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Macroscopic and microscopic spray characteristics of fatty acid esters on a common rail injection system
Fuel 203 (2017) 370–379
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Full Length Article
Macroscopic and microscopic spray characteristics of fatty acid esters on a common rail injection system Dong Han ⇑, Jiaqi Zhai, Yaozong Duan, Dehao Ju, He Lin, Zhen Huang Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s Macroscopic and microscopic spray features of three fatty acid esters are studied. Light intensity distributions of different fuel sprays are compared. Macroscopic spray parameters are influenced by fuel physical properties. Droplets of methyl laurate are of smaller SMD than methyl oleate and ethyl oleate.
a r t i c l e
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Article history: Received 25 December 2016 Received in revised form 22 April 2017 Accepted 24 April 2017
Keywords: Spray Droplet Biodiesel Fatty acid esters Common rail injection system
a b s t r a c t The composition of biodiesel is significantly influenced by the feedstock sources. This variation in biodiesel composition may cause changes in fuel physiochemical properties and further affect the engine operation processes such as the fuel injection and spray processes. In this study, the macroscopic and microscopic spray parameters of diesel fuel and three biodiesel components (methyl laurate, methyl oleate and ethyl oleate) on a diesel common rail injection system are investigated at conditions of different injection pressures and ambient pressures. The macroscopic spray parameters, including the tip penetration distance, projected spray area, spray front velocity, spray cone angle, and the maximum spray width, of different fatty acid esters are characterized and compared to those of diesel fuel. Further, the images of fuel spray development obtained by the high speed camera are converted to grayscale ones, and the grayscale values within the spray contours are extracted to evaluate the light intensity level and distribution characteristics for different fuels. Finally, the microscopic spray characteristics of test fuels measured using a split laser particle size analyzer indicated that fuel properties also play an important role in the droplet sizes, and the droplets of methyl laurate and diesel have smaller SMD than methyl oleate and ethyl oleate. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel could be made from renewable feedstock and as such is considered as a potential alternative for the petroleum-based fuels used in transportation sectors, especially as concerns about the worldwide climate warming and petroleum shortage are gradually increasing. The utilization of biodiesel in internal combustion engines does not require any engine structure modification, and can improve incomplete combustion products such as soot, hydrocarbon and carbon monoxide emissions, only with a slight sacrifice in NOx emissions [1–4]. Therefore, many researchers tried to investigate the injection [5–7], spray [8–10], combustion and emissions characteristics [11,12] of biodiesel in diesel engines. ⇑ Corresponding author at: Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China E-mail address: [email protected] (D. Han). http://dx.doi.org/10.1016/j.fuel.2017.04.098 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
Biodiesel fuels are produced by the transesterification process of a variety of biomass oils/fats with alcohols, forming mixtures of many different fatty acid esters, and their compositions are significantly influenced by the feedstock sources. For example, rapeseed, palm and jatropha biodiesel contain a large amount of long-chain fatty acid esters, the carbon chain of which generally have more than sixteen carbon atoms; in contrast, coconut biodiesel contains large fractions of light fatty acid esters such as methyl laurate (C12:0) [13]. The differences in biodiesel composition may cause changes in fuel physiochemical properties and further affect the engine operation processes. Therefore, it is necessary to identify the effects of biodiesel composition on their physical and chemical properties and on the engine performance and emissions. Knothe et al. [14,15] investigated the cetane numbers of fatty acid esters with different carbon chains, degree of unsaturation and ester structures. It was found that the number of CH2 groups and double bonds had the most pronounced impacts on the cetane
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number, while the structure of ester moiety did not greatly influence ignition properties. To characterize the chemical effects of fatty acid esters on fundamental combustion properties, the autoignition delay times [16,17] and propagating flame speeds [18] of different fatty acid esters were also investigated using different kinds of experimental facilities. These studies proved that the different ester structures caused changed fuel decomposition pathways, thus exhibiting changed global combustion features. Schöborn et al. [19,20] studied the effects of molecular structure (fatty acid carbon chain length, degree of unsaturation and alcohol moiety length) of fatty acid esters on the NOx emissions on a single cylinder engine. It was found that increased carbon chain length of fatty acid or alcohol moiety, and increased saturation resulted in a reduction in ignition delay time and decreased premixed burn fraction, which led to reduced NOx emissions provided the same injection time or ignition time for all fuels. However, as the ignition delays of all test fuels were maintained constant by selectively adding some ignition promoters, the adiabatic flame temperature played the dominant role in NOx formation. Further, Hellier et al. [21] systematically investigated the engine combustion phasing and emissions formation features of a series of stearic acid esters with varied alcohol moiety structures, by changing straight carbon chain length or carbon chain branching. The authors concluded that the alcohol moiety structure did not apparently influence the ignition quality, but pose influences on NOx given the same ignition delay time. Mueller et al. [22] suggested that the biodiesel effects on NOx emissions mainly originated from the change in equivalence ratio of the reacting mixture at the premixed auto-ignition zone. When the biodiesel fuel structure produces nearstoichiometric premixed mixture, the thermal NOx formation is promoted due to the increased local and average temperature, the reduced radiative heat loss, and the advanced combustion phasing. Pham et al. [23] investigated the effects of fatty acid methyl ester profiles on the engine-out particulate emissions, observing that lower power-based specific particulate emissions in mass and number were produced as the carbon chain length decreased and the degree of unsaturation increased. However, given the mass of particle emissions, the particle number was observed to increase with shorter carbon chain length and higher molecular oxygenate content. Barrientos et al. [24] analyzed the oxidative reactivity of soot from biodiesel surrogates with different chemical structures, and they found that soot generated from fatty acid methyl esters with shorter alkyl chain showed higher oxidative reactivity. With respect to the physical properties, Lopes et al. [25] measured the speeds of sound, which are related with isentropic bulk modulus and have important impacts on the fuel injection timing, in fatty acid methyl esters of different carbon chain lengths and degrees of unsaturation. Han et al. [26] characterized the injection rates, injection quantities and injector inlet pressure fluctuation characteristics of methyl laurate, methyl oleate and ethyl oleate on a common rail injection system. They found that the changed fuel physical properties could result in slight injection delay times, shapes of injection rate curves and pressure fluctuations at the injector inlet. As mentioned above, in spite of plenty of research work on the engine ignition and combustion characteristics for fatty acid esters, the spray development processes of different fatty acid esters have received little attention [27]. In a direct injection compression ignition engine, the fuel spray and atomization characteristics directly determine the in-cylinder fuel/air mixture formation, and further substantially influence the in-cylinder combustion quality, engine emissions and thermal efficiency. Therefore, in this study, the macroscopic and microscopic spray characteristics of three fatty acid esters, including methyl laurate, methyl oleate and ethyl oleate were experimentally obtained on a diesel common rail injection system and compared to those of diesel fuel.
