, ' 

DOE/NASAl0266-1 

NASA CR-1 746.44 

NASA-CR-174644 

19840015707 

Flame Propagatic)n in Heterogeneous 

Mixtures of Fuel Drops and Air 

G. D. Myers and A.. H. Lefebvre 

Purdue University 

_ May 1984 

Prepared for 

NATIONAL AERONAUTICS AND SpACE ADMINISTRATION 

Lewis Research Center 

Under Grant NAG 3-266 

for 

U.S. DEPARTMENT 01: ENERGY 

Conservation and Renewable Energy 

Office ()f Vehicle and Engine R&D 

r.·; ;\',' '''' (\ 'I'(I0LI ":,,. :.- \ ........ ;;o·t 

LP.NGLl:Y RESEARCH 1.t::I'J ricH 

LI8rU,RY, NASA 

111111111111111111111111111111111111111111111 

NF00420 
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DOE/NASAl0266-1 

NASA CR-1 74644 

Flame Propagation in Heterogeneous 

Mixtures of Fuel Drops and Air 

G. D. Myers and A. H. Lefebvre 

Purdue University 

West Lafayette, Indiana 47907 

May 1984 

Prepared for 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, OH 44135 

Under Grant NAG 3-266 

for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Vehicle and Engine R&D 

Washington, D.C. 20585 

Under Interagency Agreement DE-AI01-81 CS50006 
SUMMARY 

An experimental study is conducted on the influence of fuel chemistry 

on the flame speeds of flowing mixtures of fuel drops in air at atmospheric 

pressure. Air is supplied at room temperature to a 10 cm square working 

section, fitted with, removable quartz windows which provide optical access 

to the flame. The flame is ignited and stabilized using a sma1l diameter 

stainless steel tube carrying a premixed supply of propane and oxygen. 

This pilot flame system is located at the center of the test section 

entrance, and is ignited by a high-voltage spark. Sixty-four evenly-spaced 

fuel injectors are provided to ensure a uniform fuel distribution in the 

mixture entering the flame zone~ Each fuel injector comprises a plain-jet 

a~rblast atomizer, 1.83 mm in diameter., By varying the air flow rate to 

these atomizers the mean fuel drop size can be varied over wide ranges 

while maintaining all other flow parameters constant. The atomization 

quality of the spray is measured at several different locations in the 

test section using a light-scattering technique. This technique yields 

the Sauter mean diameter (SMD) of the spray from analysis of the forward 

scattering of a monochromatic beam of light resulting from its passage 

through the spray. The main advantage of using airblast fuel injection 

is that it allows the influence of mean drop size on flame speed to be 

readily determined. 

Schlieren pictures of the flame provide the basic data for the 

measurement of flame speed. A protractor is applied to enlarged prints 

of the flame photographs to obtain the flame ang~e to the nearest half 

degree. The flame speed is then calculated by multiplying the mainstream 

velocity by the sine of half the corresponding flame angle. The absolute 

values of flame speed obtained by this method are accurate only to within 

±-10 percent. However, the relative values of flame speed are considered 

to be appreciably more accurate. 

The fuels employed include a conventional No.2 fuel oil plus various 

blends of JP 7 with stocks containin.9 single-ring and multi-ring aromatics. 

The results obtained confirm theoretical predictions in regard to the 

influence on flame speed of mean drop size. For all fuels the measureq 
flame speed is found to be inversely proportional to SMD above some 

critical size, thus indicating that in this range of large drop sizes 

evaporation rates are controlling to flame speed. For SMD's below 

the critical siz~ flame speeds are Sssentially independent of mean drop size, 

which demonstrates that for well-atomized sprays evaporation rates are 

not controlling the flame speed. 

The fuels exhibiting the highest flame speeds are those containing 

multi-ring aromatics. This is attributed to the higher radiative heat 

flux emanating from their soot;"bearing flames which enhances the rate of 

evaporation of the fuel drops approaching the flame front. There is also 

evidence to suggest that the phenomenon of "micro-explosion" may accelerate 

the flame speeds of multi component fuel sprays injected in close proximity 

·to the flame zone. 

2 
INTRODUCTION 

The combustion of heterogeneous fuel-air mixtures occurs in a 

variety of practical combustion systems, including diesel and gas turbine 

engines and industrial furnaces. However, despite these broad applications, 

little is known about flame propagation in heterogeneous mixtures because 

few experimental studies have been co~ducted, and these have yielded 

incomplete and sometimes conflicting results (refs. 1 to 7). Although 

the effects of atomization quality on flame speed are not well understood, 

the influences of fuel spray properties on the performance of diesel and 

gas turbine engines have been observed for some time. Lefebvre (ref. 8), 

for example, shows that atomization quality affects the combustion 

efficiency, stability, operating temperature, emissions rate, and pattern 

factor of gas turbine engines. Taylor (ref. 9) reports that spray 

characteristics have a significant effect on the rate of pressure rise 

in diesel engines, as well as their fuel economy and pollutant emissions 

rates. Changes in fuel type also affect the performance of these engines. 

Physical fuel properties such as viscosity and surface tension influence 

atomization quality, while chemical properties affect chemical reaction 

rates, fuel volatility, and the concentration and identity of species in 

the exhaust gases. 

The work described in this report represents the first phase of a 

comprehensive program of research on the effects of fuel type and 

composition on various important combustion characteristics such as flame 

speed and the main pollutant emissions, namely carbon monoxide, unburned 

hydrocarbons, oxides of nitrogen, soot and particulates. Its objective 

is to examine the separate effects of atomization quality, fuel type, 

fuel/air ratio, and mixture velocity on the structure and burning 

velocity of flames supported by flowing heterogeneous mixtures. A unique 

feature of the research is separation of the effects of fuel physical 

properties and chemical composition on burning velocity. This is 

accomplished via an ability to control and systematically vary the 

Sauter mean diameter of the fuel spray regardless of the fuel type. 

3 
Thus, fuels with different atomization characteristics can be examined 

at the same mean drop size by suitable adjustment of the atomizer, 

thereby emphasizing the differences in burning velocity attributable to 

changes in chemical composition alone. 

This work was performed under NASA Grant NAG 3-266 in the Combustion 

Laboratory of the Thermal Science and Propulsion Center, School of 

Mechanical Engineering, Purdue University. The project manager was 

John S. Clark of the Alternative Fuels Utilization Project Office, 

NASA Lewis Research Center. Funding for the project was provided by 

the Heavy Duty Transport and Fuels Integration Branch, Office of 

Vehicle and Engine R&D, Department of Energy, Washington, D.C. 

Previous Related Work 

The results of previous experimental studies show that flame speeds 

in two-phase mixtures of fuel drops and air are governed by the following 

parameters - fuel type, mean drop size, fuel/air ratio, flow velocity, 

turbulence intensity, and average droplet interspatial distance. These 

are discussed below in turn. 

Fuel Type 

The reported flame speed data encompass the following fuels: 

tetralin, iso-octane, normal octane, ethanol, gasoline, kerosine, diesel 

oil and heavy fuel oiL Generally it is found that the heavier, less 

volatile fuels have lower flame speeds. Such fuels are also more 

sensitive to changes in mean drop size (SMD)w Quantitative results are 

reported only in 'reference 7, where flame was found to be directly . . related toth~ function ln (1 + B), where B is Spalding's mass transfer 

number. B increases as the volatility of the fuel increases. Octane, 

with a high transfer number of 6.1, as compared with 2.8 for diesel fuel, 

has nearly twice the flame speed at the same values of SMD and equivalence 

ratio. Although H~yashiet al. (ref. 6) and Ballal and Lefebvre (ref. n 

employed similar fuels, comparison of results is difficult because the 

former employed a closed combustion bomb whereas Ballal and Lefebvre used 

an open combustion chamber. 

4 
Another difficulty in analyzing flame speed data with regard to fuel 

type is the lack of adequate characterization of the fuels in each case. 

Although this would not be significant for pure fuels, the same single 

component fuel was not examined in any two of the experiments where 

comparison can be made. In the case of mUlticomponent fuels, only general 

labels such as 'fuel oil' or 'kerosine ' are given,without regard to the 

fact that wide variations in both chemical structure and evaporation 

characteristics can Qccur among fuels so labelled. For instance, Jet A, 

JP 4, JP 5 and JP 7 are all considered kerosines, yet have flash points 

ranging from 311K for Jet A to 342K for JP 7. Chemical composition also 

varies. ,Jet A contains approximately 20 percent aromatic hydrocarbons, 

while JP 7 contains only 2 percent (ref. 10). The physical and chemical 

properties of less refined fuels such as heavy oils may vary even more 

s i gnifi cantly. 

Mean Drop Size 

Drop size effects on flame speed and flame structure are found to 

vary depending on the fuel type, fraction of fuel vaporized, and the 

aerodynam"j c properties of the fresh mi xture. For the fl owi ng heterogeneous mixtures examined in references 1 to 5, increases in 

beyond a mean size of 3011m tended to lower the flame speed. 

