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Faced with dual challenges of “carbon neutral” and emission control, fossil fuel-based internal combustion engines need to explore new ways and technical paths to reduce harmful emissions and Carbon dioxide emissions simultaneously. Fuel injection process is playing a significant role not only in traditional engines but also in new low/zero carbon engines. Multi-hole nozzles have a wide range of applications in the fuel supply system. While the accepted spray study work and jet break-up models are usually developed under the quasi-steady-state of fuel injection by a single-hole nozzle. There are rare models that can describe the whole break-up processes of multi-hole nozzle spray, including complex internal flow factors, plume interaction, and the effect of start/end of injection. In this chapter, characteristics of spray morphology, evolution processes, and evaporation characteristics, emerging from the practical diesel multi-hole nozzles, were discussed and analyzed during the transient injection processes in detail. Moreover, the relationship between multi-hole nozzle internal flow properties and the corresponding spray behaviors was investigated by numerical simulation method systematically. Therefore, multi-hole spray modeling processes under engine operating conditions and the optimized design of diesel multi-hole nozzles are expected to get some benefits and clues from the current results.
School of Energy and Power Engineering, Dalian University of Technology, DaLian, China
*Address all correspondence to: pengbo.dong@dlut.edu.cn
1. Introduction
The research on fuel sprays of Internal Combustion Engines has been drawing attention for last century. As for Diesel Engines, spray evolution, which includes the jet breakup, fuel atomization, air entrainment, and mixture formation processes, is regarded as one of the determinants of engine performance and emission formation. Furthermore, nozzle geometry can directly affect the characteristics of the complicated internal flow patterns inside the nozzles, such as the cavitated turbulence. As a result, in order to improve the quality of the atomization and mixture homogeneity, researchers have spared no effort to investigate the internal flow [1, 2, 3] and spray behaviors [4, 5, 6], and many fundamental and classic theories were formed over the past decades. Originally, due to the simple structure and the easiness of applying diagnostic techniques and arranging instruments surrounding the nozzle and the spray plume, single-hole nozzle was widely used in fundamental research. Consequently, spray and combustion models, which are adopted in the numerical simulation study are usually developed from the experimental results of the single-hole nozzle spray.
On the other hand, it is known that multi-hole nozzles, which can emerge several spray plumes simultaneously, are generally applied in the engineering field of practical Diesel Engines. In this case, the orifices are normally aligned around the sac symmetrically and located off-axis to the nozzle axis. As a result, the optical access to the complex configuration and the mutual interference between the spray plumes create a difficulty for scholars to explore the internal flow and spray characteristics of multi-hole nozzles.
In recent years, there is a trend that considerable efforts have been exerted to conduct an experimental and computational study about the internal flow and sprays of the multi-hole nozzles. Different kinds of verisimilar scaled-up or real-size multi-hole nozzle models were designed to reveal the flow pattern properties inside the nozzles and the initial emerging spray dynamics [7, 8, 9, 10, 11, 12]. The application of the synchrotron x-ray sources with high energy pulses has also been extended to the study of fuel sprays during the last decade [13, 14]. At the same time, a series of studies using the three-dimensional computational fluid dynamics simulations were also conducted to link the experimental results to the numerical calculation [15, 16, 17, 18]. Based on all the approaches mentioned above, the vortex flow and string-type cavitation inside the sac, the counter-rotating vortices, film-type and string-type cavitation inside the hole volume, the needle lateral oscillation effect, and the unstable spray behaviors were found in succession.
Moreover, smaller hole diameter and more holes, accompanied by higher injection pressure are becoming prevalent in dealing with the strict emission regulations and economic demand [19, 20], because it is proven to be able to generate better fuel atomization and more homogeneous fuel/air mixture [21, 22]. In the conventional study, hole diameter is usually larger than 0.10 mm. Even though a few researchers applied the micro-hole nozzle to conduct the fundamental experimental study about the mixture formation and combustion processes, mainly single-hole nozzles were used in their experiments. As a result, the limited nozzle types and experimental conditions hindered a more satisfactory situation in understanding the flow dynamics of the real multi-hole nozzles under practical operating conditions thoroughly. There is rare information about the spray properties of multi-hole nozzles with micro orifices in the current archives, and it is worthwhile to carry out a monographic study on this issue.
