Mcdonald, Glen E

Schalla, Rose L

English

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C
	 NACA 
RESEARCH MEMORANDUM 
EFFECT OF PRESSURE ON THE SMOKING TENDENCY

OF DIFFUSION FLAMES

By Rose L. Schalla and Glen E. McDonald 
Lewis Flight Propulsion Laboratory

Cleveland, Ohio
DEPT. LIPRRy 
CHANCL'QUr Hi' AIRCRAFT 
DALLAS, TEXAS 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON 
September 17, 1953
RM 
https://ntrs.nasa.gov/search.jsp?R=19930087566 2020-06-17T09:25:14+00:00Z
NACA RN E53E05
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORANDUM 
EFFECT OF PRESSURE ON THE SMOKING TENDENCY OF DIFFUSION FLAMES 
By Rose L. Schalla and Glen E. McDonald 
SUMMARY 
The effect of pressure on smoke formation was investigated by 
burning nine hydrocarbon fuels as diffusion flames from a modified wick 
lamp in an enclosed chamber. The maximum relative rate at which each 
fuel could be burned without smoking was determined over a pressure 
range of about 1/2 to 4 atmospheres, and up to 12 atmospheres In the 
case of one fuel. The results indicate that over this pressure range 
the maximum smoke-free fuel flow was inversely proportional to the pres-
sure. The relative variation in smoking tendency among the different 
fuel types was approximately constant at all pressures. From an analysis 
of the data it has been tentatively proposed that the variations in smoke 
formation with pressure result from changes in the rate of diffusion and 
mixing of the fuel and air.
INTRODUCTION 
The variations in smoking tendency with fuel type at atmospheric 
pressure have been reported by several investigators (refs. 1, 2, and 3). 
However, investigations of smoking tendencies at pressures other than 
atmospheric have been very limited. Parker and Wolfhard (ref. 4) have 
observed that smoke formation from an acetylene flame decreased with 
decreasing pressure, and Smith (ref. 5) noted in a spectroscopic study 
of an ethylene flame that smoke formation became more pronounced at 
elevated pressures. 
In order to obtain a more comprehensive understanding of the effect 
of pressure on smoke formation, an investigation was conducted at the 
NACA Lewis laboratory to determine the variations in smoking tendency 
over a wide range of pressures for a variety of fuels. 
In the present study, six pure hydrocarbon compounds, a JP-4 fuel, 
and two blends of octane and toluene were investigated over a pressure 
range of about 1/2 to 4 atmospheres. For two of the hydrocarbons, 
namely, octene-1 and n-octane, the pressure range was extended to 9 and 
12 atmospheres, respectively. Smoking tendencies were determined by 
burning the fuels as diffusion flames from a modified wick lamp in an 
enclosed chamber. The maximum relative rate at which the fuels could be 
burned without smoking was used as the criterion of smoking tendency.
2
	
