Atomization characteristics of a compact disc-type ...

ILASS–Europe 2019, 29th Conference on Liquid Atomization and Spray Systems, 2-4 September 2019, Paris, France

Atomization characteristics of a compact disc-type ultrasonicatomizer unit

J. T. Nithin, M. Lokesh, N. Balasubramanian, T. N. C. Anand∗

Department of Mechanical Engineering,Indian Institute of Technology Madras, India.

*Corresponding author: [email protected]

AbstractThis paper reports results of an experimental study performed to understand the atomization characteristics ofa miniaturized disc-type ultrasonic atomizer unit, which was developed for the purpose of fueling a two-wheelerengine. A new, compact arrangement of fueling system using a disc-type ultrasonic atomizer was developed, foreasy packaging of the fuel system in the intake path of the engine. While miniaturizing the fueling system, theneed for maintaining a minimum liquid column above the disc had to be examined, which if not maintained, couldcause failure due to overheating of the disc. This reduction in the fuel level above the disc could possibly vary theliquid surface wave morphology and also its atomization characteristics. Thus, to understand the effect of small fuellevels or liquid films above the disc, visualization experiments were conducted, to study the near-disc liquid surfacewave morphology and its break up process using backlit imaging technique. The experiments showed that havinga thin liquid film above the disc produced multiple small crests and troughs on the liquid surface and also producedwidespread breakup of drops and formation of fine mist. More importantly, a central tall plume usually seen whenused a higher liquid column, was not seen with this compact system. This observation of absence of the centrallarge plume ensures that this compact ultrasonic atomizer system, when used on the engine, is very unlikely toproduce wall wetting. Thus, the study gives enough confidence for applying this compact ultrasonic atomizer on anengine, with simplified packaging and less-complicated controls.

KeywordsGasoline spray, ultrasonic atomization, High speed imaging.

IntroductionUltrasonic atomizers are known to produce very fine droplets using high frequency vibrations. When a disc is madeto vibrate at a very high frequency, with liquid above it, it leads to the production of mist. Higher frequencies ofoperation lead to lower droplet sizes. The atomization of various liquids using ultrasonic vibrations has always beenof interest for scientific research. There had been a number of fundamental studies to understand and correlatethe atomization characteristics to the liquid and vibration properties [1],[2],[3]. This paper looks into ultrasonicatomization of gasoline fuel. The mist formed for gasoline is reported to mostly contain droplets of less than 10 µmin size and a narrow droplet size distribution [1],[4].Ultrasonic vibrations can be produced by piezoelectric transducers, acoustic vibrators, or mechanical vibratorsactuated at high frequencies (∼MHz). Depending on the construction and interaction with the liquid, there aredifferent configurations such as horn type [5] and disc type [6]. There are two theories postulated for dropletformation mechanism in ultrasonic atomizers: break up due to inertial forces generated by vibration waves, andcollapse of cavitation-induced gas bubbles. According to the former theory, the vibrations induced into the liquidcolumn travel to its surface and develop as surface capillary waves. The crests of these surface waves develop intodroplets, once the surface tension forces are overcome by the inertia. Many experimental studies have shown goodcorrelation between Kelvin’s capillary wave length [7] and droplet size [8],[9]. In the latter theory, it is believed thatvibration causes regions of compression and expansion within the fluid column. Cavitation bubbles can be formedwhen a negative pressure wave propagates through a region. During the compression cycle, the positive pressurecan lead to an implosion of the bubbles. The resulting shock waves could lead to the production of drops from theliquid free surface [10]. Observations of sonoluminescence support the cavitation theory [11].Ultrasonic atomizers find wide application in medical nebulizers [12], drying [13] and automotive fields [14],[15],[16],[17],[18],[19]. The objective of this work is to study the feasibility of ultrasonic atomiser as a manifold fuel injectionsystem for gasoline powered internal combustion engines. Conventional fuelling system uses solenoid injectorswith pressurized fuel to atomize the fuel by the aerodynamic drag breakup mechanism. Ultrasonic atomizers haveseveral advantages over conventional fuel injectors. They produce droplets of small size with narrower dropletsize distribution, which can help in preparation of homogeneous fuel-air mixture. The low momentum spray fromultrasonic atomisers reduce manifold wall fuel impingement, which is critical in small engines whose manifoldsare narrow. Since, they do not require the liquid to be pressurized, they can work with gravity fed fuel. Sucha system, without a fuel pressurising pump helps in lowering the cost of the system. Despite these advantages,ultrasonic atomisers have the drawbacks of heating to self destruction when operated dry (without liquid layer), loweratomization flow rates, and inability to withstand high temperature operation. This study focuses on addressing thesechallenges for applying ultrasonic atomization for manifold fuelling of a two wheeler engine. While studies have beendone earlier to use disc-type atomizers to fuel internal combustion engines, leveraging the advantage of small drop

