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Quenching distances of propane-air flames in a constant-volume bomb
Combustion
and Fhne
189
LETTERS TO THE EDITORS
Quenching Distances of Propane-Air Flames in a Constant-Volume Bomb 3y means of two flat plates inserted in a 5-k. spherical cavity, quenching dkances have been measuwd for five propane-$1 mixtures as a function of pressure from 3 to 90 ;~tm. The resuhs show that at high pressures the quenching distance becomes independent of pressure.
Introduction
studies of quenching phenomena, the term “quenching distance” has been applied to the minimunl distance between the flat plates of a slot burner through which a flame will propagate for a given pressure and piate temperature. Several authors fl, 21 have applied this technique to the experimental investigation of the quenchirg phenomena. They were able to study the effects of pressure, temperature, and surface conditions on the quenching of flames for various mixture ratios of air and propane. During the present study, quenching data for various mixture ratios of propane and air were measured at much higher pressures than those previously investigated. The experiments were conducted in a closed spherical combustion bomb. Two flat plates with a variable slot opening were inserted into the cavity through the wall of the vessel, A fast-response-surface thermocouple pb:ed on the inner wall of one of the plates was used to determine whether the flame succeeded in propagating between the plates. By recording the pressure and temperature trace on an oscilloscope and varying the distance between the plates, the critical or In
I4
PRViOUS
CF15
quenching distance could be determined a.s 3 function 0 f pressure.
Experimenta
Apparatus and Procedure
A schematic diagram of the experimental apparatus is shown in Fig. 1. Premixed gropaneair mixtures were used in these experimtnts.
7
.
Tl?ANSI)UCER EXCITATIO1J BATTERY
-&-
IDIAL
-t+
WHITEY
VALVE VALVE
BATTERY RESISTOR
-M--PROPANE PRESSURE TRIINSOUCER
OIODE
a C5NDLNSER - bl!a V4CUUM WMP
THERMOCOUPLE
MLXIW TANK
COMBUSTION BOM8 I
OSCILLOSCOPE
I BOURMIN GAUGE
CdERERP.
Figure 1.
Schematic
arrungcmcnl
1 MAHOMETER
of eqxrimentaf
apparuus.
The actual quenching plates were $ in. wide and were constructed so that they extended f in. into the cavity. One of the plates was permanently mounted on a base plug and contained a surface thermccouple of the type designed by Bendersky [3-j. The second plate
K. A. 0rm-1 and f. T. Agnew
190
was movable and parallel to the first. The distance between the plates could be varied from 0 to Q in. and maintained by rnear~~ of set screws, The spacing of the quenching plates was accomphshed with straight leaf thickness gauges. The slot opening was adjusted, and then the mavable plate was secured. When the setting nad been made and checked, the entire quenching assembly was inserted into the bomb ild rhc tllermocouple connected to the oscil-
Curve C shows the results obtained from the richest mixture tested, A/F = 11.5. The data indicate a constant slope from 2 to 40 atm. At 40 atm the quench and nonqucnch diskmces lewl ortt iit 0,005 and o.oM in,, mpcclivcly,
lOSCOpe*
After evacuating the cavity for iS mm, the bomb was loaded to the desired initial pressure and allowed to sit for 5 min. This time interval was coosidcred adaquate to reduce the turbulsucr lrvrl to where the flame could be considered Inminar. The combustibIe mixture was then ignited at thz center of the cavity, and the rasults were recorded. By continuing to rvm these tests and decreasing the slot opening, in point would br reached at which no temper;tturc deflection would occur, that is, the flame would extinguish before reaching the thermocouple. It was found that t.he point of quenching and nonquenching could be determined to I/INK1 in. and alwrtys occurred at peak pressur?. Ar low pressures it was more convenient to fis the piate spacing and vary the initial ~rcss~rilr EO find thr: conditions of quench L!II~rIot~tlul:nch. Tixe &Ha points for each mixtots: raticr \verc IIM taken in any particular tW&2r’. rltlJ 0,ftcrl tr’sls rvouk! be FCp~i-&tl #I? SOVWI! difkr~~~t ditys to confirm the repcatt1bliit~ of rhu rxpcrimental technique.
The results obtained for the five mixtures tA -EI ore shown in Fig. 2. It is readily noticed Ibat ‘::rn mixtures have si3nificant!y larger rmcuching distances than do stoichiometric 0r ricft trrixlurcs al any pressure level. In particHIM, 31 pressure levels around 60 atm the ratio nf fl~c Iargcst queuching distance obtained iair,fuel = 23) to the smallest (A/F = 13.5) can % as much as a factor of 3.