A comparison between the spray properties of the nozzles with different hole diameter is made in the present chapter to provide deep insights into the multi-hole nozzles sprays, which are formed in the real scenario of Diesel Engines, and sufficient analyses about the effect of the hole diameter were conducted systematically as well. In addition, the CFD simulation results for these different nozzle configurations were also presented in this chapter with the aim of correlating the observed spray behaviors to the internal flow properties inside the nozzles.
2.1 Experimental apparatus, procedures, and conditions for spray observation
High-speed video camera observations based on the Mie scattering were made for the sprays injected by the multi-hole injectors, and the specific experimental apparatus is shown in Figure 1. Figure 1(a) presents the global experimental arrangement, including the high-pressure chamber, fuel injection system, and optical system. A common rail injection system could generate the injection pressure up to 220 MPa, and high-pressure constant volume vessels with optically accessible quartz windows were employed to create the high-pressure ambient environment of Diesel fuel injection. A delay pulse generator (Stanford Inc., DG535) and an electronic control unit (ECU) were applied to control the image timing, injection time, and injection quantity. The optical path layout is shown in Figure 1(a). A xenon lamp (USHIO Corp., SX-UID501XAMQ) and two high-speed video cameras (Nac MEMRECAM HX-3, Photron FASTCAM-APX RS) were used to record the fuel injection processes.
Figure 1.
Experimental apparatus.
When it comes to the installation of the multi-hole injectors, the detailed information about the specially designed chamber head is shown in Figure 1(b). It is well known that the observation of the multi-hole nozzle sprays is difficult due to the conical structure formed by the plumes. In this study, the multi-hole injector was installed into this specialized chamber cover obliquely, as shown in Figure 1(c), to prevent spray interference. An appropriate angle between the axis of the multi-hole nozzle and the horizontal plane was designed to maintain that one of the spray plumes could be observed as vertically as possible.
The experimental apparatus for the injection rate measurement are presented in Figure 2.
Figure 2.
Schematic of injection rate measurement experiment apparatus.
In this study, the Bocsh rate of injection meter [23] was applied. The experimental conditions corresponded to the spray observation experiments, which will be introduced in the upcoming sections. The timing of the start of injection was determined by comparing the injection pause signal with the injection rate curve signal recorded in the oscilloscope.
Figure 3 shows the schematics of the nozzles applied in this study, the two multi-hole nozzles (10 holes) have the same configuration except for the hole diameter (D = 0.10 and 0.07 mm). The experimental conditions, which are shown in detail in Table 1, were determined in consideration of the real operation conditions of small Diesel engines.
Figure 3.
Schematic of different nozzles applied in experiments.
The setting of cameras is also presented in Table 1, and the experimental measurement was conducted at least 10 times for each condition. The test fuel was the JIS #2 diesel. The injection quantity was 2 mm3/hole. In order to keep the ambient gas density similar to that of the combustion conditions, for fundamental spray research of non-evaporation conditions, the ambient temperature and pressure were 300 K and 1.5 MPa, respectively.
2.2 Methods of image processing
The typical image processing processes and the definitions of spray properties in the current study are shown in Figure 4. The same processing method was applied to the spray images of different nozzles. The spray image taken under the baseline condition is shown here as an example. The central spray of each frame was characterized as the spray tip penetration (i.e., the maximum penetration distance of the spray, S), the corresponding angle of 100 times hole diameter length (i.e., the spray cone angle, θc), and the corresponding angle at the half point of S (i.e., the spray angle, θs). The parameters are plotted as a function of the time after start of injection and nozzles with different hole diameters.
Figure 4.
Image processing for spray properties measurement.
The spray images were processed to calculate their properties by the following steps. First, each frame was converted to an effective image by subtracting the background images taken without the spray injection. After that, the spray edge could be detected by using the binarization image, which is converted by a threshold algorithm. The colored edges of different recording times show that the spray contour and the temporal variations can be well captured spatially. The spray tip penetration of the central spray plume is determined by scanning the contours from the corresponding orifice point. In this way, the morphological algorithm could also be used to extract the parameters of spray cone angle and the spray angle. The near-field spray properties around the nozzle exit region were also captured by the same method introduced above.
