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Oxides of nitrogen in combustion. Premixed flame
Oxides of Nitrogen in Combustion. Premixed Flame G. N. RICHTER, H. C. WIESE and B. H. SAGE Chemical Engineering Laboratory,
Calilornia Institute oI Technology,
Pasadena, Calilornia
(Received February 196l) Measurements were made oI the residual quantity oI the oxides of nitrogen in samples taken as a function of the spatial position in a cylindrical combustor. The influence of mixture ratio and of rate oI flow o I the reactants 1or a premixed, natural gas-air flame was studied. I n addition the apparent temperature and mole fraction oI carbon dioxide, oxygen and carbon monoxide were determined as a 1unction of spatial position in the combustor. I t was found that the local perturbation o I pressure encountered in the combustion was one oI the principal factors in increasing the residual quantities o] oxides oI nitrogen in the products o I reaction. The results arc pr,'.~ented in graphieal and tabular form,
Introduction THE oxides of nitrogen are an undesirable product of combustion processes insofar as air pollution is concerned. The present investigation was undertaken to determine the effect of combustion conditions in premixed flames upon the formation of the oxides of nitrogen. The formation of these oxides in flames and other types of combustion processes has been the subject of earlier investigation. The reaction of the oxides of nitrogen in flames was studied at the Royal Aircraft Establishment ~. A. I. ROZLOVSKI-~ reported limited kinetics data upon the behaviour of nitric oxide in hydrogen flames. H. WISE and M. F. FRECH3'4 and F. KAUFMAN and J. R. KELSO~ contributed information on the kinetics of the decomposition of nitric oxide. It is well established that perturbations in the combustion process 6-s, as well as contact with cool surfaces, are environmental factors which are conducive to the formation of the oxides of nitrogen. Pebble beds or a water quench to cool combustion products can be arranged to yield concentrations of the oxides of nitrogen of industrial interest 9. Little detailed information concerning the effect of turbulent mixing or other perturbations associated with unstable combustion ~°-~5 upon the formation of oxides of nitrogen appears to be available. Some limited information uPon a turbulent diffusion flame has been obtained 16, and data upon the effect of turbulence on flame
velocity are available 17-v~. Unstable combustion at elevated pressures or even at atmospheric pressure is not well understood. Empirical information upon unstable atmospheric combustion is appearing TM but does not contribute to an understanding of the microscopic aspects of the phenomenon. For the present experimental investigation, the quantities of the oxides of nitrogen remaining in a sample from a premixed flame of natural gas and air were measured as a function of spatial position. A range of mixture ratios and entrance velocities was investigated. Most of the measurements were made in the region in which stable, and therefore quiet, combustion was encountered, and a few measurements were made in the region of unstable combustion. The quantities of the major components were established and apparent temperatures were measured as a function of spatial position.
Equipment and Methods A combustion tube 16,21 consisting of a watercooled copper tube 3"83in. in diameter and 162 in. long was used for this study. Ports for withdrawing samples were provided in its walls. The natural gas and air were ignited upon a conventional perforated-plate flameholder. Two such flameholders were employed. Grid IV consisted of a brass plate 0-250 in. in thickness having 0.188 in. diameter, sharp-edged holes on 0"500 in. centres. Grid VI differed by having
2
G.N. Richter, H. C. Wiese and B. H. Sage
0.062in. diameter holes on 0.250in. centres. The investigations could not be carried out over an extensive range of mixture ratios because unstable or screaming combustion x5 was encountered in a relatively wide range. In m a n y instances the screaming combustion was intense enough to cause an objectionable noise level in the surrounding community. The natural gas was obtained from the local public utility and contained 88"0 mole per cent methane, 6"4 mole per cent ethane, 2-8 mole per cent propane, and 1-3 mole per cent heavier hydrocarbons, with the balance consisting of nitrogen. Specific weight measurements made gravimetrically or with an Edwards gas density balance varied from 0'044966 to 0.046668 pound per cubic foot. The variation was small, and the same composition was used for calculations relating to the entire series of measurements. The equipment 22 for the air supply consisted of a pair of blowers connected to a d.c. motor. The speed of the motor was controlled at a fixed value by a quartz oscillator and a preset counter 23 within less than 0'10 per cent or five degrees of shaft rotation, whichever was the larger measure of uncertainty. The humidity of the air was determined from wet and dry bulb temperature measurements, and corrections to the weight rate of flow of air were made for the small variations in humidity observed. Tabular values 24 of the properties of air at standard conditions were used for the calculations. Both the natural gas and air were treated as perfect gases insofar as deviations from the chosen standard conditions were involved. The weight rates of flow of natural gas and air were determined with Venturi meters, using conventional relations to calculate the results. The discharge coefficients were taken from information available in the literature 25. It is believed that the discharge coefficients were known with an accuracy such that the weight rates of flow of natural gas and air were determined under steady conditions with an uncertainty of not more than one per cent. In the following discussion the relative quantities of air and natural gas are described in terms of
mixture ratio, stoichiometric.
Vol. 6
,/, expressed in per cent of
Auxiliary equipment Apparent temperatures in the combustion zone were measured with a probe thermocouple, consisting of a platinum/platinum-rhodium thermocouple located within an aluminium oxide shroud 0.188in. in inside diameter. The bias in the apparent temperature m a y well v a r y from a few degrees at temperatures below 1 000°F to at least 100 degrees at temperatures in the major combustion zone of the order of 2000°F. It was not feasible to carry out apparent temperature measurements at thermocouple temperatures in excess of 2 000°FX% After steady state was approached, samples of the products of combustion were withdrawn from the combustor through a sampling tube introduced through one of the ports. During the sampling the gases were allowed to flow slowly into an evacuated glass bulb for a period of about two minutes. It was found unnecessary to cool the sampling tube inside the combustion tube. However, the sampling tube was heated outside the combustion zone in order to avoid condensation of water and the associated absorption of oxides of nitrogen. Temperatures were measured and samples taken at various longitudinal positions and at several radial positions. Longitudinal positions were measured as distance from the flameholder and radial positions, as distance from the centreline.
