Hao Wu， Fujun Zhang,2 and Zhenyu Zhang,?

（1. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China；2. Shenzhen Research Institute, Beijing Institute of Technology, Shenzhen 518057, China）

Abstract: Spray atomization of liquid fuel plays an important role in droplet evaporation, combustible mixture formation and subsequent combustion process. Well-atomized liquid spray contributes to high fuel efficiency and low pollutant emissions. Gasoline direct injection(GDI) has been recognized as one of the most effective ways to improve fuel atomization. As a special direct injection method, the air-assisted direct injection utilizes high-speed flow of high-pressure air at the injector exit to assist liquid fuel injection and promote spray atomization at a low injection pressure. This injection method has excellent application potential and advantages for high performance and lightweight engines. In this study, the hollow cone spray emerging from an air-assisted injector was studied in a constant volume chamber with the ambient pressures ranging from 5 kPa to 300 kPa.External macro characteristics of spray were obtained using high speed backlit imaging. Phase Doppler particle analyzer(PDPA) was utilized to study the microcosmic spray characteristics. The results show that under the flash boiling condition, the spray will generate a strong flash boiling point which causes the cone shape spray to expand both inwards and outwards. The axisymmetric inward expansion would converge together and form a lathy aggregation area below the nozzle and the axisymmetric outward expansion greatly increases the spray width. The sauter mean diameter(SMD) of flash boiling condition can be reduced to 5 μm compared to the level close to 10 μm in the non-flash boiling condition.

Key words: ambient pressure；air-assisted injector；spray characteristics；flash boiling；phase Doppler particle analyzer(PDPA)

Gasoline direct injection(GDI) is one of well-recognized breakthroughs in internal combustion engine technology to improve thermal efficiency, reduce fuel consumption and pollutant emissions[1]. Today’s modern GDI engines can work on both homogenous and stratified modes with the help of advanced electronic control unit[2-3]. Fuel injection plays a key role in achieving the right amount of liquid fuel delivery at right time and correct pressure, as well as providing well-atomized spray droplets for combustible mixture preparation[4-5]. At present, common GDI uses high-pressure atomization methods, that is,spray atomization is achieved by establishing a sufficiently high fuel pressure. Increasing fuel injection pressure does help improve atomization.However, establishing high fuel pressure puts forward high demands on the fuel supply system,such as the requirement of high-pressure fuel pump and fuel rail. These pose great challenges for the design of lightweight high performance engines used for all-terrain vehicles(ATV) and light aircraft[6].

Air-assisted direct injection was first proposed by Orbital Inc. and has been applied to both two-stroke and four-stroke engines[7-8]. Related research has shown that this twin-fluid injection method can achieve a satisfactory level of fuel atomization at a low injection pressure[9-11].Consequently, it has an excellent performance in improving engine efficiency and reducing pollutant emissions[12]. The principle of air-assisted fuel injection is to utilize high-speed air flow at the injector nozzle exit to overcome the surface tension of liquid fuel, therefore promote spray atomization[13]. Generally, liquid fuel is firstly injected into a premixing chamber filled with compressed air. The liquid fuel inside the premixing chamber experiences primary breakup and then is decelerated by the compressed air while being mixed with compressed air before finally injected into the combustion chamber[14].

Flash-boiling sprays can generate a significant effect on spray formation and its characteristics due to bubble nucleation, growth, and phase change, producing explosive-like atomization and complex spray structures[15]. In recent years, flash-boiling spray has attracted widespread attentions around the world because of its unique improvement in atomization, especially in the application of engine spray technology. Flash boiling spray can occur in GDI engines under part load conditions, particularly when operating in late inlet valve opening strategies[16]. For traditional GDI applications, numerous research work about flash boiling spray has been carried out to investigate both the influential factor on the flash boiling and its mechanism with multihole or single-hole GDI injectors[17-19]. In addition,much work about flash boiling phenomenon of hollow cone spray has been conducted recently[20-21].

Most of the previous studies about flash boiling spray of GDI system, however, were conducted with traditional vehicle-type injectors, which principally employ the atomization method of single-fluid liquid jet disintegration. Air-assisted injection is essentially a two-fluid atomization with a striking feature of low injection pressure,whereby the high-pressure fuel pump and rail can be removed[11]. In addition, the nozzle geometry of the air-assisted injector concerned in this study is an annular conical orifice through which the fuel-air mixture tends to form a progressively expanding spray, which will thin out as a result of the conservation of mass when the mixture departs from the injector nozzle and subsequently disintegrates into tiny droplets. Therefore, this spraying method has a very significant effect on improving atomization and the spatial distribution of droplets[21].

