SP Technical Research Institute of Sweden
Raú
l Ochote
erena, M
Michael
Försth (
SP),Mat
ttias Elfs
S
sberg (F
Fire Tech SP Report 20FOI)
hnology 011:66Electrical currents and breakdown
voltages as a diagnostic tool for fires
Raúl Ochoterena, Michael Försth (SP),
Mattias Elfsberg (FOI)
Abstract
Electrical currents and breakdown voltages as a
diagnostic tool for fires
Two types of electrical measurements have been investigated in order to perform diagnostics of the fire dynamics in the ISO 5660 cone calorimeter. The rationale of the study is to take advantage of the pilot ignition electrodes that are already in place and use these to collect additional information such as emission of pyrolysis gases and time to ignition.
The first part of the project was a refinement of the method for measuring the so called ion current, which has already been investigated in a pilot study. It was found that thorough shielding and grounding gives an excellent signal to noise ratio. An expression for the correlation between measured current and conductivity was also developed and validated experimentally.
The second part of the project consisted of measuring the breakdown voltage, that is the voltage when dielectric failure occurs. It was found that this method was more sensitive to the fire dynamics before ignition, such as pyrolysis, but that the response to ignition was more ambiguous for the breakdown voltage than for the ion current.
Key words: electrical discharge, fire, ignition, cone calorimeter, Paschen's law
Sökord: elektriskt överslag, brand, antändning, konkalorimeter, Paschens lag
SP Sveriges Tekniska Forskningsinstitut
SP Technical Research Institute of Sweden
SP Report 2011:66 ISBN 978-91-86622-97-8 ISSN 0284-5172
Contents
Abstract 3
Contents 4
Preface
6
Notations 7
Sammanfattning 9
Summary 10
1
Introduction 11
2
Theory 12
2.1 Current in electrode gaps without electric breakdown 13
2.2 Electric breakdown 15
3
Experimental methods
17
3.1 Electrical measurements 17
3.1.1 DC measurements of ion currents without electric breakdown 18
3.1.2 Measurements of breakdown voltage 20
3.2 Fire sources 23
3.2.1 Propane burner 23
3.2.2 Cone calorimeter 24
3.2.2.1 Fuels used in cone calorimeter 25
3.2.3 Smouldering combustion 27
4
Results and discussions
28
4.1 DC measurements of ion currents without electric breakdown 284.1.1 Improved signal and signal to noise ratio 29
4.1.2 Measurements on pyrolysis gases 33
4.1.3 Ion current vs. conductivity 33
4.2 Measurement results of breakdown voltage 35
4.2.1 Measurements in a 1 kW propane flame 35
4.2.2 Measurements in the cone calorimeter 36
4.2.2.1 Particle board 36 4.2.2.1.1 Ubreakdown vs. HRR 36 4.2.2.1.2 Ubreakdown vs. SPR 39 4.2.2.2 PUR-foam 41 4.2.2.2.1 Ubreakdown vs. HRR 42 4.2.2.2.2 Ubreakdown vs. SPR 45 4.2.2.3 Black PMMA 47 4.2.2.3.1 Ubreakdown vs. HRR 47 4.2.2.3.2 Ubreakdown vs. SPR 50
4.2.3 Attempt to measure smouldering fires 53
5
Conclusions 54
6
Future Work
54
Preface
The Swedish Board for Fire Research (Brandforsk) sponsored this project with reference number 605-091 which is gratefully acknowledged. Brandforsk is owned by the Swedish government, assurance companies, local authorities and industry and has as mission to initiate, finance and follow-up different types of fire research.
Acknowledgment is given to the staff at SP who has contributed to this project. Special thanks to Brith Månsson, Sven-Ove Vendel, Allan Bergman, Sixten Bergman, and Anders Bergman. Anders Larsson at FOI is gratefully acknowledge for sharing his knowledge on high voltage physics.
Notations
Abbreviation Quantity Unit Explanation/comment
A prefactor in expression for [m-1Pa-1]
B exponential factor in [Vm-1Pa-1]
expression for
D prefactor in expression for [Am-2K-2]
thermionic emission
d distance between electrodes [m]
E electric field [Vm-1]
HAB Height Above Burner [m]
HRR Heat Release Rate [W]
I current [A]
Ie,0 initial (no secondary [A]
ionization) electron current
at cathode
Ie total electrode current [A]
at cathode
Iion,0 initial (no secondary [A]
ionization) ion current at [A]
cathode
Iion, total ion current at cathode [A]
J current density [Am-2]
kB Boltzmann’s constant [JK-1] kB = 1.38110-23 JK-1
ne electron density [m-3]
p pressure [Pa]
PUR polyurethane
PMMA poly(methyl methacrylate),
R resistance []
S surface area of electrode [m2]
SPR Smoke Production Rate [m2s-1] tignition time to ignition [s]
T temperature [K]
U voltage [V]
Ubreakdown voltage required to overcome [V]
the dielectric strength
Townsend’s coefficient [m-1]
for ionization
effective secondary [ ]
ionization coefficient
degree of ionization [ ]
work function for thermionic [J]
emission from metal surface
resistivity [m]
Sammanfattning
Denna rapport är en fortsättningen på en förstudie där möjligheterna studerades för att använda jonströmsmätningar som diagnostisk metod inom brandteknik, främst ISO 5660 konkalorimetern. De positiva resultaten från förstudien ledde till detta
fortsättningsprojekt.
Projektet bestod av två delar:
förfinad metod att mäta jonströmmen mätningar av överslagspänningen
Den första delen gick ut på att mäta strömmen mellan elektroderna utan närvaro av gnista. Genom att skärma och jorda den utrustning som användes i förstudien åstadkoms en stor förbättring i signal/brus förhållandet. Trots detta var det fortfarande enbart möjligt att mäta själva antändningen. Detektion av pyrolysgaserna innan antändning var inte möjlig pga. alltför låga signalnivåer, även med relativt hög pålagd spänning (~1000 VDC). Ett uttryck för förhållandet mellan den uppmätta strömmen och konduktiviteten i
elektrodgapet togs fram och validerades experimentellt. Kunskap om konduktiviteten är viktig om man vill gå vidare och göra uppskattningar om gasens tillstånd såsom
temperatur, elektrontäthet och liknande.
