HYDROGEN ATOMEXCESSES IN SOMEPROPANE FLAMES
March, 1961
527
HYDROGEN ATOM EXCESSES IN SORIE PROPANE FLANES BY ROBERT REID’AND ROBERTWHEELER Department of Chemistry, Queen’s Cniberszty, Kingston, Ontario Receaved October Y, 1960
Concentrations of atomic hydrogen have been determined a t various heights in some pre-mixed propane-air flames. Excesses were noted early in the burnt gases, these being relatively larger for lean (fuel) flames and smaller for rich flames, the direct reverse of observations in hydrogen flames. The rise-velocities of the burnt gases have been measured and used to establish reaction velocity parameters consistent with ternary recombination of the excesses.
Introduction Sugden and co-workers, 2-5 have applied various emission spectrophotometric techniques to measure the atomic hydrogen and hydroxyl radical concentrations in the burnt gases of atmospheric hydrogen-air flames. Their results show that [HI becomes increasingly excessive of the equilibrium amount, as the flames are made more hydrogen rich. This is paralleled by similar excesses in [OH] and [O] measured by independent m e a n ~ . ~ Fenimore J and Jones8v9 have sampled through a fine quartz probe the combustion products of organic as well as inorganic fuels. i4dding small quantities of heavy water to the gas supply, they analyzed samples from the burnt gases massspectrographically and were able to determine [HI a t several positions. Their results for Hzair mixtures corroborate those of Sugden’s group. I n contrast, their findings for the very fuel-rich, lowtemperature, hydrocarbon flames showed the theoretical equilibrium or lower concentration throughout. Apparently, the carrying over of an excess of organic fuel into the burnt gases is not accompanied by large amounts (greater than equilibrium) of atomic hydrogen. Whether or not the value of [HI entering the burnt gases is above its thermodynamic equilibrium value, [HI,. a similar relationship t o its equilibrium concentration is expected for each member of the group, OH, 0 2 and 0. Very soon after departing the primary zone, a partial chemical equilibrium should establish t o a degree that
molecular exchanges H
+ Oz
OH
+ Oz
(4)
I n flames of organic fuels, the quasi-equilibrium of equation 1 will certainly be assisted by the reaction COz
+ OH
CO
+ OH
(5)
this’l being only somewhat slower than 2. A photometric technique developed by Sugden and collaborators has been applied here to some propane-air flames burning at atmospheric pressure. These results show some evidence that [HI is initially in excess even in very rich mixtures.
Experimental
Burner and Supply.-Metered quantities of propane and air with added nitrogen or argon were mixed and burned at a M&ker type burner of 7.8 em. diameter. This burner was mounted on a telescoping brass tube, permitting movement in the vertical direction. Very small but equal amounts of lithium and sodium chloride solutions were atomized into a n isolated gas supply to the very center of the flame (3.8 em. diam.). This resulted in a uniform flame with respect to the fuel and air supply but in the very center of which appeared minute quantities of evaporated sodium and lithium atoms whose partial pressures in the burnt gases were each approximately 10-7 atm. The outer annular flame acted as thermal shield against self-absorption of the radiation at the flame edges. Photometric System.-Radiation from selected heights in the very center of the flame was focused a t the entrance slit of a direct reading Farrand grating monochromator. The width of the entrance slit was 0.01 cni. and its height for the flames investigated was found to be maximally 0.05 cm. At a magnification of 1.0 the source of radiation was limited to 0.04 cm.2 by suitably situated optical stops. A 1P 21 electron multiplier photo-tube operating a t about 100 v. per stage was mounted behind the exit slit. A sectored disc operating at 465 c./sec. chopped the radia[HI and [OH] are equilibrated by the fast re- tion falling on the entrance slit with provision made to prevent draughts. The resulting alternating signal was act.ion2B9 amplified, rectified and measured on a damped sensitive but selective voltmeter. Because the flame front was not H IhO HS OH (2) totally flat, attention was directed to the burnt gases at while the reaction some distance above the burner top and upwards on the central flame axis. M$aeurements of the intensities of tlte 0 IIzO JJ Hz 0 2 (3) sodium-D-lines a t 5893 A. and the lithium red lines at 6707.4. should be balanced1° through the very fast bi- were made a t identical points and the ratio of these used with the measured temperature to deduce [HI a t each point. (1) Deyartnicnt of Physical Chemistry, University of Cambridge, Temperature Measurements.