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Mixing and combustion mechanisms within lateral swirl combustion system (LSCS) in a DI diesel engine
Applied Thermal Engineering 123 (2017) 7–18
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Research Paper
Mixing and combustion mechanisms within lateral swirl combustion system (LSCS) in a DI diesel engine Xiang-Rong Li a,⇑, Wei Yang a, Li-Wang Su b, Fu-Shui Liu a a b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China China North Engine Research Institute (Tianjin), Tianjin 300400, China
h i g h l i g h t s A three-stage mechanism of the wall-impinging for the LS chamber is illustrated. The fuel spray diffuses quickly away from the wall in the LS chamber. Two swirling flames are formed in the LS chamber accelerating the combustion. The stack region will degenerate into a stack point in the LS chamber.
a r t i c l e
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Article history: Received 7 March 2017 Revised 7 May 2017 Accepted 16 May 2017 Available online 17 May 2017 Keywords: DI engine LSCS Constant-volume spray experiment Wall jet performances Mixing and combustion
a b s t r a c t The fuel diffusion around the combustion chamber wall is the key to affecting the engine performance. The current enveloping combustion systems (e.g. x combustion systems) mainly utilize the air in the radial direction of the combustion chamber along the spray track. In these systems, since the combustion chamber wall only envelops the spray without obvious guiding effect, it will cause difficulty in the fuel diffusion around the chamber wall and deteriorate the combustion. On the contrary, the LSCS, as a guiding combustion system, could effectively improve the diffusion around the chamber wall. Therefore, it is necessary to further understand the mixing and combustion mechanisms of the LSCS. In this study, the spray and combustion characteristics of the wall-impinging jet in the lateral swirl (LS) combustion chamber were investigated in a constant-volume combustion vessel through the high-speed photography. These characteristics were compared between the x and LS combustion systems through the image processing and the two-color method. The results show that the spray process in the x combustion system contains free jet and wall jet, while the spray process in LS combustion system contains free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. The LSCS forces the fuel spray to swirl along the circumference of the combustion chamber. As a result, it could strengthen the diffusing and mixing process, and avoid a large quantity of fuel burning near the wall to form a thermos constraint. The LSCS could improve the distribution of the air-fuel mixing and consequently accelerate the combustion and reduce the soot emission through the guiding effect of convex edge on the wall jet. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Strict emission regulations and decreasing fossil fuels bring a great challenge to the application of diesel engines. Thanks to the technologies such as high-pressure injection system, high boost turbocharger, and after-treatment system, the new generation of diesel engines can meet the requirements of emission and fuel economy. Meanwhile, the technical approaches will lead to the ⇑ Corresponding author. E-mail address: [email protected] (X.-R. Li). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.089 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
increase of manufacture cost and engine weight. In these technologies, it is effective but challenging to improve the combustion process only by changing the combustion chamber shapes. The performance of the direct injection (DI) diesel engines mainly depends on a quick combustion process near the top dead center. In this process, fuel spray, air motion, and combustion chamber shape are coupled together to produce power and emissions. Therefore, a geometrical matching of combustion chamber shape with the given air motion and fuel injection parameters is essential to improve the diesel engine performance. One key challenge to improve the diesel engine performance is how to utilize
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Nomenclature ASOI BSFC DI DSCS ECS GCS
after start of injection brake specific fuel consumption direct injection double swirl combustion system enveloping combustion system guiding combustion system
LS LSCS PM
lateral swirl lateral swirl combustion system particulate matter
the diesel engine. In the present work, spray and combustion characteristics of the wall-impinging jet in the LS combustion chamber were studied in a constant-volume combustion vessel through high-speed photography and two-color method. Specifically, the processes of fuel diffusion, mixing and combustion were studied to explain how the LSCS to improve fuel accumulation, accelerate mixing and burning, and reduce soot emission. 1.1. The formation of LSCS concept
Fig. 1. The schematic diagram of LSCS.
Fig. 2. BSFC, soot and NOx comparison of FSCS for various excess air ratio.
the combustion chamber shape to promote the mixing and combustion process. To solve this problem, Liu and Li proposed the lateral swirl combustion system (LSCS) [1], as shown in Fig. 1. Su [2,3] experimentally and numerically indicated that the LSCS can improve air-fuel mixing, and reduce soot emission and BSFC as shown in Fig. 2. Li [4] studied the combustion and emissions characteristics of the LSCS. The results show that the LSCS can obtain a good performance under the low excess air ratio (=1.3) conditions. In summary, the LSCS can optimize the fuel-air mixing process to improve soot emission and fuel economy. This is because that LSCS’ special wall shapes influence the fuel spray and air motion, and have a positive effect on the mixing and combustion process. Hence, this study was conducted to develop an understanding of how the LSCS can affect the mixing and combustion process in
According to the interaction between the spray and combustion chamber, the authors categorize the DI combustion system into the enveloping combustion system (ECS) and the guiding combustion system (GCS). The ECS mainly uses the combustion chamber wall to envelop fuel spray. In order to improve the air utilization, the air is confined to the fuel spray region in the chamber, such as x [5] and dumbbell combustion chambers [6]. The combustion chamber geometry parameters including the combustion chamber diameter, the combustion chamber depth, and the cone height were optimized by Wickman through the genetic algorithm [7]. The optimization results show that the optimal combustion chamber has the features of large diameter and shallow depth. This main reason is that the large diameter of shallow combustion chamber reduces the chance for the fuel spray to impinge the wall, which makes most fuels burn far away from the wall (i.e. spatial combustion). The Wickman’s research proves one technical approach to optimize the ECS. In order to improve fuel atomization and reduce soot emission, the injection pressure of the fuel injection system has been continuously increased. Meanwhile, the diesel engines have been gradually downsized to increase the power density. Both factors make the wall-impinging jet to become an inevitable issue for diesel engines. Katsura et al. [8,9] systematically studied the characteristics of the wall jet which can be divided into the main jet region and the wall-main jet region. The results show that most of the fuel piles around the wall, and this is because the emergence of the wall jet vortex leads to the formation of the stagnate region between the main jet region and the wall-main jet region. Su [10] studied the effect of the chamber wall confinement on spray characteristics. The results show that the initial impinging phase of the spraywall can strengthen the disturbance of fuel spray on the air which promotes the air-fuel mixing. However, the subsequent impinging process can cause an accumulation of fuel near the wall and deteriorate the air-fuel mixing. Moreover, the over-rich mixture in the core of the fuel spray cannot be solved in the ECS. Therefore, through passively restricting air into the fuel spray region, the ECS cannot solve the above problems of fuel accumulation and further improve the mixing rate. However, these problems can be solved by the GCS through the wall guiding effect on fuel spray. Many researchers have focused on the GCS such as TRB [11], OSKA-D [12], NICS-MH [13], DSCS [14,15]. Based on the ECS, the GCS guides the fuel spray to develop along a specific trajectory through special wall shapes, which can promote the fuel diffusion and mixing. Li [16] analyzed the matching of the DSCS (double swirl combustion system) and split injections, the results show that the