Analytical methods Quantities of carbon dioxide, carbon monoxide and oxygen in the gas samples were determined by conventional Orsat methods. It appears that the mole fraction of these components was established within 0.003. Periodic duplicate analyses indicate that the standard deviation of the mole fraction was not greater than 0"002. A phenol-disulphonic acid method ~e was used to determine the quantities of the oxides of nitrogen, which are reported in mole fraction of nitrogen dioxide equivalent. It is to be expected that the quantity of the oxides of nitrogen in the sample withdrawn should be smaller than
March 1962
Oxides of nitrogen in combustion. Premixed flame
that in the flame. Rates of cooling of the sample were of the order of 100 degrees per millisecond. The rate of approach to equilibrium of the reactions associated with the formation and decomposition of the oxides of nitrogen is significant at flame temperatures, and the equilibrium shifts markedly with temperature. However, near the exit of the combustor, the temperatures of the products of reaction are lower and the composition of samples from this point should nearly reflect that of the products of reaction. Furthermore, the quantities of the oxides of nitrogen in the flame are small and far from equilibrium, and the reaction rates are apparently much slower than are encountered with a system made up of nitrogen and oxygen in equal atomic proportions. Experiments with a ballistic piston apparatus 27 confirm the marked influence of diluents on the rates of these reactions. For these reasons, it is believed that the results obtained are indicative of the conditions existing in the combustor, Significant uncertainties exist in the absolute value of the quantity of the oxides of nitrogen. Agreement of duplicate samples taken at the same time from a turbulent diffusion flamC G and from the premixed flame indicates that the standard deviation in the evaluation of the mole fraction of nitrogen dioxide equivalent was not more than 5 x 10 -~. However, the variation with time at a given point was such that the overall standard deviation of the duplicate samples Table I.
obtained in the region of stable combustion was about 6 × 10 -~ mole fraction nitrogen dioxide equivalent.
~ 1463
Entrance velocity* [t/sec
Weight rate o I flow lb/sec
102-4
1800 Mixture ratio 1 40(
,.%
Wall~
Centre fine
i Longitudinal position 52 in. I
1 00(
I
r
I
I
I
I
I
I
I
I
I
1 800 o_
E 140(
7-866 Longttudina I position 76 in.
ct.
< 1 000
I
l
I
1800
140(
1000
"? 77
I
0'4
08 12 16 Radial position in. lqgure I. Radial variation in apparent temperature for three longitudinal positions at an entrance velocity oI 8 It ~see
Experimental conditions Gas
Temp. °F
o-
~~66'6 per c e n t
Air Test No.
3
Entrance velocity* F--- I t / s e c
Weight rate of flow lb / sec
7089 7006 6998
20"44 22"07 22"26
0'1073 0'1162 0-1156
126"4 128"7 129'0
1-279 1.310 1-381
7057 7049 7025 7033 7041 7015 7065 7081
8-005 8"059 8"039 8"040 8"014 8"046 8'064 8"041
0"04300 0"04290 0"04290 0"04310 0"04310 0"04300 0"04307 0-04304
112'5 117'5 113"2 113"4 115"5 118"6 118"5 116"4
0.4879 0.4925 0"7525 0"7338 0"7301 0-7331 1"019
Grid IV 0.004622 0.004789 0.004981 Grid VI 0.001734 0.001732 0'002660 0"002668 0"002681 0'002669 0"003574
1"050
0'003812
*Average velocity of fluid stream in combustion tube approaching combustion zone. tMixture ratio expressed as per cent combustibles relative to the stoichiometric quantity required.
Temp. oF
Mixture ratiot per cent stoichiometric
Weight fraction water in air stream
78.8 87.4 87.8
65"2 68.1 71"2
0"0034 0.0018 0.0082
85-0 88.4 83"5 85'1 86"7 88"3
66.6 66"7 102"4 102'7 102"2 102'5 137"0 146"3
0'0061 0.0071 0"0054 O.OO54 0.0048 0.0035 0.0016 0.0014
95"6 84"4
4
G.N. Richter, H. C. Wiese and B. H. Sage
Experimental Results Table I records the experimental conditions for
investigations in the premixed flame. The apparent temperatures measured with the probe thermocouple are available 28. The radial variation in temperature is shown in Figure 1 for three longitudinal positions to indicate the results obtained. The highest temperatures, which occur immediately above the flameholder, could not be measured due to limitations of the thermocouple. Both the decrease of temperature as the wall is approached, and the fact that the observed temperatures are much lower than the theoretical flame temperature, show that the water-cooled wall exerts a significant influence on the course of the combustion process. With the stoichiometric mixture, unstable combustion was encountered, but this behaviour did not influence the apparent temperature appreciably. Composition of products
The mole fractions of carbon dioxide, oxygen and carbon monoxide at the centreline for conditions involving the use of Grid VI and an entrance velocity of 8 f t / s e c are illustrated in Figure 2 as a function of longitudinal position.
l +_ +,,,,+x,u,.+ ,-,02,+ ]
,+.+ ~ ~
0+08I"
.--'o-
~I_
y ++37.0
y~66"6
,",,
],~146.3 +
o.mp o
°+°°2f ~, 1 0081
lo24
..~463
+
7-137"0
~
+
~~1463
++
• +-0.04[-
~ 0'021- oMixtureratio ,t,~102.4 X~~.6 I0_
o
20
,
I,
7 "
I
~'~l,
40 60 80 Longitudinal position
IO0 in.
Figure 2. Effect ot longitudinal position on mole traction o] carbon dioxide, oxygen and carbon monoxide at centreline for an entrance velocity of 8 tt/sec
Vol. 6
It is apparent that there is no significant variation in the quantities of the primary reaction products with longitudinal position, indicating that the greater part of the combustion process occurs in the immediate vicinity of the flameholder. Details of the measurements of the concentrations of the major constituents are available -08. The mole fractions of these components are of expected values and their variation with mixture ratio is normal, indicating that the unstable combustion exerted no appreable influence on the primary combustion processes. No analyses were made for incompletely oxidized hydrocarbons. However, it appears that there were significant amounts of these compounds at the higher mixture ratios since balances of atomic species show discrepancies. Oxides of nitrogen
Experimental values of the mole fraction of nitrogen oxides determined as a function of radial and longitudinal position are available 28. Standard deviations were calculated as the rootmean-square of the deviation of replicates from the average of the replicates at that condition of combustion and location. The standard deviation of all the duplicate samples taken at substantially the same time was 6"03x 10 -6 mole fraction. For samples taken from the same position for the same entrance conditions at different times, however, the standard deviation was 18"81 x 10 -6 mole fraction. Such a difference is indicative of the degree of reproducibility obtained during combustion and the associated limitations in any detailed analysis of the results. Minor changes in the composition of the gas, the humidity of the incoming air, the surface temperature of the combustor, and possibly in the mixture ratio, materially influence the nature of local perturbations. Variations in local conditions or irregularities in the nature of the perturbations influence the residual quantities of the oxides of nitrogen sufficiently to cause the above-mentioned increase in standard deviation. The experimental data were smoothed with respect to longitudinal and radial position, and the results are presented in Table 2. The standard errors of estimate of the experimental
March
Oxides
1962
ol
nitrogen
Table 2.
in combustion.