Since flash boiling has been considered as an effective means to further improve spray atomization, the work involved with flash boiling spray issuing from non-swirl hollow cone air-assisted fuel injector has been rarely concerned. Therefore, this study primarily focuses on the phenomenon of the flash-boiling spray of air-assisted injection. Experimental investigation was carried out to study the spray characteristics of a hollow cone air-assisted injection system under both flash boiling and non-flash boiling conditions.During the experiment, the ambient pressure varied from 5 kPa to 300 kPa. The time-resolved development of spray morphology was captured by a high-speed camera. The micro spray characteristics including droplet size and velocity were measured using phase Doppler particle analyzer(PDPA).

1 Methodology

1.1 Test system specification

Experimental research was adopted to study spray characteristics of air-assisted sprays under different injection ambient pressures. Fig.1 shows the schematic diagram of the test system, which contains high-pressure constant volume chamber(CVC), PDPA system, spray visualization system, air-assisted injection system and electronic control system. The constant volume chamber(max pressure: 6.0 MPa) is mainly designed for spray visualization research and PDPA measurement. Two quartz windows (viewing range of 90 mm) were mounted at two sides with the intersection angle of 110° to achieve the best signalto-noise ratio for PDPA measurement. The top of the chamber is detachable and can be used for the adaptation of different injectors as required.The high-pressure air container and the vacuum pump were connected to the chamber through air flow valves respectively for adjusting the CVC pressure. A digital pressure gauge was used to obtain the pressure value inside the chamber.

Fig.1 Schematic of test system

Droplet size and velocity were measured by a PDPA system(from Dantec Inc.). A transmitter and a receiver were placed on each side of chamber facing the quartz window. Laser was first generated by argonion laser and then separated into a pairs of laser beams with different wavelengths by a Bragg-cell beam separator.After that, the laser beams were focused by the transmitting lens and the focus point determined the measurement volume of PDPA. Spray shadowgraph images illuminated by an LED light source were taken by a high-speed camera(Phantom V7.3) with the recording speed of 10 000 frame/s, exposure time of 50 μs and resolution of 512×512 pixels. The injection system is a self-developed air-assisted injection system. An electronic control unit was used to generate the drive signal for the injector and the trigger signal for the high-speed camera and PDPA. The relevant test conditions and parameters are shown in Tab. 1.

Tab. 1 Test conditions

Fig.2 illustrates the working principle of the adopted air-assisted injection system. In this system, the fuel injection pressure was set to 700 kPa,which is 100 kPa higher than that of compressed air filled in the mixture chamber. During the injection, the fuel injector was first opened and liquid fuel was injected into the mixing chamber.The air pressure was stabilized by a regulator chamber before entering into the mixing chamber. After liquid fuel was broken into tiny droplets partially and mixed with the compressed air, the air injector, which was opened after a certain interval of fuel injector opening, was employed to control the fuel/air mixture injection.Then the fuel/air mixture burst out rapidly to generate air-assisted spray.

Fig.2 Air-assisted injection system

Fig.3 shows the control signal sequence of fuel injector and air injector, respectively. Both injectors were driven by the current with a“Peak-Hold” waveform, which can be configured by drive signals. Both the fuel injection duration Tfand air injection duration Taare composed of“Peak” and “Hold”. The signal of “Peak” is used to open the injector nozzle and the “Hold” signal is used to maintain the opening state of the nozzle. Tiis the interval between fuel injection and fuel/air mixture injection.

Fig.3 Injection drive signal sequence

The fuel used in this study is isooctane and its properties are listed in Tab. 2. The vapor pressure of isooctane denotes the saturation pressure at the corresponding temperature. Antoine equation is adopted to estimate the vapor pressure which can be expressed as

where A, B and C are the constant values relevant to the liquid properties. The values of these constants under experimental conditions are listed in Tab. 2. To analyze the spray under the flash boiling condition, superheat degree Rpis generally defined as the ratio of ambient pressure to saturation pressure, given by Rp=Pa/Ps.For a state above boiling point (super-heated),Rp＜1. For a state below boiling point (subcooled), Rp＞1.