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acceleration effect of pilot injection forms more intensive double swirls that could enhance the air-fuel mixing. Lee [17] performed a ULPC combustion system optimization for engine-out PM reduction in a heavy duty diesel engine. The combustion system utilizes the lower lip to split the fuel into two parts, acquiring a high air utilization and reducing soot emission without hardware changes. Wei [18] proposed a swirl chamber combustion system, which can accelerate the swirl mixing of the fuel spray to reduce emissions. For the ECS and GCS mentioned, the air utilization in the combustion chamber is mainly concentrated in the radial direction of the combustion chamber along the spray trajectory. However, the air of the region in the circumference direction of the combustion chamber (i.e. the region between sprays) has not been sufficiently utilized [2]. Although the introduction of the swirl can improve the uniformity of the fuel distribution in the circumference direction of the combustion chamber, the swirl can lead to the loss of intake air flow and the decrease of intake mass. Based on these problems, the LSCS is proposed to improve the mixing and combustion without more manufacturing cost. 2. Experimental equipment and data processing 2.1. Experimental setup The spray experiment of the wall-impinging jet was conducted in the constant-volume testing system (as shown in Fig. 3), which consists of a high-pressure common rail system, a constantvolume vessel without the heating device, a high-speed camera,
and two light sources. The high-pressure common rail system with the 170 MPa pressure (as shown in Fig. 4(a)), consisted of a highpressure pump that supplied fuel to the common rail with four outlets. One outlet connected to the injector with the orifice diameter of 0.28 mm. The injector was mounted on the constantvolume vessel (with the design pressure of 2 MPa) through the injector linker as shown in Fig. 4(b). The chamber bracket under the injector was fixed in the constant-volume vessel which consists of quartz windows, sealing gaskets, and intake & exhaust pipes. Through the quartz window, the pictures were captured by the PHANTOM v7.3 high-speed camera (as shown in Fig. 4(c)) which is produced by USA TRI Company. In the experiment, the high-speed camera applied the operating frame rate of 10,000 f/s, the aperture of f/8, and the exposure time of 98 us. The applied light source were two UXL-500SX xenon lamps (from Japan USHIO Company), which were set at the angle of 90° to the camera, as shown in Fig. 4(d). The experiment applied a x chamber module and two lateral swirl (LS) chamber modules with the diameter of 100 mm, as shown in Fig. 5. Two LS chamber modules have different heights of convex edge (6 mm and 3 mm), and their radius of the circle are 8.8 mm and 12.7 mm respectively. On the sides of the modules, the support plate with two holes was designed for fixing the modules. In the experiment, the injection pressure was 140 MPa and the injection pulse was 2.5 ms. The ambient gas was sulfur hexafluoride with the density of 52.6 kg/m3. To ensure the gas purity, the pressure in the constant-volume vessel should be pumped to the negative pressure through a gas pump. Next, the vessel was scavenged by sulfur hexafluoride gas. After repeating the process
Common rail
Injector
High pressure pump
Light source
Light source
High speed camera
Fuel tank
Constant volume vessel without heating device
Fuel line Singal line
ECU Computer Fig. 3. The constant-volume testing system without heating device.
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Fig. 4. The experimental equipment.
6 mm Chamber
3 mm Chamber
45°
Convex edge
3 mm
6 mm
ω Chamber
R 50
Fig. 5. Combustion chamber modules.
two times, the sulfur hexafluoride gas in the vessel met the experimental requirements. The injecting and the shooting process were synchronized. To ensure the accuracy, the pictures of every condition were repeatedly captured for three times. The combustion experiment of the wall-impinging jet was conducted in the constant-volume testing system (as shown in Fig. 6), which is basically similar to the spray experiment system. The constant-volume testing system used in this experiment has a heating device which consists of many heating wires with the rated heating power of 9 kW [19]. The constant-volume testing system (with the maximum heating temperature of 900 K and the maximum pressure of 6 MPa), consisted of four windows with the diameter of 100 mm to visual the spray and combustion. Around the windows, the window cooling device was fixed. In the experiment, the high-speed camera applied the operating frame rate of 10,000 f/s, the aperture of f/16 and the exposure time of 10 us. The injection pressure was 140 MPa, and the injection pulse was 1.0 ms. The ambient gas was the compressed air with the density of 26.6 kg/m3 and the temperature of 850 K. Firstly part of compressed air was pumped into the vessel and then the compressed air was heated to the aimed temperature. Finally, the other part of compressed air was pumped again into the vessel until the
temperature and pressure in the vessel met the experiment requirements. Besides, the shooting process in the combustion experiment was similar to the shooting process in the spray experiment. 2.2. Data processing The pictures were processed by the Matlab program. The picture processing is shown in Fig. 7. First, the original pictures were denoised. Then the spray pictures were processed to calculate spray area, spray radius, and spray height. The flame pictures were processed by the two-color method. The two-color method is based on the radiation emitted by soot particles at two different wavelengths, to calculate the temperature of the soot particles. The soot particles can instantaneously reach thermal equilibrium with the environment due to the small size of soot particles [20,21]. Hence the temperature of soot particles can represent the temperature of the flame. The soot concentration was represented by the KL factor, where K was the soot absorption coefficient proportional to soot concentration and L was the geometrical thickness in the detecting direction of flame [21–23]. The two-color method can conveniently acquire the combustion
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Fig. 6. The constant-volume testing system with heating device.
features without disturbing the combustion process. To study the propagation of spray and flame, their profiles were obtained through the Matlab program. 3. Results and discussions 3.1. Spray experiment of wall impinging jet Fig. 8 shows the spray impinging process under different combustion chambers. At 0.7 ms ASOI, the spray shapes under different chambers are basically the same, since the fuel spray has not been in contact with the combustion chamber. At 1.1 ms ASOI, part of the fuel spray has impinged the chamber. Under the guiding action of the convex edge, the fuel spray in the LS chamber obviously swirls to both sides of the convex edge. On the contrary, the fuel spray in the x chamber is obstructed by the adducent wall of the x chamber. The diffusion speed of the fuel spray to both sides is restricted. Compared with the fuel spray in the x chamber, the velocity direction of the fuel spray in the LS chamber has been obviously changed. During the process, the kinetic energy loss in the LS chamber is less than the kinetic energy loss in the x chamber since the fuel spray in the LS chamber is not obstructed by the wall [2]. At 1.1 ms ASOI, the fuel spray in the LS chamber has begun to separate from the chamber wall, but the fuel spray in the x chamber still develops along the chamber wall. This is the fuel adhesion phenomenon which is one of the reasons for the increase of PM and HC emissions in diesel engines. Therefore, the separation of the fuel spray from the chamber wall in the LS Chamber plays a significant role in improving emission performance of diesel engines.
