effect of droplet size on the burning characteristics of ...€¦ · effect of droplet size on the...

Paper # 070HE-0036 Topic: Heterogeneous combustion, sprays & droplets

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8th

U. S. National Combustion Meeting

Organized by the Western States Section of the Combustion Institute and hosted by the

University of Utah

May 19-22, 2013

Effect of Droplet Size on the Burning Characteristics of

Liquid Fuels with Suspensions of Energetic Nanoparticles

Saad Tanvir1 and Li Qiao

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School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN, 47907

The objective of this paper is to understand the effect of droplet size (decreasing from a

millimeter scale to a micron scale) on the combustion characteristics of nanofluid fuels

(liquid fuels with suspensions of energetic nanoparticles). An experiment was developed

to produce a droplet stream with droplet sizes ranging from 100-500 µm and spacing

between 200-600 µm. The droplet stream was ignited using a heated coil, producing a

stable droplet stream flame. Pure ethanol and ethanol with the addition of aluminum

nanoparticles at varying concentrations were tested. Macroscopic visualization of the

flames showed micro-explosions to appear in the flame as the nano aluminum burns and

escape the flame front. Ethanol burned with a blue flame indicating little or no soot

formation inside the flame. Residue analysis on the stream showed that the aggregation

intensity increases with increasing particle concentration. The aggregate structures are

dominated by chain like structures and spherical clusters. The burning rate increased with

increasing particle concentration. For low concentrations nanofluids (up to 2wt.%

aluminum), the burning rates remained stable, following the D2-Law of droplet burning.

For higher concentrations, the burning rate reduces as a function of time hence deviating

from the D2-Law. Increasing particle concentration increased the maximum temperature

of stream flame. The nanofluid droplet stream burned at a higher temperature

downstream as compared to upstream due to the burning of nanoparticles which burned

at a higher temperature then that of ethanol.

1. Introduction

Metallic materials, such as aluminum, with their high combustion energies have been used

as additives in propellants and explosives [1]. With recent advances in nanoscience and

nanotechnology, the production, control and characterization of nanoscale energetic materials is

possible. Due to their high surface areas, these nanoparticles offer shortened ignition delays,

reduced burning times and more complete combustion than micron sized particles [1, 2].

Recently, the combustion and propulsion communities have shown an increased interest in

developing high performance nanofluid-type fuels. The idea is to suspend nanomaterials (fuel

additives such as energetic nanoparticles or nanocatalysts) in traditional liquid fuels to enhance

performance. The unique features of the additives, could improve power output of propulsion

systems and possibly reduce ignition delay [3, 4].


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Little work has been done on studying the ignition and burning behavior of nanofluid-type

fuels. Tyagi et al. [5] explored the ignition properties of aluminum/diesel and aluminum

oxide/diesel nanofluid fuels using a simple hot plate experiment. Results showed enhancement in

ignition probability for nanofluid fuels as compared to pure diesel fuels alone. Beloni et al. [6]

studied the effect of adding metallic additives ( pure aluminum, alloyed Al0.7Li0.3, and

nanocomposites 2B+Ti) to decane on flame length, flame speed, emissions and temperature over

a lifted laminar flame burner. Similar studies by Jackson et al. [7] and Allen et al. [8] found that

the addition of a small amount of aluminum nanoparticles to n-dodecane and ethanol in a shock

tube significantly reduces ignition delay of both nanofluid type fuels. Young et al. [9, 10]

explored the potential of using nano-sized boron particles as fuel additives for high-speed air-

breathing propulsion. It was observed that boron particle can be successfully ignited in an

ethylene/oxygen pilot flame. However, sustained combustion of boron particles can be achieved

only over a critical temperature of around 1770 K. Van Devener et al. [3, 4] conducted one of the

first works in studying the catalytic combustion of JP-10 using CeO2 nanoparticles and later

boron nanoparticles coated with a CeO2 catalytic layer. Results showed a significant reduction in

the ignition temperature of JP-10. The boron core of the particles also increased the energy

density of the JP-10 fuel. Rotavera et al. [11] found that the addition of CeO2 nanoparticles in

toluene significantly reduced the soot deposition on the shock tube walls under high fuel

concentration conditions.

