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Atomization and combustion of canola methyl ester biofuel spray

Jaime A. Erazo Jr., Ramkumar Parthasarathy *, Subramanyam Gollahalli

Combustion and Flame Dynamics Laboratory, School of Aerospace and Mechanical Engineering, 865 Asp Ave Room 212, University of Oklahoma, Norman, OK 73019, USA

a r t i c l e i n f o

Article history:

Received 14 July 2009

Received in revised form 10 June 2010Accepted 15 July 2010

Available online 25 July 2010

Keywords:

Biofuel

Combustion

Spray

Emission

Drops

a b s t r a c t

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared

to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically

accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel wasdelivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300lm. A two-com-

ponent phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and

radial velocity distributions. Radial and axial distributions of NO, CO, CO2 and O2 concentrations were also

obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R)

thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for

both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller

than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame tem-

perature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame

in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak

concentrations than No. 2 diesel fuel spray flames.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Because of the uncertain petroleum prices and the impetus to

develop renewable energy sources, biofuels are emerging as alter-

natives to petroleum fuels with practical applicability to diesel en-

gines, gas turbines, and industrial continuous combustors.

Biodiesel fuel has many important advantages over conventional

petroleum based fuels. Biodiesel is renewable, carbon-neutral from

an environment standpoint, and is sulfur-free. However, one draw-

back in the use of biodiesel fuels seems to be the increase in NO by

1–14% that has been reported from biodiesel fuelled compression–

ignition engines [1–3]. A variety of reasons have been cited for this

increase in NO emissions. Increasing iodine number has been cor-

related with increasing NO emissions from biodiesel fuelled en-

gines [1,4]. Another recent study attributed the increased NO

emissions to the increased presence of double bonds in biodieselfuels [4]. It has also been suggested that the bulk modulus differ-

ence between biodiesel and No. 2 diesel fuel causes an advance

in the fuel injection when using biodiesel [4–6], resulting in higher

temperatures and higher NO. However, the results of experiments

with continuous combustion systems such as gas turbine combus-

tors and oil furnaces show the opposite effect: NO x seems to be

lowered when certain biofuels are substituted for petroleum fuels,

either in the pure form or as blends [7–9].

The laser imaging studies by Dec [10] have revealed that the

mechanisms and processes in the combustion of a fuel spray in a

diesel engine significantly differ from the earlier model proposed

by Faeth [11] that was also applicable to continuous spray combus-

tors. Therefore, the NO emission increases observed in biodiesel

fuelled engines may not occur in continuous combustors such as

gas turbines. To understand this discrepancy, studies on flame

structure of sprays, in a more controlled environment than the

complex thermo-chemical environment existing in engines are

needed; this idea formed the basis of the present study.

In this paper, combustion characteristics of canola methyl ester

(CME) biodiesel were documented in a continuous combustor set-

up. In a companion project, biodiesel combustion in a laminar

flame was studied to isolate fuel chemistry effects [12], the results

of which provide baseline data for comparison. The specific goal of

this paper was to investigate the differences in the combustion andemission characteristics between No. 2 diesel and CME spray

flames. Parameters, such as air-preheat temperature, atomization

air, and global equivalence ratio, were controlled to provide direct

comparison. In-flame temperature, in-flame concentrations of NO,

CO, CO2 and O2, and spray droplet size and mean droplet axial/ra-

dial velocities were measured.

2. Experimental apparatus

The experiments were conducted in a large, steel combustion

chamber, shown in Fig. 1. A preheated, air co-flow system was used

0016-2361/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2010.07.022

* Corresponding author.

E-mail addresses: [email protected] (J.A. Erazo Jr.), [email protected]

(R. Parthasarathy), [email protected] (S. Gollahalli).

Fuel 89 (2010) 3735–3741

Contents lists available at ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l

http://dx.doi.org/10.1016/j.fuel.2010.07.022

mailto:[email protected]




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to deliver combustion air obtained from the lab supply line. Heat-

ing was accomplished using a 10 kW electrical resistance heater in

conjunction with a temperature controller. A settling chamber wasused to provide a uniform flow of air surrounding the spray. The

flame was contained in a stainless steel test unit with Vycor glass

windows for optical accessibility.