At 1.9 ms ASOI, the fuel has already swirled obviously in the LS chambers and the LS motion forms in the chamber. The comparison of the fuel spray process in the LS and x chambers indicates that the least fuel is distributed around both sides of the convex edge due to the fierce wall-diversion effect in the 6 mm chamber. With the decrease of the height of the convex edge, the walldiversion effect is weakened. In the 3 mm chamber, the fuel is increased around both sides of the convex edge, and in the x chamber, the most fuel is distributed around the wall. However, the fuel distribution beyond the chamber module just shows the opposite trend. During the operation of the diesel engine, part of the fuel is burned near the wall (e.g. the condition in the x chamber), resulting in insufficient fresh air in this area and relatively little use of air on both sides of the fuel spray. In the LS chamber, the fuel spray is forced to split into two bunches, which not only improves the over-concentration in the fuel spray core but also effectively utilizes the air on both sides of the fuel spray. Fig. 9 shows the spray areas under different chambers. At the early stage of the spray process, the fuel sprays under the three chambers are in the state of free jet and their spray areas are basically the same before 1 ms ASOI. After 1.2 ms ASOI, the fuel sprays begin to impinge the combustion chamber wall and their spray areas begin to change obviously. Because of the obvious walldiversion effect in the 6 mm chamber, the fuel spray can rapidly develop in the combustion chamber and the spray area is relatively large. According to the experimental results, the adducent wall in the x chamber has a significant hindrance effect on the spray process after impinging the wall. The spray area in the x chamber is always smaller than the spray area in the LS chamber. In the spray impingement process, the increase rate of the spray area is high for
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[8]. The impinging spray is studied through the spray radius and the spray height. Fig. 11 shows the spray radius under different chambers. It can be seen that the fuel spray impinges the wall at 1.1 ms ASOI and begins to develop toward both sides. Due to the diversion effect of the LS chamber, the spray radius in the LS chamber is larger than the spray radius in the x chamber. In the spray process, the spray radius in the 3 mm chamber changes obviously and is always larger than the spray radius in the x chamber. From 1.1 to 1.9 ms ASOI, the spray radius in the 6 mm chamber grows more slowly. After 1.9 ms ASOI, the spray radius in the 6 mm chamber is less than spray radius in the x chamber. This indicates that the higher height of the convex edge will constrain the spray development along the radial direction. Fig. 12 shows the spray height under different chambers. From 1.1 to 1.4 ms ASOI, the spray height in the x chamber is slightly higher than the spray height in the LS chamber. After 1.5 ms ASOI, the spray height in the LS chamber increases linearly and is significantly higher than the spray height in the x chamber. This indicates that the LS chamber can obviously accelerate the diffusion of the fuel spray away from the wall. From 2.4 to 3 ms ASOI, the obvious fluctuation occurs for the x chamber, which is mainly because the stagnate region has not been formed and the initial wall jet vortex is unstable. After 3 ms ASOI, the wall jet vortex causes the formation of a stagnate region at the end of the spray near the wall. The stagnate region hinders the spray flow until a balance is formed. Therefore the spray height grows steadily. 3.2. Combustion experiment of wall-impinging jet
Fig. 7. The picture processing flow.
the 6 mm chamber, but the increase rate is basically unchanged for the 3 mm and x chambers after 3 ms ASOI. Fig. 10 shows a schematic diagram of an impinging spray. The structure of the impinging spray has been analyzed by Katsura
3.2.1. Flame pictures Fig. 13 shows the flame development under different chambers. It can be seen that the obvious flame appears at 0.6 ms ASOI and the flame has touched the wall due to the strong spray penetrability ability under the small ambient density. At 0.8 ms ASOI, the swirling flame has begun to appear on both sides of the convex edge in the LS chamber. At 1 ms ASOI, the LS flame has basically formed. Subsequently (1.2 ms ASOI), the jet flame head separates
Fig. 8. The spray impinging under different chambers.
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Fig. 9. The spray areas under different chambers.
Fig. 10. A schematic diagram of an impinging spray.
Fig. 11. The spray radius under different chambers.
from the end of the convex edge under the action of the diversion arc. At 1.6 ms ASOI, the flame is completely distributed on both sides of the convex edge. Since the flame is not constrained by the wall after separating from the convex edge, the head area of LS flame increases rapidly. Compared to the x chamber, the combustion process in the LS chamber is obviously different.
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Fig. 12. The spray height under different chambers.
In the LS chamber, the combustion flame is distributed on both sides of the convex edge. Because of the existence of arcs on both sides, two swirling flames are formed on both sides of the convex edge, and the few flame exists in the arcs region. In the x chamber, since the adducent surface has a hindrance effect on the flame propagation, the flame can only spread along the wall after reaching the wall. As a result, the flame gradually becomes thick leading to the phenomenon of thermos constraint [24]. At 2 ms ASOI, the flame basically disappears in the central area of the LS chamber, but the flame is still fierce in the central region of the x chamber. This phenomenon shows that the fuel of the x chamber is more than the fuel of the LS chamber in the central region of the combustion chamber. Most of the fuel in the x chamber will be directly sprayed into the burning region. A large amount of fuel concentrated will burn in the central region of the wall. Consequently, the air in the central region is less than the air on both sides of the central region. Compared to the x chamber, the flame in the LS chamber is swirled to both sides of the central region, which will utilize the air in the combustion chamber. At the late stage of the combustion, two strong flame centers are formed in the LS chamber and the spray height also increases obviously. However, only one flame center is formed in the x chamber. Because of the same injection parameters, the ignition time is the same in the three combustion chambers. At 2.7 ms ASOI, the obvious flame has disappeared in the LS chamber, and the combustion has finished. On the contrary, the large area of the bright flame still exists in the x chamber. Therefore, the combustion duration in the x chamber is longer than the combustion duration in the LS chamber, which indicates that the combustion speed in the LS chamber has been improved significantly. 3.2.2. Flame temperature Fig. 14 shows the flame temperature under different chambers. From 0.6 to 1.1 ms ASOI, the flame temperature in the LS chamber is higher than the flame temperature in the x chamber. After 1.4 ms ASOI, the flame temperature in the LS chamber is lower than the flame temperature in the x chamber. This indicated that the LS chamber can accelerate the combustion. Fig. 15 shows the temperature distribution under different chambers. Before 1.5 ms ASOI, the flame area in the LS chamber is larger than the flame area in the x chamber. For the LS chamber, the temperature of the local zone has reached 2300 K at 0.9 ms ASOI, and the flame temperature and area are significantly increased at 1.2 ms ASOI. After 1.5 ms ASOI, the high-temperature zone in the LS chamber is rapidly reduced. For the x chamber, the slow burning at the early stage causes an accumulation of premixed gas and a large area of high-temperature flame is formed near the wall. This is mainly
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ω Chamber
3 mm Chamber
6 mm Chamber 0.5 ms
0.6 ms
0.8 ms
1.0 ms
1.2 ms
2.0 ms
1.6 ms
2.2 ms
2.7 ms
Fig. 13. The flame development under different chambers.
Fig. 14. The flame temperature under different chambers.
Fig. 16. The total KL factor under different chambers.
2300K
2200K
ω Chamber
2100K
2000K
1900K
3 mm Chamber 1800K
1700K
6 mm Chamber
0.6 ms
1600K
0.9 ms
1.2 ms
1.5 ms
1.9 ms
Fig. 15. The temperature distribution under different chambers.
2.2 ms
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Fig. 17. The KL factor distribution under different chambers.
because the flame development is obstructed by the adducent wall of the x chamber. 3.2.3. KL factor Fig. 16 shows the total KL factor under different chambers. Before 1.3 ms ASOI, the total KL factor in the LS chamber is slightly higher than the total KL factor in the x chamber. Subsequently, the total KL factor in the x chamber increases rapidly at 1.2 ms ASOI and then becomes much higher than the total KL factor in the LS chamber. The main reason is that the stagnate region has been formed in the high-temperature flame at 1.2 ms ASOI, and a large amount of fuel accumulates in the stagnate region. Then the area of the stagnant region continues to expand and the fuel concentration continues to increase and generate a large amount of soot. Fig. 17 shows the KL factor distribution under different chambers. At 0.9 ms ASOI, the area with the KL factor greater than 2.5 has appeared in the LS chamber, but this area just appears in the x chamber at 1.2 ms ASOI. At 1.5 ms ASOI, the large area with the
Stagnate region
KL factor greater than 2.5 appears near the wall in the x chamber, but this area appears away from the wall in the LS chamber. After 2.2 ms ASOI, the area with KL factor greater than 2.5 basically disappears in the LS chamber, but this area still exists in the x chamber. 3.3. Mechanism of the LS combustion Through the above research, it is found that the LS chamber can effectively promote the air-fuel mixing, accelerate the burning rate and reduce soot emission. In order to further explain the characteristics of the LSCS, the profile development of the spray and the flame is analyzed through processing the spray and combustion pictures. Fig. 18 shows the development of the spray profile. After the spray impinges the wall, the spray front moves away from the wall. In the 6 mm chamber, the spray front moves in the normal direction of the wall, while the spray front in the x chamber basically moves along the wall. Fig. 18 and the spray animation show
Boundary
Stagnate region
Boundary Stagnate region
1.1ms
1.1ms
6 mm Chamber
1.1ms
3 mm Chamber
Fig. 18. The development of the spray profile (the interval between two profiles is 0.2 ms).
