different initial temperatures: 1,2 – 800K; 3 – 700K; 4 – 600K. 1,3,4 – Plasma-induced ignition. 2 – heating- induced ignition
Main idea pushed forward by this paper is that the plasma assistance for combustion enhancement occurs on sophisticated multistage manner. Under some conditions the multistage combustion is observed without plasma of electrical discharge;
see [2-3] for example. But at the plasma assisted combustion the kinetic mechanism of ignition looks to be principally multistage process (at least, two stages), as it is pointed in some last publications, for example in [5-7]. The idea may be briefly described as follows: in case of hydrocarbon fueling and low temperature, flame stabilization by non-equilibrium plasma occurs by means of a two- step process. During the first step the plasma induces active radicals production and so called
“preflame” (or fuel reforming in terms of Stanford’s team), which may be simplified as production of H2, CH2O, and CO. In spite of bright luminescence, this zone does not experience significant temperature and pressure increase. This
“preflame” or “cool flame” [2-4] appears as a source of active chemical species that initiates (under favorable conditions) the second step of normal “hot” combustion, characterized by high temperature and pressure rise. Now this idea is promoted as one of the most important features of plasma method for the combustion enhancement.
To prove this idea experimentally some additional diagnostic technique possessing high temporal and space resolution is needed. A tunable diode laser absorption spectroscopy (TDLAS) was developed and applied to measure spatial distribution of the H2O temperature and concentration.
Pulsed wind tunnel PWT-50H of JIHT has been described earlier in details, for example in [5- 6]. The hydrogen and ethylene fuels are injected directly into the supersonic air flow M=2 from the combustion section wall. Combustors cross-section is 72*72 mm2. Experimental conditions are: static pressure Pst = 100-150 Torr, air flow rate mair = 0.6-0.9 kg/s, fuel mass flow-rate in the range mfuel
= 0.05-2 g/s for hydrogen and mfuel = 0.1-4 g/s for
ethylene, discharge power Wpl = 1-10kW, test duration <0.5 s.
Fig.3. Main scheme of the test section arrangement
The facility is equipped with pressure scanner (16 sensors), a schlieren system (1000 fps, texp=100 ns), devices for optical and spectroscopic observations, current-voltage sensors, laser- absorption spectroscopy of water vapor, station of chemical analysis of exhaust gases, etc. The fuel was injected through 5 circular (d=3,5mm) orifices all in a row across the span and inclined at 25 degrees from the normal in the upstream direction, Fig.3. The row of injectors is 15 mm downstream from the row of electrodes, just downstream of the ceramic block; each injector is inline with an electrode in the configuration that includes 5 electrodes. The fuel mass flow rate was balanced between the orifices using a fuel plenum.
Fuel injection was started 20 ms after discharge initiation and was switched off after completion of the discharge. Typically, the fuel injection continued 10-20 ms after the discharge to observe whether the flame was held or extinguished.
The discharge appears in the form of oscillating plasma filaments as it is shown in Fig.4a, b. Initial electrical breakdown occurs not far from the electrodes location. The individual filaments are blown down due to main flow at velocity a bit less than the core value. The frequency of oscillations depends on flow speed, inter-electrodes gap, and parameters of power supply. In the most cases this value was F=10- 30kHz under the experimental conditions. The regulation of power release in a range Wpl= 3-17kW was performed by means of electrical current change Ipl=2-20A. If the current is increased by a factor of 10, the voltage is decreased, but only by a factor of 5. It is resulted in rise of the power in about 2 times. Such a method leads to some variation in the reduced electric field E/N. Usually it is mentioned that nonequilibrium plasma (characterized by higher level of E/N) occurs more effective in terms of fast fuel ignition.
In our particular case it should be considered two main factors of successful fuel ignition and
flameholding: (1) the discharge power; and (2) length of the discharge filaments (reflects a time of interaction). All other factors appeared as much less important. Special experimental series shown a generation of sequence of active zones of reacting gas moved downstream from the place of immediate plasma-fuel interaction that is well seen in Fig.4.
a
b
Fig.4. Discharge appearance. a – discharge in flow; b – discharge at the ethylene injection.