One of the most important factors that can introduce uncertainty into the measurement results is the selection of the threshold. Compared with the single-hole nozzle, the hole-to-hole variation and the cycle-to-cycle variation of multi-hole nozzle sprays cause the image processing to be more sensitive to the algorithm. Hence, the assessment of the threshold value is necessary for the current study. Principles that are suitable for the threshold selection are summarized as (1) The threshold should ensure that the spray profiles are as similar to the raw images as possible; (2) The threshold should remove the background noises in the images; (3) The error should be within the cycle-to-cycle injection variations. As a result, the intensity threshold of 5 (the maximum intensity: 255) was selected in this study after a series of statistics.
2.3 Computational setting for internal flow study
In the current study, to aid the interpretation of the experimental results, the influences of the micro-hole diameter on the internal flow and cavitation characteristics have been numerically investigated by the commercial CFD Code FIRE Version 2017 (AVL). Figure 5 shows the computational meshes of the multi-hole nozzle with ten holes, and only one-tenth of the entire volumetric domain was selected considering the geometric periodicity, symmetry, and calculation timing.
Figure 5.
Computational meshes.
The specific settings for this computation have been listed in Table 2. The same setting was applied to the two different nozzle configurations to make comparisons between the internal flow patterns. The Reynolds Averaged Navier–Stokes Simulation (RANS) model and a four-equation k-ζ-f model developed from the standard k-ε model were adopted to simulate the turbulent flow. The k-ζ-f model introduces new transportation equations to describe the variable ζ which has a relationship with the turbulence viscosity. As a result, the property of anisotropic turbulence can be taken into the consideration. A multi-phase flow model was selected to approximate the fluid conditions inside the nozzles. Furthermore, a Linearized Rayleigh model [24] was used to express the cavitation bubble behaviors within the nozzle.
Item
Classification
Setting/Value
Model selection and Initial values
Turbulence model
K-ζ-F
Turbulence energy (m2/s2)
0.1
Turbulence length scale (m)
0.000001
Cavitation model
Linear cavitation model
Cavitation bubble density number
1.5 × 1018
Diesel saturated vapor pressure (Pa)
892
Initial boundary
Inlet boundary
Injection Pressure (MPa)
120
Export boundary
Ambient Pressure (MPa)
1.5
Mesh information
Needle-holder gap (mm)
0.002
The minimum cell size in the hole region (mm)
4.413 × 10−3
The maximum grid number (cells)
132,796
Table 2.
Computational setting.
Validation was conducted by taking the experimental results published by Blessing et al. [25] as the criteria, in which the characteristics of nozzles and boundary conditions covered many features of the current study, and the turbulence and cavitation models were proved reasonable before further computational studies. Moreover, the effect of mesh size was also taken into the consideration by making comparison between the different computational results from the meshes with different mesh sizes (7.769, 4.413, and 2.896 μm). During all the verification processes, it was proven that the distribution and occurrence of the cavitation could be predicted accurately relatively by this simulation.
Furthermore, aiming to make comparisons, the same transient needle-lift curve, as shown in Figure 6, measured from a similar type of multi-hole injector was applied to the two nozzle meshes.
Figure 7 shows the injection rate results of the two different nozzles under the conditions of Qinj = 2.0 mm3/hole. These two curves present apparent distinctions. It can be seen that the micro-hole conspicuously changes the previous regularity of the fuel injection. The injection rate of the micro-hole (0.07 mm) nozzle is much lower than that of the nozzle with 0.10 mm hole diameter when maintaining the same injection quantity per hole. While the injection duration of the micro-hole nozzle is prolonged a lot. Moreover, the initial stage of the injection duration attracts attention to analysis in detail, which is emphasized and enlarged in Figure 7. The injection rate of the nozzle with a micro-orifice is a little higher than that of the other one with larger orifices in the initial stage of injection. In fact, it is known that the fuel injection velocity and the effective flow area alter the injection rate simultaneously. Furthermore, the fuel injection velocity mainly depends on the upstream pressure in the sac, and the effective flow area is affected by the hole numbers, hole diameter, and the discharge coefficient. The theoretical flow area of the micro-hole nozzle, whose diameter is 0.07 mm, is much smaller than that of the normal one (0.10 mm), and its sac pressure discharge rate should also be much lower than that of the nozzle with normal holes. As a result, all the phenomena described above reveal that the effect of micro-holes plays entirely different roles in the injection rate at different injection stages. Specifically, the higher sac pressure is mainly caused by the relatively lower sac pressure discharge rate at the initial stage of injection, and the injection rate of this stage is dominated by the consequent higher flow velocity inside the micro-holes. However, at the middle and post-stage injection, the relatively larger effective flow area and the consequent higher mass flow rate inside the nozzle with 0.10 mm hole diameter mainly dominate the injection rate of the nozzles.