ω Chamber
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Stagnate region
Stagnate region
0.6ms
0.6ms
6 mm Chamber
Stagnate region
0.6ms
3 mm Chamber
ω Chamber
Fig. 19. The development of the flame profile (the interval between two profiles is 0.2 ms).
Fig. 20. A schematic diagram of the spray impinging wall in the LS and x chambers.
that there is a stagnate region at the root of the spray. The stagnate region under different chambers shows the different shapes. The stagnate region in the x chamber has the largest area in all three chambers, but the stagnate region in the 6 mm chamber has the smallest area. In all three sprays, there is an available space between the main spray and the spray after impinging the wall. The available space in the x chamber has the smallest area in all three chambers, and the available space in the 6 mm chamber has the largest area. This is because the diversion effect of the LS chamber is higher than the diversion effect of the x chamber. Fig. 19 shows the development of the flame profile. In the LS chamber, the flame front moves in the opposite direction of the spray movement, and the jet flame and the fame after impinging
the wall are separated by a narrow space. On the contrary, the flame front in the x chamber moves along the wall, and the jet flame and the fame after impinging the wall are connected through the stagnate region, resulting in the accumulation of fuel. Fig. 20 shows a schematic diagram of the spray impinging wall in the LS and x chambers. The spray process in the x chamber is divided into two stages: free jet and wall jet. At the early stage of wall jet, the spray moves along the wall, and spray height and spray radius both increase. Due to the poor diversion effect of the x chamber wall, a stack region is formed near the wall. At the late stage of wall jet, the fuel in the stack region continues to accumulate and hinders the diffusion of the fuel. This results in the emergence of a large area of the stagnate region at the root
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Fig. 21. LS and intervening wall jet after the separation of jet head from the convex edge.
of the spray. At this time the spray height and spray radius basically do not increase, which causes a large accumulation of fuel around the wall, finally producing much more soot. According to Figs. 20 and 21, the spray process in the LS chamber is divided into three stages: free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. At the stage of free jet, the spray in the LS chamber is not obviously different from the spray in the x chamber. At the stage of formation of LS, the convex edge forces the stack region to degenerate into the stack point, which greatly reduces the accumulation of fuel and the production of soot. Under the diversion action of the convex edge, spray height and radius both increase, and the spray height increases more obviously than the spray radius. The increase rate of the spray height in the LS chamber is much larger than the increase rate of the spray height in the x chamber. At the late stage of formation of LS, the spray will separate from the convex edge and then diffuse quickly in the direction away from the wall. This results in a small area of the stagnate region. At the last stage, the relationship between two sprays is shown in Fig. 21. After the spray separates from the wall under the action of the convex edge, the adjacent spray will promote each other and be quickly diffused to the unused space for a better mixing. 4. Conclusion The mixing and combustion mechanisms of LSCS were investigated in a constant-volume vessel. Through the high-speed technique and two-color method, the data of spray impinging and wall jet combustion was obtained to analyze the combustion and mixing performance of LS and x combustion systems. The following conclusions can be drawn: 1. The spray area in the LS chamber is larger than the spray area in the x chamber. Moreover, the spray height and spray radius in the LS chamber are larger than the spray height and spray radius in the x chamber. At the early stage of wall jet, the spray radius in the LS chamber is larger than the spray radius in the x chamber. At the late stage of wall jet, the spray radius in two chambers are basically the same, but the spray height in the LS chamber increases quicker than the spray height in the x chamber during the whole process of wall jet. In the LS chamber, the convex edge accelerates the diffusion in the direction away from the wall and reduces a large accumulation of fuel.
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2. In the LS chamber, two flame centers are formed away from the wall under the effect of the convex edge. On the contrary, only one flame center in the x chamber is formed around the wall and the fuel is sprayed into the flame, deteriorating the combustion. Hence the combustion speed in the LS chamber is quicker than the combustion speed in the x chamber. 3. The flame temperature in the LS chamber is higher than the flame temperature in the x chamber at the early stage of the combustion, but the flame temperature in the LS chamber is lower than the flame temperature in the x chamber at the late stage of the combustion. In the x chamber, more soot is produced at the late stage and mainly distributed around the wall. Contrarily, more soot is produced at the early stage and mainly distributed away from the wall in the LS chamber. Hence the soot in the LS chamber is much easy to mix with the air and be oxidized. 4. In the LS chamber, the spray and combustion processes of the wall-impinging jet are divided into three stages: free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. The convex edge splits the fuel spray from the core of the spray and forms LS, which could solve the over-rich mixture in the core of the fuel spray. The development of spray and flame profiles indicates that LSCS can effectively utilize the air in the circumferential space of the combustion chamber, and promote the development of the fuel spray away from the wall, avoiding the accumulation of fuel around the wall. The main reason is that the stack region is not formed around the wall in the LS chamber and the area of the stagnate region is always small during the development of the spray.
References [1] Fushui Liu, Xiangrong Li, Yong Shang, A lateral swirl combustion chamber, China patent No 101839166 B. [2] LiWang Su, XiangRong Li, Zheng Zhang, FuShui Liu, Numerical analysis on the combustion and emission characteristics of forced swirl combustion system for DI diesel engines, Energy Convers. Manage. 86 (2014) 20–27. [3] LiWang Su, XiangRong Li, Xu He, FuShui Liu, Experimental research on the diffusion flame formation and combustion performance of forced swirl combustion system for DI diesel engines, Energy Convers. Manage. 106 (2015) 826–834. [4] XiangRong Li, Haiqin Zhou, et al., Combustion and emission characteristics of a lateral swirl combustion system for DI diesel engines under low excess air ratio conditions, Fuel 184 (2016) 672–680. [5] Sung Wook Park, Optimization of combustion chamber geometry for stoichiometric diesel combustion using a micro genetic algorithm, Fuel Process. Technol. 91 (1) (2010) 1742–1752. [6] Shengli Wei, Feihu Wang, Xianyin Leng, et al., Numerical analysis on the effect of swirl ratios on swirl chamber combustion system of DI diesel engines, Energy Convers. Manage. 75 (1) (2013) 184–190. [7] D.D. Wickman, P.K. Senecal, R.D. Reitz, Diesel engine combustion chamber geometry optimization using genetic algorithms and multi-dimensional spray and combustion modeling, SAE Paper 2001-01-0547, 2001. [8] Naohito Katsura, Masahiro Saito, Jiro Senda, et al. Characteristics of a diesel spray impinging on a flat wall, SAE Paper 890264, 1989. [9] Naohito Katsura, Masahiro Saito, Jiro Senda, et al., Characteristics of a diesel spray impinging on a flat wall (2nd report) Nippon Kikai Gakkai Ronbunshu, B Hen/Trans. Jpn. Soc. Mech. Eng., Part B 56 (521) (1990) 227–234. [10] Wanhua Su, Kuihang Zhao, Jinghua Wang, An investigation of effects of injection parameters and chamber wall confinement on spray characteristics in constant volume bomb, Chin. Inter. Combust. Engine Eng. 13 (1) (1992) 1–5. [11] Ichiro Sakata, Kazuyoshi Ishisaka, Hiromichi Yanagihara, et al., Development of TOYOTA reflex burn (TRB) system in DI diesel, SAE 900658. [12] Satoshi Kato, Shigeru Onishi, New type of diesel engine by impingement of fuel jet (OSKA-D), SAE 901618. [13] Bai-fu Lin, Masaru Ogura, A new multi-impingement-wall head diffusion combustion system (NICS-MH) of a DI diesel engine, SAE 950851. [14] R. Wei, X. Li, A new double swirl combustion system for DI diesel engine, SAE Paper 2000-01-2915, 2000. [15] Xiangrong Li, Zuoyu Sun, Wei Du, et al., Research and development of double swirl combustion system for a DI diesel engine, Combust. Sci. Technol. 182 (8) (2010) 1029–1049. [16] Xiangrong Li, Haobu Gao, et al., Combustion and emission performance of a split injection diesel engine in a double swirl combustion system, Energy 114 (2016) 1135–1146.