At low current, the discharge is unstable, while at high current, electrode erosion is significant. Temperature measurements based on optical emission spectroscopy of the N2 second- positive system (i.e., C3Пu→B3Gg emission) yield a rotational temperature of Tg=3500±300 K, independently of the power release from the discharge under conditions of this experiment.
The basic effect: plasma-based flameholding in high-speed flow can be found in publications [5- 7] Ignition and flameholding were realized for H2 and C2H4 fuelling on a plane wall by using a transversal electrical discharge at relatively low power deposition (<2% of flow enthalpy). The power threshold for a hydrogen flameholding was measured to be Wpl<3kW; with ethylene fuelling it was measured to be Wpl≥4kW. The combustion
efficiency was estimated and it is found sufficiently high, about 0.9, for both hydrogen and ethylene.
The ignition effect of the gas discharge was compared for different levels of the power, power density, and reduced electrical field (characterizing the departure from equilibrium for the discharge).
Here it was found that the effectiveness of the flameholding is determined primarily by the level of power deposition, and secondarily by the power density. In this experiment the effect of reduced electrical field was not an important factor. In comparison with hydrogen fuelling, a main difference with ethylene fuelling was that thermal choking was not observed, even at the maximum discharge power of Wpl>10 kW. Furthermore, the completeness of the ethylene combustion decreases with increased fuel mass flow rate.
Schlieren images in Fig.5 and wall pressure distributions in Fig.6 for ethylene combustion illustrate details of plasma-fuel-flow interaction.
One can conclude that the combustion zone locates not in immediate vicinity of the zone of discharge and the fuel feeding.
Tunable diode laser (DL) absorption spectroscopy (TDLAS) is a widely used spectroscopic technique for the detection of various parameters of heated zones. This technique provides remote, nonperturbing measurements of the parameters of a hot zone with time resolution in the µs-ms range depending on the specific experimental conditions. The technique is usually based on the measurements of the ratio of the absorption line intensities of a test molecule. If Boltzmann distribution of the energy levels is established, the ratio of the line intensities depends only on the kinetic temperature of the object. The technically easiest and most straightforward version of TDLAS is the detection of direct absorption. It works well in the case of high signal-to-noise ratio (SNR). For weak lines the well developed wavelength modulation (WM) technique can be
Fig.5. Schlieren pictures of ethylene combustion. Left – discharge inflow; right – ethylene combustion
applied for reducing low-frequency noise (flicker noise) in the baseline and correcting for nonspecific absorption.
Fig.6. Evolution of wall pressure. Ethylene injection G=1.3g/s, discharge power Wpl=8.4kW. X=0 –
electrodes line.
An original differential registration scheme was developed to reduce baseline fluctuations and amplitude modulation of the DL intensity not connected with water vapor absorption. An optimal model of the experimental spectra fitting was found in laboratory experiments with stable temperature and pressure. The best coincidence of T and P values measured by TDLAS and by thermocouple and pressure sensor was obtained using the multi- line spectrum fitting model. The parameters of the probed medium are obtained as a result of experimental spectra fitting. In most cases the individual absorption lines are fitted using Voigt profiles. This approach does not require the exact mechanism of line broadening. The temperature is
inferred from the ratio of the integrals (or amplitudes) of the selected lines using the ratio calculated from spectroscopic databases. The alternative is the fitting of a whole spectral interval, which includes the selected lines. In this case the spectroscopic parameters of the lines from the databases are used. One should correctly account for different mechanisms and fit experimental line profiles using selected models of line broadening.
Molecular water was used as a test molecule.
Water vapor is one of the major combustion products and key indicators of the extent of combustion and is therefore widely used as a tracer of combustion processes in mixed gas flows.
Initially, the optimal TDLAS strategy was developed in laboratory experiments with stable conditions in an evacuated cell filled with the air.
The optimized and validated version of the TDLAS technique was then applied to the measurement of the temperature, total gas pressure, and H2O concentration in a plasma-assisted supersonic combustion flow. The selection of the specific spectral lines was dictated by several reasons: the energies of low levels should be different, the lines should be reasonably resolved and the spectral interval should be free from lines of other gas components. The following H2O absorption lines were selected: 7189.344 cm-1 (E'' = 142 cm-1), 7189.541 cm-1 (E'' = 1255 cm-1), 7189.715 cm-1 (E'' = 2005 cm-1). All lines could be recorded in a single scan of the DL wavelength across a Δλ~ 1 cm-1 spectral interval.
