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Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel
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Original research paper
Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel Limin Geng a,*, Yanjuan Wang a, Yueying Wang a, Huimei Li b a Key Laboratory of Shaanxi Province for Development and Application of New Transportation Energy, Chang'an University, Xi'an 710064, China b Department of Military Vehicle, Academy of Military Transportation, Tianjin 300161, China
highlights Effects of the injection pressure and orifice diameter on biodiesel spray were studied. STP and SCA increased with an increasing injection pressure. STP increased and SCA reduced with a decreasing orifice diameter. SMD decreased with an increasing injection pressure and decreasing orifice diameter. Orifice diameter reduction improved atomization; however, the droplet size distribution was not homogenous.
article info
abstract
Article history:
The purpose of this study was to analyze the influence of the injection pressure and orifice
Received 26 June 2018
diameter on the spray characteristics of soybean biodiesel. The macroscopic spray char-
Received in revised form
acteristics of the spray tip penetration (STP) and spray cone angle (SCA) were tested with a
3 December 2018
high-speed camera system. The microscopic spray characteristics, such as the statistical
Accepted 5 December 2018
size distribution, Sauter mean diameter (SMD), representative diameters and dispersion
Available online xxx
boundary, were obtained using a Malvern laser particle size analyzer (PSA). The test results showed that with an increasing injection pressure, the STP and the SCA of the biodiesel
Keywords:
increased, but the curves of size-volume distribution and cumulative volume distribution
Biodiesel
of the atomized droplets shifted to smaller diameters. The SMD and representative di-
Injection pressure
ameters decreased, and the dispersion boundary was reduced. Moreover, with a decreasing
Orifice diameter
orifice diameter, longer STP and smaller SCA values were observed. Similarly, the size
Spray characteristics
distribution curves of the atomized biodiesel droplets shifted to smaller diameters. The
Size distribution
SMD and representative diameters were reduced, and the relative size range of the atomized biodiesel droplets was enlarged. Higher injection pressures and smaller orifice diameters improved the biodiesel atomization; however, the smaller orifice diameters caused an inhomogeneous size distribution of the atomized biodiesel droplets. © 2019 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author. Tel.: þ86 13891436220. E-mail address: [email protected] (L. Geng). Peer review under responsibility of Periodical Offices of Chang'an University. https://doi.org/10.1016/j.jtte.2018.12.004 2095-7564/© 2019 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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Abbreviations STP SMD SCA PSA DBE DME ALR HC CO PM NOx EGR BSFC BTE Re We
1.
Spray tip penetration Sauter mean diameter Spray cone angle Particle size analyzer Di-n-butyl ether Dimethyl ether Air-to-liquid mass ratios Unburned hydrocarbon Carbon monoxide Particulate matter Nitrogen oxide Exhaust gas recirculation Brake specific fuel consumption Brake thermal efficiency Reynolds number Weber number
Introduction
In the last two decades, energy shortage and environmental pollution have increased with the rapid increase of automobiles in China. Therefore, research on substitute biofuel has attracted more attention. Biodiesel is a green, renewable, and environmentally friendly energy, which has wide sources of raw materials, such as edible vegetable oil, non-edible vegetable oil, animal fats, waste cooking oils, microalgae oils, etc. The production process of biodiesel is simple. The process includes transesterification of triglycerides with alcohols. Biodiesel can be blended with fossil diesel in various proportions, and the blends can be used in a diesel engine without significant structural modification (Attia and Hassaneen, 2016; Chauhan et al., 2012; Cheung et al., 2015; McCarthy € lu et al., 2018). et al., 2011; Ozener et al., 2014; Uyarog Compared to fossil diesel, biodiesel is renewable, environmentally friendly, and biodegradable, and contains no aromatics or sulfur. Thus, it has attracted considerable attention from researchers in recent years. Numerous researchers have reported that in most cases, unburned hydrocarbon (HC), carbon monoxide (CO), and particulate matter (PM) emissions are lower with biodiesel fuel. However, nitrogen oxide (NOx) emissions of biodiesel are higher than those of diesel owing to the higher oxygen content and bulk modulus (Agarwal et al., 2015; Aldhaidhawi et al., 2017; Ashrafu et al., 2015; Behc¸et et al., 2015; Chauhan et al., 2013; Sanjid et al., 2016; Shahira et al., 2015; Sun et al., 2010). Furthermore, some researchers have reported that injection timing retardation and exhaust gas recirculation (EGR) are effective methods to control the NOx emission of biodiesel (Hoekman and Robbins, 2012; Palash et al., 2013). The fuel spray characteristics have a significant influence on the combustion characteristics and exhaust emissions of internal-combustion engines. In recent years, some researchers have conducted experimental and numerical investigations on the spray characteristics of biodiesel fuel. The
influences of the fuel temperatures and ambient gas conditions on the spray and atomization behavior of soybean biodiesel were investigated. The results indicate that the fuel temperature and ambient air temperature increased, and the fuel vapor mass also increased. Biodiesel fuel can create a high quality mixture with air when the fuel temperature and ambient air temperature are high (Park et al., 2009). The spray characteristics of soybean biodiesel, di-n-butyl ether (DBE)/ biodiesel blends, and 0# diesel were examined by a high pressure common-rail injection system. A smaller Sauter mean diameter (SMD) was observed when DBE was mixed with the biodiesel. The DBE addition can improve the atomization of the biodiesel (Guan et al., 2015). The fuel spray characteristics of waste cooking oil biodiesel (B100) and its blend with diesel (B20) were compared with that of diesel fuel. The investigation results indicated that B100 had a greater spray tip penetration (STP), faster velocity, and smaller spray cone angle (SCA) than those of B20 and diesel. The STP was reduced, and the SCA increased significantly under a high ambient pressure (Mohan et al., 2014). The effects of the fuel injection pressures and injection timings on the particulate size-number distribution and spray characteristics were studied. A longer STP and larger spray area were observed with an increasing injection pressure. The average size of the particulates increased when the start of the injection was delayed (Agarwal et al., 2014). The spray characteristics of the biodiesel and dimethyl ether (DME) fuels were tested under different ambient pressures. The test results showed that the spray characteristics of the biodiesel were inferior to those of the DME at the same injection and ambient conditions. The ambient pressure had an important effect on the spray performances of the fuels. The STP and spray area decreased with an increasing ambient pressure (Kim et al., 2010). The spray and atomization of soybean biodiesel in a blurry injector were tested using three nozzle exit structures: cylindrical, conical, and conical-cylindrical. The results indicated that the mean diameters of the atomized droplets decreased obviously with an increasing air-to-liquid mass ratio (ALR), while the SCAs did not change with an increasing ALR. For a given liquid mass flow rate, the conical nozzle exhibited a higher atomization quality with smaller representative diameters than those of the cylindrical and conical-cylindrical nozzles (Azevedo et al., 2016). The influence of the orifice size and injection pressure on the spray morphology, velocity distribution, and SCA were experimentally studied by a pressure-swirl nozzle. The test results showed that a higher pressure widened the SCA and increased the spray velocity magnitude. Inadequate atomization would occur if the orifice diameter was too small or too large (Zhang et al., 2017). The compositions of diesel and biodiesel are different, thus, their physicochemical properties have some differences. Previous studies have shown that biodiesel has a higher density, viscosity, and surface tension, and its evaporation property is less than that of diesel fuel. These factors created an inferior atomization quality for biodiesel. In this study, the injection parameters were adjusted to improve the atomization quality of biodiesel. The macroscopic spray characteristics of biodiesel at different injection pressures and orifice diameters with respect to the STP and SCA were measured by
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
a high-speed camera system. In addition, the microscopic spray characteristics, such as the statistical size distribution, SMD, representative diameters and dispersion boundary, were tested and analyzed using a Malvern laser particle size analyzer at different injection pressures and orifice diameters.