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[17] J. Lee, S. Lee, J. Kim, D. Kim, Bowl shape design optimization for engine-out PM reduction in heavy-duty diesel engine, SAE 2015-01-0789, 2015. [18] Shengli Wei, Kunpeng Ji, et al., Numerical simulation on effects of spray angle in a swirl chamber combustion system of DI (direct injection) diesel engines, Energy 114 (2014) 289–294. [19] HaoBu Gao, XiangRong Li, JiYe Xue, Honglin Bai, Xu He, FuShui Liu, A modification to the WAVE breakup model for evaporating diesel spray, Appl. Therm. Eng. 108 (2016) 555–566. [20] Y. Matsui, T. Kamimoto, S. Matsuoka, A study on the time and space resolved measurement of flame temperature and soot concentration in a D.I. diesel engine by the two-color method, SAE Paper 790491, 1979. [21] Marco Bakenhus, Rolf D. Reitz, Two-color combustion visualization of single and split injections in a single-cylinder heavy-duty D.I. diesel engine using an endoscope-based imaging system, SAE Paper 1999-01-1112, 1999.
[22] Wei-lin Zeng, Fu-shui Liu, Xu He, Hai Liu, Sen-jia Jin, Guo-hong Tian, Xiangrong Li, Visualization study on the in-cylinder combustion temperature and soot characteristics of biodiesel, Trans. CSICE 31 (4) (2013) 296–301. [23] Weilin Zeng, Xu He, Senjia Jin, Hai Liu, Xiangrong Li, Fushui Liu, Analysis of the influence of pilot injection timing on diesel combustion in an optical engine by the two-color method, in: ASME 2013 Internal Combustion Engine Fall Technical Conference, Dearborn, Michigan, USA, October 13–16, 2013. [24] XiangRong Li, Wei Yang, LuMing Zhao, FuShui Liu, The influence of pilot-main injection matching on DI diesel engine combustion using an endoscopic visualization system, Fuel 188 (2017) 575–585.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Mixing and combustion mechanisms within lateral swirl combustion system (LSCS) in a DI diesel engine Xiang-Rong Li a,⇑, Wei Yang a, Li-Wang Su b, Fu-Shui Liu a a b
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China China North Engine Research Institute (Tianjin), Tianjin 300400, China
h i g h l i g h t s A three-stage mechanism of the wall-impinging for the LS chamber is illustrated. The fuel spray diffuses quickly away from the wall in the LS chamber. Two swirling flames are formed in the LS chamber accelerating the combustion. The stack region will degenerate into a stack point in the LS chamber.
a r t i c l e
i n f o
Article history: Received 7 March 2017 Revised 7 May 2017 Accepted 16 May 2017 Available online 17 May 2017 Keywords: DI engine LSCS Constant-volume spray experiment Wall jet performances Mixing and combustion
a b s t r a c t The fuel diffusion around the combustion chamber wall is the key to affecting the engine performance. The current enveloping combustion systems (e.g. x combustion systems) mainly utilize the air in the radial direction of the combustion chamber along the spray track. In these systems, since the combustion chamber wall only envelops the spray without obvious guiding effect, it will cause difficulty in the fuel diffusion around the chamber wall and deteriorate the combustion. On the contrary, the LSCS, as a guiding combustion system, could effectively improve the diffusion around the chamber wall. Therefore, it is necessary to further understand the mixing and combustion mechanisms of the LSCS. In this study, the spray and combustion characteristics of the wall-impinging jet in the lateral swirl (LS) combustion chamber were investigated in a constant-volume combustion vessel through the high-speed photography. These characteristics were compared between the x and LS combustion systems through the image processing and the two-color method. The results show that the spray process in the x combustion system contains free jet and wall jet, while the spray process in LS combustion system contains free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. The LSCS forces the fuel spray to swirl along the circumference of the combustion chamber. As a result, it could strengthen the diffusing and mixing process, and avoid a large quantity of fuel burning near the wall to form a thermos constraint. The LSCS could improve the distribution of the air-fuel mixing and consequently accelerate the combustion and reduce the soot emission through the guiding effect of convex edge on the wall jet. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Strict emission regulations and decreasing fossil fuels bring a great challenge to the application of diesel engines. Thanks to the technologies such as high-pressure injection system, high boost turbocharger, and after-treatment system, the new generation of diesel engines can meet the requirements of emission and fuel economy. Meanwhile, the technical approaches will lead to the ⇑ Corresponding author. E-mail address: [email protected] (X.-R. Li). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.089 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
increase of manufacture cost and engine weight. In these technologies, it is effective but challenging to improve the combustion process only by changing the combustion chamber shapes. The performance of the direct injection (DI) diesel engines mainly depends on a quick combustion process near the top dead center. In this process, fuel spray, air motion, and combustion chamber shape are coupled together to produce power and emissions. Therefore, a geometrical matching of combustion chamber shape with the given air motion and fuel injection parameters is essential to improve the diesel engine performance. One key challenge to improve the diesel engine performance is how to utilize
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Nomenclature ASOI BSFC DI DSCS ECS GCS
after start of injection brake specific fuel consumption direct injection double swirl combustion system enveloping combustion system guiding combustion system
LS LSCS PM
lateral swirl lateral swirl combustion system particulate matter
the diesel engine. In the present work, spray and combustion characteristics of the wall-impinging jet in the LS combustion chamber were studied in a constant-volume combustion vessel through high-speed photography and two-color method. Specifically, the processes of fuel diffusion, mixing and combustion were studied to explain how the LSCS to improve fuel accumulation, accelerate mixing and burning, and reduce soot emission. 1.1. The formation of LSCS concept
Fig. 1. The schematic diagram of LSCS.
Fig. 2. BSFC, soot and NOx comparison of FSCS for various excess air ratio.