Figure 7.
The injection rate of different injectors.
Typical false-colored and temporal spray images of different nozzles are shown in Figure 8. According to the theoretical foundation of Mie scattering, the scattered light intensity is a symbolic characteristic of the droplet size and fuel concentration. The spray contours can help elucidate the effects of ambient gas entrainment and interactions between spray plumes [26].
Figure 8.
False-colored spray images of different injectors.
As analyzed before, even the total injection mass per hole is held constant, the fuel injection quantity of the 0.10 mm hole is larger than that from the micro-hole at the same timing ASOI. Based on the Mie-scattering principle, the intensity is in proportion to the droplet size and concentration. Hence, the global intensity of the central sprays of the multi-hole nozzle with normal hole diameter, which are shown in Figure 8(a), is much higher than that of the micro one, particularly at the beginning stage of fuel injection. The high-intensity area can even extend to the downstream region of the central spray. It follows that the fuel concentration of the micro-hole condition is leaner, and the atomization effect is better [27].
As for the spray morphology, the edges of the spray of 0.10 mm holes are irregular compared with the micro-hole one, and the hole spray plumes are flanked by evident wavelike contours. In the case of micro holes, the edges of the spray upstream regions are neater and more orderly than those of nozzles with larger hole diameters.
Moreover, central sprays show a large eccentricity in the spray tip around the end of injection (0.8 and 1.0 ms ASOI) under the larger-hole nozzle condition, and the adjacent two plumes also present asymmetrical morphology. The low-pressure regime between the sprays generated by the air entrainment can enhance the sprawling diffusion of the multi-hole nozzle sprays. The Coanda effect [28] plays significant role in these phenomena. However, under the 0.07 mm hole diameter condition, the sprays have well symmetry. Therefore, it is safe to say that decreasing the hole diameter can supposedly reduce the uncontrollability and instability of the sprays emerging from multi-hole nozzles. A more specific investigation and discussion about this phenomenon will be introduced in the upcoming sections.
It is known that the spray propagation distance is governed by the upstream pressure and the ambient conditions. The calculated sac pressure and the measured spray tip penetration variation of different nozzles is shown in Figure 9. The Bernoulli equation is used to calculate the corresponding averaged sac pressure variations based on the injection rate measurement results in Figure 7. The equation is written as below, where Qf is the injection rate, α is the average discharge coefficient, A represents the theoretical flow area, and Ps is the sac pressure.
Figure 9.
Calculated sac pressure and spray pemetration variation of different injectors.
Qf=αA2Ps−Pa/ρfE1
As for α, it has much relationship with the local flow area, which is a direct reflection of the cavitation intensity of nozzle orifices. Nurick [29] and Payri et al. [30] conducted their experiments under quasi-steady conditions and concluded that the flow discharge coefficient of the nozzle hole mainly depended on its cavitation number under the cavitated conditions.
CN=Pinj−PV/Pinj−PaE2
However, it is still difficult to get accurate quantitative results from the internal flow of a practical Diesel multi-hole nozzle, which is usually high pressure, high velocity, turbulent, and micro size. The theory developed under the quasi-steady condition was attempted to be expanded to the transient condition in the current study, and a comparative analysis was made between the different nozzles qualitatively to provide a reference for explaining different spray behaviors.
Throughout the injection duration, the pressure in the sac of the nozzle with micro-holes is all higher than that in the nozzle with normal holes. The different effective flow area, caused by the different hole diameter plays a significant role in this issue. As for the corresponding penetration result, it is in accordance with the injection rate variations analyzed previously. As the color arrows emphasize, the micro-hole nozzle spray tip penetration is longer at the initial stage. As the time elapses, the penetration of 0.10 mm hole nozzle passes over the micro-hole nozzle spray tip penetration. In different injection stages, it is the different factors (effective flow area or sac pressure) that dominate the spray propagation of nozzles with different holes. In the calibration processes of combustion system of Diesel engines, the fuel injection quantity, injection timing, and injection times/cycle are usually adjusted as the fuel supply strategy. The diffusion and deceleration of the multi-hole nozzle spray are usually associated with the transfer of the spray momentum to the turbulence energy [31]. Consequently, when concerning the optimization of the Diesel engine preference, effects of the micro-hole on fuel injection of multi-hole nozzles should be given attention emphatically.