The fuel tank was pressurized with nitrogen, while the atomiza-

tion air was supplied from an air cylinder. An air-blast atomizer

with an injector diameter of 300lm was used to produce the

spray. A schematic diagram of the air/fuel tubing is presented in

Fig. 2.

The spray droplet size and velocity distributions were measured

using a 2-channel phase Doppler particle analyzer (PDPA) [13,14].

The source of light was an argon-ion laser operated at 225 mW,

which was split into the green (514 nm) and blue (488 nm) light.

Bragg cells were used to frequency shift one beam of each color

to facilitate the measurement of reversed flows. The receiving op-tics was set-up in the off-axis (30) forward-scatter mode. The

diameter-measurement system was calibrated with a mono-dis-

perse droplet generator. In general, approximately 10,000 data

points were collected at each measurement location in the spray

flame and averaged. At certain locations in the spray flame, such

as at the edges and far downstream of the injector, it was not pos-

sible to collect such a high number of data points within a reason-

able amount of time. On an average, approximately 120 s were

needed to collect the data points at a given location. The PDPA

transmitting and receiving optics were mounted on three-way tra-

verses to move the measuring volume to different locations in the

spray flame. The flame was almost symmetric about the vertical

axis, therefore, only half-width profiles are presented.

Species concentration profiles of CO, CO2, NO, and O2 were mea-sured using a portable gas analyzer. CO and CO2 concentrations

were measured using a non-disperse infrared detector (NDIR)

based on the attenuation of the infrared wavelength beam specific

to the species [15], while NO x and O2 concentrations were mea-sured using electro-chemical detectors [16]. The samples were col-

lected using a 1 mm diameter orifice, uncooled quartz probe. An

inline filter and a condenser were used to filter the moisture and

particles before passing the flue gases through the analyzers. The

probe was mounted on a two-way traverse. Access into the flame

was accomplished by using two custom cut Vycor glass pieces

which provided a narrow slot through which the probe was in-

serted. A thin ceramic gasket material was used to cover the excess

slot area. The flame temperature was measured using a silica-

coated type R thermocouple (Pt/Pt–Rh 13%), with a 0.35 mm bead

diameter. Data acquisition was accomplished using LabView soft-

ware and a personal computer. All thermocouple data were cor-

rected for radiation and convection errors. Radial profiles of

temperature and species concentration profiles were recorded ataxial locations at 25%, 50%, and 75% of the visible flame length from

the burner exit; the flame length was recorded by a digital camera

with a long-time exposure (1 s) in background lighting.

The properties of the fuels are provided in Table 1. Test condi-

tions including the fuel and atomization air flow rates are pre-

sented in Table 2. A global equivalence ratio of nominal value

0.68 was used to simulate lean burning combustors. All test condi-

tions were held constant with the exception of the air-preheat

temperature. CME has a higher initial boiling point than No. 2 die-

sel fuel (Table 1). Preheating the fuel would aid the atomization

and evaporation processes; however, overheating could result in

the fuel coking and clogging the atomizer tip. Therefore, the fuel

was not preheated. The heat release and pollutant emissions are

dependent on the vaporization and modes of combustion of thedrops in the spray. In order to minimize the influence of the differ-

1 2

3 4

5

67 8

9

10

1 – Air Filter and

Rotameter

2 – Co Flow Air Heater

3 – Fuel Rotameter4 – Air Rotameter

5 – Settling Chamber

6 – Flame Chamber

7 – Fuel Tank

8 – Nitrogen Tank

9 – Steel Chamber

10 – Exhaust Vent

Not to Scale

Fig. 1. Schematic drawing of combustion chamber.

1

2

3

4

5

67

8

1 – Fuel Inlet2 – Air Inlet

3 – Settling Chamber

with Marbles4 – Screen

5 – Air Co- Flow Inlet6 – Flame Chamber

7 – Injector8 – Set Screw

Not to Scale

20.3 cm

51.8 cm

Fig. 2. Schematic drawing of air and fuel tubing.

