Some optical studies with the air-acetylene flame

Talmta, 1972, Vol. 19, pp. 927 to 936. Pcrimmon Pmas. Printed in Northan Ireland

SOME OPTICAL STUDIES WITH THE AIM-ACE~LE~E FLAME K. M. ALDOUS*, R. F. BROWNER?, D. CLARK, R. M. DAGNALL and T. S. WEST Chemistry Department, Imperial College, London SW.7 2AY, U.K. (Receiued 25 October 1971. Accepted 2 December 1971)

Summary-The application of Schlieren and shadow techniques for the study of flame processes is diiussed in relation to analytically useful premixed flames. The information obtained, particularly with shadowgraphs, may be correlated with the measured signals and signal noise in atomic-absorption and flame-emission spectrometry. OPTICAL methods employing Schlieren and s~dow~aph techniques have been used extensively in combustion research for examining refractive-index fieIds,l particularly when information on such fundamental parameters as burning velocities or energyrelease profiles has been required. However, there appears to have been no attempt to correlate the photographic records obtained by these methods with quantitative observations on the behaviour of metal atoms when nebulized into flames. It is known that turbulent d~~~s~onflames, such as those produced with Beckman burners, have a higher emissive background than the corresponding laminar flames2 and it has also been observed that atomic species introduced into them are subject to more severe interelement interferences than in laminar flames. Though, as an adjunct to these observations, it is generally agreed that turbulence in pre-mixed flames is also undesirable, and various workers have published Schlierens**~6and shadows records of several flames, the studies were purely qualitative and no conclusions were drawn indicating just how turbulence in premixed flames infIuences analytica measurements, The purposes of this study were therefore to examine Schlieren and shadow techniques as a means of observing flame processes (particularly the effect of flame separation) in some an~ytically useful flames, and to correlate this i~ormation with signals obtained from ato~c-abso~tion and flame-emission measurements. In this context, changes in signal magnitude and signal noise are of particular interest as they are likely to be influenced directly by the flame processes, The work described in this communication indicates that net changes in free atom fractions due to variations in the temperature or chemical composition of the ftame gases can be a direct consequence of flame turbulence in premixed flames, as can the signal noise due to fluctuations of the atomic concentration in the flame. INFORMATION

PROVIDED

BY OPTICAL

TECHNIQUES

General theory The theory and applications of Schlieren, shadow and interferometric techniques have been reviewed most ~oroughly by Weinberg1 and here we summarize only the salient features which are of direct relevance to the present study. The optical * Present address: Division of Laboratories and Research, N.Y. State Health Department, Albany,N.Y. 12201,U.S.A. 7 Present address: Department of Chemistry, University of Florida, Gainesville 32601, U.S.A. 927

928

K. M. ALDOUS, R. F. BROWNER,D. CLARK, R. M. DAGNALL and T. S. WEST

methods mentioned provide means of measuring refractive index gradients in gases by photographic techniques and hence of defining regions over which the processes which produce such changes are occurring. In the shadow method, fight from a point source of illumination is passed through the test zone onto a screen. Any optical disturbance which takes place within the zone and which produces a refractive index gradient normal to the direction of the lightbeam results in a local change of illumination on the screen. Refractive index gradients parallel to the light-beam cause no change in screen illumination. In Schlieren photography, a parallel beam of light, after passing through the test zone, is brought to a focus at a “marking aperture” (which may take the form of a knife-edge, a pin-hole or a neutral optical wedge) by another lens. The aperture removes from the image (which in the absence of the marking aperture would be a normal image of the flame) the rays undeflected by the flame, whilst allowing some of the deflected rays to pass. An optical arrangement which may be used for shadow work and converted for use in Schlieren photography by the incorporation of only one further component (the marking aperture) is shown in Figs. 1 and 2. With these arrangements, the positioning of the flame with respect to the lens (F) is varied, depending on whether shadowgraph or Schlieren records are required. Both optical methods provide the following information. They indicate the positions and dimensions of the primary and secondary reaction zones, as these are regions over which rapid refractive index changes are occurring. By the shapes of the zones they indicate the contours of the refractive index gradients in the burning or burned gases. A wrinkled appearance of a zone shows turbulence. They indicate the rate of refractive index change by the sharpness and degree of darkening of the zone (e.g., primary reaction zones of Iaminar flames give much sharper images than do the zones of secondary diffusion flames), Time variable processes The optical observation of rapid fluctuations in the flame is important even though the higher frequency fluctuations may not register as noise on the recorder read-out (owing to its limited frequency bandpass) when the radiative emission from the flame is observed. Nevertheless, these fluctuations wiII give rise to overall changes in the