the combustion chamber shape to promote the mixing and combustion process. To solve this problem, Liu and Li proposed the lateral swirl combustion system (LSCS) [1], as shown in Fig. 1. Su [2,3] experimentally and numerically indicated that the LSCS can improve air-fuel mixing, and reduce soot emission and BSFC as shown in Fig. 2. Li [4] studied the combustion and emissions characteristics of the LSCS. The results show that the LSCS can obtain a good performance under the low excess air ratio (=1.3) conditions. In summary, the LSCS can optimize the fuel-air mixing process to improve soot emission and fuel economy. This is because that LSCS’ special wall shapes influence the fuel spray and air motion, and have a positive effect on the mixing and combustion process. Hence, this study was conducted to develop an understanding of how the LSCS can affect the mixing and combustion process in
According to the interaction between the spray and combustion chamber, the authors categorize the DI combustion system into the enveloping combustion system (ECS) and the guiding combustion system (GCS). The ECS mainly uses the combustion chamber wall to envelop fuel spray. In order to improve the air utilization, the air is confined to the fuel spray region in the chamber, such as x [5] and dumbbell combustion chambers [6]. The combustion chamber geometry parameters including the combustion chamber diameter, the combustion chamber depth, and the cone height were optimized by Wickman through the genetic algorithm [7]. The optimization results show that the optimal combustion chamber has the features of large diameter and shallow depth. This main reason is that the large diameter of shallow combustion chamber reduces the chance for the fuel spray to impinge the wall, which makes most fuels burn far away from the wall (i.e. spatial combustion). The Wickman’s research proves one technical approach to optimize the ECS. In order to improve fuel atomization and reduce soot emission, the injection pressure of the fuel injection system has been continuously increased. Meanwhile, the diesel engines have been gradually downsized to increase the power density. Both factors make the wall-impinging jet to become an inevitable issue for diesel engines. Katsura et al. [8,9] systematically studied the characteristics of the wall jet which can be divided into the main jet region and the wall-main jet region. The results show that most of the fuel piles around the wall, and this is because the emergence of the wall jet vortex leads to the formation of the stagnate region between the main jet region and the wall-main jet region. Su [10] studied the effect of the chamber wall confinement on spray characteristics. The results show that the initial impinging phase of the spraywall can strengthen the disturbance of fuel spray on the air which promotes the air-fuel mixing. However, the subsequent impinging process can cause an accumulation of fuel near the wall and deteriorate the air-fuel mixing. Moreover, the over-rich mixture in the core of the fuel spray cannot be solved in the ECS. Therefore, through passively restricting air into the fuel spray region, the ECS cannot solve the above problems of fuel accumulation and further improve the mixing rate. However, these problems can be solved by the GCS through the wall guiding effect on fuel spray. Many researchers have focused on the GCS such as TRB [11], OSKA-D [12], NICS-MH [13], DSCS [14,15]. Based on the ECS, the GCS guides the fuel spray to develop along a specific trajectory through special wall shapes, which can promote the fuel diffusion and mixing. Li [16] analyzed the matching of the DSCS (double swirl combustion system) and split injections, the results show that the
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acceleration effect of pilot injection forms more intensive double swirls that could enhance the air-fuel mixing. Lee [17] performed a ULPC combustion system optimization for engine-out PM reduction in a heavy duty diesel engine. The combustion system utilizes the lower lip to split the fuel into two parts, acquiring a high air utilization and reducing soot emission without hardware changes. Wei [18] proposed a swirl chamber combustion system, which can accelerate the swirl mixing of the fuel spray to reduce emissions. For the ECS and GCS mentioned, the air utilization in the combustion chamber is mainly concentrated in the radial direction of the combustion chamber along the spray trajectory. However, the air of the region in the circumference direction of the combustion chamber (i.e. the region between sprays) has not been sufficiently utilized [2]. Although the introduction of the swirl can improve the uniformity of the fuel distribution in the circumference direction of the combustion chamber, the swirl can lead to the loss of intake air flow and the decrease of intake mass. Based on these problems, the LSCS is proposed to improve the mixing and combustion without more manufacturing cost. 2. Experimental equipment and data processing 2.1. Experimental setup The spray experiment of the wall-impinging jet was conducted in the constant-volume testing system (as shown in Fig. 3), which consists of a high-pressure common rail system, a constantvolume vessel without the heating device, a high-speed camera,
and two light sources. The high-pressure common rail system with the 170 MPa pressure (as shown in Fig. 4(a)), consisted of a highpressure pump that supplied fuel to the common rail with four outlets. One outlet connected to the injector with the orifice diameter of 0.28 mm. The injector was mounted on the constantvolume vessel (with the design pressure of 2 MPa) through the injector linker as shown in Fig. 4(b). The chamber bracket under the injector was fixed in the constant-volume vessel which consists of quartz windows, sealing gaskets, and intake & exhaust pipes. Through the quartz window, the pictures were captured by the PHANTOM v7.3 high-speed camera (as shown in Fig. 4(c)) which is produced by USA TRI Company. In the experiment, the high-speed camera applied the operating frame rate of 10,000 f/s, the aperture of f/8, and the exposure time of 98 us. The applied light source were two UXL-500SX xenon lamps (from Japan USHIO Company), which were set at the angle of 90° to the camera, as shown in Fig. 4(d). The experiment applied a x chamber module and two lateral swirl (LS) chamber modules with the diameter of 100 mm, as shown in Fig. 5. Two LS chamber modules have different heights of convex edge (6 mm and 3 mm), and their radius of the circle are 8.8 mm and 12.7 mm respectively. On the sides of the modules, the support plate with two holes was designed for fixing the modules. In the experiment, the injection pressure was 140 MPa and the injection pulse was 2.5 ms. The ambient gas was sulfur hexafluoride with the density of 52.6 kg/m3. To ensure the gas purity, the pressure in the constant-volume vessel should be pumped to the negative pressure through a gas pump. Next, the vessel was scavenged by sulfur hexafluoride gas. After repeating the process
Common rail
Injector
High pressure pump
Light source
Light source
High speed camera
Fuel tank
Constant volume vessel without heating device
Fuel line Singal line
ECU Computer Fig. 3. The constant-volume testing system without heating device.
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Fig. 4. The experimental equipment.
6 mm Chamber
3 mm Chamber
45°
Convex edge
3 mm
6 mm
ω Chamber
R 50
Fig. 5. Combustion chamber modules.
two times, the sulfur hexafluoride gas in the vessel met the experimental requirements. The injecting and the shooting process were synchronized. To ensure the accuracy, the pictures of every condition were repeatedly captured for three times. The combustion experiment of the wall-impinging jet was conducted in the constant-volume testing system (as shown in Fig. 6), which is basically similar to the spray experiment system. The constant-volume testing system used in this experiment has a heating device which consists of many heating wires with the rated heating power of 9 kW [19]. The constant-volume testing system (with the maximum heating temperature of 900 K and the maximum pressure of 6 MPa), consisted of four windows with the diameter of 100 mm to visual the spray and combustion. Around the windows, the window cooling device was fixed. In the experiment, the high-speed camera applied the operating frame rate of 10,000 f/s, the aperture of f/16 and the exposure time of 10 us. The injection pressure was 140 MPa, and the injection pulse was 1.0 ms. The ambient gas was the compressed air with the density of 26.6 kg/m3 and the temperature of 850 K. Firstly part of compressed air was pumped into the vessel and then the compressed air was heated to the aimed temperature. Finally, the other part of compressed air was pumped again into the vessel until the
temperature and pressure in the vessel met the experiment requirements. Besides, the shooting process in the combustion experiment was similar to the shooting process in the spray experiment. 2.2. Data processing The pictures were processed by the Matlab program. The picture processing is shown in Fig. 7. First, the original pictures were denoised. Then the spray pictures were processed to calculate spray area, spray radius, and spray height. The flame pictures were processed by the two-color method. The two-color method is based on the radiation emitted by soot particles at two different wavelengths, to calculate the temperature of the soot particles. The soot particles can instantaneously reach thermal equilibrium with the environment due to the small size of soot particles [20,21]. Hence the temperature of soot particles can represent the temperature of the flame. The soot concentration was represented by the KL factor, where K was the soot absorption coefficient proportional to soot concentration and L was the geometrical thickness in the detecting direction of flame [21–23]. The two-color method can conveniently acquire the combustion
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Fig. 6. The constant-volume testing system with heating device.
features without disturbing the combustion process. To study the propagation of spray and flame, their profiles were obtained through the Matlab program. 3. Results and discussions 3.1. Spray experiment of wall impinging jet Fig. 8 shows the spray impinging process under different combustion chambers. At 0.7 ms ASOI, the spray shapes under different chambers are basically the same, since the fuel spray has not been in contact with the combustion chamber. At 1.1 ms ASOI, part of the fuel spray has impinged the chamber. Under the guiding action of the convex edge, the fuel spray in the LS chamber obviously swirls to both sides of the convex edge. On the contrary, the fuel spray in the x chamber is obstructed by the adducent wall of the x chamber. The diffusion speed of the fuel spray to both sides is restricted. Compared with the fuel spray in the x chamber, the velocity direction of the fuel spray in the LS chamber has been obviously changed. During the process, the kinetic energy loss in the LS chamber is less than the kinetic energy loss in the x chamber since the fuel spray in the LS chamber is not obstructed by the wall [2]. At 1.1 ms ASOI, the fuel spray in the LS chamber has begun to separate from the chamber wall, but the fuel spray in the x chamber still develops along the chamber wall. This is the fuel adhesion phenomenon which is one of the reasons for the increase of PM and HC emissions in diesel engines. Therefore, the separation of the fuel spray from the chamber wall in the LS Chamber plays a significant role in improving emission performance of diesel engines.