NACA RN E53E05
APPARATUS AND PROCEDURE 
A sketch of the apparatus used to study the effect of pressure on 
smoke formation is shown in figure 1. The chamber in which the fuels 
were burned was approximately 30 centimeters in diameter and 70 centi-
meters high. To obtain pressures above atmospheric, service air was 
admitted at the bottom of the chamber through perforated holes in a 
circular copper manifold. To remove the combustion products, a constant 
air exhaust of 220 milliliters per second was maintained at all pressures. 
The exhaust rate was measured by 'a flowmeter connected to the exhaust 
valve on the lid of the chamber. It was found that appreciable varia-
tions could be made in the exhaust rate without changing the smoking 
tendency of the flame. To obtain pressures below atmospheric, a vacuum 
line was Connected to the exhaust valve and the flowineter was attached 
to the air intake line. As a safety precaution and as a means of extin-
guishing the flame, gaseous carbon dioxide could be admitted to the 
chamber through a valve in the center of the lid. 
The fuels were burned from a wick lamp which consisted of a 1.5-
centimeter-inside-diameter glass tube 8 centimeters high to which was 
sealed a calibrated side arm. A hydraulic jack, operated from outside 
the chamber, permitted the raising and lowering of the wick lamp through 
a supporting collar. The rate of fuel flow needed to attain the smoking 
point could be controlled by adjusting the height that the wick extended 
above this support collar. A chimney 18 centimeters high and 4.7 centi-
meters in diameter was placed with the bottom edge level with the wick 
tip to keep. the flame erect and stable. The smoking point of the diffu-
sion flame was observed visually through a 2.54-centimeter Lucite window 
on the front of the chamber. 
The rate at which the fuel could be burned smoke-free was deter-
mined by measuring the time required for the level of the fuel in the 
side arm of the wick lamp to drop a unit distance while the fuel was 
being burned just below its incipient smoking point. The side—arm scale 
was calibrated against water; the fuel flow rates in grams per second 
could be obtained from this calibration and the specific gravity of the 
fuel. Reproducibility of the order of ±3 percent was obtained by this 
procedure. 
Since variation in the size and position of the chimney and other 
changes in the geometry of the setup will change the smoking tendency of 
the flame, the data from this study are relative. 
Fuels. - All the hydrocarbon compounds studied were of a purity of 
97 percent or greater. The two blends of n-octane and toluene were 
based on volume percent. Pertinent properties of the JP-4 fuel studied 
are shown in the following table:
NACA RM E53E05
	 3 
Fuel properties MIL-F-5624A, 
grade JP-4 
(NACA fuel 52-53) 
A.S.T.M. distillation D86-46, OF 
Initial boiling point 136 
Percent evaporated 
5 183 
10 200 
20 225 
30 244 
40 263 
50 278 
60 301 
70 321 
80 347 
90 400 
Final boiling point, OF 498 
Residue, percent 1.2 
Loss, percent 0.7 
Specific gravity at 600F/600F 0.757 
Reid vapor pressure, lb/sq in. 2.9 
Hydrogen-carbon ratio by weight 0.170 
Net heat of combustion, Btu/lb 18,700 
Aromatics, A.S.T.M. D-875-46T, 
percent by volume 8.5
RESULTS AND DISCUSSION 
The fuels employed in the present investigation and the maximum 
smoke-free fuel flow at each pressure studied are given in table I. In 
figure 2(a) the maximum rates (g/sec).at which the six pure hydrocarbon 
compounds could be burned smoke-free are plotted against pressure. The 
pressure range was extended for two of the compounds and these results 
are shown in figure 2(b). In figure 2(c) the maximum rates for 100 per-
cent n-octane, two blends of n-octane and toluene, and a JP-4 fuel are 
plotted against pressure. As can be observed from figure 2, the rates 
at which the fuels can be burned smoke-free consistently decreases with 
increasing pressure. The JP-4 fuel and the two blends of n-octane and 
toluene show the same variation in smoke formation with pressure as do 
the pure hydrocarbon compounds. 
The general shape of the curves in figure 2 suggests that they might 
fit the equation for a rectangular hyperbola, xy = k. The product of 
the pressure and smoke-free fuel flow should therefore be a constant (k) 
at all pressures. The values of k obtained from the data are presented 
in table I. If some allowance is made for experimental errors, it would 
4	 NACA EM E53E05 
appear that the values of k are reasonably constant for each fuel. The 
n-octane data, however, indicate that above 5.5 atmospheres, the xy = k 
relation is no longer satisfactory. Consequently, the use of this equa-
tion should be restricted to the pressure range of about 1/2 to 5 atmos-
pheres. Knowing the smoke-free fuel flow of a given fuel at any pressure 
within this range will permit an evaluation of k for that fuel. The 
smoke-free fuel flow at 1 atmosphere would, of course, be equal to k. 
Once k is obtained, the smoke-free fuel flow at other pressures may be 
readily calculated with an accuracy sufficient for most practical pur-
poses. 
In table II a comparison is made between the smoke-free fuel rate 
of n-octane and octene-1 over a, pressure range of 1 to 10 atmospheres. 
At 1 atmosphere, twice as much n-octane can be burned smoke-free as 
octene-1, and at all pressures up to 10 atmospheres this same proportion 
is found to be approximately constant. A similar comparison among the 
other fuels also showed a fairly constant ratio over the pressure range 
investigated. Consequently, the same relative variations in smoke forma-
tion among different fuel types existed at all the pressures investigated. 
In an attempt to explain the variations in smoke formation with pres-
sure, several features of the diffusion flame were studied. Consideration 
was first given to the effect of pressure on flame size. In agreement 
with the postulations of Parker and Wolfhard (ref. 4) it was observed 
that the height of a nonsmoking diffusion flame was essentially inde-
pendent of pressure and proportional to the mass flow. For example, at 
1 atmosphere a fuel flow of 1.8 grams per second supported a nonsmoking 
flame of n-octane 3.5 centimeters high. At both 1/2 and 2 atmospheres 
this same fuel flow again supported a nonsmoking flame 3.5 centimeters 
high. Thus, increases in pressure, over the range investigated, do not 
cause a change in the flame height providing a constant mass fuel flow is 
maintained. 
Since the flame volume and surface area are roughly proportional to 
the flame height, the volume and surface area of the flame cone are also 
independent of pressure and proportional to the mass flow. Consequently, 
the variation in smoke formation with pressure cannot be directly attri-
buted to changes in the surface area or volume of the flame. 
One of the factors which changes with pressure is the rate of diffu-
sion and, consequently, the rate of mixing (ref. 4). Since the fuels 
were burned as diffusion flames in this study, the change in rate of 
diffusion and mixing should be taken into consideration in any explanation 
regarding increased smoke formation with pressure. It is reported in ref-
erence 4 that the diffusion coefficient is inversely proportional to the 
pressure, D 1/P. If diffusion processes are the controlling factor, 
the smoke-free fuel flow should also be inversely proportional to the 
pressure. To observe how closely this relation is followed, a plot of
NA.CA RM E53E05
	 5 
smoke-free fuel flow against i/ p is shown in figure 3. The fairly 
straight lines obtained in these plots establish that the smoke-free fuel 
flow is inversely proportional to the pressure. Therefore, the change in 
the diffusion process, which controls the rate of mixing, may account for 
the variation in smoke formation with pressure. 
SUMMARY OF RESULTS 
An investigation of the variation in smoke formation among hydro-
carbon fuels over a pressure range of 1/2 to 12 atmospheres gave the 
following results: 
1. Over the pressure range studied, the amount of fuel which could 
be burned smoke free was inversely proportional to the pressure; how-
ever, the relation appeared somewhat less precise above 5 atmospheres. 
2. The same relative variation in the smoke-free fuel flow among 
the different fuel types existed at all pressures. 
3. The decrease in the rate at which the fuels could be burned 
smoke free with increasing pressure was attributed to the decrease in 
the rate of diffusion and mixing of the fuel and air. 
Lewis Flight Propulsion Laboratory 
National Advisory Committee for Aeronautics 
Cleveland-, Ohio, April 16, 1953 
1. Clarke, A. E., Hunter, T. G., and Garner, F. H.: The Tendency to 
Smoke of Organic Substances on Burning, Part I. Jour. Inst. 
Petroleum, vol. 32, no. 274, Oct. 1946, pp. 627-642. 
2. Minchin, S. T.: Luminous Stationary Flames: The Quantitative Rela-
tionship between Flame Dimensions at the Sooting Point and Chemical 
Composition, with Special Reference to Petroleum Hydrocarbons. 
Jour. Inst. Petroleum Technologists, vol. 17, 1931, pp. 102-120. 
3. Schalia, Rose L., and McDonald, Glen E.: Variation in Smoking Tendency 
Among Low Molecular Weight Hydrocarbons. MACA RM E52122, 1952. 
4. Parker, W. G., and Wolfhard, H. G.: Carbon Formation In Flames, 
'	 Part III. Jour. Chem. Soc. (London), Aug. 1950, pp. 2038-2044; 
discussion, pp. 2045-2049. 
5. Smith, E. C. W.: The Emission Spectrum of Hydrocarbon Flames. , Proc. 
Roy. Soc. (London), Ser. A, vol. 174, no. A956, Jan. 12, 1940, 
pp. 110-125.
6
	