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2. Experimental methods 2.1. Test facility As described in Fig. 1, the test facility used for the spray development observation mainly consisted of a diesel common rail injection system, a two-stage fuel pump, an electronic control unit, a constant volume vessel and a FASTCAM-series high speed photography system with a halide lamp as the light source. A high pressure nitrogen gas bottle was connected to the constant volume vessel to set up the ambient pressure and scavenge the residual fuel after each test condition. The fuel injector mounted on the top of the constant volume vessel was a single-hole injector of 0.28 mm diameter. The quartz windows on three sides of the vessel for the spray development observation had areas of 8 16 cm2. The spray images were captured in 15,000 fps (frames per second), with 66.7 ls interval between two consecutive recorded images. The image resolution was 1024 128 pixels and the camera shutter rate was set to 1/15,000 s. A split laser particle size analyzer was used for droplet size measurement, in which the particle sizes were evaluated by analyzing the spatial distribution of diffraction and scattered lights. The measurement range of the laser particle size analyzer is 0.5–1000 lm. The trigger signals for the high speed camera and the injector were synchronized using a digital delay/ pulse generator (Stanford Research Instrument, Model DG535). The test was conducted at the ambient temperature of 293 K and 0.1 MPa, 0.5 MPa and 1.0 MPa ambient pressures, with the ambient gas density being 1.15 kg/m3, 5.75 kg/m3 and 11.5 kg/m3, respectively. The injection pressures were 40 MPa and 60 MPa, respectively and the energizing pulse width was held at 1.0 ms. Spray development processes for each test fuel and condition were measured three times and the macroscopic spray parameters were calculated and averaged based on the captured images. 2.2. Test fuels The spray development processes of three fatty acid esters, including methyl laurate, methyl oleate, ethyl oleate were investigated in this study and compared to diesel fuel. The methyl oleate and ethyl oleate are typical long-chain constituents in canola, rapeseed, palm and jatropha biodiesel fuels [13], while methyl laurate, with a shorter carbon chain, is a major content in coconut biodiesel [28]. The purity of test fatty acid esters is above 98%. The fuel molecular structure, density, kinetic viscosity, surface tension and vapor pressure are listed in Table 1. Fuel densities of different fatty acid esters are similar but higher than that of diesel; meanwhile, methyl oleate and ethyl oleate have higher viscosity and surface tension than diesel and methyl laurate. 2.3. Definition of spray parameters Macroscopic spray parameters derived from the acquired highspeed images include the spray tip penetration distance, the spray cone angle, the spray front velocity, the projected spray area, and the maximum spray width. The definition of the abovementioned parameters is as follows and shown in Fig. 2: the vertical distance from the nozzle tip to the farthest spray front is defined as the spray tip penetration, and the angle included between the two lines connecting the nozzle tip and the two periphery points at the half of the tip penetration distance from the nozzle tip is defined as the spray cone angle. Spray front velocity is calculated from the derivative of the spray tip penetration distance versus time. The area of total cells covered by the spray contour is defined as the projected spray area. Also, the original images obtained by the high speed camera are converted to grayscale images. Further, these grayscale images
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Fig. 1. Schematic of fuel spray test system.
Table 1 Properties of test fuels. Fuel
Diesel
Methyl Laurate
Methyl Oleate
Ethyl Oleate
Molecular structure Density @ 313 K (g/cm3) Viscosity @313 K (mm2/s) Surface tension @ 293 K (mN/m) Vapor Pressure @ 298 K (Pa)
– 0.8035 2.48 27.1 –
CH3(CH2)10COOCH3 0.8605 2.38 29.3 1.4
CH3(CH2)7CH@CH(CH2)7COOCH3 0.8640 4.20 30.9 5.47 E03
CH3(CH2)7CH@CH(CH2)7COOCH2CH3 0.8585 4.76 31.0 4.89E04
Fig. 2. Definitions of macroscopic spray characteristics (diesel, rail pressure: 80 MPa, ambient pressure: 1.0 MPa, time after start of injection: 1.133 ms).
were inverted and the specific grayscale value (varied from 0 to 255) of each pixel inside the spray contour is extracted to reflect the relative spray brightness intensity, which is closely correlated with the local fuel droplet distribution [29,30]. 3. Results and discussion 3.1. Macroscopic spray characteristics of fatty acid esters and diesel at different injection pressures In the compression stroke of diesel engines, fuel is injected from a nozzle into the high pressure environment, and the interaction between fuel jet and the high-pressure surrounding gas leads to
spray breakup and atomization. Fuel viscosity and surface tension were found to influence the spray stability and the breakup process [31,32]. Therefore, it is necessary to characterize the effects of fuel properties of different biodiesel components on the spray development processes. The macroscopic spray parameters, including the tip penetration distance, projected spray area, spray front velocity, spray cone angle and maximum spray width of fatty acid esters and diesel fuel at 40 MPa and 60 MPa injection pressures, 1.0 MPa ambient pressure conditions are illustrated in Fig. 3. It is easily observed that injection pressure places obvious effects on these macroscopic parameters: given a test fuel, tip penetration distance (Fig. 3a), projected spray area (Fig. 3b), spray cone angle (Fig. 3d) and maximum spray width (Fig. 3e) increase with injection pressure because higher initial injection momentum promotes the spray spatial dispersion. In Fig. 3c, the occurrence of the peak front velocity is considered as the spray breakup time, before which the tip penetration distance rapidly increases and after which the spray development slows down. It is observed that the spray breakup time for all fuels is advanced with increased injection pressure. This behavior is in accordance with the Hiroyasu empirical equation, in which the breakup time is inversely proportional to the square root of the ambient density and pressure difference [33]. Fuel property also influences the spray development, especially in tip penetration distance, projected spray area, spray cone angle and the maximum spray width, albeit to a less extent than the injection pressure. The higher viscosity and surface tension of methyl oleate and ethyl oleate increase the difficulty in spray breakup, thus resulting in the spray development more along the axial direction but less in the radial direction. Therefore, it can be observed that at 40 MPa injection condition, the spray tip penetration distances of methyl oleate and ethyl oleate are slightly higher than diesel and methyl laurate (Fig. 3a), while the projected spray
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Fig. 3. Macroscopic spray parameters of different test fuels at different injection pressures (40 MPa and 60 MPa) and 0.5 MPa ambient pressure condition: (a) tip penetration distance (b) projected spray area (c) spray front velocity (d) spray cone angle (e) maximum spray width.
area (Fig. 3b), spray cone angle (Fig. 3d) and the maximum spray width (Fig. 3e) of methyl oleate and ethyl oleate are generally less than those of diesel and methyl laurate. As the injection pressure
increases to 60 MPa, the effects of fuel properties are conquered by higher injection pressure, reducing the variation in the macroscopic spray parameters for different fuels. Also, it is noted
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that methyl laurate at 60 MPa injection pressure condition produces less spray area, spray cone angle and the maximum spray width because its higher vapor pressure aggravates the cavitation phenomenon in the injection system and causes increased injection instability.
the deteriorated fuel atomization performances. In contrast, increased ambient gas pressure decreases the fuel/gas density ratio, and as such promotes the fuel atomization process.
