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An investigation of the minimum ignition energies of some C1 to C7 hydrocarbons
1. rntfoduction
Data on minimum ignition energies are necessary for fuel ignition systems and for safety standards in relation to possMe explosbn hazards in fuel handling and in industrial situations, In the case of hydrocarbon fuels, while considerable information is availableon the influence of mixture composition, fuel type, and pressure on the minimum ignition energies of quiescent mixtures at room temperature [I -83 , a consi&rab~~ number of problems remain. Information in confkting in relation to the absolute v&es of the minimum ignition energies for a number of hydrocarbons, and there is uncertainty on the influence of molecular weight and of structure. In addition, few investigations have k3n concerned with measurements at elevated temperatures, althuugh data have been reported over a range of temperatures for five fuels in stuichiometric mixtures with air and data for one fuel, n-pm&m, have been presented for various temperatures and mixture strength [4,8]. Vtious experimental techniques have been
used in the determination of minimum ignition energies but in general measuremenb of this @pe invoke the use of capacitance sparks. IXfferen~s in technique, which may influence tb v&es ob*Pre~x~t address: British Gas Corporation bM&md~ Research Statiou, SoliN& Er@nd.
J. MOORHOUSE, A. WILLIAMS, and T.. E. MAIXMSQM
Fig. 1. Diagrammatic representation of the expanding plate capacitor apparatus,
ve~;.sel (9 cm diam X 20 cm.long) by means of
their part&l pressuras. The vessel was fitted with diametrically opposed electrodes insulated from &t3bomb by PTFE; one of the electrodes could b%rrr~4 to vary the electrode spacing and its w&don measured by a dial micrameter. The explosion vesselcould be heated to temperatures up to 125QC with an accuractf of O.l’C. Be spark was generated by,means of the ~ti&le capacitor &ice described by Cheng [ I.01 ssnd thsi~ated in Fig. 1, The “apacitor consisted 039twosgsrltllel plates, each f(30 X 120 mm, with WC R+& Ed ih,eather movab!e and separated By 343tir flp, The mnvMe plate was springIf~tiarl. ,#P&at ILccruIr$,when triggered, spring r$W fWra:its ulosest separation (ca. Imm) tu abaut 4U aan: apart and the position of the pla*ecould be determined by means of a dis&~?ment traducer. The fixed, &arged plate RQSco-ted to one eIectrodP;in the explosion ~3~1~ the ather de&rode I-zingconnected to Ihe RWvkI@ plMt% an4 tu 43&h, A Smallloop of tire HWUI~tk input. tead NWused as s capacitmce tit* diti&er to #ve WLindication of the instant Qfd&harge. The fixed@atecould be charged bm a high-voltagesljuiLlTct3 and the voltage r& by a &oq9ed electrostatic voltmeter.
The ends of the cylindrical explosion vessel were fitted tith parallel windows to permit single shot schlieren photographs to be obtained. A microsecond argon jet light (Lunartron Electronics Ltd) was used which was triggered after an appropriate time interval after the spark discharge by means of a delay unit [l I 1. The explosion vessel was also fitted with a pressure transducer to monitor whether ignition had occurred. Radiation from a spark or from ignition could also be detected by means of an IP28 photomultipler. The gases (Nlathesori)were used without further purification. Their minimum purities were: methane 99.9%; ethane, propane, and butane 99.0%. In the case of the liquids, laboratory grade reagents were used (Koch Light Laboratories Ltd.) and these were degassed and vacuum.distilled prior to use, Pure (99.9%) tungsten wife of 1 mm diam was used for the electrodes; the cathode was oxidized in air at 800°C for 20 miu before mounting in the explosion vessel. 2B. Determination of tie Energy of the &bark IMEchskrge
The expanding plate capacitor allows the storage of the charge at iow voltage and a discharge between the electrodes occurs when the plate separation has caused a sufficient rise in lroltage,
J. MOORHBUSE, A. WILLIAMS, and T. E. ~ADDNX4
Eig. 3. Stereoscan photographs of electrodes showing (a) used oxidized tungsten cathode showing surfw oxklation (magnification 65x), (b) used tungsten anode Wowing edge melting and erosion after discharge (magnification 260x 1.
