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Evaporation characteristics of fuel adhesion on the wall after spray impingement under different conditions through RIM measurement system
Fuel 258 (2019) 116163
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Full Length Article
Evaporation characteristics of fuel adhesion on the wall after spray impingement under different conditions through RIM measurement system
T
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Hongliang Luoa,b, , Keiya Nishidab, Youichi Ogatab a b
College of Mechanical and Power Engineering, China Three Gorges University, Yichang, Hubei 443002, China Department of Mechanical System Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Evaporation characteristics Wall impingement Lifetime Fuel adhesion RIM
In direct-injection spark-ignition (DISI) engines, spray-wall impingement affects the mixture formation as well as combustion and exhaust emissions, making it difficult to satisfy the regulation of particle number (PN) in the future standards. In order to understand the mechanisms deeply, not only the formation but the evaporation characteristics of fuel adhesion should be investigated in detail. In this study, the fuel adhesion thickness was measured using the refractive index matching (RIM) method, then mass, area and thickness of the fuel adhesion were calculated. The evaporation evolution of fuel adhesion on the wall was discussed. Moreover, the lifetime of fuel adhesion was compared at the fixed conditions. The results showed fuel adhesion firstly evaporates from periphery, then the “hollow” occurs and develops, which facilitates the fuel evaporation. The increased injection pressure favors the droplets evaporation owing to the better atomization, leading to the decreased fuel adhesion mass and area on the wall. However, due to the strong momentum exchange between fuel droplets and air, the increased ambient pressure increases the fuel adhesion mass and area on the wall. Moreover, high injection pressure shortens the fuel adhesion lifetime by increasing evaporation rate, but high ambient pressure prolongs it due to the liquid/vapor phase changing.
1. Introduction Compared to port fuel injection (PFI) engines, direct-injection spark-ignition (DISI) engines offer many advantages, such as high efficiency and specific power, thanks to the better air-fuel mixture [1–4]. To fully exploit the potential, the reduction in particular matter (PM) emissions is the major concern owing to the spray impingement on cylinder wall and piston top surface, resulting in “wet-wall” [5–7]. Therefore, a particular attention should be paid to the formation of the fuel adhesion that makes DISI engines difficult to meet the Euro 6 emission regulations [8,9]. To clarify the spray-wall interaction, numerous investigations about fuel adhesion on the wall were performed by applying various advanced optical techniques. Among them, refractive index matching (RIM) method is widely used to measure the fuel adhesion thickness on the wall under various conditions. This method was developed by Drake et al. [10–12] to investigate fuel adhesion mass, area, and thickness with milli-second temporal resolution. Then, it become popular through the application of Yang and Ghandhi [13], Maligne and Bruneaux [14], Zheng et al. [15], Henkel et al. [16], Ding et al. [17], He et al. [18], and Luo et al. [19–21]. Although researches on fuel adhesion under non-evaporation conditions were well
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investigated, such as the effect of wall roughness[19], impingement distance[20], injection pressure [14,15,20], ambient pressure [13,15,17], the fuel adhesion under evaporation condition, especially lifetime of fuel adhesion on the wall was seldom reported. When evaporation occurs, the lifetime is a significant parameter to evaluate how much fuel survives during the engine operating condition, which is responsible for the PN emission. Moreover, the prediction of fuel lifetime may shorten the design cycle of engine with the consideration of the emission regulations. While, for the “lifetime”, it was firstly applied to study the single droplet on the wall. Tamura et al. [22] showed the droplet lifetime curve of both gasoline and diesel fuels, and they found the five stages for evaporation: evaporation, vaporization, transition, spheroidal evaporation, and spheroidal combustion. Hiroyasu et al. [23] described the droplet lifetime ranging the engine operating condition by using diesel light oil, and the results showed that lifetime curve shifted to the direction of higher plate temperature. Chin et al. [24] investigated the effect of ambient pressure on single droplets evaporation rate of n-heptane fuel, and results showed that the evaporation rate decreases with ambient pressure when the ambient temperature is lower than 600 K. Then Senda et al. [25] optimized the diesel droplet lifetime curves through setting different heat transfer
Corresponding author. E-mail address: [email protected] (H. Luo).
https://doi.org/10.1016/j.fuel.2019.116163 Received 12 June 2019; Received in revised form 1 August 2019; Accepted 5 September 2019 Available online 16 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Notation ASOI BTDC DISI fps fM (h) Δh L L/D M (h) PM
PN PFI Pinj Pamb Ra RI RIM SOI Tamb Tsat W
After Start of Injection Before Top Dead Center Direct Injection Spark Ignition Frames Per Second Probability of M (h) Thickness Fraction Fuel Adhesion Length Length To Diameter Sum of Fuel Adhesion Mass Particle Matter
Particular Number Port Fuel Injection Injection Pressure Ambient Pressure Arithmetical Mean Deviation of the Profile Refractive Index Refractive Index Matching Start of Injection Ambient Temperature Saturated Temperatures Fuel Adhesion Width
can be involved in this study. Fuel adhesion evaporation behavior is further described: it firstly evaporates from periphery, then the “hollow” occurs and develops, until all evaporates. The fuel adhesion lifetime can be shortened by increasing the injection pressure and decreasing the ambient pressure.
models and regimes by experimental results. The sub-model was established by considering the breakup behaviors of impingement and the dispersion process of breakup-droplets. Kim et al. [26] investigated the effect of ambient pressure on the evaporation of a single n-heptane fuel droplet, and results showed that the droplet lifetime increased with pressure at a low ambient temperature, but it decreased at high temperature. Although considerable works have been done on droplets evaporation, it is well known that the evaporation behavior of the fuel adhesion is more complex in contrast to that of single droplet. After impingement, both formation and evaporation processes of fuel adhesion occur, leading to complex phenomena. Therefore, the mechanism of fuel adhesion evaporation is far from fully understood. Recently, Maligne and Bruneaux observe the iso-octane fuel adhesion evaporation occurred from periphery to the center under gasoline engine-like conditions at a quite short duration [14]. But no details about lifetime of fuel adhesion on wall were reported until now. In the present work, the evaporation characteristics of fuel adhesion under different injection and ambient pressures were discussed based on the RIM method. Firstly, the evaporation of fuel adhesion on the wall was discussed, followed by the mass, area and thickness distribution. Then, the “hollow” was observed on the fuel adhesion with its development and effect on adhesion evaporation analyzed in detail. Finally, the lifetime of the fuel adhesion at various conditions were compared and discussed. Specifically, all the results were repeated at ten times, and the averaged results were shown in this paper. Overall, two contributions
2. Experimental details 2.1. Experimental apparatus The experimental apparatus for fuel adhesion measurement in the current study is shown in Fig. 1. Fuel is injected into the chamber through a mini-sac injector. A high-speed video camera (Photron FASTCAM SAZ) set at 10,000 frames per second (fps) with a resolution of 512 × 512 pixels is utilized for fuel adhesion observation from the bottom view. A xenon lamp (Ushio SX-131 UID501XAMQ) placed at a position perpendicular to the camera is used to illuminate the chamber with an incident angle of 15 deg. Both injector and camera work synchronously by the help of a delay generator. A flat plate made of quartz glass is placed under the injector as an impingement wall. Beneath it, a reflection mirror is installed so as the camera can capture the fuel adhesion image from the bottom view.
