(Burning Velocity of UnconJined Turbulent Flames)
EXPERIMENTS WITH BUTANE-AIR AND METHANE-AIR FLAMES KURT WOHL AND LEON SHORE Department of Chemical Engineering, University of Delaware, Newark, Del.
T
HE number of experiments suitable for a check of the theories of turbulent burning velocity advanced by Karlovitz, Denniston, and Wells (6) and Scurlock and Grover (8, 9) is very scarce. The burning velocity of turbulent flames burning from tubes has been studied mainly by Williams and Bollinger ( l a ) , Karlovitz and coworkers (6),and Wohl and coworkers (16). EXPERIMENTAL WORK
Flames were studied up to an average velocity in the burner tube, U , of 49 feet per second. Burner tubes with an inner diameter of 1 inch were used unless otherwise stated. The tube head was water-cooled. The flames were prevented from blowing off the burner tube by a wreath of stoichiometric fuel-oxygen pilot flames around the tube rim. The volume flow of the pilot mixture never exceeded O.5y0of the volume flow of the main stream and was usually much less than that. Just as in the work of Wohl and coworkers (16),an average burning velocity, g , was determined from the total area of maximum luminosity. This avoids error due to incomplete knowledge of the flow pattern. Luminosity was measured with a densitometer equipped with a viewing hole 0.5 mm. in diameter. Actually, in order to obtain the true area of maximum local luminosity, a correction should be made for the cylindrical symmetry of the flame. The directly observed area lies inside the true area. Such corrections have been carried through for very diffuse confined low-pressure flames by evaluating the densitometric curves in steps. For the flames reported here the correction was found t o be slight and has been omitted. The significance of the directly measured area of maximum luminosity has been checked by a chemical analytical measurement of local flame composition. A water-cooled probe 0.25 inch in outside diameter and 1 mm. in inside diameter was applied; samples were analyzed for carbon dioxide, carbon monoxide, and oxygen. The samples were taken along the flame axis, and in a horizontal direction a t a distance above the tube port which equaled one half of the height a t which Soy0of the oxygen was consumed in the flame axis. Examples of an axial and a horizontal traverse through the flame are shown in Figures 1 and 2 in which the points a t which the luminosity has its maximum are indicated. These points agree, within the limit of errors, with those a t which the oxygen concentration of the primary combustible mixture has dropped t o 50%. The use of the area of maximum luminosity for determining turbulent burning velocity is thus well justified from the point of view of progress of the combustion process. I n addition, instantaneous schlieren pictures have been taken in order to correlate average flame behavior with the structure of the flame front. In experiments with the 1-inch tube, velocity, U , and intensity of turbulence, v ‘ , of the approach stream have been varied independently with the help of screens. The tube was used either bare or with two types of arrangements of screens. For the purpose of creating nearly streamlined flow three 100mesh screens were placed 0.25 inch apart, with the uppermost screen 1.5 inches from the burner rim. It was ascertained by Eipecial experiments that an increase of the number of screens
from three to four had no effect on the burning velocity of a butane-air flame, while a reduction from three to two caused a hardly perceptible increase of the burning velocity of rich mixtures. For lean and stoichiometric mixtures even a single 100mesh screen gave the same result as four. For the purpose of creating turbulent flow, two 100-mesh screens and, above them, a coarse screen were placed in the tube, with the coarse screen 1.5 inches from the burner rim. For the approach stream passing through bare tubes, v‘ was assumed to be 5.3% of U (18). The per cent turbulence produced by the uppermost screens was calculated for the distance of one tube diameter above the tube rim-Le., 2.5 inches above the screen. The values have been interpolated from the data of von Karman (11) and Simmons and Salter (10)for the ratios of wire diameter to mesh size of the screens used. The Eulerian scale of turbulence, Zs, for the bare tube was taken as equal to 5% of the tube diameter. The scale a t 2.5 inches behind grids was computed from Dryden’s formula ( 1 ) in the manner of Scurlock and Grover (9)-i.e., in the case of the 10mesh and 4-mesh screens, for which the critical distance of 80 wire diameters is larger than 2.5 inches, the scale predicted a t 80 wire diameters was used, and anisotropy was not taken into consideration. Values of intensity and scale of turbulence are given in Table I. Normally screens 9 and 2 are used for producing low and high approach stream turbulence, respectively.
