ILASS Americas, 25th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013

Spray Visualization and Characterization of an Ultra Fine Homogeneous Atomizer Cou-

pled with a Swirl Adapter for Diesel Reformer Applications

Karthik Nithyanandan1, Ming Huo

1, Deyang Hou

2 and Chia-Fon Lee

*1,3

1Department of Mechanical Sciences and Engineering

University of Illinois at Urbana-Champaign

Urbana, IL 61801 USA

2QuantLogic Corporation

5111 Avondale Drive,

Sugar Land, TX 77479, USA

3Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy

Tsinghua University,

Beijing, China

Abstract

This paper focuses on the spray and atomization characteristics of an Ultra Fine Homogenous Atomizer (UFHA) coupled with a unique swirl adapter. The UFHA uses a Micro-Variable Circular Orifice (MVCO) to inject fuel.

The spray characteristics produced from this configuration, such as spray penetration length, spray velocity and the

droplet size distribution were evaluated under different injection pressure and air inlet pressure. Diesel fuel was

injected at pressures ranging from 300 bar to 700 bar at a back pressure of 1 bar, while compressed air at pressures

of 2 bar and 4 bar was supplied to the swirl adapter. High speed Mie scattering images were recorded to capture the

spray evolution, as seen from both the front view and the bottom view. Phase Doppler Anemometry (PDA) meas-

urements were conducted at different locations in the spray for the acquisition of droplet sizes and velocity distribu-

tions. Unlike traditional multi-hole injectors which normally inject six to eight fuel jets, the MVCO fuel injector

presented a unique fan spray pattern displaying more than twenty jets in an approximately axisymmetric fashion.

Due to the interaction between the spray jets, the breakup was enhanced compared to conventional injectors, causing

shorter spray tip penetration and finer droplet size. Coupling the swirl adapter with the injector caused the fuel

spray to become widened and more uniform, while also achieving a higher degree of atomization. PDA results dis-played a clear rotational motion indicating a strong tangential velocity component imparted to the fuel spray by the

adapter. The above characteristics favor a highly atomized, uniform mixture formation, thereby reducing coke for-

mation and auto-ignition hot-spots, which could potentially increase diesel reformer efficiency.

___________________________

*Corresponding author: cflee@illinois.edu

INTRODUCTION The US transport sector is more than 97%

dependent on petroleum-based fuels and consumes

approximately two-thirds of the nation’s oil demand. Additionally, increasing levels of carbon dioxide

(CO2) and other greenhouse gas emissions are be-

coming a global issue. In the US, the transport sector

account for one-third of CO emissions, and is pro-

jected to become the leading carbon emitter over the

next 20 years. Analysis of the US Department of

Commerce’s Vehicle Inventory and Use Survey indi-

cates that approximately 400,000 long-haul Class 7

and 8 trucks routinely travel over 500 miles from

their home base, consuming approximately 685 mil-

lion gallons of diesel fuel per year during overnight idling [3]. Assuming a higher vehicle population of

500,000 long-haul vehicles, emissions during over-

night idling have been estimated to be 10.9 million

tons of carbon dioxide (CO2) and 190,000 tons of

oxides of nitrogen annually [4]. Therefore, break-

throughs in transport technologies are clearly re-

quired to meeting these energy and environmental

challenge.

It is believed that fuel cells will be one of

the main energy conversion systems in the future.

Fuel cells have higher energy conversion efficiencies

and lower amounts of emission gases than internal combustion engines. Fuel cell Auxiliary Power Units

(APUs) supply on-board power to replace the idling

of the main engine. These fuel cells can most readily

be made to operate on reformed diesel fuel. Diesel

has been chosen as the reformate fuel because of its

high gravimetric and volumetric hydrogen density

and a well-established delivery infrastructure [2].

Catalytic onboard reforming technologies are consid-

ered feasible alternatives for supplying APUs with

hydrogen.