wdes to obtah measurements of the minimum i@litttan energies for mixtures of various composiWI$, ignition was determined by means of a
pressure transducer or by the voltage pickup signal which indicated the occurrence of ignitiola as illustrated in Fig. 4. Minimum ignition energies
f
3. MOORHQWSE, A. WILLIAMS, and T. E. MADDISUN
Fig. 6. Ignition d&rams at 1 atm pressure. TACLE 1 Minimum Ignition Ener&zs and the Corresponding Ssoi~hiometries for Hydrocarbons CT = 22°C) Pressura -___ Fuei --. metbarle ethane propane n-butane n-pentane 2-pentene n-hexane 2-hexene n-heptane Hz
1.0 atm
0.75 atm 4
@min
Ca 1.4 0.99 0.96 0.91 0.55 0.85 0.95
1.28 1.31 1.4iI 1.41 1.19 1.44
Et
9 rnti
O&J 0.41 0.46
0.9 1.17 1.30
0.5
1.48
0.61
stoichiometry are summarized in Table 1. Data for sume mixtures at 1 atm pressure and 22°C were also obtained. The addition of 7% Hz to methane reduced Ei from 0.63 to 0.50 m3. Mixtures of methane and ethane showed a linear variation of El between the values for the two components. The effect of pressure is shown by the data in Table 1 for n-pentane and by the curves in Fig. 5. The increased ignition energy at the llower prese sure, the greatly reduced ignitable range and the slight shift of the curve to kanq mixturei are irnmediately obvious.
L?T-1
+p--Q--~---~-~~~-~~-’ ~:H~E’ ~*“~ I
i wLw2
$
Fig. 8. Ignition diagrams fm rr-pentsm i~t 0.73 otm g% temperatures in lM ~ang# 22.-t 25”C. 3D. Sfudks of E?mmd
Tsmqm-tim
Measurementsof minimtrm ignitjaI3ene@cs we~3 made at elevated ten~peratums in a sitik~ n~axme~ to those at 22°C. However the range of temperatures that could be studied was hmitr?d rry ths lower energy limit Of the apparatus, thus in order to widen the temperature range the fuels ware studied at 0.75 atm presswre. At the temperature at which a measuretlrent WAS made, a number of measurements were first made to ensure that the electrode spacing was greafez than the quenching distance. In genera1the% ranged from 2.3 to 23 mm. In this way, cures for a number of fueis were obtained at temperatures up to 125°C. The effect of the increase i.n temperature is to decreax? the minimum energy a~ illustrated for rzgentane in Fig. 8. AlI the fuel:: tested, with the sxcepticsn of n-butane+shgi’~a
J. MUORHOUSE,A. WILLtAMS,ial T, E, MADDESfJN
214
xtunrb~ wftenrgara~es, Evidence from theories of gpiirkQnition in quiescent mixtures as well as HQSX~WM atidence from studies in which tern~rrture and pressure have been treated as variables -~fbg#~tt,hat, within experimental error, the minituum @tiitSrrnenergy Ed can be expressed in the ~~H!K fii =
AP?,
&MW,d is a constant, P the pressure, and T the
(3)
temperature. From the data obtained in these experiments it is fo,und that or= 2.13 and fl= -2.03. l+wever the data obtained for the pressure dependency is related solely to n-psntane and to measurements at two firessurest although the values for each pressure are the outcome of a number of experiments. Metzler [2,3’j found thet Q = 1.82 with a maximum variation of 1.65 to 2.00. He observed also that the por;itfons of the minimum point of the ignition curves are only slightly pressure dependent over the range 0.1 to 1 atm, this is also consistent with the presetit work. Enthe case of the temperature dependency all the fuel: here show the same variation with temperature. The value obtained pf 0 = -2.03 is cunsistent with the work of King and Calcote [8] who found fl= -2.00. It is also consistent with a number of theoretical analyses [5,15,16] based on thermal thearies of which the expression due to Fenn [l S] is given bdcw: 1.5
PO
log Ei(Tf -
Tfj)
=c1 +21og G - 2logB
v/h&e:POis the oxygen partial pressure, Tf and Te are the flame temperature and initial temperature respectively, Cr and C, are constants, P is
the pmmlre, temperature,
tima
axxdTf@ean,isIthe lean timia IJfxauss af shesimlk~rity afall
12~~
pimental data ta ihe exponents few pxansure ami
tempctature in Eq. (a)$ it isimirmed EEkaF rkrilhh