Premixed
ttame
Smooth values o] oxides oI nitrogen Mixture ratio, per cent stoichiometric
Longitudinal position in.
Radial position in,
68 66.6
102.4"
137
(;rid VI Entrance velocity 22 [t/sec
146"3
(;rid 1 V Entrance velocity 8 It/sec
4 8 8 8 8 12 16 22 28 40 52 52 52 52 76 IO0 100 100 IOO 04
0 0 0.47 0.95 1"43 O
2×10 3 5 10 17 3 4 4 4
o O 0
61-
51× 10-6 82 83 66 20 92 96 1 O0 100
o
4
93
0 0.47 0.95 1"43 (I 0 0.47 0.95 1 '43
5 6 8 9 5 5 5 6 6 1
69 68
9)<10-6 12 13 16 20 10 9 6 6
----------
5
--
----
5l) 16 47 34 34 28 12 18
2)<10 -6 2 4 7 12 3 3 4 4 4
5 6 8 13 5 5 6 6 7 l
----3
4 6 9 4 4 5 6 6 1
*Unstable combustion was encountered under these conditions. ?Nitrogen oxides reported in mole fraction of nitrogen dioxide ecluivalent, $Standard error of estimate assuming all the uncertainty associated with the mole fraction oxides of nitrogen.
data from the smoothed values are included. In evaluating this error, the arithmetic average of all data obtained at a single point in the combustor for a given set of conditions was employed. It was assumed that all the uncertainty lay in the mole fraction of the oxides of nitrogen. The standard error of estimate is of the order of 2× 1 0 -G mole fraction oxides of nitrogen, except in the region of unstable combustion where it is many times larger. This small standard error of estimate is within the experimental uncertainty of determining the quantity of nitrogen dioxide in the samples.
,~o
3O
~ .co 2 0 - - - - o
cU
o
Figure 3 shows the longitudinal distribution of the oxides of nitrogen under conditions of stable combustion with no apparent perturbations, obtained using Grid IV. Under these conditions, the experimental data were consistent and reproducible within the uncertainty of the analytical procedure. The greatest quantity of nitrogen oxides occurred near the wall of the combustor at small longitudinal positions. This behaviour apparently resulted from the fact that some recirculation occurs in this boundary region and there is an opportunitv for rapid cooling of portions of the
-
'R
/
t
i
o9 ---o 6
OOin, 0-47 095 143
10 o-
o
20
40
60
Longitudinal posdlon
80
~oo In.
t'zgurc 3. Longitadtnal d2stribution of oxides o] nitrogen at a mixture ratio of 68 per cent stoichiometric and an entrance w, locity of 22 ft /sec
(;. N. R i c h t e r , H. C. Wiese a n d B. H. Sage
6
combustion products which does not exist near the centreline of the combustor. At the greater longitudinal distances from the flameholder, the effects of radial diffusion are significant, and the radial variation in the oxides of nitrogen becomes small and appears to be negligible at 100 in. from the flameholder.
Vol. 6
0 m
20 I
I
~15 (J
tI
G~
.9o
z
8_u~
>
5O
--
/
, ~
~
Diffusion _ _ .. position Oand 1,~3in.
.0
E
o
.9_ x 4 0
f-
0 Z
W
c ~ 30
tl/
o
--~--l ZE
~ ~--
, Premi~O,f!ame~
10
20
40 60 Longitudinal position
80
100 in,
Figure 4. Comparison of oxides of nitrogen in premixed and turbulent diffusion flames at a mixture ratio o/ 66 per cent stoichiometric and an entrance velocity of 8 f t / sec
0 50 100 150 Mixture ratio per cent stolchiometric Figure 5.
Regions of stable and unstable combustio~
~toO
"~
]
~//
>; ¢-
The behaviour depicted in Figure 3 is contrary to that encountered in a turbulent diffusion flame '~. Figure 4 shows that the quantity of oxides of nitrogen in a stable premixed flame is approximately ten per cent of the quantity found to be typical in a turbulent diffusion flame under the same conditions ~. In the premixed flame the average quantity was of the order of 5× 10 -~ mole fraction, and in the turbulent diffusion flame the quantity was nearly 40 × 10 -~ mole fraction. This marked difference in behaviour illustrates the effect of perturbations in the combustion zone upon the formation of the oxides of nitrogen. The regions of stable and unstable combustion in the premixed flame are delineated roughly in Figure 5 in terms of mixture ratio and entrance velocity. Stable combustion occurred at mixture ratios below 80 per cent stoichiometric and above 125 per cent stoichiometric. As shown in Figure 6, the stability of combustion exerted a marked influence upon the quantity of nitrogen oxides formed. The quantity at the centreline was nearly 30 times
a
.o
*68O
I
I
]
....?,Grid VI unstable Mixture ratio ~' ~ t02'4 / per cent~entrance velocity, Uoo=8ft/sec
Qa
-6
vE 60 u~ 0
c 40
Z2o _ .
. . . .
20 Figure 6.
.
40 60 Longitudinal position
_
80
100
=n.