Det andra delen av projektet initierades av oförmågan att mäta på pyrolysgaser med hjälp av strömmätningar utan elektriskt överslag. Två elektriska kretsar designades och
tillverkades: En för att skapa en välkontrollerad gnista och en för att mäta
överslagsspänningen hos gnistan. Det visade sig att överslagsspänningen svarade väl på förändringar i gasen ovanför provkroppen även före antändning. Dessutom gav själva antändningen ytterligare en påverkan på urladdningsspänningen, dock inte lika tydlig som påverkan på strömmen i den första delen av projektet.
Ett logiskt nästa steg är att även beakta fasförskjutningen mellan ström och spänning vilket rimligtvis ger en tydligare signal om förhållandet i elektrodgapet.
Summary
This report is a follow up to a pilot study where the possibilities of using current measurement for fire diagnostics, primarily in the ISO 5660 cone calorimeter, was investigated. The positive results from the pilot study led to this project which consisted of two parts:
a refined method to measure the ion current measurement of the breakdown voltage
In the first part of the project the current between the electrodes was measured without a spark. This means that the electrodes could not be used as a spark igniter at the same time. By thoroughly shielding and grounding the equipment from the pilot study a major improvement was obtained in the signal to noise ratio. Despite this is was still not possible to measure pyrolysis gases since the signal was too weak, even with a relatively high applied voltage (~1000 VDC). An expression for the relationship between the measured current and the conductivity in the electrode gap was developed and validated experimentally. Knowledge about the conductivity is important in estimations of gas properties such as temperature, electron density, etc.
The second part of the project was initiated from the inability to measure pyrolysis gases from current measurements without electric breakdown. Two circuits were designed and constructed: One for producing well defined high voltage pulses and one for measuring the breakdown voltage. It was found that the breakdown voltage responded clearly to changes in the gas composition above the tested sample even before ignition. When ignition occurred an additional change in the breakdown voltage could be observed, although not as distinct as the current pulses measured in the first part of the project.
A logical next step would be to also measure the phase difference between current and voltage. This is expected to give a signal which more clearly characterizes the status of the gas/plasma in the electrode gap.
1
Introduction
This project is a follow up to a pilot study regarding the use of ion current measurements as a tool for ignition detection in the ISO 5660 cone calorimeter [1, 2]. In the pilot study it was found that ignition could easily be detected by applying a DC voltage of 200 V over the pilot ignition electrodes in the cone calorimeter [3] and measuring the ion current in the ~3 mm air gap between the electrodes. It was found that the ion current was
vanishingly small before ignition and that ignition could easily be detected since one or several current pulses occurred when the tested sample ignited. The pulse height was typically on the order 1-10 A and its length was on the order of some 10 ms. One drawback of this method was that it was not sensitive enough to detect for example the onset of pyrolysis. Another drawback was the fact that the electrodes of the cone calorimeter become assigned for the ion current measurements, using a DC voltage of 200 V, and therefore they cannot be used to create the pilot ignition spark that is prescribed in the cone calorimeter standard [3].
Current measurements as a method to monitor flame behaviour is not a new concept in the combustion sciences. Ionic flame monitoring is the measurement of ion currents due to an applied voltage between two electrodes in a flame. This is commonly used as a safety mechanism in burners [4, 5]. The function is to close the gas supply to the burner if the ion current disappears, that is, if the flame is extinguished. The objective is to avoid the risk that a malfunctioning burner might fill up a space with a combustible or explosive gas mixture. More advanced versions of these so called flame rods have been presented where the ion current is characterized by its DC amplitude, AC amplitude, and flickering frequency. This gives more detailed information concerning the status of the flame and it has been proposed that these three parameters combined can give an early warning that a problem is developing in the combustor [6]. From a fire safety perspective conductivity of flames is also important in various other fields such as for example when assessing risk for electrical breakdown between power lines and earth during forest fires [7].
In recent years ion current sensors in internal combustion engines have gained considerable interest [8-10]. Measurement of the ion current over the gap of the spark plug is a cost effective alternative to more expensive pressure sensors used for on board engine diagnostics. In a recent study the relationship between ion current and
temperature was explored [11].
Conductivity of flames [12] and hot air [13] has been studied for over 100 years and is an area of on-going research. It is easy to understand the complexity of the subject given the fact that the chemistry of combustion, not including ions, is still far from well-known for most fuels and combustion conditions. Including the ion chemistry makes the feat even more difficult. Using electric fields to control the combustion has been proposed by several authors for different applications such as gas turbine control [14, 15] and for metallurgical processes [16] for example. In a recent study [17] laser diagnostics were used to do fundamental research on the effect of electric fields on premixed methane-air flames. Direct numerical simulations [18] and experimental measurements [19] have been performed to study the ability of electric fields to stabilize flames. An exponential
relation between applied DC voltage and the change in burning velocity of premixed methane/air flames has been reported [20] while another study indicated a rather linear relationship between AC voltage and velocity in a propane flame [21]. The effect of electric field on soot was studied in reference [22] and one conclusion was that the majority of soot particles were positively charged. Microwaves were used to enhance flame stability in a methane-air stagnation flame in reference [23]. Finally flame flickering induced by magnetic fields was observed in reference [24].
One rationale for exploring the possible use of the electrodes in the cone calorimeter for flame diagnostics is to obtain an objective and well defined method for detecting ignition [25, 26]. In the current standard procedure for the cone calorimeter [3] an operator visually determines when ignition occur. This will by necessity be a subjective measure. Especially for flame retarded materials the flame can be indistinct and unstable [27] and when smoke is obscuring the test object it can be very difficult to objectively determine when ignition occurs. According to the standard for the smoke chamber test method [28] it is required that the inspection window, used be the operator to observe the test, is closed when a certain smoke density is reached. This obviously makes it impossible to detect ignition visually after this point. Thus the detection of ignition can be a weak link in the study of the fire properties of a material. By introducing an automatic and objective ignition detection system more accurate information could be obtained. It is suggested that measurement of ion current or dielectric breakdown voltage could be the input signal for such a system.
In this report two electrical phenomena were explored:
The ion current that flows between the electrodes under moderate voltages. This is a refinement of the previous pilot study [1, 2].
The voltage that is required to overcome the dielectric strength of the medium between the electrodes. In other words, the voltage required to create an electric breakdown.
Section 2 of this report contains the basic physical theory for the explored phenomena. The experimental materials and methods are described in Section 3 while the results are presented and discussed in Section 4. The report ends with conclusions in Section 5 and a discussion on suggested future work in Section 6.
Since this project was a direct continuation of the pilot study presented in reference [1] some parts in the present report are overlapping with the previous report.