& Cohen (ref. 1) studied monodisperse flowing aerosols only. 

drop size 

Burgoyne 

They found 

that suspE~nsions of very fine droplets « 1011m) burned in the same manner 

as a gaseous fuel under similar conditions. As the drop size was 

increased toward 30 11m, the flame speed increased, and individual drops 

encased in diffusion flame envelopes were observed superimposed on the 

gaseous laminar flame structure. However, in these experiments fuel/air 

ratio was varied concurrently with drop size, so it is difficult to draw 

conclusions on the effect of drop size alone. Burgoyne and Cohen also 

observed that the presence of drops extended the flammability limits 

leanward in comparison to the completely vaporized fuel. These 

investigators explained both phenomena in terms of a change in the 

mechanism of flame propagation. In the case of fine atomization, flame 

was thought to exist in the interdroplet space, whereas for larger drops 

5 
flame was relayed from one droplet encased in a diffusion flame envelope 

to the next, with little or no combustion occurring in the interdroplet 

region. In the case of the larger drops, the overall flammability limits 

were extended leanward, since each drop supports a diffusion flame 

burning in stoichiometric proportions. 

These findings are echoed in the work of Cekalin (ref. 2), who used 

similar arguments to explain flame behavior in gasoline drop-vapor-air 

mixtures. They obser.ved that when the mean drop size was small 

(around 20~m), and vapor was present in the interdroplet space, the 

flame speed first diminished with increases in drop size, and then began 

to increase as the fraction of fuel present in liquid form was increased. 

In general, Cekalin studied only a limited range of drop sizes, (20-30~m) 

and concluded that the presence of vapor exerted a greater influence on 

flame speed than mean drop size. A potential drawback to Cekalin's 

apparatus was the use of electric heating within the mixture preparation 

tube to vary and control the concentration of vapor. This could also 

have varied the initial mixture temperature from one test to another, 

hence influencing the burning velocity. 

Mizutani and Mishimoto (ref. 3) found that flame speed was inversely 

proportional to SMD, regardless of fuel type, fuel/air ratio, or aerodynamic properties. The mean drop size was not reduced below the 30~m 

level where a decrease in flame speed was observed in references 1, 2, 

and 5. The flame speed was found to be inversely proportional to mean 

drop size for lean mixtures and for mainstream velocities above 2 m/s. 

The work of Po1ymeropou1as and Das (ref. 5) reinforces the conclusions 

of reference 3, but suffers from a lack of drop size measurements. A 

single spray with 54~m laverage l diameter was sized; then the atomization 

quality was steadily improved, producing first an increase in burning 

velocity, and then a decrease as the SMD was further reduced. Although 

no size measurements were made beyond those for the initial spray, the 

authors estimate that p~ak flame speed was attained with a spray of 

30 microns SMD. 

The flame speed in stagnant mixtures was studied by Balla1 and 

Lefebvre (ref.?) and Hayashi et a1. (ref. 6). The latter, working with 

6 
a closed combustion bomb which was subject to large pressure increases 

as the reaction progressed, found that flame speed increased with drop 

size in the Y'ange from 16 to 40j1m, provided that the ratio of liquid 

fuel to total air was greater than the ratio of vaporized fuel to air. 

These investigators also found that the flame speed peaked on the lean 

side of the stoichiometric fuel/air ratio regardless of drop size, and 

that increasing the mean drop size shifted the peak value toward richer 

fuel/air ratios. Fur,ther, their results showed that the presence of 

drops can increase the flame speed over that for a completely vaporized 

fue 1. 

This last finding is in conflict with the results of Balla1 and 

Lefebvre (ref. 7), who studied the propagation of flat flames through 

stagnant, drop-vapor-ai r suspensions in free-fall. These workers 

recorded the highest burning velocities under conditions of complete 

vaporization, and otherwise found that flame speed was inversely proportional 

to SMD, as in reference 3. 

Wi th the exception of reference 7 the presence of drops is found to 

raise the burning velocity over that of completely vaporized fuel. 

Mizutani and Nishimoto (ref. 3), for example, found the flame speed of 

kerosine drop-air mixtures to be consistently higher than for the 

propane-air mixtures of Lefebvre and Reid (ref. 11) burned in a similar 

apparatus, for all mean drop sizes less .than lOOj1m. A later study by 

Mizutani and_Nakajima (ref. 4), conducted in the inverted-cone flame 

apparatus of reference 3, showed that the addition of kerosine drops 

to a gaseous propane-air mixture i.ncreased the flame speed at lean 

overall equivalence ratios, and extended the flammability limit.s leanward. 

They also discovered that the Phenomenon ,could be optimized with respect 

to both the amount and the drop size of the liquid kerosine added. The 

optimum mean drop size was found to be in the neighborhood of 40j1m, and 

the enhancing effec~ was reduced as the drop size was increased to 60j1m. 

Mizutani and Nakajima explained the phenomenon by proposing that the 

presence of drops serves to wrinkle and lacerate the flame surface, 

thereby increasing the flame speed. They also proposed that the drops 

serve as high temperature ignition sources, extending the flammability 

limits and accelerating the burning velocity of adjacent flame elements. 

7 
They speculated further that burning velocity is increased because the 

flame propagates through regions of optimum fuel air/ratio formed around 

evaporating drops, and because the randomly located burning drops cause 

local gas expansions, generating turbulence which further intensifies the 

transport processes. Additional experiments in which vaporized kerosine 

when added to propane yielded no increase.in burning velocity, led them 

to conclude that the phenomenon was a consequence of the two-phase nature 

of the mixture, rather than the result of some chemical or thermal property 

change. 

From Mizutani and Nakajima's results it would appear that flame 

speed is enhanced by the two-phase nature of the flow.. With the exception 

of Ballal and Lefebvre, all other investigators found that the presence 

of droplets widens flammability limits and enhances flame speeds. However, 

in Ballal and Lefebvre's experiments the drops were at rest with respect 

to the gas, since the settling velocity of the drops was eliminated by 

conducting the experiments under conditions of zero gravity. In 

reference 6 the mixture was also assumed stagnant. However, some 

droplet motion may have occurred due to settling and because the charge 

was drawn into the combustion chamber only milliseconds prior to ignition. 

One can use the standard droplet motion equation to estimate the 

time necessary for the relative velocity between drop and airstream to 

become sensibly zero. Assuming a large drop size of 100~m, with an 

initial relative velocity of 40 mis, this time is in the order of 30 ms, 

which is short compared with the injector-to-flame residence time 

encountered in the experiments reported in references 1 to 5. It can 

be assumed, therefore, that the relative velocity between drop and gas 

is near zero (neglecting the fluctuating velocity) in all these experiments. 

Hence, the motion of the drops with respect to the flame, rather than 

their motion relative to the gas, must be partly responsible for the 

increases in burning velocity of drop-vapor-air suspensions over vapor-air 

mixtures observed in references 1 to 6. 

8 
Fuel/Air Ratio 

Of the various definitions of fuel/air ratio, the simplest, and 

perhaps the most practical, is the lIoverallll fuel/air ratio, qov' which 

is found by dividing the total mass flow rate by the total air flow rate. 

Other fuel/air ratios of importance are the mass ratio of fuel drops to 

air, qL' and the ratio of fuel vapor to air, qv' 

One would expect that burning velocity should increase as a function 

of qov from a minimum'value at the lean limit to a peak just richer than 

stoichiometric, and then decrease as the mixture is enriched. However, 

the local fuel/air ratio in the vicinity of an evaporating drop mayor may 

not be related to qov' and the fuel/air ratio in the vapor phase, qv' may 

vary independently of qov' depending on the evaporation rate. In general, 

flame speeds in two-phase mixtures were found to increase with qov' but 

only one investigation encompassed a full range of equivalence ratios 

(Burgoyne and Cohen studied rich mixtures, but the fuel/air ratio and 

drop size varied concurrently). Hayashi et al. (ref. 6) found that 

burning velocities peaked on the lean side of the overall stoichiometric 

value for ethanol mixtures, and that the peak moved to richer mixture 

strengths as the drop size increased. When qov was fixed at a lean value, 

but qL was increased, a decrease in the burning velocity for small (7~m) 

drop sizes was observed, but the reduction for larger drop sizes was not 

as pronounced. Cekalin (ref. 2) observed similar behavior for gasolineair mi xturE~S. When qL was increased from zero to 0.2 a tal ean va 1 ue of 

overall equivalence ratio, a decrease in flame speed was observed for 

both 30~m and 20~m drop sizes. Beyond this minimum value, the flame 

speed increased with qL until a maximum value was obtained near 0.5. 

Miz~tani and Nishimoto (ref. 3) found that flame speed increased 

linearly with qL' with the slope being steeper for smaller drop sizes 

and higher air flow rates, but they did not distinguish between qL and 

qov' so thE~i r resul ts cannot be used to confi rm the work of Ceka 1; n. 

Ballal and Lefebvre (ref. 7) studied the effect of vapor fraciion, fv' 

(where fv 'is the fraction of the total fuel in the form of vapor) on flame 

speeds in stagnant aerosols. They found th'at flame speed increased to 

a maximum at a vapor fracti on of 1. 0 at both 1 ean and stoi chiometric 

9 
values of overall equivalence ratio. This apparent conflict with the 

results of other workers can be resolved by noting that Ballal and 

Lefebvre's study was confined to the left of Cekalin's minima, where the 

flame speed was observed to decrease as qL increased (or fV decreased). 

