The flame velocities were measured of 20 percent oxygen and 80 percent carbon monoxide mixtures containing either light water or heavy water. The flame velocity increased from 34.5 centimeters per second with no added water to about 104 centimeters per second for a 1.8 percent addition of light water and to 84 centimeters per second for an equal addition of heavy water. The addition of heavy water caused greater increases in flame velocity with equilibrium hydrogen-atom concentration than would be predicted by the Tanford and Pease square-root relation. The ratio of the flame velocity of a mixture containing light water to that of a mixture containing heavy water was found to be 1.4. This value is the same as the ratio of the reaction rate of hydrogen to that of deuterium and oxygen. A ratio of reaction rates of 1.4 would also be required for the square-root law to give the observed ratio of flame-velocity changes

Mcdonald, Glen E

English

NASA Technical Reports Server

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RESEARCH MEMORANDUM 
E F F E C T  O F  WATER ON CARBON MONOXIDE - OXYGEN 
FLAME VELOCITY 
B y  Glen E. McDonald 
Lewis  Flight  Propulsion  Laboratory 
Cleveland,  Ohio 
NATIONAL ADVISORY COMMITTEE 
WASHINGTON . . -  .. 
February 5,1954 - .  " 
https://ntrs.nasa.gov/search.jsp?R=19930088134 2020-06-17T11:49:45+00:00Z
m eo w 
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NACA RM E53L08 
NATIONAL ADVISORY COMMlTTEZ FOR AERoNAtlTICS 
'EFFIECT Ol? WATER ON CARBON MONOXIDE - OXYGEN FLAME VELOCITY 
By Glen E. McDonald 
SUMMARY 
The flame velocities were measured of 20 percent oxygen and 80 per- 
cent carbon monoxide m i x t u r e s  containing either light water or heavy 
water. The flame velocity increased from 34.5 centimeters per second 
w i t h  no added water t o  about 104 centimeters per second f o r  a 1.8 per- 
cent addition of light m t e r  and t o  84 centimeters per second for an 
equal addition of heavy water. The addition of light water caused 
greater  increases  in flame velocity with equilibrium hydrogen-atom con- 
centration  than would be  predicted  by  the Tanford  and  Pease  square-root 
relation. The r a t i o  of the flame velocity of a mixture containing light 
This value i s  the same as the   ra t io  of the reaction rate of hydrogen and 
oxygen t o  that of deuterium and oxygen. A r a t i o  of reaction rates of 
1.4 would also be required for the square-root law t o  give the observed 
r a t io  of flame-velocity changes. 
u water t o  that of a  mixture  containing heavy water was found t o  be 1.4, 
-1 
The flame-velocity  theory of Tanf'ord and Pease (ref.  1) , which re-  
l a t e s  flame velocity t o  the diffusion of f ree   raaicals  from the flame 
zone, has led t o  considerable interest in flames of various hydrogen- 
atom concentrations. Although this flame-velocity theory of Tanford and 
Pease w a s  originally develaped f o r  carbon monoxi&, it was l a t e r  extendea 
t o  hydrocarbon f'uels. In other investigations, the flame velocit ies of 
hydrocarbons have been correlated w3th those changes in hydrogen-atom 
concentration which are produced by changes i n   i n i t i a l   t e w e r a t u r e  (ref. 
Z ) ,  hydrocarbon structure (ref. 3), and fuel concentration (ref. 4). 
However, it is  an advantage t o  use carbon monoxide flames t o  determine 
the  effect  of hydrogen atoms on flame velocity, because the radical con- 
centration can be easi ly  changed by the  addition of sma l l  amounts of 
m t e r ,  without producing excessive chawes i n  flame temperature or f u e l  
3 concentration. 
In this research conducted at the RACA L e w i s  laboratory,  the  effect 
.+ on flame velocity of adding  various amounts of water t o  a carbon 
monoxide - oxygen mixture was studied. The water produced changes in the 
2 WCA RM E53LOB 
calculated  equilibrium  concentration of hydrogen atoms without producing 
significant changes i n  the thermal properties of the mixture. The a 
changes i n  flame velocity produced by the addition of water are correlated 
with the thermal equilibrium concentrations of hydrogen atom6 which exis t  
in   the  flames. 
I n  addition, the effect of isotopes on flame velocity was studied 
by adding either heavy or light water t o  a carbon monoxide flame. The 
light water contains hydrogen and the heavy water contains deuterium. 
