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Experimental study on the effect of nozzle geometry on string cavitation in real-size optical diesel nozzles and spray characteristics
Fuel 232 (2018) 562–571
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Full Length Article
Experimental study on the effect of nozzle geometry on string cavitation in real-size optical diesel nozzles and spray characteristics
T
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Zhou Chena, Zhixia Heb, , Weiwei Shanga, Lian Duanb, Han Zhoua, Genmiao Guoa, Wei Guana a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel injector nozzle Visualization experiment String cavitation Nozzle geometry Spray characteristics
Cavitation is quite important for the diesel spray atomization and the combustion of air–fuel mixture. In this study, a high-speed CMOS camera equipped with a long-distance microscope was utilized to capture the transient cavitating flow and spray characteristics in real-size optical nozzles with needle motion. The transient cavitation images, including geometry-induced cavitation and vortex-induced string cavitation, were captured clearly in cylindrical-orifice nozzles and tapered-orifice nozzles, respectively. Besides, the agglomerated phenomenon of geometry-induced cavitation was visually captured and analyzed for the first time. It was found that the string cavitation in nozzle excites the instability of spray cone angle and it is synchronized with increase of spray cone angle. In addition, it is the string cavitation but not geometry-induced cavitation has a much larger contribution to the increase of spray cone angle. It is interesting that the influence of agglomerated geometryinduced cavitation on spray cone angle was prominent. Furthermore, both the nozzle orifice L/D ratio and sac types have significant influences on string cavitation and spray characteristics. The smaller L/D ratio and VCOtype nozzles are prone to incur the stronger string cavitation, and then spray cone angle is obviously larger.
1. Introduction Pollutant emission reduction is currently considered to be one of the most important challenges of our society. The spray atomization characteristics of the diesel injector plays an essential role for the good spray combustion and pollutant emissions performance [1]. New combustion modes have been suggested for energy-saving and emission-reduction, such as premixed charge compression ignition (PCCI) [2,3], partially premixed combustion (PPC) [4] and homogeneous charge compression ignition (HCCI) [5] and low temperature combustion (LTC) [6]. Certainly, it is indispensable for realizing these new combustion modes to prepare the suitable combustible mixture. Actually, diesel injector nozzles have a significant influence on the quality of spray and preparation of air–fuel mixture [7]. It is well known that fuel atomization processes is directly influenced by the nozzle geometry and cavitating two-phase flow [8,9]. Especially, the near-field spray atomization is extremely influenced by cavitation and turbulence in nozzles [10]. Therefore, it is indispensable to conduct the investigation of cavitating flow in diesel nozzles. Especially, the string cavitation, has a great contribution to spray cone angle, deserves more attention in the future. Focus on the geometry-induced cavitation in diesel nozzles, many
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Corresponding author. E-mail address: [email protected] (Z. He).
https://doi.org/10.1016/j.fuel.2018.05.132 Received 4 April 2018; Received in revised form 24 May 2018; Accepted 26 May 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
studies have been reported [11–13]. Gavaises et al. [14,15] utilized the optical nozzles to explore the effects of the needle motion on the transient cavitating flow. Experiments indicated the links between cavitation and turbulence in the sac, moreover, the anticipated enhancement of turbulence through the onset of cavitation was identified only at the entrance of the nozzle orifice [14]. In addition, the differences of flow characteristics between the real-size nozzles and largescale transparent nozzles were stressed [15]. Based on the large-scale nozzle, Xin Zhang et al. [16] reported that the higher fuel temperature means a lower critical injection pressure for cavitation inception, in addition, the higher fuel temperature could induce the larger cavitation region under the same injection condition. However, the scale effects could not be negligible in large-scale transparent nozzles. Therefore, the experiments and simulations based on the real-size diesel nozzle were performed. W. Yu et al. [17,18] proved that the injection pressure and ambient pressure could influence both the internal nozzle flow and the spray propagation. Mitroglou et al. [19,20] studied the cavitating flow in diesel optical nozzles as well and found that the initial bubbles exist inside the nozzle before start of injection. Besides, the life time and the most probable appearance location of string cavitation have been estimated. Besides, the effects of several factors on geometry-induced cavitation were investigated by CFD [21], including injection pressure,
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Nomenclature
Pinjection
K-factor D L Istring ρ V A Dsac Sstring Sorifice
Subscripts
conicity factor of nozzle hole nozzle hole diameter nozzle hole length intensity of string cavitation mean density of fuel mean velocity of fuel cross section area of nozzle orifice sac diameter of nozzle area of string cavitation in 2D image area of nozzle orifice in 2D image
o in
injection pressure
outlet of nozzle orifice inlet of nozzle hole orifice
Abbreviation VCO L/D
valve closed orifice nozzle length-diameter ratio of nozzle orifice
frames per second with 384 × 160 pixels. For the sake of capturing higher resolution images and larger shooting rate, the shooting rate is 100,000 frames per second and the images have a resolution of 640 × 280 pixels in this experiment. The 0# fossil diesel fuel (Automobile diesel fuelV, China) was employed in this experiment. The properties of diesel fuel and test conditions are given in Table 1. The ambient pressure of all injections is atmospheric pressure. Under such an injection condition, the turbulence and cavitation in nozzle were considered to be the main reasons of near-field spray atomization [10].
roundness of nozzle inlet, discharge coefficient of orifice, the L/D ratio, and the roughness of orifice inner wall. Although the investigations based on the real diesel nozzle have made large progress, there is little mention of vortex-induced string cavitation and its effect on spray characteristics. Focus on string cavitation, He et al. [22] subsequently found that the string cavitation has a strong relationship with the location of needle, the injection pressure, and the sac size. Applying a VCO nozzle (Valve closed orifice nozzle) [23], the effect of multi-injection strategy on cavitation development in diesel injector nozzle orifices was studied. After that, Watanabe et al. [24] explored the effect of needle tip shape on string cavitation and the spray characteristics. Furthermore, the spray characteristics based on metal nozzles were discussed in order to get a closer operation condition to diesel engine. Westlye and Payri [10,25] found that a tapered orifice had a higher spray tip penetration compared to a cylindrical nozzle orifice. However, it showed that the spray cone angle was inversely proportional to the spray tip penetration. Sedarsky et al. [26] researched the spray velocity profile from a high-pressure single-orifice diesel injector, and the results revealed a strong asymmetry in the spray profile of the test injector, there are distinct fast and slow regions on opposite sides of the spray. Crua and Ding [27,28] used single-orifice metal nozzles to investigate diesel spray formation, spray transient fluctuation and mushroom spray tip at initial stage of injection. However, because of absence of visible data about nozzle transient flow, above spray phenomena cannot be well explained and all spray characteristics have not been well connected with the nozzle internal flow. In general, previous studies have made numerous interesting achievements in multi-phase nozzle flow. But several aspects, such as the agglomerated phenomenon of geometry-induced cavitation, the occurrence mechanism of string cavitation and their effects on subsequently initial spray characteristic, are still not clear and deserve further investigations. In this paper, the real-size optical nozzle tips, equipped to the high pressure common rail fuel injector, were made for visual investigations of string cavitation characteristics with considering the effects of geometric parameters of nozzle, including L/D ratio, conicity factor (Kfactor), and nozzle sac geometry. Furthermore, the effects of those parameters and the agglomerated phenomenon of geometry-induced cavitation on near-field spray characteristics were analyzed in details. Finally, the ensemble average images of string cavitation were analyzed as an important statistical description.
2.2. Optical nozzle tips The metal nozzle tip of solenoid injector was cut and replaced by the real-size optical acrylic nozzle tip shown in Fig. 2, so that the transient cavitating flow inside the nozzle could be captured visually. For guaranteeing the good sealing performance between needle valve and needle seat of nozzle during the injection process, the cutting position of origin nozzle tip must be below the sealing line (Fig. 2). All of the optical nozzles were made of raw acrylic cubes, because of its good light transmittance (> 92%), good compression resistance and a similar refractive index (1.48–1.52) to diesel fuel (1.49–1.51). The optical nozzles were manufactured by precision drilling machine. Subsequently, the optical nozzle tips and injector could be assembled by a fixture. Specifications of the test nozzles were shown in Table 2. It must be noted that when investigate effects of nozzle orifice L/D ratio on string cavitation, the nozzle 1 and nozzle 3 were reworked with shortening the orifice length after finishing experiment of nozzle 2 and nozzle 4, respectively. So that guarantee the same geometry of other nozzle
2. Experimental setup and methodology 2.1. Facilities and fuel Based on the 250 MPa common rail injection system (BITEC-GY2502), the nozzle flow and spray visualization experiment system was established (Fig. 1). According to the shadow photography, the tested nozzles were placed between the LED light and the high-speed COMS camera (FASTCAM SA-Z) equipped with a long-distance microscope (QM-1, QUESTAR). The maximum camera shooting rate is 210,000
Fig. 1. Schematic of visualization system for internal flow. 563
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utilizing of the background image without spray subtract the image with spray, the domain of spray could be got for the binary image of spray (Fig. 4a II). Based on the interface of fuel spray and ambient air in the binary image, the two fitting lines could be established (Fig. 4a III), and then according to the slopes of fitting lines, the spray cone angle can be solved successfully. The intensity of string cavitation (Istring) can be calculated by the following equation: Istring = Sstring/Sorifice (Sstring means the area of string cavitation in nozzles, Sorifice means the area of nozzle orifice in the nozzle flow 2d images). Certainly, area of nozzle hole can be easily obtained on the basis of calibration. Therefore, if obtained the area of string cavitation means that the intensity of string cavitation was obtained. As the Fig. 4b shown, the image without cavitation subtracts the image with cavitation to get cavitation region. According to threshold value (20) of MATLAB code, the binary image of string cavitation could be acquired (Fig. 4b III). The area of string cavitation was solved with accumulated the number of white pixels. Finally, the intensity of string cavitation (Istring) was resolved in accordance with the former equation. In order to obtain the ensemble average images of string cavitation, more than one hundred images of string cavitation were utilized to get the ensemble average images of string cavitation by postprocessing. At first, all grey images of string cavitation were accumulated (Fig. 4c I). After that, the cumulative grey value of each pixel divided the image number. Therefore, the average grey image could be resolved (Fig. 4c II). Finally, selecting an appropriate range of RGB, the pseudo-color image can be acquired (Fig. 4c III).