2
Theory
An electric force is exerted on electrons and ions in an electric field. Due to these forces there will be a flux of charged particles, creating a current. If two metal plates separated by air are connected to a voltage difference on the order of 10 V no visible effect will occur [29]. However, with a very sensitive ampere meter a current on the order of 10-15 A would be detected. The source of this current is electrons and ions created by natural radioactivity and cosmic rays. If a flame zone passes through the electrode gap the current will increase considerably. Charged species have been studied in a methane-oxygen flame [30, 31]. The most important of these species are electrons, CHO+, H
3O+, C2H3O+,
CH5O+, O2-, OH-, O-, CHO2-, CHO3-, and CO3- [32]. Due to these electrons and ions, the
current increases and for the electrode gap in the cone calorimeter typical currents on the order of 10-6 A have been observed with an applied voltage of 200 V [1, 2]. Section 2.1
presents the most important parameters affecting this current.
If the applied voltage is further increased the current between the electrodes will rise once a certain voltage is reached, Ubreakdown, and a discharge will be seen. This happens when
the electrons gain sufficient energy, due to the electric field, between collisions with other species. At this point, when the kinetic energy of the electrons reaches the atomic
ionization potential of the involved elements, each electron will knock out one additional electron upon collision. Immediately after the collision there will therefore be two slow
electrons that again will accelerate in the electric field and then knock out two more electrons, and so on. In other words there will be an electron avalanche and a self-sustained electric discharge will remain as long as the high voltage is applied. The basic theory for Ubreakdown is given in Section 2.2.
2.1
Current in electrode gaps without electric
breakdown
It has previously been shown [1, 2] that when the applied voltage is below Ubreakdown the
current between the cone calorimeter electrodes follows Ohm’s law:
(1)
For a homogeneous electric field in an area A with electrode distance d the resistance R is
(2)
where
is the conductivity in the gas between the electrodes [Sm-1] (or [-1m-1]).
The current is therefore
(3)
The conductivity is due to charged particles, that is electrons, positive ions, and negative ions. Since electrons are much lighter than ions they are most easily accelerated by the electric field. Therefore it is the electron concentration the determines the conductivity, as long as the electron density is not much lower than ion concentrations. Negative ions affect the conductivity negatively since they are electron depleting [9].
Experiments have shown that for air [29]:
9.6 ∙ 10 ∙ (4)
where
ne is the concentration of electrons [m-3], and
p is the pressure [Pa].
Combining Equations (3) and (4) yields:
This indicates that for a given air pressure it is the electron density that mostly influences the current. Simulations have shown that for a flat laminar lean methane-oxygen flame [32] the molar fractions of electrons, the degree of ionization , is on the order of 10-9.
This information can be used in expression (5) by using the ideal gas law:
(6)
which gives the electron density ne as:
(7)
Expression (5) transforms into:
9.6 ∙ 10 ∙ (8)
Making the very bold assumption that = 10-9 is valid in the flame zone between the
electrodes in the cone calorimeter and that the flame temperature is 1300 K this can be evaluated numerically (see Section 4.1.3 on the geometry of the electrodes):
9.6 ∙ 10 ∙ 10 ∙ 200 ∙ ∙ 1.2 ∙ 10 ∙ 1.65 ∙ 10
1.38 ∙ 10 ∙ 1300 ∙ 3 ∙ 10 22 μA
(9)
This is more or less in the same order of magnitude as the results of the measurements in the cone colorimeter, where the current was in the range 1 – 10 A [1, 2]. The current in expression (9) is not more than an indication since the degree of ionization may vary significantly between different flames [32, 33] and the contact area between flame and electrode may be smaller than the full area of the electrode [9, 34].
Other parameters also affect the current, for instance the availability of electrons. If the electron emission from the negative electrode is dominated by thermionic emission the current density J on the surface is given by [9, 29]
(10)
Where
C is a constant [Am-2K-2]
is the work function of the metal, that is the energy required to leave the metal surface [J]
The work function in its turn depends on the external electric field [35]. Other parameters affecting the current is gas flow [36] and gas composition [29].
2.2
Electric breakdown
If electron losses, due to for example recombination and attachment to walls, are ignored the current to the anode will equal the current of emitted electrons from the cathode, I0.
This is valid as long as the voltage over the electrode gap is low enough that no ionization due to collisions between accelerated electrons and molecules occur. If the voltage increases further ionization will subsequently start. This is characterized by Townsend’s coefficient for ionization, [m-1].
(11)
where
A is a constant [m-1Pa-1] B is a constant [Vm-1Pa-1] E is the electric field [Vm-1]
is the number of ionization events caused by one electron per unit length [29]. Due to the ionizations the current at the anode becomes:
(12)
Obviously the total current will be the same at the cathode. The current at the cathode consists of the initial electron current I0 and an ion current which is
1 (13)
In other words each electron in the initial electron current generates 1 ions in the electrode gap. For sufficiently high electric fields the positive ions hitting the cathode will knock out electrodes. The number of so called secondary electrons that each ion hitting the cathode knocks out is denoted .
The total electron current Ie from the cathode therefore becomes
, (14)
Where I ion, means that it is the total ion current, that is
, 1 (15)
In expression (15) the ion current is calculated based on the total electron current Ie and
not based on the initial electron current I0. This reason for this is obvious; when secondary emission is taking place I0 should be replaced by Ie in both expressions (12)
1 1
(16)
Finally the total current at the anode becomes, taking into account the emitted electron current from the cathode and the ionization in the electrode gap:
1 1
(17)
A transition from a non self-sustained current to a self-sustained current (that is an electric breakdown) occurs when the denominator becomes zero:
1 1 (18)
that is
1
1 (19)
Substituting (11) into (19), and using E=U/d gives
1 1 (20) that is ln ln 1 1 ln (21)
Expression (21) is known as Paschen’s law [37]. Whereas reasonably well defined experimental data of the gas phase properties A and B exist in the literature, information on is very scattered since this is a quite complex parameter depending on, among many factors, the state of the cathode surface. Often values of 10-1 – 10-2 are assumed [29].
