Sampling Studies of Cool Flames K. G . WILLIAMS, J. E. JOHNSON, AND H. W. CARH-IRT lliaval Research Laboratory, Washington 26, D . C.
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EXPERIMENTAL APPARATUS AND PROCEDURE
HEORETICAL interpretation of the phenomena observed with cool flames has been impeded by the apparent complexity of the process and the resulting difficulty of proving those reactions which have been postulated as parts of the over-all mechanism. These reactions are generally presumed t o proceed sequentially in a chain-branching process. Existing data on cool flame products generally do not seem to show any clear-cut separation of the process into specific reactions, although some data suggest partial separation of broad phases. This situation probably results from the overlapping of chains in time, each proceeding to its conclusion even as another is commencing. In the firm belief that a physical separation of separate zones of the cool flame could be achieved by probing into a cool flame stabilized in a long heated tube, a program was established t o make a preliminary evaluation of such a technique. The hope for success was based on the idea that increased surface has a decidedly inhibitory effect on cool flames. Prettre ( g ) , for example, obtained curves for the pentane cool flame which indicated that it is impossible t o initiate a cool flame in tubes less than 7 to 10 mm. in diameter. Spence and Townend (3) state that 6 mm. "seems to be the limiting diameter below which no 'cool' flames could be propagated." Therefore, it was not believed that any appreciable reaction would occur within sampling probes of small diameters. Very early in the experimentation, however, certain anomalies were observed in the data which led to the conclusion that extensive reaction was indeed occurring within the sampling probes, including those with diameters as small as 0.6 mm. Among conditions necessary for these reactions is a temperature within the probe on the order of 400" C. or more, a range in which Prettre states that the wall inhibition effects tend to disappear. These probe reactions presumably occur on the surface of the probes under conditions which suggest that some initiator, which must be developed in the gas phase, is required to establish the reaction.
The experimental apparatus is shown graphically in Figure 1. Tank oxygen is metered directly into a turbulence mixer a t the input end of the combustion tube. Tank nitrogen serves as a carrier for the n-hexane (Phillips pure grade, 99+ mole %), which is carbureted into the stream a t a constant rate by means of a modified Rideal Microdoser ( 1), an electrolytic pump. The gases are blended in the mixer and expanded into the combustion tube, a 100-cm. length of borosilicate glass tubing 25 mm. in inside diameter, surrounded by a heavily lagged electrically heated furnace. The furnace is supplied with a regulated power input and no attempt is made to control its temperature precisely. Total reactant flow is standardized a t 2780 ml. per minute, giving a linear velocity within the combustor of 9.5 cm. per second, calculated a t standard conditions. The true linear flow would be subject to a varying temperature correction and an unknown correction for change in volume with reaction. All linear flow velocities reported were calculated a t standard conditions. The normal reactant mixture contained 4% n-hexane, 9% oxygen, and 8'7'% nitrogen. Under these conditions a stable cool flame could be maintained in the combustion tube for extended periods of time. The position of the flame front varied slightly from day to day with changes in ambient temperature and variations in the flow rate, but it was consistently maintained within the central third of the combustion tube. A temperature profile dong the length of the tube is shown in Figure 2 with and without a cool flame. The cool flame raised the temperature about 100" C. Contents of the combustion tube were sampled along its longitudinal axis by inserting a glass tube connected to a controlled vacuum. The sampled gases passed through a calcium sulfate drying tower and a dry ice-acetone cold trap into a paramagnetic oxygen analyzer (Arnold 0. Beckman, Inc.). An iron-constantan thermocouple sealed in a length of borosilicate glass tubing 3 mm. in diameter was attached to the sampling probe so that the couple measured the temperature immediately adjacent to the probe inlet. T o produce the oxygen and temperature profiles as shown in subsequent figures, the probe was inserted into the exhaust end of the furnace and moved upstream by increments varying from 1 $0 20 mm., depending on the rate of change of the produced curve. Enough time was permitted after each positioning of the probe to permit re-establishment of equilibrium conditions.
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Data from an actual run are plotted in Figure 3. Data points have been omitted from subsequent curves for clarity. MIXING
CHAMBER
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EXPLORATORY STUDIES
TEMPERATURE MEASURING
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Figure 1. Diagram of combustion apparatus
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I n the preliminary phase of this investigation, curves typified by those of Figure 3 were reproducible from day to day if the furnace was heated continuously. During these experiments the rate of sampling was not controlled precisely and slight variations in the shape of the oxygen profiles seemed to occur with random variations in the sampling rate. This obsesvation led to an investigation of the effect of sampling rate.
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I ' N O COOL F L A U t
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Figure 3.