H F E D AB c FIG. 1.-Shadow optics. A-Real source. B-Condenser lens, C-Aperture (effective source). D-Collimating lens. E-T& zone. F-Magnifying lens. H-Film.

AB Era. 2.-Schlieren

D-Schlieren

C optics.

D

E

Optical components:

lens. F-Projection

F

G

H

A, B, C, E, H-as

lens. G-“Marking

in Fig. 1.

aperture”.

composition of the burned gases, because of the di%xent combustion processes induced. Changes in gas composition, which will also be associated with changes in gas temperature, may alter the free atom fraction and the magnitude of thermal emission signals fram so&&as nebulized into the %me.

fn ato~~-abso~t~~~ s~~orn~~y, when ah the ~~v~o~svocables whi& ef%xt the total cancentration of the element (free and c~rn~~ned) in the &me (such as ~ebul~~atio~rate, flame pa~-~~~~, etc) are fixed, then the signal is a dire& cemparative measure of the free atom fraction, & In &me-emission s~trorn~~~y, the signal rn~~~~d~ is again dependent on 8, but is also further fluent by the 3ame tern~rat~e~ because of the nature of the radiative process,

The noise in any rn~asur~ sign&, whether from atomic abso~tion or flame emission, is a comb~natjo~ of many com~unents~ Those noise components which are unrelated to iiame processes are of no relevance to this work and can be e~~~~~d from measurements of the total signal noise by the ~~~~~c~on of quadratic camp+ nents. For a spectrometric system with a fixed e~~tro~~ (noise) bedpan (AD, t&etot& root mean square noise voltage, NT, may be related to the j~~~end~t root mean square noise components, A$)by the ~~a~o~s~p :r

This Sharon includes &altthe inde~~de~t root rnoa~ square noise ~orn~~e~~ which ~ont~bute to N T.* In this particuhir instance only one noise ~ornp~ne~t is of interest, the noise [from atoms in the f&me) inducecfby flame processes, and this may be represented as Iv,. As a direct corollary of the signal magnitude in atomic absorption and atomic emission, the signal noise, Iv,, will measure fluctuations in the free atom ~on~nt~~o~, @. In atomic emission it will aho measure ~~~~atia~s in the &txw~ temperature. ~x~~rn~~~l~~ A$& measnred as the total roet. mean square noise in the ~a~~~ s@aI and _iVz is ob~ned by s~b~~~~~g from this the mrn~~ng noise ~om~onents as measnred by the root mean square noise of the basetine, thus:

930

K. M. A~nous, R. F. BROWNBR,D. CLARK, R. M. DAVNALLand T. S. WBST

optical observations of the flame gases, in all subsequent discussion this noise alone will be considered and will be referred to as “signal noise”. The apparent frequency of this noise as it registers on a recorder will be limited by the effective bandpass of the whole read-out system. However, the root mean square value of this waveform is a valid and convenient comparative measure of noise resulting from ffuctuating processes in the flame. For the purpose of this discussion, it is convenient to consider signal noise as originating from two separate sources. First there is nebulization noise. This noise should remain a constant proportion of the signal magnitude and is not amenable to measurement by Schlieren or shadow records of the flame gases although the spray supply itself can be studied by these means. Secondly there are fluctuations in the burned gases themselves. In a stable laminar flame this will be reduced to a minimum and will be of much smaller magnitude than in a turbulent flame. However stable and laminar a flame may appear to be, there will exist within it local variations in temperature and gas composition which will produce noise both in the background emission from the flame (flame flicker noise) and in any signal which is dependent on atomic concentrations in the flame. When the signal noise is not a constant proportion of the signal magnitude, even though the same solution is being nebulized, then it may be assumed that the deviation from propo~onality is a measure of the flame fluctuations. EXPERIMENTAL