2.
Experimental setup and procedure
2.1.
Biodiesel compositions and properties
The biodiesel used in this study was produced from soybean oil by transesterification with methanol. The biodiesel was made by a biotechnology company. The soybean oil was converted to biodiesel via a chemical reaction process of transesterification, where the soybean oil was combined with ethanol or methanol in the presence of a catalyst to generate fatty acid esters and glycerin. In general, methanol is used instead of ethanol because it reduces the production cost of biodiesel, making biodiesel more competitive in the market of petroleum diesel fuel. The biodiesel was composed of various fatty acid methyl esters with different carbon chain lengths, and its composition was tested by gas chromatography, as listed in Table 1. The soybean biodiesel contained 89.76% fatty acid methyl ester, where greater than 30.26% was saturated fatty acid esters, and unsaturated fatty acid esters accounted for 59.51%. Linoleic acid esters, oleic acid esters, and palmitic acid esters were the major compositions of soybean biodiesel. The physicochemical properties of the biodiesel and diesel were tested using the national standard methods, as listed in Table 2. The density of the soybean-based biodiesel was approximately 4.60% higher than that of conventional diesel. The viscosity of the biodiesel was approximately 62.80% higher than that of diesel. The surface tension of the biodiesel was approximately 7.10% higher than that of diesel. The distillation temperature of the biodiesel was higher than that of diesel. These factors resulted in a smaller SCA and longer STP for the biodiesel fuel than those of the diesel fuel, and the SMD and the representative
Table 1 e Major compositions of soybean biodiesel. Type of fatty acid esters Dodecanoic Myristic Palmitic Palmitoleic Heptadecanoic Stearic Oleic Linoleic Linolenic Arachidic Eicosenoic Behenic Docosadienoic Lignoceric Tetracosenoic Others Total
Carbon chain
Mass (%)
C12:0 C14:0 C16:0 C16:1 C17:0 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1
0.21 0.71 15.26 0.80 8.90 3.92 17.83 34.45 4.67 0.35 0.38 0.65 0.22 0.26 1.16 10.23 100.00
Table 2 e Physicochemical properties of soybean biodiesel and petroleum diesel. Item
Diesel Biodiesel
Density (20 C) (kg/m3) Kinematic viscosity (20 C) (mm2/s) Surface tension (20 C) (103N/m) Flash point ( C) 10% distillation temperature ( C) 50% distillation temperature ( C) 90% distillation temperature ( C)
836 4.81 26.55 64 218 272 338
874 7.83 28.43 160 328 337 354
Test method GB/T1884 GB/T265 GB/T1884 GB/T261 GB/T6536 GB/T6536 GB/T6536
diameters of the biodiesel were greater than those of diesel. Meanwhile, the size-volume distribution curve and the cumulative volume distribution curve of the atomized biodiesel droplets shifted to larger diameters (Geng et al., 2014), which showed that the spray and atomization quality of the biodiesel was inferior to that of diesel. The injection pressures and orifice diameters should be adjusted to improve the spray quality of the biodiesel.
2.2.
Experimental apparatus and procedure
Fig. 1 shows a schematic of the high-speed camera system, which was used to measure the SCA and STP. The system was composed of a fuel tank, a fuel injection pump, a highpressure fuel pipe, an injector, a light source, a high-speed camera (PhantomV 9.1), the black background with a ruler, and a computer. The model of the high-speed camera was PhantomV 9.1, which is produced by American Vision Research, Inc. The highest resolution of the high-speed camera was 1632 1200 pixels. The minimum exposure time was 2 ms, and the maximum photographic speed was 153,846 frames/s at the resolution of 96 8 pixels. In this study, the high-speed camera was applied to record the spray images at the photographic speed of 1200 frames/s and the resolution of 1632 1000 pixels. The recorded spray images were processed by MATLAB software. First, the images were converted to grayscale images. Second, a fixed threshold was chosen for all spray images. Finally, the spray images were extracted from the black background, and the SCA and STP were measured.
Fig. 1 e Schematic of the high-speed camera system.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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Fig. 2 shows a schematic of the Malvern test system, which was used to test the statistical size distribution and SMD of the atomized droplets. The system was composed of a fuel tank, fuel injection pump, high-pressure fuel pipe, injector, Malvern laser particle size analyzer (PSA), and computer. The measurement principle of the Malvern laser PSA is based on the theory of Fraunhofer diffraction, and its measurement range for the droplet diameters was 0.02e2000 mm. The size distribution and the mean diameters of the atomized droplets were automatically measured and calculated from the energy distribution of laser diffraction, and the measuring accuracy for the smallest particles was 0.02 mm. The particles size measured by the method was not affected by the position of the particles in the spray domain; therefore, the Malvern laser PSA could be applied to measure a high-density spray domain, and high test accuracy could be ensured for the fast moving particles. The Malvern test method measured the fuel spray characteristics in a short time with various nozzles under different ambient conditions, and the test method is widely used in the measurement of particle size distribution with considerable savings in time and manpower. A single-hole nozzle was used in the fuel injection system. The orifice length was constant at 1.5 mm, and the orifice diameters were 0.260, 0.315, and 0.366 mm. The fuel injection pressures were separately adjusted to 16, 20, and 24 MPa. The macroscopic and microscopic spray characteristics of the biodiesel were investigated by the high-speed camera and PSA under different injection pressures and orifice diameters.
3.
Results and discussion
3.1.
SCA and STP
The SCA and STP have a significant influence on the spatial distribution of the atomized droplets in the combustion chamber. In order to avoid colliding and bonding of the spray droplets in the movement process, the fuel sprays injected from the multi-orifice nozzle should not intersect. This is an important factor to consider when designing the SCA of a multi-orifice nozzle. Moreover, the SCA and STP should match with the geometry of the combustion chamber. If the STP length allows most of the fuel droplets to collide with the cylinder wall, the fuel film of the cylinder wall will be too thick. The thick fuel film on the cylinder wall may not
evaporate and combust completely, which could create heavy combustion chamber deposits. Using different injection pressures and orifice diameters, the test results of the SCA and STP are listed in Table 3. With an increasing injection pressure, the SCA of the biodiesel enlarged gradually. This was because the fuel injection velocity was accelerated, and the Reynolds number of the biodiesel flowing in the nozzle was enhanced with an increasing injection pressure. The growing disturbance and turbulence inside the nozzle, promoted the breakup and atomization of the primary spray, and ultimately increased the SCA. With an increasing injection pressure, the STP of the biodiesel increased. This was because the high injection pressure increased the fuel injection velocity and extended the STP. In addition, with an increasing orifice diameter, the SCA of the biodiesel increased, and the STP decreased gradually. This may be because the large orifice diameter increased the contact area between the fuel spray and ambient air, thus increasing the air disturbance and aerodynamic drag. In addition, breakup of the primary spray was promoted. Therefore, the SCA of the biodiesel was enlarged with an increasing orifice diameter. In addition, with a decreasing orifice diameter, the STP of the biodiesel increased. The STP was mainly influenced by the injection velocity and aerodynamic drag. The smaller the orifice diameter of the nozzle, the faster the flow speed of the fuel in the nozzle, and the STP would be increased owing to the higher fuel injection velocity. Meanwhile, the smaller orifice diameter created a smaller SCA, which resulted in a smaller surface area of the fuel spray front that contacted the ambient gas. The aerodynamic drag for the fuel spray affected by the ambient gas was significantly reduced; therefore, the STP was further increased.