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Temperature profile of combustion tube
Typical temperature and measured oxygen profiles showing experimental points
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Using the original probe ( 3 mm. in outside diameter, 1.8 mm. in inside diameter), the sampling rate was adjusted to give linear flow velocities within the probe of 33, 50, and 66 cm. per second for each of three consecutive runs. The resulting curves, shown in Figure 4, demonstrate a remarkable dependence of the measured oxygen level of the rate of sampling. T o extend the sampling rate to increased velocities, a probe having an inside diameter of 0.6 mm. was used to make runs a t linear sampling velocities of 167 and 580 cm. per second. As is shown in Figure 5, the higher velocity of sampling (curve 4) eliminated the dip from the oxygen profile (curve 5 ) . The sampling study was extended to lower linear flows by using a constant volume rate of 97.5 ml. per minute with the above probes and with probes having inside diameters of 2.3, 4.4, and 5.9 mm. The results of sampling under these conditions are shown in Figure 6 (curves 6 to 10).
librium with the gases within the combustion tube and that the walls of the sampling probe have no effect whatever on the course of the reactions, it is possible to consider the hypothetical case in which the flow rate within the sampling probe is infinite. This would result in zero time for probe reactions and so, excluding distortion of the flame due to flow disturbances, the measured oxygen level would correspond precisely to that in the combustion tube a t the inlet to the probe. I n the more realistic situation in which the sampling rate is fast compared to the rate of reaction within the probe, the extent of probe reaction would be small and the measured oxygen level would be a good approximation of the actual level. A second case offered for consideration is that in which the flow rate within the sampler is precisely equal to that within the combustion tube. I n this case, the reactions proceeding within the probe would simply duplicate those occurring within the combustion tube and the measured oxygen level would be the same as that prevailing a t the exhaust of the combustion tube. The measured oxygen level then would be completely independent of the probe inlet position and the resulting plot would be a straight horizontal line. An examination of Figure 6 shows that these cases do not represent the actual data precisely. While the high velocity curve (No. 6 ) may conceivably be a fairly close approximation of the actual oxygen levels within the combustion tube, curve 9, for which the linear sampling rate was roughly the same (1.25 to 1f as the linear flow within the combustion tube, shows a measured oxygen level of zero upstream from the flame front. Curve 8, in which the sampling velocity was about four times the combustion tube velocity, is closer to the horizontal line predicted for a 1 to 1 flow ratio, but it exhibits a marked minimum just ahead of the flame front shown by the temperature measurements. T o account for the zero measured oxygen levels found in curves 9 and 10, for the minima of curves 7 and 8, and for the decrease in measured oxygen level downstream from the flame front with decreasing sampling velocity, it was assumed that the reactions occurring within the probe proceed faster than do those which take place in the combustion tube itself. As the surface-volume ratios of the probes exceed that of the combustion tube, it is reasonable t o assume that the probe reactions are surface reactions. The minima of curves 7 and 8, the peculiar rise of curve 9, and the agreement between the measured oxygen levels ahead of the flame front of curves 6 and 7 present additional difficulties in interpretation. These can be resolved by recognizing that:
DEVELOPMENT O F PROBE REACTION HYPOTHESIS
From the foregoing data it is obvious that the measured oxygen level cannot in all cases represent the actual oxygen concentration existing within the combustion tube at the inlet of the sampling probe. As the measured oxygen concentrations tend toward lower values than can reasonably be expected and the extent of the deviation increases with increasing residence time of the sample within the probe, it is not unreasonable to assume that aome oxygen-consuming reaction is occurring within the probes. Starting with this basic assumption and adding for simplification the further assumptions that the probe is in thermal equi-
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Figure 5. Effect of varying linear sampling velocity on measured oxygen profile using 0.6-mm. probe
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As the probe inlet is moved upstream from the flame front, the inlet moves through a zone in which the concentration of chain initiators existing within the combustion tube, added to those formed within the probe itself, becomes great enough to cause the chain-branching reaction to occur within the probe. I n this case the measured oxygen level does not bear any specific relation to the actual oxygen concentration in the combustion tube a t the point of sampling and represents instead the extent of reaction occurring within the probe. Additional forward movement of the probe inlet reduces the concentration of chain initiators taken into the probe and presents a measure of choice. If the net rate of initiators formed within the probe is small, the critical concentration is not reached and the probe reaction does not occur. The memured oxygen level then rises to the actual level within the combustion tube. With small probes and rapid flows, the very unfavorable surfacevolume ratio and low residence times sufficiently inhibit chain initiator formation, the probe reactions cease, and the measured oxygen level rises to agree with that existing in the combustion tube. This is illustrated by curve 7 of Figure 6 . With larger probes in which the inhibition of chain initigtor formation is not as severe, a critical concentration can be reached despite the absence of significant contributions from the combustion tube itself. A chain-branching reaction initiates within the probe and the subsequent surface reactions occur to an extent which is not possible in the larger diameter combustion tube. This is illustrated by curves 9 and 10. TEST OF PROBE REACTION HYPOTHESIS
Figure 6. Effect of varying probe diameter on measured oxygen profiles with constant-volume sampling rate of 97.5 ml. per minute
The net rate of formation of chain initiators is reduced by the presence of surfaces. The concentration of chain initiators within the Combustion tube decreases in a n orderly fashion upstream from the flame front. The concentration of chain initiators falls to essentially zero at some point within the flame front. The rate of formation of chain initiators increases with increasing temperature (in the range encountered within the combustion tube and probes). I n addition, it must be assumed that the surface reactions occurring within the probes are concerned with materials which are associated with the chain-branching reaction, as either intermediate or end products. With the foregoing conditions in mind, it should be obvious that as any probe is inserted from the exhaust end of the combustion tube, the measured oxygen level will represent the oxygen level existing within the combustion tube at the inlet of the probe, less that which has been utilized in the probe itself. With the probe inlet between the exhaust and the flame front, the amount of oxygen utilized in probe reactions will be a function of the specific surface of the probe, the time available for reaction, and other factors such as temperature and reactant concentrations that are fixed by the specific conditions in the combustion which, for the examples given, were essentially the same for all runs.