3.2. Macroscopic spray characteristics of fatty acid esters and diesel at different ambient pressures
The relative light intensity distribution derived from the grayscale spray images can provide a qualitative analysis for the bulk spatial distribution of the fuel spray [29,30]. In Fig. 5, the light intensity levels at four different axial locations are analyzed for all fuel sprays, at 40 MPa injection pressure and 0.5 MPa ambient pressure condition. The images chosen are the ones at 1.0 ms after the time when fuel is first injected out of the nozzle. From Fig. 5, it is observed that for all test fuels, the light intensity distribution range along the radial direction is extended as the axial position moves away from the nozzle tip, indicating that the spray is mostly spread at the end area. Further, the light intensity level along the nozzle tip centerline is the highest and does not show obvious variation at different axial positions, indicating the fuel droplets within these areas are most concentrated. The most pronounced difference among different test fuels is the distribution range of the light intensity at the spray end area. In spite of similar light intensity distribution ranges for different test fuels at 20 mm and 40 mm positions in the axial distance from the nozzle tip, diesel and methyl laurate are found to have wider light intensity distribution ranges than methyl oleate and ethyl oleate at 60 mm and 80 mm positions to the nozzle tip, which suggests increased diffusion and atomization tendency for diesel and methyl laurate relative to methyl oleate and ethyl oleate. Because the differences in light intensity distribution among test fuels are the most significant at locations far away from the nozzle tip, the light intensity levels of different test fuels at 80 mm from the nozzle tip are directly compared in Fig. 6, under different test conditions. The spray images at different test conditions are similarly of 105 ± 2 mm tip penetration distances. The injection pressure and ambient pressure apparently influence the light intensity level and distribution at a given position. As seen in Fig. 6, the most pronounced difference in light intensity distribution for all test fuels occurs in 40 MPa injection pressure and 0.5 MPa ambient pressure condition, in which diesel and methyl laurate produce wider light intensity range (Fig. 6a). However, as the ambient pressure increases to 1.0 MPa but the injection pressure is kept at 40 MPa, as shown in Fig. 6b, methyl oleate and ethyl oleate are found to produce wider light intensity distribution. This is because with increased ambient pressure, spray penetration distance is significantly shortened, and the 80 mm position from the nozzle tip is close to the periphery area of the spray end, where the spray light intensity level significantly decreases due to the air entrainment and fuel atomization. The reduction in light intensity distribution range is more obvious for diesel and methyl laurate because their lower viscosity and surface tension produce shorter spray penetration distance, and the chosen position is probably much closer to the spray boundary. Effect of increased injection pressure on the light intensity distribution is illustrated in Fig. 6c, where the injection pressure is 60 MPa and the ambient pressure is still held at 0.5 MPa. Compared to 40 MPa injection pressure condition, the variation in light intensity distribution ranges of different fuels is reduced, and the ranges with high light intensity values are broadened. This indicates that the chosen position is in the body area of the spray, where droplet concentration is quite high and the fuel atomization is not obvious.
The tip penetration distance, projected spray area, spray front velocity, spray cone angle and maximum spray width of fatty acid esters and diesel fuel at 40 MPa injection pressures, 0.1 MPa, 0.5 MPa and 1.0 MPa ambient pressure conditions are described in Fig. 4. The effects of fuel properties on macroscopic spray parameters are in agreement at each ambient pressure condition, that is, more viscous fuels such as methyl oleate and ethyl oleate produce higher tip penetration distance but less spray area, spray cone angle and the maximum spray width, as discussed in the previous section. Also, it is noted that the ambient pressure poses significant influences on the macroscopic spray parameters. Increased ambient gas pressure leads to higher resistance for fuel jet development and droplet diffusion, so the spray tip penetration distance is reduced and the liquid fuel is constricted within a limited area, as illustrated in Fig. 4a and b. Also, the spray breakup time for all fuels is advanced with increased ambient pressure (Fig. 4c), which agrees with the findings by Hiroyasu et al. [33]. Meanwhile, restricted spray penetration in the axial direction is accompanied with increased spray development towards the radial direction, thus leading to increased spray cone angle (Fig. 4d). The maximum spray width versus the ambient pressure does not follow the trend as spray cone angle; as seen from Fig. 4e, the maximum spray widths at 0.1 MPa and 0.5 MPa ambient pressure conditions are similar, and higher than that at 1.0 MPa ambient pressure condition. This is because the location of the maximum spray width is generally at the spray end area, and at higher ambient pressure condition, the spray penetration is greatly reduced and the droplet diffusion at the spray end area is also sufficiently restricted. The above experimental observation could also be explained based on the changes in Reynolds number, Weber number and the fuel/gas density ratio, which are closely related with the fuel and gas mixing process and defined as the following equations [34]:
Re ¼
qVD l
We ¼
u¼
qa V 2 D ql ¼ 2r u qa
ql qa
ð1Þ
ð2Þ ð3Þ
where V is the fuel jet velocity, D is the orifice diameter, l is the fuel kinematic viscosity, r is the fuel surface tension, ql is fuel density and qa is the ambient gas density. Reynolds number and Weber number describe the ratio of inertia force and viscous force and the ratio of inertia force and surface tension, respectively, and are commonly used to describe the mutual interaction between the fuel jet and surrounding gases [35]. Increased Reynolds number and Weber number and reduced fuel/gas density ratio increase the instability of fuel jet, and thus promote the fuel atomization and the mixing with the surrounding gases. Given a test condition, increased fuel surface tension and viscosity of fatty acid esters reduce Reynolds number and Weber number, but their higher densities increase the fuel/gas density ratio. The changes in Reynolds number, Weber number and fuel/gas density ratio contribute to
3.3. Light intensity characteristics of fatty acid esters and diesel spray
3.4. Microscopic spray characteristics of fatty acid esters and diesel In addition to the macroscopic spray characteristics, this study also studies the microscopic spray characteristics of fatty acid
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Fig. 4. Macroscopic spray parameters of different test fuels at 40 MPa injection pressure and different ambient pressures (0.1 MPa, 0.5 MPa and 1.0 MPa) conditions: (a) tip penetration distance (b) projected spray area (c) spray front velocity (d) spray cone angle (e) maximum spray width.
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Fig. 5. Light intensity distribution of different test fuels at different axial positions from the nozzle tip: (a) diesel fuel (b) methyl laurate (c) methyl oleate (d) ethyl oleate.
esters, which refer to the parameters describing the fuel atomization quality by the droplet size and distribution in the fuel spray. Since it is relatively difficult to measure the droplet size within the area of high concentration of fuel droplets, the measurement positions in this study were set to be 80 mm and 100 mm away from the nozzle tip. Sauter Mean Diameter (SMD) is used as the parameter to evaluate the fuel atomization quality. The calculation formula of SMD is given in Eq. (4):
R Dmax D
f ðDi ÞD3i dD
Dmin
f ðDi ÞD2i dD
SMD ¼ R Dmin max
ð4Þ
where Dmin is the minimum droplet diameter, Dmax is the maximum droplet diameter, Di is the droplet diameter between Dmin and Dmax , f ðDi Þ is the corresponding probability density function of Di , and dD is the integration step. The droplet size distributions of methyl laurate, methyl oleate, ethyl oleate and diesel, as well as their SMD with the change of time are shown in Fig. 7. As seen in Fig. 7a, the droplet size distributions of methyl laurate and diesel are similar, while those of methyl oleate and ethyl oleate are similar. In addition, methyl laurate and diesel have higher probability densities in the area of
small droplet sizes. The differences on viscosity and surface tension are the main reasons which lead to their different droplet size distributions [36]. The difference in droplet size distribution affects the SMD of the fuel. As is shown in Fig. 7b, the SMDs of the four test fuels are always in the range of 20–27.5 lm. The SMD of methyl laurate is the smallest, while that of diesel is the second smallest. The SMD of ethyl oleate is the largest among the four test fuels. From the properties of test fuels shown in Table. 1, the viscosity and surface tension of methyl laurate and diesel are lower, so it is easier for the fuel droplets to overcome the molecular viscous force and droplet surface tension, breaking into droplets with smaller diameters. As seen in Fig. 8, the droplet size distribution of fatty acid esters and diesel are compared at the measure position of 100 mm away from the nozzle tip and the rail pressures of 60 MPa and 80 MPa. It can be easily observed that the droplet size distributions of methyl laurate and diesel are located within the range of small droplet size. At 60 MPa rail pressure, the probability density functions of methyl laurate and diesel reach the peak value (14%) as the droplet size is about 30 mm, while those of methyl oleate and ethyl oleate reach the peak value when the droplet size is about 35 mm. However, at 80 MPa rail pressure, the probability density function
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Fig. 6. Relative light intensity of fatty acid esters and diesel at 80 mm position from the nozzle tip: (a) 40 MPa injection pressure, 0.5 MPa ambient pressure (b) 40 MPa injection pressure, 1.0 MPa ambient pressure (c) 60 MPa injection pressure, 0.5 MPa ambient pressure.