Optics There are numerous possible experimental arrangements for Schlieren and shadow photography. The optical systems employed in this work were shown in Figs. 1 and 2. Lenses were used throughout. In the shadow system the flame position (E) is not critical, provided that it is not optically conjugate with the flhn plane, i.e., a focused image of (E) is not formed at (H) by the lens (F). For Schlieren photography, the flame mustbe optically conjugate with the fiIm plane. In addition, a pinhole matched to (C) is placed at (G) and this acts as the marking aperture. Exposure times of 1~1000set nominal (1.5 rnsec actual, el~troni~lly measured) were used for al1 the photo~ap~c records and this effectively “froze” most flame fluctuations. Photographs were taken on Ilford F.P.4 120 Professional Roll Film. Flames

Circular flames of ca. 10 mm diameter provided adequate photographic contrast and were also easier to align than long-path flames. Nevertheless conclusions drawn from their behaviour can be related to the behaviour of such flames. The influence of shielding gases on the stability of long-path flames should be more pronounced than with the cylindrical flame, as the area of contact and mixing of gases will occupy a greater proportion of the whole flame. However, the reduction in the background emission signal will be less, as the outer mantle corresponds to a smaller proportion of the total optical path through such a flame. The flames were produced on both capillary burnersla and burners of the Meker type (manufact~ by Beckman Research and Industrial Instrument Corporation Limited, Worsley Bridge Road, London S.E.26). Both types of burner had provision for gas sheathing. Records were obtained for both air-acetylene and nitrous oxide-acetylene flames on burners of this type, with varying degrees of nitrogen separation. However, the information provided by the nitrous oxide-acetylene tlame was no different in nature from that obtained with the air-acetylene flame. For all further work, including atomic-absorption and flame-emission signal measurements, the air-acetylene flame alone was used. RESULTS

Comparison of results obtained by shadow and Schlieren photography All optical records of flame disturbances were made alternately with both the shadow and Schlieren methods. As shadowgr~ were especially sensitive to rapid