Table I. Screen No.
1 2 3 4
5 6
7 8
9
..
Intensity of Turbulence and Eulerian Scale, 1 Inch above R i m of 1 Inch Tube Mesh Width, Inch 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.10 0.01 Bare tube
Wire Diameter, Inch
0.125 0.105 0,080
0.072 0.047 0.041 0.035 0.041 0.003
100 u’/
%
u,
13.3 10.0 7.2 6.6 5.0 4.8 4.5 6.3 0.9 5.3
12,
Mm. 1.64 1.37 1.04 0.94 0.61 0.54 0.46 0.53 0.22 1.27
AVERAGE BURNING VELOCITY O F METHANE-AIR AND BUTANEAIR FLAMES
Comparison of Methane and Butane Flames. The experiments on the burning velocity of methane-air flames are presented in Figure 3, as well as one curve for butane-air from Figure 7 . The velocity has been kept constant a t 540 cm. per second and the turbulence of the approach stream has been varied by either using a bare tube (100 v’/U = 5.3) or inserting three 100mesh screens (100 v’U = 0.9, Table I). The Reynolds number in the bare tube was 8860. The curve for the normal burning velocity, So, of methane-air mixtures has been taken from ( 2 ) . The gt curves for methane are roughly symmetrical with respect to the SOcurve, as should be expected from the theory. Butane behaves entirely differently, in that the curve is shifted toward the rich side.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1955
The effect of turbulence on gt is shown more clearly in Figure 4, in which are presente4:pairs of curves pertaining to mixtures of equal normal burning velocity, and in addition the curve for the methane-air mixture of maximum burning velocity. (The
4L o ,
t
la
‘a
4L-dELJ 0
1
Y
6
8
10
13
1Y
16
18 2U 22
Distance from tube porf,cm.
Figure
1. Axial composition traverse through,butane-air flame
Velocity 640 cm. per second, 100-mesh screen. equivalence ratio 1.25 ratio (equivalence ratio meana ratio of per cent fuel to pei. cent fuel in stoichiometric mixture)
normal burning velocity of the butane-air mixture is shown in Figure 7.) The curve-parameters in Figure 4 indicate the composition in terms of equivalence ratio-i.e., mole per cent fuel divided by the mole per cent fuel of the stoichiometric mixture. A comparison of the data with the values given by the equation of Karlovitz and coworkers for passive flame frontsLe., for flame fronts not “generating” turbulence; see Equation 3-can be made by inserting into Figure 4 the theoretical points for only the highest value of 5.3% turbulence. For methane theory and experiment agree moderately well. The values for the very lean mixture are slightly higher than those for the corresponding very rich mixture. With butane, the effect of approach stream turbulence is strongly suppressed in the lean mixture and strongly augmented in the rich one. A s u p p r e s s i o n of “normally expected” 11 flame distortions does not seem to have been r e p o r t e d previously. This behavior is assumed to be connected 16 with t h e different B 14 stability of the laminar flame front (6, 16, p. 630). The stability is decreased (increased) when the diffusivity of the scarce component is the higher (lower) one. As butane has a much lower diffusivity t h a n oxygen, rich butane flame f r o n t s 0 have a tendency to Disfance from flame axis. fm. form wrinkles or even to be disrupted under Figure 2. Horizontal composition traverse through butanethe influence of exair flame 5.5 cm. above tube Dort ternal d i s t u r b a n c e s Velocity 540 em. per second 100-mesh like approach s t r e a m screen, equivalence ratio’ 1.25
829