Fig. 7. Comparison of microscopic spray characteristics of fatty acid esters and diesel (rail pressure: 60 MPa, energizing time: 1.0 ms, measure position: 80 mm).
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Fig. 8. The effect of rail pressure on the droplet size distributions of fatty acid esters and diesel (energizing time: 1.0 ms, measure position: 100 mm).
Fig. 9. The effect of rail pressure on the SMD values of fatty acid esters and diesel (energizing time: 1.0 ms, measure position: 100 mm).
curves of methyl oleate and ethyl oleate and their corresponding droplet sizes move towards the smaller size region, while the distribution curves of methyl laurate and diesel droplets almost remain the same. The result indicates that the differences in droplet size distribution caused by fuel properties are reduced as rail pressure increases. Fig. 9 shows the SMDs calculated by the fuel droplet size distributions in Fig. 8. It is found that the rail pressure affects the fuel droplet size. At 60 MPa rail pressure, the SMDs of the four test fuels are in the range of 20–27 lm. The droplet sizes of four test fuels are in the following sequence: ethyl oleate > methyl oleate > diesel > methyl laurate, and the influence of fuel property on the SMD remains the same. However, when the rail pressure increases to 80 MPa, the SMDs of the fuels decrease slightly to the range of 19–24 lm. In addition, the differences between methyl laurate and diesel, as well as between methyl oleate and ethyl oleate are reduced, and the SMD of methyl oleate is slightly larger than that of ethyl oleate. The result indicates that increased rail pressure promotes the breakup of fuel droplets and decreases the influence of fuel property on SMD.
4. Conclusions In this study, the macroscopic and microscopic spray parameters of three biodiesel components, methyl laurate, methyl oleate and ethyl oleate were investigated and compared to diesel fuel under different injection pressure and ambient pressure conditions. The studied spray parameters include the tip penetration distance, projected spray area, spray front velocity, spray cone angle, the maximum spray width and the droplet size distribution. The images from the high speed camera are also converted to grayscale ones and the grayscale values within the spray contour for different fuels are extracted to evaluate the light intensity level and distribution. Macroscopic spray parameters are influenced by fuel properties. Compared to diesel and methyl laurate, higher viscosity and surface tension of methyl oleate and ethyl oleate lead to slightly higher tip penetration distance, but produce less projected spray area, spray cone angle and the maximum spray width. The variation in the macroscopic spray parameters caused by fuel properties could be reduced with increased injection pressure. Through the
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analysis of light intensity characteristics in spray contours, diesel and methyl laurate are found to produce wider light intensity distribution ranges at the spray end area far away from the nozzle tip, indicating their higher diffusion and atomization tendency relative to methyl oleate and ethyl oleate. The microscopic spray characteristics of test fuels revealed that fuel properties have influences on the droplet size distribution, and droplets of methyl laurate and diesel are with smaller SMD than methyl oleate and ethyl oleate. Acknowledgement This study is supported by the National Natural Science Foundation of China (Grant no. 51306114). Reference [1] Szybist JP, Song JH, Alam M, Boehman AL. Biodiesel combustion, emissions and emission control. Fuel Process Technol 2007;88(7):679–91. [2] Zhang Y, Boehman AL. Impact of biodiesel on NOx emissions in a common rail direct injection diesel engine. Energy Fuels 2007;21(4):2003–12. [3] Knight BM, Bittle JA, Jacobs TJ. The role of system responses on biodiesel nitric oxide emissions in a medium-duty diesel engine. Int J Engine Res 2011;12 (4):336–52. [4] Cheng AS, Upatnieks A, Mueller CL. Investigation of the impact of biodiesel fuelling on NOx emissions using an optical injection diesel engine. Int J Engine Res 2006;7(4):297–318. [5] Suh HK, Park SH, Lee CS. Experimental investigation of nozzle cavitating flow characteristics for diesel and biodiesel fuels. Int J Auto Technol 2008;9 (2):217–24. [6] Torres-Jimenez E, Dorado MP, Kegl B. Experimental investigation on injection characteristics of bioethanol-diesel fuel and bioethanol-biodiesel blends. Fuel 2011;90(5):1968–79. [7] Kegl B, Hribernik A. Experimental analysis of injection characteristics using biodiesel fuel. Energy Fuels 2006;20(5):2239–48. [8] Kim HJ, Park SH, Suh HK, Lee CS. Atomization and evaporation characteristics of biodiesel and dimethyl ether compared to diesel fuel in a highpressure injection system. Energy Fuels 2009;23:1734–42. [9] Wang XG, Huang ZH, Kuti OA, Zhang W, Nishida K. Experimental and analytical study on biodiesel and diesel spray characteristics under ultra-high injection pressure. Int J Heat Fluid Flow 2010;31(4):659–66. [10] Mo J, Tang CL, Li JG, Guan L, Huang ZH. Experimental investigation on the effect of n-butanol blending on spray characteristics of soybean biodiesel in a common-rail fuel injection system. Fuel 2016;182:391–401. [11] Northrop WF, Madathil PV, Bohac SV, Assanis DN. Condensational growth of particulate matter from partially premixed low temperature combustion of biodiesel in a compression ignition engine. Aerosol Sci Technol 2011;45 (1):26–36. [12] Liu HF, Lee CFF, Huo M, Yao MF. Combustion characteristics and soot distributions of neat butanol and neat soybean biodiesel. Energy Fuels 2011;25(7):3192–203. [13] Hoekman SK, Broch A, Robbins C, Ceniceros E, Natarajan M. Review of biodiesel composition, properties, and specifications. Renew Sustain Energy Rev 2012;16:143–69. [14] Knothe G, Bagby MO, Ryan TW. Cetane numbers of fatty compounds: Influence of compound structure and of various potential cetane improvers. SAE Technical Paper 971681, 1997. [15] Knothe G, Matheaus AC, Ryan III TW. Cetane numbers of branched and straight-chain fatty esters determined in an ignition quality tester. Fuel 2003;82(8):971–5. [16] Walton SM, Wooldridge MS, Westbrook CK. An experimental investigation of structural effects on the auto-ignition properties of two C-5 esters. Proc Combust Inst 2009;32(1):255–62.