Tab. 2 Main characteristics of the isooctane (25 ℃)

1.2 Image processing method and uncertainty

Fig.4a shows one original image of air-assisted spray. The PDPA measurement point was set at the injector axis with 50 mm vertical distance respect to the injector nozzle. The processing software attached to the high-speed camera was used to read the original cine video frame by frame and extract each original spray image.Subsequently, a spray image processing program was developed by using Matlab. The processing procedure mainly included background subtracting, contrast adjustment, binarization and edge detection, which has been illustrated definitely in the previous publication[11]. The macro characteristics of the spray that can be obtained after processing include the spray penetration length,spray width(the distance from the left edge to the right edge of the spray at different positions on the spray axis), and spray diffusion area rate,as shown in Fig.4b. It should be noted that the spray diffusion area rate denotes the ratio of the number of pixels occupied by the spray in the image to the total number of the image pixels,i.e. Ra=Nspray/Nimage, and is used to evaluate the diffusion characteristics of the spray in the twodimensional plane.

Fig.4 Location of PDPA measurement point(red dot) and definition of spray macro characteristics

In this study, test under each case of ambient pressure was repeated for three times. To show the experimental uncertainty, spray raw images from the three tests were processed. In addition, for each spray characteristic, three representative time instants or locations were selected for uncertainty analysis based on standard error(SE). The uncertainty(U) can be expressed as[21]

Tab. 3 Experimental uncertainties

2 Spray Macro Characteristics

2.1 Spray images

Fig.5 shows the images of typical spray development under certain ambient pressure conditions: 5 kPa, 50 kPa, 100 kPa and 300 kPa. It is noticed that drastic flash boiling spray occurs when the ambient pressure is 5 kPa. In terms of phenomena, the spray undergoes drastic lateral expansion immediately after issuing from the nozzle. In this experiment, fuel temperature was fixed at 25°C, the corresponding vapor pressure is about 6.58 kPa as shown in Tab. 2. Hence the superheat degree corresponding to the case of Pa=5 kPa is Rp=0.76. Under this superheated condition, the liquid fuel emerging from the annular nozzle exit of the hollow cone injector rapidly enters deeply into a metastable state.That is, the ambient pressure value is substantially below the saturation pressure corresponding to the initial injection fuel temperature. The flash boiling of the hollow cone spray is believed to cause the spray to expand both inwards and outwards. The inward expansion of the axisymmetric spray forms a certain aggregation area which shows a noticeable dark color near the nozzle axis while the outward expansion of the spray greatly enhances spray spreading in horizontal direction.

For the case of Pa=50 kPa and Pa= 100 kPa,no flash boiling spray occurred. However, due to this special two-fluid spray method, a large amount of air is mixed in the liquid fuel actually sprayed. In addition, due to the special nozzle geometry of the fuel-air mixture injector, the compressed air will generate high-speed airflow at the nozzle exit. The difference between the flow velocity of liquid fuel and compressed air will be conducive to the fuel atomization. Unlike the stable liquid sheet formed by a conventional single-fluid hollow cone spray, the spray produced by air-assisted injector is difficult to maintain a stable structure due to the action of compressed air and therefore appears a turbulent state. Furthermore, the near-nozzle spray morphology is changeless while the far-nozzle spray morphology appears variable under the combined action of compressed air and ambient air.

Fig.5 Selected typical spray development images

When the ambient pressure increases to Pa=300 kPa, the resistance of the ambient air to the spray increases significantly. At the same time, the spray tip penetration length is significantly reduced and the front end of spray begins to produce a distinct lateral diffusion.

2.2 Spray penetration length

The spray penetration length is a spray characteristic that determines the ability of the spray to diffuse in the axial direction of the nozzle. However, large spray penetration length is likely to cause spray-wall impingement in the cylinder, which will affect the formation of a homogeneous combustible mixture and cause abnormal combustion. Fig.6 shows the air assisted spray penetration length under different ambient pressures. It is noted that the penetration length at the same time after the start of injection decreases with the increasing of ambient pressure when Paexceeds 20 kPa. For the cases of Pa=5 kPa, 10 kPa and 20 kPa, the spray penetration lengths are almost the same. This indicates that under low ambient pressure conditions, the effect of ambient pressure on spray penetration is negligible. When Pais larger than 50 kPa, the ambient pressure begins to have a significant effect on spray penetration. This is due to the increase of ambient air density caused by the increase of ambient pressure. As a result, higher density air tends to show a stronger resistance effect to spray.