turbulence, while lean flame fronts keep flat in spite of disturbances. Because methane has a slightly higher diffusivity than oxygen, the tendencies discussed show in methane flames to a slight degree in the opposite direction. The opposite behavior of methane and butane flames is well illustrated by the trend of flame front distortions visible in the instantaneous schlieren photographs (Figures 5 and 6). From left to right through the four horizontal rows of these flame pictures one moves essentially a t constant composition and velocity in the direction of increasing approach stream turbulence. I n this direction the lower “laminar” part of the flame becomes shorter and shows more pronounced striations, and the upper part becomes more distorted and even disrupted, and also shorter. (The first steps from left to right in Figure 5 are accompanied by a composition change that obscures the effect mentioned.) In moving through the six vertical pairs of Figures 5 and 6 in the direction from top to bottom, one passes, a t constant velocity and approach turbulence, from a lean mixture of a certain normal burning velocity to a rich mixture of the same normal burning velocity. In the case of methane, the distortions of the flame front are about equal for each pair (though a slightly greater disturbance will be noticed with the lean flames); in the case of butane, the distortions are much stronger with rich flames. The fact that the rich methane flame is not more “turbulent” than the lean one proves immediately that participation of secondary air in the combustion process does not cause the effect observed with butane. Another indirect proof of this fact is furnished by the observation of Schilly (IS),that the ratio fft/t/so increases toward the rich side in the case of propane-air flames which are confined to a channel so that theaccessof secondary air is excluded. Butane-Air Flames. EFFECTOF INTENSITY AND SCALEOF TURBULENCB. Figure 7 gives a more comprehensive survey of the data obtained for butane-air mixtures. The figure contains the six possible combinations among three conditions of turbulence (100-mesh, bare tube, 4-mesh, screen 2, Table I) and two velocities (540 and 1490 em. per second) a curve obtained with a 10-mesh screen, and the curve of the normal burning velocity. Three of the curves are taken from Wohl and Shore (15). With the exception of the older data a t 540 cm. per second with a bare tube and with 10-mesh, the maxima of the curves shift toward the rich side as increases, in agreement with the above discussion. The dependence of the burning velocity of a large number of
901
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‘08
I
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0:7 0:s 0:9 110 111 112 1 3 1[Y
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Equivalence ratio Figure 3. Burning velocity of methane-air and butane-air flames as function of equivalence ratio Velocity 540 om. per second Normal burning velocity of methane-air flame 2. Methane, 100-mesh screen 3. Methane, bare tube 4. Butane, bare tube 1.
INDUSTRIAL AND E N G INEERING CHEMISTRY
830 901
,
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Figure 4. Burning velocity of methane-air and butane-air flames as function of per cent turbulence Velocity 540 om. per second. Numbers near curves signify equivalence ratios. Pairs of equal normal burning velocity (except methane curve with equivalence ratio 1.06)
butane-air flames on approach stream turbulenc?e is shown in Figure 8. The screens used to obtain the various intensities of turbulence are indicated in Table I. There is an initial relatively small but steep rise of &, increasing from lean to rich, which is not predicted by theory and is entirely a t variance with Shelkin's equation (Equation 4 above). Subsequently of rich flames rises linearly with V I , a t least up to 10% turbulence, while gtof lean and stoichiometric flames rises with v' first more slowly, later on more steeply, and in total less than S, of rich d
b
d
e B A R E TUBE
100-MESH
Picture No. 100 f / U
Equivalence Sa, cm./sec. Picture No. 100 y'/U Equivalence So, cm./sec.