As a partial test of the validity of the foregoing conclusions, the following experiments were run. 1. By sampling from the inlet rather than the exhaust end of the combustion tube, samples taken upstream from the flame front would be subjected to less heat than is the case when the sampling probe is inserted from the exhaust end. I n addition, samples taken downstream from the flame front would be subjected to more heat in this case. This procedure would then tend to lessen the magnitude and extent of the minimum in the oxygen level curve and should cause a more rapid fall-off in the measured oxygen level downstream from the flame front. In Figure 7, the results of sampling with a 1.S-rnm. probe inserted from the inlet end are compared with those obtained by sampling with the same probe inserted from the exhaust end of the combustion tube. I n both cases the linear flow within the probes was 66 cm. per second. The data of curves 11 and 12 bear out the conclusion reached above.
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Figure 7. Effect of sampling from inlet end of combustor on measured oxygen profile at estimated sampling rate of 97.5 ml. per minute
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Effect of water-cooling probe on measured oxygen profile
Using 4.44-mm. probe with a sampling rate of 97.5 ml. per minute and linear sampling velocity of 25 em. per second
2 . A jacketed probe was built so that the probe could be cooled to within 35 mm. of the inlet end. Two runs were made with this probe, the first with no cooling, the second with a continual flow of tap water through the jacket. Sampling was done from the exhaust end of the furnace. It will be noticed from Figure 8 that water cooling (curve 13) substituted a slight plateau for the obvious minimum in curve 14 obtained with the uncooled probe. This plateau undoubtedly represents reactions which occurred in the uncooled 35-mm. end section of the probe. 3. A composite probe was made by joining a length of tubing 4.4 mm. in inside diameter to the inlet end of a 0.6-mm. capillary probe. For successive runs, the length of the large diameter section was shortened stepwise from 35 to 0.6 cm. Sampling was done from the exhaust end of the furnace at such a rate that the linear flow within the large diameter section was equal to the linear flow within the combustion tube. This should cause a minimum of distortion of the flame due to sampling. Under these conditions the sampled gases within the 4.4mm. section would be exposed to about the same temperature profile as that in the combustion tube and for about the same
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period of time. The principal difference would be in the increased surface of the probe. The flow in the capillary section was presumed to be high enough (424 cm. per second) to eliminate significant reaction. Results of these experiments are shown in Figure 9. It may be seen that a length of 4.4mm. tubing as short as 0.6 cm. (curve 15)was enough to cause a decided plateau in the oxygen consumption curve and that increasing the length of the large diameter section permitted progressively more reaction to occur (curves 16 to 19). An upstream displacement in the position of the first break in the oxygen curve occurs with increasing length of the large diameter section. This displacement, as would be expected, is approximately equal to the length of the section. 4. T o test the proposal that chain-branching reactions are a necessary precursor to the probe surface reactions, a probe similar to that used above ( a 0.6-mm. capillary within 6 cm. of 4.4-mm. tubing sealed to the inlet end) was packed with glass wool, in the hope that this would eliminate the chain reactions by inhibition of initiator formation. A packing of copper turnings was also used. The results of runs made with this probe are shown in Figure 10. It will be seen that the glass wool packing (curve 20) completely eliminated the minimum in the oxygen curve obtained with the unpacked probe (curve 22). The copper catalyzed the oxygen consumption once the reaction started (curve 21) but did not displace the initial break in the oxygen curve any appreciable distance upstream from that noted with the glass wool packing. This would seem to indicate that it too served as an inhibitor for initiator formation.