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[17] Rodriguez A, Herbinet O, Battin-Leclerc F, Frassoldati A, Faravelli T, Ranzi E. Experimental and modeling investigation of the effect of the unsaturation degree on the gas-phase oxidation of fatty acid methyl esters found in biodiesel fuels. Combust Flame 2016;164:346–62. [18] Dayma G, Halter F, Foucher F, Mounaim-Rousselle C, Dagaut P. Laminar burning velocities of C-4-C-7 ethyl esters in a spherical combustion chamber: experimental and detailed kinetic modeling. Energy Fuels 2012;26 (11):6669–77. [19] Schöborn A, Ladommatos N, Williams J, Allan R, Rogerson J. The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion. Combust Flame 2009;156:1396–412. [20] Schöborn A, Ladommatos N, Allan R, Williams J, Rogerson J. Effect of the molecular structure of individual fatty acid alcohol esters (biodiesel) on the formation of NOx and particulate matter in the diesel combustion process. SAE Technical Paper 2008–01-1578, 2008. [21] Hellier P, Ladommatos N, Allan R, Rogerson J. The influence of fatty acid ester alcohol moiety molecular structure on diesel combustion and emissions. Energy Fuels 2012;26:1912–27. [22] Mueller C, Boehman A, Martin G. An experimental investigation of the origin of increased NOx emissions when fueling a heavy-duty compression-ignition engine with soy biodiesel. SAE Int J Fuels Lubr 2009;2(1):789–816. [23] Pham PX, Bodisco TA, Ristovski ZD, Brown RJ, Masri AR. The influence of fatty acid methyl ester profiles on inter-cycle variability in a heavy duty compression ignition engine. Fuel 2014;116:140–50. [24] Barrientos E, Maricq M, Boehman A, Anderson J. Impact of ester structures on the soot characteristics and soot oxidative reactivity of biodiesel. SAE Technical Paper 2015–01-1080, 2015. [25] Lopes AFG, Talavera-Prieto MDC, Ferreira AGM, Santos JB, Santos MJ, Portugal ATG. Speed of sound in pure fatty acid methyl esters and biodiesel fuels. Fuel 2014;116:242–54. [26] Han D, Duan YZ, Wang CH, Lin H, Huang Z. Experimental study on injection characteristics of fatty acid esters on a diesel engine common rail system. Fuel 2014;123:19–25. [27] Gutierrez LG, Fatouraie M, Xiao SB, Wooldridge MS, Han D, Lin H, Duan YZ, Huang Z. The effects of ester structure on transient fuel spray characteristics including novel image analysis to quantify air entrainment. Proceedings of the 2015 ASME Internal Combustion Engine Division Fall Technical Conference, Paper No. ICEF2015-1025, pp. V001T02A003, 2016. [28] Knothe G. ‘‘Designer” biodiesel: optimizing fatty ester composition to improve fuel properties. Energy Fuels 2008;22(2):1358–64. [29] Kim HJ, Park SH, Lee CS. A study on the macroscopic spray behavior and atomization characteristics of biodiesel and dimethyl ether sprays under increased ambient pressure. Fuel Process Technol 2010;91:354–63. [30] Agarwal AK, Chaudhury VH. Spray characteristics of biodiesel/blends in a high pressure constant volume spray chamber. Exp Thermal Fluid Sci 2012;42:212–8. [31] Ejim CE, Fleck BA, Amirfazli A. Analytical study for atomization of biodiesels and their blends in a typical injector: surface tension and viscosity effects. Fuel 2007;86:1534–44. [32] Zhu HY, Bohac SV, Huang Z, Assanis DN. Defeat of the soot/NOx trade-off using biodiesel-ethanol in a moderate exhaust gas recirculation premixed lowtemperature combustion mode. J Eng Gas Turb Power 2013;135:091502. [33] Hiroyasu H and Arai M. Structures of fuel sprays in diesel engines. SAE Technical Paper 900475, 1990. [34] Zigan L, Schmitz I, Flugel A, Wensing A, Leipertz A. Structure of evaporating single- and multicomponent fuel sprays for 2nd generation gasoline direct injection. Fuel 2011;90:348–63. [35] Sapit A, Nagayasu S, Tsuboi Y, Nada Y, Kidoguchi Y. A study on improvement of diesel spray characteristics fueled by rape-seed oil. SAE Int J Fuels Lubr 2012;5 (1):529–39. [36] Chen PC, Wang WC, Roberts WL, Fang TG. Spray and atomization of diesel fuel and its alternatives from a single-hole injector using a common rail fuel injection system. Fuel 2013;103:850–61.
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Full Length Article
Macroscopic and microscopic spray characteristics of fatty acid esters on a common rail injection system Dong Han ⇑, Jiaqi Zhai, Yaozong Duan, Dehao Ju, He Lin, Zhen Huang Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s Macroscopic and microscopic spray features of three fatty acid esters are studied. Light intensity distributions of different fuel sprays are compared. Macroscopic spray parameters are influenced by fuel physical properties. Droplets of methyl laurate are of smaller SMD than methyl oleate and ethyl oleate.
a r t i c l e
i n f o
Article history: Received 25 December 2016 Received in revised form 22 April 2017 Accepted 24 April 2017
Keywords: Spray Droplet Biodiesel Fatty acid esters Common rail injection system
a b s t r a c t The composition of biodiesel is significantly influenced by the feedstock sources. This variation in biodiesel composition may cause changes in fuel physiochemical properties and further affect the engine operation processes such as the fuel injection and spray processes. In this study, the macroscopic and microscopic spray parameters of diesel fuel and three biodiesel components (methyl laurate, methyl oleate and ethyl oleate) on a diesel common rail injection system are investigated at conditions of different injection pressures and ambient pressures. The macroscopic spray parameters, including the tip penetration distance, projected spray area, spray front velocity, spray cone angle, and the maximum spray width, of different fatty acid esters are characterized and compared to those of diesel fuel. Further, the images of fuel spray development obtained by the high speed camera are converted to grayscale ones, and the grayscale values within the spray contours are extracted to evaluate the light intensity level and distribution characteristics for different fuels. Finally, the microscopic spray characteristics of test fuels measured using a split laser particle size analyzer indicated that fuel properties also play an important role in the droplet sizes, and the droplets of methyl laurate and diesel have smaller SMD than methyl oleate and ethyl oleate. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel could be made from renewable feedstock and as such is considered as a potential alternative for the petroleum-based fuels used in transportation sectors, especially as concerns about the worldwide climate warming and petroleum shortage are gradually increasing. The utilization of biodiesel in internal combustion engines does not require any engine structure modification, and can improve incomplete combustion products such as soot, hydrocarbon and carbon monoxide emissions, only with a slight sacrifice in NOx emissions [1–4]. Therefore, many researchers tried to investigate the injection [5–7], spray [8–10], combustion and emissions characteristics [11,12] of biodiesel in diesel engines. ⇑ Corresponding author at: Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China E-mail address: [email protected] (D. Han). http://dx.doi.org/10.1016/j.fuel.2017.04.098 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
Biodiesel fuels are produced by the transesterification process of a variety of biomass oils/fats with alcohols, forming mixtures of many different fatty acid esters, and their compositions are significantly influenced by the feedstock sources. For example, rapeseed, palm and jatropha biodiesel contain a large amount of long-chain fatty acid esters, the carbon chain of which generally have more than sixteen carbon atoms; in contrast, coconut biodiesel contains large fractions of light fatty acid esters such as methyl laurate (C12:0) [13]. The differences in biodiesel composition may cause changes in fuel physiochemical properties and further affect the engine operation processes. Therefore, it is necessary to identify the effects of biodiesel composition on their physical and chemical properties and on the engine performance and emissions. Knothe et al. [14,15] investigated the cetane numbers of fatty acid esters with different carbon chains, degree of unsaturation and ester structures. It was found that the number of CH2 groups and double bonds had the most pronounced impacts on the cetane