flames. A comparison of these data with the equation of Karlovitz and coworkers is given in Table 11, which contains information on a t the highest value of approach flow turbulence common to all curves of Figure 8 (10%). If the shape of the theoretical curve of Karlovitz and coworkers is taken into consideration, Table 11,in conjunction with the shape of the experimental curves of Figure 8, discloses the relation between the experimental and theoretical curves over the whole range. The experimental curves for rich mixtures lie entirely above the theoretical curves; this is due to flame-generated turbulence. The experimental curves for the lean and stoichiometric mixture lie entirely or partly below the theoretical one. The special curvature of the curves for lean and stoichiometric flames, which is convex in a downward direction, indicates that, at a given velocity, the resistance against flame front distortions lessens and turns into active response when the disturbances or accelerations are increased (6). For most of the curves, points have been determined only with the bare tube and screens 9 and 2 of Table I. With the mixture of equivalence ratio 1.25, several additional screens have been employed. All the data are grouped in a satisfactory manher around smooth curves, in spite of the fact that the scales of turbulence vary from screen to screen. Especially a t a velocity of 540 cm. per second, there are five data points with an abscissa value of about 5% turbulence which lie very closely together, though their scale values vary, according to Table I, from 0.46 to 1.27 mm. This insensitivity of St to approach stream scale d
b
d 100-MESH
B A R E TUBE
C
c
I 4-MESH
Figure 5. Instantaneous schlieren pictures of methane-air flames burning above 1-inch tube Velocity 540 om. per second a b 0.9 5.3 ratio 0.947 0.843 25 33 d e 5.3 0.9 1.27 ratio 1.16 33 25
Vol. 47, No. 4
C
10 0.843 25
f
10 1.27 25
e
f 4-MESH
Figure 6. Instantaneous schlieren pictures of butaneair flames burning above 1-inch tube Velocity 540 cm. per second. Picture No. 100 t"/u Equivalence ratio Picture No. I 100 y ' / U Equivalence ratio
Normal burning velocity 25 cm. per second b C a 5.3 10 0.9 0,747 0.747 0.747 d e f 10 0.9 5.3 1.373 1.373 1.373
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1955 1701
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1 0 1 1 1 2 1 3 1 4
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Equivalence ratio Figure 7.
Burning velocity of butane-air flames as function of equivalence ratio 1. Normal burning velocity 2. 100-mesh screen, velocity 540 cm./seo. 3. 100-mesh screen, velocity 1490 cm./sec. 4. Bare tube. velocitv 540 cm./sec. 5 . 10-mesh screen, velocity 540 cm./sec. 6. Bare tube, velocity 1490 cm./sec. 7. 4-mesh screen velocity 540 cm./sec. 8. 4-mesh screen: velocity 1490 cm./sec.
is a t variance with Scurlock and Grover's theoretical equations. The reason may be that for these flames the "flame front scale" or "cell size"-Le., the average diameter of the base of flame front distortions-is not proportional to the approach scale but rather is determined by the nature of the laminar flame front, as charactgrized by Markstein (6). This is confirmed by observations on the schlieren photographs of Figures 5 and 6 and many similar ones. At the lowest approach turbulence of 0.9yo, the scale is 0.22 mm.; the average cell size in the upper flame region, however, is about 6 mm. In this case, the cell sizes are roughly uniform and their sequence is nearly periodical, so that the impression is not that of turbulent or random phenomena. Figures 5 and 6 show that on passing to higher approach stream turbulence, the flame front scale decreases, although the approach stream scale increases in the same direction from 0.22 mm. via 1.27 to 1.37 mm. The observation holds for both methane and butane flames, and can be explained by the interaction of the traveling distortions with approach turbulence (7). The trend described is confirmed by the observation of Karlovitz ( 4 ) that the flame scale decreases with increasing distance from the tube port, as flame-generated turbulence increases in this direction. In the case of butane flames (Figure 6 and unpublished photographs) a decrease of flame front scale is further noticed in passing from a lean mixture to a rich one of equal burning velocity. This is the result of the greater instability of the laminar flame front in butane-rich mixtures. Scurlock and Grover (9) find that their theoretical values for (inclusive flame-generated turbulence) far exceed the experimental values obtained by Wohl and Shore (l4,15)for the specific