C-f1.5.
D=l5.7,E=f3.5.
This condition continues from 40 to 70 atm. Curve E contains the yesults for an air/fuel ratio of 13.5. This mixture is slightly rich and has the highest burning velocity of all the mixtures tested. It will also hc noted that for any given pressure it gives the lowest quenching distances. The results again indicate a curve of constant slope from 2. to 60 atm. At hiph pressures, above 60 atm, autoignition occurred, and it was irnpossibte to obtain data beyond this point. Data for the stoichiometric mixture (D) again seem to jevel out zt high pressures. This particular curve, however, has two bends, one occurring at 80 atrn and the other at 5 atm. This was the only air/fuel ratio in which two changes in slope occurred. It is of interest to point out that Agnew and Graiff [4] also observed a change in slope for the burning velocity-pressure relatiunship kw stoichiomctric propane-air mixtures. This occurred at the same
191
value of pressure (5 atm) for which rhe Lhange in slope of the quenching distance-pressure curve was observed in this investigation. With 8n air-fuel ratio of i9.0 (Bf B constant slope was again obtained from 3 to 70 atm. with the curye leveling out from 70 to 100 atm. Curve A shows the results with the leanest mixture tested. This mixture gave the largest quenching distance for a given pressure. Again a cat~tan~ slope was obtained from 4 to 70 atm. At this poini it leveled ou! and remained constant to 90 atm. In Fig. 3 ,he results for the two extreme A/F ratios used are compared with the data of Friedman and Johnstot] [ I], It cm bc seen that the slopes of the curves are lower for the ~_irr*-_C-___;_
! ] /
I
I
present i., vestigation. At pressures for which data are available from both studies, it cau be seen that this investigation gives quenching distances slightly lower than those of Friedman and Johnston. This cuuk! be attributed to the fact that the present work was done at cvtxtmt volume, whereas that ofFriedmn and Johnston was done at constant pressure. A constanivolumeprocessgives flame temper.aturesslightIy higher than the constant-pressure CSS~, This couiia account for the quenching distances being smakr.
References
i ~1
PRESSURE -&TM.
__..__-.-. _ _.. * Deceased.
~.~
.-- --
and Fhne
189
LETTERS TO THE EDITORS
Quenching Distances of Propane-Air Flames in a Constant-Volume Bomb 3y means of two flat plates inserted in a 5-k. spherical cavity, quenching dkances have been measuwd for five propane-$1 mixtures as a function of pressure from 3 to 90 ;~tm. The resuhs show that at high pressures the quenching distance becomes independent of pressure.
Introduction
studies of quenching phenomena, the term “quenching distance” has been applied to the minimunl distance between the flat plates of a slot burner through which a flame will propagate for a given pressure and piate temperature. Several authors fl, 21 have applied this technique to the experimental investigation of the quenchirg phenomena. They were able to study the effects of pressure, temperature, and surface conditions on the quenching of flames for various mixture ratios of air and propane. During the present study, quenching data for various mixture ratios of propane and air were measured at much higher pressures than those previously investigated. The experiments were conducted in a closed spherical combustion bomb. Two flat plates with a variable slot opening were inserted into the cavity through the wall of the vessel, A fast-response-surface thermocouple pb:ed on the inner wall of one of the plates was used to determine whether the flame succeeded in propagating between the plates. By recording the pressure and temperature trace on an oscilloscope and varying the distance between the plates, the critical or In
I4
PRViOUS
CF15
quenching distance could be determined a.s 3 function 0 f pressure.
Experimenta
Apparatus and Procedure
A schematic diagram of the experimental apparatus is shown in Fig. 1. Premixed gropaneair mixtures were used in these experimtnts.
7
.
Tl?ANSI)UCER EXCITATIO1J BATTERY
-&-
IDIAL
-t+
WHITEY
VALVE VALVE
BATTERY RESISTOR
-M--PROPANE PRESSURE TRIINSOUCER
OIODE
a C5NDLNSER - bl!a V4CUUM WMP
THERMOCOUPLE
MLXIW TANK
COMBUSTION BOM8 I
OSCILLOSCOPE
I BOURMIN GAUGE
CdERERP.
Figure 1.
Schematic
arrungcmcnl
1 MAHOMETER
of eqxrimentaf
apparuus.