-Flame trmperntures w ~ er England. measured by the usuaI sodium+ line reversal met hod,12usmq . (2) E. hl. Bulewioz, C . G. James and T. M. Sugden, F‘roc. 1 2 0 ~SOC. a tungsten strip filament background source. The reversal (London), 8236, 89 (1966:. point was determined by a photometric scanning technique (3) P. J. Padley and T. 11. Sugden. ibid., 8 2 4 8 , 248 (1958). rather thnn by eye. The burnt gas temperatures measured (4) E. M. Bulewioz and T. 11. Sugden, Trans. Faradnu Sac., 64, in this manner, ranging from 185@2100”K., T\ ere vcrificd b\ 1855 (1958). a similar reversal of the lithium doublet. (5) C. G. James and T. XI. Sugden, Proe. R o y . Sac. (London), A248, Rise-Velocity Determinations .-In order to convert ob238 (1958). servations of intensity from a distance t o a time scale, a (6) E. M. Bulewics and T. M. Sugden, Trans. Faraday Soc., 62, measure of the velocity with which the burnt gases ascend 1481 (1956). vertically from the reaction zone was made. -1simple rotat(7) C . G. James and T. hI. Sugden, Nature, ilS, 252 (1955). ing drum camera was used to photograph the traces of small
+
+
+
+
(8) C. P. Fenimore and G. W. Jones, J . Phys. Chem.. 62, 693 (1958). (9) C. P. Fenimore and G. W. Jones, ibid., 63, 1834 (1959). (10) C. P. Feniinore and G . W.Jones. ibid., 62. 178 (1968).
-__
(11) C. I’ Fenimore and G. W. Jones, zbzd., 62, 1578 (1958). (12) A. G. Gaydon and H. G. Wolfhard, “Flames, Their Structure, Radiation and Temperature,” Chapman and Hall, London, 1960.
ROBERTREIDAND ROBERTWHEELER
528
Vol. 65
M solutions of !ithiurn and sodium chlorides at which dilution the effect of self-reversal of the radiation waa negligible in a shielded flame. The free lithium atom population is depleted by
+
+
Li HsO LiOH H (6) forming stable gaseous LiOH and reducing the intensity of the red lines. Moreover, this reaction is expected to be equilibrated in the burnt gas region. Therefore
0
5 Height above burner (em.). Fig. 1.-Axial hydrogen atom concentrations in flames of various pre-burnt mixture strengths (propane/stoichiometric propane:), absolute temperatures are shown for the steady state.
quantities of aluminum particles introduced in the flame gas supply to the very center of the burner only. The drum of the camera was 64 cm. in circumference and rotated a t 78 rev./min. with its surface in the focal plane of the camera. A vertical slit system was used to record only those traces of particles ascending near the center of the flame. The veocity Vmof the traces can be calculated from V , = (64 r T tan e)/s cm./sec., where r = the distance between the camera lens and the flame, s = the distance between the lens and the film, T .= the angular speed of the drum in rev./sec., and 0 = the angle between the trace marks and the perpendicular to the axis of rotation of the drum. Four different grades of aluminum particles were used in a series of experiments. Three of the samples were graded flat particles with surface areas of 1.5 X lo4,2.8 X lo4,! X lo4 crn.z/g., the fourth was an atomized sample of considerably smaller specific surface. Particles were introduced in the gas su ply by suspending them on a wire gauze fitted to a detachabye Pyrex container just beneath the burner. Slight vibrations of the container were sufficient t o carry the particles into the flame. In the measurements, we found no variations in tan e with particle size, indicating that the particles employed were sufficiently small to obviate a size factor correction. A simple graphical method was used to establish the most frequently occurring value of tan e for each flame. Values of e were distributed over about 5" in each flame, the error of measurement in this angle being about 1%. Both this and the systematic error resulting from the effect of gravitational force on the aluminum particles (