Figure 8. Burning velocity of butane-air 'flames a8 function of per cent turbulence Numbers 1490 and 540 near curves signify approach stream velocities in cm./sec.; other numbers signify equivalence ratios A 100-mesh screen X IO-mesh screen 0 4-mesh screens (see Table I) 0 Bare tube
case of low turbulence (0.9yo) and small scale (0.22 mm.) of the approach stream, The discrepancy is due to the smallness of the scale which enters their equation. It is concluded again that the approach scale has no decisive influence on flame behavior. Hottel, Williams, and Levine ( 3 ) have measured the width, D,and amplitude, h, of flame front distortions from instantaneous shadow photographs of mildly turbulent Cambridge City gas flames burning a t reduced pressure (480 mm. of mercury) and relatively low velocities (up to 305 em. per second). They chnclude that D is proportional to the scale I (which they define in a special way), and represent their data by the equation D = 51. This agrees in principle with Scurlock and Grover's suppoeition, but is different from the authors' experience, which, how-
Table 11. Comparison of Average Turbulent Burning Velocities of Butane-Air Flames of Figure 8 at 10% Approach Turbulence with Karlovitz' Predictions for Passive Flame Front Equivalence Ratio 0.80
1 .oo 1.25 1.30 1.50
Velocity, Cm ./Sec. 540 1490 540 1490 540 1490 540 1490 540 1490
Normal Burning Velocity, Cm./Sec. 30 40 33 30 11
$-
g a t 100 10, Cm./Sec. Exptl. Theory 73 72 88 115 102 a4 139 134 125 76 166 122 123 72 166 115 109 42 154 66
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
832
ever, pertains to different fuels and to flames of higher velocity. The data of Hottel and coworkers, however, do not exclude the possibility that D finally tends toward a constant value of about 5 mm. when I approaches zero.
Vol. 47, No. 4
ure 9), all curves start with a steep rise which increases strongly with increasing turbulence and also with increasing fuel concentration, and continue linearly between about 300 and 1490 cm. per second. The curves have a striking similarity to the curves of Scurlock and Grover, which represent the theoretical values of S t , including the maximum contribution of flame-generated turbulence, as a functien of approach velocity. Such curves can be constructed from Scurlock and Grover’s Figure 19 (9) for t = m, and are directly given in Figure 21 for finite times corresponding to the conditions of Williams and Bollinger’s experiments ( l a ) . I n either case, the curve for g of the passive flame front rises gradually. But the theoretical maximum contribution of flamegenerated turbulence to is larger a t small than a t large values of U , which makes for an initial steep rise and a subsequent slow rise of the final gt U-curve, just as is noticed, though in a much more outspoken manner, in Figures 9 to 11. This peculiar form of the theoretical curves stems from the use of Equation 11 for computing the total intensity of turbulence from the approach turbulence and the flame-generated turbulence. The trend described is not observed with the data of Williams and Bollinger. The reason may be that the rich butane-air flames which show this trend most strongly have a greater tendency to create the maximum possible flame turbulence than the nearly stoichiometric propane, ethylene, and acetylene flames of Williams and Bollinger. It follows from Figures 9 to 11 that the turbulent burning velocity a t a given absolute intensity of turbulence of the approach stream is larger if the approach velocity is small (and v’/U large) than if the approach velocity is large (and v ‘ / U small). Asimilar situation is recognized in Table I1 a t the equivalence ratios 0.80 and 1.00. I n either case the absolute difference between the experimental value of gcand the theoretical value of Karlovitz and coworkers is larger a t the lower velocity.