aacuncy Q = .+Dand 8 CC~(*ZOO. The vdws of the cunstmt,A t ia Eq. (3”1dep~~l upon the natureof the fuel; tlrtricvaluesobtohk! frumthcw experiments, assuming that in all CaMs U” 2DandP = * 2.0 and with 7 g&tin ia K zad P in atmospheres, fire metbane, W? X 10’ ; alkm~, 422 x I@; propane, 4,74 X lob ; tt*butanr, 1.88 X IO”; n-p~tans, I >I12X 10’ ; 2-pentorlre~, 1.73 X 105;n-hexme, r,8 I X IOs; bhmw, 1 .B2 X 10’ ; and n-haptane, 1.8 li X IQ’ v The values uf the mfnimum fgri%n errcr@~ given by these expressions af i‘,5”c:ar~i 1 alna are higher t&n the values quoted by ti~is nnd van Elbe [I ] a Their results, wi~&h ate freatznq taken as standards in relation to safety stindards, relate to the case when one ignition ~cu15: in a hundred tests, i.e., an ignition probability of 0.81. The present results, those of IRwis and van Efbe El] aad M&kr [2,3] and some ofthe xesu~Fs given by CaZcote et al. 141 are given in Table 2. The more extensive data of Calcote et al., relating to the influence of molecular structtire cannut be used here since they aU refer to a stoichiometry of # = 1. It should be noted that the results of Lewis and van Elbe and Met&t and C&ate refer to the same experimental technique, namely capacitor discharges and an ignition probability of 0.01. A significant feature of the present pork is that the data refer to a boundary below which no ignitions were observed. Points above the boundary did not relate to a unity probability bug, in the range of Ei to 2E1, to a probability of -0.8. Here there is a good agreement with other measurements based on a similar probability. rhu$ for methane the present work yielded a minimum ignition energy of 0.63 mJ while Sayers et al. [7j found 0.55 mJ, simiiarly Komaror and Belikova [I 71 found 0.47, snd Toriyama [18] obtained 0~57 and 0.58 mJ. kiwis and von Elbe [I] obtained 0,29 mJ based on the low ignition probability criterion. No measurements are available for higher hydrocarbons with which csmptison may be made, The fact that no ignitions were ever observed bc.-
experimental
212
1. MOORHOUSE, A. WILLIAMS, and T+ E., MADDISON
plotting the tnitimum ignition energies as a function of H/U, where H is the number of hydrogen atoms ti tie molecules and 0 is the number of oxygen atoms in the mixture. This type of cxplanatfon is consistent with the high temperaturo data here.
noted that in low-temperature (I 500-9XIO K) studies of hydrocarbon ignition, trace amounts of ethane or butane [20] markedly sensitize @&ion. Once the flame kernel reaches the cri%icalsize, whether propagation or degeneration occurs depends upon the rate of enthalpy production by the combustible mixture, but it is also a function ofthe rate of energy released by the discharge and of any applied voltage still applied to the electrodes after the discharge which may influence the movement of the flame kernel. In the case of n-per&me studied here (Fig. 10) the critical radius is consistent with other measurements of critical radii. For example, Mozer and Sherburne 1221 found that ethane, propane, and n-butane had minimum critical radii of 2 mm at 1 bar pressure similar to the value drtained here for n-pentane. Thus flames produced using the expanding capacitor technique are essentially similar to those produced by the fixed capacitor discharge.
elf spark ignition involves the folstages: (i) the spark discharge itself, (ii) the transfer of energy from the spark channel to the combustible mixture, (iii) the oxidation initiation praceu4s and formation of the initial flame kernel &critical radius, and (iv) the occurrence of flame propagation or flame degeneratioa y of the procezes of ig Re ma.jor unatainties niticn relate to stage (i) and (ii). Firstly, the spark &charge process is not rqmducible in these or in o::E!rexpariments wHtb * regard to the spatial posit:aouofthe spark, and observations by means of thr: &Aicren system showed that the discharge followed diff’ercntpaths in all experinlents. This Sstsrgety determined by the nature of rhe elecfrodb surface and is consistent with thz observati-n that clectrod~ reconditioning was necassary &cr about fifteen dkcharges when presumably all the &ive s&tashad been destroyed. T~a~saturtis in the plasma are high for the first I&Vm~cmsecand.suntil all the energy is ttansfiXTedto ths initS flame kernel. Energy losses due t.0 s&k wave formation are small [23] but coWder&ie heat losses to the electrodes may NW, Thus it WAS observed that distortion to the flrc~ek~ttel by the ekctrodes readily occurred jlfCQg. abrlut # hundred mictus~c~nd~,this corresj~~~Crt#lo the ~xr;?rturbatXun at this time shown in #& IQ, ‘%a snrrgy releas;ozd by the spark itself is sufficknl: ‘lomaintain temperatures greater than %#I K throughout the flame kernel even before OX@ flame bs iFUy developed. Thus differences in fltictt~v i&ndpz&cuIariy the C-H bond W9&& Lr@nof inlpQ%tm#nnd &nltion energ& The process b&,g
*
1.
