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Comparison of hot surface and hot gas ignition temperatures
348
Letters
presumably contain carbon dioxide.
substantial
amounts
to the Editors
of
R. A. RHEIN Jet P7o@4lsion Laboratory, California Institute of Technology, Pasadena, California (Received October 1964) Comparison of Hot Surface and Hot Gas Ignition Temperatures IGNITION temperatures of combustible mixtures are generally classified according to the nature of the heat source employed to effect ignition. Unfortunately, minimum ignition temperature values reported for a given mixture and initiating source frequently vary because of the various methods and apparatuses used by different investigators. In practice, the temperatures required for autoignition (AIT) tend to be lower than those for wire ignition. However, where the heat source dimensions are equivalent, we find that hot surface ignition temperatures do not vary greatly and tend to correlate over a wide range of heat source dimensions, particularly with hydrocarbon combustibles that have high AIT valuesI. Furthermore, hot gas ignition temperatures obtained with jets of hot air appear to correlate in a similar manner. Therefore, they tend to be in agreement with wire ignition temas cited by other investigators2s3; peratures, Table
I.
MIL-L-7808 JP-6 fuel n-Decane n-Octane n-Hexane n-Butane Ethane Methane Benzene Hydrogen
, oil
*Reference to trade names is for information only nnd endorsement by the U.S. Bureau of Mines is not implied.
Ignition
404 232 208 220 234 405 515 537 562 554
(Ref. (Ref. (Ref. (Ref. (Ref. (Ref. (Ref.
temperature,
Cylindrical Pyrax vessel 0.5 cm radzus 15 cm long
Pyrex Erlenmeyer (292 cm radius)* 13 cm long
4) 4) 4) 4) 4) 4) 4)
8
they also may not necessarily be much greater than those identified with heated vessel ignitions. A comparison of autoignition, wire ignition and hot gas ignition temperatures of various hydrocarbon combustibles and of hydrogen mixed with air is made in Table 1 for cylindrical heat sources of about 0.5 cm radius. The autoignition temperatures were obtained under static conditions where the liquid fuel was injected into heated cylindrical Pyrex* vessels; also included in the table are minimum AIT data reported for these combustibles in 200 cm3 Pyrex Erlenmeyer9. The wire ignition temperatures were determined under near-stagnant conditions (< 1 cm /set) where the combustible mixture was passed over a heated Inconel wire mounted perpendicular to the flow in a cylindrical tube. The hot gas ignition temperatures were also determined with near-stagnant combustible-air mixtures but with hot air jets at flow rates of about 50 cm” jsec and 100 cm3 /set in a cylindrical chamber of about 1Ocm diameter; data listed at the latter flow rate are from ref. 2 and were obtained by injecting the hot air jet into the pure fuel. It is seen that the minimum AIT values obtained with the Pyrex Erlenmeyer are noticeably lower than the corresponding values found
Comparison of hot surface and hot gas ignition temperatures combustibles and hydrogen with air at atmospheric pressure Fuel-air ratio-ptimum for ignition Ignition criterion-appearance of flame
Heat source Combustible \
Vol.
555 560 585 585 605 630 580 745 685 635
of
hydrocarbon
“C
Inconel wire 0.5 cm radius 5 cm long
Aiv jet 0.5 cm radius >I0 cm long
585 695 650 660 670 -
750 so5 750 755 765 910t 840 1 040 1 020 640
-
(Nitrogen jet)
AIT values obtained in 200 cm’ Erlenmeyer feauivalent cylinder radius Of 2.2 cm). +Values from ref. 2 obtained with hot air iet it&ted into pure fuel except for benzene: values of obtained with hot air jet iniected into fuel VaPour-air mixture.
(Ref. (Ref. (Ref. (Ref. (Ref.