Diesel reforming poses several technical challenges. The reforming reactor design and the

reforming catalyst employed are two critical factors

that can determine the product gas distribution. On

one hand, the proper selection of catalyst can signifi-

cantly affect the hydrogen conversion rate, reformer

life and cost, etc., and on the other hand, the optimal

reactor design, i.e. the injection system design, is

essential to ensure complete fuel conversion, maxi-

mum hydrogen selectivity, and low amounts of car-

bon monoxide. Diesel fuel is prone to pyrolysis upon

vaporization; meanwhile, the atomization of the die-

sel fuel from direct fuel injection can also be crucial as poor atomization could cause slow reforming ki-

netics as well as catalyst deactivation. According to

Karatzas [10], high injection pressure may result in

large build-ups of unconverted liquid fuel on the

catalyst surface which can cause mechanical damages

to the catalyst, disturbance in the flow pattern and

reactor plugging. Reformer catalysts suffer degrada-

tion due to the build-up of carbon deposits and inade-

quate feed mixing and vaporization, which cause

non-uniform temperature distributions within the

reformer reactor. Different reforming technologies

can be employed in which diesel is blended with air, pure oxygen or steam as main reactant. The gas mix-

ture has to be well-blended to avoid local variations,

e.g. air/fuel ratios, in order to prevent hot-spots that

can cause undesired side reactions such as carbon

formation and damage the catalyst. The fuel delivery

is the main focus of this study as in a diesel reformer,

the mixing of fuel with air and steam should be as

uniform as possible prior to reaching the catalyst bed

in order to minimize the hot spot and coke formation.

In this sense, a fuel injector system that fully

vaporizes and mixes the reactants, thus resulting in a

uniform flow field impinging upon the catalytic bed, is critical to achieve optimal reforming performance.

A number of studies have been done towards optimi-

zation of the reformer fuel delivery system.

Karatzas et. al [10] used a single fluid pres-

surized-swirl nozzle to inject liquid fuel as a fine hol-

low cone spray into the mixing zone for evaporation

and blending with a mixture of superheated air and

steam that enters the reformer from perpendicularly

arranged injection holes about 40 mm downstream.

Fuel was injected at about 10 bar. Mao et. al [17]

surveyed and modeled a few fuel delivery systems for diesel reformer applications, such as the mul-

tipoint impingement injector, gas-assisted simplex

injector and a high energy piezoelectric simplex in-

jector. The gas-assisted simplex injector has a sim-

ple, more robust design and less prone to internal

coking. It is easily adaptable to a different reformer

due to a narrow spray angle to minimize carbon dep-

osition on the chamber wall thus providing good at-

omization when there is adequate gas inlet pressure;

however, mixing devices are required for uniform

mixture distribution. The above injectors produced

droplets’ Sauter Mean Diameters (SMDs) in the range of 10-30 µm (3 inches downstream from injec-

tor). As far as the piezoelectric simplex injector is

concerned, it has excellent atomization for low flow

rate applications though at the expense of minimized

power consumption. Kang et. al [18] devised an ul-

trasonic injector (UI) for effective diesel delivery.

The UI can atomize diesel into droplets (~40um) by

using a piezoelectric transducer and consumes much

less power than a heating-type vaporizer. The big-

gest drawback of this configuration was found to be

lack of proper mixing; consequently, carbon deposi-tion progressed very quickly in the reformer and de-

activated the reforming catalyst after just 15 minutes.

Yolanda et. al [13] have designed and built a facility

to test reformer concepts having the capability of

providing 10 kW power. Tangential swirl is generat-

ed upstream of the diffuser/mixing chamber.

To date, further research is still required

before a diesel Polymer Electrolyte Fuel Cell – Aux-

iliary power Unit (PEFC-APU) system becomes a

practical and feasible option for commercialization. A higher efficiency of the fuel processor can be

achieved by smarter reactor designs, better integra-

tion of each segment and better understanding of the

overall process. One key segment though is the con-

version efficiency of the reformer, which forms the

‘heart’ of the system. Poor reformer performance can

result in a negative “domino effect” causing degener-

ation and deactivation of the other components in the

subsequent segments. A good fuel reformer design

could enhance the reforming efficiency and hydrogen

production greatly.

The aim of the current study is to investigate the potential application of the MVCO injector cou-

pled with a novel adapter for diesel reformers. Previ-

ous studies have shown that the MVCO fuel injector

is capable of providing variable spray angles, varia-

ble orifice areas, and variable spray patterns.[6] Such

optimized spray patterns are favorable to minimize

combustion chamber surface-wetting, oil dilution and

emissions. The MVCO injector thus has great poten-

tial to be used for diesel reformer applications due to

its unique spray characteristics. High injection pres-

sures may be applied which greatly enhance atomiza-tion of the diesel fuel, yet prevent wetting of the cata-

lyst. Meanwhile, swirl flow is commonly used as a

potential means of promoting mixing efficiency [14].