Table 1 Properties of diesel fuel (293 K) and test conditions. Units Density Kinematic viscosity Distillation temperature (95%) Cetane number Sulfur Fuel temperature Ambient temperature Injection pressure Ambient pressure
Value 3
kg/m mm2/s K / mg/kg K K MPa MPa
832 5.8 629 52 10 293 293 50–80 0.1
areas. In other word, the only difference between nozzle 1 and nozzle 2, also nozzle 3 and nozzle 4 is the orifice L/D ratio. Hence, the studies on L/D ratio could be credible and be of more significance. The K-factor of nozzle orifice was calculated by the equation (the units are micron): Kfactor = 100 ∗ (Din − Do)/L. 2.3. The typical image and postprocessing methodology The obtained typical image of cavitating flow by shadow photography is shown in Fig. 3. When light goes through the interface between vapor phase (cavitation and initial bubbles) and liquid phase, the light refraction happens so that vapor phase presents black as a result of different refractive index between diesel liquid and vapor phase. On the contrary, the diesel fuel liquid phase represents white on account of similar refractive index between liquid diesel and acrylic nozzle tip. In addition, the spray presents black because light is overshadowed by spray droplets. In this paper, in order to obtain a reliable experimental dataset, the experiments were conducted more than 20 times at each operation condition. Compared with the results under the same operation condition, the time differences of cavitation development are not more than 20 μs (two images corresponding to 10 μs under shooting rate of 100,000 fps). Furthermore, the string cavitation images from the 20time injections were used for the ensemble average images of string cavitation. As Fig. 4 shown, all experimental data, including needle lift, spray cone angle, intensity of string cavitation (Istring) and ensemble average images of string cavitation, were post-processed by MATLAB code. The post-processing are as follows in details. The needle lift could be easily obtained according to calculated the pixel number of needle motion and combination with the calibration of pixel size. The postprocessing of spray cone angle was shown in Fig. 4a. Firstly, the MATLAB code selected the area of spray within two millimeters distance from the nozzle outlet for postprocessing (Fig. 4a I). And then,
3. Results and discussion 3.1. Transient cavitating flow in cylindrical orifice nozzles The transient cavitating flow and spray images during a whole injection event with the injection duration of 2000 μs under the injection pressure of 60 MPa in nozzle 1 were shown in Fig. 5. The graphs below the purple line show the enlarged local nozzle flow images of 4 typical injection times (430 μs, 900 μs, 2070 μs and 2090 μs) for illustrating a special agglomerated phenomenon of cavitating flow. Three optical nozzle replicates were fabricated for verifying the agglomerated phenomenon of cavitating flow. Paper chose the stage when the bubble suction happened as the start of injection. It is common to see that the initial bubbles are prevailing in the nozzle during initial injection stage, and the injection would experience the bubble suction and bubble compression on account of suddenly movement of needle valve at the opening stage. Hereafter, the geometry-induced cavitation appears and extends to the outlet (270 μs). And it is interesting to see that the geometry-induced cavitation could be agglomerated in the vicinity area of nozzle axis (360 μs),accompanied with markedly increasing of spray
Fig. 2. The raw acrylic cube, origin diesel nozzle and real-size optical nozzle. 564
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Table 2 Specifications of the test nozzles. Nozzle
Orifice number
Din/mm
Do/mm
L/mm
L/Do ratio
Dsac/mm
Nozzle type
K-factor
1 2 3 4 5 6
1 1 2 2 1 1
0.18 0.18 0.29 0.29 0.31 0.31
0.18 0.18 0.21 0.22 0.22 0.22
1.9 1.2 2.0 1.75 2.3 2.3
10.5 6.5 9.5 8.0 10.5 10.5
1.0 1.0 1.0 1.0 1.0 /
Min-sac Min-sac Min-sac Min-sac Min-sac VCO
0 0 4 4 4 4
(500 μs), the spray cone angle further increase to 31°, which is almost three times spray cone angle (10°, 900 μs) of higher needle lift stage. Although the geometry-induced cavitation always exists in nozzle hole during higher needle lift stage (700–1800 μs), the spray cone angle remains approximate 10°. The cavitating flow and spray at the needle closing stage (Fig. 5c) shows the similar results with that of the needle opening stage (430–480 μs). Besides, when the string cavitation occurs in nozzle sac (Fig. 5d, 2090 μs), the geometry-induced cavitation is also agglomerated in the vicinity area of nozzle axis and shows a similar shape feature to the cavitating flow at the needle opening stage (Fig. 5a). Therefore, the agglomerated geometry-induced cavitation is also regarded as a special string cavitation which is transformed from geometry-induced cavitation under the function of vortex flow in the nozzle orifice. Owing to the lower needle lift causing the narrow flow channel, so that the vortex flow intensity cold be stronger at lower needle lift stage. In other words, the vortex flow and string cavitation (vortex cavitation) in nozzle sac could agglomerate the geometry-induced cavitation to form this kind of string cavitation (agglomerated
Fig. 3. The typical image of cavitating flow by shadow photography.
cone angle, just like Fig. 5a shown. When the cavitating flow develops to 430–480 μs, the agglomerated phenomenon of geometry-induced cavitation becomes more obvious and the spray cone angle increases to 29° (Fig. 5a). Owing to occurrence of string cavitation in nozzle sac
Fig. 4. Postprocessing methodology for spray cone angle, intensity of string cavitation and ensemble average images of string cavitation. 565
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Fig. 5. Transient cavitating flow in single cylindrical orifice nozzle with mini sac (Pinjection = 60 MPa).
spray characteristics was firstly investigated by nozzle 1 and nozzle 2 with cylindrical orifices. The experiment was conducted with injection duration of 2000 μs under injection pressure of 50 MPa (Fig. 7). The results clearly indicate that the string cavitation is more prevailing with decreasing of the L/Do ratio. Besides, the spray cone angle with smaller L/Do ratio is almost larger than the spray cone angle with larger L/Do ratio. When the fuel goes through the small-L/Do-ratio nozzle orifice, owing to lower flow resistance, the velocity of fuel would be higher that
geometry-induced cavitation) in the axis vicinity of nozzle hole. Just similar to Andriotis’ study [29] that the pressurized air was sucked into vortex flow and finally developed into the string-like structure. The variations of the spray cone angle and string cavitation with needle movement are shown in Fig. 6. Considering that the geometryinduced cavitation dominates the nozzle flow (Fig. 5), it is unsuitable to describe string cavitation with intensity of string cavitation (Istring) which was defined before. Therefore, occurrence of string cavitation (Fig. 6) was utilized to explore the relationship among needle lift, string cavitation, and spray cone angle. It is not hard to find that the increase of spray cone angle is synchronized with occurrence of string cavitation. However, although geometry-induced cavitation exists all the time during the higher needle lift stage (just like 900 μs & 1500 μs in Fig. 5), the spray cone angle was evidently smaller than that of the lower needle lift stage. The results manifest that it is the string cavitation but not geometry-induced cavitation has a much larger contribution to the increase of spray cone angle. Essentially, the string cavitating flow originates from strong vortex flow where nozzle flow is accompanied by large rotating velocity. When the vortex flow runs from nozzle orifice into atmosphere and would lose the constraint of nozzle wall, the centrifugal force would enhance fuel spray atomization. As a result, the spray cone angle increased evidently.