At 1.9 ms ASOI, the fuel has already swirled obviously in the LS chambers and the LS motion forms in the chamber. The comparison of the fuel spray process in the LS and x chambers indicates that the least fuel is distributed around both sides of the convex edge due to the fierce wall-diversion effect in the 6 mm chamber. With the decrease of the height of the convex edge, the walldiversion effect is weakened. In the 3 mm chamber, the fuel is increased around both sides of the convex edge, and in the x chamber, the most fuel is distributed around the wall. However, the fuel distribution beyond the chamber module just shows the opposite trend. During the operation of the diesel engine, part of the fuel is burned near the wall (e.g. the condition in the x chamber), resulting in insufficient fresh air in this area and relatively little use of air on both sides of the fuel spray. In the LS chamber, the fuel spray is forced to split into two bunches, which not only improves the over-concentration in the fuel spray core but also effectively utilizes the air on both sides of the fuel spray. Fig. 9 shows the spray areas under different chambers. At the early stage of the spray process, the fuel sprays under the three chambers are in the state of free jet and their spray areas are basically the same before 1 ms ASOI. After 1.2 ms ASOI, the fuel sprays begin to impinge the combustion chamber wall and their spray areas begin to change obviously. Because of the obvious walldiversion effect in the 6 mm chamber, the fuel spray can rapidly develop in the combustion chamber and the spray area is relatively large. According to the experimental results, the adducent wall in the x chamber has a significant hindrance effect on the spray process after impinging the wall. The spray area in the x chamber is always smaller than the spray area in the LS chamber. In the spray impingement process, the increase rate of the spray area is high for
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[8]. The impinging spray is studied through the spray radius and the spray height. Fig. 11 shows the spray radius under different chambers. It can be seen that the fuel spray impinges the wall at 1.1 ms ASOI and begins to develop toward both sides. Due to the diversion effect of the LS chamber, the spray radius in the LS chamber is larger than the spray radius in the x chamber. In the spray process, the spray radius in the 3 mm chamber changes obviously and is always larger than the spray radius in the x chamber. From 1.1 to 1.9 ms ASOI, the spray radius in the 6 mm chamber grows more slowly. After 1.9 ms ASOI, the spray radius in the 6 mm chamber is less than spray radius in the x chamber. This indicates that the higher height of the convex edge will constrain the spray development along the radial direction. Fig. 12 shows the spray height under different chambers. From 1.1 to 1.4 ms ASOI, the spray height in the x chamber is slightly higher than the spray height in the LS chamber. After 1.5 ms ASOI, the spray height in the LS chamber increases linearly and is significantly higher than the spray height in the x chamber. This indicates that the LS chamber can obviously accelerate the diffusion of the fuel spray away from the wall. From 2.4 to 3 ms ASOI, the obvious fluctuation occurs for the x chamber, which is mainly because the stagnate region has not been formed and the initial wall jet vortex is unstable. After 3 ms ASOI, the wall jet vortex causes the formation of a stagnate region at the end of the spray near the wall. The stagnate region hinders the spray flow until a balance is formed. Therefore the spray height grows steadily. 3.2. Combustion experiment of wall-impinging jet
Fig. 7. The picture processing flow.
the 6 mm chamber, but the increase rate is basically unchanged for the 3 mm and x chambers after 3 ms ASOI. Fig. 10 shows a schematic diagram of an impinging spray. The structure of the impinging spray has been analyzed by Katsura
3.2.1. Flame pictures Fig. 13 shows the flame development under different chambers. It can be seen that the obvious flame appears at 0.6 ms ASOI and the flame has touched the wall due to the strong spray penetrability ability under the small ambient density. At 0.8 ms ASOI, the swirling flame has begun to appear on both sides of the convex edge in the LS chamber. At 1 ms ASOI, the LS flame has basically formed. Subsequently (1.2 ms ASOI), the jet flame head separates
Fig. 8. The spray impinging under different chambers.
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Fig. 9. The spray areas under different chambers.
Fig. 10. A schematic diagram of an impinging spray.
Fig. 11. The spray radius under different chambers.
from the end of the convex edge under the action of the diversion arc. At 1.6 ms ASOI, the flame is completely distributed on both sides of the convex edge. Since the flame is not constrained by the wall after separating from the convex edge, the head area of LS flame increases rapidly. Compared to the x chamber, the combustion process in the LS chamber is obviously different.
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Fig. 12. The spray height under different chambers.
In the LS chamber, the combustion flame is distributed on both sides of the convex edge. Because of the existence of arcs on both sides, two swirling flames are formed on both sides of the convex edge, and the few flame exists in the arcs region. In the x chamber, since the adducent surface has a hindrance effect on the flame propagation, the flame can only spread along the wall after reaching the wall. As a result, the flame gradually becomes thick leading to the phenomenon of thermos constraint [24]. At 2 ms ASOI, the flame basically disappears in the central area of the LS chamber, but the flame is still fierce in the central region of the x chamber. This phenomenon shows that the fuel of the x chamber is more than the fuel of the LS chamber in the central region of the combustion chamber. Most of the fuel in the x chamber will be directly sprayed into the burning region. A large amount of fuel concentrated will burn in the central region of the wall. Consequently, the air in the central region is less than the air on both sides of the central region. Compared to the x chamber, the flame in the LS chamber is swirled to both sides of the central region, which will utilize the air in the combustion chamber. At the late stage of the combustion, two strong flame centers are formed in the LS chamber and the spray height also increases obviously. However, only one flame center is formed in the x chamber. Because of the same injection parameters, the ignition time is the same in the three combustion chambers. At 2.7 ms ASOI, the obvious flame has disappeared in the LS chamber, and the combustion has finished. On the contrary, the large area of the bright flame still exists in the x chamber. Therefore, the combustion duration in the x chamber is longer than the combustion duration in the LS chamber, which indicates that the combustion speed in the LS chamber has been improved significantly. 3.2.2. Flame temperature Fig. 14 shows the flame temperature under different chambers. From 0.6 to 1.1 ms ASOI, the flame temperature in the LS chamber is higher than the flame temperature in the x chamber. After 1.4 ms ASOI, the flame temperature in the LS chamber is lower than the flame temperature in the x chamber. This indicated that the LS chamber can accelerate the combustion. Fig. 15 shows the temperature distribution under different chambers. Before 1.5 ms ASOI, the flame area in the LS chamber is larger than the flame area in the x chamber. For the LS chamber, the temperature of the local zone has reached 2300 K at 0.9 ms ASOI, and the flame temperature and area are significantly increased at 1.2 ms ASOI. After 1.5 ms ASOI, the high-temperature zone in the LS chamber is rapidly reduced. For the x chamber, the slow burning at the early stage causes an accumulation of premixed gas and a large area of high-temperature flame is formed near the wall. This is mainly
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ω Chamber
3 mm Chamber
6 mm Chamber 0.5 ms
0.6 ms
0.8 ms
1.0 ms
1.2 ms
2.0 ms
1.6 ms
2.2 ms
2.7 ms
Fig. 13. The flame development under different chambers.