2. 3. 4. 5. 6. 7, 8. 9. 10. 11. 12. 13. 14. 15.
kvis, B. and van Eibg, G., CQF~2bUAX2, Flames and’ Explosions of Cases, Academic Press, New York, 1951. Metzler, A. 9.) NACA, WM E52F27, August, 1952. Metzler, A, J., NACA, RM E53H31, October, 1953. Calcote, H. F., Gregory, C. A., Barr&f, C. M., and Giber, R B., Itid. Eng. Chem 44,2656 (1952). SwEt, C. C., NACA, Res. Mem. ES5 I 16 (1955). pamai, G., ~ombust. Flame 2,171 (1958). !4ayers, .I. P., Tewari, G. P., and Wason 3. R., Gas Council Research Comm. GC 171 (1970). King, I. R., and C&ate, ti F., J. Chem. Phys. 23, 2444 (1955). Weinberg, P. J., and W&on, L R, Froc. Roy, Sm. A&U, 41 f1971). Cheng, D. Y., Combusm.P;aamc11,517 (1967). Schtieren Methods, Notes on Applied Science, No. 31, HMSO London (1963). cheng, D. Y., privateCommunication (1969). BiddIestone, H. G., and Nefdhercolt, W., ZEE Pape,r No. 3967M (1962). WoX, T. W., and Burke& ‘6. T,, Cornbust. Flcrme 1, 330 C1957), Pm, J. B.,ind. Erag. Chem 43,1865 (1951).
Data on minimum ignition energies are necessary for fuel ignition systems and for safety standards in relation to possMe explosbn hazards in fuel handling and in industrial situations, In the case of hydrocarbon fuels, while considerable information is availableon the influence of mixture composition, fuel type, and pressure on the minimum ignition energies of quiescent mixtures at room temperature [I -83 , a consi&rab~~ number of problems remain. Information in confkting in relation to the absolute v&es of the minimum ignition energies for a number of hydrocarbons, and there is uncertainty on the influence of molecular weight and of structure. In addition, few investigations have k3n concerned with measurements at elevated temperatures, althuugh data have been reported over a range of temperatures for five fuels in stuichiometric mixtures with air and data for one fuel, n-pm&m, have been presented for various temperatures and mixture strength [4,8]. Vtious experimental techniques have been
used in the determination of minimum ignition energies but in general measuremenb of this @pe invoke the use of capacitance sparks. IXfferen~s in technique, which may influence tb v&es ob*Pre~x~t address: British Gas Corporation bM&md~ Research Statiou, SoliN& Er@nd.
J. MOORHOUSE, A. WILLIAMS, and T.. E. MAIXMSQM
Fig. 1. Diagrammatic representation of the expanding plate capacitor apparatus,
ve~;.sel (9 cm diam X 20 cm.long) by means of
their part&l pressuras. The vessel was fitted with diametrically opposed electrodes insulated from &t3bomb by PTFE; one of the electrodes could b%rrr~4 to vary the electrode spacing and its w&don measured by a dial micrameter. The explosion vesselcould be heated to temperatures up to 125QC with an accuractf of O.l’C. Be spark was generated by,means of the ~ti&le capacitor &ice described by Cheng [ I.01 ssnd thsi~ated in Fig. 1, The “apacitor consisted 039twosgsrltllel plates, each f(30 X 120 mm, with WC R+& Ed ih,eather movab!e and separated By 343tir flp, The mnvMe plate was springIf~tiarl. ,#P&at ILccruIr$,when triggered, spring r$W fWra:its ulosest separation (ca. Imm) tu abaut 4U aan: apart and the position of the pla*ecould be determined by means of a dis&~?ment traducer. The fixed, &arged plate RQSco-ted to one eIectrodP;in the explosion ~3~1~ the ather de&rode I-zingconnected to Ihe RWvkI@ plMt% an4 tu 43&h, A Smallloop of tire HWUI~tk input. tead NWused as s capacitmce tit* diti&er to #ve WLindication of the instant Qfd&harge. The fixed@atecould be charged bm a high-voltagesljuiLlTct3 and the voltage r& by a &oq9ed electrostatic voltmeter.