3.2. The effects of nozzle orifice L/Do ratios on string cavitation and spray characteristics The nozzle orifice L/Do ratio is considered to be one of the most important factors affecting nozzle flow and subsequent spray characteristics [30,31]. However, the former investigations mostly focused on the geometry-induced cavitation. Rarely did researchers report the effect of L/Do ratio on string cavitation in diesel nozzles. Thus, the effect of nozzle orifice L/Do ratio on string cavitation and
Fig. 6. The spray cone angle and the string cavitation in single cylindrical orifice nozzle with mini sac (Pinjection = 60 MPa). 566
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ratio nozzle, there is no obvious hole-to-hole string cavitation to be captured in large-L/Do-ratio nozzle during the high needle lift stage (1000 μs). The reason for the flow difference probably is that the higher flow resistance for the large-L/Do-ratio nozzle means a lower fuel velocity and a weaker intensity of vortex and turbulence. For the purpose of well understanding the occurrence position of string cavitation, more than 100 images of string cavitation were postprocessed for getting the ensemble average images of string cavitation by MATLAB code. The Fig. 10 shows the statistical average descriptions of string cavitation, the red area represents cavitation and white area represents liquid fuel. As mentioned above, different L/Do ratios get rise to the differences of string cavitation intensity. Although there still remains large difference in string cavitation intensity between two nozzles, the string cavitation mostly appears at the sac area and nozzle outlet area with stronger cavitation intensity but a weaker intensity at inlet area of nozzle hole. During the lower needle lift period, the fuel from sac area to nozzle inlet would experience a large flow transition, hence the vortex intensity gets naturally stronger in sac area. When the local pressure in sac was below the saturated vapor pressure of fuel, the string cavitation prominently appeared at sac area with a stronger intensity. After that, when fuel comes to nozzle outlet area, the fuel velocities of different nozzle position should be different in tapered nozzle orifices. According to the continuity equation: ρo Vo Ao = ρin Vin Ain , the cross section of nozzle outlet ( AO ) is smaller in tapered orifices. Furthermore, owing to the larger gas volume fraction in outlet, the mean density ( ρo ) could be smaller in outlet areas. Certainly, the outlets of tapered orifices have the largest fuel velocity (Vo ) for the same mass flow. Therefore, the strongest vortex and turbulence would form at outlet areas and stimulate the strongest string cavitation. It well illuminates why outlet areas have the strongest string cavitation. Simultaneously, the stronger string cavitation would further narrow the flow channel cross section, this phenomenon may enlarge the velocity differences in tapered nozzle orifice.
Fig. 7. Effects of L/Do ratios on string cavitation in cylindrical nozzle with mini sac (Pinjection = 50 MPa).
could strengthen the intensity of vortex and turbulence. Reasonably, it presents prevailing string cavitation and larger spray cone angle. Different from nozzle 1 and nozzle 2, nozzle 3 and nozzle 4 were designed to have the tapered orifices. And the influence of orifice L/Do ratio on string cavitation and spray characteristics was investigated once again in these tapered-orifice nozzles with the injection duration of 1800 μs under injection pressure of 80 MPa (Fig. 8). The curves manifest a similar result as former study shown in cylindrical-orifice nozzles (Fig. 7). The fluctuation of spray cone angle is strongly influenced by string cavitation. Obviously, the intensity of sting cavitation has been strengthened a lot inside the nozzle orifice with smaller L/Do ratio. Certainly, the spray cone angle is larger as well on account of stronger sting cavitation. It is worth stressing that the hole-to-hole string cavitation, which is analyzed in Fig. 9(a) in detail, only occurs in smaller L/D ratio nozzle at full needle lift stage. In conclusion, the L/Do ratio of nozzle orifice plays a crucial role in string cavitation, whether it is the cylindrical-orifice nozzles or tapered-orifice nozzles. Benefited from that the tapered nozzle orifice could well suppress the geometry-induced cavitation [32], the string cavitation was clearly visualized without the interference of geometry-induced cavitation (Fig. 9). Therefore, it would be better to utilize the intensity of string cavitation (Istring) to discuss the relationship among needle lift, string cavitation and spray cone angle. It is easy to see that the geometryinduced cavitation could be preferably suppressed by the tapered orifice nozzle (Fig. 9). Moreover, the intensity of string cavitation in small-L/Do-ratio nozzle was much stronger (Fig. 9a). Besides, the string cavitation is originated from needle seat during the lower needle lift stage (500 μs & 1700 μs), this string cavitation could be called needleoriginated string cavitation. Nevertheless, the other kind of string cavitation, which is originated from one orifice and extends through sac volume into the other orifice during the full needle lift stage (1000 μs in Fig. 9a), could be called hole-to-hole string cavitation [14]. In contrast, the fluctuation of spray cone angle induced by hole-to-hole string cavitation is smaller than that induced by needle-originated string cavitation on account of weaker string cavitation. One of reasons could be that the higher needle lift made a larger flow channel for fuel flow in nozzle sac. Therefore, the vortex intensities and turbulence could be weaker in nozzle sac. Furthermore, the hole-to-hole string cavitation is hard to extend to nozzle outlet so that it could not directly affect the spray. As a result, the obtained spray cone angle is smaller. By contrast, things are different for a large-L/Do-ratio nozzle as the Fig. 9(b) shown. Except that the needle-originated string cavitation is apparently weaker at the low needle lift stage in comparison with that in the small-L/Do-
3.3. Transient cavitating flow characteristics of Min-sac and VCO nozzles Nozzle sac types could also largely influence the cavitating flow characteristics [33,34], and its influence on string cavitation is worth further investigation on account of lacking experimental investigation. Two Min-sac and two VCO nozzle replicates were respectively fabricated for verifying the effects of sac type on string cavitation. Utilizing nozzle 5 (Min-sac type) and nozzle 6 (VCO type) with the fuel injection
Fig. 8. Effects of L/Do ratios on string cavitation in tapered nozzle with mini sac (Pinjection = 80 MPa). 567
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Fig. 9. String cavitation and spray characteristics of tapered nozzles with different L/Do ratios (Pinjection = 80 MPa).
Fig. 10. Ensemble average images of string cavitation in diesel nozzles with different L/D ratios (Pinjection = 80 MPa).
duration of 2000 μs under injection pressure of 60 MPa, the effect of nozzle sac geometry on transient cavitating flow characteristics was investigated (Fig. 11). Both optical nozzles only have one tapered orifice, so that only the needle-originated string cavitation exists at low needle lift stage (Fig. 12) (The hole-to-hole string cavitation only occurs in multi-hole nozzle). As Fig. 11 shown, the string cavitation in the VCO nozzle occurs earlier and maintains a longer time than that in the Minsac nozzle. Accordingly, the spray cone angle is comparatively larger in the VCO nozzle than that in Min-sac nozzle. The flow experiences extremely violent flow transition at the inlet location of VCO nozzle orifice. Additionally, the VCO nozzle provides a narrow flow channel for fuel flow, so that the vortex and turbulence intensities and the local low pressure were strengthened here. As a result, the stronger string cavitation spontaneously appears and last a longer time in VCO nozzle. The transient internal flow and corresponding spray images of the Min-sac nozzle and VCO nozzle with a single tapered orifice were shown in Figs. 12 and 13, respectively. First of all, as the Fig. 12 shown, the string cavitation inception appears at 240 μs almost simultaneously with the compression process of sucked bubbles in the VCO nozzle. Moreover, it is interesting that the initial bubbles could be sucked into the vortex flow and form a part of string cavitation (240 μs, 250 μs in Fig. 12). When the string cavitation develops to the outlet, the spray cone angle is remarkably increased immediately (300 μs in Fig. 12). In contrast, the string cavitation inception appears much later at 360 μs in the Min-sac nozzle (Fig. 13). The stronger vortex and turbulence flow in VCO nozzle were considered to be the main reason for this difference between two type nozzles. Certainly, the initial bubbles, also provide numerous gas nuclei, could stimulate the string cavitation inception. Compared to the VCO nozzle geometry, the Min-sac nozzle has a comparatively larger sac volume, and then the flow transition is much
weaker. Besides, the string cavitation gradually becomes weaker with the increasing of needle lift after injection time of 300 μs in the VCO nozzle. And then, the string cavitation gradually vanishes after injection of 400 μs. The string cavitation firstly vanishes at the middle location of nozzle orifice, later at the orifice outlet, and the orifice inlet followed (400 μs, 450 μs, 550 μs in Fig. 12). However, the string cavitation only
Fig. 11. String cavitation and spray cone angle of the Min-sac and VCO nozzles (Pinjection = 60 MPa). 568
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Fig. 12. Transient cavitating flow in a single tapered orifice VCO nozzle (Pinjection = 60 MPa).
would last for the ending stage of injection and the intensity of string cavitation becomes increasingly stronger with large spray cone angles as time goes on (Figs. 11 and 12). Conversely, the string cavitation does not obviously appear in Min-sac nozzle at the needle closing stage. However, because of more and more strong vortex and turbulence flow caused by the reduced flow channel, the spray cone angle still become larger than before (Figs. 11 and 13). Afterwards, the needle valve closed rapidly, the fuel pressure of sac immediately decreases to the saturation pressure of diesel fuel, so that fuel vapor instantaneously generates and fills up sac and nozzle orifice. The ensemble average images of string cavitation were post-processed by MATLAB code as well (Fig. 14). Firstly, what needs to be emphasized is that the dark color in outlet was caused by the boundary of optical nozzle. Thanks to different sac geometries, the string cavitation intensity has a quite large difference. It is obviously to see that the string cavitation of Min-sac nozzle only occurs in the sac volume with slight intensity and rarely occurs at the outlet (Fig. 14a).
develops a little to the inlet of nozzle orifice and keeps a very short life time, then vanishes in Min-sac nozzle (Fig. 13). The weaker vortex and turbulence flow in Min-sac nozzle should be the main reason for this phenomenon. The current investigation results presents some differences with the results of T. Hayashi [35] that only the film-type cavitation was observed within the VCO cylindrical orifice nozzle and the prevailing string cavitation was observed within the Min-sac cylindrical orifice nozzle. In addition, M. Gavaises [36] published that the cylindrical orifice nozzle mainly exhibits the formation of cloud cavitation in the nozzle orifice and the string cavitation dominates the flow in tapered orifice at low needle lift, is similar to current results. The different nozzle orifice geometries maybe get rise to the diversities of results. For well understanding these diversities, there is still need the further experiment to be carried out and to verify it. When injection events come to the needle closing stage, the distinct differences between Min-sac and VCO nozzles still remain. Once the string cavitation reformed at time of 2090 μs in VCO nozzle (Fig. 12), it
Fig. 13. Transient cavitating flow in a single tapered orifice Min-sac nozzle (Pinjection = 60 MPa). 569
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Fig. 14. Ensemble average images of string cavitation in diesel nozzles with different sac (Pinjection = 60 MPa).
a longer time with a stronger intensity than that inside the Min-sac nozzle. 5) The ensemble average images of string cavitation were post-processed by MATLAB code and analyzed in details. Based on different flow transition positions, reasons for the intensity diversity and occurrence positions of string cavitation in different nozzles were provided.