*Minimum
present
research
2) 2) 2) 2) 2)
December
1964
Letters to the Editors
with the heated cylindrical vessel, Inconel wire and air jet of O-5 cm radius; the variation is greatest (330” to 575°C) for combustibles with low AIT values such as n-hexane, n-octane, n-decane and JP-6 fuel, which are least resistant to oxidation. By comparison, the cylindrical autoignition temperatures and wire vessel ignition temperatures (0.5 cm radius heat sources) do not vary by more than 75°C for n-octane and n-decane. Even less n-hexane, variation is found with heat sources of equal surface areas. It is further noted that the hot gas ignition temperatures of the various hydrocarbon combustibles are noticeably higher (2 160°C) than the autoignition temperatures obtained with the cylindrical vessel with a 0.5 cm radius. However, the hot gas ignition temperatures were measured at the jet base, whereas the axial temperature of the jet was at least 100 “C lower at the plane where ignition occurred above the jet base. In addition, pseudo or ‘cool’ flames were not considered as evidence of hot gas ignition; in some cases, such flames were observed at jet temperatures between 50” and 100°C below those required for ‘hot’ flame ignition. Thus, the hot gas ignition temperatures can be as much as 200°C lower than the values reported here, in which case they would not differ greatly from the autoignition or the wire ignition temperatures. Admittedly, some variation should be expected between these different temperatures since the controlling reaction mechanisms for hot surface and hot gas ignition do not necessarily display the same temperature dependence: also, .the wall effects present in autoignition are essentially negligible in the hot wire and gas ignitions. Although mass transport can be important, particularly with hot gas ignition, its influence on ignition temperature cannot account for the large variations observed when heat source dimensions differ greatly. In comparing ignition temperatures, a number of variables besides the nature of the heat source and its dimensions must be considered. Generally, the ignition temperatures increase with decreasing pressure and oxygen concentration, the increase being greatest at reduced pressures and in small size vessels (as used above) where wall quenching effects are most evident. They
349
vary slightly with combustible concentration except at near-limiting concentrations for ignition where they increase noticeably. They also increase with decreasing contact time or fuel residence time and, therefore, are usually higher under flow conditions; the influence of contact time may also account for part of the variation observed between the hot surface and hot gas ignition temperatures cited above. However, for a given contact time, the ignition temperature of a combustible mixture tends to be the same under static and laminar flow conditions. With increasing flow or with decreasing heat source dimensions, the rate of heat input also becomes important. Obviously, rate of heat input can be expected to be of little influence in minimum AIT determinations where the heat source is large and the fuel contact times are maximum. As mentioned earlier, the use of light emission must be further qualified to distinguish between ‘cool’ and ‘hot’ flame ignitions. Similarly, the magnitude or rate of the pressure or temperature rise should be specified when either of these criteria is used to detect ignition: where one of these is employed, the ignition temperature values tend to be lowest, particularly under conditions where flames are quenched readily. These and the other factors discussed here must be considered before a valid comparison of ignition temperature data can be made. This research was sponsored by the U.S.A.F. Laboratory, Research and Aero Propulsion Technology Division, Wright-Patterson Air Force Base, under Delivery Order 33(657)-63-
37, Task 304801. J. M. KUCHTA R. J. CATO M. G. ZABETAKIS Explosives Research Center, Bureau of Mines, 4800 Forbes Avenue, Pittsburgh, Pa, U.S.A. (Received
November
1964)
References KUCHTA, J. M., BARTKOWIAK, A. and ZABETAKIS, M. G. ‘Hot surface ignition temperatures of hydrocarbon fuel vapor-a& mixtures’. Paper presented at 148th Meeting of the American Chemical Society, Chicago, Ill., 2 September 1964
350
Letters
to the Editors
2 VANPEE, M. and WOLFHARD, H. G. J. Amer. Rocket Sot. July 1959, 29, 517 3 WOLFHARD, H. G. Jet Propulsion, December 1958, 28, 798
Vol.
ti
4 ZABETAKIS, M. G., FURNO, A. L. and JONES, G. W. Zndustr. Engng Chem. (Zndustr.), October 1954. 46, 2173
Letters
presumably contain carbon dioxide.