Swirling flows can also improve and control the mix-

ing process between fuel and oxidizer streams to

achieve flame stabilization and heat release rate en-

hancement [14, 15, 16, 20-26]. According to

Karatzas [10], mixing of fuel with the air and water

vapor is of great significance in a diesel reformer

before the fuel impacts the catalyst and causes wet-

ting. Proper mixing promotes effective hydrogena-

tion and thus, higher reformation efficiency. Moti-vated in this respect, a swirl adapter has been de-

signed which provides a combination of clockwise

and counter-clockwise swirl to aid in mixing of the

gas mixture before it reaches the catalyst.

EXPERIMENTAL SETUP AND

METHODOLOGY

The Apparatus The experiment was carried out on an opti-

cal engine facility, where only the fuel injection sys-

tem is used as shown schematically in Fig. 1. A

Bosch CP1 high pressure pump (1350 bar max.) cou-

pled with electric motor was responsible for deliver-

ing the fuel into the common rail. Pressure is varied

using a pressure regulator which is controlled by a

LabView interface. The control is made possible

through a custom-designed circuit which supplies the

necessary voltage to the pressure regulator in order to

vary the fuel pressure. The injection system is controlled by a lo-

cally-made injector driver, which provides the re-

quired current to lift the needle inside the injector at a

given timing. The returned fuel from both the injec-

tor and the high pressure pump are cooled through an

air-cooled heat exchanger before being recollected in

the fuel tank so that the injection system would not

be overheated. The injection duration and frequency

of injections are controlled using a Digital Pulse

Generator, and a Function Generator, both of which

work in conjunction with the LabView interface.

Figure 1. Experimental Setup – Fuel Injection Sys-

tem

The front view of the swirl adapter is shown in Fig. 2. This adapter is fitted on the injector. It has

two sets of concentric holes for the supply of air

and/or water, which promote swirl. The outer set of

holes produces a counter-clockwise swirl while the

inner set produces a clockwise swirl. This novel con-

figuration is expected to enhance the mixing of air,

fuel and water to a complete homogeneous mixture.

Figure 2. Front view of the swirl adapter

Laser Diagnostics High-speed imaging as well as the PDA

setup is shown in Fig. 3. Back light illumination was

used for capturing the time resolved injection event. A phantom V7 high speed camera was used at a

speed of 15037 fps, corresponding to a time resolu-

tion of 67 μs. An Oxford copper vapor laser (Model:

LS20-50), synchronized with the camera, was used as

the backlight source. The injector was positioned

such that the spray direction was perpendicular to the

light source/camera to obtain the side view images,

and the injector was tilted by 90 degrees to make the

spray parallel to the light source/camera to obtain the

bottom view images. The laser light was delivered

through a fiber and projected onto a white paint coat-ed glass to form a homogeneous background. The

camera was triggered by the injection signal and the

exposure time was set to 3 μs. For each case, a min-

imum of three injections were conducted for any

quantitative analysis.

A 2-D Dantec Fiber PDA system with a

BSA P60 processor coupled with a 58N70 detector

unit were used for the measurement of the droplet

size and velocity distribution. The transmitting and

receiving optics were installed on a three dimensional

traverse which precisely controls the position of the

measuring point. An argon-ion laser with a maxi-

mum power of 8.5 W was used. A Bragg cell unit

positioned inside this fiber optical unit provided a 40

MHz frequency shift such that green beam with

wavelength of 514.5 nm and blue beam with wave-

length of 488 nm were generated. Light scattered by

the droplets was collected by a 310 mm focal length lens positioned at 30 degrees to the plane of the two

incident green beams to ensure refraction dominated

mode. The signal from the four photomultipliers was

transmitted to the processor unit where all the data

processing was carried out. The measurements were

triggered by the injection signal.

Test Matrix A summary of the experimental test matrix is shown in Table 1. The rail pressures selected were

300 bar, 500 bar, and 700 bar while the back pressure

was kept at 1 bar. These injection pressures are a

good balance between too high pressures (up to 1000

– 1200 bar, which would potentially cause long fuel

penetration and thus wetting of the catalyst) and too

low pressures (around 7-10 bar, which is not suffi-

cient for good atomization of the fuel). Injection

duration of 0.5 ms was chosen for the experiment.

