Effects of Blending Long-Chain Alcohols with Jet Fuel on the Macroscopic Spray Characteristics

Li Fengyu; Liu Yanju; Huang Qifu; Fu Wei; Shi Weidong; Lin Qizhao

(1. Department of Thermal Science and Energy Engineering, Uniνersity of Science and Technology of China, Hefei 230026; 2. Beijing Mechanical Equipment Institute, Beijing, 100854)

Abstract: Long-chain alcohols were considered to be promising alternative fuels and fuel additives. This study was aimed at figuring out the influences of blending long-chain alcohols with jet fuel on the macroscopic spray characteristic. n-Butanol and n-pentanol were chosen as tested alternative fuels. A common-rail injection system was used to create high-pressure injection conditions. The Schlieren imaging system was used to capture spray processes. Results showed that with the addition of long-chain alcohols, the spray tip penetration and the peak spray tip velocity increased, whereas the spray cone angle and the spray area decreased. Comparisons between those macroscopic spray characteristics of AKB blends and AKP blends were also conducted. Compared to AKB blends, AKP blends showed similar spray tip penetration, larger peak tip velocity, smaller spray cone angle, and smaller spray area.

Key words: macroscopic spray characteristics; jet fuel; n-butanol; n-pentanol; common-rail system

1 Introduction

Conventional fuels have been widely used in many kinds of engines for several decades, and more and more interests are being attracted to the renewable resources that can replace those conventional fuels nowadays. Jet fuel is one of the most important commercial fuels[1]. With rapid increase in the number of aircraft, more and more researchers have turned their attention to the sustainability of fuel source and the environmental problems from the exhaust emission of aircrafts during use of jet fuel.

As an effective way to solve these problems, alternative fuels and fuel additives have been coming into researchers’ sight. The key to concerns of using jet fuel could finally be summarized as two points, i.e. usage stabilization and environmental friendliness. The usage stabilization is mainly related to the sustainable resource of alternative fuels and fuel additives, while the environmental friendliness is mainly related to the inherent physical and chemical properties of those alternative fuels and fuel additives.

A lot of work had been conducted on several kinds of potential fuels[2-8], and alcohol was treated as one of the most promising alternative fuels and fuel additives, thanks to its better miscibility with conventional fuels[9-11], better stability of its blends[12]and better convenience to be stored and transported[13], as well as more similar properties to those of conventional fuels[12,14-16]. Compared with the short-chain alcohols, the long-chain alcohols (in the carbon chain of which there exist more than three carbon atoms) have been receiving more and more attention recently.

It is known to all that the combustion and emission performance of fuels are influenced by their spray characteristics to a great extent when fuels are used in engines.

Gao, et al.[17]confirmed that n-butanol showed similar spray characteristics and variation trends to those of diesel. Upon using n-butanol as an additive to soybean biodiesel, Mo, et al.[13]observed the variation of spray characteristics of blended fuels with different mixing ratios. It was inferred that the spray tip penetration and the droplet diameter decreased with the addition of n-butanol. These trends were dominated by the decreasing kinematic viscosity and surface tension of fuel. Furthermore, the addition of n-butanol also enhanced air entrainment along the edge of spray.

As one another long-chain alcohol, n-pentanol also attached many researchers’ interests. Ma, et al.[14,18]confirmed that the heat release process was restrained, and the peak of heat release increased with the combustion being advanced with the addition of n-pentanol into diesel. Because of the effects on spray characteristics, fuel’s expansion was enhanced, and air entrainment was improved. Furthermore, with the addition of n-pentanol the spray tip penetration was decreased under nonevaporation condition, however it could increase under engine-like temperature conditions. Similar to the conclusion mentioned in the literature report[14], Li, et al.[19]observed higher fuel efficiency of fuels containing n-pentanol as compared with that of diesel.