Optical studies with the air-aa%ylene flame

931

changes in the refractive index field normal to the optical path, gradual changes across the flame ends do not register strongly and a flat, seemingly two-dimensional, picture is obtained (Plates 1-4 and 6-8). The Schlieren records have a three-dimensional appearance (Plate 5), which is a result of the greater responsiveness of the technique to the small components of refractive index change, normal to the light path, which occur at the flame ends. The one instance where Schlieren photography has a clear advantage in the quality of the record produced is when flames of very high emissive background (such as nitrous oxide-acetylene) are to be studied. Here the marking aperture considerably reduces the light intensity from the flame, which would in its absence (as in shadow work) pass directly to the film and reduce the image contrast. In view of the simplicity of the shadow method it is to be recommended for visual studies of laminar flames of low emissive background, and only shadowgrams will be discussed further. Information obtained from shadowgrams of air-acetylene james General. The unseparated fuel-rich air-acetylene flame maintained on a capillary burner is exceptionally stable and laminar (Plate l)*, up to the height of ca 57 mm. The leaner flame (not shown) is more nearly cylindrical in shape, with less expansion at all levels. The same fuel-rich flame on a Beckman RIIC burner (Plate 6) has a larger initial diameter, showing greater expansion at the flame base, but reaches approximately the same diameter as the flame maintained on the capillary burner, at 30 mm above the primary zones. The initial expansion of gases, being more rapid, is less laminar. The more diffuse secondary reaction zone also indicates that the secondary combustion process is less sharply defined. The unevenness of the top of the flame illustrates what may also be visually observed, that this flame is more susceptible to draughts. FIame background. Progressive separation of the air-acetylene flame on the capillary burner with an outer sheath of nitrogen, produces changes both in flame shape and laminarity. Plates 2,3 and 4 show the effects of nitrogen flow-rates of 2,6 and 10 I./mm. From these representative photographs (selected from a set covering also all intermediate flows), the best compromise between efficient separation and induced turbulence, at a height of measurement 15 mm above the primary cones, would seem to occur at 6 I./mm. Reference to Fig. 3(i) shows that the most efficient removal of OH emission does indeed occur at 6 I./mm nitrogen flow. At a position of measurement 30 mm above the primary reaction zones, separation with flow-rates above 6 I./mm might be expected to lead to increased turbulence. Figure 3(i)(c) does show a slight increase in signal at flow-rates above 6 I./mm, indicating that the induced turbulence is producing some air entrainment. This leads to increased combustion, with a consequent increase in OH emission. From the noise curves (Fig. 3(ii)), the signal: noise ratio at 15 mm height in the flame passes through a minimum at a nitrogen flow of 6 I./ruin. In other words, a nitrogen flow of 6 I./min stabilizes the hot flame gases by diminishing their fluctuations. * This may be contrasted with results of RamP whose air-acetylene frame maintained on a Meker type burner showed extreme turbulence right from the base of the flame. This could only be reduced by flame shielding. The unshielded capillary burner flame appears as laminar as &I&S shielded flame. This shows how an unsatisfactory burner design may produce unstable flames.

932

K. M. ALDOUS, R. F. BROWNER,D. CLARK, R. M. DAGNALL and T. S. W~sr 100

a 54 m

60 40

20

Sheathing nitrogen flow, Umin FIG. 3.-Emission

and noise from fuel-rich air-CIH, flames. (i) OH emission with capillary burner, (ii) OH emission noise with capillary burner, (iii) OH emission with Beckmann RIIC burner, (iv) OH emission noise with Beckmann RIIC burner. Height of observation above primary reaction zones: u-3 mm; b-15 mm; c-30 mm.

However, this increase in stability of the flame zones is not clearly seen on the shadowgrams because the unseparated flame is so stable and only regions of greater turbulence than this show clearly on the photographs. Increasing the flow-rate to 10 l./m.in causes optical inhomogeneities in the gases, appearing as increased noise, and these are compatible with an examination of the flow patterns as shown by the shadowgrams. The noise curves also show that noise levels measured high in the flame (c) are greater than those measured in the central regions (b) at all nitrogenjlow-rates. This again accords with the photographs (Plates 2-4). Here the higher regions of the flame are seen to be more turbulent than the central regions at all separation flows. The information provided by the shadowgrams can also be correlated with the variation of OH emission signals with height (Fig. 4). With no shielding [Fig. 4(i)(u) and 4(C)(a)] the signal: noise ratio is approximately constant with height. The shadowgram (Plate 1) also shows no visible variation of optical inhomogeneities or turbulence with height. With a 6 l./min flow of shielding gas the signal: noise ratio passes through a pronounced minimum at a measurement height of 15 mm. The relevant shadowgram (Plate 3) shows some inhomogeneity low in the flame, where the gases are close to the primary zones, and also slight turbulence in the outer flame

PLATESI-2.-Shadowgrams

2 of fueI-rich air-acetylene Jlames on capillary burner.

1. Normal flame. 2. 2 l./min flow of nitrogen (for separation of flame). 932

PLATES

3-4.~Shadowgrams

4 of fuel-rich air-acetyleneftames on capillary burner.

3. 6 I./min of nitrogen. 4. 10 I./min of nitrogen.

6 PLATES

5-6.--shadow and Schlieren records of fuel-rich air-acefylene Jfames. Schlieren photograph of separated flame (10 Lfmin Ng flow) on capillary burner. Shadowgram of normal flame on Beckmann RIiC burner.