substantial
amounts
to the Editors
of
R. A. RHEIN Jet P7o@4lsion Laboratory, California Institute of Technology, Pasadena, California (Received October 1964) Comparison of Hot Surface and Hot Gas Ignition Temperatures IGNITION temperatures of combustible mixtures are generally classified according to the nature of the heat source employed to effect ignition. Unfortunately, minimum ignition temperature values reported for a given mixture and initiating source frequently vary because of the various methods and apparatuses used by different investigators. In practice, the temperatures required for autoignition (AIT) tend to be lower than those for wire ignition. However, where the heat source dimensions are equivalent, we find that hot surface ignition temperatures do not vary greatly and tend to correlate over a wide range of heat source dimensions, particularly with hydrocarbon combustibles that have high AIT valuesI. Furthermore, hot gas ignition temperatures obtained with jets of hot air appear to correlate in a similar manner. Therefore, they tend to be in agreement with wire ignition temas cited by other investigators2s3; peratures, Table
I.
MIL-L-7808 JP-6 fuel n-Decane n-Octane n-Hexane n-Butane Ethane Methane Benzene Hydrogen
, oil
*Reference to trade names is for information only nnd endorsement by the U.S. Bureau of Mines is not implied.
Ignition
404 232 208 220 234 405 515 537 562 554
(Ref. (Ref. (Ref. (Ref. (Ref. (Ref. (Ref.
temperature,
Cylindrical Pyrax vessel 0.5 cm radzus 15 cm long
Pyrex Erlenmeyer (292 cm radius)* 13 cm long
4) 4) 4) 4) 4) 4) 4)
8
they also may not necessarily be much greater than those identified with heated vessel ignitions. A comparison of autoignition, wire ignition and hot gas ignition temperatures of various hydrocarbon combustibles and of hydrogen mixed with air is made in Table 1 for cylindrical heat sources of about 0.5 cm radius. The autoignition temperatures were obtained under static conditions where the liquid fuel was injected into heated cylindrical Pyrex* vessels; also included in the table are minimum AIT data reported for these combustibles in 200 cm3 Pyrex Erlenmeyer9. The wire ignition temperatures were determined under near-stagnant conditions (< 1 cm /set) where the combustible mixture was passed over a heated Inconel wire mounted perpendicular to the flow in a cylindrical tube. The hot gas ignition temperatures were also determined with near-stagnant combustible-air mixtures but with hot air jets at flow rates of about 50 cm” jsec and 100 cm3 /set in a cylindrical chamber of about 1Ocm diameter; data listed at the latter flow rate are from ref. 2 and were obtained by injecting the hot air jet into the pure fuel. It is seen that the minimum AIT values obtained with the Pyrex Erlenmeyer are noticeably lower than the corresponding values found
Comparison of hot surface and hot gas ignition temperatures combustibles and hydrogen with air at atmospheric pressure Fuel-air ratio-ptimum for ignition Ignition criterion-appearance of flame
Heat source Combustible \
Vol.
555 560 585 585 605 630 580 745 685 635
of
hydrocarbon
“C
Inconel wire 0.5 cm radius 5 cm long
Aiv jet 0.5 cm radius >I0 cm long
585 695 650 660 670 -
750 so5 750 755 765 910t 840 1 040 1 020 640
-
(Nitrogen jet)
AIT values obtained in 200 cm’ Erlenmeyer feauivalent cylinder radius Of 2.2 cm). +Values from ref. 2 obtained with hot air iet it&ted into pure fuel except for benzene: values of obtained with hot air jet iniected into fuel VaPour-air mixture.
(Ref. (Ref. (Ref. (Ref. (Ref.