In  general,  reaction rates involvi'ng molecules which contain hydrogen 
d i f fe r  from the  reaction rates invblving molecules which contain the 
isotope deuterium. Flame velocit ies were measured by the Bunsen burner 
method using shadow photography for carbon monoxide - oxygen mixtures 
which contained 80 percent carbon monoxide, 20 percent oxygen, and up t o  
1.8 percent either heavy or lfght water. The measured differences in 
the  flame velocit ies of the "isotopic flames" are compmed x i t h  changes 
i n  the  reaction  characteristics which resulted from the s6bstitution of 
the isotopes. - . . .  . .  " 
Apparatus. - The apparatus i s  diagramtical ly  i l lustrated in  f ig-  
me 1. The experimental procedure w a s  as follows: Mixtures of gases 
were prepared by admitting the degired.p&ial  pressures of water vapor 
stored in F, carbon monoxide stored in D , and tank oxygen into an 
evacuated cylinder G. Mixing was accolqplished w i t h  a small stirrer 
which entered through the w a l l  of the cylinder. The pressure of the 
water vapor was measured w i t h  an o i l  manometer. For a determination of 
flame velocity, the gases were moved by mercury displacement through a 
flowmeter H and burned i n  a Bunsen cone above a burner 35 centimeters 
long and 0.7 centimeter i n  diameter. The flame was photographed i n  a 
parallel-beam shadowgraph system with a zirconium arc as a light source. 
Materials. - The .carbon monoxide used i n  these ex-periments was pre- 
pared by the sulfuric acid dehydration d 98- t o  100-percent formic acid. 
The apparatus i n  which-the carbon monoxide wa8 generated and collected 
m s  evacuated before- use; ana the sulfuric acid was degassed. Before 
collection, the generated carbon monoxide was passed through a t rap  at 
-78' C t o  remove traces of sulfuric acid, formic acid, and w a t e r  vapor. 
The l ight and the heavy water were degassed and stored under vacuum. 
The heavy water, which w-as obtained f r o m  the Stuart  Oxygen Company, 
had a purity of 99.8 percent. 
Drying procedure. - The carbon monoxide and oxygen were dried before 
mixing by passage through anhydrous magnesium perchlorate. The flow 
system up t o  the l i p  of the burner. was evacuated in  a high vacuum system 
t o  remove absorbed water. 
" 
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NACA RM E53L08 3 
L% w 
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Flame-velocity measurement. - The flame velocity w a s  calculated f r o m  
a Dlanometer measurement of the area of the photograph of the flame and 
th; volumetric r a t e  of mixture flow. The surface area of the conibustian 
zone S was  calculated from the approximate relation: 
S = dZ/h 
where 
A =ea, measured with the planometer, of projection of codust ion 
cone 
2 slant  height of cone 
h height of conibustion cone 
The flame-surface azeas were held nearly constant, and the  flow  rate 
was varied t o  minimize dlfferences which might a r i se  from the choice of 
a flame surface and the method of calculating flame velocity. An average 
value of 2/h was used in the calculations. This procedure improves 
the precision in the relative values of flame velocity, although the ab- 
solute flame veloc i t ies   my  d i f fe r   s l igh t ly  from those  calculated  in 
another manner. In  most cases, two determinations were made of the flame 
velocity of each mixture. 
The flame velocities of the carbon monoxide - oxygen mixtures axe 
plotted against the percentage of added water in  ffgure 2 and a re   l i s t ed  
in  tab le  I. The flame velocit ies are for d r y  carbon monoxide and f o r  
various additions of light and heavy water from zero t o  1.8 percent. 
The experimental flame-velocity values, s h m  in f igure 2 ,  increase from 
34.5 centimeters per second with no added water to about 104 centimeters 
per second for  a 1.8 percent addition of light w a t e r  and t o  about 84 
centbeters   per  second f o r  an equal addition of heavy water. 
DISCUSSION 
When water i s  added t o  the casbon monoxide flame, the calculated 
equilibrium flame temperature decreases progressively, m i l e   t h e  calcu- 
lated  equilibrium hydrogen-atom concentration  increases  progressively. 
The equilibrium flame temperatures and the thermal equilibrium hydrogen- 
atom concentrations for the additions of light water which were used i n  
these experiments are presented in table II. The temperatures and the 
atom concentrations w e r e  CdCulRted by  the method of reference 5. 