Fig. 1. Experimental setup for fuel adhesion measurement. 2
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defined as the intersection of the nozzle center axis and flat wall. The impingement angle is set at 45 deg, and the impingement distance is determined at 22 mm from the nozzle exit to the impingement point along the spray axis by considering the real distance when injection occurs in a small gasoline engine. The positive y axis is defined as the direction of spray after impingement, while the positive z axis is defined as parallel to the injector axis. The positive x axis is the perpendicular to the yz plane pointing out of the figure. The injector used is a mini-sac gasoline injector with a single hole. The diameter and length of nozzle hole is 0.155 mm and 0.65 mm, respectively, leading to the length-todiameter (L/D) ratio being 4.2. In addition, the nozzle hole is a conventional straight hole without counterbore. Table 2 lists the test conditions. Owing to the special requirement of RIM method for fuel adhesion measurement, the refractive index (RI) of tested fuel should be similar to that of the quartz glass (RI = 1.49). Therefore, toluene (RI = 1.46) is selected as a substitute for gasoline, and further explanations can be found in our previous studies [20,21]. Experiments are performed in a constant high-pressure chamber filled with nitrogen gas. An electric preheater (Toyokoatsu Kanthal AF-200 V) is installed at the bottom of the chamber. The air temperature (Tamb = 433 K) is monitored by a thermocouple placed above the wall. One thing worth noting that it is difficult to measure the temperature of the flat wall, thus the temperature near the wall obtained from the thermocouple presenting it. According to the in-cylinder conditions at different crank angles before top dead center (BTDC) during the warmup period, two different ambient pressures 0.15 and 0.74 MPa are selected at Tamb = 433 K, resulting in the ambient density at 1.95 and 5.95 kg/m3, respectively. Another thing needs to be noted that the saturated temperatures (Tsat) of toluene at 0.15 MPa is 398 K, indicating Tsat < Tamb. While at 0.74 MPa, Tsat is 472 K, indicating Tsat > Tamb. Namely, toluene changes the phase from vapor to liquid when increasing the ambient pressure from 0.15 to 0.74 MPa. For safety care, a cooling system is applied to regulate the fuel temperature (before injection) at room temperature. The injection pressure varies among 10, 20, and 30 MPa with different injection durations of 2.9, 2.1, and 1.7 ms to satisfy the requirement of constant injection mass at 4.0 mg with a consideration of the real injection mass in each hole.
Table 1 Injector parameters and impingement conditions. Impingement Conditions Impingement Wall
Quartz Glass
Diameter of the Wall (mm) Thickness of the Wall (mm) Impingement Distance (mm) Impingement Angle (deg) Surface Roughness (μm)
50 2 22 45 Ra 7.0
Injector Parameters Injector Type Hole Number Hole Type Nozzle Hole Diameter (mm) Nozzle Hole Length (mm) L/D Ratio
Mini-Sac 1 Straight Hole without Counterbore 0.155 0.65 4.2
2.3. Image processing method RIM method is applied to measure the fuel adhesion thickness. The image processing of the RIM experiment is shown in Fig. 3. First, the image without fuel adhesion named as “dry image” is acquired. After fuel adhering on the wall, it is subtracted by the “dry image” to obtain the only adhered fuel image. Using the calibration curve shown in Fig. 3, the thickness distribution can be obtained. For the adhered fuel area, the pixels whose thickness is larger than 0.1 μm can be integrated. Using the scale (0.106 mm/pixel) got from the observation, the adhesion area is calculated. While, for the adhered fuel mass, the thickness can be added up when the value is larger than 0.1 μm. Then using the scale and density of toluene (867 kg/m3), the adhesion mass can be calculated. Additional details about RIM method and calibration curve can be seen in our previous publications [20,21].
Fig. 2. Specification of the injector and flat wall. Table 2 Test conditions. Evaporation conditions Test Fuel Fuel Temperature (K) Injection Mass (mg) Ambient Gas Injection Pressure (MPa) Injection Duration (ms) Ambient Temperature (K) Ambient Density (kg/m3) Ambient Pressure (MPa)
Toluene 293 4 Nitrogen 10, 20, 30 2.9, 2.1, 1.7 433 1.19 0.15
5.95 0.74
3. Results and discussion
2.2. Experimental conditions
3.1. Evolution of fuel adhesion
Table 1 lists the impingement conditions and injector parameters. The quartz glass (Sigma Koki, DFSQ1-50CO2) with surface roughness at Ra7.0 μm is employed to represent the rough surface of the piston [19], measured by a portable high-performance surface roughness and waviness measuring instrument (Kosaka Laboratory Ltd., SE300) with a resolution of 0.0064 μm. As shown in Fig. 2, the diameter of the plate is 50 mm, and the thickness is 2 mm. The impingement point (o) is
Fig. 4 illustrates the evolution of the fuel adhesion at Pamb = 0.15 MPa. The images of adhesion on the wall at 50, 70, 100, and 150 ms ASOI are depicted. The adhered fuel thickness varying from 0 to 2.0 μm is shown by the pseudocolor, and the cross symbol represents the impingement point (o). It is evident that fuel area decreases with time due to evaporation. It is interesting to find that fuel adhesion evaporates from the impingement region to downstream. Owing to the interaction between fuel 3
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Fig. 3. Image processing for RIM.
with pseudocolor representing the thickness from 0 to 2.0 μm and the cross symbol being the impingement point (o). In contrast to Fig. 4, the fuel adhesion survives much longer when increased the ambient pressure up to 0.74 MPa. More discussion about it will be discussed in the next part. Again, it is clear to see the fuel adhesion area decreases with an increase in injection pressure. Moreover, it is worth noting that the area increases by raising the ambient pressure, when compared to Fig. 4. The increased ambient pressure decelerates the droplets by strong resistance force from the air. Then more droplets tend to “stick” on the wall instead of “splash”, leading to more fuel adhesion on the wall. Moreover, Kim et al. [26] and Luo et al. [5] pointed out that the splashing droplets after impingement tend to coalesce at higher ambient pressure. The large droplets re-deposit on the wall more easily, thus increasing the fuel adhesion on wall. Furthermore, it was demonstrated that the evaporation rate decreases with ambient pressure increasing [26], thus more fuel adhesion remaining on the wall. And more discussions about the evaporation rate at high ambient pressure can be seen in the next part. Additionally, similar to Fig. 4, it can also be observed that more fuel left near the cross symbol with an increase in injection pressure at 500 ms ASOI in Fig. 5. The definitions of fuel adhesion length (L) and width (W) are shown in Fig. 6 to discuss the detailed fuel adhesion characteristics. L is the maximum vertical distance from the top edge to the bottom edge of the
spray and wall, the adhesion located on the impingement region is thin compared to that of the downstream, which leads to quick evaporation. With time elapse, it seems the fuel adhesion “moves downward”. Moreover, the fuel adhesion area decreases with an increase in injection pressure. Injection pressure promotes better atomization, the small size droplets evaporates more easily at high temperature condition before impingement. Even though some droplets impinge on the wall, with an increase in the Weber number, the droplets behavior may change from “stick” to “splash” [27,28]. In other words, although some droplets stick on the wall, the air flow with high velocity promotes its evaporation, resulting in less fuel adhesion on the wall ultimately. It should be noted that during the evaporation, the adhesion firstly evaporates from the periphery of the adhesion as Maligne and Bruneaux [14] reported, then evaporates from the center of the adhesion, leading to a “hollow” on the adhesion, which will be discussed more in the next section. It is also interesting to find that the more fuel left near the cross symbol with an increase in injection pressure, see at 150 ms ASOI of Fig. 4. The reason can be involved as below: After impact on the wall, fuel tends to spread along the wall at low injection pressure, resulting in more fuel left at the downstream. In contrast, fuel tend to splash at high injection pressure, leading to less fuel left at the downstream. Fig. 5 shows the evolution of the fuel adhesion at Pamb = 0.74 MPa. Similar to Fig. 4, images at 50, 100, 300, and 500 ms ASOI are shown 4
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Fig. 4. Evolution of fuel adhesion at Pamb = 0.15 MPa.