This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0).

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ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France

sizes produced by it [14], one of the key challenges is in packaging such a fuel system for small two-wheelers,where the availability of space is constrained. Hence, efforts were made to arrive at a compact arrangement in thisstudy.

Material and methodsFor this study, a disc-type piezoelectric transducer made of poly-crystalline Lead Zirconate Titanate (PZT) basedferro-electric ceramic material (Table 1) was used for atomising automotive gasoline fuel (whose properties areshown in Table 2). This transducer vibrates in thickness mode at 1.67 MHz. Since the targeted application is for amanifold fuelling system and the ambient conditions in the manifold of a naturally aspirated engine involve pressuresvarying from 20 to 110 kPa, and temperatures in the range of 20 to 50 °C, this pilot study was conducted in an openchamber with atmospheric conditions of 101.325 kPa and 20 °C.

Table 1. Manufacturer specification of Disc type ultrasonic atomizer for water at 20°C and the liquid column height of 25mmabove the disc.

Oscillation frequency 1.67 MHzDisc Diameter 20 mmDisc Thickness 1.2 mmPower Rating 38 VDC, 80VA

Flow Rate 120 ml/hrDroplet size 5µm SMD

Table 2. Properties of gasoline used for the study. Values marked with an asterisk are from reference [20].

Properties Gasoline

Chemical composition C2 - C14∗

C,H wt% 85-88 , 12-15∗

Density @ 20°C 736 kg/m3

Surface Tension @ 20°C 18.93mN/m∗

Dynamic Viscosity @ 20°C 0.237 mN.s/m2

Vapour pressure 70.6 kPa∗

Molecular Mass 100-105 kg/kmol∗

Enthalpy of vapourization 316.6 kJ/kg∗

A high speed Photron FASTCAM® camera with a 105 mm macro lens was placed transverse to the spray plume asshown in Fig. 1. An LED strobe was used to illuminate the spray. Spray images were recorded at 9600 fps. Withthis setup, back-lit images of the gasoline spray formed using ultrasonic atomizer were recorded with a frame size of512 x 756 pixels and the pixel size was 0.08 mm. An Arduino® based micro-controller was used to synchronouslytrigger the atomiser, camera and strobe light. Images were recorded as 8 bit gray scale images.

Figure 1. a) Experiment setup showing the ultrasonic atomizer mounting and b) Schematic of the imaging layout [ Not to scale ]

In the proposed engine fueling system application, the piezoelectric disc will be placed at the bottom of a cylindricalmanifold and during the engine suction stroke, air will carry the atomized fuel formed above the disc into the cylinder.Although air and atomized fuel will interact in a cross flow configuration in the targeted application, in this preliminaryinvestigation, the spray characteristics were studied in a quiescent ambient only. To have optical access to thenear-disc region, an open mounting arrangement with its face flat to the disc surface as shown in Fig. 1a was