The combustion behavior of nanofluid-type fuels depend on multiple factors such as type,

size and concentration of the nanoparticles added, the nanofluid fuel’s colloidal stability as well

as the base liquid fuels used. Furthermore, the unique physical properties of nanofluids such as

enhanced thermal conductivity and optical properties [12-17] may also affect their burning

behavior. Motivated by the aforementioned work, Gan et al. [18, 19] explored the burning

characteristics of single fuel droplets (in the range of 0.5 – 2.5 mm in diameter) containing nano

and micro sized aluminum particles. The results show different burning behavior for micro

suspension and nano suspensions. For the same particle concentrations, the microexplosive

behavior was more aggressive in the microsuspension as compared to the nanosuspension. This

was attributed to the difference in the structure of the agglomerates formed during the

evaporation and combustion process. It was observed that for nanosuspension the aggregate

formed was dense and porous via Brownian motion. On the other hand, the microsuspension

resulted in an aggregate that was rigid and impermeable via fluid transport (droplet surface

regression and internal circulation). It was also noted that ethanol based nanofluids were more

stable than decane solutions.

Gan et al. [18, 19] later expanded this work by studying the combustion behavior of boron

and iron (due to their higher energy densities than aluminum) based nanofluids with both ethanol

and n-decane as the base fluids. One of the observations made was that for a high boron particle

concentration, nanofluid droplets burned inconsistently. Some boron particles burned

simultaneously with the liquid fuel, where as the rest formed an aggregate on the fiber that

burned after all the liquid fuel had completely burnt out for the n-decane case but not for the

ethanol case (because the high flame temperature melted the agglomerate). A similar trend was

observed for the iron nanoparticles. However, larger aggregates in the iron nanofluids exploded

shortly after ignition resulting in the formation of jets in multiple directions. For dilute ethanol

based suspensions, simultaneous combustion of ethanol and nanoparticles was seen; similar to

that observed earlier [18, 19].


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These results show that particle aggregation plays an important role in the combustion

behavior of nanofluid droplets. Note in these work, the diameter of the droplets is in the range of

0.5-2.5 mm. For smaller droplets such as in a spray, however, particle aggregation may be a less-

serious issue. The degree of aggregation and how it affects the overall combustion behvaior

depend on a comparison of the time scales: the characteristic droplet evaporation (or

combustion) time vs. the characteristic particle aggregation time. For a large droplet, the

characteristic time of particle aggregation may be on the same order as to that of droplet

evaporation and burning. As a result, large aggregation structures will form during the process of

evaporation and burning. Eventually, a large agglomerate can be formed that will burn during a

later stage. However, for much smaller droplets that are not significantly larger than the

nanoparticles or an agglomerate (similar to a 10-m droplet vs. 100-nm nanomaterial), the

aggregation timescale may be much longer than the characteristic droplet-burning timescale,

which means that until the droplet is completely evaporated and burned, the particles inside may

have insufficient time to form a solid aggregate. This would essentially change the distinctive

combustion stages and the overall burning characteristics. Motivated by these, we developed a

droplet stream combustion experiment which can produce a stream of droplets of micron sizes

(100-500 µm). The goal of this paper is to quantify the effect of droplet size on the combustion

behavior of nanofluid fuels.

2. Experimental Methods

2.1 Fuel preparation and characterization

The nanofluid fuels are prepared using physical and chemical (where required) dispersion

methodologies as discussed in the earlier study [18, 19]. The appropriate amounts of particles

were vigorously stirred with the base fuel. This was followed by sonication of the colloidal

mixture in an ultrasonic disrupter (Sharpertek, SYJ-450D) to avoid and delay particle

agglomeration. The sonication was performed in an ice bath to maintain a constant temperature

of the nanofluid. The nanofluid was sonicated of 8 minutes. The sonicator generates a series of 4

second long pulses with 4 second spacing.

Ethanol was used as a base fuel for the current study. Aluminum nanoparticles (averaged

size of 80 nm, from Nano-structured & Amorphous Materials, Inc.) were considered as additives

to ethanol. Figure 1 shows the SEM (Scanning Electron Microscopy) image of the nanoparticles.