Ballal and Lefebvre solve this dilemma by including both the vapor 

fraction and the overall equivalence ratio in their expression for flame 

speed, recognizing that one may vary independently of the other depending 

on the evaporation ra,te. They found the flame speed to vary with the 

inverse of the square root of (1 - fv)' The overall equivalence ratio 

affects flame speed through the laminar burning velocity of the vaporized 

fuel, which is assumed to behave like a gaseous fuel, rising from the 

lean limit and peaking just on the rich side of the stoichiometric 

equivalence ratio. This conclusion represents the concensus of all 

the investigations reviewed, namely that flame speed is sensitive to 

the amount of vapor present in the inter-droplet space, regardless of 

the overall equivalence ratio. 

Analysis of the effect of vapor fraction and fuel/air ratio on flame 

speed in references 2, 3 and 5 is complicated by the fact that some 

evaporation undoubtedly occurred between fuel injection and ignition, 

and the amount of evaporation varied with the main air flow rate and 

fuel injection pressure (both of which affect the evaporation rate 

through the velocity 'slip' between spray and airflow), and the drop size. 

One can show that evaporation did take place by conservatively estimating 

the evaporation time for a given case, and comparing this to the residence 

time of the fuel between injection and ignition. In the experiments of 

Polymeropolous and Oas (ref. 5), for example, kerosine was injected into 

an airstream moving at 2.65 mis, 1 meter upstream of the ignition point. 

The largest mean drop size reported in this experiment is 50~m. 

Conservatively estimating the evaporation rate by assuming room temperature 

and pressure, and no slip between the drop and the airstream, the 

evaporation time for a 50~ drop is 80 ms. The corresponding residence 

time is 400 ms, indicating that total evaporation occurs for at least 

some portion of the initial spray, since some drops will be larger and 

some smaller than the mean size, depending on the distribution. Hence, 

in these experiments, one would expect the vapor fraction to increase 

10 
with reduction in drop size and/or increase in main air flow rate. In 

reference 3, for instance, the observed increase in flame speed with 

increasing flow rate could be partially due to the concurrent increase 

in evaporation rate. 

Fluid Dynamics 

The flow velocity, turbulence intensity, and turbulence scale are 

interrelated parameters which can have a significant influence on burning 

velocity. The effects of flow velocity and turbulence intensity were 

examined in references 2 and 3. Mizutani and Nishimoto (ref. 3) found 

that the component of turbulence intensity normal to the flow direction 

caused an increase in flame speed regardless of flow velocity, liquid fuel/ 

air ratio, or drop size. Cekalin (ref. 2) observed that flame speed was 

very depl~ndent on turbulence intensity for small drop sizes and for high 

vapor fractions, but found no effect for the larger drop sizes. Both 

sets of experimental conditions fall roughly within the transition region 

of Balla"1 and Lefebvre (ref. 12), where u I '" 2SL, and ST is expected to 

be approximately twice the fluctuating velocity, and completely independent 

of scale. Generally, i.t is found that flame speeds in heterogeneous 

mixtures are a strong function of turbulence intensity. 

Drop Spacing 

The influence of the average. distance between drops was examined by 

Cekalin (ref. 2), who found the sensitivity of flame speed to the spatial 

distance d to be a function of the vapor fraction. When the vapor fraction 

was high, the flame speed fell gradually with increase in d, regardless of 

drop size, in the range from 20-30~m. Where the vapor fraction was low, 

the flame speed decreased rapidly with increase in d. Cekalin attributed 

this to the difference in the flame propagation mechanism between the two 

cases. Where the vapor fraction is high, flame propagates readily in 

the interdroplet space, with little regard for d. Ho\</ever, if the vapor 

fraction is low, flame must be relayed among individual drops burning in 

diffusion flame envelopes and, in consequence, the interdrop distance 

is criti ca 1. 

11 
The experimental work reviewed above indicates that flame speed in 

a two-phase mixture depends on the fraction of vapor present, the mean 

drop size, the overall fuel/air ratio, the fuel volatility, the turbulence 

intensity in the approach stream, motion of the drops with respect to the 

flame front, and the interdrop1et distance. ~10reover, the sensitivity of 

flame speed to anyone of these variables is usually dependent upon the 

values of the others. Even so, two of the above studies propose mathematical 

relationships between.flame speed and the relevant variables. Mitzutani 

and Nishimoto suggest the following empirical equation for flame speed 

in the drop size range 30-100~m and lean equivalence ratios (ref. 3). 

ST = ~ (qL - 0.012)(u ,)1. 15 m/s (1) 

where 0 is the Sauter mean diameter, ~m, qL is the liquid/ai\ mass ratio, 

ul is the fluctuating velocity normal to the mean velocity, and K is an 

empirical constant depending on fuel type. K = 6800 for 'kerosine ' 

and 4300 for diesel fuel. The above equation predicts a linear 

relationship between flame speed and the inverse of the drop size. 

It is difficult to apply in practice since the constant K is available 

for only two fuels, and the liquid/air ratio is not as easily measured 

as the overall fuel/air ratio. Nonetheless, the equation shows good 

agreement with experiment, particularly for small drop sizes (40 - 60 ~m), 

and is the only expression proposed for flowing aerosols. 

Ballal and Lefebvre (ref. 7) propose the following expression for flame 

speeds in stagnant or low turbulence drop-vapor-air mixtures: 

= ~(.l - f v) PF 0

2 

S a

+ 5L~-.0.5 g ------ 2 

8 Pg In (1 + B) SL 

(2 ) 

This equation comprises two terms; the first characterizes the evaporation 

rate, and thus depends on fuel volatility, SMO, and vapor fraction. The 

second characterizes the chemical reaction rate. When the evaporation 

12 
time is longer than the chemical reaction time, flame speed is enhanced 

by increases in gas density, fuel volatility, vapor concentration, and 

reduction in the mean drop size. Where the reaction time is negligibly 

small in comparison to the time required for evaporation, equation (2) 

also predicts that flame speed is inversely proportional to mean drop 

size. At the other extreme, when chemical reaction rates are limiting, 

the flame speed reverts to the normal burning velocity for the evaporated 

mixture. Ballal and Lefebvre used the equation successfully in predicting 

their results for a variety of experimental conditions. However, the 

relation was not developed for turbulent flowing mixtures, and cannot, 

therefore, be expected to account for the accelerated evaporation rates 

due to droplet motion and the effect of turbulence in wrinkling the 

flame front. 

13 
EXPERIMENTAL 

The apparatus employed is shown schematically in Figure 1. Air is 

drawn at room temperature through a sound attenuator and butterfly valve 

to the suction side of a centrifugal blower. At exit it enters a 

flow-metering section and then proceeds into a series of perforated 

plates, honeycombs and wire gauzes to flatten the velocity profile, 

erase any swirl, and eliminate any large-scale eddies. Downstream 

of this flow-conditioning section a transition piece converts the ducting 

from a nominal 15 cm diameter to a 10 cm square cross section which ma.tes 

with the test section. The test section comprises a 10 cm square duct, 

50 cm long, milled from mild steel, and equipped with removable quartz 

windows, 25 cm long and 10 cmwide, which provide optical access to the 

flame. The test section is followed by a simple expansion nozzle which 

conveys the combustion products outdoors via a large diameter section of 

water-cooled exhaust ducting. 

The flame is ignited and stabilized using a 0.62 mm outer diameter 

stainless steel tube carrying a premixed supply of propane and oxygen. 

The igniter tube is located at the center of the test section entrance. 

The pilot flame is ignited by a high-voltage spark. A minimum of oxygen 

is used because a high flame temperature at the igniter could disturb 

the schlieren image. It was found necessary to leave the spark on to 

achieve adequate stabilization of the main flame. 

Fuel Injection 

The fuel injection system is designed to provide a controllable, 

metered, filtered supply of fuel and air to the multi-point airblast 

atomizer. The fuel supply system consists of a fuel tank, filter, pump, 

and relief valve placed within a return loop, feeding the atomizer through 

throttling val~es. Fuel metering is accomplished through two linear 

flowmeters, arranged so that ball valves can be used to isolate either. 

The flow meters are checked in situ by closing off the fuel flow to the 

atomizer and directing it for a specified time into a calibrated glass 

container. The fuel pressure is measured just downstream of the flow 

14 
..... 

0'1 

ATOMIZING 

AIR STORAGE 

TORCH 

IGNITER SCHLIEREN 

SYSTEM 

Figure 1. Schematic Diagram of Test Rig . 

EXHAUST 

t 
meters to monitor the cleanliness of the fuel system. Provision is made 

for purging the fuel side of the atomizer with filtered shop air from 

the air side. 