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number, while the structure of ester moiety did not greatly influence ignition properties. To characterize the chemical effects of fatty acid esters on fundamental combustion properties, the autoignition delay times [16,17] and propagating flame speeds [18] of different fatty acid esters were also investigated using different kinds of experimental facilities. These studies proved that the different ester structures caused changed fuel decomposition pathways, thus exhibiting changed global combustion features. Schöborn et al. [19,20] studied the effects of molecular structure (fatty acid carbon chain length, degree of unsaturation and alcohol moiety length) of fatty acid esters on the NOx emissions on a single cylinder engine. It was found that increased carbon chain length of fatty acid or alcohol moiety, and increased saturation resulted in a reduction in ignition delay time and decreased premixed burn fraction, which led to reduced NOx emissions provided the same injection time or ignition time for all fuels. However, as the ignition delays of all test fuels were maintained constant by selectively adding some ignition promoters, the adiabatic flame temperature played the dominant role in NOx formation. Further, Hellier et al. [21] systematically investigated the engine combustion phasing and emissions formation features of a series of stearic acid esters with varied alcohol moiety structures, by changing straight carbon chain length or carbon chain branching. The authors concluded that the alcohol moiety structure did not apparently influence the ignition quality, but pose influences on NOx given the same ignition delay time. Mueller et al. [22] suggested that the biodiesel effects on NOx emissions mainly originated from the change in equivalence ratio of the reacting mixture at the premixed auto-ignition zone. When the biodiesel fuel structure produces nearstoichiometric premixed mixture, the thermal NOx formation is promoted due to the increased local and average temperature, the reduced radiative heat loss, and the advanced combustion phasing. Pham et al. [23] investigated the effects of fatty acid methyl ester profiles on the engine-out particulate emissions, observing that lower power-based specific particulate emissions in mass and number were produced as the carbon chain length decreased and the degree of unsaturation increased. However, given the mass of particle emissions, the particle number was observed to increase with shorter carbon chain length and higher molecular oxygenate content. Barrientos et al. [24] analyzed the oxidative reactivity of soot from biodiesel surrogates with different chemical structures, and they found that soot generated from fatty acid methyl esters with shorter alkyl chain showed higher oxidative reactivity. With respect to the physical properties, Lopes et al. [25] measured the speeds of sound, which are related with isentropic bulk modulus and have important impacts on the fuel injection timing, in fatty acid methyl esters of different carbon chain lengths and degrees of unsaturation. Han et al. [26] characterized the injection rates, injection quantities and injector inlet pressure fluctuation characteristics of methyl laurate, methyl oleate and ethyl oleate on a common rail injection system. They found that the changed fuel physical properties could result in slight injection delay times, shapes of injection rate curves and pressure fluctuations at the injector inlet. As mentioned above, in spite of plenty of research work on the engine ignition and combustion characteristics for fatty acid esters, the spray development processes of different fatty acid esters have received little attention [27]. In a direct injection compression ignition engine, the fuel spray and atomization characteristics directly determine the in-cylinder fuel/air mixture formation, and further substantially influence the in-cylinder combustion quality, engine emissions and thermal efficiency. Therefore, in this study, the macroscopic and microscopic spray characteristics of three fatty acid esters, including methyl laurate, methyl oleate and ethyl oleate were experimentally obtained on a diesel common rail injection system and compared to those of diesel fuel.
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2. Experimental methods 2.1. Test facility As described in Fig. 1, the test facility used for the spray development observation mainly consisted of a diesel common rail injection system, a two-stage fuel pump, an electronic control unit, a constant volume vessel and a FASTCAM-series high speed photography system with a halide lamp as the light source. A high pressure nitrogen gas bottle was connected to the constant volume vessel to set up the ambient pressure and scavenge the residual fuel after each test condition. The fuel injector mounted on the top of the constant volume vessel was a single-hole injector of 0.28 mm diameter. The quartz windows on three sides of the vessel for the spray development observation had areas of 8 16 cm2. The spray images were captured in 15,000 fps (frames per second), with 66.7 ls interval between two consecutive recorded images. The image resolution was 1024 128 pixels and the camera shutter rate was set to 1/15,000 s. A split laser particle size analyzer was used for droplet size measurement, in which the particle sizes were evaluated by analyzing the spatial distribution of diffraction and scattered lights. The measurement range of the laser particle size analyzer is 0.5–1000 lm. The trigger signals for the high speed camera and the injector were synchronized using a digital delay/ pulse generator (Stanford Research Instrument, Model DG535). The test was conducted at the ambient temperature of 293 K and 0.1 MPa, 0.5 MPa and 1.0 MPa ambient pressures, with the ambient gas density being 1.15 kg/m3, 5.75 kg/m3 and 11.5 kg/m3, respectively. The injection pressures were 40 MPa and 60 MPa, respectively and the energizing pulse width was held at 1.0 ms. Spray development processes for each test fuel and condition were measured three times and the macroscopic spray parameters were calculated and averaged based on the captured images. 2.2. Test fuels The spray development processes of three fatty acid esters, including methyl laurate, methyl oleate, ethyl oleate were investigated in this study and compared to diesel fuel. The methyl oleate and ethyl oleate are typical long-chain constituents in canola, rapeseed, palm and jatropha biodiesel fuels [13], while methyl laurate, with a shorter carbon chain, is a major content in coconut biodiesel [28]. The purity of test fatty acid esters is above 98%. The fuel molecular structure, density, kinetic viscosity, surface tension and vapor pressure are listed in Table 1. Fuel densities of different fatty acid esters are similar but higher than that of diesel; meanwhile, methyl oleate and ethyl oleate have higher viscosity and surface tension than diesel and methyl laurate. 2.3. Definition of spray parameters Macroscopic spray parameters derived from the acquired highspeed images include the spray tip penetration distance, the spray cone angle, the spray front velocity, the projected spray area, and the maximum spray width. The definition of the abovementioned parameters is as follows and shown in Fig. 2: the vertical distance from the nozzle tip to the farthest spray front is defined as the spray tip penetration, and the angle included between the two lines connecting the nozzle tip and the two periphery points at the half of the tip penetration distance from the nozzle tip is defined as the spray cone angle. Spray front velocity is calculated from the derivative of the spray tip penetration distance versus time. The area of total cells covered by the spray contour is defined as the projected spray area. Also, the original images obtained by the high speed camera are converted to grayscale images. Further, these grayscale images
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Fig. 1. Schematic of fuel spray test system.