3.2.
Size distribution curves of the atomized droplets
Some researchers have reported that increasing the injection pressure provided better brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) than those of the original injection pressure for diesel and biodiesel blends (Shehata et al., 2015). Therefore, the size-volume distribution of the atomized biodiesel droplets was tested by the Malvern laser PSA with the increased injection pressures of 16, 20, and 24 MPa. The influence of the injection pressure on the size distribution of the atomized biodiesel droplet was analyzed in this study. The test results are shown in Figs. 3 and 4. In Fig. 3, PDFV represents the volume of certain diameter droplet as a percentage of the total volume of spray droplets. In Fig. 4, VC represents the percentage of all
Table 3 e SCA and STP of the biodiesel under different injection pressures and orifice diameters. Injection 24 MPa/ 20 MPa/ 16 MPa/ 16 MPa/ 16 MPa/ pressure/ 0.366 0.366 0.366 0.315 0.260 orifice mm mm mm mm mm diameter Fig. 2 e Schematic of the Malvern test system.
SCA ( ) STP (cm)
16.10 30.6
14.30 28.4
13.10 22.0
8.56 35.7
6.33 57.6
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
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Fig. 3 e Size-volume distribution curves of the atomized droplets under different injection pressures.
Fig. 5 e Size-volume distribution curves of the atomized droplets under different orifice diameters.
droplets volumes smaller than a certain diameter to the total volume of spray droplets, which is called the cumulative sizevolume distribution of the atomized droplets. With an increasing injection pressure, the curve of the size-volume distribution and the cumulative volume distribution of the atomized biodiesel droplets moved toward smaller diameters. Increasing the injection pressure could reduce the diameter of the atomized droplets and improve the spray quality of the biodiesel fuel. The higher injection pressure resulted in a faster injection velocity, and the velocity difference between the ambient gas and fuel spray was enlarged. Therefore, the aerodynamic effects and turbulence development were enhanced, which promoted the fragmentation of the atomized fuel droplets. The orifice diameter is an significant parameter that influences the cycle fuel injection quantity and engine performance. When the needle valve is fully lifted, the orifice diameter will affect the peak injection rate and cyclic injection quantity (Han et al., 2018). Furthermore, the smaller orifice diameter can improve the air-fuel mixing, atomization, and vaporization, leading to a shorter combustion duration and higher BTE (Kumar et al., 2018). The size-volume distribution of the atomized biodiesel droplets was measured by the Malvern laser PSA under the injection pressure of 16 MPa with different orifice diameters of 0.260, 0.315, and 0.366 mm. The effects of the orifice diameter on the size distribution of the atomized droplets were analyzed. Fig. 5 shows the size-volume distribution
curves of the atomized droplets, and Fig. 6 shows the cumulative volume distribution curves of the atomized droplets using a nozzle with different orifice diameters. With a decreasing orifice diameter, the size-volume distribution curve of the atomized biodiesel droplets gradually shifted to a smaller diameter. When the orifice diameter was 0.366 mm, the peak diameter of the sizevolume distribution curve of the atomized biodiesel droplet was 72 mm, and the volume percentage corresponding to the peak diameter was 15.01%. However, when the orifice diameter was 0.26 mm, the peak diameter of the sizevolume distribution curve of the atomized biodiesel droplet was 61 mm, and the volume percentage corresponding to the peak diameter was 12.47%. In addition, with a decreasing orifice diameter, the cumulative volume distribution curves of the atomized droplets shifted towards a smaller diameter, and the corresponding size of the atomized biodiesel droplets gradually decreased when the different cumulative volumes are obtained. Therefore, reducing the orifice diameter decreased the droplet size and improved the atomization quality of the biodiesel. The flow velocity of the biodiesel inside the nozzle was accelerated owing to the reduced orifice diameter, increased Reynolds number of the fuel flowing in the nozzle, intensified disturbance of the internal flow in the nozzle, increased turbulence and fragmentation of the circular jet. Therefore, the diameters of the atomized droplets decreased, and the atomization quality of the biodiesel was improved.
Fig. 4 e Cumulative volume distribution curves of the atomized droplets under different injection pressures.
Fig. 6 e Cumulative volume distribution curves of the atomized droplets under different orifice diameters.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
3.3.
SMD of the atomized droplets
The SMD is also referred to as the volume-surface mean diameter, and it is defined as shown in Eq. (1). Z
Dmax
Dmin Dmax
Therefore, the atomization quality of the biodiesel was improved, and the SMD of the atomized droplets decreased. The combined effects of the injection pressure and orifice diameter resulted in a similar SMD for 24 MPa with the 0.366 mm nozzle and 16 MPa with the 0.260 mm nozzle.
D3 dN
D32 ¼ Z
(1) D2 dN
Dmin
The parameter N is the droplets number of diameter D. The minimum diameter was Dmin ¼ 0. The parameter D32 was most frequently used to evaluate the fuel spray quality, and it is also a common calculation parameter in the combustion reaction. The SMDs of the atomized biodiesel droplets were measured by the Malvern laser PSA under different injection pressures with different orifice diameters. The test results are listed in Table 4. The SMD of the atomized biodiesel droplets gradually decreased and the atomization quality of the biodiesel was improved with the increase of the injection pressures. This was because the disturbances in the nozzle and the aerodynamic force were enhanced with the increase of the injection pressure. When the fuel injection pressure was increased, the jet speed was accelerated, the Reynolds number (Re) increased, and the turbulence and internal disturbance were strengthened. In addition, the fragmentation of the circular jet was promoted. Furthermore, the relative velocity between the gas phase and liquid phase was accelerated with an increasing injection pressure. The Weber number (We) increased, and the gas disturbance was strengthened. The fuel atomization was improved, and the SMD of the atomized biodiesel droplets decreased owing to the higher injection velocity and greater aerodynamic force. In addition, the SMDs of the atomized biodiesel droplets gradually decreased with a decreasing orifice diameter. This was because the flow velocity of the fuel in the nozzle was accelerated with the reduction of the orifice diameter, and the internal disturbance in the nozzle was enhanced, which promoted of the turbulence and fragmentation of the circular jet. Therefore, the SMDs of the atomized biodiesel droplets decreased with a decreasing orifice diameter. When the injection pressure was decreased from 24 to 16 MPa, the injection velocity decreased. The Re and We values decreased, and the turbulence and gas disturbance were weakened. Therefore, the spray quality of the biodiesel deteriorated. When the nozzle diameter was reduced from 0.366 to 0.260 mm, the flow velocity of the fuel in the nozzle increased. The Re and We values were enhanced, and the turbulence and internal disturbance were strengthened.
3.4.