Table 1 Coefficients in different gases for Townsend’s coefficient for ionization (11) and for Paschen’s law (21). Gas A [m-1Pa-1] [29] B [Vm -1Pa-1] [29] [ ] N2 9 257 >1.3·10-6 [29] O2 7 206 10-7 - 4.5·10-2 [29] Air 11 274 8·10-6 - 1.5·10-4 10-2 [29] [38] H2 4 99 10-6 – 2.4·10-3 [29] H2O 10 218 CO2 15 220
As an example, the breakdown voltage for a 3 mm electrode gap (such as in the cone calorimeter) at atmospheric pressure (101 kPa) would become
274 ∙ 101 ∙ 10 ∙ 3 ∙ 10
ln 11
ln 101 1 ln 101 ∙ 10 ∙ 3 ∙ 10
13 kV
(22)
The can be compared with typical values for the dielectric strength of air which is 3.2 kVmm-1 at atmospheric pressure [29].
Strictly speaking the breakdown voltage in expression (21) is rather dependent on the molecule concentration than on pressure. This means that if the pressure is constant and temperature increases the breakdown voltage will decrease.
Furthermore, in a flame environment the gas properties obviously differs from the properties of air. For example electrons and ions much more abundant in flame zones than in air. In one study [22] it was found that the flame reduced the dielectric field strength to one seventh that of air. In other words the breakdown voltage is expected to drop when ignition occurs.
The theory for breakdown voltage described here is valid for moderate products of pressure and gap distance, pd < 300 Pa·m [29]. Since the experiments in this study has been performed at atmospheric pressure and with a gap distance of 3 mm this is really at the limit of the applicability of the theory. For high products of pd the breakdown is better described by the faster processes of spark discharges (streamers) [29, 39]. However, the theory above is only used for a qualitative interpretation of the experimental results so the physics of streamers will not be described.
3
Experimental methods
3.1
Electrical measurements
The majority of tests were performed with the type of electrode assembly originally used in the cone calorimeter. Figure 1 shows such an electrode, supplied by Fire Testing Technology Limited, East Grinstead, UK.
Fi
3.
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Figure 6. Schematic of the electronics used to create the high voltage to the electrodes. T2 is an IRF740, D3 and D4 are a 1N4005, C3 has a value of 8.2 nF. The inductance of the primary and secondary windings of X1 are 9.9 mH and 44 mH, respectively.
Table 2 Component list for the electronics used to create the high voltage pulse to the electrodes. See Figure 6. Symbol Value/Type R1 & R2 250 Ω R3 & R8 10 kΩ R4 1-10 kΩ R5 & R6 100 Ω R7 10 Ω C1 4700 µF C2 1 µF C3 8.2 nF T1 BC337NPN T2 IRF740 D1 L-7104GD D2 L-7104YD D3 & D4 1N4005 X1 B 0221119027
The potential across the electrodes was measured using a resistive voltage divider and commercial Tektronix P6015A. The resistive voltage divider consisted of two resistors, one of 1GΩ and other of 1MΩ which were connected in series across the electrodes. The latter resistor and the oscilloscope (1 MΩ) were connected in parallel. A schematic of the voltage divider is shown in Figure 7.
VDC C1 R1 R2 R3 4 3 2 1 5 6 7 8 LM555 C2 R4 R5 D1 T1 R6 D2 R7 R8 T2 D3 D4 C3 X1 Spark Gap
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burner
surements of as shown in ndard [41]. W ane and 10±0 esulting in a b ircuit shown in ted by the ab he electrodes the electrod otential falls a of them sam system. The each set of da tered using a f breakdown Figure 10. T Well ventilate 0.5 l/min air blue flame. n Figure 6. ove describe s is plotted a des rises until as a consequmpled at a rat ereafter, the m
ata, thus obta a low-pass fil voltage a we The burner co ed combustio fed to the bu ed circuit, is s as a function l an electrica ence of the i e of 100 MS maximum vo aining the br ter. ell controlled omply with th on conditions urner, corresp shown in of time. It ca al discharge ionisation of S/s, was oltage betwee reakdown d 1 kW he s were used ponding to a an f en n
Fi
3.
Th ill is m Th SP po to HC stu gure 10 T.2.2
C
he cone calor lustrated in F subjected to material. The i he cone calor PR (Smoke P ossible to sam measure for CN for exam udy. The propane buCone calo
rimeter [3] w Figure 11. Th o irradiation f irradiation le rimeter can b Production R mple the exh r example un mple. No exteurner used in t
rimeter
was used for his is a test w from an elect evels used in be used to m Rate), and ML haust gases to nburned hydr ernal gas cha
the measureme most of the m where a 0.01 trically heate n this study w measure time LR (Mass Lo o an FTIR sp rocarbons an aracterisation ents of breakdo measuremen m2 specimen ed conical sp were 25 and 5 to ignition, H oss Rate) of t pectrometer o nd toxic gases n measureme own voltage. nts, schematic n, horizontall piral above th 50 kW/m2. HRR (Heat R the tested ob or other exter s such as NO ents were per
cally ly positioned he tested Release Rate) bject. It is als rnal analyser O, HCl, and rformed in th d, ), so rs his
Fi No Th sh ele wi
3.
Th Th gure 11 S ormal operat his is achieve hown in Figu ectrodes sim ithout electri.2.2.1
F
hree fuel typ Polyur where Particl 100 m Black area o he fuels are s chematic pictu tion of the co ed by a spark ure 1. For the mply acted as ical breakdow
Fuels used i
e were used rethane foam e 100 mm x 1 le board with mm x 100 mm PMMA, pol of the specim shown in Figure of the cone
one calorime k igniter actu e measuremen the pilot. Fo wn, no pilot
in cone calo
in the tests w m with a dens 100 mm and h a density o m and the thicly(methyl me mens where 10 gure 12 to Fig e calorimeter. eter requires ually consisti nts of breakd or the measur was used.
orimeter
with the cone
sity of 21±1 the thicknes of 680±50 kg ckness was 1 ethacrylate), 00 mm x 100 gure 14. pilot ignition ing of the sam down voltage rement of ion e calorimeter kg/m3. The s about 35 m g/m3. The are 12 mm. with a densi 0 mm and th n of the pyro me spark gap e in Section 4 n current in S r. These were area of the sp mm. ea of the spec ity of 1180±5 e thickness w olysis gases. p as that 4.2.2 the test Section 4.1.1 e: pecimens cimens wher 50 kg/m3. T was 10 mm. t 1, re he
Fi Fi gure 12 P 10 gure 13 P m olyurethane fo 00 mm x 100 m article board u mm x 12 mm an oam used in th mm x 35 mm an
used in the exp nd a density of he experiments nd a density of periments. The f 680±50 kg/m3 . The specimen f 21±1 kg/m3. e specimens ha 3. n in the picture ve dimensions e has dimensio s 100 mm x 100 ons 0
Fi
3.