Although many efforts had been done to study the possibility of using long-chain alcohols as alternative fuel or fuel additive to diesel and biodiesel, using them as alternative fuels or fuel additives to jet fuel was unseen to the author’s knowledge. This work was aimed at figuring out the effects of using n-butanol or n-pentanol in jet fuelbased fuels on the spray characteristics and the difference of spray characteristics between fuels with different long-chain alcohols. This research would determine the possibilities of using n-butanol or n-pentanol as alternative fuels of jet fuel from the viewpoint of the spray field, and further, may give explanations for their performance in the real engine during application process.

2 Experimental

2.1 Test rig

In the last several years, many researchers conducted their studies on jet fuel under engine-like conditions[20-24], and these investigations confirmed that the study of jet fuel could be done under high injection pressure and high ambient pressure conditions. In this work, a commonrail injection system was used to produce fuel spray under a high injection pressure reaching up to 120 MPa. A constant volume chamber (with four optical accessing windows having a diameter of 90 mm) with a maximum designed pressure reaching up to 16.0 MPa was used to provide a high pressure engine-like condition by using nitrogen cylinder to pressurized the inner part of the chamber. The experimental system is shown in Figure 1.

A LED light was used as light source, and the Schlieren light path was used to project spray images and a highspeed camera (FASTCAM SA5 1000K-M3, Photron USA Inc.) with 20 000 fps and 1/20 000 s of exposure time was used to capture those images. After having been captured by the high-speed camera, spray images of a complete spray process were processed by a software written by Pascal language under Delphi development environment. The analysis mainly included the following steps：

Figure 1 Schematic diagram of experimental system

Firstly, an image before the presence of spray was chosen and was settled as the background image; secondly, the timing corresponding to the image right before presence of spray was determined as injection timing and then spray images from the image of injection timing to the image in which spray penetrated to the bottom of optical accessing window was chosen as a complete spray process; thirdly, those spray images were scanned and their gray information was compared with that in background image; and finally, the macroscopic spray characteristics were detected and measured. Schematic diagram of image processing is shown in Figure 2, in which the left image was the raw image, the middle image was the project image obtained by eliminating background image from raw image and the right image was the gray image.

Figure 2 Schematic diagram of image processing

2.2 Tested fuels & conditions

In this study, the fuel used as the base fuel was jet fuel, and the fuel additives included n-butanol and n-pentanol, as representatives of long-chain alcohols. The chosen blending ratios (volume proportion between n-butanol (or n-pentanol) and jet fuel) covered 0%, 10%, 20% and 40%, which were labeled as AK100, AKB10 (or AKP10), AKB20 (or AKP20) and AKB40 (or AKP40), respectively. The parameters dominating macroscopic spray characteristics were fuel density and fuel kinematic viscosity, with these two parameters of tested fuels shown in Table 1. The fuel density was calculated by dividing the fuel mass with the volume, and the kinematic viscosity was measured with a kinetic viscometer (SYD-265H). Table 2 shows the test conditions for this study. The injection pressures and ambient pressures chosen for this study were 80, 90, 100 and 1.0, 2.0, 3.0, respectively.

Table 1 Properties of tested fuels

Table 2 Test conditions used in experiments

3 Results & Discussion

In this section, the effects of blending long-chain alcohol with jet fuel on the spray characteristics, i.e. spray tip penetration, spray cone angle and spray area, were presented and analyzed.

Figure 3 shows the definitions of main spray characteristics. Spray tip penetration, abbreviated to S, was the axial distance between fuel nozzle exit and tip of spray. Spray cone angle, abbreviated to θ, was the angle formed by two lines that passed through nozzle exit and the two points of spray edge at S/2. Spray area was the projected area of the spray body.

Figure 3 Definitions of spray characteristics

3.1 Spray tip penetration

Spray tip penetration is an important indicator of the axial development capacity of a spray.