Conversely, the VCO nozzle presents remarkable difference that string cavitation occurs in whole nozzle nearby orifice axis (Fig. 14b). Especially, the string cavitation of inlet area and outlet area is much stronger. Compared with the situations of Min-sac nozzle, the highest occurrence probability of string cavitation in VCO nozzle is not located in the sac area but located in the inlet area of nozzle orifice. Perhaps, the diversity of relative positions between sac and needle valve caused the flow difference. The Min-sac nozzles have a larger sac volume in comparison to Min-sac nozzles and VCO nozzles, so that the needle valve cone of sac nozzle could guide fuel flow to sac area fluently until the fuel flow encounters the fierce flow transition at the inlet area. Therefore, the string cavitation may not occur. Relatively, the Min-sac nozzle has a smaller sac volume. As a consequence, the fuel flow is not yet fluently guided to sac area by needle valve cone, it turns to the inlet and lead to flow transition, so that the string cavitation only occurs in sac area. In the same way, due to the lack of sac volume, the fierce flow transition occurs at the inlet area so that the string cavitation is prominent at inlet in VCO nozzle.
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51776088), a Project Funded by the Priority Academic Program Development of Jiangsu High Education Institutions and Natural Science Foundation of Jiangsu Province (BK20161349). References [1] Wei H, Chen X, Wang G, Zhou L, An S, Shu G. Effect of swirl flow on spray and combustion characteristics with heavy fuel oil under two-stroke marine engine relevant conditions. Appl Therm Eng 2017;124:302–14. [2] Natarajan S, Shankar SA, Sundareswaran AUM. Early injected PCCI engine fuelled with bio ethanol and diesel blends – an experimental investigation. Energy Procedia 2017;105:358–66. [3] Zhang F, Liu HF, Yu J, Yao M. Direct numerical simulation of n-heptane/air autoignition with thermal and charge stratifications under partially-premixed charge compression ignition (PCCI) engine related conditions. Appl Therm Eng 2016;104:516–26. [4] Su W, Yu W. Effects of mixing and chemical parameters on thermal efficiency in a partly premixed combustion diesel engine with near-zero emissions. Int J Engine Res 2012;13(3):188–98. [5] Turkcan A, Ozsezen AN, Canakci M. Experimental investigation of the effects of different injection parameters on a direct injection HCCI engine fueled with alcohol–gasoline fuel blends. Fuel Process Technol 2014;126(5):487–96. [6] Jacobs TJ, Assanis DN. The attainment of premixed compression ignition lowtemperature combustion in a compression ignition direct injection engine. Proc Combust Inst 2007;31(2):2913–20. [7] Schmidt DP, Corradini ML. The internal flow of diesel fuel injector nozzles: a review. Int J Engine Res 2001;2(1):1–22. [8] Martínez-Martínez S, Sánchez-Cruz FA, Riesco-Ávila JM, Gallegos-Muñoz A, Aceves SM. Liquid penetration length in direct diesel fuel injection. Appl Therm Eng 2008;28(14–15):1756–62. [9] Payri R, García JM, Salvador FJ, Gimeno J. Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel 2005;84(5):551–61. [10] Westlye FR, Battistoni M, Skeen SA, Manin J, Pickett LM, Ivarsson A. Penetration and combustion characterization of cavitating and non-cavitating fuel injectors under diesel engine conditions. SAE 2016 World Congress and Exhibition. 2016:380–3. [11] Badock C, Wirth R, Fath A, Leipertz A. Investigation of cavitation in real size diesel injection nozzles. Int J Heat Fluid Flow 1999;20(5):538–44. [12] Jiang G, Zhang Y, Wen H, Xiao G. Study of the generated density of cavitation inside diesel nozzle using different fuels and nozzles. Energy Convers Manage 2015;103:208–17. [13] Qiu T, Song X, Lei Y, Liu X, An X, Lai M. Influence of inlet pressure on cavitation flow in diesel nozzle. Appl Therm Eng 2016;109:364–72. [14] Gavaises M, Roth H, Arcoumanis C. Cavitation initiation, its development and link with flow turbulence in diesel injector nozzles. SAE Int J Engines 2002;111(3):561–80. [15] Gavaises M, Arcoumanis C, Flora H, Badami M. Cavitation in real-size multi-hole diesel injector nozzles. SAE Int J Engines 2000;109(3). [16] Zhang X, He Z, Wang Q, Tao X, Zhou Z, Xia X, et al. Effect of fuel temperature on cavitation flow inside vertical multi-hole nozzles and spray characteristics with different nozzle geometries. Exp Therm Fluid Sci 2017. [17] Yu W, Yang W, Mohan B, Tay K, Zhao F, Zhang Y, et al. Numerical and
4. Conclusions In this paper, the real-size optical nozzle tips with cylindrical and tapered orifice were designed and equipped to the high pressure common rail injector for visual investigation of string cavitation and spray characteristics. The agglomerated phenomenon of geometry-induced cavitation and the effects of nozzle geometry on string cavitation and spray characteristics were investigated in details. The investigation could provide abundant experimental data for well understanding the effects of string cavitation on diesel spray fluctuation. The results could also provide experimental data for further simulation which could reduce experimental costs and shorten the development cycle of nozzle design. Based on the better spray atomization induced by string cavitation, it maybe provides a new idea for better spray atomization by means of controlling the string cavitation instead of developing the higher-pressure fuel pump and smaller orifice nozzle. The conclusions are summarized as follows: 1) The vortex flow and string cavitation could agglomerate the geometry-induced cavitation in axis vicinity of the nozzle orifice. And the agglomerated geometry-induced cavitation contributes to the larger spray cone angle, as the string cavitation does. 2) The increase of spray cone angle is synchronized with occurrence of string cavitation. It is the string cavitation but not geometry-induced cavitation has a larger contribution to the increase of spray cone angle. 3) The tapered nozzle could preferably suppress the geometry-induced cavitation. Moreover, the nozzle orifice L/Do ratio plays an essential role for intensity of string cavitation. A small L/Do ratio means a more prevailing string cavitation in the nozzle, and then get rise to a larger spray cone angle. 4) The string cavitation is also directly influenced by nozzle sac types. Owing to the stronger vortex and turbulence intensity, the string cavitation inside the VCO nozzle appears earlier and could maintain 570
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[26] Sedarsky D, Idlahcen S, Rozé C, Blaisot JB. Velocity measurements in the near field of a diesel fuel injector by ultrafast imagery. Exp Fluids 2013;54(2):1–12. [27] Crua C, Heikal MR, Gold MR. Microscopic imaging of the initial stage of diesel spray formation. Fuel 2015;157:140–50. [28] Ding H, Wang Z, Li Y, Xu H, Zuo C. Initial dynamic development of fuel spray analyzed by ultra high speed imaging. Fuel 2016;169:99–110. [29] Andriotis A, Gavaises M, Arcoumanis C. Vortex flow and cavitation in diesel injector nozzles. J Fluid Mech 2008;610(610):195–215. [30] He Z, Shao Z, Wang Q, Zhong W, Tao X. Experimental study of cavitating flow inside vertical multi-hole nozzles with different length–diameter ratios using diesel and biodiesel. Exp Therm Fluid Sci 2015;60(60):252–62. [31] Suh HK, Chang SL. Effect of cavitation in nozzle orifice on the diesel fuel atomization characteristics. Int J Heat Fluid Flow 2008;29(4):1001–9. [32] Tao XC, Zhi-Xia HE, Zhong WJ, Guo GM, Zhang ZY. Experimental study of the effect of nozzle hole conicity on internal flow and spray characteristics. J Eng Thermophys 2016;71:1–8. [33] Moro A, Zhou Q, Xue F, Luo F. Comparative study of flow characteristics within asymmetric multi hole VCO and SAC nozzles. Energy Convers Manage 2017;132:482–93. [34] Salvador FJ, Carreres M, Jaramillo D, Martínez-López J. Comparison of microsac and VCO diesel injector nozzles in terms of internal nozzle flow characteristics. Energy Convers Manage 2015;103:284–99. [35] Hayashi T. Analysis of Internal Flow and Spray Formation with Real Size Diesel Nozzle. 微粒化 = Atomization: J ILASS-Japan 2012;21:92–6. [36] Gavaises M, Andriotis A, Papoulias D, Mitroglou N, Theodorakakos A. Characterization of string cavitation in large-scale Diesel nozzles with tapered holes. Phys Fluids 2009;21(5):114.