Representative diameters and dispersion boundary
The representative diameters are defined as the ratio of the volume of the atomized droplets below a certain diameter to the volume of all atomized droplets in the cumulative volume distribution curve of atomized droplets (Mohamad et al., 2015). The subscripts of the representative diameters indicate the volume percentage, where D0.5 is the mass median diameter, and D0.999 is the largest diameter of the atomized droplets. The representative diameters of the atomized droplets are often used in fuel spray and combustion analyses. The representative diameter is an essential parameter for calculating the dispersion boundary of the atomized droplets. The representative diameters of D0.999 are significant when they are used to analyze the HC and soot emissions of the engine. D0.999 represents the largest diameter of the atomized droplets. The large droplets can be generated for the fuel injected later in the cycle or because of overfuelling of the engine. The fuel left in the injector sac volume and the nozzle holes at the end of injection may not fully vaporize during the combustion process (Mohamad et al., 2015). The large fuel droplets enter the engine cylinder during the expansion stroke, which results in undermixing of the fuel with air; thus, the fuel is not fully burned. This is one of the main sources of HC in the diesel engine. In addition, it is difficult for the large droplets to evaporate completely. When the temperature in the cylinder is high, the fuel may not be uniformly mixed, and some fuel-rich zones may exist. Incomplete combustion and pyrolysis of the fuel droplets will produce soot emission. Table 5 lists the representative diameters of the atomized biodiesel droplets under different injection pressures and different orifice diameters. When the injection pressure was increased from 16 to 24 MPa, the representative diameters of the atomized biodiesel droplets decreased. The size of the atomized droplets decreased with an increasing injection pressure. The amount of small fuel droplets increased, and the amount of large fuel droplets decreased. The atomization quality of the biodiesel was improved. In addition, most of the representative diameters decreased
Table 5 e Representative diameters of the atomized droplets under different injection pressures and orifice diameters. Table 4 e SMDs of atomized droplets under different injection pressures and orifice diameters.
Injection pressure/ orifice diameter
Injection 24 MPa/ 20 MPa/ 16 MPa/ 16 MPa/ 16 MPa/ pressure/ 0.366 0.366 0.366 mm 0.315 0.260 orifice mm mm mm mm diameter
24 20 16 16 16
D32 (mm)
36.23
39.10
49.13
42.29
35.86
MPa/0.366 MPa/0.366 MPa/0.366 MPa/0.315 MPa/0.260
mm mm mm mm mm
Representative diameter D0.1
D0.5
D0.9
D0.999
24.52 26.89 33.42 27.10 20.35
49.87 52.75 63.11 57.47 52.42
85.12 89.83 107.00 101.50 96.37
141.86 167.94 198.82 198.82 167.94
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
Table 6 e Relative size range Ds and the dispersion boundary Db under different injection pressures and orifice diameters. Injection pressure/orifice diameter 24 20 16 16 16
MPa/0.366 MPa/0.366 MPa/0.366 MPa/0.315 MPa/0.260
mm mm mm mm mm
Ds
Ds
1.22 1.19 1.16 1.29 1.45
1.84 2.18 2.15 2.46 2.20
with a decreasing orifice diameter. However, the maximum diameters of the atomized droplets were the same at 16 MPa with the 0.366 mm nozzle and 16 MPa with the 0.315 mm nozzle. However, according to the statistical results for the cumulative volume distribution of the atomized droplets, the volume ratio of largest droplets was different in both cases. The volume ratio of largest droplets was 0.1% at 16 MPa with the 0.366 mm nozzle, while the volume ratio was 0.01% at 16 MPa with the 0.315 mm nozzle. Therefore, the number of largest droplets was greater with the 0.366 mm nozzle under the 16 MPa injection pressure, and the spray quality of the biodiesel was improved by reducing the orifice diameter under a constant injection pressure. Moreover, based on the decline degree of the representative diameters, the influence of the injection pressure was greater than that of the orifice diameter. The dispersion degree is used to describe the size range of the atomized droplets, which is commonly represented by the relative size range Ds and the dispersion boundary Db (Cao, 2013). The relative size range is defined as shown in Eq. (2). Ds ¼
D0:9 D0:1 D0:5
(2)
It represents the range of the droplet diameter relative to the mass median diameter. The dispersion boundary is defined as shown in Eq. (3). Db ¼
D0:999 D0:5 D0:5
(3)
It represents the dispersion degree of the maximum droplet diameter relative to the mass median diameter. With an increasing injection pressure, the relative size range of the atomized biodiesel droplets increased slightly, and the dispersion boundary decreased. Thus, with an increasing injection pressure, the size distribution range of the atomized biodiesel droplets showed minimal change. The diameter of the largest droplets decreased, and the size distributions of the droplets became more homogeneous. The atomization quality of the biodiesel was improved. In addition, the relative size range and dispersion boundary of the atomized biodiesel droplets increased with a decreasing orifice diameter. The size-volume distribution curves of the atomized biodiesel droplets became wider and flatter, and the relative size range of the atomized droplets was larger. The size distributions of the atomized biodiesel droplets became uneven (see Table 6).
7
Although increasing the injection pressure and reducing the orifice diameter could improve the atomization of the biodiesel fuel, the increase of the injection pressure was more significant for the improvement of the spray quality owing to the larger SCA and a more uniform size distribution of the atomized droplets. In addition, the spray characteristics of the diesel fuel were tested at 16 MPa with the 0.366 mm nozzle. The test results showed that the STP of the diesel was 18.73 mm. The SCA was 15.3 . The SMD was 35.86 mm, and the relative size range and dispersion boundary were 1.17 and 1.91, respectively. The spray parameters of the diesel fuel were similar to those of the biodiesel fuel at 24 MPa with the 0.366 mm nozzle. Therefore, the optimized injection pressure for the biodiesel fuel was 24 MPa, and the same spray quality of the diesel fuel was achieved at 16 MPa with the 0.366 mm nozzle.
4.
Conclusions
The physicochemical property differences between diesel and biodiesel were compared using the test method. The effects of the injection pressure and orifice diameter on the spray characteristics of biodiesel were investigated with the high-speed camera and Malvern PSA. Finally, the macroscopic and microscopic spray characteristics of the biodiesel, such as the STP, SCA, SMD, statistical size distribution, representative diameters, and dispersion boundary, were analyzed in the study. The main conclusions are as follows. (1) Based on the test of the physicochemical properties, the soybean biodiesel contained 89.76% fatty acid methyl ester. The density of the soybean-based biodiesel was approximately 4.60% higher than that of conventional diesel. The viscosity was approximately 62.80% higher than that of diesel. The surface tension was approximately 7.10% higher than that of diesel, and the distillation temperature of the biodiesel was significantly higher than that of diesel. Therefore, the evaporation properties of the biodiesel were inferior to those of diesel. (2) With an increasing fuel injection pressure, the SCA and STP of the biodiesel gradually increased. The size-volume distribution curves and the cumulative volume distribution curves of the atomized biodiesel droplets moved towards smaller diameter, and the SMD and representative diameters decreased with an increasing injection pressure. The size distribution range of the atomized biodiesel droplets showed minimal change, while the dispersion boundary decreased. Therefore, an increasing injection pressure could reduce the number of large fuel droplets and improve the atomization of biodiesel fuel. (3) With a decreasing orifice diameter, the SCA of the biodiesel decreased and the STP increased. The size-volume distribution curves and cumulative volume distribution curves of the atomized biodiesel droplets
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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shifted to a smaller diameter, and the SMD and representative diameters decreased. The relative size range and dispersion boundary of the atomized biodiesel droplets increased significantly. Although the decreasing orifice diameters could reduce the mean diameter of the atomized droplets, the droplet size distribution was not homogenous.
Conflict of interest The authors do not have any conflict of interest with other entities or researchers.
Acknowledgments This project is financially supported by the National Natural Science Foundation of China (51806020), the Youth Innovation Team of Shaanxi Universities (Energy Saving and New Energy Vehicles), and the Special Funds for Basic Scientific Research of Central Colleges, Chang’an University (310822172203 and 300102228403).