Effect of stability of combustion on tormatio~ of oxides of nitrogen
greater for unstable than for stable combustion. In m a n y cases unstable combustion in the premixed f a m e yielded higher mole fractions of nitrogen oxides than were encountered in a turbulent diffusion flame 1~. In one or two isolated cases during screaming combustion in the
March 1962
Oxides of nitrogen in c o m b u s t i o n . P r e m i x e d flame
premixed flame, a fiftyfold increase in the oxides of nitrogen for the overall macroscopic conditions occurred. The maximum concentration of oxides of nitrogen during unstable combustion in the premixed flame was at the centreline of the combustor. Such a situation was also found with the turbulent diffusion flametL Near the wall of the combustor there exists more damping of the perturbations than at the centreline, and hence the much lower concentration of oxides of nitrogen. Unstable combustion can be induced by impressing a perturbation on the combustion process. For the same set of conditions stable reaction may be realized if the perturbation is omitted. In the absence of an impressed perturbation it is often possible to maintain stable combustion for extended periods under the same conditions as it is possible to maintain unstable combustion if perturbation is introduced, Such a situation may well account for the large standard error of estimate of experimental measurements of the mole fraction of the oxides of nitrogen obtained in samples from such an unstable combustion process. During screaming or oscillatory unstable combustion in premixed flames there are regular fluctuations in the measured pressure which sometimes are as large as 20 per cent of the average pressure. Such perturbations introduce a corresponding local change in temperature. As a result of the equilibrium and kinetic characteristics of the reaction of nitrogen and oxygen, the increase in temperature associated with the local rise in pressure is favourable to the formation of nitric oxide. The subsequent decrease in pressure tends to freeze the reaction, and the net effect is an increase in the average concentration of the oxides of nitrogen because of the non-linear effect of temperature on the rate of reaction. It is to be expected that both the magnitude of the pressure fluctuation and the frequency are of importance. However, it is probable that at frequencies of the order of S00 to 1 0 0 0 c / s , the effect of frequency is greater than the effect of an increase in the magnitude of the pressure fluctuations.
7
The financial support o[ the Air Pollution Control District of Los Angeles County, Calilornia, made possible the experimental work described here. The interest o~ H. H. Reamer contributed materially to the experimental programme. Joan Jacobs assisted in the reduction of the experimental data, and Ann Taylor contributed to preparation of the manuscript. References I FOGARTY, B. B., PARKER, ~r. G. and ~vVOLFHARD, H. G. 'Combustion processes in flames. Part El. T h e reactions of nitric oxide in flames', Royal AircraJt Establishment Report Chem. 483, D e c e m b e r 1951 2 R o z e o v s K l , A. I. Zh. ]iz. Khim. 1956, 30, 912 WISE, H. a n d FRECH, M. F. J. chem. Phys. 1952, 20, 22 •t \VISE, H. a n d FRECH, M. F. J. chem. Phys. 1953, 21, 752 5 KAUFMAN, F. a n d KELSO, J. R. ]. chem. Phys. 1955, 23, 1702 P o p o v , V. A. Dohl. Ahad. Nauh S.S.S.R. 1953, 91, 305 7 RICHARDSON, J. M., HOWARD, H. C. a n d SMITH, R. W . ' T h e relation b e t w e e n s a m p l i n g - t u b e m e a s u r e m e n t s a n d c o n c e n t r a t i o n f l u c t u a t i o n s in a t u r b u l e n t gas j e t ' , Fourth Symposium (International) on Combustion, p 814. W i l l i a m s a n d W i l k i n s : B a l t i m o r e , 1953 SCHOLEFIELD, D. A. a n d GARSIDE, J. E. ' T h e s t r u c t u r e a n d s t a b i l i t y of diffusion flames', Third
b'vmposium on. Combustion and Flame Explosion Phenomena, p 102. W i l l i a m s
and
and \Vilkins: B a l t i m o r e , 1949 ~' (;~LBEWr, N. a n d DANIELS, F. lndustr. Engng (;hem. (Industr.), 1948, 40, 1719 '" CROCCO, L., GREY, J. a n d HARRJE, O. T. J. Amer. Rocket Soc. 1960, 30, 159 tl ZUCROW, M. J. a n d OSBORN, J. R. Jet Propulsion, 1958, 28, 654 ,2 ZUCROW, M. J., OSBOR~, J. R. a n d PtNCHAK, A. C. J. Amer. Rocket Soc. 1960, 30, 758 t:~ ('HV:~G, StN-I. ' C o n c e p t s a n d t h e o r y of c o m b u s t i o n
i i
instability', The James Forrestal Research Center, Princeton University, Rep. No. 251 [Contract NOa(s)-53-817c. ASTIA AD-72 687], 1955 ROt;ERS, D. E. and MARBLE,F. E. 'On mechanics for high frequency combustion instability', 1955 Heat Transfer and Fhdd Mechanics Institute, held at University of California, Los Angeles, 23 to 25 Iune 1955. Reprints of Papers, Stanford University Press, 1955
'.-' Sixth Symposium (International) on Combustion a t Yale U n i v e r s i t y , N e w H a v e n , Conn., 19 to 24 A u g u s t 1956, ' I n s t a b i l i t y in c o m b u s t i o n c h a m b e r s ' , p 487. R e i n h o l d : New York, 1957 It; RICHTER, G. N., \¥IESE, H. C. a n d SAGE, B. H. diffusion flame', J. Chem. Engng Data, 1961, 6,377 'Oxides of n i t r o g e n in c o m b u s t i o n . T u r b u l e n t ,7 [)A~K(iHL~':R, (;. Z. Elektrochem. 1940, 46, 601
8
G . N . Richter, H. C. Wiese and B. H. Sage
Js North Atlantic Treaty Organization. Advisory Group for Aeronautical Research and Development, Selected Combustion Problems and Selected Combustion Problems, II. Butterworths: London, 1954 and 1956 J~ Symposia on Combustion, Third to Sixth, published for the Combustion Institute during 1949 to 1957 20 BARNHART, D. H. and DIEHL, E. K. 'Control of nitrogen oxides in boiler flue gases by two-stage combustion', The Babcock and Wilcox Company, Research Center, Alliance, Ohio, June 1959 21 BERRY, V. J., MASON, D. M. and SAGE, B. H. Industr. Engng Chem. (Industr.), 1953, 45, 1,596 ',2 Hsu, N. T., REAMER, H. H. and SAGE, B. H. Amer. Docum. Inst., Wash., Document 4219, 1954
Vol. 6
2~ REAMER, H. H. and SAGE, B. H. Rev. sci. Instrum. 1953, 24, 362 '-,l Hsu, N. T. and SAGE, B. H. Amer. Docum. Inst., Wash., Document 5311, 1957 '-':' JORISSEN, A. L. Trans. ,4met. Soc. mech. Engrs, 1952, *]4,905 2s BEATTY, R. L., BERGER, L. B. and SC~RENK, H. H. 'Determination of the oxides of nitrogen by the phenol-disulfonic acid method', Rep. Invest. U.S. Bur. Min. No. 3687, February 1943 ~ OLIN, J. B. and SAGE, B. H. J. Chem. Engng Data, 1960, 5, 16 2~ RICHTER, G. N., WIESE, H. C. and SAGE, B. H. Amer, Docum. Inst., Wash., Document 7121, 1962
Calilornia Institute oI Technology,
Pasadena, Calilornia