Figure 10 shows the temporal variation of the spray angle and spray cone angle. It is evident that the spray angle is wider under the 0.1 mm hole condition. The maximum deviation appears at 0.1 ms ASOI, up to 11°. Furthermore, it seems that the micro-hole can exert more influence on the spray angle reduction compared with that on the spray tip penetration. On the other hand, different from the spray tip penetration results, no overlap happens among the two spray angle curves of nozzles with different holes. The simulation results in the upcoming section can be used to explain this phenomenon in depth.
Figure 10.
Spray angle and cone angle variation of different injectors.
The difference between the spray cone angles of the nozzles with different holes is also evident. It should be noted that the penetration of the larger-hole nozzle is shorter than 100 times the hole diameter at 0.1 ms ASOI; hence, only the spray angle can be measured, as shown in the figure. Because the position (10 mm) of 100 times of hole diameter is around the spray tip area of the 0.10 mm hole, it results in a small value of spray cone angle, and the spray cone angle difference between the two nozzles is not too much at 0.2 ms ASOI. After that, when the spray penetrates long enough, attributed to the more completed internal flow inside the larger holes [32], the 0.10 mm hole nozzle spray cone angle increases a great deal suddenly.
When it comes to the micro-hole condition, the value of the spray cone angle is much smaller and the variation is steadier. This can be explained like that since the cavitation collapse and turbulence flow inside different nozzles are two of the major mechanisms of the spray primary break-up [31, 33], the fuel jet enters the chamber with a less cavitation level, reduced mass flow rate, momentum, and less turbulence caused by the increased ratio of nozzle hole length to diameter, which can result in the narrower spray cone angle.
According to the above discussion, because of the unique geometric structure, the micro-hole nozzle has a lower injection rate, higher sac pressure, and wider spray angle and spray cone angle compared to those of the nozzle with normal holes. Combing these phenomena with the interlaced relationship between the trends of the spray tip penetration of different nozzles, it is concluded that the effect of the micro-hole on different spray properties is discrepant.
3.2 Characteristics of the near-field spray
In order to investigate the spray behaviors near the nozzle tip region in detail, high-speed imaging of 100,000 fps was applied to take a close-up view of this regime. The Higher spatial and temporal resolutions allowed a more detailed observation of the very emergence of the fuel from the nozzle orifice. Figure 11 shows the close-up gradient spray images of different nozzles, respectively.
Figure 11.
Close-up view of sprays injected from different injectors.
According to the images of typical timings, the normal-hole nozzle sprays pulsate out from the nozzle tip to the radial direction, and the edges of them fluctuate seriously. The perturbation of the spray is marked and emphasized by different color arrows in the figure. On the other hand, since the injection duration is longer under the micro orifices condition when maintaining the same injection quantity, the selection for the typical timing of the images is a little different. With the same results under the imaging rate of 10,000 fps, the spray illumination intensity becomes weaker under the micro-hole condition. Of interest is that the spray pulsating phenomenon almost disappears, and the profiles of the sprays become much neater and more stable.
The angle, determined by 10 times the hole diameter away from the nozzle tip, is defined as the spray dispersion angle. The average spray dispersion angle and the single-shot results are shown in Figure 12. Generally, the spray dispersion angle under the hole diameter of 0.10 mm condition is much wider than that of the micro-hole one. As for the single-shot result, corresponding to the pulsating phenomenon of the near-field spray, as the capital letters and color arrows indicate in the figures, the spray dispersion angle curve waves and fluctuates strongly, especially in the initial stage of the injection (0.1–0.25 ms ASOI). The integrated speculations and reasonable explanations for this phenomenon can be excavated by linking the previous results [34] and the current study.
Figure 12.
Close-up spray behaviors injected from different injectors.