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Figure 10. Effect of packing 6-cm. length of 4.44-mm. tubing at intake of 0.6-mm. probe on oxygen profiles Sampling rate of 71 ml. per minute and linear sampling velocities of 8.7 om. per second in 4.44-mm. section and 424 cm. per second in 0.6-mm. section
Figure 9.
Effect of varying length of 4.44-mm. tubing at intake end of 0.6-mm. probe
Sampling rate of 71 ml. per minute and linear eampling velocity of 8.7 om. per second in 4.4-mm. section and 424 om. per second in 0.6-mm. section
5 . In the hope that it would be possible t o establish the place a t which reaction within the probes was occurring, the flame was sampled with a 4.5-mm. probe. After the measured oxygen and temperature profiles were established in the usual manner, the probe was left in a fixed position within the combustion tube with its inlet well ahead of the flame front. A pair of thermocouples sealed in glass were then arranged in such a manner that they could be moved forward simultaneously, one in the combustion tube, the other inside the now stationary probe. In addition to giving a direct measure of the temperature within the probe as compared to that in the combustion tube, the volume of the thermocouple would cause a considerable increase in the flow rate within the sampling probe downstream from the sensing end of the thermocouple. This would therefore constitute a tech-
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nique for sampling the contents of the stationary probe. This experiment was run a t two flow rates within the sampling probe. Results of probing the flame by the conventional technique of moving the probe forward by increments are shown in Figure 11 (upper). The flame was located further forward than usual in the combustion tube b y virtue of a difference in the hexane
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with the measured temperature maximum within the probe. The flatness of these curves downstream from the abrupt fall-off indicates that little further reaction is occurring and that the bulk of the oxygen-consuming reaction takes place in a relatively short length of the probe. The flat tail portion of the curve represents the measured oxygen level as determined by the unobstructed probe in the position in which it was fixed. DISCUSSION
Figure 11. Results of sampling using 4.48-mm. probe
concentration, but the plot is not otherwise unusual. I n Figure 11 (center) are shown the measured temperatures of the combustion tube and within the fixed probe. The temperature profiles of the combustion tube in both cases were identical. It will be noticed that the temperature maxima within the probe are displaced downstream from the temperature maximum in the combustion tube and that the maxima are higher within the probe than within the tube, suggesting exothermic reaction. The downstream displacement increases with increased flow through the sampling tube. I n Figure 11 (lower) are plotted the oxygen profiles measured within the Gxed probe as affected by moving the internal thermocouple forward incrementally. It may be seen that the falloff in oxygen level is rather abrupt and in both cases coincides
From the results of this study, a tentative explanation for the observed phenomena was developed, based on certain precepts which may be summarized briefly as follows: Chain-branching reactions analogous to or identiral with cool flame reaction can be produced in glass tubes of diameters as small as 0.6 mm. under conditions which permit the formation of chain initiators in a gas system having a more favorable surface-volume ratio prior to its introduction. Intermediate or end products of the chain-branching reaction are extremely susceptible to further oxidation on surfaces. T h e rate of this surface reaction is faster than the rate of oxidation of the same products in the gas phase. While the formation of chain initiators is inhibited by surfaces, the extent of inhibition decreases with increasing temperatures. Several experiments were devised to test the validity of the explanation and the results of these qualitativelj supported the original proposal. The complexities involved in the system prevent accurate quantitative description of the observed phenomena and so the rightness of these precepts must still be considered open to review. A strong suggestion remains implicit in the results, however, that the over-all cool flame reaction consists of at least two phases which, by proper selection of experimental conditions, can be either partially separated or individually exaggerated. The first of these phases is believed to be the commonly postulated chain-branching reaction. The second phase is apparently a surface reaction which seems to arise from either intermediate or end products of the branching reaction. As i t is virtually impossible to study cool flames in the absence of surfaces, the surface reactions must be regarded as a contributing factor in most of the experimental and anal) tical work which has been done on cool flames. The contribution of the surface reaction to the over-all reaction will depend on the surfacevolume and temperature relationships of the particular system employed. The fact that extensive reaction can occur within small probes suggests caution in the design and interpretation of experiments in which flames are sampled by probing techniques. LITERATURE CITED
(1) King, R. O., and Davidson, R. R., Can. J . Research, 21A, 65 (July 1943). ( 2 ) Prettre, M., “Third Symposium on Combustion. Flame and ExDlosion Phenomena,” Williams & Wilkins, Baltimore,
1949.
(3) Spence, K., and Townend, D. T. A,, “Third Symposium on Combustion, Flame and Explosion Phenomena,” p. 405, Williams & Wilkins, Baltimore, 1949. RECEIVED for review July 8, 1955.
ACCEPTED August 10, 1956.