results in the fuel adhesion “moving downward”, the same phenomena can be observed in Figs. 4 and 5. Then from a certain value (for Pamb = 0.74 MPa, when L decreases to 10 mm; while for Pamb = 0.15 MPa, when L decreases to 7.5 mm), both L and W decrease at a similar rate. This is attributed to the “hollow”. When “hollow” occurs, the fuel adhesion evaporates change from the periphery to the “hollow”, leading to the similar decreasing rate in L and W, which will be discussed more in the next part.
fuel adhesion. W is the maximum horizontal distance from the left edge to the right edge of the fuel adhesion. And all the results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data in Fig. 7. Fig. 7(a) and (b) show the fuel adhesion length and width under different injection and ambient pressures, respectively. Obviously, both L and W decrease with time due to evaporation. For all case at different conditions show similar tendencies although some differences can be observed. All curves can be divided into two stages by different decrease rates: Stage Ⅰ and Ⅱ. And the transition point was marked by a star symbol. It reveals the transition time decreases with decreased ambient pressure and increased injection pressure due to the strong evaporation. For L, the opposite tendencies can be recognized. When Pamb = 0.15 MPa, L decreases firstly sharply at Stage Ⅰ, then normally at Stage Ⅱ. While, for Pamb = 0.74 MPa, L decreases firstly normally at Stage Ⅰ, then sharply at Stage Ⅱ. The high ambient pressure hiders the fuel evaporation to some extent, thus decelerating the fuel adhesion “moving downward” may be one possible reason for it. However, for W, all results show the same tendency: W decreases firstly normally at Stage Ⅰ, then sharply at Stage Ⅱ, indicating little effect of both injection and ambient pressures on W decreasing transition. Fig. 7(c) shows the fuel adhesion length to width under different injection and ambient pressures. Both L and W decreases from the maximum value at right side to 0 along different curves. It has to be noticed that for all case, L decreases more sharply than W at first, which
3.2. Fuel adhesion mass and area Fig. 8 shows fuel adhesion mass at left vertical axis, with right one being the ratio of fuel adhesion mass to injection mass. Results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data. It reveals that mass decreases with injection pressure. The better atomization and quick evaporation rate should be responsible for it. However, the mass increases with ambient pressure. It should be attributed to the more fuel adhering on the wall and low evaporation rate. One interesting thing is that under Pinj = 10 MPa, Pamb = 0.74 MPa, the mass increases until 200 ms ASOI, even the injection duration is just 2.9 ms. The lowest velocity of spray and largest size of the droplets should be responsible for it. Owing to the high ambient pressure and low injection pressure, the incoming droplet impinge on the wall relatively lately, resulting in mass increase. Furthermore, the high ambient pressure 5
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Fig. 5. Evolution of fuel adhesion at Pamb = 0.74 MPa.
coalesces the droplets, making the droplets difficult to evaporate before impingement. Additionally, the splashing droplets re-deposit on the wall easily [21], which should be another reason for that. Results also provide that the maximum ratio of fuel adhesion mass to injection mass is about 4% under all cases, indicating most of the fuel evaporates or splashes off the wall. Fig. 9 shows the fuel adhesion area with time. Results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data. The same as mass, the adhesion area increases with ambient pressure, but decreases with injection pressure. One interesting observation is that under Pinj = 10 MPa, Pamb = 0.74 MPa condition, almost no increase can be seen in the area, although mass increase can be observed in Fig. 8. The incoming and re-depositing droplets impinge on the same region of the adhesion may be one possible explanation for it. Additionally, from the observation in Figs. 8 and 9, the time for total evaporation of fuel adhesion at different conditions are totally different, more analyses can be seen in the next section. 3.3. “Hollow” on the fuel adhesion Fig. 10 illustrates the “hollow” in fuel adhesion under Pinj = 10 MPa, Pamb = 0.15 MPa. From 60 to 120 ms ASOI, the fuel adhesion evaporates from periphery, leading to fuel adhesion “moving downward”. In fact, the evaporation occurs at all fuel adhesion,
Fig. 6. Definition of fuel adhesion length and width.
6
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Fig. 8. Fuel adhesion mass.
Fig. 9. Fuel adhesion area.
adhesion structure and evaporation dynamic. As the fuel adhesion is formed by the spray impingement, the fuel adhesion is thin at the periphery owing to the shear force. Therefore, the heat transfer from the ambient gas to the fuel adhesion results in a faster increase of temperature, leading to a faster evaporation than that in the center of the adhesion. Specially, owing to the roughness of the impingement wall, the fuel adhesion is non-uniform [19]. Then, with evaporation, “hollow” occurs at the relative thin region in the center of the adhesion, resulting in the adhesion “breakup”. Then “hollow” develops owing to the stronger heat transfer and faster evaporation, facilitating the fuel evaporation. Finally, the fuel adhesion becomes much less uniform until all evaporates. The mass at different adhesion thickness under Pinj = 10 MPa, Pamb = 0.15 MPa was applied for further investigation, as depicted in Fig. 12. The horizontal axis represents fuel adhesion thickness, with fuel adhesion mass being as the vertical axis. The adhesion mass is determined by the values of each pixel calculated from the equation below: ∞
Fig. 7. Fuel adhesion length and width.
M (h) =
∑ h (i) i=0
resulting in the thick region becomes thin with time. Then “hollow” occurs around 140 ms, and develops, leading to less uniformity of fuel adhesion. Finally, the fuel adhesion evaporates from the “hollow” until all evaporated. A conceptual model for fuel adhesion evaporation is proposed here to explain this mechanism. Fig. 11 shows the correlation between fuel
(1)
where M (h) is defined as the sum of fuel adhesion mass in the thickness fraction between h ± 0.5Δh and h, and Δh is 0.05 μm. Fig. 12 shows the mass distribution along thickness with time under Pinj = 10 MPa, Pamb = 0.15 MPa. It can be observed that the curve decreases due to evaporation. Furthermore, the peak value decreases and moves to right from 50 to 150 ms, owing to the evaporation occurring 7
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Fig. 10. “Hollow” occurs during evaporation (Pinj = 10 MPa, Pamb = 0.15 MPa).