Experimental Study on Internal Nozzle Flow and Macroscopic Spray Characteristics of a Kind of Wide Distillation Fuel (WDF) - Kerosene. SAE 2016 World Congress and Exhibition. 2016. Yu W, Yang W, Mohan B, Tay KL, Zhao F. Macroscopic spray characteristics of wide distillation fuel (WDF). Appl Energy 2015. Mitroglou N, Gavaises M, Nouri JM, Arcoumanis C. Cavitation inside enlarged and real-size fully transparent injector nozzles and its effect on near nozzle spray formation. Proc Dipsi Workshop Droplet Impact Phenom Spray Investig 2011;10(4):525–89. Mitroglou N, Mclorn M, Gavaises M, Soteriou C, Winterbourne M. Instantaneous and ensemble average cavitation structures in Diesel micro-channel flow orifices. Fuel 2014;116(1):736–42. Sun ZY, Li GX, Chen C, Yu YS, Gao GX. Numerical investigation on effects of nozzle’s geometric parameters on the flow and the cavitation characteristics within injector’s nozzle for a high-pressure common-rail DI diesel engine. Energy Convers Manage 2015;89(9):843–61. He Z, Zhang Z, Guo G, Wang Q, Leng X, Sun S. Visual experiment of transient cavitating flow characteristics in the real-size diesel injector nozzle. Int Commun Heat Mass Transfer 2016;78:13–20. Gavaises M, Arcoumanis C, Roth H, Gianadakis E, Omae K, Sakata I, et al. Effect of multi-injection strategy on cavitation development in diesel injector nozzle holes. SAE Int J Engines 2005;114(3):1029–45. Watanabe H, Nishikori M, Hayashi T, Suzuki M, Kakehashi N, Ikemoto M. Visualization analysis of relationship between vortex flow and cavitation behavior in diesel nozzle. Int J Engine Res 2014;16(1):5–12. Payri R, Viera JP, Gopalakrishnan V, Szymkowicz PG. The effect of nozzle geometry over internal flow and spray formation for three different fuels. Fuel 2016;183:20–33.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Experimental study on the effect of nozzle geometry on string cavitation in real-size optical diesel nozzles and spray characteristics
T
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Zhou Chena, Zhixia Heb, , Weiwei Shanga, Lian Duanb, Han Zhoua, Genmiao Guoa, Wei Guana a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Diesel injector nozzle Visualization experiment String cavitation Nozzle geometry Spray characteristics
Cavitation is quite important for the diesel spray atomization and the combustion of air–fuel mixture. In this study, a high-speed CMOS camera equipped with a long-distance microscope was utilized to capture the transient cavitating flow and spray characteristics in real-size optical nozzles with needle motion. The transient cavitation images, including geometry-induced cavitation and vortex-induced string cavitation, were captured clearly in cylindrical-orifice nozzles and tapered-orifice nozzles, respectively. Besides, the agglomerated phenomenon of geometry-induced cavitation was visually captured and analyzed for the first time. It was found that the string cavitation in nozzle excites the instability of spray cone angle and it is synchronized with increase of spray cone angle. In addition, it is the string cavitation but not geometry-induced cavitation has a much larger contribution to the increase of spray cone angle. It is interesting that the influence of agglomerated geometryinduced cavitation on spray cone angle was prominent. Furthermore, both the nozzle orifice L/D ratio and sac types have significant influences on string cavitation and spray characteristics. The smaller L/D ratio and VCOtype nozzles are prone to incur the stronger string cavitation, and then spray cone angle is obviously larger.
1. Introduction Pollutant emission reduction is currently considered to be one of the most important challenges of our society. The spray atomization characteristics of the diesel injector plays an essential role for the good spray combustion and pollutant emissions performance [1]. New combustion modes have been suggested for energy-saving and emission-reduction, such as premixed charge compression ignition (PCCI) [2,3], partially premixed combustion (PPC) [4] and homogeneous charge compression ignition (HCCI) [5] and low temperature combustion (LTC) [6]. Certainly, it is indispensable for realizing these new combustion modes to prepare the suitable combustible mixture. Actually, diesel injector nozzles have a significant influence on the quality of spray and preparation of air–fuel mixture [7]. It is well known that fuel atomization processes is directly influenced by the nozzle geometry and cavitating two-phase flow [8,9]. Especially, the near-field spray atomization is extremely influenced by cavitation and turbulence in nozzles [10]. Therefore, it is indispensable to conduct the investigation of cavitating flow in diesel nozzles. Especially, the string cavitation, has a great contribution to spray cone angle, deserves more attention in the future. Focus on the geometry-induced cavitation in diesel nozzles, many
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Corresponding author. E-mail address: [email protected] (Z. He).
https://doi.org/10.1016/j.fuel.2018.05.132 Received 4 April 2018; Received in revised form 24 May 2018; Accepted 26 May 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
studies have been reported [11–13]. Gavaises et al. [14,15] utilized the optical nozzles to explore the effects of the needle motion on the transient cavitating flow. Experiments indicated the links between cavitation and turbulence in the sac, moreover, the anticipated enhancement of turbulence through the onset of cavitation was identified only at the entrance of the nozzle orifice [14]. In addition, the differences of flow characteristics between the real-size nozzles and largescale transparent nozzles were stressed [15]. Based on the large-scale nozzle, Xin Zhang et al. [16] reported that the higher fuel temperature means a lower critical injection pressure for cavitation inception, in addition, the higher fuel temperature could induce the larger cavitation region under the same injection condition. However, the scale effects could not be negligible in large-scale transparent nozzles. Therefore, the experiments and simulations based on the real-size diesel nozzle were performed. W. Yu et al. [17,18] proved that the injection pressure and ambient pressure could influence both the internal nozzle flow and the spray propagation. Mitroglou et al. [19,20] studied the cavitating flow in diesel optical nozzles as well and found that the initial bubbles exist inside the nozzle before start of injection. Besides, the life time and the most probable appearance location of string cavitation have been estimated. Besides, the effects of several factors on geometry-induced cavitation were investigated by CFD [21], including injection pressure,
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Nomenclature
Pinjection
K-factor D L Istring ρ V A Dsac Sstring Sorifice
Subscripts
conicity factor of nozzle hole nozzle hole diameter nozzle hole length intensity of string cavitation mean density of fuel mean velocity of fuel cross section area of nozzle orifice sac diameter of nozzle area of string cavitation in 2D image area of nozzle orifice in 2D image
o in
injection pressure
outlet of nozzle orifice inlet of nozzle hole orifice
Abbreviation VCO L/D
valve closed orifice nozzle length-diameter ratio of nozzle orifice
frames per second with 384 × 160 pixels. For the sake of capturing higher resolution images and larger shooting rate, the shooting rate is 100,000 frames per second and the images have a resolution of 640 × 280 pixels in this experiment. The 0# fossil diesel fuel (Automobile diesel fuelV, China) was employed in this experiment. The properties of diesel fuel and test conditions are given in Table 1. The ambient pressure of all injections is atmospheric pressure. Under such an injection condition, the turbulence and cavitation in nozzle were considered to be the main reasons of near-field spray atomization [10].
roundness of nozzle inlet, discharge coefficient of orifice, the L/D ratio, and the roughness of orifice inner wall. Although the investigations based on the real diesel nozzle have made large progress, there is little mention of vortex-induced string cavitation and its effect on spray characteristics. Focus on string cavitation, He et al. [22] subsequently found that the string cavitation has a strong relationship with the location of needle, the injection pressure, and the sac size. Applying a VCO nozzle (Valve closed orifice nozzle) [23], the effect of multi-injection strategy on cavitation development in diesel injector nozzle orifices was studied. After that, Watanabe et al. [24] explored the effect of needle tip shape on string cavitation and the spray characteristics. Furthermore, the spray characteristics based on metal nozzles were discussed in order to get a closer operation condition to diesel engine. Westlye and Payri [10,25] found that a tapered orifice had a higher spray tip penetration compared to a cylindrical nozzle orifice. However, it showed that the spray cone angle was inversely proportional to the spray tip penetration. Sedarsky et al. [26] researched the spray velocity profile from a high-pressure single-orifice diesel injector, and the results revealed a strong asymmetry in the spray profile of the test injector, there are distinct fast and slow regions on opposite sides of the spray. Crua and Ding [27,28] used single-orifice metal nozzles to investigate diesel spray formation, spray transient fluctuation and mushroom spray tip at initial stage of injection. However, because of absence of visible data about nozzle transient flow, above spray phenomena cannot be well explained and all spray characteristics have not been well connected with the nozzle internal flow. In general, previous studies have made numerous interesting achievements in multi-phase nozzle flow. But several aspects, such as the agglomerated phenomenon of geometry-induced cavitation, the occurrence mechanism of string cavitation and their effects on subsequently initial spray characteristic, are still not clear and deserve further investigations. In this paper, the real-size optical nozzle tips, equipped to the high pressure common rail fuel injector, were made for visual investigations of string cavitation characteristics with considering the effects of geometric parameters of nozzle, including L/D ratio, conicity factor (Kfactor), and nozzle sac geometry. Furthermore, the effects of those parameters and the agglomerated phenomenon of geometry-induced cavitation on near-field spray characteristics were analyzed in details. Finally, the ensemble average images of string cavitation were analyzed as an important statistical description.