The optical scheme was modified for the actual experimental conditions of the test section.
The results of the preliminary experiments have shown that DL beam transport to the test chamber of the aerodynamic tunnel using a free-space approach was absolutely unsuitable. Firstly, the strong acoustic vibrations of the facility caused
Fig.7. Typical schlieren pictures of ethylene combustion. Combination of images from two windows.
DLAS measurements was made for different sections of the flow, the red line in Fig.6 shows a sample section.
serious problems in reproducibly coupling the DL beam to the focusing optics. Secondly, the absorption of the DL by the water vapor of laboratory atmosphere dramatically increased the absorption signal of the “room temperature” line 7189.344 cm-1. In addition, strong electric noise of the pulsed plasma facility increased the instability of the DL drivers and, hence, the instability of DL operation. All these reasons dictated the location of the sensitive electronics (drivers, optical block with DL, recording system) in a separate room away from the facility and transporting the DL beam to the chamber by a fiber.
DLAS measurements was made for different sections of the flow, the red line in Fig.7 shows a sample section.
Distribution of H2O temperature and concentration. The DLAS measurements were fulfilled in typical operation modes: plasma power was Wpl=8kW, hydrogen mass flow rate was GH2=0.3g/s, and the ethylene GC2H4=0.8g/s. The typical absorption spectra are shown in Fig.8b for
“cold” and “hot” conditions. They were averaged over 30 scans. In some cases a high signal-to-noise ratio enabled spectral fitting with fewer averaged scans. In reality the fluctuations of gas parameters is observed strong, especially in vicinity of fuel injection place. Figures 8a give some impression on the combustion dynamics: it is a 2D image of DLAS spectra before combustion and after it.
The temperature distribution along the combustor for hydrogen obtained as a result of such fitting is shown in Fig.9. Each point in the figure was obtained in individual run of the facility. The values of the temperature inferred from both slopes
coincide reasonably well. The water vapor partial pressure measured in a parallel way is presented in Fig.10. The uncertainty of the temperature evaluation associated with the experimental errors and fitting procedure was estimated as σ = 40 K.
Fig.9 Temperature distribution for H2 combustion measured by DLAS. X axis is along the flow direction. Y
is the distance from the wall.
To explain the above observations, we propose the following two-zone (it reflects the two-stage ignition process) scheme of the plasma assisted flameholding. Zone 1, in which the “cold”
combustion takes place accompanied by plasma- induced fuel conversion and relatively small power release. Note, that the combustion layer in this zone is rather thin. It is actually the shear layer, where the mixing is additionally promoted by the plasma filaments. Despite of high gas temperature here a total heat release is not big. Zone 2, in which the combustion is completed or almost completed with Fig.8. Sample of real spectral dynamics. Typical absorption spectra of water vapors for two zones of flowfield (b).
high energy release. Intensive mixing limits the gas temperature elevation. We emphasize that the plasma is the key element of this scheme: it launches the cold combustion inside of the first zone by generating high amount of active species.
The lengths of the first zone in our tests were measured by the schlieren and schlieren-streak technique in the range of from 50mm to 150mm. It corresponds to the induction time range τind=0.1- 0.3ms.
The combustion completeness was estimated based on three independent methods. The first method includes sampling measurements of chemical composition of exhaust gases, namely, the concentrations of CO2, CO, NOx, CxHy, and O2.
In the second method the measurements of the water vapor concentration in zone of combustion and downstream by means of laser absorption spectroscopy are utilized for estimation. The third method compares the experimental data and the results of the 3D Navier-Stokes simulation. All three methods gave the value of combustion completeness higher then η=0.9.
Acknowledgments
The experimental study is supported through EOARD-ISTC project #3057p (special thanks to Dr. Julian Tishkoff and Dr. Campbell Carter).
Some part of this work was also supported by the Russian Academy of Science (Program #9, Project 9.2 and OFN-10, Project 4.6).
References.
1. Leonov S. B., Yarantsev D. A., Napartovich A. P., Kochetov I. V. “Plasma-Assisted Combustion of Gaseous Fuel in Supersonic Duct”, Plasma Science, IEEE Transactions on Plasma Science, 2006, Volume: 34, Issue: 6, pp.2514-2525
2. Basevich, V. Ya. Chemical kinetics in the combustion processes. In: “Handbook of heat and mass transfer”. V. 4 (Ed.
N.Cheremisinoff), Houston: Gulf. 1990, p.