Fig. 11. Schematic conceptual model of fuel adhesion evaporation dynamic.
the influence of the “hollow” and explaining this phenomenon above, the probability of fuel adhesion was employed in Fig. 13. The probability curves must satisfy the normalization condition:
from periphery. While, from 150 to 400 ms, the peak value decreases but moves to left. This should be attributed to the “hollow” occurrence and development, resulting in thick fuel adhesion “breakup” and evaporation. For further understanding the uniformity of the fuel adhesion under 8
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time. From 50 to 100 ms ASOI, the uniformity changes little. However, when “hollow” occurs, the curve becomes wide, suggesting worse uniformity of fuel adhesion on the wall. From it the conclusion can be obtained that fuel adhesion evaporates from periphery with less uniformity changing, then “hollow” facilitates the evaporation of the fuel adhesion, resulting in non-uniformity of the adhesion, which agrees well with the observation in Fig. 10. 3.4. Fuel adhesion lifetime Fig. 14 shows the fuel adhesion lifetime on the wall. It should be noted that the lifetime in this article is the time duration from the start of injection (SOI) to the fuel adhesion totally evaporation. One thing should be pointed that although the adhesion lifetime is quite longer compared to the real working condition in gasoline engine, where the adhesion not completely evaporates before the combustion occurs. Lifetime is significantly important to evaluate evaporation characteristics, especially for optimization and verification of the model in simulation. Results show that injection pressure decreases the lifetime, which can be explained by the enhanced atomization and high evaporation rate. The decrease in lifetime is more obvious by increasing the injection pressure from 10 to 20 MPa, when compare to that of injection pressure elevating from 20 to 30 MPa. Rather, the increased ambient pressure prolongs the fuel adhesion lifetime. As described before, due to the transient from vapor to liquid phase by enhancing the ambient pressure from 0.15 to 0.74 MPa, the evaporation rate decreases, resulting in long lifetime of the fuel adhesion on the wall. As a result, fuel adhesion on the wall increases. However, the injection pressure has no effect on the phase change. Additionally, this should be another reason for the fuel adhesion mass increases with an increase of ambient pressure. In order to decrease the soot emission, the less fuel adhesion is deposited on the wall, the better. As a result, from this measurement results, the injection pressure should be increased but the ambient pressure should be taken care to decrease. Although in this experiment, the selected ambient pressures of 0.15 MPa and 0.74 MPa lead to the liquid/vapor phase changing, Kim et al. [26] and Tao et al. [29] also pointed the increased ambient pressure inhibits the fuel evaporation by their experiment and simulation results, respectively. Frankly speaking, owing to the limitation of the experimental equipment, the ambient temperature is far lower than that in the real engine, resulting in the lifetime of the fuel adhesion much longer than the real engine working time. Nevertheless, with the help of the curves in Fig. 14, the trends can be identified, and the evaporation behaviors can be estimated. The experimental results can provide guidance for engine design and basis for simulation work.
Fig. 12. Fuel adhesion mass distribution along thickness (Pinj = 10 MPa, Pamb = 0.15 MPa).
Fig. 13. Probability of fuel adhesion mass distribution along thickness (Pinj = 10 MPa, Pamb = 0.15 MPa).
4. Conclusion The evaporation characteristics of fuel adhesion under different pressures were investigated experimentally in this paper. The fuel adhesion evolution was discussed, followed with mass, area and thickness of the fuel adhesion. Furthermore, the “hollow” of adhesion was analyzed with observation, mass distribution and probability of thickness. Finally, the lifetime of the fuel adhesion was compared. Based on these quantitative data provided in this study, the major conclusions are summarized as follows: 1. The fuel adhesion firstly evaporates from periphery, leading to fuel adhesion “moving downward”. Then, “hollow” occurs and develops, resulting in less uniformity of fuel adhesion. Finally, the fuel adhesion evaporates from the “hollow” until all evaporated. Furthermore, L decreases more sharply than W at first, which leads to the fuel adhesion “moving downward”. When “hollow” occurs, both L and W decrease at a similar rate. 2. The increased injection pressure promotes better atomization and
Fig. 14. Fuel adhesion lifetime. ∞
∑ fM (hi) = 1 i=0
(2)
where the probability of M (h) is defined as fM (h) . Fig. 13 illustrates the probability along fuel adhesion thickness with 9
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increases the momentum of the spray, thus changing the droplets behavior from “stick” to “splash”, resulting in less fuel adhesion on the wall. Moreover, the strong flow of the air favors the evaporation of the fuel on the wall. The increased ambient pressure increases the density of the air, leading to stronger air drag force. Therefore, the velocity of spray decreases significantly, resulting in more fuel adhering on the wall instead of “splash”. In addition, owing to the transient from vapor to liquid phase, the higher ambient pressure decreases the evaporation rate, thus increasing the fuel adhesion on the wall. 3. The fuel adhesion lifetime is shortened by increasing injection pressure owing to the better atomization and quick evaporation. And the lifetime extends with an increase in ambient pressure because of more fuel adhesion, hard evaporation and liquid/vapor changing.
[10] Drake MC, Fansler TD, and Rosalik ME. Quantitative high-speed imaging of piston fuel films in direct-injection engines using a refractive-index-matching technique. In: The 15th Annual Conference on Liquid Atomization and Spray Systems, Madison, WI; 15–17 May 2002. p. 1–8. [11] Drake MC, Fansler TD, Solomon AS, et al. Piston fuel films as a source of smoke and hydrocarbon emissions from a wall-controlled spark-ignited direct-injection engine. SAE Technical Paper 2003-01-0547; 2003. [12] Drake MC, Haworth DC. Advanced gasoline engine development using optical diagnostics and numerical modeling. Proc Combust Inst 2007;31(1):99–124. [13] Yang B, Ghandhi J. Measurement of diesel spray impingement and fuel film characteristics using refractive index matching method. SAE Technical Paper 2007-010485; 2007. [14] Maligne D, Bruneaux G. Time-resolved fuel film thickness measurement for direct injection SI engines using refractive index matching. SAE Technical Paper 2011-011215; 2011. [15] Zheng Y, Xie X, Lai MC, et al. Measurement and simulation of DI spray impingements and film tics. In: The 12th Triennial Int. Conf. on Liquid Atomization and Spray Systems, Heidelberg, Germany; 2–6 Sep. 2012. p. 1–8. [16] Henkel S, Beyrau F, Hardalupas Y, et al. Novel method for the measurement of liquid film thickness during fuel spray impingement on surfaces. Opt Express 2016;24(3):2542–61. [17] Ding CP, Sjöberg M, Vuilleumier D, Reuss DL, He X, Böhm B. Fuel film thickness measurements using refractive index matching in a stratified-charge SI engine operated on E30 and alkylate fuels. Exp Fluids 2018;59(3):59. [18] He X, Li Y, Sjöberg M, Vuilleumier D, Ding CP, Liu F, et al. Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel. Proc Combust Inst 2019;37(4):4955–63. [19] Luo HL, Uchitomi S, Nishida K, Ogata Y, Zhang W, Fujikawa T. Experimental investigation on fuel film formation of spray impingement on flat walls with different surface roughness. Atomization Sprays 2017;27(7):611–28. [20] Luo HL, Nishida K, Uchitomi S, Ogata Y, Zhang W, Fujikawa T. Effect of spray impinging distance on piston top fuel adhesion in direct injection gasoline engines. Int J Engine Res 2018. (Online First). [21] Luo HL, Nishida K, Uchitomi S, Ogata Y, Zhang W, Fujikawa T. Effect of temperature on fuel adhesion under spray-wall impingement condition. Fuel 2018;234:56–65. [22] Tamura Z, Tanasawa Y. Evaporation and combustion of a drop contacting with a hot surface. Symposium on Combustion 1958;7(1):509–22. [23] Hiroyasu H, Kadota T, Senda T. Droplet evaporation on a hot surface in pressurized and heated ambient gas. Trans JSME 1973;39–328:3779. [24] Chin J, Lefebvre A. The role of the heat-up period in fuel drop evaporation. 21st Aerospace Sciences Meeting; 1983. [25] Senda J, Kobayashi M, Iwashita S, Fujimoto H. Modeling of diesel spray impingement on a flat wall. SAE Trans 1994;103(3):1918–31. [26] Kim H, Sung N. The effect of ambient pressure on the evaporation of a single droplet and a spray. Combust Flame 2003;135:261–70. [27] Bai C, Gosman AD. Development of methodology for spray impingement simulation. J Engines 1995;104(3):550–68. [28] Bai C, Rusche H, Gosman AD. Modeling of gasoline spray impingement. Atomization Sprays 2002;12(1–3):1–27. [29] Tao M, Ge H, Van Der Wege B, Zhao P. Fuel wall film effects on premixed flame propagation, quenching and emission. Int J Engine Res 2018. (Online First).