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Limin Geng is an associate professor, doctor of transportation engineering. Limin Geng has a deep interest in spray, combustion and pollutant emissions of alternative fuels such as biodiesel, methanol and ethanol. In her research experiences, she has accomplished more than 4 research projects, published more than 15 journal papers, and co-edited 2 textbooks.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
Available online at www.sciencedirect.com
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Original research paper
Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel Limin Geng a,*, Yanjuan Wang a, Yueying Wang a, Huimei Li b a Key Laboratory of Shaanxi Province for Development and Application of New Transportation Energy, Chang'an University, Xi'an 710064, China b Department of Military Vehicle, Academy of Military Transportation, Tianjin 300161, China
highlights Effects of the injection pressure and orifice diameter on biodiesel spray were studied. STP and SCA increased with an increasing injection pressure. STP increased and SCA reduced with a decreasing orifice diameter. SMD decreased with an increasing injection pressure and decreasing orifice diameter. Orifice diameter reduction improved atomization; however, the droplet size distribution was not homogenous.
article info
abstract
Article history:
The purpose of this study was to analyze the influence of the injection pressure and orifice
Received 26 June 2018
diameter on the spray characteristics of soybean biodiesel. The macroscopic spray char-
Received in revised form
acteristics of the spray tip penetration (STP) and spray cone angle (SCA) were tested with a
3 December 2018
high-speed camera system. The microscopic spray characteristics, such as the statistical
Accepted 5 December 2018
size distribution, Sauter mean diameter (SMD), representative diameters and dispersion
Available online xxx
boundary, were obtained using a Malvern laser particle size analyzer (PSA). The test results showed that with an increasing injection pressure, the STP and the SCA of the biodiesel
Keywords:
increased, but the curves of size-volume distribution and cumulative volume distribution
Biodiesel
of the atomized droplets shifted to smaller diameters. The SMD and representative di-
Injection pressure
ameters decreased, and the dispersion boundary was reduced. Moreover, with a decreasing
Orifice diameter
orifice diameter, longer STP and smaller SCA values were observed. Similarly, the size
Spray characteristics
distribution curves of the atomized biodiesel droplets shifted to smaller diameters. The
Size distribution
SMD and representative diameters were reduced, and the relative size range of the atomized biodiesel droplets was enlarged. Higher injection pressures and smaller orifice diameters improved the biodiesel atomization; however, the smaller orifice diameters caused an inhomogeneous size distribution of the atomized biodiesel droplets. © 2019 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author. Tel.: þ86 13891436220. E-mail address: [email protected] (L. Geng). Peer review under responsibility of Periodical Offices of Chang'an University. https://doi.org/10.1016/j.jtte.2018.12.004 2095-7564/© 2019 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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Abbreviations STP SMD SCA PSA DBE DME ALR HC CO PM NOx EGR BSFC BTE Re We
1.
Spray tip penetration Sauter mean diameter Spray cone angle Particle size analyzer Di-n-butyl ether Dimethyl ether Air-to-liquid mass ratios Unburned hydrocarbon Carbon monoxide Particulate matter Nitrogen oxide Exhaust gas recirculation Brake specific fuel consumption Brake thermal efficiency Reynolds number Weber number
Introduction
In the last two decades, energy shortage and environmental pollution have increased with the rapid increase of automobiles in China. Therefore, research on substitute biofuel has attracted more attention. Biodiesel is a green, renewable, and environmentally friendly energy, which has wide sources of raw materials, such as edible vegetable oil, non-edible vegetable oil, animal fats, waste cooking oils, microalgae oils, etc. The production process of biodiesel is simple. The process includes transesterification of triglycerides with alcohols. Biodiesel can be blended with fossil diesel in various proportions, and the blends can be used in a diesel engine without significant structural modification (Attia and Hassaneen, 2016; Chauhan et al., 2012; Cheung et al., 2015; McCarthy € lu et al., 2018). et al., 2011; Ozener et al., 2014; Uyarog Compared to fossil diesel, biodiesel is renewable, environmentally friendly, and biodegradable, and contains no aromatics or sulfur. Thus, it has attracted considerable attention from researchers in recent years. Numerous researchers have reported that in most cases, unburned hydrocarbon (HC), carbon monoxide (CO), and particulate matter (PM) emissions are lower with biodiesel fuel. However, nitrogen oxide (NOx) emissions of biodiesel are higher than those of diesel owing to the higher oxygen content and bulk modulus (Agarwal et al., 2015; Aldhaidhawi et al., 2017; Ashrafu et al., 2015; Behc¸et et al., 2015; Chauhan et al., 2013; Sanjid et al., 2016; Shahira et al., 2015; Sun et al., 2010). Furthermore, some researchers have reported that injection timing retardation and exhaust gas recirculation (EGR) are effective methods to control the NOx emission of biodiesel (Hoekman and Robbins, 2012; Palash et al., 2013). The fuel spray characteristics have a significant influence on the combustion characteristics and exhaust emissions of internal-combustion engines. In recent years, some researchers have conducted experimental and numerical investigations on the spray characteristics of biodiesel fuel. The
influences of the fuel temperatures and ambient gas conditions on the spray and atomization behavior of soybean biodiesel were investigated. The results indicate that the fuel temperature and ambient air temperature increased, and the fuel vapor mass also increased. Biodiesel fuel can create a high quality mixture with air when the fuel temperature and ambient air temperature are high (Park et al., 2009). The spray characteristics of soybean biodiesel, di-n-butyl ether (DBE)/ biodiesel blends, and 0# diesel were examined by a high pressure common-rail injection system. A smaller Sauter mean diameter (SMD) was observed when DBE was mixed with the biodiesel. The DBE addition can improve the atomization of the biodiesel (Guan et al., 2015). The fuel spray characteristics of waste cooking oil biodiesel (B100) and its blend with diesel (B20) were compared with that of diesel fuel. The investigation results indicated that B100 had a greater spray tip penetration (STP), faster velocity, and smaller spray cone angle (SCA) than those of B20 and diesel. The STP was reduced, and the SCA increased significantly under a high ambient pressure (Mohan et al., 2014). The effects of the fuel injection pressures and injection timings on the particulate size-number distribution and spray characteristics were studied. A longer STP and larger spray area were observed with an increasing injection pressure. The average size of the particulates increased when the start of the injection was delayed (Agarwal et al., 2014). The spray characteristics of the biodiesel and dimethyl ether (DME) fuels were tested under different ambient pressures. The test results showed that the spray characteristics of the biodiesel were inferior to those of the DME at the same injection and ambient conditions. The ambient pressure had an important effect on the spray performances of the fuels. The STP and spray area decreased with an increasing ambient pressure (Kim et al., 2010). The spray and atomization of soybean biodiesel in a blurry injector were tested using three nozzle exit structures: cylindrical, conical, and conical-cylindrical. The results indicated that the mean diameters of the atomized droplets decreased obviously with an increasing air-to-liquid mass ratio (ALR), while the SCAs did not change with an increasing ALR. For a given liquid mass flow rate, the conical nozzle exhibited a higher atomization quality with smaller representative diameters than those of the cylindrical and conical-cylindrical nozzles (Azevedo et al., 2016). The influence of the orifice size and injection pressure on the spray morphology, velocity distribution, and SCA were experimentally studied by a pressure-swirl nozzle. The test results showed that a higher pressure widened the SCA and increased the spray velocity magnitude. Inadequate atomization would occur if the orifice diameter was too small or too large (Zhang et al., 2017). The compositions of diesel and biodiesel are different, thus, their physicochemical properties have some differences. Previous studies have shown that biodiesel has a higher density, viscosity, and surface tension, and its evaporation property is less than that of diesel fuel. These factors created an inferior atomization quality for biodiesel. In this study, the injection parameters were adjusted to improve the atomization quality of biodiesel. The macroscopic spray characteristics of biodiesel at different injection pressures and orifice diameters with respect to the STP and SCA were measured by
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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a high-speed camera system. In addition, the microscopic spray characteristics, such as the statistical size distribution, SMD, representative diameters and dispersion boundary, were tested and analyzed using a Malvern laser particle size analyzer at different injection pressures and orifice diameters.