used. The motive of this study is to determine the spray behaviour for different heights of the gasoline liquidcolumn above the disc. An acrylic mounting was made with CNC machined circular seats for the piezoelectricdisc, at various depths and gasoline fuel feeding passages. The piezoelectric disc was clamped at its edges inthe seat by a screw arrangement. By providing a continuous flow of gasoline by gravity feed, the liquid columnavailable above the disc depends on the depth of the seat minus the disc thickness [this is referred to as the liquidcolumn height] and the excess gasoline will overflow through a bleed passage located above the flat surface of themounting. With this arrangement, liquid column heights of 1, 2, 3 and 4 mm could be maintained with an accuracyof ±0.2mm in static conditions [in the absence of piezoelectric vibrations]. During the operation of the piezo crystal,the liquid film surfaces are not horizontal and surface waves are observed, which will be discussed in the nextsection. Experiments were also conducted for a thinner liquid film, which just wetted the surface of the disc. But,the film thickness at this condition could not be determined exactly and it is hence referred to as "wetted condition(<0.5mm)" in the paper.

Results and discussionExperiments were conducted for different liquid column heights of 4, 3, 2, and 1 mm, and wetted disc conditions tostudy its effect on spray characteristics. Figures 2 and 3 show frame by frame images of the captured spray eventfor wetted condition (<0.5mm) and 3 mm liquid column height over the atomiser disc, respectively. At the highercolumn height, the pressure wave lifted up the liquid and it resembles a fountain jet. A similar observation wasreported by Matsuura et al.[21].

Figure 2. Time evolution of the spray while maintaining a gasoline liquid column height of less than 0.5 mm.

Figure 3. Time evolution of the spray while maintaining a gasoline liquid column height of 3 mm.

Patterns observed in the liquid free surface for different column heights are depicted in Fig. 4. For very low filmthickness, an evenly distributed crest and trough pattern is observed (4a). A very dense droplet cloud is seenduring the start of atomization, which eventually drifts upward with low momentum. Owing to the latent heat ofvaporization of gasoline which causes cooling, buoyant currents are formed, which distorts the droplet cloud. Incontrast, for a higher liquid column, a central liquid plume (Fig. 4b) is observed and surface waves ride over it. Inthis condition, ligaments are formed at the tip of the plume, which eventually fall back into the liquid pool. Somelarger droplets are also observed due to the secondary breakup of these ligaments.To get a detailed insight into the spray topography, these high speed images were post-processed using the MAT-LAB® software. To ascertain the area of the spray in the image at each time step, 8 bit gray scale images wereconverted into black and white images using 50% as the threshold level. To remove the effect of non-uniform il-lumination of the background, all the images were subtracted from the image at t = 0 (ie. start of spray) beforethresholding. The spray penetration is defined as the distance from free liquid surface at static condition, along thedirection perpendicular to disc surface, which en-composes 90% of the spray area in the processed image.The spray plumes observed for all the cases studied was narrow, and the width of the spray region was under 20mm. The spray penetration was found to be increasing with liquid column height (Figure 5a). This suggests that as



the pressure waves travels longer through the liquid column, they impart higher momentum to the droplets formedat the surface. This could be due to the interference between the waves. Attenuation of the pressure waves will alsooccur as the distance travelled to reach the surface increases. No atomization and surface waves were observedfor column heights above 40 mm, as the energy imparted by piezoelectric crystal gets attenuated within the liquidcolumn.The atomizer is targeted for manifold fuelling application for a 200 cc two-wheeler engine. The atomiser will beplaced at the bottom of the intake manifold of diameter ∼25mm. This arrangement will lead to a cross flow systemfor the spray with the intake air. In the actual engine arrangement with the cross flow of intake air interacting withthe spray, the spray tip penetration will be further lower. Lower spray penetration will result in less wall impingementon the inner walls of the intake manifold and this will be reflected as reduced wall film thickness. This is one of themajor advantages expected for the ultrasonic atomiser system over conventional solenoid operated nozzle injectorswhich generates high momentum droplets and have penetrations of over ∼80mm [22].