Acknowledgements This study is supported by the Mazda Motor Corporation, and special thanks to them. References [1] Zhao F, Lai MC, Harrington DL. Automotive spark-ignited direct-injection gasoline engines. Prog Energy Combust Sci 1999;25(5):437–562. [2] Gold M, Stokes J, Morgan R, Heikal M, Sercey GD, Begg S. Air-fuel mixing in a homogeneous charge DI gasoline engine. SAE Technical Paper 2001-01-0968; 2001. [3] Montanaro A, Malaguti S, Alfuso S. Wall impingement process of a multi-hole GDI spray: Experimental and numerical investigation. SAE Technical Paper 2012-011266; 2012. [4] Lacey J, Kameshwaran K, Sathasivam S, et al. Effects of refinery stream gasoline property variation on the auto-ignition quality of a fuel and homogeneous charge compression ignition combustion. Int J Engine Res 2017;18(3):226–39. [5] Luo HL, Nishida K, Uchitomi S, Ogata Y, Zhang W, Fujikawa T. Microscopic behavior of spray droplets under flat-wall impinging condition. Fuel 2018;219:467–76. [6] Berggren C, Magnusson T. Reducing automotive emissions—the potentials of combustion engine technologies and the power of policy. Energy Policy 2012;41:636–43. [7] Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog Energy Combust Sci 2009;35(5):398–437. [8] He X, Ratcliff MA, Zigler BT. Effects of gasoline direct injection engine operating parameters on particle number emissions. Energy Fuels 2012;26(4):2014–27. [9] Senda J, Kanda T, Al-Roub M, V.Farrell P, Fukami T, Fujimoto H. Modelling spray impingement considering fuel film formation on the wall. SAE Technical Paper 970047; 1994.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Evaporation characteristics of fuel adhesion on the wall after spray impingement under different conditions through RIM measurement system
T
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Hongliang Luoa,b, , Keiya Nishidab, Youichi Ogatab a b
College of Mechanical and Power Engineering, China Three Gorges University, Yichang, Hubei 443002, China Department of Mechanical System Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Evaporation characteristics Wall impingement Lifetime Fuel adhesion RIM
In direct-injection spark-ignition (DISI) engines, spray-wall impingement affects the mixture formation as well as combustion and exhaust emissions, making it difficult to satisfy the regulation of particle number (PN) in the future standards. In order to understand the mechanisms deeply, not only the formation but the evaporation characteristics of fuel adhesion should be investigated in detail. In this study, the fuel adhesion thickness was measured using the refractive index matching (RIM) method, then mass, area and thickness of the fuel adhesion were calculated. The evaporation evolution of fuel adhesion on the wall was discussed. Moreover, the lifetime of fuel adhesion was compared at the fixed conditions. The results showed fuel adhesion firstly evaporates from periphery, then the “hollow” occurs and develops, which facilitates the fuel evaporation. The increased injection pressure favors the droplets evaporation owing to the better atomization, leading to the decreased fuel adhesion mass and area on the wall. However, due to the strong momentum exchange between fuel droplets and air, the increased ambient pressure increases the fuel adhesion mass and area on the wall. Moreover, high injection pressure shortens the fuel adhesion lifetime by increasing evaporation rate, but high ambient pressure prolongs it due to the liquid/vapor phase changing.
1. Introduction Compared to port fuel injection (PFI) engines, direct-injection spark-ignition (DISI) engines offer many advantages, such as high efficiency and specific power, thanks to the better air-fuel mixture [1–4]. To fully exploit the potential, the reduction in particular matter (PM) emissions is the major concern owing to the spray impingement on cylinder wall and piston top surface, resulting in “wet-wall” [5–7]. Therefore, a particular attention should be paid to the formation of the fuel adhesion that makes DISI engines difficult to meet the Euro 6 emission regulations [8,9]. To clarify the spray-wall interaction, numerous investigations about fuel adhesion on the wall were performed by applying various advanced optical techniques. Among them, refractive index matching (RIM) method is widely used to measure the fuel adhesion thickness on the wall under various conditions. This method was developed by Drake et al. [10–12] to investigate fuel adhesion mass, area, and thickness with milli-second temporal resolution. Then, it become popular through the application of Yang and Ghandhi [13], Maligne and Bruneaux [14], Zheng et al. [15], Henkel et al. [16], Ding et al. [17], He et al. [18], and Luo et al. [19–21]. Although researches on fuel adhesion under non-evaporation conditions were well
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investigated, such as the effect of wall roughness[19], impingement distance[20], injection pressure [14,15,20], ambient pressure [13,15,17], the fuel adhesion under evaporation condition, especially lifetime of fuel adhesion on the wall was seldom reported. When evaporation occurs, the lifetime is a significant parameter to evaluate how much fuel survives during the engine operating condition, which is responsible for the PN emission. Moreover, the prediction of fuel lifetime may shorten the design cycle of engine with the consideration of the emission regulations. While, for the “lifetime”, it was firstly applied to study the single droplet on the wall. Tamura et al. [22] showed the droplet lifetime curve of both gasoline and diesel fuels, and they found the five stages for evaporation: evaporation, vaporization, transition, spheroidal evaporation, and spheroidal combustion. Hiroyasu et al. [23] described the droplet lifetime ranging the engine operating condition by using diesel light oil, and the results showed that lifetime curve shifted to the direction of higher plate temperature. Chin et al. [24] investigated the effect of ambient pressure on single droplets evaporation rate of n-heptane fuel, and results showed that the evaporation rate decreases with ambient pressure when the ambient temperature is lower than 600 K. Then Senda et al. [25] optimized the diesel droplet lifetime curves through setting different heat transfer
Corresponding author. E-mail address: [email protected] (H. Luo).
https://doi.org/10.1016/j.fuel.2019.116163 Received 12 June 2019; Received in revised form 1 August 2019; Accepted 5 September 2019 Available online 16 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Notation ASOI BTDC DISI fps fM (h) Δh L L/D M (h) PM
PN PFI Pinj Pamb Ra RI RIM SOI Tamb Tsat W
After Start of Injection Before Top Dead Center Direct Injection Spark Ignition Frames Per Second Probability of M (h) Thickness Fraction Fuel Adhesion Length Length To Diameter Sum of Fuel Adhesion Mass Particle Matter
Particular Number Port Fuel Injection Injection Pressure Ambient Pressure Arithmetical Mean Deviation of the Profile Refractive Index Refractive Index Matching Start of Injection Ambient Temperature Saturated Temperatures Fuel Adhesion Width
can be involved in this study. Fuel adhesion evaporation behavior is further described: it firstly evaporates from periphery, then the “hollow” occurs and develops, until all evaporates. The fuel adhesion lifetime can be shortened by increasing the injection pressure and decreasing the ambient pressure.