An co in fo fo gure 14 B m.2.3
S
n attempt wa ombustion. F ncluding an u or breakdown oam, see FiguBlack PMMA u mm x 10 mm an
Smoulderi
as made to de For this purpo upholstered fu
n voltage. Th ure 12 clad w
used in the exp nd a density of
ing comb
etect change ose EN 1021 furniture moc he upholstere with blue cotteriments. The f 1180±50 kg/m
ustion
es in the brea -1 was used ck-up, cigare ed furniture m ton textile. specimens hav m3. akdown volta [42]. Figure ettes, and the mock-up con ve dimensions age during sm 15 shows th measureme nsisted of stan 100 mm x 100 mouldering he setup ent equipmen ndard PUR-ntFi
4
4
Th ra de py fla sm ele gure 15 T4
R
.1
D
e
he purpose o atio could be etermine whe yrolysis gase ames with a mouldering fi ectrode gap wThe test setup f
Results
DC meas
electric b
of these tests improved as ether the new es. In the prev DC-circuit b fire. Finally th was calculat for smoulderin
and dis
suremen
breakdow
were twofol s compared to w circuit, wit vious study i but no attemp he relation b ed and the ca ng combustionscussion
nts of ion
wn
ld. Firstly, to o the previou th increased a it was found pts were mad between the i alculations c according to Ens
n current
o investigate w us, unshielde applied volta to be a straig de to study py ion current an could be valid EN 1021-1.ts withou
whether the ed circuit. Se age, could be ghtforward ta yrolysis gase nd the condu dated by exput
signal to noi econdly, to e used to dete task to detect es or a uctivity in the periment. ise ect t e4.
In un Fi Fi to.1.1
I
n order to ass nshielded, cir gure 16. Io ir an igure 17 show noise ratio hmproved
sess the new rcuit is show on current mea rradation [1]. T nd the shunt r ws the ion cu has improved
signal an
circuit a repr wn in Figure 1 asured at the i The measurem esistance was 1 urrent measu d considerabnd signal t
resentative re 16 [1]. ignition of a pa ment circuit wa 100 k.ured with the bly.
to noise r
esult obtaine article board su s unshielded. T new circuit.atio
ed with the pr ubjected to 50 The applied vo It is clear th revious, kWm-2 oltage was 200Vhat the signal V
Fi Te cu th is ra Fi gure 17. Io ap ests were als urrent to pass he voltage sig no significan atio. gure 18. Io th re on current mea pplied voltage o performed s through the gnal in the os nt evidence t on current mea here was no ex esistance of the asured with th was 200V and d with the 100 e 1 M resist scilloscope. A that removin asured with th ternal shunt re e oscilloscope. he new shielded d the shunt resi 0 k shunt r tance of the A representa ng the shunt r he new shielded esistance, mea d circuit, see Fi istance was 10 resistance rem oscilloscope ative result is resistance inc d circuit. The a aning that all cu
igure 3 and Fig 0 k. moved. This e, and thereby shown in Fi creases the s applied voltage urrent passed gure 4. The forces all th y increasing igure 18. The signal to nois e was 200V and the 1 M e ere se d
In re se on be is Fi 10 [1 Fi On ha wi io ex n order to inc epresentative een by compa nly selected, ehaviour of th 108 . This igure 17 with 000 V / 10×1 , 40]. gure 19. Io th ne proposal as been to inc
ith the large n current is r xample, Figu
crease the ion result is sho aring with th albeit fairly he system. A is obtained u h R = 200V / 10-6 A. It has on current mea he shunt resista for increasin crease the su electrode su relatively hig ure 19. n current the own in Figure he typical res representativ A rough estim using Ohm’s / 2×10-6A, or previously b asured with th ance was 100 k ng the ion cur urface size of urfaces shown
gh but not sig
applied volt e 19. The ion sult using 200 ve, results ar mate of the m s law, R=U/I r approximat been shown t he new shielded k. rrent, thereby f the electrod n in Figure 5 gnificantly d
tage was incr n current clea
0 V, shown i re shown her minimum resi
I, and approx
ing the resul that the ion c
d circuit. The a y making the des. Tests we 5. A result is different from reased to 100 arly increase in Figure 17. re they descri istance in the ximating the r t in Figure 1 current follow applied voltage e method mo ere therefore shown in Fig m the current 00 V. A ed as can be . Although ibe the gener e electrode g result in 9 with R = ws Ohm’s la e was 1000V an ore sensitive, performed gure 20. The t in, for ral gap aw nd e
Fi U ele th sig Fu ele Th Fi gure 20. Io ci m
sing such lar ectrodes the he tested sam gnificant this urthermore, t ectrodes, wh herefore this gure 21. S on current mea ircuit. The app meaning that al
rge electrode irradiation f mple, see Figu
s might affec the flames m hich could ex approach w hadow effect o asured with th plied voltage w ll current passe es will introd from the resis ure 21. Since ct, for examp might be quen xplain the abs as not found on the exposed he large electro was 1000V and ed the 1 M r duce some pr stively heate e the shadow ple, the HRR nched when t sence of ion d to be interes sample due to
odes, see Figure there was no e esistance of th roblems. Due ed spiral will ed (the non-d R, which is an they enter th current incre sting for furt
o the large elec
e 5, and the ne external shunt e oscilloscope. e to the large not reach ce darkened) ar n unwelcome e gap betwee ease using th ther developm trodes in Figur ew shielded resistance, e size of the ertain areas o rea is e effect. en the hese electrod ment. re 5. of des.
4.1.2
Measurements on pyrolysis gases
Despite several attempts it was not possible to measure a current due to pyrolysis gases. The applied voltage was increased to 1000 VDC and the large electrodes in Figure 5 were tested but no signal above the noise level could be detected. A plausible explanation is that the increase in electron density, or ion density, is simply not large enough to give a measurable change in the conductivity of the pyrolysis gases.
For this reason another approach was investigated in order to measure pyrolysis gases, i.e. measurement of the breakdown voltage. As will be seen below this method can be used both for measuring the onset of pyrolysis as well as for detecting ignition.
4.1.3
Ion current vs. conductivity
When measurements are performed the result is a current. This is not a direct property of the gas between the electrodes. Rather, in order to characterize the gas in the gap the conductivity is the physical property of choice. Therefore the relation between current and conductivity is investigated here.