The different locations in the spray at which PDA

measurements were taken, is shown in Table 2.

Figure 3. Experimental Setup – Optics & Laser Diagnostics

Roughly 25000 samples were acquired at

every location for every experimental condition.

Note that the settings are targeted at investigating the

conical spray mode of MVCO injector along with the

adapter, since the multi-jet mode of MVCO is similar

to conventional multi-jet sprays which are well known. These tests were done using diesel fuel, with

shop air delivered to the swirl adapter.

In Auto-Thermal Reforming (ATR), the

most important operating parameters are the steam-

to-carbon (H2O/C) and oxygen-to-carbon (O2/C) rati-

os in the feedstock composition. These ratios are

derived by dividing the reactants moles (H2O and O2)

with the carbon moles present in the employed fuel. Practically speaking, excess air, meaning higher O2/C

ratios, are often implemented in order to trigger and

sustain the ATR reactions as well as to compensate

heat losses. Also, the ratios are often empirically

derived as the optimal values of the H2O/C and O2/C

ratios can differ significantly depending on the quali-

ty of the diesel fuel, reforming catalyst and reactor

design employed. To simulate these different ratios, spray characteristics were measured at different fuel

injection pressures, coupled with different air inlet

pressures (30 psi, 2 bar and 60 psi, 4 bar), as summa-

rized in Table 1.

Image Post-Processing The spray penetration length, Sp is obtained

directly from the front view measurement, based on

the displacement between the orifice and the furthest tip point, or geometrically the hypotenuse of the tri-

angle as illustrated in Figure 4. It should be men-

tioned first that the acquired spray image is “line of

sight” accumulation of the signals scattered by the

liquid spray jet, lets us consider an imaginary "view

plane" on which all the collected signals are imposed.

Then the "hypotenuse" in the front view image

should be interpreted as the projection of the actual

penetration length on the view plane. A major as-

sumption of this method is that there exists one indi-

vidual fuel jet whose axis is exactly located on the

front view "hypotenuse" which is hard to validate, as the rotational orientation of the injector is completely

random; however considering that the number of jets

generated are well above twenty, depending on the

injection pressure, the angle φ between the left-most

fuel jet axis in the front view image and the view

plane is really within 10 degrees, leading to a differ-

ence between the projected penetration length and

actual length to be within 3% as illustrated in Figure

5. Therefore, it is reasonable to determine the spray

penetration using this method. Detection of the "hy-

potenuse" of the spray is, similar to a number of pre-vious studies [20-26], based upon an intensity thresh-

old criterion to be met to distinguish the spray edge

from the background. The spray cone angle is decid-

ed based upon the location of the injector tip and the

two spray tips as illustrated in Figure 4.

Figure 4. Spray characteristics representation

Figure 5. Determination of the spray penetration Sp

Injection Pres-

sure (Pinj, bar)

Air Pressure

(Pair, psi)

300

0

30 (2 bar)

60 (4 bar)

500

0

30

60

700

0

30

60

Table 1. Experimental conditions

Table 2. PDA Measurement Locations

RESULTS AND DISCUSSION

MVCO Injector without Swirl Adapter The spray evolution of an individual injec-

tion event at an injection pressure of 500 bar, from

the front view and bottom view is shown in Figure 6

and Figure 7 respectively. The most apparent differ-

ence is the multiple fine jet plumes produced by the

MVCO injector, which manifests an “umbrella” shape, while for the conventional multi-jet injectors

[20-26], the individual jet plumes usually can be

clearly observed. 30 fuel jets on the periphery are

observed from the images, which is a substantially

higher number of jets compared to six to eight jets

from the conventional multi-hole injector. Therefore

the MVCO injector provides a much greater volumet-

ric coverage for the same amount of fuel injected,

which in turn provides a more homogeneous mixture,

compared to the conventional diesel injector. The

front view images of the spray show a slightly

"bumped" spray in the radial direction about halfway between the injector tip and the end of the spray jet,

which is likely a head vortex. The bottom view (Fig-

ure 7) also shows the hollow cone feature of the

spray, which is of crucial importance for air entrain-

ment. As the closely spaced fuel jets interact with

each other as the spray develops, there remains a ma-

jor concern that insufficient air entrainment would be

caused. The hollow cone shape provides a path from

inside for air/fuel contact which should ameliorate

the issue.