2.2. Optical nozzle tips The metal nozzle tip of solenoid injector was cut and replaced by the real-size optical acrylic nozzle tip shown in Fig. 2, so that the transient cavitating flow inside the nozzle could be captured visually. For guaranteeing the good sealing performance between needle valve and needle seat of nozzle during the injection process, the cutting position of origin nozzle tip must be below the sealing line (Fig. 2). All of the optical nozzles were made of raw acrylic cubes, because of its good light transmittance (> 92%), good compression resistance and a similar refractive index (1.48–1.52) to diesel fuel (1.49–1.51). The optical nozzles were manufactured by precision drilling machine. Subsequently, the optical nozzle tips and injector could be assembled by a fixture. Specifications of the test nozzles were shown in Table 2. It must be noted that when investigate effects of nozzle orifice L/D ratio on string cavitation, the nozzle 1 and nozzle 3 were reworked with shortening the orifice length after finishing experiment of nozzle 2 and nozzle 4, respectively. So that guarantee the same geometry of other nozzle
2. Experimental setup and methodology 2.1. Facilities and fuel Based on the 250 MPa common rail injection system (BITEC-GY2502), the nozzle flow and spray visualization experiment system was established (Fig. 1). According to the shadow photography, the tested nozzles were placed between the LED light and the high-speed COMS camera (FASTCAM SA-Z) equipped with a long-distance microscope (QM-1, QUESTAR). The maximum camera shooting rate is 210,000
Fig. 1. Schematic of visualization system for internal flow. 563
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utilizing of the background image without spray subtract the image with spray, the domain of spray could be got for the binary image of spray (Fig. 4a II). Based on the interface of fuel spray and ambient air in the binary image, the two fitting lines could be established (Fig. 4a III), and then according to the slopes of fitting lines, the spray cone angle can be solved successfully. The intensity of string cavitation (Istring) can be calculated by the following equation: Istring = Sstring/Sorifice (Sstring means the area of string cavitation in nozzles, Sorifice means the area of nozzle orifice in the nozzle flow 2d images). Certainly, area of nozzle hole can be easily obtained on the basis of calibration. Therefore, if obtained the area of string cavitation means that the intensity of string cavitation was obtained. As the Fig. 4b shown, the image without cavitation subtracts the image with cavitation to get cavitation region. According to threshold value (20) of MATLAB code, the binary image of string cavitation could be acquired (Fig. 4b III). The area of string cavitation was solved with accumulated the number of white pixels. Finally, the intensity of string cavitation (Istring) was resolved in accordance with the former equation. In order to obtain the ensemble average images of string cavitation, more than one hundred images of string cavitation were utilized to get the ensemble average images of string cavitation by postprocessing. At first, all grey images of string cavitation were accumulated (Fig. 4c I). After that, the cumulative grey value of each pixel divided the image number. Therefore, the average grey image could be resolved (Fig. 4c II). Finally, selecting an appropriate range of RGB, the pseudo-color image can be acquired (Fig. 4c III).
Table 1 Properties of diesel fuel (293 K) and test conditions. Units Density Kinematic viscosity Distillation temperature (95%) Cetane number Sulfur Fuel temperature Ambient temperature Injection pressure Ambient pressure
Value 3
kg/m mm2/s K / mg/kg K K MPa MPa
832 5.8 629 52 10 293 293 50–80 0.1
areas. In other word, the only difference between nozzle 1 and nozzle 2, also nozzle 3 and nozzle 4 is the orifice L/D ratio. Hence, the studies on L/D ratio could be credible and be of more significance. The K-factor of nozzle orifice was calculated by the equation (the units are micron): Kfactor = 100 ∗ (Din − Do)/L. 2.3. The typical image and postprocessing methodology The obtained typical image of cavitating flow by shadow photography is shown in Fig. 3. When light goes through the interface between vapor phase (cavitation and initial bubbles) and liquid phase, the light refraction happens so that vapor phase presents black as a result of different refractive index between diesel liquid and vapor phase. On the contrary, the diesel fuel liquid phase represents white on account of similar refractive index between liquid diesel and acrylic nozzle tip. In addition, the spray presents black because light is overshadowed by spray droplets. In this paper, in order to obtain a reliable experimental dataset, the experiments were conducted more than 20 times at each operation condition. Compared with the results under the same operation condition, the time differences of cavitation development are not more than 20 μs (two images corresponding to 10 μs under shooting rate of 100,000 fps). Furthermore, the string cavitation images from the 20time injections were used for the ensemble average images of string cavitation. As Fig. 4 shown, all experimental data, including needle lift, spray cone angle, intensity of string cavitation (Istring) and ensemble average images of string cavitation, were post-processed by MATLAB code. The post-processing are as follows in details. The needle lift could be easily obtained according to calculated the pixel number of needle motion and combination with the calibration of pixel size. The postprocessing of spray cone angle was shown in Fig. 4a. Firstly, the MATLAB code selected the area of spray within two millimeters distance from the nozzle outlet for postprocessing (Fig. 4a I). And then,
3. Results and discussion 3.1. Transient cavitating flow in cylindrical orifice nozzles The transient cavitating flow and spray images during a whole injection event with the injection duration of 2000 μs under the injection pressure of 60 MPa in nozzle 1 were shown in Fig. 5. The graphs below the purple line show the enlarged local nozzle flow images of 4 typical injection times (430 μs, 900 μs, 2070 μs and 2090 μs) for illustrating a special agglomerated phenomenon of cavitating flow. Three optical nozzle replicates were fabricated for verifying the agglomerated phenomenon of cavitating flow. Paper chose the stage when the bubble suction happened as the start of injection. It is common to see that the initial bubbles are prevailing in the nozzle during initial injection stage, and the injection would experience the bubble suction and bubble compression on account of suddenly movement of needle valve at the opening stage. Hereafter, the geometry-induced cavitation appears and extends to the outlet (270 μs). And it is interesting to see that the geometry-induced cavitation could be agglomerated in the vicinity area of nozzle axis (360 μs),accompanied with markedly increasing of spray
Fig. 2. The raw acrylic cube, origin diesel nozzle and real-size optical nozzle. 564
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Table 2 Specifications of the test nozzles. Nozzle
Orifice number
Din/mm
Do/mm
L/mm
L/Do ratio
Dsac/mm
Nozzle type
K-factor
1 2 3 4 5 6
1 1 2 2 1 1
0.18 0.18 0.29 0.29 0.31 0.31
0.18 0.18 0.21 0.22 0.22 0.22
1.9 1.2 2.0 1.75 2.3 2.3
10.5 6.5 9.5 8.0 10.5 10.5
1.0 1.0 1.0 1.0 1.0 /
Min-sac Min-sac Min-sac Min-sac Min-sac VCO
0 0 4 4 4 4
(500 μs), the spray cone angle further increase to 31°, which is almost three times spray cone angle (10°, 900 μs) of higher needle lift stage. Although the geometry-induced cavitation always exists in nozzle hole during higher needle lift stage (700–1800 μs), the spray cone angle remains approximate 10°. The cavitating flow and spray at the needle closing stage (Fig. 5c) shows the similar results with that of the needle opening stage (430–480 μs). Besides, when the string cavitation occurs in nozzle sac (Fig. 5d, 2090 μs), the geometry-induced cavitation is also agglomerated in the vicinity area of nozzle axis and shows a similar shape feature to the cavitating flow at the needle opening stage (Fig. 5a). Therefore, the agglomerated geometry-induced cavitation is also regarded as a special string cavitation which is transformed from geometry-induced cavitation under the function of vortex flow in the nozzle orifice. Owing to the lower needle lift causing the narrow flow channel, so that the vortex flow intensity cold be stronger at lower needle lift stage. In other words, the vortex flow and string cavitation (vortex cavitation) in nozzle sac could agglomerate the geometry-induced cavitation to form this kind of string cavitation (agglomerated
Fig. 3. The typical image of cavitating flow by shadow photography.
cone angle, just like Fig. 5a shown. When the cavitating flow develops to 430–480 μs, the agglomerated phenomenon of geometry-induced cavitation becomes more obvious and the spray cone angle increases to 29° (Fig. 5a). Owing to occurrence of string cavitation in nozzle sac
Fig. 4. Postprocessing methodology for spray cone angle, intensity of string cavitation and ensemble average images of string cavitation. 565
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Fig. 5. Transient cavitating flow in single cylindrical orifice nozzle with mini sac (Pinjection = 60 MPa).
spray characteristics was firstly investigated by nozzle 1 and nozzle 2 with cylindrical orifices. The experiment was conducted with injection duration of 2000 μs under injection pressure of 50 MPa (Fig. 7). The results clearly indicate that the string cavitation is more prevailing with decreasing of the L/Do ratio. Besides, the spray cone angle with smaller L/Do ratio is almost larger than the spray cone angle with larger L/Do ratio. When the fuel goes through the small-L/Do-ratio nozzle orifice, owing to lower flow resistance, the velocity of fuel would be higher that
geometry-induced cavitation) in the axis vicinity of nozzle hole. Just similar to Andriotis’ study [29] that the pressurized air was sucked into vortex flow and finally developed into the string-like structure. The variations of the spray cone angle and string cavitation with needle movement are shown in Fig. 6. Considering that the geometryinduced cavitation dominates the nozzle flow (Fig. 5), it is unsuitable to describe string cavitation with intensity of string cavitation (Istring) which was defined before. Therefore, occurrence of string cavitation (Fig. 6) was utilized to explore the relationship among needle lift, string cavitation, and spray cone angle. It is not hard to find that the increase of spray cone angle is synchronized with occurrence of string cavitation. However, although geometry-induced cavitation exists all the time during the higher needle lift stage (just like 900 μs & 1500 μs in Fig. 5), the spray cone angle was evidently smaller than that of the lower needle lift stage. The results manifest that it is the string cavitation but not geometry-induced cavitation has a much larger contribution to the increase of spray cone angle. Essentially, the string cavitating flow originates from strong vortex flow where nozzle flow is accompanied by large rotating velocity. When the vortex flow runs from nozzle orifice into atmosphere and would lose the constraint of nozzle wall, the centrifugal force would enhance fuel spray atomization. As a result, the spray cone angle increased evidently.