Different from the single-hole nozzle, due to the off-axis arrangement of the orifices, there is usually vortex flow inside the sac of multi-hole nozzles, especially under the condition of low needle lift. Moreover, with the needle moving, the location variation of the unstable vortex core results in the unstable spiraling flow pattern emerging through the hole with the vortex. During these processes, there is usually the generation of string-type cavitation in the sac and hole flow field. All the properties, only belonging to the internal flow of multi-hole nozzles, play significant roles in the phenomenon of near filed spray pulsating [7, 35]. When the hole diameter is reduced to 0.07 mm, the fluctuation of the spray dispersion angle curve decreases dramatically, and the angle becomes much narrower than that of the normal hole diameter condition. The reduced flow transverse can suppress the cavitation and vortex level under the micro-hole condition, and it is also impeded the fuel to enter into the micro-holes from the sac volume of the multi-hole nozzle. These could be used to explain the reason why the micro-hole multi-hole nozzle has a relatively narrower spray width and steady spray morphology.
3.3 Internal flow inside different nozzle configurations
The computational study is used to illustrate the different internal flow characteristics between the two nozzles. In this transient simulation analysis, according to the needle lift curve applied in the study, three typical timings (0.1, 0.318, and 0.5 ms ASOI) are selected to represent the initial, full needle lift, and post stages of the injection, respectively.
Figure 13 shows the pressure variation inside different nozzles at the typical injection stages. The time-resolved averaged sac pressure variation of the two nozzles is also plotted in this figure. Attention should be paid to the nozzle with micro holes. Its sac and hole pressures are all higher than those of the one with larger orifices throughout the injection duration, which coincides with the experimental analysis shown in Figure 9. The lower theoretical effective flow area and sac pressure discharge rate of the micro-hole nozzle are mainly attributed to this issue.
Figure 13.
Pressure distribution inside the nozzles at the typical timings during the injection duration.
The temporal velocity variation on the inlet and outlet sections of the two nozzles is shown in Figure 14. Four monitoring points (P1, P2, P3, and P4) were set along the horizontal diameter line (Line A-B) symmetrically.
Figure 14.
Temporal variation of fuel velocity at the monitoring points on the hole sections.
The injection velocity on the outlet of the micro-hole nozzle is higher than that of the normal-hole one, especially in the initial stage of injection, which agrees with the discussion about the injection rate and sprays penetration results in the experiments. Furthermore, no matter on the inlet or on the outlet sections, the flow velocity fluctuation of the nozzle with larger orifices is the most intense one, which corresponds with the spray pulsating instance. Moreover, a reverse variation tendency appears at the symmetrical monitoring points with the needle moving up, which implies that there is a spiral and asymmetrical flow pattern inside the hole. However, under the micro-hole diameter conditions, the amplitude of the velocity wave decreases dramatically, which coincides with the neater spray profiles and the reduced spray pulsating phenomenon discussed in the experimental results.
The streamlines inside different holes, which are shown in Figure 15, can interpret the instance analyzed in the optical experiments. There are much more complicated streamlines with stronger curvatures and counter-rotating flow inside the hole volume under the larger orifice condition. The vorticity distributions results along the orthogonal orifice lines also show higher stream-wise vorticity under 0.1 mm hole diameter conditions. This spiral flow is also observed by Gavaises and Andriotis [11], Lai et al. [32], and Hayashi et al. [35]. It has been proven that there are close correlations between this swirling motion and the wider spray cone angle. However, when attention is paid to the nozzles with micro-orifices, the vorticity decreases, and the streamline is stable and smooth relatively. This agrees with the reduced fluctuation of its spray behaviors observed in the experiments.
Figure 15.
Streamline and streamwise vorticity distribution along the horizontal and vertical diameter lines on the outlets of different multi-hole nozzles at full needle lift timing.
As shown in Figure 16, the velocity resolution of fuel jet injection on the hole exit section of the multi-hole nozzle is conducted, and Va is the vector on the hole axis. The other velocity component Vr is on the outlet section, which could be divided into two components. In the current result, the ratio of Vr to Va along the horizontal hole diameter line is shown in this figure. The normal-hole nozzle at three typical timings all has a much higher ratio than that of the micro-hole nozzle, which is coincided with the result observed in the experiments that the nozzle with 0.10 mm holes has a wider spray cone angle.
Figure 16.
Distribution of the ratio of velocity components (Vr/Va) along the horizontal diameter of the exit of different nozzles at typical timings.