The 64-point airb1ast atomizer employed in this study was constructed 

from various sizes of hypodermic stainless-steel tubing, machined and 

joined with silver solder. The face of the injector is slightly less 

than 10 cm square, so that it fits snugly in the transition piece just 

upstream of the test section. Each p1ain~jet atomizer consists of a 

fuel tube 1.08 mm outside diameter and 0.68 mm inside diameter, surrounded 

concentrically by an air tube 1.83 mm outside diameter, and 1.37 mm inside 

diameter. Each fuel tube is sleeved with a smaller tube to reduce the 

inside diameter to 0.33 mm, thus increasing the necessary pressure drop 

across the nozzle. Each of the atomizers so constructed is fed by 

separate supplies of fuel and air introduced at both ends of a given 

feeder tube. The sixteen feeder tubes are connected to four manifold 

tubes which are in turn joined to four fuel supply tubes and four air 

supply tubes. Fuel and air are conveyed from the systems described above 

to the fuel and air supply tubes vi.a two horseshoe-shaped manifolds 

positioned outside the rig. 

The atomization quality of the injector as a function of fuel, 

airblast, and main air flow rates, was characterized at a variety of 

downstream positions using the light scattering technique first suggested 

by Dobbins et al. (ref. 13) and later improved by Lorenzetto(ref. 14). 

This technique yields the Sauter mean diameter of the spray from analysis 

of the forward scattering of a monochromatic beam of light resulting 

from its passage through the spray. By traversing a receiving photo 

multiplier tube from the scattered beam's center outward, plots can be 

generated of the light intensity as a function of radial distance from 

the beam centerline. Analysis of these plots yields the SMD as a 

function of the wavelength of the light source, in this case a 2 mW 

He-Ne laser. The atomizer was tested by traversing a fuel oil spray at 

five different elevations across its height, six different positions 

downstream of the atomizer, and several main air flow rates. The SMD 

was found to increase slightly (~5%) as the downstream distance increased, 

16 
probably due to evaporation and coalescence. It also increased about 5 

percent, at any given fuel and atomizer air flow rate, as the main air 

velocity was increased four-fold. No consistent variation of SMD was 

observed from the upper to the lower portion of the spray; SMD increased 

slightly, around 2 percent, toward the bottom of the test section, perhaps 

due to settling of the larger drops. The uniformity of the spray was 

checked across its width by visual observation of the beam. No changes in 

intensity of the light.as it interacted with the beam were observed, 

indicating general uniformity. Particular attention was paid to the 

regions above and below the test section centerline where the flame ang"'e 

was measured for calculation of the burning velocity. 

A widely-used equation for correlating SMD data from plain-jet 

airblast atomizers is the following due to Rizk and Lefebvre (ref. 15) 

(3) 

where do is the inside diameter of the fuel nozzle, a is the surface 

tension, PA is the atomizing air density at the nozzle exit, Ur is the 

relative velocity between the air and the liquid, ALR is the mass ratio 

of atomiz"ing air to liquid flow per nozzle, )1L is the kinematic viscosity 

of the liquid, PL is the density of the liquid, and Cl and C2 are 

empirically-determined constants. The above equation.was installed in a 

computer program developed to yield the Sauter mean diameter as a function 

of fuel and atomizer air flow· rates. The correlation was immediately 

successfu"' in predicting the overall trend ~f the data, but some 

adjustment of the two'constants was required to match actual SMD values. 

Best fit of the experimental data was accomplished using Cl = 0.940 and 

C2 = 0.290. Measured and calculated values of SMD are depicted in 

Figure 2, showing the satisfactory fit obtained. 

17 
18 

E 

:t. 

.. o 

:E 

CJ) 

180~------------------------------------' 

FUEL OIL 

o 

140 

120 

~MEASURED 

60 0 CORRELATION OF 

LEFEBVRE a RIZK 

40 MAIN AIRSTREAM VELOCITY= 10.4 m/s 

o 1.0 20 3.0 4.0 5.0 6.0 

AIR ROTAMETER READING 

Figure 2. Correlation of Drop-Size Data. 
Measurement of Flame Speed 

The system for determining the flame angle for the measurement of 

burning velocity is based upon a two-mirror schlieren system, whose 

optical components include a 100 watt high pressure mercury vapor arc, 

two 2-meter focal length parabolic mirrors silvered on the front surface, 

and a knife edge stop that is adjustable horizontally and vertically. 

The schlieren image is recorded using a 35 mm camera with the lens 

removed. The camera is equipped with a horizontal travel focal 'plane 

shutter capable of exposure times of 1/2000 s. It is mounted 

on a tripod 25 cm behind the knife edge, producing a rectangular image 

on the film 1.25 cm wide and 3.4 cm long. Lines are marked directly 

on the enlarged prints with a fine ball-point pen, and the angles are 

measured to the nearest half-degree with a protractor. The flame 

speed is calculated by multiplying the mainstream velocity by the sine 

of the corresponding angle. The average flame speed is then: 

(4) 

where the U and L subscripts represent upper and lower positions, 

respectively. 

19 
RESULTS 

The fuels employed in this study are listed in Table 1 along with 

the properties most relevant to atomization, evaporation and soot formation. 

Fuel oil was selected for the 'base1ine ' study, since large quantities 

were available from a central reservoir. As indicated in Table 1, fuel oil 

has atomization characteristics very similar to the other fuels, except 

for a slightly higher. viscosity. The effective evaporation coefficients 

iisted in the table are for 1200 K, normal atmospheric pressure, and 

50~m SMD. The value for fuel oil ;s somewhat lower than for the other 

fuels, despite its lower flash point and initial boiling point, indicating 

that fuel oil is less volatile in the mean. Chemically, the fuel oil 

contains an average amount of aromatic hydrocarbons, but is composed 

predominantly of saturated hydrocarbons, or alkanes. The fuel oil is 

very similar to blend 3 in this respect. The aromatic content indicates 

the presence of unsaturated hydrocarbons having as a common feature one 

or more six-carbon benzene ring structures .. Aromatic classification 

includes both monocyc1ic arenes, such as benzene, and polycyclic compounds 

such as naptha1ene. The fuel oil also, contains some sulphur, unlike the 

other fuels studied. 

Flame Structure 

The following discussion is based on fuel oil flames, but the other 

fuels exhibited very similar flames except for certain aspects of flame 

luminosity. 

The flame was quite bright throughout the core region at all equivalence 

ratios. This incandescent region extended from the igniter tip through 

the test section and some distance downstream of the nozzle into the exhaust 

duct. Within the exhaust tube, the bright core was surrounded by a brushlike sneath. For good atomization quality (SMD < 60~m), the structure was 

as described above, very similar to that observed in the investigations of 

Solado and Yule, (ref. 16), and Onuma and Agasawara (ref. 17). Reduction 

in fuel/air ratio reduced the size of the flame, but did not affect its 

structure. As the drop size increased from fine (30-40~m) toward 

20 
Table 1. Some key fuel properties. 

Atomization Evaporati on Chemical 

Properties Properties Properties 

CJ v p 50% BP A AROMATICS H/e 

FUEL kg/s2 m2

/s kg/m3 K mm2

/s wt % ratio 

xl06 

FUEL OIL 0.0276 -2.44 841.65 529 0.35 27.6 1.9 

JP 7 0.0261 1.57 793.77 484 0.44 2.0 2.04 

BLEND 2 0.0301 1.22 837.08 473 0.46 45.0 1.7 

BLEND 3 0.0303 1. 55 842.07 . 492 0.41 26.8 1. 73 

BLEND 4 0.0320 1. 58 885.88 500 0.40 48.2 1.5 

21 
coarse (80-l00~) the flame lengthened, and individual droplets could be 

seen burning in the brush section of the flame. In general, the distinction 

between the brush 1 i kesheath and the inner core became 1 ess well defi ned 

as the drop size increased. Very near the drop size where extinction 

occurred the bright core disappeared completely and the flame appeared 

to consist of individually burning drops. A sheet of flame burned 

intermittently in the interdroplet space, propagating within no definite 

boundaries. The amount of soot and smoke also increased dramatically 

as the drop size was increased. 

To the naked eye, the core of the flame just downstream of the 

igniter, in the region where angle measurements were made, appeared to be 

a solid and continuous sheet. Viewed through dark cutting goggles, the 

core region was seen to be constructed of many ellipse-shaped flameletr. 

which disappeared into less coherent structures downstream of the bright 

zone. These flamelets became larger and more distinct as the drop size 

increased, and it was surmised that each was associated with individual 

drops or groups of drops. 

Preliminary schlieren photographs were taken at both long and short 

(1/60 - 1/1000 s) exposures to gain further insight int~ the flame structure. 

The schlieren images taken at short exposures yielded no new information 

about the core region of the flame. At mean drop sizes below 60~m, the 

schlieren pictures showed a continuous cone expanding downstream, analogous 

to the bright core region seen directly. Occasionally, when the scale 

of the flame (at lower air flow rates) allowed some of the brush-like 

structure to form within the test section, cellular blisters of combusting 

gas could be seen in the schlieren image, surrounding the solid, nearly 

featureless core with a rugged, lacerated sheath reminiscent of schlieren 

pictures taken through gaseous turbulent flames. Cellular structure was 

never observed in the core region and was confined to the brush-like region 

outside the bright core. Although the lacy, cellular structure is an 

interesting detail, it was not considered in calculations of the flame speed. 

Only boundaries of the well defined core region of the flame were used in 

determining the cone angle. 