4 NACA RM E53L08 
Since addition of water t o  a carbon monoxide - oxygen flame pro- 
duces a decrease i n  the temperature of the flame, it is evident that the 
concurrent increase i n  flame velocity is  not of thermal origin. The 
higher flame velocity is the result of the substitution of a m o r e  rapid 
chemical react ion  for  the slower reaction which occurs in   t he  dry carbon 
monoxide flame. Tanford and Pease (ref. 1) postulated that the faster 
reaction which occurs when water is  added is a result of the formation 
of hydrogen atom i n  the flame zone which diffuse into the unburned gas 
where they  in i t ia te  the flame reactions. These authors related flame 
velocity to the thermal equilibrium concentration of hydrogen atoms in 
the combustion zone. The variation found in the present investigation 
is  shown i n  figure 3. Tanford and Pease predicted that the relat ion be- 
tween the flame velocity and the hydrogen-atom concentration would have- 
the form . .  
where 
Uf flame  velocity, cm/sec 
C1 constant 
k reaction  rate  constant 
D diffusion  coefficient 
pH partial  pressure of hydrogen atom 
They suggested that the constant C1 be considered the flame velocity 
of a dry carbon monoxide flame where p~ = 0. Since k and D are not 
changed by the  addition of water, 
w h e r e  
Uf" flame velocity of mixture of dry carbon monoxide and oxygen 
C2 constant 
The straight l ine  obtained i n  a log-log plot  of Uf - $ against pH 
is  shown i n  figure 4.. Instead of the predicted slope of 0.5, however, 
a slope of 1.26 i s  found; the resulting equation 
. .. . 
? 
I. 
EIACA RM E53L08 5 
e 
L 
indicates a much greater dependence of flame velocity on hydrogen-atom 
concentration  than is  predicted by Tanford and Pease. 
In order t o  investigate  further t he  reactions which might control 
the flame propagation, the light water was replaced by heavy water i n  a 
series of flame-velocity measurements. When the heavy water is added t o  
a carbon monoxide flame, the velocity is increased by an amount dFf- 
ferent from tha t  produced by the  addition of light water (see fig. 2).  
Only small thermal differences result from the  isotopic  substitution. 
The over-all thermal conductivlty of the preflame gases i s  not 
changed a significant amount by the  addition of heavy water in   place 
of light water, because the  total   conductivity is proportional only t o  
the sum of the products of conductivity and mole fraction of each com- 
ponent, that is, 
The change in  conductivity K of the isotopic water i s  
portionalto  the  square root  of the   ra t io  of the masses 
Therefore, no significant change i s  made i n   t he  thermal 
the preflame gases by the isotopic substitution. 
inversely  pro- 
conductivity of 
In the  flame itself ,   the  isotopic  substi tutjon produces isotopic 
radicals which have a larger  percentage  difference i n  mass than  the 
isotopic waters in the preflame gas. However, the two groups which 
have the  greatest  aifference i n  thermal conductivity - the  isotopic 
hydrogen atoms and the isotopic hydrogen molecules - have a combined 
equilibrium concentration of less than 0.5 percent. Consequently, the 
thermal conductivity of the flame gases is not changed significantly. 
The flame temperature i s  changed by the isotopic substitution; but, 
since  the amount of water added t o  the flame i s  less  than 2 percent, the 
flame temperature i s  about lo K lower f o r  the heavy-water flame. 
Since there i s  no difference between the isotopic flames i n   t h e  
amount of thermal energy transferred from the flame t o  the preflame gas, 
any difference  in  flame velocity must be due to  the  differences between 
the rates of chemical react ion in  the two flames. One  may then look 
for  chem€cal reactions which w i l l  show the  proper  isotopic  effect. 
6 NACA RM = S O 8  
The steps 
of water vapor 
mary reaction 
may take place 
i n  flames. 
t 
in  the  direct   oxidation of  carbon monoxide in   t he  presence 
have been reported (ref. 6).  They suggest that the pr i -  
L 
an the surface a t  low temperatures and i n  the gas phase 
The hydrogen  which is  formed, i n  turn, sets up reaction  chains - w  
which have an estimated length of 106. The init iation reaction for the  
chain propagation is the oxidation of the hydrogen 
03 
Hz + 02-(initiation) (5) 
which i s  followed by reactions of the  type 
EO2 + CO- C02 + OH 
OH + CO-C02.+ H 
I n  a Btudy of the  inhibit ion of the chain propagation reactions 
(6, 7, and 8), it was found tha t  the carbon monoxide reaction i n  the 
presence of w a t e r  involves the same intermediate products a6 the 
hydrogen-oxygen reaction (ref. 6).  Therefore, it may be expected tha t  
the relative effects of light and heavy water on the  over-all  cabon 
monoxide - oxygen reaction might be the same as the   re la t ive  effect  of 
substituting deuterium for hydrogen i n  the hydrogen-oxygen reaction, 
It has been determined (ref, 7) that the hydrogen-oxygen and deuterium- 
oxygen slow oxidations have ve loc i t ies  i n  the r a t io  of approximately 
1.4. The r a t i o  of the increase in flame velocity resulting from adding 
light rather than heavy water t o  the carbon monoxide flame also has a 
value of 1.4 (table I o r  Big. 2). The difference between the light and 
the heavy w a t e r  is approximately the same as is observed between the 
hydrogen-oxygen and deuterium-oxygen reactions. 