-
u
0 ZOO 900 600 800,1000lZ00 14001600
U,cmJsec. Figure 9. Burning velocity of butaneair flames as function of average velocity in burner tube 100-mesh screen. Numbers near curves signify equivalence ratios; crosses signify normal burning velocity
Hottel and coworkers further find that the amplitude of flame front distortions, h, is independent of scale. This is to be expected from theory at relatively small “times” t. The specific equation given by the above authors is h v‘/So. A similar dimensionally correct equation may be obtained asfollows: Equation 7 gives h = v‘t; Equation 9 gives t = L / U cos ,8 ( L = length of flame front, U = mean approach velocity, 0 = mean angle between approach velocity and flame front). However, L = r/sin ,8 ( r = distance of point of interest from axis), and U sin ,8 = E; therefore t = r/g cos @r/g, and h = r v’/gt. EFFECT OF APPROACH STREAMVELOCITY.The burning velocity is presented as a function of average approach velocity a t given values of v’/U and butane concentration in Figures 9 to 11. Except for low turbulence and stoichiometric composition (Fig-
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i 1.50
o am
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I,$ YOO 600 800 1000 POO iyo~i~lo
U,crn/sec Figure 10. Burning velocity of butane-air flames as function of average velocity in burner tube Bare tube.
See Figure 9
0
1.50
u 200 400 600 800 1000 UOO 1’tOO 1600
U,cm/s ec. Figure 11. Burning velocity of butaneair flames as function of average velocity in burner tube &mesh screen.
See Figure 9
April 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
Table 111. Constants of Equation SO) - 1 = a bU for Linear Part of Butane-Air Curves of Figures 9 to 11 100 v’/U 5.3
0.9 Equivalence Ratio
a
b X lo*
a
1’ 1.25 1.50
-0,092 0.185 1.56
3.40
0.225 0.824 4.04
4.02
8.40
10
b X 104 6.08 10.9
b X 10’
a 1.01 1.91
9.83 13.4 42.7
6.66
32.8
possibly that beyond a certain size of distortions the flame front is disrupted. FLAME THICKNESS
It seems safe to say that the thickness of the flame zone, H , will increase with the increasing scale of flame turbulence. ’ For continuous flame fronts H can be assumed to be proportional to N
the root mean square displacement of a flame front element Y. There follows from Equation 2 above
H Table IV. Flame Thickness and Relative Flame Scale for Butane-Air Flames Burning above 1-Inch Tube Equivalenre Ratio 1 25
1 00
U,
Cm./Sec.
100 v’/U
540 540 540 1490
0.9 6.3 10 5.3
St, cm./sec.
44 62
102
84
H, cm.
l’al
cin.
0.40 4 . 0 0.74 1.33 0.84 0.54 0.67 0.61
St,
cni./sec. 47
82
125 117
H,
om.
1’2.
om.
0.40 1.0 0.62 0.43 0.80 0.29 0.62 0.25
The linear part of the gt curves of Figures 9 to 11 is reprebU. The coefficients sented by the equation (st/So) - 1 = a a and b are assembled in Table 111. The increase of a and b with increasing per cent turbulence and fuel content is striking. On moving from stoichiometric to rich, intercept a grows more strongly than slope b, which indicates an increase of flame-created turbulence (16). In detail there is, of course, no agreement between Figures 9 to 11 and Scurlock and Grover’s predictions. For instance, intercept a, as approximated from Figure 21 (9), is not very different for different fuels. This agrees very well with the findings of Williams and Bollinger. But in the present case, the variation of a with fuel concentration is very strong indeed. EFFECT OF TUBE DIAMETER. The influence of tube diameter on has been investigated for various butane-air flames burning above bare tubes of diameters of 0.4, 1.0, and 1.5 inches. One group of experiments was conducted a t a constant velocity of 540 cm. per second; another a t a constant Reynolds number of 8850, which corresponds t o the above velocity in a 1-inch tube. The results are shown on a double-logarithmic scale in Figure 12. with increasing diameter occurs a t conA strong increase of stant velocity, and a small one a t constant Reynolds number. This means that the effect of diameter on burning velocity is larger than that of velocity, as was also found by Williams and Bollinger. This poses a severe problem, as no quantitative theory predicts the effect and, because of it, experimental data of St should actually be judged only on a relative basis. The diameter effect has obviously nothing to do with entrainment of outside air, as in this case the effect should be opposite for lean and rich mixtures. It is assumed that the effect is due to the fact