22 
Test Conditions 

Fuel oil was burned over wide ranges of test conditions. The range 

of each variable studied was divided into a large number of specific values 

in order to provi de a good background for work with the other 1 iqui d 

fuels in short supply. Data were taken at fuel/air ratios from 0.02 to 

0.09, in steps of 0.01, while main air flow rates were varied from 0.12 

to 0.37 kg/s in 10 steps at each fuel/air ratio. Data could not be 

taken at high air flow rates for the weaker fuel-oil mixtures due to 

flame instability. The blended fuels and JP 7 were examined over the 

same range of variables as fuel oil, but each range was divided into 

fewer specific values, since work with fuel oil has shown nothing could 

be gained from finer graduation. JP 7 and all of the blends were examined 

at mainstream velocities of 10.4, 18, 24 and 33 m/s for each of four 

fuel/air ratios. Mean drop sizes generally ranged from the minimum 

possible at the given fuel flow rate through at least six values, the 

last of which was near the extinction point. 

Flame Speed Data 

The main parameters investigated were fuel/air ratio, mainstream 

velocity, mean drop size, and fuel type. The results of these studies 

are discussed below in turn. 

Fuel/Air Ratio 

Figures 3 to 5 are typical of the results obtained on the influence 

of fuel/air ratio on flame speed. It should be noted that the fuel/air 

ratio quoted in these figures is the overall value, i.e. the ratio of 

total fuel flow rate (liquid plus va~or) to the total air flow rate 

(mainstream air plus atomizing air). Since the fuel is injected only 

12 cm upstream of the flame, little evaporation occurs prior to combustion. 

The stoichiometri.c fuel/ai.r rati'o is very similar for all fuels, 

ranging from 0.067 for JP 7 to 0.071 for blend 4. The curves of flame 

speed versus fuel/air ratio in Figures 3 to 5 exhibit the same general 

characteri sti. cs as the correspondi ng curves for gaseous mi.xtures. They 

show that flame speeds increase with fuel/air ratio and attain their 

maximum value at mixture strengths slightly richer than stoichiometric. 

23. 
24 

6-------------------------------------, FUEL OIL 

5 

4 

~ 

E 3 .. 

ten 

2 

MAINSTREAM VELOCITY = 10.4 m/s 

SMD,l±m 

o 40 

o 60 

A 

o 

FUEL lAIR RATIO 

o 

o 

Figu're 3. Influence of Fuel/Air Ratio and Mean Drop Size on 

Flame Speed for Fuel Oil. ' 
~ 

E 

FUEL BLEND 3 

6 - MAINSTREAM VELOCITY = 33 m/s 

SMD, ~m 

o 70 

5 6. 90 

4-

.. 3 .... en 

2 

1-

FUEL/AIR RATIO 

Figure 4. Influence of Fuel/Air Ratio and Mean Drop Size on 

Flame Speed for Fuel Blend 3. 

25 
26 

6----------------------------------------~ FUEL BLEND 4 

MAINSTREAM VELOCITY= 33 m/s 

SMD, ~m 

5 0 40 

en 

"-

E 

4 

.. 3 

ten 

2 

o 70 

A 90 

FUEL/AIR RATIO 

Figure 5. Influence of Fuel/Air Ratio and Mean Drop Size on 

Flame Speed for Fuel Blend 4. 
Further evidence on the effect of mixture strength on flame speed 

is contained in Figures 6 to 9, in which flame speed is plotted against 

the reciprocal of mean drop s'ize. Comparison of tnese figures illustrates 

that for the heavier fuels such as fuel oil and blend 4 the flame speed 

is more sensitive to fuel/air ratio. The family of curves at different 

fuel/air ratios for JP 7 are closely knit, while those for fuel oil and 

blend 4 are spread over a wider range of flame speed values. These figures 

also show that although mixture strength affects flame speeds at all 

drop sizes its influence becomes more pronounced with increase in mean 

drop size. 

Mainstream Velocity 

Some indication of the influence of flow velocity on flame speed may 

be gained by comparing Figs. 6 and 7. The fuel and test conditions for 

these two figures are identical except for velocity, which is 24 m/s in 

one case and 33 m/s in the other. The effect of velocity on flame speed 

is also shown, more directly, in Fig. 10. It is clear from these figures 

that increase in mainstream velocity enhances flame speed. The same effect 

has been observed previously for gaseous mixtures (refs. 11 and 12) and is 

attributed to the increase in turbulence intensity that accompanies an 

increase 'in velocity. 

Mean Drop Size 

The analytical study of Bal1al and Lefebvre (ref. 7) showed that the 

rate of flame propagation in mixtures of fuel drops, fuel vapor, and air 

can be described by a model based on considerations of evaporation rates 

and chemical reaction rates which yields the relationship for flame speed 

denoted above as equation (2). 

Under conditions where evaporation rates are controlling the flame 

speeds, and the mixture is comprised solely of fuel drops and air, 

equation (2) simplifies to 

(5) 

27 
N 

co 

6ri----------------------------------------------------------------~ 

~ 

E 

5 

4 

.. 3 ~ 

CJ) 

2 

00 

MAINSTREAM VELOCITY= 24 rn/s 

FUEL=JP7 

I, 

/1/ 

I;~/ // 

/1/ // 

/1// 

/i~ , 

.005 .010 

o 

.015 

(SMD, porn)-I 

FUEL/AIR RATIO 

o 0.08 

A 0.06 

o 0.04 

V 0.02 

.020 .025 

Figure 6. Influence of Fuel Air Ratio on Flame Speed for 

Mainstream Velocity of 24 m/s. 
N 

'-0 

6~1-------------------------------------------------------------' 

~ 

E 

J 

4 

~ 3 .... (J) 

2 

0

0 

MAINSTREAM VELOCITY = 33 m/s 

FUEL =JP 7 

.005 .010 

v 

.015 

{SMD, Jl-m)-I 

v 

.020 

FUEL/AIR RATIO 

o 0.08 

A 0.06 

o 0.04 

V 0.02 

.025 

Fi gure 7. Influence of Fuel Air Ratio on Flame Speed for 

Mainstream Velocity of 33 m/s. 
w 

o 

~ 

E 

.. 

Ien 

5J- MAINSTREAM VELOCITY=24 m/s 

FUEL OIL 

4 

3 

2 

00 .005 .010 .015 

(SMD, JLm)-1 

.020 

FUEL/AIR RATIO 

o 0.08 

l:l. 0.06 

o 0.04 

'V 002 

.025 

Figure 8. Influence of Fuel/Air Ratio on Flame Speed for Fuel Oil. 
w 

5~1--------------------------------------------------~----~ MAINSTREAM VELOCITY= 24 m/s 

FUEL= BLEND 4 

4 

(/) 3 

'" E 

roo 

2 

00 

FUEL /AIf~ RATIO 

o 0.08 

o 0.04 

.005 .010 .01 .020 .025 

(SMD, fLm ) - I 

Figure 9. Influence of Fuel/Air Ratio on Flame Speed for Blend 4. 
en 

...... 

E 

.. 

too 

32 

7 

FUEL/AIR RATIO =0.020 

FUEL= JP 7 

6 

MAINSTREAM 

AIR VELOCITY 

0 33 m/s 

l:1 24 II 

0 cl8 II 

'V 10.4 II 

3 

40 50 60 70 80 

SMD, fLm 

Figure 10. Influence of Mainstream Velocity and Mean Drop Size 

on Fl arne Speed. 
In the above expression the mass transfer number, B, provides a 

measure of the volatility of the fuel. Typical values of B for combustion 

at atmospheric pressure are 3.8 for kerosine, 2.8 for diesel oil, and 

1.5 for heavy fuel oil. Some workers prefer to use the evaporation 

constant, A, instead of B, in the calculation of evaporation rates and 

drop lifetimes. This presents no problems in practice since A and B 

can be readily interchanged using the equation 

(6) 

Experimental values of evaporation constant for use in calculations 

of evaporation rates and drop lifetimes are fairly sparse and are usually 

confined to conditions of steady-state evaporation in quiescent mixtures 

at normal atmospheric pressure. However, Chin and Lefebvre (ref. 18) 

have provided "effective" values of evaporation constant that cover wide 

ranges of ambient air pressure, temperature and velocity, and which also 

take into account the influence of the heat-up period in lowering "the 

overall evaporation rate. The average boiling point (50 percent recovered) 

is the physical property selected to characterize the volatility of the 

fuel. Their results for normal atmospheric pressure are shown in 

Figure 11 as plots of Aeff versus Tbn for various values of UDo' and 

three levels of ambient temperature, namely 500, 1200 and 2000K. They 

show that, in general, Aeff increases with increase in ambient temperature, 

pressure, velocity and drop size, and diminishes with increase in normal 

boiling temperature. 

The concept of an effective value of evaporation constant considerably 

simplifies calculations of the evaporation characteristics of fuel drops. 

For example, for any given conditions of pressure, temperature, and 

relative velocity, the lifetime of a fuel drop of any given size is 

obtained as 

(7) 

33 
tJ) ",' 

1.6.-..,---,--,----.---r----r----..----I 

1.0 

0.8 

P= 100 kPa 

UDot 

(m/s)(jL) 

10000 

5000 

2000 

1000 

-:::::::::::::::::::::;==== 500 ----_ 200 

100 

o 

E E 0.6 10000 

5000 

..... 