8 PLATES7-8.-Shadow and Schlieren records of fuel-rich air-acetyleneflames. 7. Shadowgram of flame on Beckmann RIIC burner with 4 l./min NP for separation. 8. Shadowgram of flame on Beckmann RIIC burner with 10 I./min Na for separation.

Optical studies with the air-acetylene flame

80

6

I6

30

80

1

6

18

30

6

18

30

Height of observation, mm

of signal and noise with height. fi) OH emission, fuel-rich, (ii) ON emission noise, fuel-~cb, {iii) OH emission, leaner flame, (iu) OH emission noise, leaner fIame. a--No sheathing gas; b-6 I./min sheathing gas flow-rate.

Fr0.4.-Variation

regions at heights above 20 mm. The central flame regions appear to be relatively laminar. The leaner air-acetylene flame is physically smaller and “stiffer” than the fuel-rich flame. Whatever turbulence is present, as indicated by the shadowgrams (not illustrated here), it varies little with height, whether the flame is unseparated or separated. Figures 4(E) and (iu) confirm this behaviour in terms of the signals and their associated noise. The effect of sep~ation on the air-acetylene flame maintained on the Beckman RIIC burner is different from the effect when the flame is stabilized on a capillary burner. At a height of observation of 15 mm [Figs. 3(G) and (iv)], the noise level actually increases at first as the nitrogen separation reduces the signal. The peak noise occurs at a flow-rate of 4 I./mm. Plate 7 shows that at this flow there is a sharp de~n~tion between the lower part of the flame, which is separated, and the part above it, which is unseparated, and where gases are burning. This transitional region occurs at a height of about 15 mm in the flame and of course leads to a noisy signal. The ~haviour of the Beckman RUG burner and separator should be con~asted with that of the capillary burner and separator. With this capillary arrangement, all flow-rates of nitrogen give progressive separation up the flame without any sharp boundaries between separated and unseparated regions.

934

K. M. ALWUS, R. F. BROWNER,D. CLARK, R. M. DAGNALL and T. S. WEST

Atomic-absorption signals. Copper is nearly completely atomized in the airacetylene flame. l3 Therefore, the atomic-absorption signal from a copper solution nebulized into such a flame is a measure of the steady-state atomic concentration which lies in the radiative path between the atomic line source and the detector, Any fluctuations in the signal must then only reflect fluctuations in the local steady-state atomic concentration plus nebulizer noise, which will remain a constant proportion of the signal. Figure S(iii) shows the variation in copper atomic absorption with increasing 100

I

II

b

I

I

2

6

IO

2

IO

2

6

IO

100

m 80

E 6 a

60 40 20

I

I

2

6

Sheathing nitrogen FIG. 5.-Capper (i) (ii) (iii) (iu)

I

flow,

I

6 LJmin

atomic-absorption and emission at 324.1 nm.

Cu emission signal. Lean flame Cu emission noise. Lean flame Cu atomic-absorption signal. Fuel-rich flame Cu atomic-absorption noise. Fuel-rich flame.

nitrogen separation. As expected, the signal is negligibly altered. On separation at a flow-rate of 6 l./min of nitrogen the noise drops by a factor of 2. This confirms the previous deduction that separation at this flow rate not only reduces fluctuations in the outer flame mantle, but actually stabilizes the whole body of the flame. Again, separation at a flow-rate of 10 I./mm causes turbulence, and the noise returns almost to its unseparated level, as with the OH emission. Atomic-emission signals. The copper flame-emission signals in a lean air-acetylene flame are shown in Figs. 5(i) and (ii) in comparison with the OH emission in the same