769
3. Sokolik A. S., “Self-ignition and combustion in gases”, UFN (rus), XXIII, issue 3, 1940, pp.209-250
4. Kim W., Mungal M. G., and Cappelli M. A.,
“Formation and Role of Cool Flames in Plasma-assisted Premixed Combustion,” Appl.
Phys. Lett., vol. 92, 051503, Feb. 2008.
5. Leonov S.B., Carter C., Yarantsev D.A.
“Experiments on Electrically Controlled Flameholding on a Plane Wall in Supersonic Airflow”, Journal of Propulsion and Power, 2009, vol.25, no.2, pp.289-298
6. Sergey Leonov, Dmitry Yarantsev, Vladimir Sabelnikov, Electrically Driven Combustion near Plane Wall in M>1 Duct, 3rd EUCASS Proceedings, July 2009, Versailles, France 7. Leonov S. B., Sabelnikov V.A., Yarantsev D.
A., Napartovich A. P., Kochetov I. V.
“Plasma-Induced Ethylene Ignition and Flameholding in Confined Supersonic Air Flow at Low Temperatures”, Plasma Science, IEEE Transactions on Plasma Science, 2011, February, accepted for publishing.
Fig.10. Water vapor partial pressure in plasma assisted combustor for hydrogen.
6.2. NONEQUILIBRUM PLASMA ACCOMPANYING THE IGNITION OF METHANE-OXYGEN MIXTURES
K.V. Artem’ev, N.K. Berezhetskaya, A.M.Davydov, S.Yu.Kazantsev, I.G.Kononov, I.A. Kossyi, N.I.Malykh, N.A.Popov
1, M.I.Taktakishvili, N.M. Tarasova, K.N.Firsov,
E.A.Filimonova
2, M.R.Shakarov
3A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russia
1D.V. Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia
2OIVT Russian Academy of Sciences, Moscow, Russia
3Moscow Institute of Electronic Technology, Moscow, Russia
The problem of plasma production in processes of ignition of combustible gas mixtures has been much investigated in both experimental and theoretical works. The presence of charged particles in flames is a well-known fact established early in the last century. In the forties, flame conductivities have been studied in some detail. It has been shown that the conductivity value is several orders greater than values calculated assuming equilibrium ionization.
Such “overequilibrium” ionization is also known as
“chemical” ionization. Considerable progress in research on chemical ionization was achieved in the 1960-70s. As a result, a considerable amount of experimental and theoretical data was gathered, for the greater part, on hydrocarbon flames. When analyzing the current state of the problem, we have to note that the investigations concern only a limited range of phenomena (in most cases, shock waves and steady-state flames) and diagnostic methods (usually, probes). For a number of hydrocarbons, adequate kinetic schemes are as yet unavailable to explain experimental values of the electron density of the plasma accompanying oxidation processes in chemically active gases.
The experiments described in this work were stimulated by a series of investigations carried out at the GPI with the aim to reveal peculiarities of the ignition process initiated in combustible gas
mixtures by high-power electric discharges locally in small volumes (which are much less than the reactor volume). As initiators in our experiments, we used high-current gliding surface discharges, microwave discharges, laser sparks on solid target surfaces and laser sparks in space inside the reactor.
We present results of measurements of the electron density in the plasma accompanying the ignition of stoichiometric methane-oxygen mixtures in closed volumes. To initiate the ignition, we used discharges of a different type: high-current gliding surface discharges produced by a multielectrode system, microwave discharges (‘microwave arc’), and laser sparks from a focused HF laser. The electron density ne was measured with the help of two microwave interferometers (wavelengths λ1 = 0.2 cm and λ2 = 0.8 cm). A series of measurements was performed with a single Langmuir probe.
The experiments have revealed certain peculiar features of the spatiotemporal behavior of the chemically ionized nonequilibrium plasma. The results afford a basis for further experimentation. It is proposed to use microwaves to exert influence upon the ionized component of the flame and, through it, upon the combustion process, when irradiating different zones of the flame with relatively strong microwaves, where these waves will be absorbed by the ionized medium.