Figure 4. Patterns observed in liquid free surface when the piezoelectric disc vibrates in longitudinal direction at f = 1.67 MHz fora) small liquid column (d<0.5mm) and b) large liquid column (0.5mm < d < 40mm) [ Not to scale ]

The effect of the liquid column height on atomization rate is summarised in Figure 5b. The atomization rate forgasoline is higher than that for water (Table 1) and it increases with liquid column height, for the conditions studied.There is a substantial higher atomization rate for th column height of "1 mm" compared to that of "< 0.5 mm". Thiscould be due to the central liquid plume observed in the former. This stretching of liquid surface and the resultingincrease in the surface area improved the atomization.Engine controllers meter the fuel from cycle to cycle. To understand the working of the atomizer in intermittentoperation, an experiment was conducted to track the dynamics associated with atomization of a single droplet. Adroplet of initial mass 60 mg was used for this study, (the maximum fuel demand per cycle for the 200cc engineis 15 mg). Figure 6 shows the different regimes of its atomization. The droplet was dropped onto the atomizerand the time just before it struck the surface was considered as the t=0. Within a time span of half a millisecond,capillary waves appeared on its surface and caused it to wrinkle. As the time progressed, ligaments evolved andbroke into smaller primary droplets. Some of these primary droplets wetted the disc and merged into the liquid layerand a fine mist cloud was ejected from its surface. As the height of the liquid layer decreased to a critical heightbelow which the forces of vibration were lower than the surface tension forces (of the smaller droplets), atomization



Figure 5. Spray behaviour at different liquid column heights (a) Atomization rate (b) Spray penetration

ceased. Similar observations were reported by Peng et. al.[6] who studied the atomization of a water droplet. Theycalled these regimes as (a) unperturbed droplet, (b) capillary wave regime, (c) ligament regime (d) catastrophic dropbreakup and atomization, and (e) tail-off regime respectively.

Figure 6. Different regimes of atomization for an initial droplet diameter of 5.4 mm [60mg] and f = 1.67 MHz.

Although some residual gasoline was left above the disc without atomizing, the majority of the atomization processwas completed within 50 ms, which is within the 60 ms cycle time available for 2000 rpm engine operating speed.To meet the fuel demand at higher speeds two atomizers can be used in tandem.

ConclusionsThe aim of this study was to evaluate the potential of using a disc-type ultrasonic atomizer as a compact manifoldfuelling system for single cylinder SI engine used in automobile application. Many studies have shown that ultrasonicatomizers can produce small droplets and have a narrower droplet size distribution. This was the motivation, asreducing the droplet size helps in improving the mixture preparation inside the engine cylinder.Since the disc type atomizer will be mounted at the bottom of the intake manifold pipe in the fuelling system design,the primary focus was to determine the height of the gasoline liquid column required to be maintained. Previousstudies on ultrasonic atomization were carried out with liquid column heights of 20 to 30 mm for maximum atomiza-tion rate [1], [14]. Such a high liquid gasoline pool is not desirable in the manifold of the engine. This study was ableto understand the trade-off between the liquid column height and atomization rate. In this work, an insight in theminimal column height possible was also obtained. Even with very low column heights (lower than 0.5 mm), reason-able atomization was obtained. Lowering the column height also resulted in reduced spray penetrations. Shorterpenetration and lower momentum sprays will help in avoiding intake manifold wall impingement, which will help inreducing hydrocarbon emissions due to fuel film entering the cylinder and also improve the transient response ofthe engine. There are some drawbacks associated with this system which still need to be overcome for successfulimplementation of this technology: the ultrasonic atomizer requires a minimum liquid level for atomization to occur.This means that a residual amount of fuel will be left above it during engine shutdown, and this will add to thenon-engine hydrocarbon emissions. However, the system looks promising over conventional solenoid injectors, andthis pilot study has given enough confidence to test it on an engine.

AcknowledgementsThis work was funded by the Centre for Industrial Consultancy and Sponsored Research, IIT Madras under Inno-vative Project ME5-2018. The authors also acknowledge Mr. V Lakshmi Narasimhan, TVS Motor Company, for hisencouragement to pursue studies on ultrasonic atomization for small engines.



Nomenclatured Liquid film thickness or liquid column height [mm]fps Frames per secondf Ultrasonic resonance frequencyCNC Computer Numerical control (machine)cc centimeter cubeSI Spark ignited

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