The temporal variations of the liquid volume fraction are shown in Figure 17. Affected by its special configuration, the cavitation distribution in the multi-hole nozzle is asymmetrical. Under the low need lift condition, when the injection starts, the film-type cavitation generates at a lower field of the hole inlet because of the aspects changing the flow direction. With the needle lifting up, the cavitation moves to the upper flow field and gradually develops into the string-type cavitation, which is mainly affected by the spiral and streamwise counter-rotating vortices flow structure inside the hole. Finally, the string-type cavitation can even reach the central area of the exit section, where intense mass and momentum transfer happens in the downstream region. The issues discussed above are all conducive to the wider spray cone angle [36]. However, under the micro-hole diameter condition, the cavitation intensity is much smaller, which is attributed to the higher pressure and smoother flow structure inside the hole volume of the nozzle.
Figure 17.
Temporal variation of liquid volume fraction distribution inside different nozzle holes.
The turbulence intensity increases with the enhancement of the dissipation of the spray momentum transferring to the turbulence energy, resulting in more intense liquid/gas interactions, lower spray velocity, and wider spray diffusion [37]. Figure 18 shows the turbulence kinetic energy (TKE) distribution along the horizontal hole diameter line of different nozzles at the typical timing. The gradient of this value between the hole boundary locations and the hole central area of different nozzles is all prominent during the injection durations. However, they present different relations in the three injection stages. Initially, affected by the higher pressure increasing rate in the sac and the consequent higher velocity in the hole, the TKE value in the boundary location of the micro-hole is a little higher than that of the normal hole, but it is lower in the central area of the hole. After that, the TKE value under the normal-hole diameter condition increases a lot, and maintains a high level. However, the TKE value under the micro-hole diameter condition does not increase so much in the following injection stages, which indicates that the spray behaviors of the micro-hole nozzle would be more stable, resulting in the narrower spray angle and cone angle observed in the experiments.
Figure 18.
Distribution of turbulence kinetic energy along the horizontal diameter of the exit of different nozzles at typical timings.
According to the results of the internal flow of the different nozzles, except for the aerodynamic factors, it is the hydrodynamic factors and the unique properties of the internal flow in different nozzles that dominate their spray behaviors. The different characteristics of the flow patterns inside different nozzles mainly contribute to the deviation between their spray properties.
In the current work, the differences in the spray morphology between the two realistic multi-hole diesel nozzles under different hole diameter conditions were analyzed by the high-speed video observation method during the transient injection processes. The relationship between the internal flow, cavitation variation, and spray behaviors was investigated by the numerical simulations. The main conclusions are summarized as follows:
Different from normal single-hole nozzles, which are usually adopted in fundamental fuel injection research, unique and unstable spray behaviors of the multi-hole nozzles are observed in the experiments. The pulsating spray instance, and the wide spray angle and spray cone angle of multi-hole nozzles imply that the spray development can be affected greatly by its complex nozzle configurations. Sprays from the multi-hole nozzles are mainly dominated by the sac pressure, vortex flow in the sac, complicated spiral, and turbulent flow structure inside the hole except for the spray to spray interaction. The injection rate and spray tip penetration have a strong relationship with the pressure-increasing rate in the sac and the effective flow area of the nozzles.
The numerical simulation results about the internal flow of multi-hole nozzles with different hole diameters show that the increasing rate of sac pressure is faster inside micro-hole nozzles. Compared with the micro-hole, larger holes of the multi-hole nozzle can enhance the generation of dense swirling motion, where the string-type cavitation usually forms. Consequently, stronger vorticity, higher turbulence, and larger velocity components at hole exits are produced by these complex flow patterns, and the corresponding enhanced interfacial instability and wider spray propagation are observed in the optical experiments.
Influence of hole diameter on the internal flow, injection processes, and spray development of multi-hole nozzles is prominent. The reduced effective flow area suppresses the cavitation and turbulence flow, alters the injection rate, and prolongs the injection duration. Moreover, the effect of hole diameter plays different roles in the spray properties (penetration, spray angle, and cone angle) of the multi-hole nozzles. The implications of these results have practical significance when considering the diesel fuel spray trajectory within the combustion chamber.
The authors would like to thank the Mazda Corporation for technical support and AVL-Japan for providing the numerical software.
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Written By
PengBo Dong
Submitted: 09 February 2022Reviewed: 21 March 2022Published: 27 April 2022
Open access peer-reviewed chapter