The resistivity is given by
(23)
Where
R is the resistance []
A the cross sectional area of the electrode gap [m2], and d the separation between the electrodes [m]
and the conductivity is simply the inverse of the resistivity
(24)
The geometrical details of the electrode pair are given in the upper part of Figure 22. The distanced between the electrode surfaces is 3 mm and the two radii required to calculated the exposed electrode area in the gap are r1 = 2.4/2=1.2 mm and r2=3.3/2=1.65 mm. The
area is, therefore
Fi Th In vo Th [S Ex co Th Th m Ex th th cu gure 22 D he resistance n Figure 16, F oltage was 20 ≅ he ion curren Sm-1] by mult xpression (26 onductivity herefore his is within measurement u xpression (26 heoretical atte he electrodes, urrent at elec Details of the el e is simply ca ∙ Figure 17, Fi 00 V. Theref ≅ 482 4 nt in these fig tiplying the i 6) was valida =1.390 Sm -≅ 482 7% of the tru uncertainties 6) will not be empt to expla , the express trical breakd ectrode pair a alculated usin ∙ igure 18, and fore 482m-1 200 gures can the
ion current [A ated by a cal 1. The applie 2 ∙35 ∙ 10 13 ue conductiv s. e used furthe ain the ion cu ion is probab down. nd the measur ng Ohm’s la ≅ 482m-1∙ d in most figu 2.41m-1 erefore simpl A] by a facto libration in a ed voltage wa 1.3 Sm-1 vity of the sa er in this repo urrent result bly not valid
rement of cond w, R=U/I, yi ures in refere 1V-1∙ ly be transfo or 2.41. saline soluti as 13 V and alinity which ort but is an s. Due to eff for measure ductivity. ielding a con (26) ence [1] the a (27) rmed to cond
ion with kno the current 3 (28) is acceptable important re fects of the s ements of vol nductivity of applied ductivity own 35 mA. le given the eference for sharp edges o ltage and f of
Fi th
4
Re de pr bu lim4.
M sta se be he Bu co Fi inally, if a sig he cathode th.2
M
esults from t escribed in S resented belo urner. The la mitations of t.2.1
M
Measurements andardised p etup is shown efore the mix eight above t urner), the br onducted out gure 23. B bu fo gnificant par e current wilMeasure
the measurem ection 3.1.2 ow were condtter was emp the measurin
Measurem
s of the break propane flam n Figure 10. xing zone, th the burner. It reakdown vo tside the reacBreakdown volt urner in the fl or the heights 5 rt of the curre ll depend on
ment re
ments conduc are presente ducted using ployed to imp ng system bements in a
kdown poten me at two heig It is possible e breakdown t is also obse oltage increa ction borderstage for a prop ame center, an 55 mm and 105 ent is due to the cathode
sults of b
cted using th d in the follo g the cone cal prove the un efore it was aa 1 kW pr
ntial done alo ghts, are show e to observe t n voltage is h erved that aro ases due to th s of the flame pane flame. Ub nd for several o 5 mm. thermionic e temperature
breakdo
he intermitten owing section lorimeter and nderstanding applied to theropane fla
ong the symm wn in Figure that, at the bo high and decr ound 180 mm he fact that th e but measur breakdown is show off-center radi emission of e , see Eq. (10
own volta
nt spark disp ns. Measurem d a well char of the behav e cone calorimame
metry axis, an e 23. The me ottom of the reases as a fu m HAB (Heig he measurem e in hot gasewed for several ii (distance to t electrons fro 0).
age
positive ments racterised viour and meter. nd across the easurement flame end ju unction of ght Above ments are thenes.
l height above the flame cente
m e ust n the er)
4.
A t = t = t = Th th4.
4. Fi wi ig ig on Fi.2.2
M
fixed protoc = 0 s = 10 s = 20 s he time to ig he results belo.2.2.1
P
2.2.1.1 U igure 24 show ith 50 kWm -gnition occurs gnition. The m n particle boa gure 24. PMeasurem
col was follo
data acqui the sample protected t the radiatio irradiation nition was o ow.
Particle boa
Ubreakdown vs. H w the breakd -2 irradiation s at 50 s. Fig measurement ard. For othearticle board.
ments in th
wed in all m sition started e was introdu the sample fr on shield wa n level; 25 kW bserved visuard
HRR down voltage level. It is cl gure 25 show ts continued er measureme 50 kWm-2. t ignihe cone ca
measurements d uced under th from direct ir as removed a Wm-2 or 50 k ually in addit e and HRR fo learly seen th ws a close-up for approxim ents the typicition=50 s.
alorimete
s:
he cone heate rradiation
and the specim kWm-2. tion to the dr or a measure hat Ubreakdown of the dynam mately 1000 cal sampling
er
er but the rad
men was exp
op in Ubreakdo ement on a pa drops signif mics around s, for the ca g time was ar diation shield posed to the own observed article board ficantly when the time of ase 50 kWm -round 200 s. d set in d n 2
Fi In wi is gure 25. P n Figure 26 a ith different seen when i article board. nd Figure 27 scales on the gnition takes 50 kWm-2. t ign 7 the results f e time-axis. t s place. nition=50 s. Clos for an irradia tignition is now se-up around ig ation level of w 144 s and a gnition. f 25 kWm-2 a a distinct drop are shown, p in Ubreakdowwn
Fi Fi gure 26. P gure 27. P article board. article board. 25 kWm-2. t ign 25 kWm-2. t ig nition=144 s.
Th wh fo m vo br wi 4. Fi th Fi he initial Ubr hen it is 50 k or this is uncl means lower m oltage, see Eq reakdown vo ith Figure 35 2.2.1.2 U igure 28 show he right ordin gure 28. P reakdown is som kW. This is e lear since in molecule con q. (12) and th oltage for low 5 (PUR-foam Ubreakdown vs. S ws the same nate. Figure 2 article board. me kV lower easily seen by
fact the oppo ncentrations, he subsequen wer radiation m) or compar SPR Ubreakdown as 29 shows a cl 50 kWm-2. t igni
when the irr y comparing osite trend co
which typic nt discussion
level can als ring Figure 4 in Figure 24 lose-up arou ition=50 s. radiation leve g Figure 25 w ould be expe ally would le n. The trend t so be seen by 40 with Figur
4 but this tim und tignition. el is 25 kW c with Figure 2 ected. Higher ead to a lowe towards a low y comparing re 42 (black P e compared t compared to 27. The reaso r temperature er breakdown wer g Figure 33 PMMA). to the SPR o on e n on
Fi Fi gure 29. P gure 30. P article board. article board. 50 kWm-2. t ign 25 kWm-2. t ign nition=50 s. Clos nition=144 s.