3.2. The effects of nozzle orifice L/Do ratios on string cavitation and spray characteristics The nozzle orifice L/Do ratio is considered to be one of the most important factors affecting nozzle flow and subsequent spray characteristics [30,31]. However, the former investigations mostly focused on the geometry-induced cavitation. Rarely did researchers report the effect of L/Do ratio on string cavitation in diesel nozzles. Thus, the effect of nozzle orifice L/Do ratio on string cavitation and
Fig. 6. The spray cone angle and the string cavitation in single cylindrical orifice nozzle with mini sac (Pinjection = 60 MPa). 566
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ratio nozzle, there is no obvious hole-to-hole string cavitation to be captured in large-L/Do-ratio nozzle during the high needle lift stage (1000 μs). The reason for the flow difference probably is that the higher flow resistance for the large-L/Do-ratio nozzle means a lower fuel velocity and a weaker intensity of vortex and turbulence. For the purpose of well understanding the occurrence position of string cavitation, more than 100 images of string cavitation were postprocessed for getting the ensemble average images of string cavitation by MATLAB code. The Fig. 10 shows the statistical average descriptions of string cavitation, the red area represents cavitation and white area represents liquid fuel. As mentioned above, different L/Do ratios get rise to the differences of string cavitation intensity. Although there still remains large difference in string cavitation intensity between two nozzles, the string cavitation mostly appears at the sac area and nozzle outlet area with stronger cavitation intensity but a weaker intensity at inlet area of nozzle hole. During the lower needle lift period, the fuel from sac area to nozzle inlet would experience a large flow transition, hence the vortex intensity gets naturally stronger in sac area. When the local pressure in sac was below the saturated vapor pressure of fuel, the string cavitation prominently appeared at sac area with a stronger intensity. After that, when fuel comes to nozzle outlet area, the fuel velocities of different nozzle position should be different in tapered nozzle orifices. According to the continuity equation: ρo Vo Ao = ρin Vin Ain , the cross section of nozzle outlet ( AO ) is smaller in tapered orifices. Furthermore, owing to the larger gas volume fraction in outlet, the mean density ( ρo ) could be smaller in outlet areas. Certainly, the outlets of tapered orifices have the largest fuel velocity (Vo ) for the same mass flow. Therefore, the strongest vortex and turbulence would form at outlet areas and stimulate the strongest string cavitation. It well illuminates why outlet areas have the strongest string cavitation. Simultaneously, the stronger string cavitation would further narrow the flow channel cross section, this phenomenon may enlarge the velocity differences in tapered nozzle orifice.
Fig. 7. Effects of L/Do ratios on string cavitation in cylindrical nozzle with mini sac (Pinjection = 50 MPa).
could strengthen the intensity of vortex and turbulence. Reasonably, it presents prevailing string cavitation and larger spray cone angle. Different from nozzle 1 and nozzle 2, nozzle 3 and nozzle 4 were designed to have the tapered orifices. And the influence of orifice L/Do ratio on string cavitation and spray characteristics was investigated once again in these tapered-orifice nozzles with the injection duration of 1800 μs under injection pressure of 80 MPa (Fig. 8). The curves manifest a similar result as former study shown in cylindrical-orifice nozzles (Fig. 7). The fluctuation of spray cone angle is strongly influenced by string cavitation. Obviously, the intensity of sting cavitation has been strengthened a lot inside the nozzle orifice with smaller L/Do ratio. Certainly, the spray cone angle is larger as well on account of stronger sting cavitation. It is worth stressing that the hole-to-hole string cavitation, which is analyzed in Fig. 9(a) in detail, only occurs in smaller L/D ratio nozzle at full needle lift stage. In conclusion, the L/Do ratio of nozzle orifice plays a crucial role in string cavitation, whether it is the cylindrical-orifice nozzles or tapered-orifice nozzles. Benefited from that the tapered nozzle orifice could well suppress the geometry-induced cavitation [32], the string cavitation was clearly visualized without the interference of geometry-induced cavitation (Fig. 9). Therefore, it would be better to utilize the intensity of string cavitation (Istring) to discuss the relationship among needle lift, string cavitation and spray cone angle. It is easy to see that the geometryinduced cavitation could be preferably suppressed by the tapered orifice nozzle (Fig. 9). Moreover, the intensity of string cavitation in small-L/Do-ratio nozzle was much stronger (Fig. 9a). Besides, the string cavitation is originated from needle seat during the lower needle lift stage (500 μs & 1700 μs), this string cavitation could be called needleoriginated string cavitation. Nevertheless, the other kind of string cavitation, which is originated from one orifice and extends through sac volume into the other orifice during the full needle lift stage (1000 μs in Fig. 9a), could be called hole-to-hole string cavitation [14]. In contrast, the fluctuation of spray cone angle induced by hole-to-hole string cavitation is smaller than that induced by needle-originated string cavitation on account of weaker string cavitation. One of reasons could be that the higher needle lift made a larger flow channel for fuel flow in nozzle sac. Therefore, the vortex intensities and turbulence could be weaker in nozzle sac. Furthermore, the hole-to-hole string cavitation is hard to extend to nozzle outlet so that it could not directly affect the spray. As a result, the obtained spray cone angle is smaller. By contrast, things are different for a large-L/Do-ratio nozzle as the Fig. 9(b) shown. Except that the needle-originated string cavitation is apparently weaker at the low needle lift stage in comparison with that in the small-L/Do-
3.3. Transient cavitating flow characteristics of Min-sac and VCO nozzles Nozzle sac types could also largely influence the cavitating flow characteristics [33,34], and its influence on string cavitation is worth further investigation on account of lacking experimental investigation. Two Min-sac and two VCO nozzle replicates were respectively fabricated for verifying the effects of sac type on string cavitation. Utilizing nozzle 5 (Min-sac type) and nozzle 6 (VCO type) with the fuel injection
Fig. 8. Effects of L/Do ratios on string cavitation in tapered nozzle with mini sac (Pinjection = 80 MPa). 567
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Fig. 9. String cavitation and spray characteristics of tapered nozzles with different L/Do ratios (Pinjection = 80 MPa).
Fig. 10. Ensemble average images of string cavitation in diesel nozzles with different L/D ratios (Pinjection = 80 MPa).
duration of 2000 μs under injection pressure of 60 MPa, the effect of nozzle sac geometry on transient cavitating flow characteristics was investigated (Fig. 11). Both optical nozzles only have one tapered orifice, so that only the needle-originated string cavitation exists at low needle lift stage (Fig. 12) (The hole-to-hole string cavitation only occurs in multi-hole nozzle). As Fig. 11 shown, the string cavitation in the VCO nozzle occurs earlier and maintains a longer time than that in the Minsac nozzle. Accordingly, the spray cone angle is comparatively larger in the VCO nozzle than that in Min-sac nozzle. The flow experiences extremely violent flow transition at the inlet location of VCO nozzle orifice. Additionally, the VCO nozzle provides a narrow flow channel for fuel flow, so that the vortex and turbulence intensities and the local low pressure were strengthened here. As a result, the stronger string cavitation spontaneously appears and last a longer time in VCO nozzle. The transient internal flow and corresponding spray images of the Min-sac nozzle and VCO nozzle with a single tapered orifice were shown in Figs. 12 and 13, respectively. First of all, as the Fig. 12 shown, the string cavitation inception appears at 240 μs almost simultaneously with the compression process of sucked bubbles in the VCO nozzle. Moreover, it is interesting that the initial bubbles could be sucked into the vortex flow and form a part of string cavitation (240 μs, 250 μs in Fig. 12). When the string cavitation develops to the outlet, the spray cone angle is remarkably increased immediately (300 μs in Fig. 12). In contrast, the string cavitation inception appears much later at 360 μs in the Min-sac nozzle (Fig. 13). The stronger vortex and turbulence flow in VCO nozzle were considered to be the main reason for this difference between two type nozzles. Certainly, the initial bubbles, also provide numerous gas nuclei, could stimulate the string cavitation inception. Compared to the VCO nozzle geometry, the Min-sac nozzle has a comparatively larger sac volume, and then the flow transition is much
weaker. Besides, the string cavitation gradually becomes weaker with the increasing of needle lift after injection time of 300 μs in the VCO nozzle. And then, the string cavitation gradually vanishes after injection of 400 μs. The string cavitation firstly vanishes at the middle location of nozzle orifice, later at the orifice outlet, and the orifice inlet followed (400 μs, 450 μs, 550 μs in Fig. 12). However, the string cavitation only
Fig. 11. String cavitation and spray cone angle of the Min-sac and VCO nozzles (Pinjection = 60 MPa). 568
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Fig. 12. Transient cavitating flow in a single tapered orifice VCO nozzle (Pinjection = 60 MPa).