Fig.6 Effect of ambient pressure on spray penetration length

2.3 Spray width

To demonstrate the lateral spreading of spray, the evolution of spatial-resolved spray width under different Pacases along the nozzle axis at t=2.0 ms is plotted in Fig.7. It is significant that the spray width for Pa=5 kPa is larger than all the other cases in each axial position,which indicates that flash boiling substantially promotes spray outward moving. Intense flash boiling spray (Pa=5.0 kPa) achieves a rapid increase in spray width even the liquid fuel is just ejected from the nozzle exit. For the cases of Pa=10 kPa, 20 kPa and 50 kPa, spray width in near-nozzle field is quite stable. However, in the far-nozzle field, spray width appears significant fluctuations. When Paincreases to 100 kPa, the effect of high air density on spray resistance begins to show. It is noted that after a period of fluctuation, the spray width decreases rapidly at around 50 mm from the nozzle. For the high ambient pressure of 300 kPa, a sharp increase in spray width occurs at 17.5 mm from the nozzle.This is due to the generation of vortex-like droplet cloud caused by the high ambient pressure. Under this condition, the radial spread of the spray will increase greatly but the penetration in the vertical direction will be reduced.

Fig.7 Effect of ambient pressure on spray width

2.4 Spray dispersion rate

To quantitatively study the spray dispersion characteristic, the spray dispersion rate against time is calculated and shown in Fig.8.Here spray dispersion rate is obtained by calculating the ratio that the number of pixels occupied by the spray to the number of total pixels in the spray image. It can be seen that under flash boiling condition (Pa=5.0 kPa), spray dispersion rate is the larger than all the other cases for the whole spray process. When the ambient pressure increases from 5 kPa to 300 kPa, the spray dispersion rate nearly shows a monotonous decreasing tendency.

Fig.8 Effect of ambient pressure on spray dispersion rate

3 Spray Micro Characteristics

In most practical applications of combustion systems, well-atomized spray leads to higher volumetric heat release rate, a wider range of stabilized operating condition and lower pollutant emissions. Therefore, the research on the liquid fuel atomization is important and spray microscopic characteristics should be conducted to obtain the spray atomization performance.

In this study, the droplet velocity and diameter of the spray were measured using PDPA and statistical calculations were performed to obtain the mean velocity and sauter mean diameter(SMD, also known as D32) of spray droplet.Here the SMD is defined as follows

Fig.9 shows the droplet mean velocity and SMD under different injection pressures. As can be seen that with the increase of ambient pressure, the droplet mean velocity and SMD show a monotonic decrease and a monotonic increase, respectively. Under the flash boiling condition, the droplet mean velocity is larger than 100 m/s.Meanwhile, the SMD of Pa=5.0 kPa is close to 5 μm. This illustrates that flash boiling spray can greatly increase the velocity of droplet movement and reduce the droplet diameter, which is conducive to the atomization of the spray.

Fig.9 Droplet mean velocity and size under different ambient pressures

In addition, under a wide range of injection ambient pressures (Pa＜300 kPa), the air-assisted injection system can realize a relatively satisfactory level of spray atomization. The maximum SMD is not greater than 15 μm. This result implies that this two-fluid injection method has a significant effect on improving spray atomization with a low injection pressure.

4 Conclusions

In this study, spray visualization measurements and PDPA test were undertaken to analyse the impact of ambient pressure on spray macro and micro characteristics of an air-assisted outwardly opening injector. The temporal and spatial evolution of air-assisted spray were captured and the droplet mean velocity together with SMD were obtained. The main findings are summarized as follows.

Flash boiling spray produced by a low ambient pressure (5 kPa) can greatly increases spray penetration length, spray width and spray dispersion rate. The mean velocity of the spray droplet is more than 100 m/s and the SMD is close to 5 μm. The inward expansion of the spray produced by flash boiling point will create a darkcolour aggregation area on the axis below the nozzle.

Under the ambient pressures of 10 kPa, 20 kPa and 50 kPa, noticeable spray width fluctuation occurs in the far-nozzle field which is absent in the near-nozzle field. With the increasing of ambient pressure, the spray dispersion rate decreases. The mean velocity and SMD of spray droplet decreases and increases respectively with the increasing of ambient pressure.