W is ex se we av th Fi
4.
Th co se When studying completely xhaust hood, ee Figure 11. ell localized veraging of 6 hat is observe gure 31. P.2.2.2
P
he results for ompared to th econds after r g results like spatially ave mixed, and m By compari to the electr 64 samples, c ed at 80 – 11 article board.PUR-foam
r PUR-foam he particle bo removal of th e those shown eraged since measured in ison, the mea rode gap, alth correspondin 0 s, which ha 25 kWm-2. t ig are shown in oard is that i he radiation wn in Figure 3 all smoke fro the smoke o asurement of hough in the ng to 1.2 s. Th as no clear c gnition=144 s. Cl n Figure 32 t gnition takes shield at t = 30 it should om the samp optical densit f Ubreakdown is presented fig his could exp orresponding ose-up around to Figure 39. s place much 20 s. be observed ple is collecte ty measurem an in-situ m gures there i plain the dip g peak in the d ignition. The major d h faster, just o d that the SPR ed in the ment system, measurement is a temporal p in Ubreakdown e SPR curve. difference as one or a few R s w4. Fi It t > is in 2.2.2.1 U gure 32. P is interesting > 100 s altho made with t n the noise am Ubreakdown vs. H UR-foam. 50 k g to observe ough the HRR the SPR, see mplitude. HRR kWm-2. t ignition= that Ubreakdow R drops. The Figure 36. T =21 s. wn stays at a r e same pheno The major inf
relatively low omena is obs fluence on U w level in Fig erved when Ubreakdown is ra gure 32 for a compariso ather a decrea on ase
Fi In th gure 33. P n Figure 34 th he drop in Ub UR-foam. 50 k he irradiation reakdown, espec kWm-2. t ignition= n level was 2 cially in Figu =21 s. Close-up 25 kWm-2 an ure 35. p around ignit d tignition = 24 ion.
Fi Fi gure 34. P gure 35. P UR-foam. 25 k UR-foam. 25 k kWm-2. t ignition= kWm-2. t ignition =24 s.
4. Th bu Fi 2.2.2.2 U he same Ubre ut this time c gure 36. P Ubreakdown vs. S eakdown as in Fi compared to t UR-foam. 50 k SPR igure 32 to F the SPR. kWm-2. t ignition= Figure 35 is s =24 s.
Fi Fi gure 37. P gure 38. P UR-foam. 50 k UR-foam. 25 k kWm-2. t ignition= kWm-2. t ignition= =24 s. Close-up =24 s. p around ignition.
Fi
4.
4. Fi ig gure 39. P.2.2.3
B
2.2.3.1 U inally results gnition occurs UR-foam. 25 kBlack PMM
Ubreakdown vs. H s for black PM s at t = 51, al kWm-2. t ignitionMA
HRR MMA are sh lthough this n=24 s. Close-u hown. In Figu drop is not s up around ignit ure 40 there i so distinct. tion.Fi Fi gure 40. B gure 41. B Black PMMA. 5 Black PMMA. 5 50 kWm-2. t ignit 50 kWm-2. t igni tion=51 s.
In be Fi n Figure 42, w etween igniti gure 42. B
where the irr ion and a dist
Black PMMA. 2 radiation leve tinct drop in 25 kWm-2. t igni el was 25 kW Ubreakdown is ition=151 s. Wm-2 and t igni
very good. ition
Fi 4. Th ca gure 43. B 2.2.3.2 U he correlatio ase, see Figur
Black PMMA. 2 Ubreakdown vs. S n between U re 47. 25 kWm-2. t ign SPR Ubreakdown and nition=151 s. Clo SPR is relati ose-up around ively good, e ignition.
Fi Fi gure 44. B gure 45. B Black PMMA. 5 Black PMMA. 5 50 kWm-2. t ignit 50 kWm-2. t igni tion=51 s.
Fi Fi gure 46. B gure 47. B Black PMMA. 2 Black PMMA. 2 25 kWm-2. t igni 25 kWm-2. t ign ition=151 s.
4.2.3
Attempt to measure smouldering fires
Finally an attempt was made to measure a change in the breakdown voltage when the electrodes were positioned above a smouldering cigarette in an upholstered furniture mock-up, see Section 3.2.3. No significant change in Ubreakdown could be measured.
There is one major difference between the measurement of smouldering fire, Figure 15, and the measurements in the propane flame and the cone calorimeter, Figure 10 and Figure 8 respectively. The temperature is very high in the flames of the propane burner and the cone calorimeter. Even for the cone calorimeter before ignition the temperature is high and increasing. By contrast, the temperature above the smouldering fire is quite constant and not significantly higher than the ambient room temperature. This means that the change in Ubreakdown due to changed air density does not come into play for the
smouldering fire in the same way as it does for the propane burner and the cone calorimeter. On the other hand the results show that increased temperature, due to removed radiation shield, is not the supreme parameter affecting Ubreakdown. For example,
in Figure 30, Ubreakdown does not start to decrease significantly at 20 s when the radiation
shield is removed, but rather at ~75 s when the pyrolysis gases, as measured by the SPR, start to appear. This is in contrast to the results in Figure 29 where the decrease in
Ubreakdown starts immediately at the time of shield removal, but where also the SPR starts
to increase at the time of shield removal. Furthermore the correlation between temperature (or density) and Ubreakdown is not trivial, as discussed at the end of Section
4.2.2.1.1.
Although measurement of smouldering fires did not succeed in this project this does not mean that there is no hope for using the electrodes for this purpose. There are many possible refinements possible for the method as will be discussed below.
5
Conclusions
It has been shown that the signal to noise ratio of DC current measurements can be greatly improved by careful shielding and grounding of the measurement equipment. This enables more sensitive detection of the ignition phase. It was also shown that the current can straightforwardly be translated into electric conductivity which is a property that better lends itself to a description of the status of the gas between the electrodes.
It was not possible to detect a current above the noise level for the pyrolysis phase, neither by increasing the applied voltage to 1000 VDC nor by increasing the size of the electrodes. The explanation for this is probably that the concentration of electrons and ions is much lower for the pyrolysis gases than for the flame front. In order to measure pyrolysis gases with the electrodes another property was investigated, the breakdown voltage, Ubreakdown.