2.
Experimental setup and procedure
2.1.
Biodiesel compositions and properties
The biodiesel used in this study was produced from soybean oil by transesterification with methanol. The biodiesel was made by a biotechnology company. The soybean oil was converted to biodiesel via a chemical reaction process of transesterification, where the soybean oil was combined with ethanol or methanol in the presence of a catalyst to generate fatty acid esters and glycerin. In general, methanol is used instead of ethanol because it reduces the production cost of biodiesel, making biodiesel more competitive in the market of petroleum diesel fuel. The biodiesel was composed of various fatty acid methyl esters with different carbon chain lengths, and its composition was tested by gas chromatography, as listed in Table 1. The soybean biodiesel contained 89.76% fatty acid methyl ester, where greater than 30.26% was saturated fatty acid esters, and unsaturated fatty acid esters accounted for 59.51%. Linoleic acid esters, oleic acid esters, and palmitic acid esters were the major compositions of soybean biodiesel. The physicochemical properties of the biodiesel and diesel were tested using the national standard methods, as listed in Table 2. The density of the soybean-based biodiesel was approximately 4.60% higher than that of conventional diesel. The viscosity of the biodiesel was approximately 62.80% higher than that of diesel. The surface tension of the biodiesel was approximately 7.10% higher than that of diesel. The distillation temperature of the biodiesel was higher than that of diesel. These factors resulted in a smaller SCA and longer STP for the biodiesel fuel than those of the diesel fuel, and the SMD and the representative
Table 1 e Major compositions of soybean biodiesel. Type of fatty acid esters Dodecanoic Myristic Palmitic Palmitoleic Heptadecanoic Stearic Oleic Linoleic Linolenic Arachidic Eicosenoic Behenic Docosadienoic Lignoceric Tetracosenoic Others Total
Carbon chain
Mass (%)
C12:0 C14:0 C16:0 C16:1 C17:0 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1
0.21 0.71 15.26 0.80 8.90 3.92 17.83 34.45 4.67 0.35 0.38 0.65 0.22 0.26 1.16 10.23 100.00
Table 2 e Physicochemical properties of soybean biodiesel and petroleum diesel. Item
Diesel Biodiesel
Density (20 C) (kg/m3) Kinematic viscosity (20 C) (mm2/s) Surface tension (20 C) (103N/m) Flash point ( C) 10% distillation temperature ( C) 50% distillation temperature ( C) 90% distillation temperature ( C)
836 4.81 26.55 64 218 272 338
874 7.83 28.43 160 328 337 354
Test method GB/T1884 GB/T265 GB/T1884 GB/T261 GB/T6536 GB/T6536 GB/T6536
diameters of the biodiesel were greater than those of diesel. Meanwhile, the size-volume distribution curve and the cumulative volume distribution curve of the atomized biodiesel droplets shifted to larger diameters (Geng et al., 2014), which showed that the spray and atomization quality of the biodiesel was inferior to that of diesel. The injection pressures and orifice diameters should be adjusted to improve the spray quality of the biodiesel.
2.2.
Experimental apparatus and procedure
Fig. 1 shows a schematic of the high-speed camera system, which was used to measure the SCA and STP. The system was composed of a fuel tank, a fuel injection pump, a highpressure fuel pipe, an injector, a light source, a high-speed camera (PhantomV 9.1), the black background with a ruler, and a computer. The model of the high-speed camera was PhantomV 9.1, which is produced by American Vision Research, Inc. The highest resolution of the high-speed camera was 1632 1200 pixels. The minimum exposure time was 2 ms, and the maximum photographic speed was 153,846 frames/s at the resolution of 96 8 pixels. In this study, the high-speed camera was applied to record the spray images at the photographic speed of 1200 frames/s and the resolution of 1632 1000 pixels. The recorded spray images were processed by MATLAB software. First, the images were converted to grayscale images. Second, a fixed threshold was chosen for all spray images. Finally, the spray images were extracted from the black background, and the SCA and STP were measured.
Fig. 1 e Schematic of the high-speed camera system.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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Fig. 2 shows a schematic of the Malvern test system, which was used to test the statistical size distribution and SMD of the atomized droplets. The system was composed of a fuel tank, fuel injection pump, high-pressure fuel pipe, injector, Malvern laser particle size analyzer (PSA), and computer. The measurement principle of the Malvern laser PSA is based on the theory of Fraunhofer diffraction, and its measurement range for the droplet diameters was 0.02e2000 mm. The size distribution and the mean diameters of the atomized droplets were automatically measured and calculated from the energy distribution of laser diffraction, and the measuring accuracy for the smallest particles was 0.02 mm. The particles size measured by the method was not affected by the position of the particles in the spray domain; therefore, the Malvern laser PSA could be applied to measure a high-density spray domain, and high test accuracy could be ensured for the fast moving particles. The Malvern test method measured the fuel spray characteristics in a short time with various nozzles under different ambient conditions, and the test method is widely used in the measurement of particle size distribution with considerable savings in time and manpower. A single-hole nozzle was used in the fuel injection system. The orifice length was constant at 1.5 mm, and the orifice diameters were 0.260, 0.315, and 0.366 mm. The fuel injection pressures were separately adjusted to 16, 20, and 24 MPa. The macroscopic and microscopic spray characteristics of the biodiesel were investigated by the high-speed camera and PSA under different injection pressures and orifice diameters.
3.
Results and discussion
3.1.
SCA and STP
The SCA and STP have a significant influence on the spatial distribution of the atomized droplets in the combustion chamber. In order to avoid colliding and bonding of the spray droplets in the movement process, the fuel sprays injected from the multi-orifice nozzle should not intersect. This is an important factor to consider when designing the SCA of a multi-orifice nozzle. Moreover, the SCA and STP should match with the geometry of the combustion chamber. If the STP length allows most of the fuel droplets to collide with the cylinder wall, the fuel film of the cylinder wall will be too thick. The thick fuel film on the cylinder wall may not
evaporate and combust completely, which could create heavy combustion chamber deposits. Using different injection pressures and orifice diameters, the test results of the SCA and STP are listed in Table 3. With an increasing injection pressure, the SCA of the biodiesel enlarged gradually. This was because the fuel injection velocity was accelerated, and the Reynolds number of the biodiesel flowing in the nozzle was enhanced with an increasing injection pressure. The growing disturbance and turbulence inside the nozzle, promoted the breakup and atomization of the primary spray, and ultimately increased the SCA. With an increasing injection pressure, the STP of the biodiesel increased. This was because the high injection pressure increased the fuel injection velocity and extended the STP. In addition, with an increasing orifice diameter, the SCA of the biodiesel increased, and the STP decreased gradually. This may be because the large orifice diameter increased the contact area between the fuel spray and ambient air, thus increasing the air disturbance and aerodynamic drag. In addition, breakup of the primary spray was promoted. Therefore, the SCA of the biodiesel was enlarged with an increasing orifice diameter. In addition, with a decreasing orifice diameter, the STP of the biodiesel increased. The STP was mainly influenced by the injection velocity and aerodynamic drag. The smaller the orifice diameter of the nozzle, the faster the flow speed of the fuel in the nozzle, and the STP would be increased owing to the higher fuel injection velocity. Meanwhile, the smaller orifice diameter created a smaller SCA, which resulted in a smaller surface area of the fuel spray front that contacted the ambient gas. The aerodynamic drag for the fuel spray affected by the ambient gas was significantly reduced; therefore, the STP was further increased.
3.2.