(Received February 196l) Measurements were made oI the residual quantity oI the oxides of nitrogen in samples taken as a function of the spatial position in a cylindrical combustor. The influence of mixture ratio and of rate oI flow o I the reactants 1or a premixed, natural gas-air flame was studied. I n addition the apparent temperature and mole fraction oI carbon dioxide, oxygen and carbon monoxide were determined as a 1unction of spatial position in the combustor. I t was found that the local perturbation o I pressure encountered in the combustion was one oI the principal factors in increasing the residual quantities o] oxides oI nitrogen in the products o I reaction. The results arc pr,'.~ented in graphieal and tabular form,
Introduction THE oxides of nitrogen are an undesirable product of combustion processes insofar as air pollution is concerned. The present investigation was undertaken to determine the effect of combustion conditions in premixed flames upon the formation of the oxides of nitrogen. The formation of these oxides in flames and other types of combustion processes has been the subject of earlier investigation. The reaction of the oxides of nitrogen in flames was studied at the Royal Aircraft Establishment ~. A. I. ROZLOVSKI-~ reported limited kinetics data upon the behaviour of nitric oxide in hydrogen flames. H. WISE and M. F. FRECH3'4 and F. KAUFMAN and J. R. KELSO~ contributed information on the kinetics of the decomposition of nitric oxide. It is well established that perturbations in the combustion process 6-s, as well as contact with cool surfaces, are environmental factors which are conducive to the formation of the oxides of nitrogen. Pebble beds or a water quench to cool combustion products can be arranged to yield concentrations of the oxides of nitrogen of industrial interest 9. Little detailed information concerning the effect of turbulent mixing or other perturbations associated with unstable combustion ~°-~5 upon the formation of oxides of nitrogen appears to be available. Some limited information uPon a turbulent diffusion flame has been obtained 16, and data upon the effect of turbulence on flame
velocity are available 17-v~. Unstable combustion at elevated pressures or even at atmospheric pressure is not well understood. Empirical information upon unstable atmospheric combustion is appearing TM but does not contribute to an understanding of the microscopic aspects of the phenomenon. For the present experimental investigation, the quantities of the oxides of nitrogen remaining in a sample from a premixed flame of natural gas and air were measured as a function of spatial position. A range of mixture ratios and entrance velocities was investigated. Most of the measurements were made in the region in which stable, and therefore quiet, combustion was encountered, and a few measurements were made in the region of unstable combustion. The quantities of the major components were established and apparent temperatures were measured as a function of spatial position.
Equipment and Methods A combustion tube 16,21 consisting of a watercooled copper tube 3"83in. in diameter and 162 in. long was used for this study. Ports for withdrawing samples were provided in its walls. The natural gas and air were ignited upon a conventional perforated-plate flameholder. Two such flameholders were employed. Grid IV consisted of a brass plate 0-250 in. in thickness having 0.188 in. diameter, sharp-edged holes on 0"500 in. centres. Grid VI differed by having
2
G.N. Richter, H. C. Wiese and B. H. Sage
0.062in. diameter holes on 0.250in. centres. The investigations could not be carried out over an extensive range of mixture ratios because unstable or screaming combustion x5 was encountered in a relatively wide range. In m a n y instances the screaming combustion was intense enough to cause an objectionable noise level in the surrounding community. The natural gas was obtained from the local public utility and contained 88"0 mole per cent methane, 6"4 mole per cent ethane, 2-8 mole per cent propane, and 1-3 mole per cent heavier hydrocarbons, with the balance consisting of nitrogen. Specific weight measurements made gravimetrically or with an Edwards gas density balance varied from 0'044966 to 0.046668 pound per cubic foot. The variation was small, and the same composition was used for calculations relating to the entire series of measurements. The equipment 22 for the air supply consisted of a pair of blowers connected to a d.c. motor. The speed of the motor was controlled at a fixed value by a quartz oscillator and a preset counter 23 within less than 0'10 per cent or five degrees of shaft rotation, whichever was the larger measure of uncertainty. The humidity of the air was determined from wet and dry bulb temperature measurements, and corrections to the weight rate of flow of air were made for the small variations in humidity observed. Tabular values 24 of the properties of air at standard conditions were used for the calculations. Both the natural gas and air were treated as perfect gases insofar as deviations from the chosen standard conditions were involved. The weight rates of flow of natural gas and air were determined with Venturi meters, using conventional relations to calculate the results. The discharge coefficients were taken from information available in the literature 25. It is believed that the discharge coefficients were known with an accuracy such that the weight rates of flow of natural gas and air were determined under steady conditions with an uncertainty of not more than one per cent. In the following discussion the relative quantities of air and natural gas are described in terms of
mixture ratio, stoichiometric.
Vol. 6
,/, expressed in per cent of
Auxiliary equipment Apparent temperatures in the combustion zone were measured with a probe thermocouple, consisting of a platinum/platinum-rhodium thermocouple located within an aluminium oxide shroud 0.188in. in inside diameter. The bias in the apparent temperature m a y well v a r y from a few degrees at temperatures below 1 000°F to at least 100 degrees at temperatures in the major combustion zone of the order of 2000°F. It was not feasible to carry out apparent temperature measurements at thermocouple temperatures in excess of 2 000°FX% After steady state was approached, samples of the products of combustion were withdrawn from the combustor through a sampling tube introduced through one of the ports. During the sampling the gases were allowed to flow slowly into an evacuated glass bulb for a period of about two minutes. It was found unnecessary to cool the sampling tube inside the combustion tube. However, the sampling tube was heated outside the combustion zone in order to avoid condensation of water and the associated absorption of oxides of nitrogen. Temperatures were measured and samples taken at various longitudinal positions and at several radial positions. Longitudinal positions were measured as distance from the flameholder and radial positions, as distance from the centreline.