The flame-velocity r a t io  Uf(E#)/Uf(D2c)) can thus be taken t o  be 
governed. only  by the r a t io  of the reaction rates of the isotopes H2 
and D2, and the cantroll ing reaction in moist carbon monoxide - oxygen 
flames can be taken t o  be the molecular reaction E2 + 02, which is  
independent of diffusion. Thus, the  r a t i o  of the flame velocities is 
determined by the  direct   ra t io  of the-react ion  ra tes  of the rate con- 
troll ing steps i n  the  two isotopic reactions. It should be pointed 
.. . 
.. . 
NACA RM E53L08 7 
W 
4 
out, however, that  most current theoretical treatments relate flame 
veloci ty   to   the square root of the reaction rate (ref.  9).  
Tanford and Pease concentrated on a  different  set of controlling 
reactions and placed the emphasis on the hydrogen-atom concentration 
being controlled by diffusion. For this isotopic substitution, the 
changes which would affect   the flame velocity  are  the  concentration, 
the  diffusion  coefficient, and the specific  reaction rate of the i so-  
topic atoms. The ra t io  of the  change i n  flame velocity produced by 
adding l igh t  and heavy water i s  given by 
If D20 is substi tuted for Hz0 i n  sslounts less  than 2 percent,  the 
equilibrium deuterium-atom concentration is approximately equal t o   t he  
equilibrium hydrogen-atom concentration (ref. 8) so that it can be 
assumed that 
The diffision coefficient of the hydrogen isotope depends on the square 
root of masses so  the  ra t io  i s  given by 
The effect  of the isotopic substitu-bion on the  reaction rates is  more 
diff icul t  to  assess .  Firs t ,  it i s  not certain with what species the 
hydrogen atom reacts, although Tanford and Pease have intimated that the 
reaction is  with the fuel molecule. Second, there has been no Wect  
measurement of the  re la t ive  ra tes  of reaction of the isotopic-hydrogen 
atoms. Turd, while a possible ratio of reaction rates might be cal- 
culated, the effective teqperature at which the reaction occurs is 
uncertain. 
If the known ra t ios  of diffusion  coefficients and atom concentra- 
t i on  which are produced 'by the isotopes (eqs. (10) and (11) ) are sub- 
stituted into equation (9) then  the  ratio of the specific-reaction-rate 
constant re-ed will have t o  be 1.4 t o  give  the  experimental  ratio 
However, since the relative reaction rates of the isotopic-hydrogen atoms 
have not been measured and cannot be unanibiguously calculated, it i s  not 
known whether the isotopic-reaction rates have the  ra t io  of 1.4. There- 
fore, i-b is not known whether the  ra t io  of the velocit ies at  the isotopic 
flames have the  value which would  be  predicted by the theory of Tanford 
and Pease. 
8 NACA RM E53L08 
It appears tha t   the   e f fec t  of substituting heavy w a t e r  fo r  light 
water on the  velocity of the moist carbon monoxide - oxygen flame i s  
consistent  with  the changes observed i n  the hydrogen-oxygen and 
deuterium-oxygen reactions when a direct  proportionality between flame 
velocity  and  reaction  rate i s  assumed, and it may be consistent  with 
the Tanford and Pease equation i f  t he   r a t io  of rate constants 
kH/kD = 1.4. 
. .  " 
L e w i s  Flfght Propulsion Laboratory 
National Advisory C o d t t e e  f o r  Aeronautics 
Cleveland, Ohio,  November 30, 1953 
1. .Tanford, Charles, and Pease, Robert N. : Theory of Burning Velocity. 
11. The Sapme Root Law f o r  Burning Velocity. Jaur. Chem. Phys., 
vol. 15, no. 12, Dee. 1947, pp.  861-865. 
2. Dugger, Gordon L.: Effect of Initial Mixtwe Temperature on Flame 
Speed of Methane-&, Propane-Air, and Ethylene-Air Mixtures. 