Figure 4 Effect of adding n-butanol or n-pentanol into jet fuel on the spray tip penetration of tested fuel

Figure 4 shows the influence of adding long-chain alcohols into jet fuel on the spray tip penetration of the tested fuels. It can be seen from Figure 4 that compared with blended fuels, pure jet fuel showed the shortest spray tip penetration. With the addition of n-butanol or n-pentanol, the measured spray tip penetration showed an increasing tendency. This phenomenon was dominated by the differences of density and kinematic viscosity between those tested fuels, as shown in Table 1. In Table 1, with the increase of the longchain alcohols proportion, the values of density and kinematic viscosity were increased. With a higher density, a larger axial momentum was given to the tested fuel, and thus a stronger ability to go through the ambient air downstream, this could also be verified via the comparison between tip velocities (which would be shown in next section).The larger value of kinematic viscosity also contributed to the longer spray tip penetration. The tested fuel with greater kinematic viscosity tended to show a less preference to be atomized due to the stronger constraint effect.

Furthermore, the influence of adding long-chain alcohols on the spray tip penetration was also affected by the change of the ambient pressure. It can be seen from Figure 4 that under the same injection pressure, the difference of spray tip penetration between different tested fuels decreased with an increasing ambient pressure. For example, with respect to the n-butanol-jet fuel blended fuels, when the experimental data of pure jet fuel were used as the benchmarks for comparison, and the injection pressure was set at 80 MPa and the n-butanol proportion was increased to 40%, the spray tip penetration increased by 27.08% under an ambient pressure of 1 MPa, and increased by 9.19% under an ambient pressure of 2 MPa, while still increasing by 3.47% under an ambient pressure of 3 MPa. The enhancement of air-resistance dominated this trend. With the increase of ambient pressure, the density of ambient air increased, and the axial development of the spray was impeded, so the change in the range of spray tip penetration decreased.Figure 5 shows the comparison of spray tip penetration between n-butanol-jet fuel blends and n-pentanol-jet fuel blends. It can be inferred that the difference between spray tip penetrations of those two kinds of blended fuels was negligible.

Figure 5 Comparison of spray tip penetration between tested fuels with different long-chain alcohol

3.2 Spray tip velocity

Figure 6 shows the variation of spray tip velocity caused by the addition of long-chain alcohols. With the addition of n-butanol or n-pentanol, the spray tip velocity was increased. This was caused by the increased density and kinematic viscosity, as mentioned before. Due to the higher density, the deceleration effect of the ambient air on the tip of spray was weakened, and upon comparing to the enhanced ability to go through the ambient air downstream, the importance of this effect decreased. The increased kinematic viscosity also played an important role, it made the tip of spray denser, and thus the tip of spray was more likely to maintain its peak velocity.

Figure 6 Variation of spray tip velocity with addition of long-chain alcohol

In order to demonstrate the difference of spray tip velocity between different tested fuels more clearly, the comparison of peak tip velocity between different tested fuels was conducted, as illustrated in Figure 7. As an important factor of initial axial momentum, the peak tip velocity could also be an indicator of the development ability of spray. Figure 7 shows that with the addition of long-chain alcohol, the peak tip velocity demonstrated an increasing tendency, as showed in Figure 6. Furthermore, the comparison in Figure 7 also shows that adding n-pentanol could bring about a higher peak tip velocity than that brought about by adding n-butanol.

Figure 7 Peak tip velocity of tested fuel during spray process

3.3 Spray cone angle

Spray cone angle is always used as an indicator of spray radial development.

Figure 8 shows the effects of adding n-butanol and n-pentanol into jet fuel on the spray cone angle of the tested fuels. In general, all the spray cone angles showed a rapid increase during the early period of the spray process, and then they all decreased to relatively stable values. This fact could be attributed to the movement of the injector needle. A great fluctuation appeared during the needle lift, at this time the spray was not stable yet. Once the needle was open fully, the spray stabilized, and then a relatively stable value of the spray cone angle was measured.