would last for the ending stage of injection and the intensity of string cavitation becomes increasingly stronger with large spray cone angles as time goes on (Figs. 11 and 12). Conversely, the string cavitation does not obviously appear in Min-sac nozzle at the needle closing stage. However, because of more and more strong vortex and turbulence flow caused by the reduced flow channel, the spray cone angle still become larger than before (Figs. 11 and 13). Afterwards, the needle valve closed rapidly, the fuel pressure of sac immediately decreases to the saturation pressure of diesel fuel, so that fuel vapor instantaneously generates and fills up sac and nozzle orifice. The ensemble average images of string cavitation were post-processed by MATLAB code as well (Fig. 14). Firstly, what needs to be emphasized is that the dark color in outlet was caused by the boundary of optical nozzle. Thanks to different sac geometries, the string cavitation intensity has a quite large difference. It is obviously to see that the string cavitation of Min-sac nozzle only occurs in the sac volume with slight intensity and rarely occurs at the outlet (Fig. 14a).
develops a little to the inlet of nozzle orifice and keeps a very short life time, then vanishes in Min-sac nozzle (Fig. 13). The weaker vortex and turbulence flow in Min-sac nozzle should be the main reason for this phenomenon. The current investigation results presents some differences with the results of T. Hayashi [35] that only the film-type cavitation was observed within the VCO cylindrical orifice nozzle and the prevailing string cavitation was observed within the Min-sac cylindrical orifice nozzle. In addition, M. Gavaises [36] published that the cylindrical orifice nozzle mainly exhibits the formation of cloud cavitation in the nozzle orifice and the string cavitation dominates the flow in tapered orifice at low needle lift, is similar to current results. The different nozzle orifice geometries maybe get rise to the diversities of results. For well understanding these diversities, there is still need the further experiment to be carried out and to verify it. When injection events come to the needle closing stage, the distinct differences between Min-sac and VCO nozzles still remain. Once the string cavitation reformed at time of 2090 μs in VCO nozzle (Fig. 12), it
Fig. 13. Transient cavitating flow in a single tapered orifice Min-sac nozzle (Pinjection = 60 MPa). 569
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Fig. 14. Ensemble average images of string cavitation in diesel nozzles with different sac (Pinjection = 60 MPa).
a longer time with a stronger intensity than that inside the Min-sac nozzle. 5) The ensemble average images of string cavitation were post-processed by MATLAB code and analyzed in details. Based on different flow transition positions, reasons for the intensity diversity and occurrence positions of string cavitation in different nozzles were provided.
Conversely, the VCO nozzle presents remarkable difference that string cavitation occurs in whole nozzle nearby orifice axis (Fig. 14b). Especially, the string cavitation of inlet area and outlet area is much stronger. Compared with the situations of Min-sac nozzle, the highest occurrence probability of string cavitation in VCO nozzle is not located in the sac area but located in the inlet area of nozzle orifice. Perhaps, the diversity of relative positions between sac and needle valve caused the flow difference. The Min-sac nozzles have a larger sac volume in comparison to Min-sac nozzles and VCO nozzles, so that the needle valve cone of sac nozzle could guide fuel flow to sac area fluently until the fuel flow encounters the fierce flow transition at the inlet area. Therefore, the string cavitation may not occur. Relatively, the Min-sac nozzle has a smaller sac volume. As a consequence, the fuel flow is not yet fluently guided to sac area by needle valve cone, it turns to the inlet and lead to flow transition, so that the string cavitation only occurs in sac area. In the same way, due to the lack of sac volume, the fierce flow transition occurs at the inlet area so that the string cavitation is prominent at inlet in VCO nozzle.
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51776088), a Project Funded by the Priority Academic Program Development of Jiangsu High Education Institutions and Natural Science Foundation of Jiangsu Province (BK20161349). References [1] Wei H, Chen X, Wang G, Zhou L, An S, Shu G. Effect of swirl flow on spray and combustion characteristics with heavy fuel oil under two-stroke marine engine relevant conditions. Appl Therm Eng 2017;124:302–14. [2] Natarajan S, Shankar SA, Sundareswaran AUM. Early injected PCCI engine fuelled with bio ethanol and diesel blends – an experimental investigation. Energy Procedia 2017;105:358–66. [3] Zhang F, Liu HF, Yu J, Yao M. Direct numerical simulation of n-heptane/air autoignition with thermal and charge stratifications under partially-premixed charge compression ignition (PCCI) engine related conditions. Appl Therm Eng 2016;104:516–26. [4] Su W, Yu W. Effects of mixing and chemical parameters on thermal efficiency in a partly premixed combustion diesel engine with near-zero emissions. Int J Engine Res 2012;13(3):188–98. [5] Turkcan A, Ozsezen AN, Canakci M. Experimental investigation of the effects of different injection parameters on a direct injection HCCI engine fueled with alcohol–gasoline fuel blends. Fuel Process Technol 2014;126(5):487–96. [6] Jacobs TJ, Assanis DN. The attainment of premixed compression ignition lowtemperature combustion in a compression ignition direct injection engine. Proc Combust Inst 2007;31(2):2913–20. [7] Schmidt DP, Corradini ML. The internal flow of diesel fuel injector nozzles: a review. Int J Engine Res 2001;2(1):1–22. [8] Martínez-Martínez S, Sánchez-Cruz FA, Riesco-Ávila JM, Gallegos-Muñoz A, Aceves SM. Liquid penetration length in direct diesel fuel injection. Appl Therm Eng 2008;28(14–15):1756–62. [9] Payri R, García JM, Salvador FJ, Gimeno J. Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel 2005;84(5):551–61. [10] Westlye FR, Battistoni M, Skeen SA, Manin J, Pickett LM, Ivarsson A. Penetration and combustion characterization of cavitating and non-cavitating fuel injectors under diesel engine conditions. SAE 2016 World Congress and Exhibition. 2016:380–3. [11] Badock C, Wirth R, Fath A, Leipertz A. Investigation of cavitation in real size diesel injection nozzles. Int J Heat Fluid Flow 1999;20(5):538–44. [12] Jiang G, Zhang Y, Wen H, Xiao G. Study of the generated density of cavitation inside diesel nozzle using different fuels and nozzles. Energy Convers Manage 2015;103:208–17. [13] Qiu T, Song X, Lei Y, Liu X, An X, Lai M. Influence of inlet pressure on cavitation flow in diesel nozzle. Appl Therm Eng 2016;109:364–72. [14] Gavaises M, Roth H, Arcoumanis C. Cavitation initiation, its development and link with flow turbulence in diesel injector nozzles. SAE Int J Engines 2002;111(3):561–80. [15] Gavaises M, Arcoumanis C, Flora H, Badami M. Cavitation in real-size multi-hole diesel injector nozzles. SAE Int J Engines 2000;109(3). [16] Zhang X, He Z, Wang Q, Tao X, Zhou Z, Xia X, et al. Effect of fuel temperature on cavitation flow inside vertical multi-hole nozzles and spray characteristics with different nozzle geometries. Exp Therm Fluid Sci 2017. [17] Yu W, Yang W, Mohan B, Tay K, Zhao F, Zhang Y, et al. Numerical and
4. Conclusions In this paper, the real-size optical nozzle tips with cylindrical and tapered orifice were designed and equipped to the high pressure common rail injector for visual investigation of string cavitation and spray characteristics. The agglomerated phenomenon of geometry-induced cavitation and the effects of nozzle geometry on string cavitation and spray characteristics were investigated in details. The investigation could provide abundant experimental data for well understanding the effects of string cavitation on diesel spray fluctuation. The results could also provide experimental data for further simulation which could reduce experimental costs and shorten the development cycle of nozzle design. Based on the better spray atomization induced by string cavitation, it maybe provides a new idea for better spray atomization by means of controlling the string cavitation instead of developing the higher-pressure fuel pump and smaller orifice nozzle. The conclusions are summarized as follows: 1) The vortex flow and string cavitation could agglomerate the geometry-induced cavitation in axis vicinity of the nozzle orifice. And the agglomerated geometry-induced cavitation contributes to the larger spray cone angle, as the string cavitation does. 2) The increase of spray cone angle is synchronized with occurrence of string cavitation. It is the string cavitation but not geometry-induced cavitation has a larger contribution to the increase of spray cone angle. 3) The tapered nozzle could preferably suppress the geometry-induced cavitation. Moreover, the nozzle orifice L/Do ratio plays an essential role for intensity of string cavitation. A small L/Do ratio means a more prevailing string cavitation in the nozzle, and then get rise to a larger spray cone angle. 4) The string cavitation is also directly influenced by nozzle sac types. Owing to the stronger vortex and turbulence intensity, the string cavitation inside the VCO nozzle appears earlier and could maintain 570
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