NACA RM E53E05 
TABLE I. - VARIATION IN SMOKE-FREE FUEL FLOW WITH PRESSURE 
Pres- Fuel k- Pres- Fuel k Pres- Fuel k 
sure, flow, sure, flow, sure, flow, 
atm g/sec atm g/sec atm g/sec 
Isooctane 90 percent n-octane 80 percent n-octane 
(2,2,4-trimethylpentane) +10 percent toluene +20 percent toluene 
0.65 2.80X10 3 1.8X10 3 0.85 13.35X10 2.9X103 0.56 2.69X10 3 1.5X103 
1.04 1.89 2.0 1.12 2.33 2.6 1.05 1.44 1.5 
1.48 1.33 2.0 1.51 1.72 2.6 1.53 .96 1.5 
2.11 .86 1.8 2.53 1.01 2.6 1.37 .67 1.6 
2,64 .78 2.1 3.18 .92 2.9 3.35 .60 2.0 
3.07 .73 2,2 3.76 .82 3,1 Average 1.6 
3.80 .65 2.5 Average 2.8 
Average 2.1 
Cyclohexene 2-iuethyloctane JP-4 
0.62 2.43X10 3 1.5X10 3 1.04 3.85X10 3 4,0X10 3 0.56 2.42X10 3 1.4X103 
1.04 1.29 1.3 1.38 2.81 3.9 1.26 1.04 1.3 
1.21 1.09 1.3 1.78 2.13 3.8 1.72 .70 1.2 
1.82 .72 1.3 2.16 1.71 3.7 2.16 .66 1.4 
2.84 .56 1.6 3.01 1.29 3.9 3.59 .53 1.9 
3.59 .47 1.7 3.46 1.21 4.2 3.79 .52 2.0 
Average 1.5 Average 3.9 Average L 
n-Octane Octene-1 Heptene-1 
1.04 5.30X10 3 5.5)cLO -3 0.79 3.37X10 3 2.7X10 3 0.50 4.38X10 3 2.2X103 
1.19 4.47 5.3 1.04 2.65 2.8 .76 2.97 2.3 
1.48 3.56 5.3 1.48 1.91 2.8 1.04 2.20 2.3 
2.02 2.56 5.2 1.82 1.34 2.4 1.21 1.91 2,3 
2.64 1.92 5.1 2.70 .95 2.6 1.68 1.25 2.1 
3.18 1.62 5.2 2.79 .95 2.7 2.43 .89 2.2 
3.76 1.44 5.4 3.11 .90 2.8 3.76 .70 2.6 
5.53 1.00 5.5 3.74 .78 2,9 Average 2.3 
8.35 .80 (6•7)b 8.82 37 
12.01 .56 (.i) Average 2.7 
Average 1	 5.3
ak ,
 product of pressure and fuel flow.
	