3736 J.A. Erazo Jr. et al./ Fuel 89 (2010) 3735–3741

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ence in boiling points on the combustion characteristics, the

co-flow air temperature was adjusted to be a constant fraction

(two-thirds) of the initial boiling point in both spray flames. The

uncertainties in the measurements were computed following stan-

dard procedures and are shown as uncertainty bars in the figures

displaying measurements. More details of the set-up and proce-

dure are presented by Erazo [17].

3. Results and discussion

The spray droplet Sauter mean diameter (SMD) profiles for No.

2 diesel fuel and CME fuel are presented in Fig. 3. The refractive

index of the drops changes due to the heat transfer in the flame;

the change in refractive index was estimated to be 6.8% [18]. The

maximum estimated error in the SMD measurement due to the

change in refractive index was 5% for the 30-lm drops and 8%

for the 60-lm drops [19]. The drop diameter calibration curve

slope was set at the initial refractive index value, as suggested by

Schneider and Hirleman [19]. In general, the droplet size increased

with increasing radial distance from the injector at all axial loca-

tions. Due to the swirl imparted by the injector, the large dropswere thrown to the spray edge, and took longer to evaporate and

burn, resulting in an increase in SMD in the outer edge of the spray,

similar to observations made in other investigations [20–22]. The

drop sizes in both spray flames are comparable in the near-injector

region. Farther downstream of the injector, the drop sizes were

comparable for both sprays. At an axial distance of 3 cm from the

injector, the SMD of CME was larger than that of the diesel spray

flame; this could be due to the smaller drops of CME evaporating

faster than the diesel drops [23], leaving the larger drops to remain

at this axial location.

The axial and radial components of velocity of the No. 2 diesel

and CME fuel droplets at varying axial locations in the spray flame

are presented in Figs. 4 and 5 respectively. Similar trends are ob-

served in both fuel sprays. The mean droplet axial velocity peaked

at the centerline and decreased with increasing radial distance

from the injector. The mean axial component of droplet velocity

decreased with increasing axial distance from the injector as thespray width increased. This behavior is similar to the gas velocity

profile in a jet. The mean axial velocity of the CME drops was high-

er than that of the diesel drops near the spray edge due to the

smaller SMD at these locations. The axial momentum injected

was constant for both fuels; the smaller SMD drops have less slip,

and therefore higher velocity near the edge.

The mean radial component of droplet velocity, on the other

hand, increased with increasing radial distance from the injector

axis. The swirl effect coupled with the preferential combustion of

smaller droplets depleted the number of smaller droplets at the

spray edges leaving only larger droplets with more momentum.

The radial velocity of the CME spray drops was smaller than the ra-

dial velocity of the diesel spray drops because of their smaller size

in the near-injector region.A comparison of the in-flame temperature profiles (Fig. 6) of the

two spray flames indicates that in the near-injector region (25%

flame height) the flame temperatures were similar, but in the

far-injector region the CME spray flame had lower temperatures.

In some places, the CME in-flame temperature was lower by as

much as 200 K than in the No. 2 diesel spray flame.

The in-flame concentration profiles of CO, CO2, O2 and NO at dif-

ferent axial locations (25%, 50%, and 75% of visible flame height)

are presented in Figs. 7 and 8. The NO concentration measure-

ments are plotted together with the in-flame temperatures to eval-

Table 1

Physical and chemical properties of No. 2 diesel fuel and canola methyl ester fuel.

Fuel Molecular formula Density (kg/m3) Boiling point (C) Viscosity ( cSt) H eating value ( MJ /k g) Iodine numb er

No. 2 diesel fuel C16H34 850 150–350 2.63 at 40 C 42.6 8.6

Canola methyl ester C19H36O2 881 340–405 4.37 at 40 C 37.4 115

Table 2

Fuel and air flow rates and temperature settings.