The amount of particles added is precisely measured using an analytical scale (Torban AGZN

100) with an accuracy of 0.1 mg. Nanofluid samples prepared (0.1-5 wt.% aluminum in ethanol)

maintained excellent suspension quality for over 2 hours without the presence of a surfactant.

This is because ethanol is a polar and hydrophilic liquid. Hence a good suspension of

nanoparticles with hydrophilic oxide surface in ethanol is maintained.


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Figure 1: SEM image of 80nm Aluminum particles

2.2 Experimental Setup – Droplet Stream generation and ignition

Figure 2 shows the schematic of the droplet stream generation system. The setup consists

of a vibrating orifice droplet generator, a mechanical syringe pump system, a wave function

generator, a linear amplifier, and a high speed camera along with a backlight. The droplet

generator (Drop Generator LHG-01), containing a piezoceramic disk and 100 µm orifice, is

oriented so that the stream is in a downward direction. A KdScientific syringe pump system

supplies the nanofluid into the droplet generator at the specified constant volumetric flow-rate

via Festo PL-6 tubing. The wave function generator (Model 519 AM/FM Function Generator) is

connected to the linear amplifier (Piezo Systems, Inc. Model EPA-104) whose signal is sent to

both the piezoceramic disk inside the droplet generator as well as to the digital oscilloscope

(Tektronix, TDS 2024B) to monitor the actual output of the amplifier.


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Figure 2. Schematic of the droplet stream generation system

As the fluid is forced through the droplet generator, the square wave signal causes the

piezoceramic disk within the droplet generator to oscillate and apply longitudinal disturbances to

the fluid jet, thus perturbing the fluid. In accordance with the Rayleigh Instability theory, the

fluid, when disturbed at the proper frequency, will break-up from a uniform jet stream into a

uniform stream of equally sized and spaced spherical droplets. Quantitative analysis was

conducted on the stream to monitor droplet size and spacing as a function of applied frequency

and volumetric flow rate using a high speed camera (a monochrome Phantom V7.3 camera with

a speed of 6688 fps at a resolution of 800×600 and a color Photron Fastcam camera with a

speed of 1000 fps at a resolution of 512×512). A frequency of 20 kHz was used for the

preliminary combustion experiments. This gives the maximum distance (550 µm) between each

droplet of diameter ~190-200 µm.

The volumetric flow rate had little impact on the droplet sizes and spacing for low applied

frequencies. The orifice assembly and droplet generator were thoroughly cleaned after each test

to ensure that no nanoparticle deposits are left on the walls of the tube and in the orifice plate.

A heated nickel coil, attached to a high voltage power supply, was used to ignite the

droplet streams. The coil was placed at a distance of 0.8 inches downstream of the orifice. A

DSLR camera was used to capture the burning behavior of the stream. A protective screen was

placed around the flame to get better imaging and to isolate the flame from external air

disturbances.

3. Results and Discussion

3.1 Physical appearance of the flames and combustion residue analysis

Flame tests were conducted for pure ethanol and ethanol with 0.1, 0.5, 1.0, 2.0 and 3.0

wt.% aluminum nanoparticles. Figure 3 (a-e) shows the comparison of the droplet stream flames

of the fuels. The initial droplet size was 176 µm and an average spacing between the droplets

was set at 550 µm for all tests conducted. Figure 3 (a) shows a blue droplet stream flame for pure

ethanol. The blue stream flame is indication of little or no soot formation inside the flame. Once


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0.1 wt% aluminum nanoparticles were added to ethanol, we saw micro-explosions to appear in

the droplet flame because the aluminum particles burned. These explosions become more

prominent as particle concentration increases.

Figure 4 shows a closer look at this phenomenon. The burning and escaping of particles

from the flame is similar to what was observed in previous work [18, 19]. As discussed earlier,

due to the larger size of droplets used by Gan et al. [18, 19], the nanoparticles had the tendency

of forming a large agglomerate and the agglomerate burned at a later stage after the liquid fuel

had been completely combusted. This, however, was not observed in present flames.