Analytical methods Quantities of carbon dioxide, carbon monoxide and oxygen in the gas samples were determined by conventional Orsat methods. It appears that the mole fraction of these components was established within 0.003. Periodic duplicate analyses indicate that the standard deviation of the mole fraction was not greater than 0"002. A phenol-disulphonic acid method ~e was used to determine the quantities of the oxides of nitrogen, which are reported in mole fraction of nitrogen dioxide equivalent. It is to be expected that the quantity of the oxides of nitrogen in the sample withdrawn should be smaller than
March 1962
Oxides of nitrogen in combustion. Premixed flame
that in the flame. Rates of cooling of the sample were of the order of 100 degrees per millisecond. The rate of approach to equilibrium of the reactions associated with the formation and decomposition of the oxides of nitrogen is significant at flame temperatures, and the equilibrium shifts markedly with temperature. However, near the exit of the combustor, the temperatures of the products of reaction are lower and the composition of samples from this point should nearly reflect that of the products of reaction. Furthermore, the quantities of the oxides of nitrogen in the flame are small and far from equilibrium, and the reaction rates are apparently much slower than are encountered with a system made up of nitrogen and oxygen in equal atomic proportions. Experiments with a ballistic piston apparatus 27 confirm the marked influence of diluents on the rates of these reactions. For these reasons, it is believed that the results obtained are indicative of the conditions existing in the combustor, Significant uncertainties exist in the absolute value of the quantity of the oxides of nitrogen. Agreement of duplicate samples taken at the same time from a turbulent diffusion flamC G and from the premixed flame indicates that the standard deviation in the evaluation of the mole fraction of nitrogen dioxide equivalent was not more than 5 x 10 -~. However, the variation with time at a given point was such that the overall standard deviation of the duplicate samples Table I.
obtained in the region of stable combustion was about 6 × 10 -~ mole fraction nitrogen dioxide equivalent.
~ 1463
Entrance velocity* [t/sec
Weight rate o I flow lb/sec
102-4
1800 Mixture ratio 1 40(
,.%
Wall~
Centre fine
i Longitudinal position 52 in. I
1 00(
I
r
I
I
I
I
I
I
I
I
I
1 800 o_
E 140(
7-866 Longttudina I position 76 in.
ct.
< 1 000
I
l
I
1800
140(
1000
"? 77
I
0'4
08 12 16 Radial position in. lqgure I. Radial variation in apparent temperature for three longitudinal positions at an entrance velocity oI 8 It ~see
Experimental conditions Gas
Temp. °F
o-
~~66'6 per c e n t
Air Test No.
3
Entrance velocity* F--- I t / s e c
Weight rate of flow lb / sec
7089 7006 6998
20"44 22"07 22"26
0'1073 0'1162 0-1156
126"4 128"7 129'0
1-279 1.310 1-381
7057 7049 7025 7033 7041 7015 7065 7081
8-005 8"059 8"039 8"040 8"014 8"046 8'064 8"041
0"04300 0"04290 0"04290 0"04310 0"04310 0"04300 0"04307 0-04304
112'5 117'5 113"2 113"4 115"5 118"6 118"5 116"4
0.4879 0.4925 0"7525 0"7338 0"7301 0-7331 1"019
Grid IV 0.004622 0.004789 0.004981 Grid VI 0.001734 0.001732 0'002660 0"002668 0"002681 0'002669 0"003574
1"050
0'003812
*Average velocity of fluid stream in combustion tube approaching combustion zone. tMixture ratio expressed as per cent combustibles relative to the stoichiometric quantity required.
Temp. oF
Mixture ratiot per cent stoichiometric
Weight fraction water in air stream
78.8 87.4 87.8
65"2 68.1 71"2
0"0034 0.0018 0.0082
85-0 88.4 83"5 85'1 86"7 88"3
66.6 66"7 102"4 102'7 102"2 102'5 137"0 146"3
0'0061 0.0071 0"0054 O.OO54 0.0048 0.0035 0.0016 0.0014
95"6 84"4
4
G.N. Richter, H. C. Wiese and B. H. Sage
Experimental Results Table I records the experimental conditions for
investigations in the premixed flame. The apparent temperatures measured with the probe thermocouple are available 28. The radial variation in temperature is shown in Figure 1 for three longitudinal positions to indicate the results obtained. The highest temperatures, which occur immediately above the flameholder, could not be measured due to limitations of the thermocouple. Both the decrease of temperature as the wall is approached, and the fact that the observed temperatures are much lower than the theoretical flame temperature, show that the water-cooled wall exerts a significant influence on the course of the combustion process. With the stoichiometric mixture, unstable combustion was encountered, but this behaviour did not influence the apparent temperature appreciably. Composition of products
The mole fractions of carbon dioxide, oxygen and carbon monoxide at the centreline for conditions involving the use of Grid VI and an entrance velocity of 8 f t / s e c are illustrated in Figure 2 as a function of longitudinal position.
l +_ +,,,,+x,u,.+ ,-,02,+ ]
,+.+ ~ ~
0+08I"
.--'o-
~I_
y ++37.0
y~66"6
,",,
],~146.3 +
o.mp o
°+°°2f ~, 1 0081
lo24
..~463
+
7-137"0
~
+
~~1463
++
• +-0.04[-
~ 0'021- oMixtureratio ,t,~102.4 X~~.6 I0_
o
20
,
I,
7 "
I
~'~l,
40 60 80 Longitudinal position
IO0 in.
Figure 2. Effect ot longitudinal position on mole traction o] carbon dioxide, oxygen and carbon monoxide at centreline for an entrance velocity of 8 tt/sec
Vol. 6
It is apparent that there is no significant variation in the quantities of the primary reaction products with longitudinal position, indicating that the greater part of the combustion process occurs in the immediate vicinity of the flameholder. Details of the measurements of the concentrations of the major constituents are available -08. The mole fractions of these components are of expected values and their variation with mixture ratio is normal, indicating that the unstable combustion exerted no appreable influence on the primary combustion processes. No analyses were made for incompletely oxidized hydrocarbons. However, it appears that there were significant amounts of these compounds at the higher mixture ratios since balances of atomic species show discrepancies. Oxides of nitrogen
Experimental values of the mole fraction of nitrogen oxides determined as a function of radial and longitudinal position are available 28. Standard deviations were calculated as the rootmean-square of the deviation of replicates from the average of the replicates at that condition of combustion and location. The standard deviation of all the duplicate samples taken at substantially the same time was 6"03x 10 -6 mole fraction. For samples taken from the same position for the same entrance conditions at different times, however, the standard deviation was 18"81 x 10 -6 mole fraction. Such a difference is indicative of the degree of reproducibility obtained during combustion and the associated limitations in any detailed analysis of the results. Minor changes in the composition of the gas, the humidity of the incoming air, the surface temperature of the combustor, and possibly in the mixture ratio, materially influence the nature of local perturbations. Variations in local conditions or irregularities in the nature of the perturbations influence the residual quantities of the oxides of nitrogen sufficiently to cause the above-mentioned increase in standard deviation. The experimental data were smoothed with respect to longitudinal and radial position, and the results are presented in Table 2. The standard errors of estimate of the experimental
March
Oxides
1962
ol
nitrogen
Table 2.
in combustion.