Atomization air flow rate (l/min) Co-flow air flow rate (l/min) Co-flow air temperature (C) Fuel flow rate (ml/min)

No. 2 diesel fuel 6.32 58 100 4.6

Canola methyl ester 6.32 58 232 4.6

Radial Position (cm)

S a u t e r M e a n D i a m e t e r ( m i c r o n

)

0 0.5 1 1.5 20

20

40

60

80

0.5 cm

1 cm2 cm

3 cm

CME FuelVf = 4.6 ml/min

Vaa = 6.32 l/minVcf = 58 l/minTcf = 232 C

Axial Distances Downstream of Nozzle


S a u t e r M e a n D i a m e t e r ( m i c r o n

)

0 0.5 1 1.5 20

20

40

60

80

0.5 cm

1 cm2 cm

3 cm

3.5 cm

No. 2 Diesel FuelVf = 4.6 ml/min

Vaa = 6.32 l/minVcf = 58 l/minTcf = 100 C


Fig. 3. Sauter mean diameter profiles for No. 2 diesel fuel and CME fuel spray flames.

J.A. Erazo Jr. et al. / Fuel 89 (2010) 3735–3741 3737

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uate the significance of NO production by the Zeldovich mecha-

nism, which is closely correlated with flame temperature. In the

diesel spray flame, the concentrations of CO, CO2 and NO peaked

at 0.75 cm in the radial direction, while the O2 concentration

was at a minimum at this location. The in-flame temperature pro-

file also correlated well with the in-flame concentration profiles of NO, with profiles having peaks at this same location. This behavior

indicates that NO formation in these flames is strongly driven by

the Zeldovich mechanism. The in-flame concentration trends im-

ply that the reaction front of the spray flame at this flame height

is at approximately 0.75 cm from the flame axis in the radial direc-

tion. This behavior was observed at all the flame heights.

The CME in-flame concentration profiles displayed trends dif-ferent from those observed in the diesel spray flame. A clear-cut


M e a n D r o p l e t A

x i a l V e l o c i t y ( m / s )

0 0.25 0.5 0.75 1 1.25 1.5 1 .75 20

3

6

9

12

15

18

0.5 cm

1 cm

2 cm3 cm

3.5 cm

Axial Distances Downstream of NozzleNo. 2 Diesel FuelVf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 100 C


M e a n D r o p l e t A

x i a l V e l o c i t y ( m / s )

0 0.25 0.5 0 .75 1 1.25 1 .5 1 .75 20

3

6

9

12

15

18

0.5 cm1 cm

2 cm

3 cm

Axial Distances Downstream of NozzleCME FuelVf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 232 C

Fig. 4. Mean droplet axial velocity profiles for No. 2 diesel fuel and CME fuel spray flames.


M e a n D r o p l e t R a d i a l V e l o c i t y ( m / s )

0 0.25 0 .5 0 .75 1 1.25 1 .5 1 .75 2-3

-2

-1

0

1

2

3

4

5

0.5 cm

1 cm

2 cm

3 cm3.5 cm

No. 2 Diesel FuelVf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 100 C



M e a n D r o p l e t R a d i a l V e l o c i t y ( m / s )

0 0.25 0 .5 0 .75 1 1.25 1 .5 1 .75 2-3

-2

-1

0

1

2

3

4

5

0.5 cm1 cm

2 cm

3 cm

Axial Distances Downstream of NozzleCME FuelVf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 232 C

Fig. 5. Mean radial droplet velocity profiles for No. 2 diesel fuel and CME fuel spray flames.


F l a m e T e m p e r a t u r e ( K )

0 0.5 1 1.5 21200

1400

1600

1800

2000

2200

25% Flame Length

50% Flame Length

75% Flame Length


No. 2 Diesel Fuel

Vf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 100 C


F l a m e T e m p e r a t u r e ( K )

0 0.5 1 1.5 21200

1400

1600

1800

2000

2200

25% Flame Length

50% Flame Length

75% Flame Length


CME Fuel

Vf = 4.6 ml/minVaa = 6.32 l/minVcf = 58 l/minTcf = 232 C

Fig. 6. In-flame temperature profiles for No.2 diesel fuel and CME fuel spray flames.