The ends of the cylindrical explosion vessel were fitted tith parallel windows to permit single shot schlieren photographs to be obtained. A microsecond argon jet light (Lunartron Electronics Ltd) was used which was triggered after an appropriate time interval after the spark discharge by means of a delay unit [l I 1. The explosion vessel was also fitted with a pressure transducer to monitor whether ignition had occurred. Radiation from a spark or from ignition could also be detected by means of an IP28 photomultipler. The gases (Nlathesori)were used without further purification. Their minimum purities were: methane 99.9%; ethane, propane, and butane 99.0%. In the case of the liquids, laboratory grade reagents were used (Koch Light Laboratories Ltd.) and these were degassed and vacuum.distilled prior to use, Pure (99.9%) tungsten wife of 1 mm diam was used for the electrodes; the cathode was oxidized in air at 800°C for 20 miu before mounting in the explosion vessel. 2B. Determination of tie Energy of the &bark IMEchskrge
The expanding plate capacitor allows the storage of the charge at iow voltage and a discharge between the electrodes occurs when the plate separation has caused a sufficient rise in lroltage,
J. MOORHBUSE, A. WILLIAMS, and T. E. ~ADDNX4
Eig. 3. Stereoscan photographs of electrodes showing (a) used oxidized tungsten cathode showing surfw oxklation (magnification 65x), (b) used tungsten anode Wowing edge melting and erosion after discharge (magnification 260x 1.
wdes to obtah measurements of the minimum i@litttan energies for mixtures of various composiWI$, ignition was determined by means of a
pressure transducer or by the voltage pickup signal which indicated the occurrence of ignitiola as illustrated in Fig. 4. Minimum ignition energies
f
3. MOORHQWSE, A. WILLIAMS, and T. E. MADDISUN
Fig. 6. Ignition d&rams at 1 atm pressure. TACLE 1 Minimum Ignition Ener&zs and the Corresponding Ssoi~hiometries for Hydrocarbons CT = 22°C) Pressura -___ Fuei --. metbarle ethane propane n-butane n-pentane 2-pentene n-hexane 2-hexene n-heptane Hz
1.0 atm
0.75 atm 4
@min
Ca 1.4 0.99 0.96 0.91 0.55 0.85 0.95
1.28 1.31 1.4iI 1.41 1.19 1.44
Et
9 rnti
O&J 0.41 0.46
0.9 1.17 1.30
0.5
1.48
0.61
stoichiometry are summarized in Table 1. Data for sume mixtures at 1 atm pressure and 22°C were also obtained. The addition of 7% Hz to methane reduced Ei from 0.63 to 0.50 m3. Mixtures of methane and ethane showed a linear variation of El between the values for the two components. The effect of pressure is shown by the data in Table 1 for n-pentane and by the curves in Fig. 5. The increased ignition energy at the llower prese sure, the greatly reduced ignitable range and the slight shift of the curve to kanq mixturei are irnmediately obvious.