Premixed
ttame
Smooth values o] oxides oI nitrogen Mixture ratio, per cent stoichiometric
Longitudinal position in.
Radial position in,
68 66.6
102.4"
137
(;rid VI Entrance velocity 22 [t/sec
146"3
(;rid 1 V Entrance velocity 8 It/sec
4 8 8 8 8 12 16 22 28 40 52 52 52 52 76 IO0 100 100 IOO 04
0 0 0.47 0.95 1"43 O
2×10 3 5 10 17 3 4 4 4
o O 0
61-
51× 10-6 82 83 66 20 92 96 1 O0 100
o
4
93
0 0.47 0.95 1"43 (I 0 0.47 0.95 1 '43
5 6 8 9 5 5 5 6 6 1
69 68
9)<10-6 12 13 16 20 10 9 6 6
----------
5
--
----
5l) 16 47 34 34 28 12 18
2)<10 -6 2 4 7 12 3 3 4 4 4
5 6 8 13 5 5 6 6 7 l
----3
4 6 9 4 4 5 6 6 1
*Unstable combustion was encountered under these conditions. ?Nitrogen oxides reported in mole fraction of nitrogen dioxide ecluivalent, $Standard error of estimate assuming all the uncertainty associated with the mole fraction oxides of nitrogen.
data from the smoothed values are included. In evaluating this error, the arithmetic average of all data obtained at a single point in the combustor for a given set of conditions was employed. It was assumed that all the uncertainty lay in the mole fraction of the oxides of nitrogen. The standard error of estimate is of the order of 2× 1 0 -G mole fraction oxides of nitrogen, except in the region of unstable combustion where it is many times larger. This small standard error of estimate is within the experimental uncertainty of determining the quantity of nitrogen dioxide in the samples.
,~o
3O
~ .co 2 0 - - - - o
cU
o
Figure 3 shows the longitudinal distribution of the oxides of nitrogen under conditions of stable combustion with no apparent perturbations, obtained using Grid IV. Under these conditions, the experimental data were consistent and reproducible within the uncertainty of the analytical procedure. The greatest quantity of nitrogen oxides occurred near the wall of the combustor at small longitudinal positions. This behaviour apparently resulted from the fact that some recirculation occurs in this boundary region and there is an opportunitv for rapid cooling of portions of the
-
'R
/
t
i
o9 ---o 6
OOin, 0-47 095 143
10 o-
o
20
40
60
Longitudinal posdlon
80
~oo In.
t'zgurc 3. Longitadtnal d2stribution of oxides o] nitrogen at a mixture ratio of 68 per cent stoichiometric and an entrance w, locity of 22 ft /sec
(;. N. R i c h t e r , H. C. Wiese a n d B. H. Sage
6
combustion products which does not exist near the centreline of the combustor. At the greater longitudinal distances from the flameholder, the effects of radial diffusion are significant, and the radial variation in the oxides of nitrogen becomes small and appears to be negligible at 100 in. from the flameholder.
Vol. 6
0 m
20 I
I
~15 (J
tI
G~
.9o
z
8_u~
>
5O
--
/
, ~
~
Diffusion _ _ .. position Oand 1,~3in.
.0
E
o
.9_ x 4 0
f-
0 Z
W
c ~ 30
tl/
o
--~--l ZE
~ ~--
, Premi~O,f!ame~
10
20
40 60 Longitudinal position
80
100 in,
Figure 4. Comparison of oxides of nitrogen in premixed and turbulent diffusion flames at a mixture ratio o/ 66 per cent stoichiometric and an entrance velocity of 8 f t / sec
0 50 100 150 Mixture ratio per cent stolchiometric Figure 5.
Regions of stable and unstable combustio~
~toO
"~
]
~//
>; ¢-
The behaviour depicted in Figure 3 is contrary to that encountered in a turbulent diffusion flame '~. Figure 4 shows that the quantity of oxides of nitrogen in a stable premixed flame is approximately ten per cent of the quantity found to be typical in a turbulent diffusion flame under the same conditions ~. In the premixed flame the average quantity was of the order of 5× 10 -~ mole fraction, and in the turbulent diffusion flame the quantity was nearly 40 × 10 -~ mole fraction. This marked difference in behaviour illustrates the effect of perturbations in the combustion zone upon the formation of the oxides of nitrogen. The regions of stable and unstable combustion in the premixed flame are delineated roughly in Figure 5 in terms of mixture ratio and entrance velocity. Stable combustion occurred at mixture ratios below 80 per cent stoichiometric and above 125 per cent stoichiometric. As shown in Figure 6, the stability of combustion exerted a marked influence upon the quantity of nitrogen oxides formed. The quantity at the centreline was nearly 30 times
a
.o
*68O
I
I
]
....?,Grid VI unstable Mixture ratio ~' ~ t02'4 / per cent~entrance velocity, Uoo=8ft/sec
Qa
-6
vE 60 u~ 0
c 40
Z2o _ .
. . . .
20 Figure 6.
.
40 60 Longitudinal position
_
80
100
=n.