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reaction front was not observed in the CME spray flame. At 25%

flame height, the CO concentration peaked at approximately

1.25 cm in the radial direction. The O2 concentration reached a

minimum at the centerline and increased in the radial direction,whereas the CO2 concentration peaked at the centerline and de-

creased in the radial direction. The NO and in-flame temperature

profiles did not display any correlation. At all flame heights, the

CME spray flame produced less NO than the No. 2 diesel fuel spray

flame. This behavior was seen at other flame heights, but in a less

pronounced fashion.

From these measurements, distinctly different combustion pro-

cesses can be discerned in the two spray flames. The No. 2 diesel

fuel burned in a heterogeneous combustion environment, where

fuel droplets of varying size were evaporating and burning. Soot

formation and oxidation was evident from the luminous, yellow

flame produced. This combustion mode resulted in an increase

in-flame temperature and NO formation, as seen in the in-flame

measurements. In contrast, increased droplet evaporation, lessluminosity, and reduced in-flame temperatures in the CME spray

flame are all indicative of significant homogenous gas phase reac-

tions. The difference is due to the fuel-bound oxygen present in the

ester functional group of the CME fuel molecule, which aids the

oxidation process and suppresses soot precursors [24].The lower NO concentrations in the CME spray flame are oppo-

site to the observations of increased NO emissions in biodiesel

fuelled, intermittent combustion, compression–ignition engines

reported in various studies [1–3]; however, recent spray flame

studies under continuous combustion conditions similar to those

simulated in this paper, have reported decreases in NO x emissions

[7–9] in agreement with our work.

This disagreement between the compression–ignition engine

and other spray flame studies, at a first glance, may be attributed

to the large differences in variability of pressure, temperature,

and residence time between compression–ignition engines and

continuous combustors. According to the long-accepted Faeth’s

model [11] for spray combustion, both spray flames (intermittent

or continuous) exhibit similar behavior; therefore, it was initiallydifficult to rationalize these observations. Besides, we were sur-

Fig. 7. In-flame concentration profiles of CO, CO2, O2 and NO of No. 2 diesel fuel spray flame.


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prised to see that NO x emissions were higher for biofuels than for

the diesel fuel when their vapors were burned in gas-flame burners

[12], in agreement with the observations made in CI engines. A

much closer examination of the more recent, laser-diagnostics

based combustion model of Dec [10], reveals that most of the fuel

injected into a compression–ignition engine burns in the homoge-

neous gas-flame mode, and hence accounts for the congruence of

observations between the gas burners and CI engines, and variance

from those in continuous spray combustors.

4. Summary and conclusions

Spray flames of No. 2 diesel fuel and CME fuel were studied un-

der conditions simulating continuous combustors. Droplet size,

velocity, in-flame temperature, and in-flame species concentration

profiles were obtained for the two flames. The CME spray flame

displayed higher rates of droplet evaporation compared to the

No. 2 diesel spray. The No. 2 diesel fuel spray produced larger drop-

lets. The smaller drops in the CME spray flame had higher mean ax-

ial velocities and lower radial velocities in the far-injector region.

The CME spray flame produced less NO at all flame heights com-pared to the No. 2 diesel flame. Also, the CME spray flame was

up to 200 K cooler than the No. 2 diesel spray flame in the far-bur-

ner region. Overall, the No. 2 diesel spray flame operated mostly in

the heterogeneous combustion mode, in contrast to more homog-

enous, gas phase combustion demonstrated by the CME fuel spray

flame. The CME spray flame behavior was in agreement with other

biodiesel spray flame studies under similar continuous combustion

conditions. Also, the variation between CI engine and continues

spray combustion in the NO x production of biofuels compared to

petroleum fuel can be explained by the differences of the dominant

combustion mode.

Acknowledgments

This work was supported by a grant from the Oklahoma Bioen-

ergy Center. The first author would also like to thank the US

Department of Education for funding received through a GAANN

fellowship.

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