Figure 8 quantifies the spray penetration calculated using the method aforementioned. It is

seen that it increases along with time and reaches a

quasi-steady state after about 1 ms. The quasi-steady

state penetration length observed is about 20 mm, 24

mm and 26 mm for the three injection pressure cases

respectively. It can be observed that the spray pene-

tration value of the MVCO injector for a given injec-

tion pressure is lower compared to previously studied

fuel delivery systems which had values between 35 –

90 mm [10, 13, 17-19, 20-26], due to the simple fact

that the total injected volume was divided into more jets, resulting in a lower penetration for each individ-

ual jet, which will be beneficial in avoiding wall wet-

ting especially under low load conditions. The quan-

tified spray cone angle for different injection pres-

sures is also summarized in Table 3. Spray cone an-

gle increases with increasing injection pressure indi-

cating the flexibility of the spray with varying engine

load. At low load condition, the spray is character-

ized by a relatively narrow spray angle and shorter

penetration so as to prevent wall wetting while at

high load, the spray was widened to suit for higher

power output.

The droplet size probability density function

(PDF) at different injection pressures are shown in

Figure 9 where the normalized counts of all samples

are plotted against their size in a 0.5 μm bin. It is

seen that distributions peaks lie at about 9-12 μm

mainly due to the more violent secondary breakup induced by the unique spray pattern of the MVCO.

The narrow bandwidth of the distribution also indi-

cates uniform drop size distribution.

Figure 6. MVCO Injector Spray Evolution (Front

view)

Figure 7. MVCO Injector Spray Evolution (Bottom

view)

Figure 8. Spray Penetration Length at different injec-

tion pressures

Table 3. Spray cone angles at different injection

pressures

(a) (b) (c)

Figure 9. PDF of the droplet size at axial 22 mm, radial 40 mm at Pinj = (a) 300 bar (b) 500 bar (c) 700 bar

(a) (b)

Figure 10. Mean droplet diameter (a) Axial variation (b) Radial variation, without air supply

Figure 10 (a) shows the axial variation of the

mean diameter at a radial position of 40 mm. No

apparent trend of the droplet size against injection

pressure is observed, which indicates uniform atomi-

zation with varying load. Figure 10 (b) shows the

radial variation of mean diameter at an axial position of 22 mm. The plot shows increasing mean diameter

as we move higher up in the radial direction. This

can be explained by the fact that at around 40-45 mm

radial position, we are almost exactly on the jet

where the droplets are yet to undergo breakup. How-

ever the lighter, already dispersed droplets move

away from the jet towards the centre, displaying a

smaller mean diameter.

Figure 11 illustrates the droplet velocity

(axial velocity component) in both axial and radial

directions at position C (22-0-40), without the swirl

adapter at injection pressures of 500 and 700 bar. At both injection pressures, the velocity is seen to be

about 80 m/s. Thus it can be seen that the velocity is

uniform with varying injection pressures (load).

Contrary to some previously studied injectors, the

measured velocity for this new MVCO injector is

lower than most of the injectors and swirl atomizers

found in the literature survey [20-29], where a maxi-

Injection Pressure (bar) Spray Cone Angle θ (degrees)

300 118 ± 1

500 127 ± 1

700 130 ± 1

mum velocity of around 100-140 m/s is detected.

Both the radial and axial velocities of the MVCO

conical spray are much lower than a conventional

multi-jet from a conventional multi-hole diesel injec-

tor, where the jet velocity is generally above 150 m/s

under similar injection pressure and back pressure. The lower velocity profile of the conical spray of

MVCO is favorable for diesel reformers, where the

back pressure is low and no catalyst wetting is de-

sired.

Weber Number,

(1)

where ρ is the density of the fluid (kg/m3), ν is its

velocity (m/s), l is its characteristic length, typically

the droplet diameter (m) and σ is the surface tension

(N/m).