Optical studies with the air-acetylene

flame

935

flame (the signals are scaled). Copper is assumed to have a j? value of unity in this flame, so the copper emission magnitude is a measure of the temperature of the flame. With increased nitrogen flow the signal drops in almost exactly the same way as the OH emission. The drop in signal is equivalent to about a 10” drop in temperature of the whole flame. The noise observed, however, drops by a greater degree than does the OH noise. Inruence of other sources of turbulence. So far only turbulent phenomena in laminar flames with nitrogen sheaths have been considered. Although these have considerable application to atomic-fluorescence spectroscopy,14 in other applications, i.e., long-path atomic-absorption spectroscopy, separation may not be considered so advantageous. In these instances the laminarity of the unseparated flame is of prime importance. In order to relate this to the analytical signal, atomic-absorption and flame-emission signals from copper in laminar flames were compared with the signals from the same flames when made turbulent by doubling the total gas supply to the burner. The acetylene: air ratio was maintained constant. The drop in signal in both atomic-absorption and flame-emission was slight, 2 % and 7 % respectively. However, the noise in both cases approximately doubled. Turbulence therefore gives rise to noise in unseparated flames in much the same way that it does in separated flames. CONCLUSIONS

It has been shown by this study that optical methods provide valuable records of the combustion processes in premixed laminar flames. The optical disturbances which these photographs reveal may be interpreted in terms of the laminarity or turbulence of the flame gases, provided that the information is used on a well-de&red comparative basis. From an analytical standpoint, the records are of value because they give insight into the processes which lead to variations in the magnitudes and noise levels of atomic-absorption (and hence atomic-fluorescence) and flame emission signals. The beneficial effect on signal noise of the correct level of nitrogen separation has been shown. Of the two optical techniques examined, both of which require relatively inexpensive equipment, the shadow method has been shown to provide as much information as Schlieren photography whilst involving a simpler experimental arrangement, and so is to be preferred. Zusammenfassung-Die Anwendung von Schlieren- und Schattenverfahren auf die Untersuchung vor? Vorglngen in Flammen wird im Hinblick auf die analvtisch niitzlichen voreemischten Flammen diskutiert. Die besonders aus Schattenphotogaphien erhaltene Information kann zu den bei der Atomabsorptionsund Flammenemissionspektrometrie gemessenen Signalen und deren Rauschen in Beziehung gesetzt werden. R&nn~On discute de I’application des techniques “Schlieren” et d’ombre pour l’etude des processus de flamme en relation avec les flammes premelangees analytiquement miles. Les informations obtenues, particulierement avec les sciogrammes, peuvent etre rattach&s aux signaux mesures et au bruit remarquable en spectrometric d’absorpticn atomique et d’emission de flamme. REFERENCES 1. F. J. Weinberg, Optics of Flames, Butterworths, London, 1963. 2. V. A. Fassel and D. W. Golightly, Anal. Chem., 1963; 39, 466. 3. W. Slavin, At. Absorption Newsletter, 1967, 6, 9.

936

K. M. ALDOUS, R. F. BROWNER,D. CLARK, R. M. DAGNALL and T. S. WEST

4. C. S. Ram, Spectrochim. Acta, 1968,23B, 827. 5. V. G. Mossotti and M. Duggau, Appl. Opt. 1968,7,1325. 6. M. P. Bratzel, Jr., R. M. Dagnall and J. D. Winefordner, International Atomic Absorption Spectroscopy Conference, Sheffield, 1969. 7. S. Goldman, Frequency AnaIysis Modulation and Noise, p. 380. McGraw-Hill, New York, 1948. 8. J. D. Wiiefordner and T. J. Vickers, Anal. Chem., 1964,36, 1939. 9. Zdem, ibid., 1965,37,416. 10. Zdem, ibid., 1964,36, 1947. 11. J. D. Winefordner, M. L. Parsons, J. M. Mansfield and W. J. McCarthy, ibid., 1967, 39,437. 12. K. M. Aldous, R. F. Browner, R. M. Dagnall and T. S. West, ibid., 1970,42,939. 13. L. de Galan and J. D. Winefordner, J. Quant. Spectry. Radiative Transfer, 1967,7,251. 14. R. F. Browner and D. C. Manning, Anal. Chem. in press.