~ 0.4 

..< 

.~ ____ ---.:===-----.~ Tggg ~ ___ -=::--_-_~ 500 

--=- T88 

0.2 

0.20 

0.12 

0.08 

o 

10000 

5000 

2000 

1000 

500 

tWo 

o 0.04'"-:-~-:--L---'-----'---50-L-O--52.L-O--5...L.4-0-l560 

Tbn, K 

Figure 11. Variation of Effective Evaporation Constant with Normal 

Boiling Point at Atmospheric Pressure. 
Using equation (6) to substitute for Aeff into equation (5) gives 

(8) 

or, (9) 

An important conclusion to be drawn from equations (5) and (9) is 

that, for conditions where evaporation rates are controlling the flame 

speed, the latter is inversely proportional to mean drop size. This 

the0retica1 finding is fully confirmed in Figures 6 to 9, and 12 to 17, 

in which flame speed is plotted against the reciprocal of mean drop 

size. for all fuels at several values of fuel/air ratio and mainstream 

velocity. These figures show over wide ranges of mean drop size, a 

straight-line relationship between ST and SMD- l , indicating that over 

this range of drop sizes evaporation rates are controlling the flame 

speeds. In theory the straight portion of the lines drawn in Figures 

6 to 9, and 12 to 17, should pass through the origin, i.e. flame speed 

should become zero for infinite fuel drop size. In practice the lines 

tend to intercept the abscissa (see Figures 6 and 14) at a finite value 

of SMD, usually around 300 to 400 microns, depending on the type of 

fuel and the fuel/air ratio. Thus although the theory might suggest that 

flame speed should reduce gradually to zero, corresponding to infinitely 

large drops, the results indicate that in practice there is a maximum 

mean drop size above which flame propagation is impossible. For the 

fuels and test conditions examined in this study, this practical limit 

on mean drop size is around 400 microns. 

The figures show that in general flame speed increases with decrease 

in mean drop size until a critical value is reached. For drop sizes 

smaller than the critical value, which is around 60 to 70 microns, the 

curves flatten out, indicating that for finely atomized sprays flame speeds 

are much less dependent on evaporation rates, and are governed primarily 

35 
w 

0) 

5. , 

4 

en 3 

""-

E 

.. 

~ 

en 

2 

00 

FUEL/AIR RATI0=0.040 

MAINSTREAM VELOCITY= 18 m/s 

.005 .010 .015 

(SM D, p.m)-I 

o FUEL OIL 

o JP7 

o BLEND 2 

'V BLEND 3 

Do BLEND 4 

,020 .025 

Figure 12. Influence of SMD and Fuel Type on Flame Speed. 
W 

'-l 

FUEL/AIR RATI0=0.040 

51- MAINSTREAM VELOCITY= 24 m/s 

4 

en 

" E 3 

.. 

~ 

(f) 

2 

00 .005 

Figure 13. 

.oro .015 .020 

(SMD, f'm}-I 

DFUEL OIL 

OJP 7 

o BLEND 2 

V BLEND 3 

l:l. BLEND 4 

.025 

Influence of SMD and Fuel Type on Flame Speed. 
w 

co 

6r.--------------------------------------------------------------, 

f/) 

" E 

5 

4 

.. 3 

ten 

2 . 

0 1 

0 

FUEL/AIR RATIO = 0.040 

MAINSTREAM VELOCITY= 33 m/s 

.~/ 

/ V 

1/ 

II 

If ,f 

II /' 

// 1/ 

I I 

.005 .010 

I 

.015 

(SMD, p,m)-I 

o JP7 

o BLEND 2 

'\7 BLEND 3 

a BLEND 4 

.L. 

.020 .025 

Figure 14. Influence of SMD and Fuel Type on Flame Speed. 
W 

\.0 

5 

4 

~ 3 

E 

.. 

ten 

2 

00 

FUEL/AIR RATI0=0.060 

MAINSTREAM VELOCITY = 24m/s 

.005 .010 .015 

(SMDt p.m)-I 

.020 

~ FUEL OIL 

OJP 7 

.025 

Figure 15. Influence of SMD and Fuel Type on Flame Speed. 
~ 

o 

5. FUEL/AIR RATI0=Q.080 i 

4 

(/) 3 ....... 

E 

. 

ten 

2 

00 

MAINSTREAM VELOCITY= 10.4 m/s 

.005 .010 .015 

(SMD, ,.,.m)-l 

" 

o FUEL OIL 

OJP7 

o BLEND 2 

"BLEND 3 

~BLEND 4 

.020 .025 

Figure 16. Influence of SMD and Fuel Type on Flame Speed. 
~ 

(/) 

" E 

.. 

ten 

5~f--------------------------------------------------------------' 

4 

3 

2 

00 

FUEL/AIR RATIO= 0.080 

MAINSTREAM VELOCITY = 24 m/s 

.005 .010 .015 

(SMD, f'm)-I 

o FUEL OIL 

o JP 7 

o BLEND 2 

\l BLEND 3 

6. BLEND 4 

.020 .025 

Figure 17. Influence of SMD and Fuel Type on Flame Speed. 
by mixing and/or chemical reaction rates. The work of Polymeropoulos and 

Das (ref. 5) and Hayashi et a 1. (ref. 6) among others, has shown that 

further improvement in atomization quality beyond the critical value would 

gradually reduce the flame speed until it reached the value corresponding 

to a premixed gaseous mixture. 

Fuel Type 

The JP 7 and blends 2, 3 and 4 used in this study were prepared by 

the Southwest Research Institute. JP 7 is the so-called Ibase l fuel, a 

low aromatic-content kerosine. The three fuel blends were prepared by 

mixing different amounts of two aromatic blending stocks, Exxon Aromatic 

150, and Marathon Marasol 45, with JP 7. The Exxon stock contains primarily monocycl i c aromati cs, 1 i ke benzene, while the Marasol contains 

primarily two-ring aromatics such as napthalene. Blend 2 is a mixture 

of JP 7 and the monocyclic stock, having a relatively high aromatic 

content of 45 percent. Blend 3 is prepared from JP 7 and the two-ring 

stock, yielding a mixture with a lower aromatic content of 26.8 percent, 

comparable to the heating oil. The final blend, number 4, is also made 

from the two-ring stock, and has the highest aromatic content of 48.2 

percent. 

At the outset, JP 7 was anticipated to have the highest flame speed, 

with its high hydrogen to carbon ratio, low aromatic content, and 

relatively high volatility. Fuel oil and blend 4 were expected to have 

lower flame speeds and greater sensitivity to mean drop size, due to their 

higher content of complex hydrocarbons and low volatility. Blend 3 was 

expected to fall somewhere in between. In general, fuel chemistry was 

anticipated to have a negligible effect on flame speed in comparison to 

evaporation characteristics. As shown in Table 1, the fuels showed no 

really striking differences in volatility, and the flame speed was expected 

to remain comparable from fuel to fuel. 

The results showed that although JP 7, the most volatile of the fuels 

studied, exhibited higher flame speeds at weaker mixture strengths, the 

highest flame speeds recorded were for fuel oil, which has a lower 

42 
volatility and nearly ten times the aromatic content. The next highest 

f1 arne speeds ·were recorded for blend 3, JP 7, blend 2, and blend 4, in 

that order. The flame speeds of JP 7 and blend 2 were comparable over 

wide ranges of SMD and fuel/air ratio values, despite the fact that the 

aromatic content is much higher for blend 2. As Table 1 shows, the 

evaporation properties cannot be used to predict the experimenta11ydetermined ranking of flame speeds. However, the listing of chemical 

properties is no more· successful in this respect. The net heat of 

combustion for each fuel is highest for blend 3 and JP 7, lower for blend 

2, blend 4 and the fuel oil, and is generally comparable for all the 

fuels. The gross heat of combustion is similarly unrelated to the ranking 

of the fuels in terms of flame speed. 

However, the two fuels that exhibited the highest flame speeds have 

a similar composition. The fuel oil is 27.6 percent aromatics, with at 

least half of these being a two-ring aromatics. Simi.1ar1y, blend 3 is 

composed to 26.8 percent aromatics, and these are also predominantly 

two-ring. The behavior cannot be explained in terms of aromatic content 

alone, since both blends 2 and 4 contain more aromatics than either fuel 

oil or blend 3. However, the fact that the fuels having the highest flame 

speed are related chemically seems more than coincidence. 

Pay't of the explanation for the unexpectedly high burning velocities 

of the less volatile fuels may be in their high luminosity. The radiant 

energy of the fuel oil and blended fuels was not measured directly, but 

was apparent from flame observation. Unfortunately, the effects of radiation were not included in the evaporation analysis, because the experimental 

work of Hottel, et a1. (ref. 19) has shown that radiative heating of fuel 

sprays is usually insignificant compared to convective heat transfer. 

Ber1ad and Hibbard (ref. 20) also found that fuels such as JP 4 did not 

absorb significant amounts of radiation in the spectral bands typically 

produced by luminous flames. Nevertheless, the fuels having a higher 

aromatic content burned more brightly, and the test section and exhaust 

duct were noticeably subject to higher radiative fluxes when these fuels 

were burned. Perhaps the convective and conductive contributions to 

evaporation rate were similar enough among the fuels studied that the 

43 
addition of higher radiative fluxes in the case of the heavier fuels was 

sufficient to overcome their slightly lower volatility. Unfortunately, 

even the most elementary radiation study required spectra and intensity 

measurements and analysis that were beyond the scope of the present work. 