Figure 3. (a) Pure Ethanol (b) 0.1wt.% Aluminum in Ethanol (c) 1.0 wt.% Aluminum in Ethanol (d, e) 2.0 wt.%

Aluminum in Ethanol


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Figure 4. Nano aluminum combustion inside a 1.0wt.% Al in ethanol nanofluid flame

Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX)

analysis was conducted on samples collected from within the burning droplet stream as well as

the escaping combusted nanoparticles that appear downstream of the flame (as shown in Figures

3 and 4). All samples were allowed to dry and their residue was scanned using SEM. Figure 5 (a),

(b) and (c) show SEM images for samples collected from within the stream for 1.0, 2.0 and 3.0

wt.% Al in ethanol respectively.


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Figure 5. SEM images of samples from within the stream flame; (a) 1 wt.% Al in Ethanol; (b) 2 wt.% Al in Ethanol; (c) 3

wt. % Al in Ethanol

Aggregation type and sizes of the three samples – Density increases: We noticed that both

aggregation density and aggregate sizes increase upon further addition of nanoparticles to

ethanol. For the 1 wt.% aluminum case (Figure 5a), we saw that aggregate sizes vary from a few

nanometers to about 30 micrometers. Majority of the aggregates took the form of an elongated

chain like structure. The reason of formation of such a structure is still unclear. There were

however some spherical clusters of variable sizes present. A similar trend was observed for the 2

wt.% aluminum case (Figure 5b). The aggregate sizes were in fact bigger with some chain like

agglomerates exceeding 50 µm. The presence of spherical clusters became more prominent as

compared to 1 wt% aluminum case. Furthermore, the density of the aggregates present within the

sample increase illustrating a larger presence of Al or Al2O3 within the sample. A much different

structure was observed however for the 3 wt.% aluminum case (Figure 5c). The structure

comprises of large chain like lumps of aggregates. The density and size of the aggregates

significantly increases from the 2wt.% case.

(a) (b)

(c)


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EDX analysis on the three samples shows a higher Al/O ratio for samples of higher

aluminum concentration. The results show that the Al/O ratio increases from 0.66 for 1 wt.%

aluminum case to as high as 1.58 for 3 wt.% aluminum. A higher Al/O can be attributed to an

increased amount of unburned aluminum aggregates due to incomplete nanofluid combustion as

the aluminum concentration in the sample increases.

Figures 6 (a), (b) and (c) show SEM images of deposits of escaped burning aluminum

particles (highlighted in Figure 6) for 1.0, 2.0 and 3.0 wt.% Al in ethanol streams respectively.

While observing these images we find a similar trend to the samples collected directly from the

stream flame. The aggregate density and sizes are much smaller for the 1 and 2wt.% aluminum

cases (less than 10µm) than that of 3wt.% aluminum case where the size of some aggregates

exceed 50µm. Another observation is that the shape of aggregates is much different in the 3wt.%

case in comparison to the others. For the 1 and 2 wt.% aluminum case the aggregates are mostly

in spherical clusters. Whereas for the 3 wt.% aluminum case the aggregates are dominated by

chain like structure with spherical clusters attached to their ends. Furthermore the amount of

aggregates per unit area increases upon addition of nanoparticles. This increase is more

significant once we increase the aluminum concentration in the sample from 2 wt.% to 3 wt.%.

EDX analysis on the three samples shows a higher Al/O ratio for samples of higher

aluminum concentration. The results show that the Al/O ratio increases from 0.64 for 1 wt.%

aluminum case to as high as 1.27 for 3 wt.% aluminum. A higher Al/O can be attributed to an

increased amount of unburned aluminum in the combusting aluminum nanoparticle aggregates

leaving the stream flame as the aluminum concentration in the sample increases. EDX analysis

was also carried out for singled out aggregate structures for all the cases above. The value of

Al/O ratio remained between 1.75 and 2.28. The higher values attributed to denser spherical

clusters and the lower to the less dense chain like aggregate structures.


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Figure 6. SEM images of samples from deposits of escaped burning particles; (a) 1 wt.% Al in Ethanol; (b) 2 wt.% Al in

Ethanol; (c) 3 wt. % Al in Ethanol

3.2 Effect of particle addition on burning rate

Backlight shadowgraphy using the high speed (Phantom V7.3, Vision Research) camera

was used to determine the burning rate - variation of droplet diameter as a function of time.