NACA Rep. 1061, 1952. (Sugersedes NaCA TN's 2170 and 2374. ) 
3. Simon, Dorothy Martin: Flame Propagation. 111. Theoretical Con- 
siderations of the Burning Velocities of Hydrocarbons. Jour. Am. 
Chem. SOC., vol. 73, no. .l, Jan. 1951,  pp.  422-425. 
4. Simon, Dorothy M., and Wong, Edgar  L.: Flame Velocities over a W i d e  
Composition Range f o r  Pentane-Air, Ethylene-&, and Propyne-Air 
Flames. NACA RM E5lH09, 1951. 
6 .  B u f f ,  Vearl N. , Gordan, Sanford, and Morrell, Virginia E. : General 
Method and Thermodynamic Tables for Computation of Equflibrium 
Composition and Temperature of Chemical Reactions. NACA Rep. 1037, 
1951. (Supersedes NACA TWs 2113 and 2161.) 
6. Hadman, G. , Thompson, E. W., and Hinshelwood, C. N. : The Oddation 
of Carbon Monoxide. Proc. Roy. SOC. (London), ser. A, vol. 137, 
no. A381, July 1, 1932, pp.  87-101. 
7. Hinshelwood, C. N. , Williamson, A. T., and Wolfenden, J. H. : Re- 
action Between Oxygen and the  Heavier Isotope of Hydrogen. Nature, 
vol. 133, June 2, 1934, pp. 836-837. 
I 
t 
NACA RM E53L08 9 
8 .  Friedman, Raymond, and Burke, Edward: Burning Velocities - Acetylene 
and Dideuteroacetylene with Air. Ind. and Eng. Chem., vol. 43, 
no. 12, Dec. 1951, pp. 2772-2776. 
9. Evans, Marjorie W.: Current Theoretical Concepts of Steady-State 
Flame Propagation. Chem. Rev., vol. 51, no. 3, 1952, pp. 363-429. 
10 NACA RM E53L08 
TABLl3 I. - FLAME S m D  OF CARBON NXiOXIDE - OXYGEN "TLTRES 
Added 
vat er , 
percent 
0.0 
0 
.12 
.31 
.50 
.50 
1.03 
1.66 
1.66 
1.78 
Flame velocity, cm/sec 
Dry 
34.0{ 34*o 
34.0 
"" 
"" 
"" 
"" 
"" 
"" 
"" 
"" 
H20 
added 
43.1 { 42*8 
43.4 
54.0 { 54m6 
53.5 
63.2 r3*2 
63.2 
63.8 
84.3 
"" 
"" 
-04.7 
D20 
added 
"" 
"" 
"" 
"" 
"" 
"" 
"" 
"" 
"I_ 
55.5 
".I 
"" 
"" 
68.4{ 68*7 
68.1 
82.2 
80.0 
c 
a 
3 
ETACA FM E53L08 11 
TABLE 11. - TEMPERATUKE OF CARBON MONOXDE - OmGCw FLAMES 
[SO .1 Percent CO, 19.5 percent 02, and 
0.4 percent I?,.] 
Added 
concentration,  temperature, water, 
atom flame light 
Hydrogen- Equilibrium 
percent atm OK 
0.0 0 2876 
.125 
.00225  2861 .5 
.00165 2865  .31 
.00085  2870 
1.03 
.0045 2837  1.78 
.0034 2850 
. . . . . . . . . . " . . . . . . . .. 
Tu 
Pump 
Figure 1. - Diagramnvltic sketch of apperetue. 
... . 
6 
A 
B 
C 
a 
Arc lanlp 
Lenses 
camera 
Carbon nmnoxide storage 
Stapcock 
Hater vial 
Mixing chamber 
Fl-ter 
NACA RM E53L08 13 
" _I 
0 .4 .e 1.2 1.6 2 .o 
Light o r  heavy water, percent 
Figure 2. - Variation of flame velocity of mixtures 
carbon monoxide and oxygen with additions of 
light or heavy water. 
14 NACA RM E53M8 
.P 
h 
Figure 3. - Variation of flame velocity of mixtures of carbon 
monoxide,  oxygen,  and water with hydrogen-atom  concentration. 
r 
c 
c 
Y 
NACA RM E53IX)8 
0 
15 
.0006 .001 .002 .004 .006 .01 
Partial pressure of hydrogen atoms, +, atm 
Figure 4. - Variation of flame-speed augmentation (U& 
where = 34.6) with hy*ogen-atom concentration. 
NACA-LPngley - 2-5-54 - 335 
. 


Effect of water on carbon monoxide-oxygen flame velocity

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