models and regimes by experimental results. The sub-model was established by considering the breakup behaviors of impingement and the dispersion process of breakup-droplets. Kim et al. [26] investigated the effect of ambient pressure on the evaporation of a single n-heptane fuel droplet, and results showed that the droplet lifetime increased with pressure at a low ambient temperature, but it decreased at high temperature. Although considerable works have been done on droplets evaporation, it is well known that the evaporation behavior of the fuel adhesion is more complex in contrast to that of single droplet. After impingement, both formation and evaporation processes of fuel adhesion occur, leading to complex phenomena. Therefore, the mechanism of fuel adhesion evaporation is far from fully understood. Recently, Maligne and Bruneaux observe the iso-octane fuel adhesion evaporation occurred from periphery to the center under gasoline engine-like conditions at a quite short duration [14]. But no details about lifetime of fuel adhesion on wall were reported until now. In the present work, the evaporation characteristics of fuel adhesion under different injection and ambient pressures were discussed based on the RIM method. Firstly, the evaporation of fuel adhesion on the wall was discussed, followed by the mass, area and thickness distribution. Then, the “hollow” was observed on the fuel adhesion with its development and effect on adhesion evaporation analyzed in detail. Finally, the lifetime of the fuel adhesion at various conditions were compared and discussed. Specifically, all the results were repeated at ten times, and the averaged results were shown in this paper. Overall, two contributions
2. Experimental details 2.1. Experimental apparatus The experimental apparatus for fuel adhesion measurement in the current study is shown in Fig. 1. Fuel is injected into the chamber through a mini-sac injector. A high-speed video camera (Photron FASTCAM SAZ) set at 10,000 frames per second (fps) with a resolution of 512 × 512 pixels is utilized for fuel adhesion observation from the bottom view. A xenon lamp (Ushio SX-131 UID501XAMQ) placed at a position perpendicular to the camera is used to illuminate the chamber with an incident angle of 15 deg. Both injector and camera work synchronously by the help of a delay generator. A flat plate made of quartz glass is placed under the injector as an impingement wall. Beneath it, a reflection mirror is installed so as the camera can capture the fuel adhesion image from the bottom view.
Fig. 1. Experimental setup for fuel adhesion measurement. 2
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defined as the intersection of the nozzle center axis and flat wall. The impingement angle is set at 45 deg, and the impingement distance is determined at 22 mm from the nozzle exit to the impingement point along the spray axis by considering the real distance when injection occurs in a small gasoline engine. The positive y axis is defined as the direction of spray after impingement, while the positive z axis is defined as parallel to the injector axis. The positive x axis is the perpendicular to the yz plane pointing out of the figure. The injector used is a mini-sac gasoline injector with a single hole. The diameter and length of nozzle hole is 0.155 mm and 0.65 mm, respectively, leading to the length-todiameter (L/D) ratio being 4.2. In addition, the nozzle hole is a conventional straight hole without counterbore. Table 2 lists the test conditions. Owing to the special requirement of RIM method for fuel adhesion measurement, the refractive index (RI) of tested fuel should be similar to that of the quartz glass (RI = 1.49). Therefore, toluene (RI = 1.46) is selected as a substitute for gasoline, and further explanations can be found in our previous studies [20,21]. Experiments are performed in a constant high-pressure chamber filled with nitrogen gas. An electric preheater (Toyokoatsu Kanthal AF-200 V) is installed at the bottom of the chamber. The air temperature (Tamb = 433 K) is monitored by a thermocouple placed above the wall. One thing worth noting that it is difficult to measure the temperature of the flat wall, thus the temperature near the wall obtained from the thermocouple presenting it. According to the in-cylinder conditions at different crank angles before top dead center (BTDC) during the warmup period, two different ambient pressures 0.15 and 0.74 MPa are selected at Tamb = 433 K, resulting in the ambient density at 1.95 and 5.95 kg/m3, respectively. Another thing needs to be noted that the saturated temperatures (Tsat) of toluene at 0.15 MPa is 398 K, indicating Tsat < Tamb. While at 0.74 MPa, Tsat is 472 K, indicating Tsat > Tamb. Namely, toluene changes the phase from vapor to liquid when increasing the ambient pressure from 0.15 to 0.74 MPa. For safety care, a cooling system is applied to regulate the fuel temperature (before injection) at room temperature. The injection pressure varies among 10, 20, and 30 MPa with different injection durations of 2.9, 2.1, and 1.7 ms to satisfy the requirement of constant injection mass at 4.0 mg with a consideration of the real injection mass in each hole.
Table 1 Injector parameters and impingement conditions. Impingement Conditions Impingement Wall
Quartz Glass
Diameter of the Wall (mm) Thickness of the Wall (mm) Impingement Distance (mm) Impingement Angle (deg) Surface Roughness (μm)
50 2 22 45 Ra 7.0
Injector Parameters Injector Type Hole Number Hole Type Nozzle Hole Diameter (mm) Nozzle Hole Length (mm) L/D Ratio
Mini-Sac 1 Straight Hole without Counterbore 0.155 0.65 4.2
2.3. Image processing method RIM method is applied to measure the fuel adhesion thickness. The image processing of the RIM experiment is shown in Fig. 3. First, the image without fuel adhesion named as “dry image” is acquired. After fuel adhering on the wall, it is subtracted by the “dry image” to obtain the only adhered fuel image. Using the calibration curve shown in Fig. 3, the thickness distribution can be obtained. For the adhered fuel area, the pixels whose thickness is larger than 0.1 μm can be integrated. Using the scale (0.106 mm/pixel) got from the observation, the adhesion area is calculated. While, for the adhered fuel mass, the thickness can be added up when the value is larger than 0.1 μm. Then using the scale and density of toluene (867 kg/m3), the adhesion mass can be calculated. Additional details about RIM method and calibration curve can be seen in our previous publications [20,21].