HACA 
 
bAverage does not include values in parentheses. 
NACA RM E53E05 	 7 
TABLE II. - RATIO OF SMOKE-FREE FUEL FLOW 
RATE OF n-OCTANE AND OCTEI'IE-1 OVER A 
PRESSURE RANGE OF 1 TO 10 ATMOSPHERES 
Pres- 
sure, 
atm
Fuel flow, 
9/sec
Ratio of 
n-octane 
to 
octene-1 n-Octane Octene-1 
1 5.5]X103 2.75X10 3 2.00 
2 2.60 1.25 2.08 
3 1.70 .88 1.93 
4 1.34 .73 1.84 
6 .98 .51 1.92 
8 .80 .40 2.00 
10 .67 a32 2.09
aEraPo1ated value. 
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Pressure gauge 
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8
	 NACA RN E53E05 
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Figure 1. - Diagram of apparatus for determining smoking tendency.
NtCA RN E53E05	 9 
5. 
4.8 
4.0 
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Hydrocarbon 
o	 a-Octane 
0	 2-Methyloctane 
0	 Octene-1 
Heptene-1 
V	 Isooctane 
Cyclohexene 
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0	 1	 2	 3	 4
Pressure, atm 
(a) Six hydrocarbons at pressures to 4 atmospheres.

Figure 2. - Variation In smoke-free fuel flow with pressure. 
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NACA RN E53E05	 11 
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Pressure, atm. 
(c) Various fuels and blends at pressures to 4 atmospheres.

Figure 2. - Concluded. Variation in smoke-free fuel flow with pressure. 
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Effect of pressure on the smoking tendency of diffusion flames

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