Table 1 Properties of test fuels. Fuel
Diesel
Methyl Laurate
Methyl Oleate
Ethyl Oleate
Molecular structure Density @ 313 K (g/cm3) Viscosity @313 K (mm2/s) Surface tension @ 293 K (mN/m) Vapor Pressure @ 298 K (Pa)
– 0.8035 2.48 27.1 –
CH3(CH2)10COOCH3 0.8605 2.38 29.3 1.4
CH3(CH2)7CH@CH(CH2)7COOCH3 0.8640 4.20 30.9 5.47 E03
CH3(CH2)7CH@CH(CH2)7COOCH2CH3 0.8585 4.76 31.0 4.89E04
Fig. 2. Definitions of macroscopic spray characteristics (diesel, rail pressure: 80 MPa, ambient pressure: 1.0 MPa, time after start of injection: 1.133 ms).
were inverted and the specific grayscale value (varied from 0 to 255) of each pixel inside the spray contour is extracted to reflect the relative spray brightness intensity, which is closely correlated with the local fuel droplet distribution [29,30]. 3. Results and discussion 3.1. Macroscopic spray characteristics of fatty acid esters and diesel at different injection pressures In the compression stroke of diesel engines, fuel is injected from a nozzle into the high pressure environment, and the interaction between fuel jet and the high-pressure surrounding gas leads to
spray breakup and atomization. Fuel viscosity and surface tension were found to influence the spray stability and the breakup process [31,32]. Therefore, it is necessary to characterize the effects of fuel properties of different biodiesel components on the spray development processes. The macroscopic spray parameters, including the tip penetration distance, projected spray area, spray front velocity, spray cone angle and maximum spray width of fatty acid esters and diesel fuel at 40 MPa and 60 MPa injection pressures, 1.0 MPa ambient pressure conditions are illustrated in Fig. 3. It is easily observed that injection pressure places obvious effects on these macroscopic parameters: given a test fuel, tip penetration distance (Fig. 3a), projected spray area (Fig. 3b), spray cone angle (Fig. 3d) and maximum spray width (Fig. 3e) increase with injection pressure because higher initial injection momentum promotes the spray spatial dispersion. In Fig. 3c, the occurrence of the peak front velocity is considered as the spray breakup time, before which the tip penetration distance rapidly increases and after which the spray development slows down. It is observed that the spray breakup time for all fuels is advanced with increased injection pressure. This behavior is in accordance with the Hiroyasu empirical equation, in which the breakup time is inversely proportional to the square root of the ambient density and pressure difference [33]. Fuel property also influences the spray development, especially in tip penetration distance, projected spray area, spray cone angle and the maximum spray width, albeit to a less extent than the injection pressure. The higher viscosity and surface tension of methyl oleate and ethyl oleate increase the difficulty in spray breakup, thus resulting in the spray development more along the axial direction but less in the radial direction. Therefore, it can be observed that at 40 MPa injection condition, the spray tip penetration distances of methyl oleate and ethyl oleate are slightly higher than diesel and methyl laurate (Fig. 3a), while the projected spray
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Fig. 3. Macroscopic spray parameters of different test fuels at different injection pressures (40 MPa and 60 MPa) and 0.5 MPa ambient pressure condition: (a) tip penetration distance (b) projected spray area (c) spray front velocity (d) spray cone angle (e) maximum spray width.
area (Fig. 3b), spray cone angle (Fig. 3d) and the maximum spray width (Fig. 3e) of methyl oleate and ethyl oleate are generally less than those of diesel and methyl laurate. As the injection pressure
increases to 60 MPa, the effects of fuel properties are conquered by higher injection pressure, reducing the variation in the macroscopic spray parameters for different fuels. Also, it is noted
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that methyl laurate at 60 MPa injection pressure condition produces less spray area, spray cone angle and the maximum spray width because its higher vapor pressure aggravates the cavitation phenomenon in the injection system and causes increased injection instability.
the deteriorated fuel atomization performances. In contrast, increased ambient gas pressure decreases the fuel/gas density ratio, and as such promotes the fuel atomization process.
3.2. Macroscopic spray characteristics of fatty acid esters and diesel at different ambient pressures
The relative light intensity distribution derived from the grayscale spray images can provide a qualitative analysis for the bulk spatial distribution of the fuel spray [29,30]. In Fig. 5, the light intensity levels at four different axial locations are analyzed for all fuel sprays, at 40 MPa injection pressure and 0.5 MPa ambient pressure condition. The images chosen are the ones at 1.0 ms after the time when fuel is first injected out of the nozzle. From Fig. 5, it is observed that for all test fuels, the light intensity distribution range along the radial direction is extended as the axial position moves away from the nozzle tip, indicating that the spray is mostly spread at the end area. Further, the light intensity level along the nozzle tip centerline is the highest and does not show obvious variation at different axial positions, indicating the fuel droplets within these areas are most concentrated. The most pronounced difference among different test fuels is the distribution range of the light intensity at the spray end area. In spite of similar light intensity distribution ranges for different test fuels at 20 mm and 40 mm positions in the axial distance from the nozzle tip, diesel and methyl laurate are found to have wider light intensity distribution ranges than methyl oleate and ethyl oleate at 60 mm and 80 mm positions to the nozzle tip, which suggests increased diffusion and atomization tendency for diesel and methyl laurate relative to methyl oleate and ethyl oleate. Because the differences in light intensity distribution among test fuels are the most significant at locations far away from the nozzle tip, the light intensity levels of different test fuels at 80 mm from the nozzle tip are directly compared in Fig. 6, under different test conditions. The spray images at different test conditions are similarly of 105 ± 2 mm tip penetration distances. The injection pressure and ambient pressure apparently influence the light intensity level and distribution at a given position. As seen in Fig. 6, the most pronounced difference in light intensity distribution for all test fuels occurs in 40 MPa injection pressure and 0.5 MPa ambient pressure condition, in which diesel and methyl laurate produce wider light intensity range (Fig. 6a). However, as the ambient pressure increases to 1.0 MPa but the injection pressure is kept at 40 MPa, as shown in Fig. 6b, methyl oleate and ethyl oleate are found to produce wider light intensity distribution. This is because with increased ambient pressure, spray penetration distance is significantly shortened, and the 80 mm position from the nozzle tip is close to the periphery area of the spray end, where the spray light intensity level significantly decreases due to the air entrainment and fuel atomization. The reduction in light intensity distribution range is more obvious for diesel and methyl laurate because their lower viscosity and surface tension produce shorter spray penetration distance, and the chosen position is probably much closer to the spray boundary. Effect of increased injection pressure on the light intensity distribution is illustrated in Fig. 6c, where the injection pressure is 60 MPa and the ambient pressure is still held at 0.5 MPa. Compared to 40 MPa injection pressure condition, the variation in light intensity distribution ranges of different fuels is reduced, and the ranges with high light intensity values are broadened. This indicates that the chosen position is in the body area of the spray, where droplet concentration is quite high and the fuel atomization is not obvious.