that with large tubes the flame fronts are longer, so that the distortions have more time to grow while traveling down the flame front. From the point of view of Scurlock and Grover’s original theory the experiments with bare tubes are not conclusive, as for this case the increase of scale with diameter would compensate for the increase of flame front length. But it has been concluded that the flame front scale is mostly independent of approach stream scale. And according to Markstein (7), the growth of distortions will be stronger than follows from Scurlock and Grover’s theory. With lean and stoichiometric flames which show the strongest dependence of gt on D,it is clearly noticed that this dependence cannot be expressed by one power term with coristant exponent. The trend with fuel concentration means that the distortions grow the more slowly the larger their average amplitude, or
+
si
833
=
klz[(SdSo)- 11
(1)
where St is local turbulent burning velocity. The authors have determined values of flame thickness by chemical analysis. These data can be interpreted by calculating relative values of This procedure is analogous to that used by Karlovitz scale (4). Chemical analytical traverses have been made in a horizontal direction a t a distance above the tube port which equaled one half of the height a t which 50% of the oxygen was consumed in the flame axis. Flame thickness has been defined as the distance between the points of 10% and 90% oxygen consumption. Data on thickness are given in Table IV. The scale values of this table have been calculated from Equation 1 with k = 1, whereby the assumption has been made that the local turbulent burning velocity a t the place of the traverse can be approximated by the measured average burning velocity, gt. The scale values computed have, of course, only relative significance.
Velocity
Reynolds No:
SYOcmlsec. 5 . I*/
8850 /
2 1
0.5
02
;2 . 0
0.v
1.0
0.4 Diomefer, inch
1.6
3.0
1.5
Figure 12. Burning velocity of butaneair flames as function of tube diameter Bare tubes.
Numbers near curves signify equivalence ratios
Flame thickness increases with increasing turbulence, and decreases, on the whole, with increasing butane concentration. The velocity a t a given value of relative turbulence has little effect on H . The dependence of scale on conditions fully confirms the conclusions which have been drawn from schlieren photographs: The scale strongly decreases, at a given approach stream velocity, with increasing intensity of turbulence, notwithstanding the fact that the scale of approach turbulence happens to increase in this direction. The decrease of these calculated scale values with increasing butane concentration is also strong. An increase of velocity a t a given value of relative turbulence causes a reduction of scale, due to the increase of the absolute intensity of turbulence.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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NOMENCLATURE
Vol. 47, No. 4
(6) Markstein, G. H., Fourth Symposium on Combustion, pp. 44-
D = average base diameter of flame front distortions H = thickness of flame zone h = measured amplitude of flame front distortions L = length of flame front 1 = scale of turbulence as defined in ( 3 ) 11 = Lagrangian scale of turbulence k Eulerian scale of turbulence 1’ = relative scale of turbulence r =distance from axis SO= normal burning velocity St = turbulent burning velocity = average turbulent burning velocity t = time U = average approach stream velocity u‘ = intensity of approach stream turbulence 6 = mean angle between flame front and approach velocity
-
(7)
59, Williams & Wilkins, Baltimore, 1953. Markstein, G. H., “Selected Combustion Problems,” Combustion Colloquium, Cambridge, England, AGARD, NATO, pp. 263-5, Butterworths Scientific Publications, London, 1954.
(8) Scurlock, A. C., and Grover, J. H., Fourth Symposium on Combustion, pp. 645-58, Williams & Wilkins, Baltimore, 1953.
Scurlock, A. C., and Grover, J. H., “Selected Combustion Problems, “Combustion Colloquium, Cambridge, England, AGARD, NATO, pp. 215-47, Butterworths Scientific Publications, London, 1954. (10) Simmons, L. F. G., and Salter, C., Proc. Roy. Soc. (London), A145, 212 (1934). (11) von Karman, T., Proc. Fifth Internatl. Congress Appl. Mechanics, p. 347, Cambridge, Mass., 1938. (12) Williams, D. T., and Bollinger, L. M., Third Symposium on Combustion, pp. 176-85, Williams & Wilkins, Baltimore, (9)
1949.