Figure 8 Influence of adding long-chain alcohol on spray cone angle of tested fuel

It can be easily found from Figure 8 that AK100 showed a largest spray cone angle, and with an increasing proportion of long-chain alcohol in blended fuels, the spray cone angle showed a decreasing tendency, the smallest value of the spray cone angle appeared when the proportion of long-chain alcohol reached 40%. The increased density and kinematic viscosity could affect the change in spray cone angle. With an increasing density, the tested fuel could go through the ambient air downstream more easily, and then the body of the spray was more likely to develop axially. At the same time, the increment of kinematic viscosity also gave a stronger constraint force for the radial diffusion of the spray, making it harder for the spray to develop radially. By combining the effects of the two aspects, the spray cone angle decreased with the addition of n-butanol and n-pentanol.

Differences in the spray cone angle between the n-butanoljet fuel blends and the n-pentanol-jet fuel blends are exhibited in Figure 9. It can be found from Figure 9 that the blends with n-pentanol tended to exhibit a smaller spray cone angle as compared with the blends with n-butanol. And Figure 9 also shows that with the increase of ambient pressure, the difference between these two kinds of blended fuels increased. For example, when the spray cone angle of AKB40 was used as the benchmark for comparison, and the injection pressure was set to 90 MPa, the spray cone angle of AKP40 was by 16.33% smaller than that of AKB40 under an ambient pressure of 3 MPa, while it was by only 6.21% smaller than that of AKB40 under an ambient pressure of 1 MPa. This fact could be attributed to the enhanced airresistance caused by the increased ambient air density. With a stronger air-resistance, the body of the spray tended to transform part of the axial momentum into the radial momentum so that the radial development was promoted, and then the difference on the spray cone angle was enlarged.

Figure 9 Comparison of spray cone angle between tested fuels blended with different long-chain alcohol

3.4 Spray area

Spray area is an intuitive concept, as the area occupied by the spray in the observation window in Figure 3.

Figure 10 exhibits the comparison of spray area between the fuels tested. Pure jet fuel showed the large spray area in all the images of Figure 10. As we know, a longer spray tip penetration would enlarge the spray area, while a smaller spray cone angle would reduce the spray area. Under the balance between the two aspects, the addition of long-chain alcohol showed a reducing effect on the spray area of the tested fuels. In Figure 10, the n-pentanol-jet fuel blends tended to show smaller spray area as compared with n-butanol-jet fuel blends. The measured data in the past sections could be used to figure out this phenomenon. Under the same test condition, and with the same mixing ratio, the n-pentanol-jet fuel blended fuel had a similar spray tip penetration and a smaller spray cone angle as compared to the n-butanol-jet fuel blended fuel. From this point of view, the result of comparison could be easily understood. Furthermore, a smaller spray area could explain why the body of spray was denser to some extent. In turn, it could be a proof for the results and discussion about spray tip penetration and spray cone angle referred to in the past sections.

4 Conclusions

In this study, n-butanol and n-pentanol were chosen as representatives of long-chain alcohols, and they were used as alternative fuels. High-pressure injection conditions were created with a common-rail injection system, and a constant volume chamber was used to create the pressurized ambient environment. Based on the spray images captured with the Schlieren imaging system, the macroscopic spray characteristics were measured. Test results could be stated as follows：

Figure 10 Comparison of spray area between tested fuels

1) Adding long-chain alcohols could increase the spray tip penetration of tested fuels. The enhanced axial momentum was a leading factor, and the larger constraint effect on reducing atomization also contributed to this tendency.

2) The tested fuel with a higher proportion of the longchain alcohol exhibited a larger value of peak tip velocity. This verified the higher initial axial momentum brought about by the addition of the long-chain alcohols

3) The spray cone angle decreased with the addition of long-chain alcohols. Larger proportion of long-chain alcohols brought about larger constraint effect for the radial diffusion of the blended fuels.

4) AKP blends showed similar spray tip penetration, along with larger peak spray tip velocity, smaller spray cone angle, and smaller spray area to those of AKB blends.

Acknowledgement:This work was supported by the National Natural Science Foundation of China (No.51376171).