Figure 7 shows the variation of the droplet diameter squared as a function of time for nanofluids

with aluminum concentration varying from 0.1 to 4 wt.%. Starting with 0.5 inches downstream

of the end of the coil, the measurements were taken in increments of 0.5 inches downstream of

the flame source. The speed of the falling droplets within the stream was calculated using the

high speed camera and was estimated to be 6.37 m/s.

(a) (b)

(c)


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Figure 7. Variation of droplet diameter as a function of time for the ethanol-based nanofluid fuels with varying aluminum

concentrations.

From Figure 7 we observe that for pure ethanol and for nanofluids with low concentrations

of aluminum (0.1-2.0 wt.%) the squared of the droplet size decreases linearly with time. If

linearly fit, the data points for pure ethanol and nanofluids with low aluminum concentrations

(0.1-1.0 wt.%) give an R2 (Coefficient of Determination) value of equal to or over 0.99;

indicating an excellent fit. Hence, for these cases we can conclude that the droplet size regresses

following the classical D2-Law for droplet combustion. The 2 wt.% case gives an R

2 value of

0.986 indicating good correlation with the D2-Law. For higher concentrations however, this is

not the case. For 3 and 4 wt. % aluminum nanofluids, the droplet size regression deviates from

the D2-Law forming bent curves. The deviation becomes more significant as the particle

concentration increases. This behavior could be attributed to multiple factors. The change in

physical properties such as thermal conductivity, viscosity, surface tension, may affect heat of

vaporization. Moreover, the deviation from the D2-Law could be the effect of particle

aggregation behavior. Particle aggregation affects fluid dynamics inside a combusting droplet

and therefore potentially impacts the combustion process. However further investigation is

required to fully understand this phenomenon.

Figure 8 shows the variation initial burning rate (measured at a location of 0.5”

downstream of the heating coil) and the burning rate further downstream (measured at a location

of 3.5” downstream of the heating coil) as a function of aluminum concentration inside the


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nanofluid. We observed that the initial burning rate increases with increasing particle

concentration. Since aluminum nanoparticles have a higher thermal conductivity and radiation

energy absorption ability, this leads to an increase in thermal conductivity and radiation

absorption of the resulting Al/ethanol nanofluid ensuing faster evaporation and hence an elevated

burning rate. As we move downstream of the initial measuring point, we notice that the rate of

burning rate increases reduces. The trend is also evident from Figure 9.

Figure 8. Variation of initial burning rate with the addition of aluminum nanoparticles

For low concentrations nanofluids (up to 1wt.% aluminum), the burning rates remain stable,

following the D2-Law of droplet burning. For higher concentration cases, the burning rate varies

throughout the entire evaporation process in situations in which the D2-law does not apply. As

we explore the burning rate variation as a function of time as we notice that the burning rate for

higher particle concentration nanofluids follows a decreasing trend. This is illustrated in Figure

9. The decreasing trend is more evident in the 3 and 4 wt.% aluminum cases. This reduction in

burning rate downstream of the flame can be attributed to the aggregation of nanoparticles inside

the nanofluid during the combustion process. In section 3.1 (Figure 5c), we witnessed a

significant increase in aggregation density in the samples collected from within the flame. The

evident increase in aggregation at higher particle concentrations (3 and 4 wt.% aluminum)

inhibits diffusion of the base fluid to the surface of the droplet. Even though the increase in

particle concentration would increase the radiation absorption of the nanofluid; in this case

however, this effect is overcome by the hindrance to fluid diffusion inside the droplet caused by

particle aggregation. Hence the rate of droplet regression decreases, reducing the overall burning

rate of the droplet.


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Figure 9. Variation of Burning Rates as a function of time

3.3 Infrared imaging and temperature distribution of the droplet stream flames

The temperature distribution of the droplet stream flames is determined using a high-speed

megapixel infrared (IR) camera (SC6100 HD Series from FLIR Systems, Inc.). An integration

time of 0.8925ms and a frame rate of 30 Hz were used for recording infrared images from the

flame. The camera was placed 0.125m from the stream flame. From the collected infrared

images, we were especially interested the effect of the addition of nanoparticles on flame

temperature.