Regardless of the role played by flame radiation in accelerating 

flame speeds, the fact remains that fuel chemistry, in some guise, does 

appear to influence flame speeds, whereas fuel volatility apparently 

does not. 

44 

, 
DISCUSSION OF RESULTS 

The comparable values of flame speed for both "light" and "heavy" 

fuels and the high flame speeds recorded in general, are results unique 

to this study. Experimentally, the apparatus employed differs from previous 

studies only in the method and positioning of the fuel jnjection. The 

multi-point injector provides a more uniform combustion charge than in 

previous investigations. where single point injectors were used. 

However, this factor alone would not be expected to raise the flame 

speed. The most probable explanation for the exceptionally high flame 

speeds lies the high level of turbulence intensity in the mixture 

approaching the flame front. This strong turbulence stems partly from 

the high flow velocities employed and also from the airblast atomizers, 

each of which g.enerates turbulence and shear due to the relative velocity 

between the atomizer air and the mainstream flow. 

Micro-explosion, the rapid fragmentation of multicomponent fuel drops 

resulting from superheating of low boiling point species trapped in the 

droplet 'interior, is another plausible explanation for the observed flame 

behavior. Law (ref. 21) observed this phenomenon initially in the 

combustion of single droplets of water/oil emulsion, but more recently 

(ref. 22) for diesel fuel as well. Although Law's experiments are 

concerned with single, isolated. relatively large (200 pm) drops falling 

freely in a furnace environment after ignition by a gaseous flame, similar 

conditions exist in the turbulent spray flame of the present work. 

The mechanism for this thermodynamic process requires a multicomponent liquid whose constituents have largely different boiling points. 

and a high energy flux to each droplet for rapid heating. Such conditions 

exist in our experiments in which multi-component fuels are injected 

directly into the flame. According to Law (ref. 21) the micro-explosion 

takes place as volatile components in the droplet interior (those near the 

surface are immediately vaporized) are heated beyond their critical 

temperature before adequate time has elapsed for their diffusion to the 

drop surface.· Thus, the less volatile constituents act as a pressure 

vessel, allowing superheating and nucleation to occur in the droplet 

45 
interior. The higher the boiling point of those species, the more 

superheating occurs. The eventual pressure explosion causes nearinstantaneous mixing and gasification of the remaining droplet mass. 

Evaporation times for micro-exploding drops are consequently much shorter. 

The intensity of the micro-explosion can be expected to increase as 

the critical temperature differential among the liquid components increases. 

JP 7, for example, beinQ composed primarily of unsaturated hydrocarbons 

having similar boiling points (the boiling point .range is 61 K), is less 

likely to precipitate micro-explosions than fuel oil, which is composed 

of both saturated and unsaturated hydrocarbons (boiling point range, 153 K). 

Also noteworthy is the difference in critical temperature between the 

monocyclic and polycyclic hydrocarbons. Benzene has a critical temperature 

of 563 K, while the value fornapthalene is 748 K (ref. 22). This suggests 

that while both the monocyclic and polycyclic blends would be susceptible 

to micro-explosion, the phenomenon would be most likely,'and more intense, 

for the polycyclic fuels. 

Thus, the phenomenon of micro-explosion seems to explain several aspects 

of the observed flame behavior. The flame speeds for JP 7 are higher than 

those for the blended fuels at lean equivalence ratios, but lower as the 

mixture strength increases. This could be due to the fact that less 

radiant energy is available for the rapid temperature rise required for 

micro-explosion at the leaner fuel/air ratios. The highest flame speeds 

are exhibited by fuel oil, which has the widest boiling point range of the 

fuels studied. Blend 3 has a boiling point range of 8DK, and similar 

composition, and also exhibits higher flame spe~ds than the remaining 

fiuels. Those fuels containing primarily polycyclic aromatics (fuel oil, 

blend 3) are characterized by higher flame speeds than blends prepared 

with monocyclic solvents (blend 2). However, blend 4, having the highest 

aromatic content, and containing a larger percentage of the polycyclics 

than any of the fuels stu,died, nonetheless shows a lower flame speed. 

This appears to contradict the microexplosion hypothesis, unless perhaps 

this blend is actually less likely to micro-explode due to its lower 

concentration of volatiles. There could be some optimum composition 

allowing maximum intensity of the event in a given environment. 

46 
The micro-explosion phenomenon isalso compatible with the flame 

structure observed in high-speed schlieren movies. Spherical and 

ellipsoid-shaped disturbances are observed to riddle the flame front 

with explosive swiftness. These disturbances often appear and disappear 

in the inter-frame period of some 0.4 ms. Law (ref. 1) has observed 

micro-explosion events of sub-millisecond duration, comparable to the 

lifetime of the disturbances observed in the spray flame. The direct 

movies also seem to suggest this behavior when the region just downstream 

of the injector is examined closely. 

Although not the primary interest of this study, the success of the 

micro-explosion hypothesis in explaining the observed flame behavior was 

sufficiently provocative to merit further, if brief, experimentation. 

Normal heptane, C7 H16 , an alkane, was burned just lean of stoichiometric, 

with SMOs ranging from 30-100vm, and the same air velocities (10.4, 18, 

24 and 33 m/s) as the mUlti-component fuels. A pure fuel, the heptane 

is not susceptible to micro-explosion. Furthermore, the fuel's high 

volatility [normal boiling point, 416 K (ref. 23)J compared to the other 

liquid fuels ,studies, suggests a higher flame speed on the basis of 

normal evaporation-rate considerations alone. 

However, the measured burning velocities for heptane are about the 

same as those for fuel oil under similar conditions. For example, at an 

'SMOof 40vm and identical conditions of air velocity and fuel/air ratio, 

the value of ST recorded for fuel oil is 4.1 m/s while that for heptane 

is 4.0 m/s. At a higher air flow rate, and an SMO of 50vm, the value 

of ST for fuel oil is 4.9 mis, while that for heptane is 4.5 m/s. 

Although heptane is less sensitive to drop size than fuel oil. flame 

speeds for the latter are higher below 80vm SMD. 

With respect to the other blends, the heptane burns more rapidly 

for similar values of SMD, although only slightly more so. The difference 

in flame speed between heptane and blend 4, the least volatile fuel burned, 

amounts to 1 m/s at low SMO values. 

47 
Heptane flames, when viewed both visually and via schlieren 

photography, show close similarity to gaseous propane flames. Visually, 

the flame appears as a transparent dark blue cone of plasma, in stark 

contract to the bright opqaue core of blended fuel flames. Carbon 

incandescence is observed, but much further downstream in the exhaust 

duct. Although short exposure schlieren pictures were not taken (the 

objective being to measure flame speed), the visible details are fewer 

for heptane flames than in similar pictures of the blended fuel flames. 

Contrast is generally more uniform within the flame zone, suggesting 

a more homogeneous field of temperature and concentration, in comparison 

to the foamy, rugged appearance of the blended fuel schlieren pictures. 

Although the higher flame speeds for the less volatile, but microexplosion-prone fuel oil, in comparison to heptane might lend credence 

to the micro-explosion hypothesis, such a limited comparison can only 

suggest that further investigation is requ"ired. The fact that the flame 

speed for heptane is comparable to those of all the fuels studied, 

despite its h"igher volatility, reinforces the inability of evaporationrate criteria alone to predict flame speed as a function of fuel type. 

The plausibility of the micro-explosion phenomena implied by the results 

presented suggests the necessity for its inclusion when considering spray 

evaporation and burning in cases where fuel is sprayed directly into a 

combustion zone. 

48 
CONCLUSIONS 

The burning velocity and structure of flames in two-phase mixtures 

has been studied for six fuels differing in chemical and physical properties 

over wide ranges of fuel/air ratio, main air velocity, and Sauter mean 

diameter. In agreement with theoretical predictions the measured burning 

velocity is found to be inversely proportional to mean drop size above 

some critical size. For SMD's below this critical size the flame speed 

is sensibly independent of drop size,. thus indicating a lower range of 

drop sizes over which flame speed is independent of evaporation rates. 

Flame speed increases with overall fuel/air ratio in the range 

studied for all fuels and all test conditions. The dependence of flame 

speed on fuel/air ratio increases with increase in air velocity, SMD, 

and fuel aromatic content, and decreases with increase in fuel volatility. 

Increase in air velocity enhances the burning velocity for any given 

SMD, fuel/air ratio and fuel type, and raises the sensitivity of the 

burning vE~locity to these variables. No simple relation appears to exist 

between fuel evaporation properties, chemical composition, and burning 

velocity, although the results suggest that flame speed is sensitive to 

the hydrocarbon structure of the fuel under conditions where evaporation, 

rather than chemical reaction, could be expected to dominate the overall 

burning rate. There is evidence to suggest that the higher radiative 

heat flux from fuels containing larger amounts of carbon may increase the 

flame speE~d of these fuels, despite their lower volatility. Further, the 

results indicate that the phenomenon of micro-explosion may accelerate the 

burning velocity of multi-component fuel sprays injected in close proximity 

to the combustion zone. 