The tip penetration distance, projected spray area, spray front velocity, spray cone angle and maximum spray width of fatty acid esters and diesel fuel at 40 MPa injection pressures, 0.1 MPa, 0.5 MPa and 1.0 MPa ambient pressure conditions are described in Fig. 4. The effects of fuel properties on macroscopic spray parameters are in agreement at each ambient pressure condition, that is, more viscous fuels such as methyl oleate and ethyl oleate produce higher tip penetration distance but less spray area, spray cone angle and the maximum spray width, as discussed in the previous section. Also, it is noted that the ambient pressure poses significant influences on the macroscopic spray parameters. Increased ambient gas pressure leads to higher resistance for fuel jet development and droplet diffusion, so the spray tip penetration distance is reduced and the liquid fuel is constricted within a limited area, as illustrated in Fig. 4a and b. Also, the spray breakup time for all fuels is advanced with increased ambient pressure (Fig. 4c), which agrees with the findings by Hiroyasu et al. [33]. Meanwhile, restricted spray penetration in the axial direction is accompanied with increased spray development towards the radial direction, thus leading to increased spray cone angle (Fig. 4d). The maximum spray width versus the ambient pressure does not follow the trend as spray cone angle; as seen from Fig. 4e, the maximum spray widths at 0.1 MPa and 0.5 MPa ambient pressure conditions are similar, and higher than that at 1.0 MPa ambient pressure condition. This is because the location of the maximum spray width is generally at the spray end area, and at higher ambient pressure condition, the spray penetration is greatly reduced and the droplet diffusion at the spray end area is also sufficiently restricted. The above experimental observation could also be explained based on the changes in Reynolds number, Weber number and the fuel/gas density ratio, which are closely related with the fuel and gas mixing process and defined as the following equations [34]:
Re ¼
qVD l
We ¼
u¼
qa V 2 D ql ¼ 2r u qa
ql qa
ð1Þ
ð2Þ ð3Þ
where V is the fuel jet velocity, D is the orifice diameter, l is the fuel kinematic viscosity, r is the fuel surface tension, ql is fuel density and qa is the ambient gas density. Reynolds number and Weber number describe the ratio of inertia force and viscous force and the ratio of inertia force and surface tension, respectively, and are commonly used to describe the mutual interaction between the fuel jet and surrounding gases [35]. Increased Reynolds number and Weber number and reduced fuel/gas density ratio increase the instability of fuel jet, and thus promote the fuel atomization and the mixing with the surrounding gases. Given a test condition, increased fuel surface tension and viscosity of fatty acid esters reduce Reynolds number and Weber number, but their higher densities increase the fuel/gas density ratio. The changes in Reynolds number, Weber number and fuel/gas density ratio contribute to
3.3. Light intensity characteristics of fatty acid esters and diesel spray
3.4. Microscopic spray characteristics of fatty acid esters and diesel In addition to the macroscopic spray characteristics, this study also studies the microscopic spray characteristics of fatty acid
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Fig. 4. Macroscopic spray parameters of different test fuels at 40 MPa injection pressure and different ambient pressures (0.1 MPa, 0.5 MPa and 1.0 MPa) conditions: (a) tip penetration distance (b) projected spray area (c) spray front velocity (d) spray cone angle (e) maximum spray width.
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Fig. 5. Light intensity distribution of different test fuels at different axial positions from the nozzle tip: (a) diesel fuel (b) methyl laurate (c) methyl oleate (d) ethyl oleate.
esters, which refer to the parameters describing the fuel atomization quality by the droplet size and distribution in the fuel spray. Since it is relatively difficult to measure the droplet size within the area of high concentration of fuel droplets, the measurement positions in this study were set to be 80 mm and 100 mm away from the nozzle tip. Sauter Mean Diameter (SMD) is used as the parameter to evaluate the fuel atomization quality. The calculation formula of SMD is given in Eq. (4):
R Dmax D
f ðDi ÞD3i dD
Dmin
f ðDi ÞD2i dD
SMD ¼ R Dmin max
ð4Þ
where Dmin is the minimum droplet diameter, Dmax is the maximum droplet diameter, Di is the droplet diameter between Dmin and Dmax , f ðDi Þ is the corresponding probability density function of Di , and dD is the integration step. The droplet size distributions of methyl laurate, methyl oleate, ethyl oleate and diesel, as well as their SMD with the change of time are shown in Fig. 7. As seen in Fig. 7a, the droplet size distributions of methyl laurate and diesel are similar, while those of methyl oleate and ethyl oleate are similar. In addition, methyl laurate and diesel have higher probability densities in the area of
small droplet sizes. The differences on viscosity and surface tension are the main reasons which lead to their different droplet size distributions [36]. The difference in droplet size distribution affects the SMD of the fuel. As is shown in Fig. 7b, the SMDs of the four test fuels are always in the range of 20–27.5 lm. The SMD of methyl laurate is the smallest, while that of diesel is the second smallest. The SMD of ethyl oleate is the largest among the four test fuels. From the properties of test fuels shown in Table. 1, the viscosity and surface tension of methyl laurate and diesel are lower, so it is easier for the fuel droplets to overcome the molecular viscous force and droplet surface tension, breaking into droplets with smaller diameters. As seen in Fig. 8, the droplet size distribution of fatty acid esters and diesel are compared at the measure position of 100 mm away from the nozzle tip and the rail pressures of 60 MPa and 80 MPa. It can be easily observed that the droplet size distributions of methyl laurate and diesel are located within the range of small droplet size. At 60 MPa rail pressure, the probability density functions of methyl laurate and diesel reach the peak value (14%) as the droplet size is about 30 mm, while those of methyl oleate and ethyl oleate reach the peak value when the droplet size is about 35 mm. However, at 80 MPa rail pressure, the probability density function
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Fig. 6. Relative light intensity of fatty acid esters and diesel at 80 mm position from the nozzle tip: (a) 40 MPa injection pressure, 0.5 MPa ambient pressure (b) 40 MPa injection pressure, 1.0 MPa ambient pressure (c) 60 MPa injection pressure, 0.5 MPa ambient pressure.
Fig. 7. Comparison of microscopic spray characteristics of fatty acid esters and diesel (rail pressure: 60 MPa, energizing time: 1.0 ms, measure position: 80 mm).
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Fig. 8. The effect of rail pressure on the droplet size distributions of fatty acid esters and diesel (energizing time: 1.0 ms, measure position: 100 mm).
Fig. 9. The effect of rail pressure on the SMD values of fatty acid esters and diesel (energizing time: 1.0 ms, measure position: 100 mm).
curves of methyl oleate and ethyl oleate and their corresponding droplet sizes move towards the smaller size region, while the distribution curves of methyl laurate and diesel droplets almost remain the same. The result indicates that the differences in droplet size distribution caused by fuel properties are reduced as rail pressure increases. Fig. 9 shows the SMDs calculated by the fuel droplet size distributions in Fig. 8. It is found that the rail pressure affects the fuel droplet size. At 60 MPa rail pressure, the SMDs of the four test fuels are in the range of 20–27 lm. The droplet sizes of four test fuels are in the following sequence: ethyl oleate > methyl oleate > diesel > methyl laurate, and the influence of fuel property on the SMD remains the same. However, when the rail pressure increases to 80 MPa, the SMDs of the fuels decrease slightly to the range of 19–24 lm. In addition, the differences between methyl laurate and diesel, as well as between methyl oleate and ethyl oleate are reduced, and the SMD of methyl oleate is slightly larger than that of ethyl oleate. The result indicates that increased rail pressure promotes the breakup of fuel droplets and decreases the influence of fuel property on SMD.
4. Conclusions In this study, the macroscopic and microscopic spray parameters of three biodiesel components, methyl laurate, methyl oleate and ethyl oleate were investigated and compared to diesel fuel under different injection pressure and ambient pressure conditions. The studied spray parameters include the tip penetration distance, projected spray area, spray front velocity, spray cone angle, the maximum spray width and the droplet size distribution. The images from the high speed camera are also converted to grayscale ones and the grayscale values within the spray contour for different fuels are extracted to evaluate the light intensity level and distribution. Macroscopic spray parameters are influenced by fuel properties. Compared to diesel and methyl laurate, higher viscosity and surface tension of methyl oleate and ethyl oleate lead to slightly higher tip penetration distance, but produce less projected spray area, spray cone angle and the maximum spray width. The variation in the macroscopic spray parameters caused by fuel properties could be reduced with increased injection pressure. Through the
D. Han et al. / Fuel 203 (2017) 370–379
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