Figures 10 and 11 show the temperature distribution inside the droplet stream flame with

and without the presence of nanoparticles upstream (measured at 3 inches downstream of the

heating coil) as well as downstream (measured at 6 inches downstream of the heating coil) when

the particles started to burn. For these experiments, the droplet diameter is 176 µm and the

thickness of the droplet stream flames was estimated to be 9-11 mm. Note that the temperatures

are not the actual flame temperatures and will be calibrated in the future. We find that the

maximum temperature of the stream flame increases as the particle concentration increases. This

is consistent with the trend of the measured burning rate, that is, the burning rate increases as the

particle concentration increases. Furthermore, the temperature distribution within the stream

flame is consistent with previous work on droplet stream flames [20]. We observe a local

minimum temperature is at the axis of the droplet. The maximum temperature is reached on

either side of the stream axis where the flame sheet is believed to be present suggesting a nearly

cylindrical flame sheet around the droplet stream. Beyond this point the temperature reduces to

ambient temperature.


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Figure 10. Upstream thermal images of the droplet stream: (a) Pure Ethanol; (b) 1wt.% Al in Ethanol; (c) 2wt.% Al in

Ethanol; (d) 3 wt.% Al in Ethanol

As the nanofluid droplet stream moves downstream (Figure 11), we observed that the flame

temperature increases as compared to the upstream temperatures. For the nanofluid cases, the red

spots indicate burning of aluminum particles at a higher temperature. Furthermore, disruption

was observed inside the flames suggesting an unsteady behavior of the droplet stream flame.

There are two reasons for this: first, due to the continuously reducing droplet size, the stream

becomes weak and the slightest ambient disturbance causes disruption in the flame. Second, the

particles and/or particle aggregates present inside the droplets escape the surface of the droplets

and start to burn causing micro-explosions within the stream. This could largely distort the

stream flame. Macroscopically visualizing the stream for nanofluids, we did witness nano

aluminum burning and particles escaping the flame (Figure 4). Another observation we see from

Figure 11 is that the hottest part of the flame is the burning of aluminum and aluminum

aggregates, reaffirming that aluminum burns at a higher temperature than ethanol hence

increasing the temperature of the flame.

Thin filament pyrometer technique will be used to determine the absolute flame

temperature from the temperature of the inserted filament of known emissivity while accounting

for the radiative heat loss using the correlation used by Blunck et al. [21]. Beside the heat release

from burning particles, which tend to increase the droplet stream temperature, there is a

possibility of an increased droplet temperature resulting from radiation absorption by

nanoparticles. These effects will be quantified in the near future.

2.5 inches


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Figure 11. Downstream thermal images of the droplet stream: (a) Pure Ethanol; (b) 1wt.% Al in Ethanol; (c) 2wt.% Al in

Ethanol; (d) 3 wt.% Al in Ethanol

4. Conclusions

A droplet stream combustion experiment was developed to understand the effect of droplet

size on the overall burning behavior of nanofluid fuels – liquid fuels with stable suspensions of

energetic nanoparticles. Ethanol with or without suspension of aluminum nanoparticles at

varying concentrations were considered. Major conclusions from this work are:

(1) A Macroscopic visualization of the flames showed micro-explosions to appear in the

flame as the nano aluminum burns and escape the flame front. The blue stream flame of

ethanol was an indication of little or no soot formation inside the flame.

(2) Residue analysis on the stream showed that the aggregation intensity increases with

increasing particle concentration. The aggregate structures are dominated by two types of

aggregate forms: chain like structures and spherical clusters. More investigation is

required on why such structures are formed.

(3) The burning rate increased with increasing particle concentration. For low concentrations

nanofluids (up to 2wt.% aluminum), the burning rates remained stable, following the D2-

Law of droplet burning. For higher concentrations, the burning rate reduces as a function

of time hence deviating from the D2-Law.

(4) The maximum temperature of the stream increased with increasing particle concentration.

The maximum temperature is reached on either side of the stream axis where the flame

sheet is believed to be present suggesting a nearly cylindrical flame sheet around the

droplet stream. The nanofluid droplet stream burned at a higher temperature downstream

2.5 inches


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as compared to upstream due to the burning of nanoparticles which burned at a higher

temperature then that of ethanol. Thin filament pyrometer technique will be used to

determine the absolute flame temperature and will help us quantify these effects.

Acknowledgements

This work has been supported by the National Science Foundation (NSF).

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