The actual quenching plates were $ in. wide and were constructed so that they extended f in. into the cavity. One of the plates was permanently mounted on a base plug and contained a surface thermccouple of the type designed by Bendersky [3-j. The second plate
K. A. 0rm-1 and f. T. Agnew
190
was movable and parallel to the first. The distance between the plates could be varied from 0 to Q in. and maintained by rnear~~ of set screws, The spacing of the quenching plates was accomphshed with straight leaf thickness gauges. The slot opening was adjusted, and then the mavable plate was secured. When the setting nad been made and checked, the entire quenching assembly was inserted into the bomb ild rhc tllermocouple connected to the oscil-
Curve C shows the results obtained from the richest mixture tested, A/F = 11.5. The data indicate a constant slope from 2 to 40 atm. At 40 atm the quench and nonqucnch diskmces lewl ortt iit 0,005 and o.oM in,, mpcclivcly,
lOSCOpe*
After evacuating the cavity for iS mm, the bomb was loaded to the desired initial pressure and allowed to sit for 5 min. This time interval was coosidcred adaquate to reduce the turbulsucr lrvrl to where the flame could be considered Inminar. The combustibIe mixture was then ignited at thz center of the cavity, and the rasults were recorded. By continuing to rvm these tests and decreasing the slot opening, in point would br reached at which no temper;tturc deflection would occur, that is, the flame would extinguish before reaching the thermocouple. It was found that t.he point of quenching and nonquenching could be determined to I/INK1 in. and alwrtys occurred at peak pressur?. Ar low pressures it was more convenient to fis the piate spacing and vary the initial ~rcss~rilr EO find thr: conditions of quench L!II~rIot~tlul:nch. Tixe &Ha points for each mixtots: raticr \verc IIM taken in any particular tW&2r’. rltlJ 0,ftcrl tr’sls rvouk! be FCp~i-&tl #I? SOVWI! difkr~~~t ditys to confirm the repcatt1bliit~ of rhu rxpcrimental technique.
The results obtained for the five mixtures tA -EI ore shown in Fig. 2. It is readily noticed Ibat ‘::rn mixtures have si3nificant!y larger rmcuching distances than do stoichiometric 0r ricft trrixlurcs al any pressure level. In particHIM, 31 pressure levels around 60 atm the ratio nf fl~c Iargcst queuching distance obtained iair,fuel = 23) to the smallest (A/F = 13.5) can % as much as a factor of 3.
C-f1.5.
D=l5.7,E=f3.5.
This condition continues from 40 to 70 atm. Curve E contains the yesults for an air/fuel ratio of 13.5. This mixture is slightly rich and has the highest burning velocity of all the mixtures tested. It will also hc noted that for any given pressure it gives the lowest quenching distances. The results again indicate a curve of constant slope from 2. to 60 atm. At hiph pressures, above 60 atm, autoignition occurred, and it was irnpossibte to obtain data beyond this point. Data for the stoichiometric mixture (D) again seem to jevel out zt high pressures. This particular curve, however, has two bends, one occurring at 80 atrn and the other at 5 atm. This was the only air/fuel ratio in which two changes in slope occurred. It is of interest to point out that Agnew and Graiff [4] also observed a change in slope for the burning velocity-pressure relatiunship kw stoichiomctric propane-air mixtures. This occurred at the same
191
value of pressure (5 atm) for which rhe Lhange in slope of the quenching distance-pressure curve was observed in this investigation. With 8n air-fuel ratio of i9.0 (Bf B constant slope was again obtained from 3 to 70 atm. with the curye leveling out from 70 to 100 atm. Curve A shows the results with the leanest mixture tested. This mixture gave the largest quenching distance for a given pressure. Again a cat~tan~ slope was obtained from 4 to 70 atm. At this poini it leveled ou! and remained constant to 90 atm. In Fig. 3 ,he results for the two extreme A/F ratios used are compared with the data of Friedman and Johnstot] [ I], It cm bc seen that the slopes of the curves are lower for the ~_irr*-_C-___;_
! ] /
I
I
present i., vestigation. At pressures for which data are available from both studies, it cau be seen that this investigation gives quenching distances slightly lower than those of Friedman and Johnston. This cuuk! be attributed to the fact that the present work was done at cvtxtmt volume, whereas that ofFriedmn and Johnston was done at constant pressure. A constanivolumeprocessgives flame temper.aturesslightIy higher than the constant-pressure CSS~, This couiia account for the quenching distances being smakr.
References
i ~1
PRESSURE -&TM.
__..__-.-. _ _.. * Deceased.
~.~
.-- --