L?T-1
+p--Q--~---~-~~~-~~-’ ~:H~E’ ~*“~ I
i wLw2
$
Fig. 8. Ignition diagrams fm rr-pentsm i~t 0.73 otm g% temperatures in lM ~ang# 22.-t 25”C. 3D. Sfudks of E?mmd
Tsmqm-tim
Measurementsof minimtrm ignitjaI3ene@cs we~3 made at elevated ten~peratums in a sitik~ n~axme~ to those at 22°C. However the range of temperatures that could be studied was hmitr?d rry ths lower energy limit Of the apparatus, thus in order to widen the temperature range the fuels ware studied at 0.75 atm presswre. At the temperature at which a measuretlrent WAS made, a number of measurements were first made to ensure that the electrode spacing was greafez than the quenching distance. In genera1the% ranged from 2.3 to 23 mm. In this way, cures for a number of fueis were obtained at temperatures up to 125°C. The effect of the increase i.n temperature is to decreax? the minimum energy a~ illustrated for rzgentane in Fig. 8. AlI the fuel:: tested, with the sxcepticsn of n-butane+shgi’~a
J. MUORHOUSE,A. WILLtAMS,ial T, E, MADDESfJN
214
xtunrb~ wftenrgara~es, Evidence from theories of gpiirkQnition in quiescent mixtures as well as HQSX~WM atidence from studies in which tern~rrture and pressure have been treated as variables -~fbg#~tt,hat, within experimental error, the minituum @tiitSrrnenergy Ed can be expressed in the ~~H!K fii =
AP?,
&MW,d is a constant, P the pressure, and T the
(3)
temperature. From the data obtained in these experiments it is fo,und that or= 2.13 and fl= -2.03. l+wever the data obtained for the pressure dependency is related solely to n-psntane and to measurements at two firessurest although the values for each pressure are the outcome of a number of experiments. Metzler [2,3’j found thet Q = 1.82 with a maximum variation of 1.65 to 2.00. He observed also that the por;itfons of the minimum point of the ignition curves are only slightly pressure dependent over the range 0.1 to 1 atm, this is also consistent with the presetit work. Enthe case of the temperature dependency all the fuel: here show the same variation with temperature. The value obtained pf 0 = -2.03 is cunsistent with the work of King and Calcote [8] who found fl= -2.00. It is also consistent with a number of theoretical analyses [5,15,16] based on thermal thearies of which the expression due to Fenn [l S] is given bdcw: 1.5
PO
log Ei(Tf -
Tfj)
=c1 +21og G - 2logB
v/h&e:POis the oxygen partial pressure, Tf and Te are the flame temperature and initial temperature respectively, Cr and C, are constants, P is
the pmmlre, temperature,
tima
axxdTf@ean,isIthe lean timia IJfxauss af shesimlk~rity afall
12~~
pimental data ta ihe exponents few pxansure ami
tempctature in Eq. (a)$ it isimirmed EEkaF rkrilhh
aacuncy Q = .+Dand 8 CC~(*ZOO. The vdws of the cunstmt,A t ia Eq. (3”1dep~~l upon the natureof the fuel; tlrtricvaluesobtohk! frumthcw experiments, assuming that in all CaMs U” 2DandP = * 2.0 and with 7 g&tin ia K zad P in atmospheres, fire metbane, W? X 10’ ; alkm~, 422 x I@; propane, 4,74 X lob ; tt*butanr, 1.88 X IO”; n-p~tans, I >I12X 10’ ; 2-pentorlre~, 1.73 X 105;n-hexme, r,8 I X IOs; bhmw, 1 .B2 X 10’ ; and n-haptane, 1.8 li X IQ’ v The values uf the mfnimum fgri%n errcr@~ given by these expressions af i‘,5”c:ar~i 1 alna are higher t&n the values quoted by ti~is nnd van Elbe [I ] a Their results, wi~&h ate freatznq taken as standards in relation to safety stindards, relate to the case when one ignition ~cu15: in a hundred tests, i.e., an ignition probability of 0.81. The present results, those of IRwis and van Efbe El] aad M&kr [2,3] and some ofthe xesu~Fs given by CaZcote et al. 141 are given in Table 2. The more extensive data of Calcote et al., relating to the influence of molecular structtire cannut be used here since they aU refer to a stoichiometry of # = 1. It should be noted that the results of Lewis and van Elbe and Met&t and C&ate refer to the same experimental technique, namely capacitor discharges and an ignition probability of 0.01. A significant feature of the present pork is that the data refer to a boundary below which no ignitions were observed. Points above the boundary did not relate to a unity probability bug, in the range of Ei to 2E1, to a probability of -0.8. Here there is a good agreement with other measurements based on a similar probability. rhu$ for methane the present work yielded a minimum ignition energy of 0.63 mJ while Sayers et al. [7j found 0.55 mJ, simiiarly Komaror and Belikova [I 71 found 0.47, snd Toriyama [18] obtained 0~57 and 0.58 mJ. kiwis and von Elbe [I] obtained 0,29 mJ based on the low ignition probability criterion. No measurements are available for higher hydrocarbons with which csmptison may be made, The fact that no ignitions were ever observed bc.-
experimental
212
1. MOORHOUSE, A. WILLIAMS, and T+ E., MADDISON
plotting the tnitimum ignition energies as a function of H/U, where H is the number of hydrogen atoms ti tie molecules and 0 is the number of oxygen atoms in the mixture. This type of cxplanatfon is consistent with the high temperaturo data here.