(a) (b)

Figure 11. Droplet Velocities - Position C, Pinj = (a) 500 bar (b) 700 bar

Figure 12. Weber Number vs Diameter - Position C,

Pinj = 300 bar

To identify the breakup scheme of the spray,

the Weber number was calculated as shown in Equa-

tion (1). The Weber number gives an indication of

the breakup regime of the droplets.

The importance of the Weber number is that it is an important parameter to reflect the droplet be-

havior, not only in deciding the breakup regime, but

also in deciding the splashing regime. The impinge-

ment of droplet in the diesel reformer is of critical

importance since catalyst wetting can cause deterio-

ration of the reforming efficiency. Higher Weber

number indicates a ‘catastrophic’ breakup of droplets,

which will provide much higher atomization and bet-

ter mixing [30]. The properties of diesel fuel were

calculated at ambient temperature (25° C) and pres-

sure (1 atm). This is a valid estimation, since fuel was injected into the atmosphere. It is important to note

that there in a PDA measurement, there will be a

large number of points which show zero velocity, and

that is why there is an accumulation of points at

We=0. From Figure 12, we see that particles in the

diameter range of 5-10 µm (the mean diameter) show

a heavy accumulation at a Weber number of 500.

This indicates that the droplets are in the ‘cata-

strophic’ breakup regime [30], ensuring a higher de-

gree of atomization and hence, enhanced mixing.

MVCO Injector with Swirl Adapter

Figure 13. MVCO Injector with Swirl Adapter -

Spray Evolution comparison at Pinj = 500 bar and Pair

= 30 psi & 60 psi (Front View)

Figure 14. MVCO Injector with Swirl Adapter -

Spray Evolution comparison at Pinj = 500 bar and Pair = 30 psi & 60 psi (Bottom View)

Figure 13 shows the spray evolution (side

view) of the MVCO injector coupled with the swirl

adapter, at an injection pressure of 500 bar, with dif-

ferent inlet air pressures (2 bar, 30 psi and 4 bar, 60

psi). It can be seen clearly that the spray becomes

much wider with the use of the adapter. At around

0.9 ms ASOI, the fuel jets are much less distinguish-

able, and only a highly dense fuel-air mist is ob-

served. Figure 14 show the same spray evolution

from the bottom view. The outline of the adapter is

visible in these images. At 0.36 ms, there is a notice-

able difference between the spray without the adapter

and that with the adapter. The penetration is seen to be shortened. The jets also seem to be fat-

tened/smeared due to the effect of the air swirl. In

these figures, the cut-off at the top is due to the ge-

ometry of the swirl adapter (covering the injector tip).

Figures 15(a) and 15(b) show velocity dis-

tribution at different injection pressures with air sup-

ply from the swirl adapter (500bar, 30 psi and 700

bar, 60 psi respectively). It is seen that the swirl re-

duces the velocity. The maximum velocity is about

60 m/s, which is 20 m/s lower than the velocity with-

out the adapter. The velocity plots show a clear dis-

tribution of points below 0 (indicating movement in the opposite direction), extending up to 20 m/s at 60

psi air pressure. This indicates a rotational swirl mo-

tion with a velocity component found in the direction

opposite to that of primary spray jet direction. In-

creasing the air pressure enhances this effect. This

swirl is expected to greatly enhance the air/fuel mix-

ing before the mixture reaches the catalyst in a real

diesel reformer. It is important to note that the fuel is

injected at relatively very high pressures compared to

the air pressure, and hence the jet would just tear

through the air mist formed. Even so, at compara-tively lower air pressures of 30 and 60 psi, we see a

strong swirling effect.

Figure 16 shows Weber Number at Position

B at 500 bar fuel injection pressure, without air sup-

ply, and air at 30 psi. Similar to Figure 12, we see

quite a large number of points with We ~ 500. Alt-

hough the swirl adapter reduces the velocity of the

spray jet and imparts a rotational motion to the jet,

the accumulation of droplets at We ~ 500 is still the

highest. Peak values of Weber number are seen to go

up with higher air pressure, indicating that the air

flow enhances the ‘catastrophic’ breakup of the drop-lets.