The breakdown voltage correlated with HRR and SPR before and at ignition. After ignition the correlation was poor. This shows that, given the simple equipment used in this report, is possible to use the pilot ignition electrodes to detect pyrolysis gases and ignition. Indeed, this is possible to do at the same time as the electrodes perform the additional role of pilot ignition. In other words, it is possible to perform measurements of
Ubreakdown without compromising with the ISO 5660 standard. In fact, according to the
standard the spark igniter should be removed after ignition. In other words the low correlation after ignition is of no relevance if the standard is followed. The drop in
Ubreakdown is not as distinct as the current pulses that appear using the DC measurements.
However, this was only a first test of measuring Ubreakdown and there is clearly a potential
for improvement of the methodology.
6
Future Work
Further research should aim at improving the response of the methods to weakly ionized gases such as pyrolysis gases and especially smoke from smouldering fire. The latter has not yet been measurable be any of the two methods tested.
The voltage divider probe did not give the same result as the commercial voltage probe. This is probably due to the relative high inductance of the voltage divider. The inductance will affect the probes accuracy when measuring dynamic processes. The performance of the voltage divider could be improved by either adding a compensating network or changing the resistors. An improved probe should be calibrated against known high voltage pulses and not against a commercial probe since the latter can give incorrect results for the type of demanding measurements performed in this study (high voltage, fast processes, EMC problems…).
One proposal for improving the response of the system is to study the time lag between the applied voltage and resulting current. This can be done quite straightforwardly with the method of electric breakdown as has been described in this report but an improved voltage probe and an appropriate current probe are required. For the DC measurements (without electric breakdown) it is clearly not possible to measure a time lag since the voltage is constant. Therefore a signal generator producing AC voltages would be required. When the atmosphere between the electrodes changes there will be a change in the capacitance, and therefore also a change in the the time lag. Measuring the change in time lag (capacitance) instead of the current (conductivity) can also be described as measuring electric dipoles instead of electric monopoles (free charges). The change in
abundance of dipoles can be expected to vary more than the change of free charges since the latter require ionization or emission of electrons from the cathode. Changes in dipole concentration can occur more easily due to non-ionized chemistry in the pyrolysis of the fuel, in the flames, or in the smoke gases.
Electric flame diagnostics is not limited to the cone calorimeter. One interesting project would be to conduct a similar study to the one presented here but applied to the smoke box method [28] where under-ventilated combustion is studied.
References
[1] M. Försth, A. Larsson, On the use of ion current measurements to detect ignition in the cone calorimeter, Brandforsk project 311-081, SP Report 2008:50, in, SP Fire Technology, Borås, 2008.
[2] M. Försth, A. Larsson, Ion current measurements as a tool for ignition detection in the cone calorimeter, Fire and Materials, 34 (2010) 421-428.
[3] ISO 5660-1:2002 Reaction-to-fire tests - Heat release, smoke production and mass loss rate - Part 1: Heat release rate (cone calorimeter method), in: International Organization for Standardization, 2002.
[4] Honeywell, C7005A,B Gas Pilot and Flame Rod Assemblies, in, Honeywell Inc. [5] B. Chian, T.J. Nordberg, Circuit diagnostics from flame sensing AC component, in, Honeywell International Inc., USA, 2007.
[6] J.M. Niziolek, W.M. Clark, J.M. Bialobrzeska, T.M. Grayson, Diagnostic ionic flame monitor, ABB, USA, 2002.
[7] K. Mphale, M. Heron, Measurement of Electrical Conductivity for a Biomass Fire, International Journal of Molecular Sciences, 9 (2008) 1416-1423.
[8] P. Mehresh, D. Flowers, R.W. Dibble, Experimental and numerical investigation of effect of fuel on ion sensor signal to determine combustion timing in homogeneous charge compression ignition engines, International Journal of Engine Research, 6 (2005) 465-474.
[9] A. Franke, Characterization of an Electrical Sensor for Combustion Diagnostics, in: Combustion Physics, Lund Institute of Technology, Lund, 2002.
[10] S. Saxena, J.Y. Chen, R.W. Dibble, Increasing the signal-to-noise ratio of sparkplug ion sensors through the addition of a potassium acetate fuel additive, Proceedings of the Combustion Institute, 33 (2011) 3081-3088.
[11] Z. Gao, X. Wu, C. Man, X. Meng, Z. Huang, The relationship between ion current and temperature at the electrode gap, Applied Thermal Engineering.
[12] T.M. Sugden, Elementary combustion reactions. Charged Species, in: 10th Symposium on Combustion, Combustion Institute, 1965.
[13] J. Prager, G. Baldea, U. Riedel, J. Warnatz, Equilibrium Composition and Electrical Conductivity of High-Temperature Air, in: 21st International Colloquium on the
Dynamics of Explosions and Reactive Systems (ICDERS), Poiters, France, 2007. [14] T.H. Dimmock, W.R. Kineyko, The electrical properties of ionized flames. Part I. Study of flame ionization, in, Defense Technical Information Center (DTIC), 1961. [15] T.H. Dimmock, G.F. Miller, J.J. Nichol, W.R. Kineyko, The electrical properties of ionized flames. Part II. Electrostatic and magnetohydrodynamic deflection, in, Defense Technical Information Center (DTIC), 1961.
[16] B. Karlowitz, Flames augmented by electrical power, Pure & Applied Chemistry, 5 (1962) 557-564.
[17] F. Altendorfner, J. Kuhl, L. Zigan, A. Leipertz, Study of the influence of electric fields on flames using planar LIF and PIV techniques, Proceedings of the Combustion Institute, 33 (2011) 3195-3201.
[18] M. Belhi, P. Domingo, P. Vervisch, Direct numerical simulation of the effect of an electric field on flame stability, Combustion and Flame, 157 (2010) 2286-2297.
[19] M.K. Kim, S.H. Chung, H.H. Kim, Effect of AC electric fields on the stabilization of premixed bunsen flames, Proceedings of the Combustion Institute, 33 (2011) 1137-1144. [20] J.D.B.J. van den Boom, A.A. Konnov, A.M.H.H. Verhasselt, V.N. Kornilov, L.P.H. de Goey, H. Nijmeijer, The effect of a DC electric field on the laminar burning velocity of premixed methane/air flames, Proceedings of the Combustion Institute, 32 (2009) 1237-1244.