Effect of stability of combustion on tormatio~ of oxides of nitrogen
greater for unstable than for stable combustion. In m a n y cases unstable combustion in the premixed f a m e yielded higher mole fractions of nitrogen oxides than were encountered in a turbulent diffusion flame 1~. In one or two isolated cases during screaming combustion in the
March 1962
Oxides of nitrogen in c o m b u s t i o n . P r e m i x e d flame
premixed flame, a fiftyfold increase in the oxides of nitrogen for the overall macroscopic conditions occurred. The maximum concentration of oxides of nitrogen during unstable combustion in the premixed flame was at the centreline of the combustor. Such a situation was also found with the turbulent diffusion flametL Near the wall of the combustor there exists more damping of the perturbations than at the centreline, and hence the much lower concentration of oxides of nitrogen. Unstable combustion can be induced by impressing a perturbation on the combustion process. For the same set of conditions stable reaction may be realized if the perturbation is omitted. In the absence of an impressed perturbation it is often possible to maintain stable combustion for extended periods under the same conditions as it is possible to maintain unstable combustion if perturbation is introduced, Such a situation may well account for the large standard error of estimate of experimental measurements of the mole fraction of the oxides of nitrogen obtained in samples from such an unstable combustion process. During screaming or oscillatory unstable combustion in premixed flames there are regular fluctuations in the measured pressure which sometimes are as large as 20 per cent of the average pressure. Such perturbations introduce a corresponding local change in temperature. As a result of the equilibrium and kinetic characteristics of the reaction of nitrogen and oxygen, the increase in temperature associated with the local rise in pressure is favourable to the formation of nitric oxide. The subsequent decrease in pressure tends to freeze the reaction, and the net effect is an increase in the average concentration of the oxides of nitrogen because of the non-linear effect of temperature on the rate of reaction. It is to be expected that both the magnitude of the pressure fluctuation and the frequency are of importance. However, it is probable that at frequencies of the order of S00 to 1 0 0 0 c / s , the effect of frequency is greater than the effect of an increase in the magnitude of the pressure fluctuations.
7
The financial support o[ the Air Pollution Control District of Los Angeles County, Calilornia, made possible the experimental work described here. The interest o~ H. H. Reamer contributed materially to the experimental programme. Joan Jacobs assisted in the reduction of the experimental data, and Ann Taylor contributed to preparation of the manuscript. References I FOGARTY, B. B., PARKER, ~r. G. and ~vVOLFHARD, H. G. 'Combustion processes in flames. Part El. T h e reactions of nitric oxide in flames', Royal AircraJt Establishment Report Chem. 483, D e c e m b e r 1951 2 R o z e o v s K l , A. I. Zh. ]iz. Khim. 1956, 30, 912 WISE, H. a n d FRECH, M. F. J. chem. Phys. 1952, 20, 22 •t \VISE, H. a n d FRECH, M. F. J. chem. Phys. 1953, 21, 752 5 KAUFMAN, F. a n d KELSO, J. R. ]. chem. Phys. 1955, 23, 1702 P o p o v , V. A. Dohl. Ahad. Nauh S.S.S.R. 1953, 91, 305 7 RICHARDSON, J. M., HOWARD, H. C. a n d SMITH, R. W . ' T h e relation b e t w e e n s a m p l i n g - t u b e m e a s u r e m e n t s a n d c o n c e n t r a t i o n f l u c t u a t i o n s in a t u r b u l e n t gas j e t ' , Fourth Symposium (International) on Combustion, p 814. W i l l i a m s a n d W i l k i n s : B a l t i m o r e , 1953 SCHOLEFIELD, D. A. a n d GARSIDE, J. E. ' T h e s t r u c t u r e a n d s t a b i l i t y of diffusion flames', Third
b'vmposium on. Combustion and Flame Explosion Phenomena, p 102. W i l l i a m s
and
and \Vilkins: B a l t i m o r e , 1949 ~' (;~LBEWr, N. a n d DANIELS, F. lndustr. Engng (;hem. (Industr.), 1948, 40, 1719 '" CROCCO, L., GREY, J. a n d HARRJE, O. T. J. Amer. Rocket Soc. 1960, 30, 159 tl ZUCROW, M. J. a n d OSBORN, J. R. Jet Propulsion, 1958, 28, 654 ,2 ZUCROW, M. J., OSBOR~, J. R. a n d PtNCHAK, A. C. J. Amer. Rocket Soc. 1960, 30, 758 t:~ ('HV:~G, StN-I. ' C o n c e p t s a n d t h e o r y of c o m b u s t i o n
i i
instability', The James Forrestal Research Center, Princeton University, Rep. No. 251 [Contract NOa(s)-53-817c. ASTIA AD-72 687], 1955 ROt;ERS, D. E. and MARBLE,F. E. 'On mechanics for high frequency combustion instability', 1955 Heat Transfer and Fhdd Mechanics Institute, held at University of California, Los Angeles, 23 to 25 Iune 1955. Reprints of Papers, Stanford University Press, 1955
'.-' Sixth Symposium (International) on Combustion a t Yale U n i v e r s i t y , N e w H a v e n , Conn., 19 to 24 A u g u s t 1956, ' I n s t a b i l i t y in c o m b u s t i o n c h a m b e r s ' , p 487. R e i n h o l d : New York, 1957 It; RICHTER, G. N., \¥IESE, H. C. a n d SAGE, B. H. diffusion flame', J. Chem. Engng Data, 1961, 6,377 'Oxides of n i t r o g e n in c o m b u s t i o n . T u r b u l e n t ,7 [)A~K(iHL~':R, (;. Z. Elektrochem. 1940, 46, 601
8
G . N . Richter, H. C. Wiese and B. H. Sage
Js North Atlantic Treaty Organization. Advisory Group for Aeronautical Research and Development, Selected Combustion Problems and Selected Combustion Problems, II. Butterworths: London, 1954 and 1956 J~ Symposia on Combustion, Third to Sixth, published for the Combustion Institute during 1949 to 1957 20 BARNHART, D. H. and DIEHL, E. K. 'Control of nitrogen oxides in boiler flue gases by two-stage combustion', The Babcock and Wilcox Company, Research Center, Alliance, Ohio, June 1959 21 BERRY, V. J., MASON, D. M. and SAGE, B. H. Industr. Engng Chem. (Industr.), 1953, 45, 1,596 ',2 Hsu, N. T., REAMER, H. H. and SAGE, B. H. Amer. Docum. Inst., Wash., Document 4219, 1954
Vol. 6
2~ REAMER, H. H. and SAGE, B. H. Rev. sci. Instrum. 1953, 24, 362 '-,l Hsu, N. T. and SAGE, B. H. Amer. Docum. Inst., Wash., Document 5311, 1957 '-':' JORISSEN, A. L. Trans. ,4met. Soc. mech. Engrs, 1952, *]4,905 2s BEATTY, R. L., BERGER, L. B. and SC~RENK, H. H. 'Determination of the oxides of nitrogen by the phenol-disulfonic acid method', Rep. Invest. U.S. Bur. Min. No. 3687, February 1943 ~ OLIN, J. B. and SAGE, B. H. J. Chem. Engng Data, 1960, 5, 16 2~ RICHTER, G. N., WIESE, H. C. and SAGE, B. H. Amer, Docum. Inst., Wash., Document 7121, 1962