Figure 17 (a) and (b) shows the axial varia-

tion (at radial position 40 mm) and radial variation (at

axial position 22 mm) of mean diameter respectively,

at an injection pressure of 300 bar with varying air

pressures. There is no noticeable axial variation,

however, the mean droplet size increases with an

increase in inlet air pressure. This is due to the fact

that the air swirl causes the fuel droplets to collide

against each other and they coalesce to form slightly

bigger particles, resulting in an increased droplet size measurement. It is also shown that the mean diameter

values are less affected by the air, as we move up

radially. At 25 mm radial position, the mean diame-

ters without and with air are furthest apart. But as we

go higher up, they approach each other, and at 45 mm

radial position, all three values converge at a single

point. This shows that the air has least impact at the

jet position itself, but has maximum effect towards

the centre of the spray (hollow cone).

(a) (b)

Figure 15. Droplet Velocities - Position B, (a) Pinj = 500 bar, Pair = 30 psi (b) Pinj = 700 bar, Pair = 60 psi

(a) (b)

Figure 16. Weber Number vs Diameter - Position B, Pinj = 500 bar with (a) No air (b) Pair = 30 psi

(a) (b)

Figure 17. Mean droplet diameter (a) Axial Variation (b) Radial variation, at Pinj=300 bar

(a) (b)

Figure 18. Mean droplet diameter – Radial Variation at (a) No air (b) Pair = 60 psi

Coalescence of fuel droplets occurs mostly

towards the centre of the spray cone. Therefore, as

breakup occurs, particles would move towards the cen-tre of the spray and have a swirl motion imparted to

them, thereby causing coalescence in the inner portion

of the spray cone. Figure 18 (a) and (b) shows the radi-

al variation (at axial position 22 mm) of mean diameter

without air supply, and air supplied at 60 psi respective-

ly, at different injection pressures.

The mean diameter and SMD do not display

any specific for varying injection pressures at all air

pressure cases, again indicating the uniform atomiza-

tion characteristics of the MVCO injector at different

loads. Mean diameter for all injection pressures (no air)

increases moving up radially. This variation with radial position is eliminated with the introduction of air. This

shows how the swirl brought in by the air enhances the

mixing of fuel and air, thereby creating a spray uniform

in drop size.

CONCLUSIONS 1. The MVCO injector coupled with the swirl adapter

produces a widespread, highly atomized, umbrella-

shaped spray with nearly 24 jets. The spray also

features adaptive spray cone angle according to dif-

ferent engine load.

2. Maximum Spray Penetration observed is 25 mm.

Mean Diameter is about 8 µm; SMD is about 25-40

µm without the swirl adapter.

3. When coupled with the swirl adapter, the spray

pattern obtained is much wider, with a shorter

spray penetration, caused by suppression of the ra-

dial component due to the swirling air motion. 4. A clear swirling motion is observed in the PDA

results. The velocity measured showed a negative

velocity component (previously unseen in meas-

urements made without the swirl adapter), indicat-

ing a rotational motion due to swirl.

5. The mean droplet diameter was found to increase

with increase in inlet air pressure, indicating coa-

lescence of fuel droplets, caused by collision of

droplets due to the swirl. These effects were more

pronounced towards the centre of the spray (hollow cone) and lesser towards the periphery

6. The highly atomized, widespread, uniform spray

with short penetration length, make this a highly

suitable fuel delivery system for diesel reformer

applications.

ACKNOWLEDGMENTS “This material is based upon work supported

by the National Science Foundation under Award

Number: NSF1113660. Any opinions, findings, and

conclusions or recommendations expressed in this pub-

lication are those of the author(s) and do not necessarily

reflect the views of the National Science Foundation.”

The authors are grateful for the support from NSF.

NOMENCLATURE ρ density of the fluid

(kg/m3)

ν fluid velocity (m/s)

l characteristic length, typi-

cally the droplet diameter

(m)

σ Surface Tension (N/m)

MVCO Micro Variable Circular

Orifice

PDA Phase Doppler Anemome-

try

CO2 carbon dioxide

APU Auxiliary Power Unit

SMD Sauter Mean Diameter

ASOI After Start of Injection

kW kilo-watt

PEFC-APU Polymer Electrolyte Fuel

Cell – Auxiliary power

Unit

ATR Auto-Thermal Reforming

H2O/C steam-to-carbon ratios in

the feedstock composition

O2/C oxygen-to-carbon ratios in

the feedstock composition

Sp spray penetration length

Pinj injection pressure

Pair air pressure

PDF Probability Density Func-

tion

We Weber number

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