Size distribution curves of the atomized droplets
Some researchers have reported that increasing the injection pressure provided better brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) than those of the original injection pressure for diesel and biodiesel blends (Shehata et al., 2015). Therefore, the size-volume distribution of the atomized biodiesel droplets was tested by the Malvern laser PSA with the increased injection pressures of 16, 20, and 24 MPa. The influence of the injection pressure on the size distribution of the atomized biodiesel droplet was analyzed in this study. The test results are shown in Figs. 3 and 4. In Fig. 3, PDFV represents the volume of certain diameter droplet as a percentage of the total volume of spray droplets. In Fig. 4, VC represents the percentage of all
Table 3 e SCA and STP of the biodiesel under different injection pressures and orifice diameters. Injection 24 MPa/ 20 MPa/ 16 MPa/ 16 MPa/ 16 MPa/ pressure/ 0.366 0.366 0.366 0.315 0.260 orifice mm mm mm mm mm diameter Fig. 2 e Schematic of the Malvern test system.
SCA ( ) STP (cm)
16.10 30.6
14.30 28.4
13.10 22.0
8.56 35.7
6.33 57.6
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
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Fig. 3 e Size-volume distribution curves of the atomized droplets under different injection pressures.
Fig. 5 e Size-volume distribution curves of the atomized droplets under different orifice diameters.
droplets volumes smaller than a certain diameter to the total volume of spray droplets, which is called the cumulative sizevolume distribution of the atomized droplets. With an increasing injection pressure, the curve of the size-volume distribution and the cumulative volume distribution of the atomized biodiesel droplets moved toward smaller diameters. Increasing the injection pressure could reduce the diameter of the atomized droplets and improve the spray quality of the biodiesel fuel. The higher injection pressure resulted in a faster injection velocity, and the velocity difference between the ambient gas and fuel spray was enlarged. Therefore, the aerodynamic effects and turbulence development were enhanced, which promoted the fragmentation of the atomized fuel droplets. The orifice diameter is an significant parameter that influences the cycle fuel injection quantity and engine performance. When the needle valve is fully lifted, the orifice diameter will affect the peak injection rate and cyclic injection quantity (Han et al., 2018). Furthermore, the smaller orifice diameter can improve the air-fuel mixing, atomization, and vaporization, leading to a shorter combustion duration and higher BTE (Kumar et al., 2018). The size-volume distribution of the atomized biodiesel droplets was measured by the Malvern laser PSA under the injection pressure of 16 MPa with different orifice diameters of 0.260, 0.315, and 0.366 mm. The effects of the orifice diameter on the size distribution of the atomized droplets were analyzed. Fig. 5 shows the size-volume distribution
curves of the atomized droplets, and Fig. 6 shows the cumulative volume distribution curves of the atomized droplets using a nozzle with different orifice diameters. With a decreasing orifice diameter, the size-volume distribution curve of the atomized biodiesel droplets gradually shifted to a smaller diameter. When the orifice diameter was 0.366 mm, the peak diameter of the sizevolume distribution curve of the atomized biodiesel droplet was 72 mm, and the volume percentage corresponding to the peak diameter was 15.01%. However, when the orifice diameter was 0.26 mm, the peak diameter of the sizevolume distribution curve of the atomized biodiesel droplet was 61 mm, and the volume percentage corresponding to the peak diameter was 12.47%. In addition, with a decreasing orifice diameter, the cumulative volume distribution curves of the atomized droplets shifted towards a smaller diameter, and the corresponding size of the atomized biodiesel droplets gradually decreased when the different cumulative volumes are obtained. Therefore, reducing the orifice diameter decreased the droplet size and improved the atomization quality of the biodiesel. The flow velocity of the biodiesel inside the nozzle was accelerated owing to the reduced orifice diameter, increased Reynolds number of the fuel flowing in the nozzle, intensified disturbance of the internal flow in the nozzle, increased turbulence and fragmentation of the circular jet. Therefore, the diameters of the atomized droplets decreased, and the atomization quality of the biodiesel was improved.
Fig. 4 e Cumulative volume distribution curves of the atomized droplets under different injection pressures.
Fig. 6 e Cumulative volume distribution curves of the atomized droplets under different orifice diameters.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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3.3.
SMD of the atomized droplets
The SMD is also referred to as the volume-surface mean diameter, and it is defined as shown in Eq. (1). Z
Dmax
Dmin Dmax
Therefore, the atomization quality of the biodiesel was improved, and the SMD of the atomized droplets decreased. The combined effects of the injection pressure and orifice diameter resulted in a similar SMD for 24 MPa with the 0.366 mm nozzle and 16 MPa with the 0.260 mm nozzle.
D3 dN
D32 ¼ Z
(1) D2 dN
Dmin
The parameter N is the droplets number of diameter D. The minimum diameter was Dmin ¼ 0. The parameter D32 was most frequently used to evaluate the fuel spray quality, and it is also a common calculation parameter in the combustion reaction. The SMDs of the atomized biodiesel droplets were measured by the Malvern laser PSA under different injection pressures with different orifice diameters. The test results are listed in Table 4. The SMD of the atomized biodiesel droplets gradually decreased and the atomization quality of the biodiesel was improved with the increase of the injection pressures. This was because the disturbances in the nozzle and the aerodynamic force were enhanced with the increase of the injection pressure. When the fuel injection pressure was increased, the jet speed was accelerated, the Reynolds number (Re) increased, and the turbulence and internal disturbance were strengthened. In addition, the fragmentation of the circular jet was promoted. Furthermore, the relative velocity between the gas phase and liquid phase was accelerated with an increasing injection pressure. The Weber number (We) increased, and the gas disturbance was strengthened. The fuel atomization was improved, and the SMD of the atomized biodiesel droplets decreased owing to the higher injection velocity and greater aerodynamic force. In addition, the SMDs of the atomized biodiesel droplets gradually decreased with a decreasing orifice diameter. This was because the flow velocity of the fuel in the nozzle was accelerated with the reduction of the orifice diameter, and the internal disturbance in the nozzle was enhanced, which promoted of the turbulence and fragmentation of the circular jet. Therefore, the SMDs of the atomized biodiesel droplets decreased with a decreasing orifice diameter. When the injection pressure was decreased from 24 to 16 MPa, the injection velocity decreased. The Re and We values decreased, and the turbulence and gas disturbance were weakened. Therefore, the spray quality of the biodiesel deteriorated. When the nozzle diameter was reduced from 0.366 to 0.260 mm, the flow velocity of the fuel in the nozzle increased. The Re and We values were enhanced, and the turbulence and internal disturbance were strengthened.
3.4.
Representative diameters and dispersion boundary
The representative diameters are defined as the ratio of the volume of the atomized droplets below a certain diameter to the volume of all atomized droplets in the cumulative volume distribution curve of atomized droplets (Mohamad et al., 2015). The subscripts of the representative diameters indicate the volume percentage, where D0.5 is the mass median diameter, and D0.999 is the largest diameter of the atomized droplets. The representative diameters of the atomized droplets are often used in fuel spray and combustion analyses. The representative diameter is an essential parameter for calculating the dispersion boundary of the atomized droplets. The representative diameters of D0.999 are significant when they are used to analyze the HC and soot emissions of the engine. D0.999 represents the largest diameter of the atomized droplets. The large droplets can be generated for the fuel injected later in the cycle or because of overfuelling of the engine. The fuel left in the injector sac volume and the nozzle holes at the end of injection may not fully vaporize during the combustion process (Mohamad et al., 2015). The large fuel droplets enter the engine cylinder during the expansion stroke, which results in undermixing of the fuel with air; thus, the fuel is not fully burned. This is one of the main sources of HC in the diesel engine. In addition, it is difficult for the large droplets to evaporate completely. When the temperature in the cylinder is high, the fuel may not be uniformly mixed, and some fuel-rich zones may exist. Incomplete combustion and pyrolysis of the fuel droplets will produce soot emission. Table 5 lists the representative diameters of the atomized biodiesel droplets under different injection pressures and different orifice diameters. When the injection pressure was increased from 16 to 24 MPa, the representative diameters of the atomized biodiesel droplets decreased. The size of the atomized droplets decreased with an increasing injection pressure. The amount of small fuel droplets increased, and the amount of large fuel droplets decreased. The atomization quality of the biodiesel was improved. In addition, most of the representative diameters decreased
Table 5 e Representative diameters of the atomized droplets under different injection pressures and orifice diameters. Table 4 e SMDs of atomized droplets under different injection pressures and orifice diameters.