LITERATURE CITED
(1) Dryden, H. L., Quart. A p p l . Math., 1 , 7-42 (1943). (2) Harris, M. E., Grumer, J., von Elbe, G., and Lewis, B., Third
Symposium on Combustion, pp. 80-9, Williams & Wilkins, Baltimore, 1949. (3) Hottel, H. C., Williams, G. C., and Levine, R. S., Fourth Symwosium on Combustion, _ww. _ 636-44, Williams & Wilkins, Baltimore, 1953. (4) Karlovitz, B., “Selected Combustion Problems,” Combustion Colloquium, Cambridge, England, AGARD, NATO, pp. 248-62, Butterworths Scientific Publications, London, 1954. ( 5 ) Karlovitz, B., Denniston, D. W,, Jr., and Wells, F. E., J. Chem. Phys., 19, 541-7 (1951).
Wohl, K., and Schilly, R., Project Squid Semiannual Progress ReDort. D. 69 (ADril 1. 1953). (14) Wohf, K.; *and Shore, L., Project Squid Semiannual Progress Report, p. 162 (Oct. 1, 1952). (15) Wohl, K., Shore, L., von Rosenberg, H., and Weil, C. W., Fourth Symwosium on Combustion. -ww. - 620-35, Williams & Wilkins, Baltimore, 1953. (13)
RECEIVED for review March 5, 1954. ACCEPTED November 24, 1954. Research conducted under auspikes of Project Squid, jointly sponsored by the Office of Naval Research, Department of the Navy. Office of Scientific Research, Department of the Air Force, and Office of Ordnance Research, Department of the Army, under Contract No. N6-ori-105.
Effects of Shear on Lithium Greases
and Their Soap Phase THEODORE A. RENSHAW Naval Air Experimental Station, Naval Air Material Center, Philadelphia 12, Pa.
ELECTRON MICROSCOPE STUDIES
...show
some unusual results contrary to accepted ideas of grease structure and rheology
...emphasize
the attractive forces between adjacent fibers a s a primary factor responsible for special properties of greases
A
CONSIDERABLE number of studies of greases have already been made with the electron microscope and much information has been revealed concerning the varied physical forms that the soap component may take depending on different compositions and different treatments. The basic objective of such studies is always to achieve understanding and, hence, control of all the variables that contribute to the properties of the bulk product. The soap phase, being responsible for the special nature of greases, is studied to determine what factors influence its capacity for gelation. One study (8)describes the mechanical breakdown of greases in terms of the disintegration of the soap fibers constituting the thickening agent.
Additional information on sheared greases is reported here, and some of the generally accepted concepts concerning the rheology and structure of greases are re-evaluated. Further study of the attractive forces acting between soap particles may clarify grease phenomena. EXPERIMENTAL
The materials studied in this investigation were all greases currently on the military qualified products list of Military Specification MIL-G-3278, Grease; Aircraft and Instruments (For Low and High Temperatures). Attention was concentrated on this class because of the widespread interest in its properties a t temperature extremes. It was also considered a favorable group to study because of the uniformity of the constituent materials. All greases thus far submitted were composed of a lithium base soap and an oil which is essentially a synthetic dibasic ester oil. The fact that small amounts of petroleum oils and other diverse additives were sometimes present did not appreciably affect the similarity found among the greases. A means for inducing extremely high rates of shear was required to investigate the effect of maximum fiber degradation on grease properties. Such a n apparatus was developed by modifying a high temperature grease performance unit of the type described in Method 33.1 of Federal Specification VV-L-791e. This modification comprises a means of forcing the test grease through the rotating ball bearing a t a controlled rate. The grease enters the rear of the bearing housing through copper tubing inserted in a hole opposite the ball cage and, after passing through the balls, is exuded onto the front surface of the housing.