49 
REFERENCES 

1 . Burgoyne, J. H., and Cohen, L., liThe Effect of Drop Si ze on Fl ame 

Propagation in Liquid Aerosols," Proc. Roy. Soc. London A. 225, 

p. 375, 1954. 

2. Cekalin, E. K., "Flame Propagation in a Two Phase Mixture," 8th 

Int. Symp. on Comb., pp. 1125-1129, 1962. 

3. Mizutani, Y., and Nishimoto, T., "Turbulent Flame Velocities in 

Pre-mixed Sprays: Part I - Experimental Study," Comb. Sci. 

and Tech., Vol. 6, pp. 1-10, 1972. 

4. Mizutani, Y., and Nakajima, A., "Combustion of Fuel Vapor -

Drop-Air Systems: Part I - Open Burner Flames," Comb. and Flame, 

Vol. 21, pp. 343-385, 1973. 

5. Polymeropoulos, C E., and Das, S., liThe Effect of Droplet Size 

on the Burning Velocity of Kerosine-Air Sprays," Comb. and Flame, 

Vol. 25, pp. 247-257, 1975. 

6. Hayashi, S., Jumagai, S., and Sakai, T., "Propagation Velocity and 

Structure of Flames in Droplet-Vapor-Air Mixtures," Comb. Sci. 

and Tech., Vol. 15, pp. 169-177,1976. 

7. Ballal, D. R., and Lefebvre, A. H., "Flame Propagation in Heterogeneous 

Mixture of Fuel Droplets, Fuel Vapor, and Air," 18th Int. Symp. on 

Comb., pp. 321-327, 1981. 

8. Lefebvre, A. H., Gas Turbine Combustion, McGraw Hill, 1983. 

9. Taylor, C. F., The Internal Combustion Engine in Theory and 

Practice: Combustion, Fuels, Materials, Design. The M.I.T. Press, 

Cambridge, Massachusetts, 1977. 

10. Faeth, G. M., "Current Status of Droplet and Liquid Combustion," 

Prog. Energy and Combust. Sci., Vol. 3, pp. 191-224,1977. 

11. Lefebvre, A. H., and Reid, R., liThe Influence of Turbulence on 

the Structure and Propagation of Enclosed Flames," Comb. and 

Flame, Vol. la, pp. 355-366, 1966. 

50 
12. Ballal, D. R. and Lefebvre, A. H., liThe Structure and Propagation 

of Turbulent Flames,1I Proc. Roy. Soc. London, A. 334, pp. 217-234. 

1975. 

13. Dobbins, R. A., Crocco, L. and Glassman,!., "Measurement of Mean 

Particle Sizes of Sprays from Diffractive1y Scattered Light,1I 

AIAA Journal, Vol. 1, No.8, pp. 1882-1886, 1963. 

14. Lorenzetto, G. E., and Lefebvre, A. H., IIMeasurements on Drop Size 

from a Plain Jet .Air Blast Atomizer,1I AIAA Journal, Vol. 15, 

No.7, pp. 1006-1010, 1977. 

15. Rizk, N. K., and Lefebvre, A. H., "Spray Characteristics of Plain-Jet 

Air Blast Atomizers,1I paper presented to ASME Conference on Gas 

Turbine Engines, Phoenix, Arizona, March 21,22,1983. 

16. Bolado, R., and Yule, A. J., liThe Relationship Between Atomization 

Characteristics and Spray Flame Structures," Proceedings, 

ICLASS-82, pp. 221-227, 1982. 

17. Onuma, Y., and Ogasawa ra, M., "Studi es on the Structure of a Spray 

Combustion F1ame,1I 15th Int. Symp. on Comb., pp. 453-465, 1978. 

18. Chin, J. S. and Lefebvre, A. H., IIEffective Values of Evaporation 

Constant for Hydrocarbon Fuel Drops,1I SAE Proceedings of the 20th 

Automotive Technology Development Contractors Coordination Meeting, 

Dearborn, MI, October, 1982. 

19. Hottel, H. C., Williams, G. C. and Simpson, H. C., "Combustion of 

Heavy Liquid Fuel Droplets," 5th Int. Symp. on Comb., pp. 101-129, 

1955. 

20. Ber1ad, A. and Hibbard, R., "Effect of Radiant Energy on Vaporization 

and Combustion of Liquid Fuels," NACA RM E-52109, Washington, D.C., 1952. 

21. Law, C. K., IIRecent Advances in Droplet and Liquid Combustion, II 

Prog. Energy and Comb. Sci., Vol. 8, pp. 171-201,1982. 

22. Randolph, A. L., and Law, C. K., IIA Preliminary Investigation of 

Soot Formation from Multi-Component Droplet Combustion," presented 

to Spring Technical Meeting, Central States Combustion Institute, 

Lexington, Kentucky, March 1983. 

23. Weast, R. C., and Astle, M. J., eds., CRC Handbook of Chemistry 

and Physics, CRC Press, Inc., Boca Raton, Florida, p. F-90, 1981. 

51 
ALR 

B 

C1, 

cp 

d 

do 

D 

fv 

H/C 

K 

k 

qL 

qov 

qv 

S 

SL 

ST 

ST 

SMD 

U 

u' 

" 

"eff 

0: 

11 

v 

P 

(J 

52 

C2 

APPENDIX 

Nomenclature 

air/liquid mass ratio 

mass transfer number 

constants in equation (3) 

specific heat at constant pressure, J/kg K 

interdroplet distance, m 

diameter of fuel discharge orifice, m 

drop diameter, or Sauter mean diameter, m 

fraction of fuel in vapor form 

hydrogen/carbon ratio 

constant in equation (1) 

thermal conductivity, J/ms K 

liquid fuel/atr mass ratio 

total fuel/air mass ratio 

fuel vapor/air mass ratio 

flame speed, m/s 

laminar flame speed, m/s 

turbulent flame speed, m/s 

mean value of turbulent flame speed, m/s 

Sauter mean diameter, m 

velocity, m/s 

r.m.s. value of fluctuating velocity, m/s 

evaporation constant, m

2

/s 

effective value of evaporation constant, m2/s 

thermal diffusivity, (k/cp p), m2/s 

dynamic viscosity, kg/ms 

k · t' , 't 2/ lnema lC V1SCOSl y, m s 

dens i ty, kg/m3 

surface tension, kg/s2 
Subscripts 

A air 

F fuel 

g gas 

L liquid 

o initial value 

r relative value 

v vapor 

53 
1. Report No. -12. Government Accession No. 

NASA CR-174644 

4. Title and Subtitle 

Flame Propagation in Heterogeneous Mixtures of Fuel 

Drops and Air 

7. Author(s) 

G. D. Myers and A. H. Lefebvre 

9. Performing Organization Name and Address 

Purdue University 

School of Mechanical Engin~ering 

West Lafayette, Indiana 47907 

12. Sponsoring Agency Name and Address 

U.S. Department of Energy 

Office of Vehicle and Engine R&D 

Washington, D.C. 20585 

15. Supplementary Notes 

3. Recipient's C/italog No. 

5. Report Date 

May 1984 

6. Performing Organization Code 

505-32-4A 

8. Performing Organization Report No. 

10. Work Unit No. 

11. Contract or Grant No. 

NAG 3-266 

13. Type of Report and Period Covered 

Contractor Report 

14. Sponsoring Agency Code. Report No. 

DOEjNASAj0266-1 

Final Report. Prepared under Interagency Agreement DE-AIOl-81CS50006. Project 

Manager, John S. Clark, Program Coordination Office, NASA Lewis Research Center, 

Cleveland, Ohio 44135. 

16. Abstract 

Photographic methods are used to measure flame speeds in flowing mixtures of 

fuel drops and air at atmospheric pressure. The fuels employed include a conventional fuel oil plus various blends JP 7 with stocks containing single-ring 

and multi-ring aromatics. The results for stoichiometric mixtures show that 

flame propagation cannot occur in mixtures containing mean drop sizes larger 

than 300 to 400 microns, depending on the fuel type. For smaller drop sizes, 

down to around 60 microns, flame speed is inversely proportional to drop size, 

indicating that evaporation rates are limiting to flame speed. Below around 60 

microns, the curves of flame speed versus mean drop size flatten out, thereby 

demonstrating that for finely atomized sprays flame speeds are much less dependent on evaporation rates, and are governed primarily by mixing and/or chemical 

reaction rates. The fuels exhibiting the highest flame speeds are those containing multi-ring aromatics. This is attributed to the higher radiative heat 

flux emanating from their soot-bearing flames which enhances the rate of evaporation of the fuel drops approaching the flame front. 

17. Key Word8 (Sugge8ted by Author(s» 

Alternative fuels 

Flame propagation 

Spray combustion 

18. Security CI888If. (of thl8 report) 

Uncl assi fi ed 

18. Distribution Statement 

20. Security CI88SIf. (of this page) 

Unclassified 

Unclassified - unlimited 

STAR Category 28 

DOE Category UC-96 

21. No. of pages 

56 

22. Price· 

A04 

*For sale by the National Technical Information Service, Springfield, Virginia 22161 

*USGPO: 1984-759-100-529 
End of Document