Injection pressure/ orifice diameter
Injection 24 MPa/ 20 MPa/ 16 MPa/ 16 MPa/ 16 MPa/ pressure/ 0.366 0.366 0.366 mm 0.315 0.260 orifice mm mm mm mm diameter
24 20 16 16 16
D32 (mm)
36.23
39.10
49.13
42.29
35.86
MPa/0.366 MPa/0.366 MPa/0.366 MPa/0.315 MPa/0.260
mm mm mm mm mm
Representative diameter D0.1
D0.5
D0.9
D0.999
24.52 26.89 33.42 27.10 20.35
49.87 52.75 63.11 57.47 52.42
85.12 89.83 107.00 101.50 96.37
141.86 167.94 198.82 198.82 167.94
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
J. Traffic Transp. Eng. (Engl. Ed.) xxxx; xxx (xxx): xxx
Table 6 e Relative size range Ds and the dispersion boundary Db under different injection pressures and orifice diameters. Injection pressure/orifice diameter 24 20 16 16 16
MPa/0.366 MPa/0.366 MPa/0.366 MPa/0.315 MPa/0.260
mm mm mm mm mm
Ds
Ds
1.22 1.19 1.16 1.29 1.45
1.84 2.18 2.15 2.46 2.20
with a decreasing orifice diameter. However, the maximum diameters of the atomized droplets were the same at 16 MPa with the 0.366 mm nozzle and 16 MPa with the 0.315 mm nozzle. However, according to the statistical results for the cumulative volume distribution of the atomized droplets, the volume ratio of largest droplets was different in both cases. The volume ratio of largest droplets was 0.1% at 16 MPa with the 0.366 mm nozzle, while the volume ratio was 0.01% at 16 MPa with the 0.315 mm nozzle. Therefore, the number of largest droplets was greater with the 0.366 mm nozzle under the 16 MPa injection pressure, and the spray quality of the biodiesel was improved by reducing the orifice diameter under a constant injection pressure. Moreover, based on the decline degree of the representative diameters, the influence of the injection pressure was greater than that of the orifice diameter. The dispersion degree is used to describe the size range of the atomized droplets, which is commonly represented by the relative size range Ds and the dispersion boundary Db (Cao, 2013). The relative size range is defined as shown in Eq. (2). Ds ¼
D0:9 D0:1 D0:5
(2)
It represents the range of the droplet diameter relative to the mass median diameter. The dispersion boundary is defined as shown in Eq. (3). Db ¼
D0:999 D0:5 D0:5
(3)
It represents the dispersion degree of the maximum droplet diameter relative to the mass median diameter. With an increasing injection pressure, the relative size range of the atomized biodiesel droplets increased slightly, and the dispersion boundary decreased. Thus, with an increasing injection pressure, the size distribution range of the atomized biodiesel droplets showed minimal change. The diameter of the largest droplets decreased, and the size distributions of the droplets became more homogeneous. The atomization quality of the biodiesel was improved. In addition, the relative size range and dispersion boundary of the atomized biodiesel droplets increased with a decreasing orifice diameter. The size-volume distribution curves of the atomized biodiesel droplets became wider and flatter, and the relative size range of the atomized droplets was larger. The size distributions of the atomized biodiesel droplets became uneven (see Table 6).
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Although increasing the injection pressure and reducing the orifice diameter could improve the atomization of the biodiesel fuel, the increase of the injection pressure was more significant for the improvement of the spray quality owing to the larger SCA and a more uniform size distribution of the atomized droplets. In addition, the spray characteristics of the diesel fuel were tested at 16 MPa with the 0.366 mm nozzle. The test results showed that the STP of the diesel was 18.73 mm. The SCA was 15.3 . The SMD was 35.86 mm, and the relative size range and dispersion boundary were 1.17 and 1.91, respectively. The spray parameters of the diesel fuel were similar to those of the biodiesel fuel at 24 MPa with the 0.366 mm nozzle. Therefore, the optimized injection pressure for the biodiesel fuel was 24 MPa, and the same spray quality of the diesel fuel was achieved at 16 MPa with the 0.366 mm nozzle.
4.
Conclusions
The physicochemical property differences between diesel and biodiesel were compared using the test method. The effects of the injection pressure and orifice diameter on the spray characteristics of biodiesel were investigated with the high-speed camera and Malvern PSA. Finally, the macroscopic and microscopic spray characteristics of the biodiesel, such as the STP, SCA, SMD, statistical size distribution, representative diameters, and dispersion boundary, were analyzed in the study. The main conclusions are as follows. (1) Based on the test of the physicochemical properties, the soybean biodiesel contained 89.76% fatty acid methyl ester. The density of the soybean-based biodiesel was approximately 4.60% higher than that of conventional diesel. The viscosity was approximately 62.80% higher than that of diesel. The surface tension was approximately 7.10% higher than that of diesel, and the distillation temperature of the biodiesel was significantly higher than that of diesel. Therefore, the evaporation properties of the biodiesel were inferior to those of diesel. (2) With an increasing fuel injection pressure, the SCA and STP of the biodiesel gradually increased. The size-volume distribution curves and the cumulative volume distribution curves of the atomized biodiesel droplets moved towards smaller diameter, and the SMD and representative diameters decreased with an increasing injection pressure. The size distribution range of the atomized biodiesel droplets showed minimal change, while the dispersion boundary decreased. Therefore, an increasing injection pressure could reduce the number of large fuel droplets and improve the atomization of biodiesel fuel. (3) With a decreasing orifice diameter, the SCA of the biodiesel decreased and the STP increased. The size-volume distribution curves and cumulative volume distribution curves of the atomized biodiesel droplets
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004
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shifted to a smaller diameter, and the SMD and representative diameters decreased. The relative size range and dispersion boundary of the atomized biodiesel droplets increased significantly. Although the decreasing orifice diameters could reduce the mean diameter of the atomized droplets, the droplet size distribution was not homogenous.
Conflict of interest The authors do not have any conflict of interest with other entities or researchers.
Acknowledgments This project is financially supported by the National Natural Science Foundation of China (51806020), the Youth Innovation Team of Shaanxi Universities (Energy Saving and New Energy Vehicles), and the Special Funds for Basic Scientific Research of Central Colleges, Chang’an University (310822172203 and 300102228403).
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Limin Geng is an associate professor, doctor of transportation engineering. Limin Geng has a deep interest in spray, combustion and pollutant emissions of alternative fuels such as biodiesel, methanol and ethanol. In her research experiences, she has accomplished more than 4 research projects, published more than 15 journal papers, and co-edited 2 textbooks.
Please cite this article as: Geng, L et al., Effect of the injection pressure and orifice diameter on the spray characteristics of biodiesel, Journal of Traffic and Transportation Engineering (English Edition), https://doi.org/10.1016/j.jtte.2018.12.004