Fig. 2. Specification of the injector and flat wall. Table 2 Test conditions. Evaporation conditions Test Fuel Fuel Temperature (K) Injection Mass (mg) Ambient Gas Injection Pressure (MPa) Injection Duration (ms) Ambient Temperature (K) Ambient Density (kg/m3) Ambient Pressure (MPa)
Toluene 293 4 Nitrogen 10, 20, 30 2.9, 2.1, 1.7 433 1.19 0.15
5.95 0.74
3. Results and discussion
2.2. Experimental conditions
3.1. Evolution of fuel adhesion
Table 1 lists the impingement conditions and injector parameters. The quartz glass (Sigma Koki, DFSQ1-50CO2) with surface roughness at Ra7.0 μm is employed to represent the rough surface of the piston [19], measured by a portable high-performance surface roughness and waviness measuring instrument (Kosaka Laboratory Ltd., SE300) with a resolution of 0.0064 μm. As shown in Fig. 2, the diameter of the plate is 50 mm, and the thickness is 2 mm. The impingement point (o) is
Fig. 4 illustrates the evolution of the fuel adhesion at Pamb = 0.15 MPa. The images of adhesion on the wall at 50, 70, 100, and 150 ms ASOI are depicted. The adhered fuel thickness varying from 0 to 2.0 μm is shown by the pseudocolor, and the cross symbol represents the impingement point (o). It is evident that fuel area decreases with time due to evaporation. It is interesting to find that fuel adhesion evaporates from the impingement region to downstream. Owing to the interaction between fuel 3
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Fig. 3. Image processing for RIM.
with pseudocolor representing the thickness from 0 to 2.0 μm and the cross symbol being the impingement point (o). In contrast to Fig. 4, the fuel adhesion survives much longer when increased the ambient pressure up to 0.74 MPa. More discussion about it will be discussed in the next part. Again, it is clear to see the fuel adhesion area decreases with an increase in injection pressure. Moreover, it is worth noting that the area increases by raising the ambient pressure, when compared to Fig. 4. The increased ambient pressure decelerates the droplets by strong resistance force from the air. Then more droplets tend to “stick” on the wall instead of “splash”, leading to more fuel adhesion on the wall. Moreover, Kim et al. [26] and Luo et al. [5] pointed out that the splashing droplets after impingement tend to coalesce at higher ambient pressure. The large droplets re-deposit on the wall more easily, thus increasing the fuel adhesion on wall. Furthermore, it was demonstrated that the evaporation rate decreases with ambient pressure increasing [26], thus more fuel adhesion remaining on the wall. And more discussions about the evaporation rate at high ambient pressure can be seen in the next part. Additionally, similar to Fig. 4, it can also be observed that more fuel left near the cross symbol with an increase in injection pressure at 500 ms ASOI in Fig. 5. The definitions of fuel adhesion length (L) and width (W) are shown in Fig. 6 to discuss the detailed fuel adhesion characteristics. L is the maximum vertical distance from the top edge to the bottom edge of the
spray and wall, the adhesion located on the impingement region is thin compared to that of the downstream, which leads to quick evaporation. With time elapse, it seems the fuel adhesion “moves downward”. Moreover, the fuel adhesion area decreases with an increase in injection pressure. Injection pressure promotes better atomization, the small size droplets evaporates more easily at high temperature condition before impingement. Even though some droplets impinge on the wall, with an increase in the Weber number, the droplets behavior may change from “stick” to “splash” [27,28]. In other words, although some droplets stick on the wall, the air flow with high velocity promotes its evaporation, resulting in less fuel adhesion on the wall ultimately. It should be noted that during the evaporation, the adhesion firstly evaporates from the periphery of the adhesion as Maligne and Bruneaux [14] reported, then evaporates from the center of the adhesion, leading to a “hollow” on the adhesion, which will be discussed more in the next section. It is also interesting to find that the more fuel left near the cross symbol with an increase in injection pressure, see at 150 ms ASOI of Fig. 4. The reason can be involved as below: After impact on the wall, fuel tends to spread along the wall at low injection pressure, resulting in more fuel left at the downstream. In contrast, fuel tend to splash at high injection pressure, leading to less fuel left at the downstream. Fig. 5 shows the evolution of the fuel adhesion at Pamb = 0.74 MPa. Similar to Fig. 4, images at 50, 100, 300, and 500 ms ASOI are shown 4
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Fig. 4. Evolution of fuel adhesion at Pamb = 0.15 MPa.
results in the fuel adhesion “moving downward”, the same phenomena can be observed in Figs. 4 and 5. Then from a certain value (for Pamb = 0.74 MPa, when L decreases to 10 mm; while for Pamb = 0.15 MPa, when L decreases to 7.5 mm), both L and W decrease at a similar rate. This is attributed to the “hollow”. When “hollow” occurs, the fuel adhesion evaporates change from the periphery to the “hollow”, leading to the similar decreasing rate in L and W, which will be discussed more in the next part.
fuel adhesion. W is the maximum horizontal distance from the left edge to the right edge of the fuel adhesion. And all the results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data in Fig. 7. Fig. 7(a) and (b) show the fuel adhesion length and width under different injection and ambient pressures, respectively. Obviously, both L and W decrease with time due to evaporation. For all case at different conditions show similar tendencies although some differences can be observed. All curves can be divided into two stages by different decrease rates: Stage Ⅰ and Ⅱ. And the transition point was marked by a star symbol. It reveals the transition time decreases with decreased ambient pressure and increased injection pressure due to the strong evaporation. For L, the opposite tendencies can be recognized. When Pamb = 0.15 MPa, L decreases firstly sharply at Stage Ⅰ, then normally at Stage Ⅱ. While, for Pamb = 0.74 MPa, L decreases firstly normally at Stage Ⅰ, then sharply at Stage Ⅱ. The high ambient pressure hiders the fuel evaporation to some extent, thus decelerating the fuel adhesion “moving downward” may be one possible reason for it. However, for W, all results show the same tendency: W decreases firstly normally at Stage Ⅰ, then sharply at Stage Ⅱ, indicating little effect of both injection and ambient pressures on W decreasing transition. Fig. 7(c) shows the fuel adhesion length to width under different injection and ambient pressures. Both L and W decreases from the maximum value at right side to 0 along different curves. It has to be noticed that for all case, L decreases more sharply than W at first, which
3.2. Fuel adhesion mass and area Fig. 8 shows fuel adhesion mass at left vertical axis, with right one being the ratio of fuel adhesion mass to injection mass. Results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data. It reveals that mass decreases with injection pressure. The better atomization and quick evaporation rate should be responsible for it. However, the mass increases with ambient pressure. It should be attributed to the more fuel adhering on the wall and low evaporation rate. One interesting thing is that under Pinj = 10 MPa, Pamb = 0.74 MPa, the mass increases until 200 ms ASOI, even the injection duration is just 2.9 ms. The lowest velocity of spray and largest size of the droplets should be responsible for it. Owing to the high ambient pressure and low injection pressure, the incoming droplet impinge on the wall relatively lately, resulting in mass increase. Furthermore, the high ambient pressure 5
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Fig. 5. Evolution of fuel adhesion at Pamb = 0.74 MPa.
coalesces the droplets, making the droplets difficult to evaporate before impingement. Additionally, the splashing droplets re-deposit on the wall easily [21], which should be another reason for that. Results also provide that the maximum ratio of fuel adhesion mass to injection mass is about 4% under all cases, indicating most of the fuel evaporates or splashes off the wall. Fig. 9 shows the fuel adhesion area with time. Results under Pamb = 0.15 and 0.74 MPa are depicted by filled and open data. The same as mass, the adhesion area increases with ambient pressure, but decreases with injection pressure. One interesting observation is that under Pinj = 10 MPa, Pamb = 0.74 MPa condition, almost no increase can be seen in the area, although mass increase can be observed in Fig. 8. The incoming and re-depositing droplets impinge on the same region of the adhesion may be one possible explanation for it. Additionally, from the observation in Figs. 8 and 9, the time for total evaporation of fuel adhesion at different conditions are totally different, more analyses can be seen in the next section. 3.3. “Hollow” on the fuel adhesion Fig. 10 illustrates the “hollow” in fuel adhesion under Pinj = 10 MPa, Pamb = 0.15 MPa. From 60 to 120 ms ASOI, the fuel adhesion evaporates from periphery, leading to fuel adhesion “moving downward”. In fact, the evaporation occurs at all fuel adhesion,
Fig. 6. Definition of fuel adhesion length and width.