Fig. 14. The flame temperature under different chambers.
Fig. 16. The total KL factor under different chambers.
2300K
2200K
ω Chamber
2100K
2000K
1900K
3 mm Chamber 1800K
1700K
6 mm Chamber
0.6 ms
1600K
0.9 ms
1.2 ms
1.5 ms
1.9 ms
Fig. 15. The temperature distribution under different chambers.
2.2 ms
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Fig. 17. The KL factor distribution under different chambers.
because the flame development is obstructed by the adducent wall of the x chamber. 3.2.3. KL factor Fig. 16 shows the total KL factor under different chambers. Before 1.3 ms ASOI, the total KL factor in the LS chamber is slightly higher than the total KL factor in the x chamber. Subsequently, the total KL factor in the x chamber increases rapidly at 1.2 ms ASOI and then becomes much higher than the total KL factor in the LS chamber. The main reason is that the stagnate region has been formed in the high-temperature flame at 1.2 ms ASOI, and a large amount of fuel accumulates in the stagnate region. Then the area of the stagnant region continues to expand and the fuel concentration continues to increase and generate a large amount of soot. Fig. 17 shows the KL factor distribution under different chambers. At 0.9 ms ASOI, the area with the KL factor greater than 2.5 has appeared in the LS chamber, but this area just appears in the x chamber at 1.2 ms ASOI. At 1.5 ms ASOI, the large area with the
Stagnate region
KL factor greater than 2.5 appears near the wall in the x chamber, but this area appears away from the wall in the LS chamber. After 2.2 ms ASOI, the area with KL factor greater than 2.5 basically disappears in the LS chamber, but this area still exists in the x chamber. 3.3. Mechanism of the LS combustion Through the above research, it is found that the LS chamber can effectively promote the air-fuel mixing, accelerate the burning rate and reduce soot emission. In order to further explain the characteristics of the LSCS, the profile development of the spray and the flame is analyzed through processing the spray and combustion pictures. Fig. 18 shows the development of the spray profile. After the spray impinges the wall, the spray front moves away from the wall. In the 6 mm chamber, the spray front moves in the normal direction of the wall, while the spray front in the x chamber basically moves along the wall. Fig. 18 and the spray animation show
Boundary
Stagnate region
Boundary Stagnate region
1.1ms
1.1ms
6 mm Chamber
1.1ms
3 mm Chamber
Fig. 18. The development of the spray profile (the interval between two profiles is 0.2 ms).
ω Chamber
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Stagnate region
Stagnate region
0.6ms
0.6ms
6 mm Chamber
Stagnate region
0.6ms
3 mm Chamber
ω Chamber
Fig. 19. The development of the flame profile (the interval between two profiles is 0.2 ms).
Fig. 20. A schematic diagram of the spray impinging wall in the LS and x chambers.
that there is a stagnate region at the root of the spray. The stagnate region under different chambers shows the different shapes. The stagnate region in the x chamber has the largest area in all three chambers, but the stagnate region in the 6 mm chamber has the smallest area. In all three sprays, there is an available space between the main spray and the spray after impinging the wall. The available space in the x chamber has the smallest area in all three chambers, and the available space in the 6 mm chamber has the largest area. This is because the diversion effect of the LS chamber is higher than the diversion effect of the x chamber. Fig. 19 shows the development of the flame profile. In the LS chamber, the flame front moves in the opposite direction of the spray movement, and the jet flame and the fame after impinging
the wall are separated by a narrow space. On the contrary, the flame front in the x chamber moves along the wall, and the jet flame and the fame after impinging the wall are connected through the stagnate region, resulting in the accumulation of fuel. Fig. 20 shows a schematic diagram of the spray impinging wall in the LS and x chambers. The spray process in the x chamber is divided into two stages: free jet and wall jet. At the early stage of wall jet, the spray moves along the wall, and spray height and spray radius both increase. Due to the poor diversion effect of the x chamber wall, a stack region is formed near the wall. At the late stage of wall jet, the fuel in the stack region continues to accumulate and hinders the diffusion of the fuel. This results in the emergence of a large area of the stagnate region at the root
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Fig. 21. LS and intervening wall jet after the separation of jet head from the convex edge.
of the spray. At this time the spray height and spray radius basically do not increase, which causes a large accumulation of fuel around the wall, finally producing much more soot. According to Figs. 20 and 21, the spray process in the LS chamber is divided into three stages: free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. At the stage of free jet, the spray in the LS chamber is not obviously different from the spray in the x chamber. At the stage of formation of LS, the convex edge forces the stack region to degenerate into the stack point, which greatly reduces the accumulation of fuel and the production of soot. Under the diversion action of the convex edge, spray height and radius both increase, and the spray height increases more obviously than the spray radius. The increase rate of the spray height in the LS chamber is much larger than the increase rate of the spray height in the x chamber. At the late stage of formation of LS, the spray will separate from the convex edge and then diffuse quickly in the direction away from the wall. This results in a small area of the stagnate region. At the last stage, the relationship between two sprays is shown in Fig. 21. After the spray separates from the wall under the action of the convex edge, the adjacent spray will promote each other and be quickly diffused to the unused space for a better mixing. 4. Conclusion The mixing and combustion mechanisms of LSCS were investigated in a constant-volume vessel. Through the high-speed technique and two-color method, the data of spray impinging and wall jet combustion was obtained to analyze the combustion and mixing performance of LS and x combustion systems. The following conclusions can be drawn: 1. The spray area in the LS chamber is larger than the spray area in the x chamber. Moreover, the spray height and spray radius in the LS chamber are larger than the spray height and spray radius in the x chamber. At the early stage of wall jet, the spray radius in the LS chamber is larger than the spray radius in the x chamber. At the late stage of wall jet, the spray radius in two chambers are basically the same, but the spray height in the LS chamber increases quicker than the spray height in the x chamber during the whole process of wall jet. In the LS chamber, the convex edge accelerates the diffusion in the direction away from the wall and reduces a large accumulation of fuel.
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2. In the LS chamber, two flame centers are formed away from the wall under the effect of the convex edge. On the contrary, only one flame center in the x chamber is formed around the wall and the fuel is sprayed into the flame, deteriorating the combustion. Hence the combustion speed in the LS chamber is quicker than the combustion speed in the x chamber. 3. The flame temperature in the LS chamber is higher than the flame temperature in the x chamber at the early stage of the combustion, but the flame temperature in the LS chamber is lower than the flame temperature in the x chamber at the late stage of the combustion. In the x chamber, more soot is produced at the late stage and mainly distributed around the wall. Contrarily, more soot is produced at the early stage and mainly distributed away from the wall in the LS chamber. Hence the soot in the LS chamber is much easy to mix with the air and be oxidized. 4. In the LS chamber, the spray and combustion processes of the wall-impinging jet are divided into three stages: free jet, formation of LS, and LS & intervening wall jet after the separation of jet head from the convex edge. The convex edge splits the fuel spray from the core of the spray and forms LS, which could solve the over-rich mixture in the core of the fuel spray. The development of spray and flame profiles indicates that LSCS can effectively utilize the air in the circumferential space of the combustion chamber, and promote the development of the fuel spray away from the wall, avoiding the accumulation of fuel around the wall. The main reason is that the stack region is not formed around the wall in the LS chamber and the area of the stagnate region is always small during the development of the spray.
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