noted that in low-temperature (I 500-9XIO K) studies of hydrocarbon ignition, trace amounts of ethane or butane [20] markedly sensitize @&ion. Once the flame kernel reaches the cri%icalsize, whether propagation or degeneration occurs depends upon the rate of enthalpy production by the combustible mixture, but it is also a function ofthe rate of energy released by the discharge and of any applied voltage still applied to the electrodes after the discharge which may influence the movement of the flame kernel. In the case of n-per&me studied here (Fig. 10) the critical radius is consistent with other measurements of critical radii. For example, Mozer and Sherburne 1221 found that ethane, propane, and n-butane had minimum critical radii of 2 mm at 1 bar pressure similar to the value drtained here for n-pentane. Thus flames produced using the expanding capacitor technique are essentially similar to those produced by the fixed capacitor discharge.
elf spark ignition involves the folstages: (i) the spark discharge itself, (ii) the transfer of energy from the spark channel to the combustible mixture, (iii) the oxidation initiation praceu4s and formation of the initial flame kernel &critical radius, and (iv) the occurrence of flame propagation or flame degeneratioa y of the procezes of ig Re ma.jor unatainties niticn relate to stage (i) and (ii). Firstly, the spark &charge process is not rqmducible in these or in o::E!rexpariments wHtb * regard to the spatial posit:aouofthe spark, and observations by means of thr: &Aicren system showed that the discharge followed diff’ercntpaths in all experinlents. This Sstsrgety determined by the nature of rhe elecfrodb surface and is consistent with thz observati-n that clectrod~ reconditioning was necassary &cr about fifteen dkcharges when presumably all the &ive s&tashad been destroyed. T~a~saturtis in the plasma are high for the first I&Vm~cmsecand.suntil all the energy is ttansfiXTedto ths initS flame kernel. Energy losses due t.0 s&k wave formation are small [23] but coWder&ie heat losses to the electrodes may NW, Thus it WAS observed that distortion to the flrc~ek~ttel by the ekctrodes readily occurred jlfCQg. abrlut # hundred mictus~c~nd~,this corresj~~~Crt#lo the ~xr;?rturbatXun at this time shown in #& IQ, ‘%a snrrgy releas;ozd by the spark itself is sufficknl: ‘lomaintain temperatures greater than %#I K throughout the flame kernel even before OX@ flame bs iFUy developed. Thus differences in fltictt~v i&ndpz&cuIariy the C-H bond W9&& Lr@nof inlpQ%tm#nnd &nltion energ& The process b&,g
*
1.
2. 3. 4. 5. 6. 7, 8. 9. 10. 11. 12. 13. 14. 15.
kvis, B. and van Eibg, G., CQF~2bUAX2, Flames and’ Explosions of Cases, Academic Press, New York, 1951. Metzler, A. 9.) NACA, WM E52F27, August, 1952. Metzler, A, J., NACA, RM E53H31, October, 1953. Calcote, H. F., Gregory, C. A., Barr&f, C. M., and Giber, R B., Itid. Eng. Chem 44,2656 (1952). SwEt, C. C., NACA, Res. Mem. ES5 I 16 (1955). pamai, G., ~ombust. Flame 2,171 (1958). !4ayers, .I. P., Tewari, G. P., and Wason 3. R., Gas Council Research Comm. GC 171 (1970). King, I. R., and C&ate, ti F., J. Chem. Phys. 23, 2444 (1955). Weinberg, P. J., and W&on, L R, Froc. Roy, Sm. A&U, 41 f1971). Cheng, D. Y., Combusm.P;aamc11,517 (1967). Schtieren Methods, Notes on Applied Science, No. 31, HMSO London (1963). cheng, D. Y., privateCommunication (1969). BiddIestone, H. G., and Nefdhercolt, W., ZEE Pape,r No. 3967M (1962). WoX, T. W., and Burke& ‘6. T,, Cornbust. Flcrme 1, 330 C1957), Pm, J. B.,ind. Erag. Chem 43,1865 (1951).