6
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Fig. 8. Fuel adhesion mass.
Fig. 9. Fuel adhesion area.
adhesion structure and evaporation dynamic. As the fuel adhesion is formed by the spray impingement, the fuel adhesion is thin at the periphery owing to the shear force. Therefore, the heat transfer from the ambient gas to the fuel adhesion results in a faster increase of temperature, leading to a faster evaporation than that in the center of the adhesion. Specially, owing to the roughness of the impingement wall, the fuel adhesion is non-uniform [19]. Then, with evaporation, “hollow” occurs at the relative thin region in the center of the adhesion, resulting in the adhesion “breakup”. Then “hollow” develops owing to the stronger heat transfer and faster evaporation, facilitating the fuel evaporation. Finally, the fuel adhesion becomes much less uniform until all evaporates. The mass at different adhesion thickness under Pinj = 10 MPa, Pamb = 0.15 MPa was applied for further investigation, as depicted in Fig. 12. The horizontal axis represents fuel adhesion thickness, with fuel adhesion mass being as the vertical axis. The adhesion mass is determined by the values of each pixel calculated from the equation below: ∞
Fig. 7. Fuel adhesion length and width.
M (h) =
∑ h (i) i=0
resulting in the thick region becomes thin with time. Then “hollow” occurs around 140 ms, and develops, leading to less uniformity of fuel adhesion. Finally, the fuel adhesion evaporates from the “hollow” until all evaporated. A conceptual model for fuel adhesion evaporation is proposed here to explain this mechanism. Fig. 11 shows the correlation between fuel
(1)
where M (h) is defined as the sum of fuel adhesion mass in the thickness fraction between h ± 0.5Δh and h, and Δh is 0.05 μm. Fig. 12 shows the mass distribution along thickness with time under Pinj = 10 MPa, Pamb = 0.15 MPa. It can be observed that the curve decreases due to evaporation. Furthermore, the peak value decreases and moves to right from 50 to 150 ms, owing to the evaporation occurring 7
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Fig. 10. “Hollow” occurs during evaporation (Pinj = 10 MPa, Pamb = 0.15 MPa).
Fig. 11. Schematic conceptual model of fuel adhesion evaporation dynamic.
the influence of the “hollow” and explaining this phenomenon above, the probability of fuel adhesion was employed in Fig. 13. The probability curves must satisfy the normalization condition:
from periphery. While, from 150 to 400 ms, the peak value decreases but moves to left. This should be attributed to the “hollow” occurrence and development, resulting in thick fuel adhesion “breakup” and evaporation. For further understanding the uniformity of the fuel adhesion under 8
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time. From 50 to 100 ms ASOI, the uniformity changes little. However, when “hollow” occurs, the curve becomes wide, suggesting worse uniformity of fuel adhesion on the wall. From it the conclusion can be obtained that fuel adhesion evaporates from periphery with less uniformity changing, then “hollow” facilitates the evaporation of the fuel adhesion, resulting in non-uniformity of the adhesion, which agrees well with the observation in Fig. 10. 3.4. Fuel adhesion lifetime Fig. 14 shows the fuel adhesion lifetime on the wall. It should be noted that the lifetime in this article is the time duration from the start of injection (SOI) to the fuel adhesion totally evaporation. One thing should be pointed that although the adhesion lifetime is quite longer compared to the real working condition in gasoline engine, where the adhesion not completely evaporates before the combustion occurs. Lifetime is significantly important to evaluate evaporation characteristics, especially for optimization and verification of the model in simulation. Results show that injection pressure decreases the lifetime, which can be explained by the enhanced atomization and high evaporation rate. The decrease in lifetime is more obvious by increasing the injection pressure from 10 to 20 MPa, when compare to that of injection pressure elevating from 20 to 30 MPa. Rather, the increased ambient pressure prolongs the fuel adhesion lifetime. As described before, due to the transient from vapor to liquid phase by enhancing the ambient pressure from 0.15 to 0.74 MPa, the evaporation rate decreases, resulting in long lifetime of the fuel adhesion on the wall. As a result, fuel adhesion on the wall increases. However, the injection pressure has no effect on the phase change. Additionally, this should be another reason for the fuel adhesion mass increases with an increase of ambient pressure. In order to decrease the soot emission, the less fuel adhesion is deposited on the wall, the better. As a result, from this measurement results, the injection pressure should be increased but the ambient pressure should be taken care to decrease. Although in this experiment, the selected ambient pressures of 0.15 MPa and 0.74 MPa lead to the liquid/vapor phase changing, Kim et al. [26] and Tao et al. [29] also pointed the increased ambient pressure inhibits the fuel evaporation by their experiment and simulation results, respectively. Frankly speaking, owing to the limitation of the experimental equipment, the ambient temperature is far lower than that in the real engine, resulting in the lifetime of the fuel adhesion much longer than the real engine working time. Nevertheless, with the help of the curves in Fig. 14, the trends can be identified, and the evaporation behaviors can be estimated. The experimental results can provide guidance for engine design and basis for simulation work.
Fig. 12. Fuel adhesion mass distribution along thickness (Pinj = 10 MPa, Pamb = 0.15 MPa).
Fig. 13. Probability of fuel adhesion mass distribution along thickness (Pinj = 10 MPa, Pamb = 0.15 MPa).
4. Conclusion The evaporation characteristics of fuel adhesion under different pressures were investigated experimentally in this paper. The fuel adhesion evolution was discussed, followed with mass, area and thickness of the fuel adhesion. Furthermore, the “hollow” of adhesion was analyzed with observation, mass distribution and probability of thickness. Finally, the lifetime of the fuel adhesion was compared. Based on these quantitative data provided in this study, the major conclusions are summarized as follows: 1. The fuel adhesion firstly evaporates from periphery, leading to fuel adhesion “moving downward”. Then, “hollow” occurs and develops, resulting in less uniformity of fuel adhesion. Finally, the fuel adhesion evaporates from the “hollow” until all evaporated. Furthermore, L decreases more sharply than W at first, which leads to the fuel adhesion “moving downward”. When “hollow” occurs, both L and W decrease at a similar rate. 2. The increased injection pressure promotes better atomization and
Fig. 14. Fuel adhesion lifetime. ∞
∑ fM (hi) = 1 i=0
(2)
where the probability of M (h) is defined as fM (h) . Fig. 13 illustrates the probability along fuel adhesion thickness with 9
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increases the momentum of the spray, thus changing the droplets behavior from “stick” to “splash”, resulting in less fuel adhesion on the wall. Moreover, the strong flow of the air favors the evaporation of the fuel on the wall. The increased ambient pressure increases the density of the air, leading to stronger air drag force. Therefore, the velocity of spray decreases significantly, resulting in more fuel adhering on the wall instead of “splash”. In addition, owing to the transient from vapor to liquid phase, the higher ambient pressure decreases the evaporation rate, thus increasing the fuel adhesion on the wall. 3. The fuel adhesion lifetime is shortened by increasing injection pressure owing to the better atomization and quick evaporation. And the lifetime extends with an increase in ambient pressure because of more fuel adhesion, hard evaporation and liquid/vapor changing.
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