*Minimum
present
research
2) 2) 2) 2) 2)
December
1964
Letters to the Editors
with the heated cylindrical vessel, Inconel wire and air jet of O-5 cm radius; the variation is greatest (330” to 575°C) for combustibles with low AIT values such as n-hexane, n-octane, n-decane and JP-6 fuel, which are least resistant to oxidation. By comparison, the cylindrical autoignition temperatures and wire vessel ignition temperatures (0.5 cm radius heat sources) do not vary by more than 75°C for n-octane and n-decane. Even less n-hexane, variation is found with heat sources of equal surface areas. It is further noted that the hot gas ignition temperatures of the various hydrocarbon combustibles are noticeably higher (2 160°C) than the autoignition temperatures obtained with the cylindrical vessel with a 0.5 cm radius. However, the hot gas ignition temperatures were measured at the jet base, whereas the axial temperature of the jet was at least 100 “C lower at the plane where ignition occurred above the jet base. In addition, pseudo or ‘cool’ flames were not considered as evidence of hot gas ignition; in some cases, such flames were observed at jet temperatures between 50” and 100°C below those required for ‘hot’ flame ignition. Thus, the hot gas ignition temperatures can be as much as 200°C lower than the values reported here, in which case they would not differ greatly from the autoignition or the wire ignition temperatures. Admittedly, some variation should be expected between these different temperatures since the controlling reaction mechanisms for hot surface and hot gas ignition do not necessarily display the same temperature dependence: also, .the wall effects present in autoignition are essentially negligible in the hot wire and gas ignitions. Although mass transport can be important, particularly with hot gas ignition, its influence on ignition temperature cannot account for the large variations observed when heat source dimensions differ greatly. In comparing ignition temperatures, a number of variables besides the nature of the heat source and its dimensions must be considered. Generally, the ignition temperatures increase with decreasing pressure and oxygen concentration, the increase being greatest at reduced pressures and in small size vessels (as used above) where wall quenching effects are most evident. They
349
vary slightly with combustible concentration except at near-limiting concentrations for ignition where they increase noticeably. They also increase with decreasing contact time or fuel residence time and, therefore, are usually higher under flow conditions; the influence of contact time may also account for part of the variation observed between the hot surface and hot gas ignition temperatures cited above. However, for a given contact time, the ignition temperature of a combustible mixture tends to be the same under static and laminar flow conditions. With increasing flow or with decreasing heat source dimensions, the rate of heat input also becomes important. Obviously, rate of heat input can be expected to be of little influence in minimum AIT determinations where the heat source is large and the fuel contact times are maximum. As mentioned earlier, the use of light emission must be further qualified to distinguish between ‘cool’ and ‘hot’ flame ignitions. Similarly, the magnitude or rate of the pressure or temperature rise should be specified when either of these criteria is used to detect ignition: where one of these is employed, the ignition temperature values tend to be lowest, particularly under conditions where flames are quenched readily. These and the other factors discussed here must be considered before a valid comparison of ignition temperature data can be made. This research was sponsored by the U.S.A.F. Laboratory, Research and Aero Propulsion Technology Division, Wright-Patterson Air Force Base, under Delivery Order 33(657)-63-
37, Task 304801. J. M. KUCHTA R. J. CATO M. G. ZABETAKIS Explosives Research Center, Bureau of Mines, 4800 Forbes Avenue, Pittsburgh, Pa, U.S.A. (Received
November
1964)
References KUCHTA, J. M., BARTKOWIAK, A. and ZABETAKIS, M. G. ‘Hot surface ignition temperatures of hydrocarbon fuel vapor-a& mixtures’. Paper presented at 148th Meeting of the American Chemical Society, Chicago, Ill., 2 September 1964
350
Letters
to the Editors
2 VANPEE, M. and WOLFHARD, H. G. J. Amer. Rocket Sot. July 1959, 29, 517 3 WOLFHARD, H. G. Jet Propulsion, December 1958, 28, 798
Vol.
ti
4 ZABETAKIS, M. G., FURNO, A. L. and JONES, G. W. Zndustr. Engng Chem. (Zndustr.), October 1954. 46, 2173